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Abstract
The rapid construction of prefabricated components of reinforced concrete structures using 3D printing concrete as a permanent formwork is a promising way to organically combine 3D printing with traditional construction technology. The bonding property of the contact interface between the 3D-printed permanent formwork and internal post-cast concrete is crucial for the 3D-printed structure to maintain the overall mechanical performance. In this study, the roughness of 3D-printed formwork (10 mm, 15 mm, 20 mm) and the age difference between the formwork and post-cast concrete (1 d, 3 d, 7 d, and 14 d) were set as variables to investigate their influence on the interfacial tensile and shearing bonding properties. Additionally, computerised tomography (CT) scanning was used to visualise the meso-structure of the interfacial zones and the pore characteristics were quantified. The microstructure and hydration products of the bonding interface were detected, and the gradient structure of the interface transition zone of the 3D printing formwork was presented. A plastic limit analytical mechanical model of the interfacial shear strength was established. The results show that the tensile and shear properties of the bonding interface were the best when the layer height of the printing formwork was 20 mm, and it is recommended to cast internal concrete materials after 7 d of 3D printing formwork moulding.
... In view of this, the introduction of 3D printed concrete as formwork has been extensively explored to promote its structural application [31]. Standard implementation procedure starts with 3D printing the outer contour of the structural components. ...
... Zhu et al. [32] investigated the 3D concrete printing of permanent formwork for concrete columns, and suggested that the concave-convex interface enabled satisfactory interface adhesion and load transfer mechanism between the cast-in-place concrete and the printed concrete permanent formwork, and thereby improved the load-bearing capacity of the composite columns. Wang et al. [31] have elucidated the influences of surface roughness of 3D-printed formwork and the age difference between the formwork and post-cast concrete on the interfacial tensile and shearing bonding properties. Accordingly, 20 mm layer thickness of the printing formwork Manuscript Prepared for Journal of Intelligent Construction 5 as well as to cast internal concrete materials after 7 d of 3D printing formwork moulding were recommended. ...
... E and G represent only the elastic constants in the specified directions. The bonding force of the contact interfaces is practically dominated by the localized chemical bonding force, mechanical interlocking force, and chemical bonding force, etc [31,44]. Cracks at aggregate interface: It happens when the interface between formwork and concrete is comparatively more robust than the aggregate interface. ...
... Formwork is a temporary or permanent mold which can be contained and shaped while wet until it hardens, and can support itself and all additional loads during construction [1]. The temporary formwork is removed when the concrete has gained sufficient strength, while the permanent types are integrated as permanent parts of the structure [2]. Formwork is a crucial aspect of concrete construction, representing a significant proportion of the total cost and is required for a major part of the time during cast-in-place projects [3]. ...
... Digital technology, such as Building Information Modeling (BIM), can improve formwork design efficiency, reduce material waste, and streamline the construction process, leading to cost savings [50,126,145]. Additionally, technologies like 3D printing offer the potential to create custom formwork components at lower costs [2,40,59]. However, these technologies also require investment in software, equipment, and training. ...
This study provides a comprehensive review of the engineering challenges of formwork in concrete construction. The paper investigates different formwork systems, their design based on form pressure, and the difficulties of form stripping. Alternative binders are gaining more and more interest by opening new opportunities for sustainable concrete materials and their impact on form pressure and concrete setting is also investigated in this paper. The discussion involves several engineering challenges such as sustainability, safety, and economy, while it also explores previous case studies, and discusses future trends in formwork design. The findings pinpoint that choosing an appropriate formwork system depends significantly on project-specific constraints and that the development of innovative materials and technologies presents significant benefits but also new challenges, including the need for training and regulation. Current trends in formwork design and use show promising possibilities for the integration of digital technologies and the development of sustainable and ‘smart’ formwork systems. Continued research within the field has the possibility to explore new formwork materials and technologies, which will contribute to the implementation of more effective and sustainable practices in concrete construction.
... A few components produced via 3DCP are utilised as permanent formwork, ornamental elements, or other non-loadbearing components. See Fig. 7 for a building method for permanent formwork structures that the Hebei University of Technology [260] has developed. This potential method may be used with the present design requirements for reinforced concrete buildings to enhance the flexibility and automation benefits of 3D printing. ...
... Hebei University of Technology's bridge components: (a) Permanent 3D printed formwork for an irregular shape of arch construction, (b) providing reinforcement cage inside the formwork, (c) pouring of concrete; reproduced from[260]. ...
3D printing of concrete is a rapidly growing additive manufacturing technology, bringing advantages like freedom of geometry, formwork-free construction, reduced construction time, and wastage, contributing towards a sustainable built environment. The technology also has the potential to mimic bio-inspired material archi-tectures with enhanced performance. Among the available additive manufacturing techniques, the scalability and the speed of construction motivate the adaptation of extrusion-based concrete 3D printing technology. For conventional concrete pumping, the rheological behaviour and the mix design have been well established. However, for 3D printing, the pumpability of the mix which is governed by its rheology and the flow mechanism has to be viewed in conjunction with the extrudability, shape-retention, and buildability. This study provides a review of significant rheological properties, the effect of various material mixes, and printer configurations on the pumping of conventional as well as printable concrete, and a general comparison between them. The requirements of a pumpable concrete mix based on rheology and the optimal hardened properties for the 3D printing application are discussed. The significant rheological properties affecting the printability of concrete are plastic viscosity, static yield stress, thixotropy, and open time. A detailed review of various factors affecting the pumpability of a printable concrete mix is presented. The key constituents of a concrete mix affecting pump-ability identified here are the type of binder(s), chemical admixture, the inclusion of nano-fillers, aggregate shape, size and grading, and aggregate-to-cement ratio. The range of yield stress and plastic viscosity are discussed for the 3D printable mixes. Recommendations are provided to improve the process and quality of 3D concrete printing. State-of-the-art applications and the potential of this technology to manufacture nature-inspired material architectures with superior toughness is highlighted.
... Scholars have proposed methods of synchronously printing steel cables on the filament or interlayer [24,25], using robots to place mesh components [26], laying prefabricated reinforcing elements [27], printing beside U-shaped wire mesh [28], planting nails inside interlayer areas [29], and barbed-wire reinforcement between layers, etc. For reinforcing after printing, 3D-printed concrete permanent formwork (3DPCPF) and then resetting the steel cages is a possible solution and is currently recognized and successfully utilized in many construction cases [8,[30][31][32][33]. 3DPCPF can achieve better advantages in manufacturing special-shaped building components and lessening material wastage. ...
... Similar phenomena also occurred in Refs. [8,30], which will be explained later. ...
... Further studies are required on other mechanical properties and problems like environmental degradation and integration of tension reinforcement for the elements. Yang Y. et al. (2022) found that the mechanical strength properties of 3DP ultra-highperformance fibre-reinforced concrete (3DP-UHPFRC) are better than the Mould Casted ultra-high-performance fiberreinforced concrete (MC-UHPFRC) due to the directional distribution of steel fibers in 3DP specimens. Though the compressive elastic modulus was anisotropic, the variation in tensile elastic modulus was insignificant. ...
... Studies are required in the direction of the structural performance of 3D printed formwork structures, which depends on coordination between the formwork and postcast concrete. Wang et al. (2022) did a study to get a relationship between the mechanical properties at the interface of formwork and the casted concrete of the final product, the roughness of 3D printed formwork, and the time gap between the printing of formwork and casting of concrete. The optimum roughness and the time gap between the printing of formwork and casting were presented. ...
Concrete 3D Printing (3DP) is a potential technology for increasing automation and introducing digital fabrication in the construction industry. Concrete 3D Printing provides a significant advantage over conventional or precast methods, such as the prospects of topologically optimized designs and integrating functional components within the structural volume of the building components. Many previous studies have compiled state-of-art studies in design parameters, mix properties, robotic technologies, and reinforcement strategies in 3D printed elements. However, there is no literature review on using concrete 3D Printing technology to fabricate structural load-carrying elements and systems. As concrete 3DP is shifting towards a large-scale construction technology paradigm, it is essential to understand the current studies on structural members and focus on future studies to improve further. A systematic literature review process is adopted in this study, where relevant publications are searched and analyzed to answer a set of well-defined research questions. The review is structured by categorizing the publications based on issues/problems associated with structural members and the recent technology solutions developed. It gives an overall view of the studies, which is still in its nascent stage, and the areas which require future focus on 3D printing technology in large-scale construction projects.
... 3DCP has gradually developed from architectures to structural practices [7,8]. The performances of adopted cementitious materials are the key to the structural application and promotion of 3DP technology [9,10]. ...
... In this experiment, samples with 0 %, 0.18 %, 0.30 %, 0.42 %, and 0.54 % of retarders were prepared, which were marked as M0, M1, M2, M3, and M4, respectively. According to the previous research results, the water-cement ratio in the range of 0.18 to 0.4, and the sand-to-binder ratio in the range of 1.1 to 1.7, were suggested to meet the requirements of the 3DPC [2,8,11,[35][36][37][38]. In this study, the water-cement ratio was selected to be 0.3, and the sand-to-binder ratio was 1.4. ...
Investigation into the mechanical performances of 3D-printed composite exposed to elevated high-temperatures is of great significance for broadening the engineering application field of 3D printing. Aluminate cement-based composite was developed for 3D printing. The setting time, flowability, and printability of the composite material were adjusted by adding proper dosages of retarder. Then the compressive and flexural properties of the 3D-printed aluminate cement-based composites were evaluated, as well as the residual mechanical properties after heat-treatment at 200, 400, 600, 800, and 1000 °C, respectively. The pore distribution characteristics and the difference between the interface and matrix areas of 3D-printed composites after heat-treatment were systematically quantified through CT microscopic examinations and their relationship with the mechanical anisotropy was clarified. Additionally, the microstructural evolution were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD) and thermogravimetric-differential scanning calorimetry (TG-DSC) technologies. The results indicate that high temperature has a more significant effect on the interfaces of 3D-printed concrete, and the increase of porosity at interfaces was 23 % higher than that at the matrix after exposure to 1000 °C. At different temperatures, the average porosity in the Y direction was lower than that in the X and Z directions, and therefore result in the damage anisotropy.
... However, the parametric design of the building should take into account the possibility of rounding of the foundation structures. For example, in the literature Wang L. et al. [41] have been printed structures that could have potential use in foundation structures. Reinforcing steel elements in the form of reinforcing baskets were placed in these structures and then concreted with commercial concrete mix to monolithize the structures. ...
... With the emergence of 3DPC technology, researchers have attempted to incorporate steel reinforcement to address its inherent weakness in bending resistance. This includes the design of steel reinforcement printing devices, synchronous insertion of rebar [12,13], interlayer reinforcement mesh embedding [14], and post-printing reinforced concrete casting [15,16]. Consequently, the study of chloride ion diffusion in 3DPC has become increasingly crucial. ...
Three-dimensional printed concrete (3DPC) is an anisotropic heterogeneous material composed of a concrete matrix and the interfaces between layers and filaments that form during printing. The overall ion transport properties can be characterized by the equivalent diffusion coefficient. This paper first establishes a theoretical model to calculate the equivalent diffusion coefficient of 3DPC. Verification through numerical calculations shows that this theoretical model is highly precise. Based on this, the model was used to analyze the effects of dimensionless interface parameters on the equivalent diffusion coefficients in different directions of 3DPC. Finally, the dynamic ionic transport properties of 3DPC were investigated through finite element numerical simulation. The results of the dynamic study indicate that interfaces have a significant impact on the ion distribution and its evolution within 3DPC. The product of the interface diffusion coefficient and interface size can represent the ionic transport capacity of an interface. The stronger the ionic transport capacity of an interface, the higher the ion concentration at that interface. Due to the “drainage” effect of lateral interfaces, the ion concentration in the middle of 3DPC with a smaller equivalent diffusion coefficient is higher than that in 3DPC with a larger equivalent diffusion coefficient.
... From the point of sustainability, this formwork type can be further subdivided according to the used material (congruent concrete material or permanent non-concrete material) or the structural function (load bearing supporting formwork or the opposite). Possibilities of using 3D-printed concrete as lost formwork can be found in various pilot projects [68][69][70][71][72][73] with the load-bearing characteristics not fully disclosed. Possible applications for non-concrete lost formwork are concretesaving slab constructions [74,75] or reinforcement for the structures, e.g. by using fibre reinforced polymers (FRP) [76][77][78][79][80][81][82]. ...
This comprehensive study delves into scientific publications, distinguishing general literature from formwork-specific findings and giving an overview of research methods and formwork systems for concrete construction. A shift towards reusable materials and the widespread use of robotics, particularly in 3D printing is noticeable. Structural research maintains a balanced focus, with little attention set on beams. While automation is attracting interest, full integration into formwork-related publications is still evolving. The need for practical implementation, applicability to mass production and sustainability aspects requires further exploration. Concluding, a list of future challenges is presented, serving as a guidance for subsequent research and developments in research and the concrete industry.
... Система опалубки представляет собой временную или постоянную строительную форму, которая включает в себя опалубку и элементы, обеспечивающие ее жесткость и устойчивость, такие как несущие конструкции, строительные леса и крепежные элементы [19]. Существует большое разнообразие систем опалубки [20]. ...
The object of the study is the building of the National Space Center in Moscow. Method. In the course of research, the provisions of system analysis, methods of expert assessments, risk theory, methods of economic and mathematical modeling, simulation modeling, comparative method, method of generalization, abstraction, induction and deduction were used. Results. The analysis of theoretical sources and calculated data based on the project of a high–rise construction object was carried out, as a result of which the criteria for the characteristics of the object were established, providing conditions for the economically profitable use of self–lifting formwork, and a financial and economic model for justifying investments in this project was built.
... The application of 3D concrete printing in construction has been extensively researched, as evidenced by various research papers [22][23][24][25]. Additionally, 3D printing of steel-based materials is gaining popularity within the industry, particularly in the structural aspect, as demonstrated in the literature [26][27][28][29][30]. Furthermore, 3D printing has been explored as a viable option for producing formwork in the construction industry, as discussed in [31][32][33]. These applications can enhance the efficiency of structural component construction by facilitating the use of optimized shapes and reinforcement layouts. ...
This study presents a comprehensive evaluation of the behavior of two large-scale reinforced concrete deep beams designed using the Generative Tie Method (GTM). Different admissible values of the loss of compressive strength in the concrete (η adm) were considered, and the results were compared to a deep beam designed with the help of the Strut-and-Tie Method (STM). The study highlights the importance of selecting an appropriate η adm value for designing reinforced concrete deep beams capable of exhibiting ductile behavior using the GTM. The results show that specimen SIIIa, designed using the GTM with a η adm of 0.5, demonstrated a highly optimized reinforcement layout with the most pronounced reinforcement yielding, indicating ductile behavior. Among the studied specimens, the one designed using GTM with an η adm of 0.7 exhibited the lowest maximum crack width and the highest ultimate load despite demonstrating brittle failure. Both GTM-designed specimens demonstrated maximized material efficiency and higher ductility ratios than the deep beam designed using STM. By integrating the bond-slip model proposed in the fib Model Code 2010 into the finite element analysis framework, the numerical simulations produced highly accurate predictions of the behavior of the deep beams. The authors suggest further investigations to enhance the proposed numerical strategy and apply the GTM for designing reinforced concrete deep beams with organic or optimized shapes.
... In addition, the use of 3D concrete printing technology to produce permanent formwork for building components can maximize the material, time, and cost savings, especially when formwork with complex geometries is required [10]. Studies have shown that casting can only take place when the 3D printed model has been molded for 7 days [11]. Appropriate addition of micro-expanders or increasing the proportion of small and medium size coarse aggregates in cast-in-place concrete to create structural elements will result in better bonding [12,13]. ...
Modular integrated construction (MiC) is a new type of assembled building structure system that consists of prefabricated concrete modules connected using post-cast concrete. To reduce material consumption and realize casting without supporting molds, thin and lightweight concrete formworks (MiC formworks) with a thickness of 30 mm are installed as part of the shear wall. Due to the thinness, concrete pouring tends to cause MiC formwork cracking, mold rising, and other problems. Its stress performance and damage mechanism are not clear. For this reason, three groups of MiC formworks with different material composition types are designed. The static load test is carried out in a graded partition loading mode, and parametric analysis is combined with numerical simulation to systematically study the influence of different material components on the mechanical properties of MiC formworks. The results show that the front cracks of the MiC formworks are mainly distributed under the truss tendons, and the back cracks are mainly distributed in the span position of the adjacent truss tendons. These cracks both occur along the span direction of the MiC formworks. Increasing the concrete strength has a significant effect on improving the load-bearing capacity of MiC formworks, while incorporating steel fibers can significantly improve its deformation and crack resistance. Parametric analysis showed that the steel fiber admixture exhibited limited improvements in the cracking resistance of the panels as the concrete matrix grade increased. The research results provide a practical basis for optimizing the production process of MiC formworks.
... Moreover, the mechanical properties and durability of printed concrete structures are significantly impacted by the interlayer bonding [31][32][33][34], since the compressive, flexural and tensile strength, as well the resistance to chloride penetration, carbonation and water movement of 3DPC elements reduce significantly compared to molded concrete. These findings were confirmed that most of these reasons are pore defects at the neighbor layers, particularly the pore fractions, sizes, shapes, interconnectivity, distribution and positions [26,[35][36][37][38][39][40][41]. Merely, apart from the above studies, it should also be noted that the effect of the stacked layers vertically on printed elements, when regarding to the mechanical properties and durability in 3DPC structures. ...
... Considering the weak tensile, shear, and flexural capabilities of concrete, 3DCP technology is limited in engineering applications due to the lack of proper reinforcement methods, which arouses much interest from researchers. As a compromised method, 3D concrete printed formworks for conventional casting have been widely implemented, especially in large-scale structures, by Wang Li [27], China Construction Second Engineering Bureau LTD [28], Winsun [29], ETH [12], Apis Cor [30], and so on. Post-tension in 3D concrete printed structures based on traditional construction technologies is another feasible method [20,[31][32][33][34]. Flexible cables embedded in the concrete filament have also been verified as an efficient method [35][36][37]. ...
As numerous impressive large-scale 3D printed concrete structures have been built with 3D concrete printing (3DCP) technology, more attention has been given to the automation, accuracy and reliability of this technology. The concept of Brain-Eyes-Hands Loop (BEH Loop) is defined for use in automatic construction, describing the relationships among design, evaluation, and construction. In this paper, an innovative continuous reinforcement method with fiber reinforced polymer (FRP) flexible textile and Engineered Cementitious Composite (ECC) is presented, that offers an effective solution for the lack of tensile reinforcement and corrosion of steel bars in 3D printed concrete structures. The reinforced 3D printed concrete walls were constructed, evaluated, and tested under quasi-static cyclic loading. By digital rebuilding and geometric evaluation, the printing quality of wall specimens was accessed and ensured. Based on the mechanical tests, the seismic performances of the wall specimens were obtained and found to be mainly bending failure. The proposed reinforcement method was demonstrated for delaying crushing and improving mechanical behaviors. The end column and smaller height-width ratio also showed much improvement in mechanical performance. Compared with the conventional cast concrete wall, the 3DCP wall specimens reinforced by the proposed reinforcement method showed better material utilization efficiency and mechanical properties. Therefore, the 3DCP structures reinforced with flexible FRP textile show great potential for the future use in fully automatic construction of large-scale complex ar-chitectures according to the frame of the BEH Loop.
... The rapid development of 3D concrete printing (3DCP) has the potential to greatly reduce labor demand, improve sustainability, reduce construction costs, and effectively overcome the dilemma faced by traditional construction methods [1][2][3][4]. In recent years, substantial achievements have been made by 3DCP in the field of architecture and civil engineering. ...
3D printed ultra-high performance concrete (3DP-UHPC) plays an important role in the realization of ultra-high compressive and tensile strengths. Considering the particular characteristics of UHPC, the conversion of UHPC to 3DP-UHPC is a complex phenomenon and has been the subject of numerous studies. It is very important to be able to design a thixotropic structure in the early hydration stage for bridging the gap between the slow setting of UHPC and the rapid setting of the 3D printing procedure. In the design and application of 3DP-UHPC, requirements such as the ratio of coagulant and flocculant, fiber alignment, reinforced 3D printed no-rebar reinforced concrete, safety, cost etc. need to be taken into account. We present a comprehensive review of 3DP-UHPC in concrete construction from preparation to application, including design method, raw materials, mechanical, reinforced methods, and applications. Finally, recommendations are provided to promote the application of 3DP-UHPC in engineering practice.
... These topics are currently part of various research efforts (e.g. [56]). ...
... In recent years, three dimensional printing (3DCP) technology has developed rapidly, and has been used increasingly not just in landscape architecture and maintenance components but also in load-bearing structures such as structural components, buildings, bridges, etc. (Xiao et al. 2021b;Wang et al. 2022;Ma et al. 2022;Wang et al. 2021a, b, c;Bai et al. 2021a, b). The high performance of concrete materials is the basis for the development and application of 3D printing in structural engineering. ...
Compared with conventional cast-in-place mortar materials, three dimensional (3D)-printed mortar has the characteristics of high cementitious material consumption, low aggregate-cement ratio, and large water evaporation area, which makes 3D-printed materials and structures more prone to plastic shrinkage and cracking. In this study, polypropylene (PP) fiber and polyvinyl alcohol (PVA) fiber were used to optimize the shrinkage and crack resistance of 3D-printed mortar. The effects of PP and PVA fibers on the printability, mechanical properties , shrinkage, and crack resistance of 3D-printed mortar were tested, and the regulation mechanism was explored. The results show that the optimal content of PP fiber for cracking resistance is 0.3%; the corresponding 28-day compressive strength and flexural strength are 39.2 and 6.9 MPa, respectively, and the shrinkage of the material can be reduced by 13.8% after 120 days. A content of PVA fiber higher than 0.2% is recommended for cracking resistance; the 28-day compressive strength and flexural strength reach 41.3 and 8.0 MPa, respectively, and the shrinkage of the material can be reduced by 13.3% after 120 days. Hence, PVA fiber is recommended to control the shrinkage and cracking of 3D-printed mortar.
... The function of 3D printing concrete is still limited and restricted by structure performance. In fact, 3D printing permanent formwork technology is promising for promoting the application of 3DCP in the field of structural engineering because the reinforcement cage can be designed and fabricated according to existing design specifications [3]. ...
In recent years, the emerging 3D printing concrete technology has been proved to be an effective and intelligent strategy compared with conventional casting concrete construction. Due to the principle of additive manufacturing strategy, this concrete extrusion technique creates great opportunities for designing freeform geometries for surface decoration since this material has a promising performance of high compressive strength, low deformation, and excellent durability. However, the structure behavior is usually questioned, defined by the thickness and printing path. At the same time, the experiments for using 3D printing elements for structural and functional parts are still insufficient. Little investigation has been made into developing reinforcement strategies compatible with 3D printing concrete. In fact, conventional formwork and easy-to-install reinforcement support structures have various advantages in terms of labor costs but can hardly be reused. Thus, using 3D concrete printing as formwork for projects in different scales is an effective solution in the mass customized prefabrication era. Considering large-scale projects, the demand to provide concrete formwork with a proper reinforcement strategy for better toughness, flexibility, and strength is necessary. In this paper, we proposed different off-site reinforced 3D printing concrete strategies and evaluated them from time and material cost, deviation, and accessibility of fabrication.
... The influence of various parameters on the load transfer between digitally fabricated elements is currently being investigated by different authors (e.g. [29,30]). However, the hydrostatic pressure inside an actual water tank would further counteract possible delamination. ...
Incorporating sufficient reinforcement to ensure a ductile structural behaviour is a persisting challenge in digitally fabricated concrete structures. This paper investigates the structural performance of a reinforcement approach for 3D concrete printed elements, consisting of an unreinforced 3D printed concrete shell and a sprayed shell reinforced with a conventional reinforcing mesh for application in water tanks. Four reinforced concrete elements produced with this approach were tested in direct tension and compared to a reference test of a monolithic specimen to analyse the behaviour of circular water tanks under hoop stresses. Two eccentric reinforcement arrangements and two different printing patterns were investigated. Despite the testing setup not perfectly representing the actual behaviour of circular water tanks, in which shell deformations are kinematically restrained, the feasibility of the fabrication method could be examined. The results did not show significant differences in the behaviour of the different fabrication methods, with similar ductility as expected in a conventionally reinforced shell. The eccentric reinforcement caused the crack formation to originate on the surface close to the reinforcement, accompanied by out-of-plane deformations. The cracks on the far side of the reinforcement opened suddenly and reduced the out-of-plane deformations. The predictions with models neglecting the eccentricity of the reinforcement overestimated the crack opening. The best predictions were obtained from the tension chord model by only considering the concrete area defined by twice the mechanical cover of the reinforcement.
... 3D concrete printing has been shown to be an efficient, economical, and intelligent construction method [1,2]. To benefit from these advantages and promote its practical application in the construction industry, research has been developed to translate architectural forms into reliable load-bearing structures [3,4]. The translation has been substantialized and promoted by intensive research efforts from many scientists and engineering practitioners [5][6][7][8]. ...
3D printed ultra-high performance concrete (3DP-UHPC) acting as the reinforcement to in-process reinforce 3D printed concrete (3DP-C) can significantly improve the load-bearing capacity of 3DP-C. This method (3DP-UIRC) is achieved via a dual 3D printing procedure in which 3DP-UHPC as inner core is wrapped by 3DP-C and extruded together. However, lack of printing synergism between 3DP-C and 3DP-UHPC in deposition parameters, such as deposition layer height, printing speed and deposition rate, may result in heterogeneity of composite materials, and thus negates the overall mechanical strength of the structure. Therefore, parameters of nominal proceeding error (NPE) and co-printing factor (CF) are suggested to characterize the simultaneous manufacturing precision of 3DP-UIRC. The two suggested indicators are practically geometrical parameters to reflect the 3D concrete printing quality. Correspondingly, the printing precision error e and area ratio r are defined based on the experimentally measured values for comparison with NPE and CF, respectively. The printing precision error e and the area ratio r are calculated to be 0.37 and 0.163 by evaluating the practical syncing printing results with different layer height and extrusion volumetric flow rates of 3DP-C and 3DP-UHPC. They are approximately equal to NPE of 0.4 and CF of 0.16 with deviations of 7.5% and 2.5%, respectively. This study provides a new concept and quality assessment method for simultaneous printing of 3DP-UIRC. It also provides ideas and scientific references for promoting the application of 3DP-UIRC in 3D printing structured applications.
... With the dexterity and fexibility construction advantages o 3D printing procedure [26][27][28], 3DP-UHPC can be deposited at a specic position inside the structure according to the stress characteristics o the structure. It is known as UHPC reinorced normal strength concrete (URN) to achieve signicant improvements in mechanics, durability, cost-eectiveness, lightweight, green and low carbon, etc. [29,30]. ...
In spite of anisotropy and layer interface weakness intrinsic to the 3D printing procedure, 3D printed ultra-high performance concrete (3DP-UHPC) exhibits excellent mechanical performance due to fiber alignment. For the first time, 3DP-UHPC slab, 3DP-UHPC reinforced normal concrete (PURN) slabs are tested against contact explosions. Explosion resistances with regard to different reinforcing methods, layer thickness ratios and construction methods for base materials are investigated. Specifically, PURN, steel bar reinforced 3DP-UHPC (RU), steel bar reinforced normal concrete (RC), and normal concrete (NC) slabs of similar sizes are constructed to compare the explosion resistances. Different reinforcing layer thickness are attempted. From the contact explosion tests, it exhibits that the extrusion-based 3D printing procedure enhances the explosion resistance substantially via fiber orientation alignment. With the layer thickness ratio of 40% (PURN6) as the watershed, both the top and bottom surface crater diameters of PURN slabs increase first and then decrease with increasing reinforcing layer thickness. In particular, crater diameters and failure modes for PURN8 and PURN15 are consistent with those of RC and RU, respectively. The underlying mechanism for the fibers to be aligned by the 3D printing procedure is theoretically analyzed to support the conclusions. The material costs of all the slabs are compared. The costs of PURN8 and PURN15 are 1.7 and 0.8 times of those for RC and RU, respectively. Thus, it can be stated that the 3D extrusion-based printing procedure will avail explosion resistance for 3DP-UHPC. Current test results prove the feasibility and cost-effectiveness of PURN for protective structures.
3D concrete printing has the potential to replace shotcrete to construct linings of tunnels in hard rock. The shear strength of the interface between rock and printed concrete is vital, especially at super-early ages. However, traditional methods for testing the shear strength of the interface, e.g. the direct shear test, are time-consuming and result in a high variability for fast-hardening printed concrete. In this paper, a new fast bond shear test is proposed. Each test can be completed in one minute, with another two minutes for preparing the next test. The influence of the matrix composition, the age of the printed matrices, and the interface roughness of the artificial rock substrate on the shear strength of the interface was experimentally studied. The tests were conducted at the age of the matrices at the 1st, the 4th, the 8th, the 16th, the 32nd, and the 64th minute after its final setting. A dimensionless formula was established to calculate the shear strength, accounting for the age of the printed matrices, the interface roughness, and the shear failure modes. It was validated by comparing the calculated results and the experimental results of one group of samples.
The stiffness and failure properties of cement-asphalt mastic-aggregate (C-AM-A) interface are among the most important factors affecting the performance of pouring semi-flexible pavement materials (SFP). Therefore, determining the characteristics of C-AM-A interface are essential for guiding the design of SFP from the perspective of interface enhancement. In this study, the failure characteristics of C-AM-A interfaces are examined experimentally and numerically. Firstly, the effects of temperature and the proportion of cement substituting limestone filler on the bonding strength of C-AM-A interface are analyzed via pull-off tests. Then, an innovative test method based on a three-point bending test of C-AM-A beam is proposed to investigate the influence of test temperature and asphalt mastic type on the fracture characteristics of the C-AM-A interface. Finally, based on the cohesive surface techniques, numerical modeling of C-AM-A beam under three-point bending was applied to study the effect of cohesive parameters on the interfacial fracture characteristics. The results show that the interface bonding strength decreased significantly with the increase of temperature. Using cement as a filler improves the bonding strength, fracture strength, stiffness and fracture energy of C-AM-A interface as compared to the case when limestone filler is used. As the temperature increases from −10 °C to 20 °C, the failure mode of the C-AM-A interface first alters from adhesive failure to mixed failure mode, and then to cohesive failure. It suggests that enhancing the adhesion of C-AM-A interface is more advantageous for improving the fracture resistance at low temperatures, while increasing the interface cohesion is more important at relatively high temperatures. The laboratory test methods, the numerical model and methodology developed in this study are useful to study the failure behavior of C-AM-A interface and optimize the performance of SFP from the perspective of interface enhancement.
As a new construction method, 3DCP (3D concrete printing) is
expected to show great potential for reducing labor and construction waste and
implementing free geometries. Research institutes and companies worldwide
are trying to recognize and develop this potential early. There are many
problems to solve, from reinforcement to long-term durability, but the
possibility is recognized as various structures have been tested. Several
researchers are proposing materials suitable for 3DCP. Since 3DCP requires
demanding material properties, the beginning of this technology is indeed the
material. However, if excellent materials are not utilized well, technology
development will be as tardy. In addition, 3DCP does not require the
intervention of the manufacturer during the manufacturing process. Therefore,
the designer should consider Design for Manufacturing (DfM) at the design
stage to determine whether it is possible to manufacture, as the designer even
acts as the manufacturer. This paper proposes a methodology that optimizes printing
parameters by considering material properties experimentally and quantifying
thresholds in the manufacturing process. The printing parameters are nozzle
speed, nozzle standoff distance, and pump speed. First, data on the material's
initial and ending setting time, flexural and compressive strength, and
rheological properties are collected. Then, the obtained material properties are
used to set the range of printing parameters compared to the reference paper.
When the range is set to specify the optimal parameters, the cross-sectional
geometry, buildability & layer deformation, and strength according to the
printing parameters are evaluated sequentially, and the parameters that fall
behind at each stage are erased. Lastly, sharp corners and overhangs are
analyzed for typical requirements of complex shapes to quantify constraints in
the manufacturing process of the selected parameters. In this paper, the
material's dynamic shear yield stress of 1274 MPa, R_thix of 11.7 Pa/s, and
A_thix of 1.2 Pa/s, and the printing parameters selected through all evaluations
were nozzle speed of 4500 mm/min, nozzle height of 8 mm, and pump speed
of 50 Hz. The combination of this material and the printing parameters showed
limits of up to 90 degrees sharp corner and up to 7.1 mm overhang.
Considering the deformation of the layer below and the top machining for
assembly, the manufacturer should add one layer to the design model. Suppose
the fundamental limitations for the parameters optimized for the introduction
of materials and equipment are quantified as follows. In that case, it is
straightforward for designers to reflect them in the design algorithm.
Furthermore, it serves as a starting point for developing the manufacturing
system in the desired direction.
3D concrete printing (3DCP) is a recent trend in the construction sector. In 3DCP, the concrete is pumped layer by layer through a nozzle attached to a robotic arm to print structural components or the entire building. The application of 3DCP is not just limited to the earth; it’s also gaining traction in space habitat construction. However, there are constraints associated with using 3DCP in construction projects. This review details the top challenges faced in the 3DCP process related to material, strength, printer and highlights the applications of 3DCP in the construction sector. By choosing the suitable materials and configuring the printing parameters, it is possible to overcome the material and structural strength-related challenges such as workability, hardening time, mechanical and durability properties. The use of 3DCP on construction sites certainly provides cost and waste reductions, faster build rates, safer working environments, and the possibility of more intricate architecture. Reinforcing methods and sustainable processes are the most frequently encountered issues with the 3DCP. Lack of technology, material variability, process optimization, and many other issues are barriers to advancing 3D printing technology for concrete. It is concluded that 3DCP has potential and opportunity for the construction if the challenges are addressed.
Under the dual constraints of industrialization and digitalization, the building skin and structure are further integrated to form standardized units to meet the requirements of architectural performance, industrial prefabrication and “complexity” aesthetic characteristics. The complex and diverse forms of today's building skin hide profound mathematical logic relations and operation rules of form generation. Crystallographic group with regular symmetry and the operation principles reflected by it is one of the most important rules and methods of form and pattern processing in skin design. The study of the mural symbols in ancient Egypt, the murals in the Alhambra, the manuscripts of Escher and the window lattice in ancient Chinese architecture profoundly reflects the basic operation principle of crystal group in shaping the skin form of architecture. Abundant and diverse architectural skin forms can be formed through the operation of symmetry group on basic graphic units. On the basis of clarifying the basic principle of crystal group action, the operation matrix of crystallographic symmetry group can be transformed into parameterized operation steps through programming language for visual operation, and then the skin form with high complexity and leap dimension can be generated by geometric algorithm, and the design method of building skin generation based on crystallographic group is constructed. In the selection of operation form, combined with the calculation of building performance and structure, the construction skin can be used in practical engineering is generated. Based on crystallographic group operation, the unifications of building skin and the classification simplification of components can meet the requirements of modular and unifications design in the process of building industrialization, and meet the requirements of current building industrialization and digitization. It has great research significance and value in the aspects of design and construction efficiency and material economic cost.
Effective reinforcing method is of great significance to the 3D concrete printing (3DCP) technology for structural application. Currently available reinforcing measures for 3DCP only provide directional instead of integral reinforcement for 3D printed constructs. To this gap, in this study, a novel reinforcing approach for 3DCP via U-type steel wire mesh (USWM) is proposed, which facilitate integrated horizontal and vertical reinforcements. Printed specimens without reinforcement, with flat steel wire mesh (FSWM) and USWM are manufactured to demonstrate the structural superiority of the currently proposed strengthening approach through flexural tests. The results show that the flexural performance of 3D printed specimens is substantially improved due to introduction of USWM, as indicated by significantly higher load-bearing capacity and deflection hardening compared to the counterparts reinforced with FSWM and plain specimen.
The bonding surface structure generated by the repair of concrete structures has been paid more attention as a weak point. The effects of old concrete age, interface roughness, and freeze-thawing (F-T) attack on adhesive interface are comprehensively investigated. In this study, six kinds of interface roughness and five different old concrete age are designed. The interfacial bonding property is mainly evaluated by splitting tensile strength (fts). Fractal analysis was used to characterize the interface roughness using laser scanning data. In general, the fts increased with the increasing value of interface fractal dimension. The relationship between fts and fractal dimension value was further analyzed, considering the old concrete ages and the F-T cycles. The results show that the effect of roughness on the bonding property of new-to-old concrete is more significant than the age of old concrete, and the influence of the F-T cycles on the bonding surface is mainly reflected in the initial stage of the F-T deterioration process. The relative dynamic elasticity modulus decreased obviously under F-T cycles, especially for the specimens with low interface roughness. In combination with the results of two non-destructive methods (ultrasonic non-destructive test and relative dynamic elastic modulus test), the larger roughness and the smaller age of old concrete can improve the bond performance.
Despite the dramatic development in digital manufacturing technologies in the recent years, 3D printing of earth materials, such as cob, still presents several challenges to the market-available 3D printing systems. This paper describes the development process of a 3D printing system for cob that fits the contemporary requirements of digital construction. The study first described the methodology of producing a revised cob recipe for the purpose of 3D printing. Then, the study conducted thorough investigations into the properties of three types of extrusion systems using both electromechanical and pneumatic methods, leading eventually to the development of a new bespoke dual-ram extruder. The study then explored systematically the relationship between the new 3DP system and the rheological properties of cob, followed by an exploration to the new geometric opportunities the new system offers. The study findings show that the new extrusion system improves greatly the 3DP process of cob in terms of extrusion rate, continuity, consistency, and mobility. The findings are expected to bring 3D printed cob construction closer to full-scale applications. On a broader scale the study contributes to the disciplines of architectural design and construction by providing a framework capable of bridging the knowledge gap between vernacular modes of building production and contemporary digital practice.
Recent developments in computational design and digital fabrication with concrete enable the realization of freeform geometries that optimize material use. 3D Concrete extrusion Printing (3DCP) is presently one of the most utilized digital fabrication methods with concrete. The expected advantages of 3DCP result from shaping concrete without formwork and from placing material only where functionally required. Although these advantages were pointed out more than 20 years ago, it is difficult to find competitive examples and their usage in real buildings. Consequently, the nonspecific character of the process acts as a shortcoming by opening up extensive possibilities without a clear direction. This paper proposes an automated 3DCP prefabrication platform for customized columns. The process-specific parameters are, therefore, fine-tuned for high-quality products with diverse forms and textures. Additionally, this paper proposes an evaluation method for geometric complexity and identifies the types of column typologies that may benefit from a 3DCP prefabrication platform.
Concrete overlays are widely used to rehabilitate bridge decks. This investigation presents parametric and sensitivity analyses of the mechanical strains due to restraint of shrinkage of the overlay that may cause it to crack. The parametric analysis identifies the significant variables and the sensitivity analysis quantifies the correlations between these variables and the magnitude of mechanical strains. The strain analysis accounts for drying shrinkage using Fick's law, and for tensile creep strains. Total strains are computed by superposition, assuming a linear-elastic concrete behavior, using a time-history approach developed in a previous investigation. Concrete overlays on concrete slabs and composite systems are considered. Compressive strength of the overlay concrete, curing period, overlay thickness and environmental humidity contribute the most to the variability of the analytically predicted mechanical strains.
Contour crafting application has been widely used in different industrial, pharmaceutical, medical, and aviation application for more than three decades. Recently, mega-sized 3-D printers were developed, with sufficient capability to print full scale construction projects, including walls, homes, bridges, and multi-story buildings. Successful 3-D printing of projects is accomplished by additive manufacturing (AM) process through the placement of successive layers of construction materials using robotic arms (3-D printers). This layered construction, known as contour crafting, is controlled by computer modelling software. This paper presents the history of contour crafting development, its current application in construction, the advantages of contour crafting applications in construction, and the main impediments to the wide spread of contour crafting in the local construction market. In addition, this paper highlights the current research efforts made to integrate building information modeling (BIM) in contour crafting construction. Based on the recent research findings, contour crafting application in construction improves job site productivity and project sustainability as a result of reduced material waste in construction sites. Finally, the automated construction through the application of contour crafting technique results in improved job site safety and increased overall quality of construction projects due to the increased automation in construction.
There are conflicting views in the literature concerning the optimum moisture state for an existing substrate prior to the application of a repair material. Both saturated-surface-dry (SSD) and dry substrates have been found to be preferable in a variety of studies. One confounding factor is that some studies evaluate bonding of the repair material to the substrate via pull-off (direct tension) testing, while others have employed some form of shear specimens as their preferred testing configuration. Available evidence suggests that dry substrate specimens usually perform equivalently or better in shear testing, while SSD ones generally exhibit higher bond strengths when a pull-off test is performed, although exceptions to these trends have been observed. This paper applies a variety of microstructural characterization tools to investigate the interfacial microstructure that develops when a fresh repair material is applied to either a dry or SSD substrate. Simultaneous neutron and X-ray radiography are employed to observe the dynamic microstructural rearrangements that occur at this interface during the first 4 h of curing. Based on the differences in water movement and densification (particle compaction) that occur for the dry and SSD specimens, respectively, a hypothesis is formulated as to why different bond tests may favor one moisture state over the other, also dependent on their surface roughness. It is suggested that the compaction of particles at a dry substrate surface may increase the frictional resistance when tested under slant shear loading, but contribute relatively little to the bonding when the interface is submitted to pull-off forces. For maximizing bond performance, the fluidity of the repair material and the roughness and moisture state of the substrate must all be given adequate consideration.
The stresses, strains, and curvatures due to the shrinkage restraint of new concrete bridge deck overlays by the underlying older substrate are investigated. A time-history analysis method is derived that, for each time increment, computes free-shrinkage and creep strains, enforces compatibility and equilibrium using a time-dependent stiffness matrix, and determines incremental mechanical and total strains. A new model for tensile creep strains has test-predicted ratios for experimental results reported by others that average 1.00, with a coefficient of variation of 11%. The resulting stresses and mechanical strains are nonlinear across the depth of the member, with large stress gradients in the top and bottom faces of the overlay at early ages of drying. Simplified analytical methods proposed by others are often not accurate: neglecting swelling of the substrate underestimates the mechanical strains, neglecting tensile creep markedly overestimates the
mechanical strains, and assuming uniform free shrinkage through the overlay thickness initially overestimates the mechanical strains but subsequently underestimates them at older ages. The studies
also found that application of a waterproofing membrane at the top of the overlay 3 days after the end of curing has very little effect on the maximum tensile stresses in the overlay. The age-adjusted equivalent modulus method accurately estimates the overlay tensile stress at early ages, but fails to predict the time of cracking.
Concrete patch repair has long been used to repair the damaged concrete structures. In cold regions, freeze-thaw cycle is one of the major damage factors. Not only the material itself is damaged by freeze-thaw cycles, but also the adhesive interface, which is regarded as the weakest part of composite system, degrades under freeze-thaw cycles. Air entraining agent has long been used to increase the freeze-thaw resistance of concrete materials. However, the effect of air entraining agent on the adhesive interface under freeze-thaw cycles has not been explored. The degradation mechanism and failure mode of concrete repair system have not been studied, either. In this study, to investigate the effects of water-cement ratio of substrate concrete and air entraining agent in substrate concrete and repairing mortars, three kinds of substrate concrete were casted and repaired by two kinds of ordinary Portland cement mortar. With certain number of freeze-thaw cycles up to 150 cycles, through splitting prism test, the splitting tensile strength and failure mode of composite specimens were experimented. The relative dynamic elastic modulus and splitting tensile strength of substrate concretes and repairing mortars were obtained as well. Results showed that air entraining agent in the repairing mortar greatly influenced the adhesive tensile strength under freeze-thaw cycles. The water-cement ratio and air entraining agent of substrate concrete insignificantly affected the adhesive interface, but affected the splitting tensile strength and the freeze-thaw resistance of substrate concrete, and thus affected the failure mode of composite specimens. © 2014 4th International Conference on the Durability of Concrete Structures.
Concrete decks are commonly rehabilitated using concrete overlays. Overlay shrinkage is restrained at the interface with the substrate. An analytical method is presented to compute the distribution of internal humidity and free shrinkage strains in the overlay and substrate for bridge decks with solid rectangular cross sections and therefore overcomes the shortcomings of generic shrinkage models. Drying humidity is modeled using Fick’s Second Law and the concrete diffusivity as a function of the internal humidity. The
prediction of internal humidity is validated using experimental data obtained by others. The distribution of free shrinkage strains through the overlay thickness is strongly nonlinear with localized maxima at the top of the overlay and at the interface with the substrate. Large swelling strains also occur in the upper fibers of
the substrate. Thin overlays develop large free shrinkage strains rapidly, whereas thick overlays produce large swelling strains in the substrate.
This paper presents a new finite element formulation of the upper bound theorem. The formulation uses a six-noded linear strain triangular element. Each node has two unknown velocities and each corner of a triangle is
associated with a specified number of unknown plastic multiplier rates. The major advantage of using a linear strain element, rather than a constant
strain element, is that the velocity field can be modelled more accurately. In addition, the incompressibility condition can be easily satisfied without
resorting to special arrangements of elements in the mesh. The formulation permits kinematically admissible velocity discontinuities at specified
locations within the finite element mesh. To ensure that finite element formulation of the upper bound theorem leads to a linear programming problem,
the yield criterion is expressed as a linear function of the stresses. The linearized yield surface is defined to circumscribe the parent yield surface
so that the solution obtained is a rigorous upper bound. During the solution phase, an active set algorithm is used to solve the resulting linear
programming problem. Several numerical examples are given to illustrate the capability of the new procedure for computing rigorous upper bounds. The
efficiency and accuracy of the quadratic formulation is compared with that of the 3-noded constant strain formulation in detail.
3D concrete printing is an emerging construction technology, and presents an opportunity for utilising materials that are otherwise considered unsuitable for concrete construction. Incorporating underutilised solids and/or waste solids as aggregates is a way of gaining the maximum environmental and economic benefits from the emerging 3D concrete printing technology. In this study, desert sand (small size), river-sediment ceramsite sand (medium size), and recycled concrete (large size) were experimentally investigated for use as aggregates in the 3D printing of concrete. Three mixtures were designed with continuous, open, and interrupted gradations of solids, respectively, based on a theory of particle interference, and aiming to meet the requirements of extrusion-based 3D printing. The influences of the particle grading characteristics on the printability-related early age behaviours, mechanical properties, and shrinkage resistance were measured and analysed. The test results demonstrate that the self-supporting skeletal effect formed by the gradated particles reduces the flowability of the mixtures, but the structural build-up/buildability performance is improved (under the premise of the desired printability). The interlayer distribution and skeleton of the gradated aggregate contribute to improving the interfacial interlocking effect and contact bonding between layers; this is visually validated through computed tomography (CT) scanning. Further, the addition of aggregates reduces the proportion of cementitious composites, and therefore effectively mitigates the shrinkage of the cement matrix. The grading characteristics of the underutilised particle resources are crucial for regulating the early-age 3D printability. This article provides feasible solutions based on experimental data for promoting the eco-utilisation of underutilised and waste solids in 3D printing, and these solutions satisfy the minimum strength and durability requirements.
The general application of 3D printed concrete (3DP-C) in practical engineering is significantly hindered by the lack of adequate reinforcement procedures, which is an intrinsic limitation of the 3D printing process. To address this limitation, this study proposes an in-process dual-printing reinforcement method, which concurrently and continuously prints ultra-high performance concrete (UHPC) and ordinary 3DP-C. The UHPC, as an integral part of the printed structure, serves as reinforcement for the overall concrete composite. Printing equipment for these dual concrete materials is developed to realize the coordination of the dual-material deposition process. Significant improvement of mechanical properties of 3D printing UHPC (3DP-UHPC) reinforced 3DP-C, which is then abbreviated as 3DP-UHPCRC, is demonstrated through four points bending tests of thin plate specimens. In comparison to plain 3DP-C, the reinforced concrete possesses an 160.5% increase in ultimate bending strength as well as a brittle to ductile bending failure mode transition, thus demonstrating the effectiveness of the proposed reinforcement method.
Cementitious composites blended with high Belite sulfoaluminate cement and medium-heat Portland cements were optimized to achieve both favorable 3D printability and durability performance. The influence of the hybrid cement ratio on the rheology and the hardened properties, such as mechanical properties, drying shrinkage, and carbonization resistance, was systematically investigated. The results revealed that the mixture incorporated with 80% CSA-HB cement exhibited excellent early strength, and a 1d compressive strength higher than 30 MPa was achieved. Hybrid mixing of CSA-HB and P·MH reduces the super-early age hydration heat by 37.8%. Meanwhile, the 28-day carbonization depth of the mixture with 80% P·MH cement was 52.4% lower than that comprising a high content of CSA-HB. The designed mixtures were compatible with the printing process and validated by the printing practice of 9.4m-span arch components using a large-scale robotic printer.
3D concrete printing has tremendous potential for construction manufacturing; however, weak interface bonding between adjacent layers remains a well-known issue that affects the mechanical properties of printed structures. The layers introduce anisotropy and reduce the capacity to resist tensile and shear loads. Reinforcements, inserted perpendicular to the printed layers to traverse the interfaces, can improve these limitations, but the insertion of reinforcements is difficult to achieve in practice, and there are few published studies exploring appropriate methods. This study presents a promising approach using U-shaped nails inserted into concrete during the printing process. The bridging effect and dowel action of the applied U-nails are visualised and analysed to elucidate the toughness improvement. The ultimate tensile strength and shear strength of 3D printed concrete are significantly increased by 145.0% and 220.0%, respectively. U-nails with a filament thickness of 2–2.5 mm are recommended to yield optimal improvement in the interlayer strength.
This paper compiles selected predictive analytical and numerical tools which can be used to model and understand the mechanisms of importance at different stages during and immediately after extrusion-based 3D printing of cementitious materials. The proposed toolbox covers different aspects of the process including mixing, material transportation, layer deposition, mechanical behavior of the fresh printed structure, and its early curing. Specifically, the paper provides basic analytical methods that should be helpful for an initial, first-order analysis of a given printing process. These methods deliver, in turn, a first estimation of some material requirements and process parameters. Limitations of these analytical methods are also discussed. Furthermore, the paper presents a review of advanced numerical tools that can be used to simulate the steps in the printing process accurately. It is shown that these tools can serve to describe complex behaviors, help in designing process parameters, or optimizing the rheological response, even though further developments are still needed to capture fully the attendant physical mechanisms.
Printability is a key parameter that affects the application of foam concrete to 3D printing. In this study, the hydroxypropyl methylcellulose (HPMC) and silica fume (SF) were doped into foam concrete as a viscosity modifier and thixotropic agent, and their effects on the stability, rheological properties, and printability of 3D printing foam concrete were investigated. Both HPMC and SF effectively reduced the volume bleeding rate of foam concrete, while HPMC was beneficial for stabilizing the foam, and SF increased the wet density of foam concrete. With the increase in the dosage of HPMC and SF and resting time, the static yield stress, dynamic yield stress, and plastic viscosity of foam concrete increased continuously. SF increased the static yield stress considerably, while HPMC affected the dynamic yield stress and plastic viscosity considerably. It is suggested to combine tanθ and stack height of the printed foam concrete together to evaluate the buildability of 3D printing foam concrete. The suitable ranges of static yield stress, dynamic yield stress and plastic viscosity for 3D printable foam concrete with a wet density from 1550 to 1850 kg/m³ are 1113–1658 Pa, 66.4–230.1 Pa, and 2.08–3.71 Pa s, respectively. The compressive strength of the 3D printed foam concrete with dry density of 1815 kg/m³ in the testing direction Z, Y, and X reached 19.9 MPa, 28.5 MPa and 24.6 MPa, respectively.
Appling fresh mortar onto the surface of old concrete has been a common repair and reinforcement method. In this work, the influence of initial moisture conditions of old mortar on the microstructure of repair mortar was checked. Two types of composites were prepared by bonding fresh repair mortar with a pre-dried or a water-saturated old cement mortar. By analyzing X-ray CT images, severe shrinkage cracking was found in the repair mortar bonded with a pre-dried old mortar. A large discontinuity was revealed between repair mortar and pre-saturated old mortar by X-ray CT images. Various porosities in the repair mortar were identified by Mercury Intrusion Porosimetry tests. Dynamic water imbibition in the two types of composites was followed by neutron radiography. Shrinkage cracks in repair mortar can significantly promote water infiltration in both repair mortar and substrate, which poses a threat to the durability of the repair system.
Three-dimensional (3D) printing construction techniques have been increasingly developed in recent years and transferred into practical applications in some instances. However, knowledge of the differences between large-scale 3D printing and laboratory 3D printing is still limited. In this review, large-scale extrusion-based 3D printing concrete (3DPC) technologies are subdivided into three categories according to different printed objects and construction processes: 3D printing elements, 3D printing formworks, and monolithic 3DPC on-site. The different printability requirements of large-scale and lab-scale 3DPC are compared in this paper. Then practical problems of large-scale 3DPC are discussed, including economic and environmental optimizations, reinforcement constructions, preparation and remix of concrete, and different large-scale 3D printing systems. The construction details of reinforcements implemented with various 3D printing technologies are reviewed based on several pioneering projects. Finally, some recommendations for future work are provided for facilitating the use of large-scale 3DPC technology in practice of construction.
Concrete with coarse aggregate in the 3D concrete printing (3DCP) has broad prospects for high strength, low cost and shrinkage. In this study, an extrusion-based 3D printer was designed and utilized to print concrete with the largest aggregate size of 20 mm. Printable concrete with coarse aggregate was designed by different volume ratios of cement to aggregate (C/A). Then the effect of C/A on the printability and direction-dependent mechanical performance was investigated. X-ray micro-computed tomography (X-CT) was used to detect and analyze the voids distribution characteristics of printed specimens. Scanning electron microscope (SEM) investigations of microstructure at the interlayer area were conducted. Results indicated that the initial flowability of printable concrete should be within 178–200 mm, and the recommended printable C/A was within 0.35–0.60. The decrease of C/A improved the maximum printing height and mechanical performance but weakened the shape-stability of the multi-layer structure. X-CT results indicated that reducing excess slurry content caused by the decline of C/A decreased the compactness of the printed structure. The compressive strength and flexural strength of 3D printed specimens showed a direction-dependent characteristic, mainly related to the non-uniform distribution of voids revealed by X-CT. SEM images revealed the “micro-bridging” morphology in the interlayer area and proved that there were carbonation and structural weakening problems at the surface and surrounding of this area.
This study investigated 3D concrete printing of permanent formwork for concrete column construction. The effect of different hydroxypropyl methyl cellulose (HPMC) contents (0, 0.0003 and 0.0006 by mass of binder) and the water-to-binder (W/B) ratios (0.27, 0.29 and 0.31) on the rheological properties, structural build-up and mechanical performance were studied using several mixtures for manufacture of the permanent formworks. The results showed that the mixture with the HPMC = 0.0006 and the W/B = 0.27 showed the maximum static yield stress, largest thixotropy and maximum green strength, and thereby selected as the optimum mixture. The plastic failure of the optimum mixture was also predicted using a thixotropy model and was compared with the experimental results. Subsequently, three concrete columns with different longitudinal steel reinforcement ratios (0.0%, 1.9% and 2.5%) were constructed using the printed concrete as the permanent formwork and tested in compression. Good bonding was observed at the interface of the cast-in-place concrete and the printed concrete permanent formwork. In addition, it was observed that the initial stiffness, the maximum bearing capacity and the corresponding longitudinal displacement of the concrete columns increased, as the longitudinal reinforcement ratio increased. The counterpart concrete columns using the conventional formworks were also constructed and tested for comparison. In comparison, the concrete columns made using the printed concrete as the permanent formwork obtained a higher stiffness and bearing capacity than the counterpart conventional concrete columns. The reasons for the differences are explained.
Currently, applying 3D concrete printing to compressed structural systems with low tensile/shear stresses is a feasible and alternative route to promote this advanced technology into engineering practice because of the lack of an effective structural reinforcing method. Modular manufacturing of segmental components and fabricated construction of large-scale structures are promising for overcoming the limitations of the working dimensions of 3D printers. In this study, four different interlocking configurations are designed with "I," "V," "π," and "s"-shaped forms. Moreover, three cementitious composites―cement-, epoxy-, and phosphate-based composites―are prepared, aiming at optimizing the mechanical integrity and capacity of prefabricated structures with mechanical bite and adhesive forces. The effects of the interlocking forms on the interfacial connection strength at the joints are investigated and evaluated by compression–tensile and compression–shear tests. The results provide experimental data and references for constructing large-scale concrete arch structures by the modular assembly of 3D-printed segments.
The performance of the interface bond between the TRC permanent formwork and cast-in-place concrete is a key factor that determines whether the formwork can exert sufficient mechanical properties to replace traditional formwork. Sufficient performance of the TRC permanent formwork can further promote its application in engineering practice. This paper presents the results of a three-point bending test performed on a TRC formwork superimposed plain concrete beam with prefabricated cracks. The influences of various interface treatment methods, interface agent types, TRC formwork thicknesses, chopped fibre contents, textile distribution rates and other factors on the bonding performance of the interface between the TRC permanent formwork and cast-in-place concrete are considered. The study found that regardless of whether the interface is processed, the interface bonding performance between the TRC permanent formwork and the cast-in-place concrete is better than that between traditional formwork and cast-in-place concrete. After the interface is processed, the ultimate bearing capacities of the composite beam specimens are improved to varying degrees, and the crack development mode is improved. Treating the interface, increasing the thickness of the TRC formwork, increasing the formwork distribution textile rate, adding chopped fibres and other treatment methods can reduce the strain of the interface between the TRC formwork and cast-in-place concrete.
TRC permanent formwork is made of a new type of cement-based material and has wide application prospects in formwork engineering. To promote the large-scale development of TRC permanent formwork and make it widely used in engineering practice, based on the single-lap shear test, this paper analyses the bonding properties between the TRC permanent formwork and cast-in-place concrete at different lengths in terms of the failure mode, load-slip curve, etc., and further elaborates on the fracture mechanics theory. The research shows that when the length of the formwork small, the failure mode of the specimen measured by the single-lap shear test mainly occurs at the textile-matrix interface in the TRC permanent formwork; when the length of the TRC formwork large, the failure mode begins to change to textile fracture. The effective bonding length of the TRC permanent formwork and the cast-in-place concrete is approximately 150 mm. When the TRC formwork length is less than 150 mm, the interface bonding force increases continuously with increasing TRC formwork length. The load-slip is mainly divided into two curves with typical characteristics. The first type of curve has a clear load softening stage, and the whole curve of the second type has a stage where the slip increment is small and the load drops sharply. Finally, the interface fracture energy can better characterize the interfacial bonding properties between TRC permanent formwork and cast-in-place concrete.
In order to avoid the shortcomings of the traditional building template, such as the cumbersome process of supporting and disassembling the mold, the low turnover, and the occupation of a lot of labor, it is proposed to replace the traditional building formwork with the textile reinforced concrete (TRC) as a permanent formwork. In this paper, the template form is used as the research parameter, and the bending performance of the U-shaped and single-plate TRC template composite beam and the comparative beam under static load is compared and studied. The test results found that during the loading process, the single-plate formwork composite beam was peeled off from the TRC formwork, and the U-shaped formwork and the post-cast concrete interface bonded well. Under the premise that the interface between the formwork and the post-cast concrete is good, the cracking load, yield load and ultimate load of the TRC formwork composite beam are improved compared with the ordinary reinforced concrete beam; the crack width of the composite beam is smaller and the development is slow; Bending rigidity becomes greater but ductility is reduced to a certain extent. Overall, the bending performance of the U-shaped formwork composite beam is better than that of the single-plate formwork composite beam. In addition, on the basis of the test, this paper puts forward the formula of the normal section bending capacity of the U-shaped TRC template laminated RC beam, and the test results are in good agreement with the theoretical calculations.
A synchronized concrete and bonding agent deposition system is proposed to address the weak interlayer bond strength issue in extrusion-based 3D concrete printing (3DCP). In the proposed system, printable concrete and bonding agent are deposited concurrently via a rotatable nozzle with two outlets. Therefore, the interlayer bond strength is improved by the bonding agent deposited on the interface between printed filaments. Various bonding agents, including water, cement strengthener, polymer solution, and cement paste, were investigated. The results indicate that when the cement paste with a 0.26 water-to-cement mass ratio is adopted as the bonding agent, a relative bond strength as high as 267% can be obtained via the proposed system. Continuous printing was conducted to demonstrate that the proposed system has great potential for practical engineering applications. The proposed system has the potential to eliminate the weakness in interlayer bond strength of 3DCP processes and to widen 3DCP applications.
This study aims to explore effects of interfacial bond properties on the anisotropic mechanical behavior of 3D printed concrete. One finite element (FE) model which fully considered the interfacial bond properties with the traction-separation law was established and verified by the experimental results. The influence of nozzle dimensions, interfacial bond strength and concrete properties on the compressive and flexural strengths was analyzed. The results show that the horizontal shear deformation between printed filaments leads to the strength reduction for the 3D printed specimen under compression, and the tensile strength at the mid-span determines the flexural strength of 3D printed specimen. The compressive strength is relatively lower while the flexural strength is much higher for the specimens loaded in Y and Z directions. The simulation shows that the number of interfaces, the tensile and shear properties of the interface between printed filaments contribute the variation of anisotropy under compression and flexure.
ECC characterized by its strain-hardening behavior in tension, high tensile and compressive strain and cracking control capacity is an ideal material for permanent formwork of reinforced concrete (RC) beams. This study investigated the shear behavior of RC slender beams without shear reinforcements by using precast U-shaped ECC permanent formwork. A total of 10 beams, including 1 RC beam for reference and 9 beams using U-shaped permanent formwork with different thicknesses and interfacial properties were fabricated and tested. ECC permanent formwork with three different thickness, namely 15, 20 and 25 mm, respectively were precast. At each thickness of permanent formwork, three different interfacial properties, namely smooth interface, anchored interface and rugged interface, respectively were prepared to investigate the effects of interfacial properties. All beams were tested by four-point loading tests to investigate the shear behavior and failure mechanism. The experimental results revealed that the shear carrying and deformation capacities of RC slender beams without stirrups could be enhanced significantly by using U-shaped ECC permanent formwork. The predictive equations for shear carrying capacities of RC beams without stirrups by using precast U-shaped ECC formwork were also proposed based on modified truss model and it agreed well with experimental result.
While interest in 3D printing of concrete (3DCP) and structures has been growing, a major obstacle for implementation of 3DP construction method is the need for steel reinforcement and the challenges this presents to the 3DP process. Engineered Cementitious Composites (ECC), also known as Strain-hardening Cement-based Composites (SHCC), hold promise to attain structural integrity, durability, reliability and robustness without steel reinforcement. This article surveys the state of the art on 3DP research with ECC and suggests needed research to direct future development. Research in Asia, Europe and the United States has demonstrated printability and buildability of 3DP-ECC that exhibits characteristic tensile ductility of cast ECC. Nonetheless, a number of outstanding research areas are identified, including those associated with more sustainable mix-design, rheology control, microstructure, filament/filament interface weakness, and long-term durability. Resolution of these challenges will better position the research community to addressing full scale construction, print speed, and print quality.
A major flaw of 3D concrete printing (3DCP) is its weak interlayer bonding. Consequently mechanical, bond behaviors and durability are compromised. The shrinkage because of excess surface water in the extrusion process and its evaporation promotes the reduction of interlayer bonding. In this study a novel additive mortar based on calcium sulphoaluminate (CSA) cement, cellulose fiber and limestone filler was proposed for application between layers, which permits extension of printing time interval and also caters for breaks in construction printing. The mortar was applied on substrate filaments whereas the overlaying filaments were printed at 60 min, 90 min and 120 min print time intervals corresponding to initial and final setting times of the 3D printing concrete. Its effect was verified through accurate measurements of interlayer tensile and shear bond strength on cross bonded samples. Microstructural investigation of interlayer structures was through SEM to cater for both EDS and XRD to study hydration products. Results suggest that the proposed additive mortar holds water and utilizes it for internal curing to attain overall enhancement in early age hydration, produces expansive ettringite to counter shrinkage and generates additional mechanical bonding between printed layers by fiber and fine aggregate interlocking. Interlayer tensile strength was boosted greater than 1.91 MPa for 60 min print time interval. The composite could possibly be utilized between each layer of a typical 3DCP operation to improve filament deposit and stacking process, to reduce voids and longitudinal flaws and to enhance durability because it is very easy to manufacture and apply while its constituent materials are plentiful, cheap, safe and environmentally friendly.
The adhesive interface in new-to-old concrete widely occurs in the concrete structure, and the adhesion assessment for freeze-thaw (F-T) resistance is hardly focused on. To evaluate the F-T resistance of adhesive interfaces in new-to-old concrete with a nondestructive method, this experiment was conducted with ultrasonic pulse velocity (UPV). Specimens with the same natural roughness were obtained using three-dimension printing technology. The surface roughness was measured via the sand replacement method. The traditional evaluation that the decrease of relative dynamic elastic modulus under 60 % represents the failure of concrete F-T resistance has a limitation in new-to-old concrete structures. A new characterization method of F-T resistance in adhesive interface for new-to-old concrete was proposed based on the variation trend of UPV in parallel direction (UPV-PD) and vertical direction (UPV-VD). The UPV-VD loss ratio had a three-stage characteristic and a steady phase, which was different from the loss ratio of splitting tensile strength in numerical value. The results showed that the UPV-PD loss ratio grew linearly in the process of F-T cycles. Moisture content was the dominant influence on the numerical difference. This difference can be eliminated based on a correction method. The variation trend of the UPV loss ratio in adhesive interfaces was similar to that of splitting tensile strength, and the obvious turning points appeared after similar F-T cycles. The characterization method of F-T resistance proposed in this article is applicable to evaluate the F-T resistance of adhesive interfaces in new-to-old concrete, which is also proved by splitting tensile test results.
Exposing bridge elements to severe environmental conditions causes a reduction in service life and durability which demands repair or total replacement. Different strategies for repair and retrofit can be chosen. These strategies include patching, crack repairs, concrete sealers, a protective layer made of concrete or steel. Ultra-high performance concrete offers an option for repairing and retrofitting different structural elements, however, the bond strength between concrete substrates and ultra-high performance concrete can still be considered a knowledge gap in the literature. In this paper, bond strength between ultra-high performance concrete and substrate made of normal concrete with different surface preparation was investigated experimentally. Thirty specimens were tested under bi-surface shear test with different surface preparation, including roughness degree, mechanical connector, and bonding agent. Furthermore, two non-contact test methods including terrestrial laser scanning and digital image processing were incorporated to evaluate the roughness of the substrate interface and correlate the roughness degree to the bond strength between the two materials. The results showed that an adequate roughness for the interfacial surface with or without mechanical connectors transferred the failure mode to the concrete substrate indicating high bond strength between the two materials if compared to inter-facial surfaces without any preparation. In addition, the use of bonding agent could harm the bond strength between the two material which is inappropriate for retrofitting. The result from scanning and image processing showed that both methods qualitatively identified the degree of interface roughness and their result can be correlated to bond strength.
Contour Crafting is a novel technology in construction industry based on 3D printing that uses robotics to construct free form building structures by repeatedly laying down layers of material such as concrete. It is actually an approach to scale up automatic fabrication from building small industrial parts to constructing buildings. However, there are little information about contour crafting (CC) in current use; present paper aims to describe the operational steps of creating a whole building by the machine reviewing relevant literature. Furthermore, it will represent the advantages of CC usage compared to traditional construction methods, as well as its applicability in construction industry.
In the present work, a great deal of importance is attached to sand concrete as a new repair material. An investigation was conducted to evaluate the bond strength and the type of failure in composite concrete bi-layers. The test specimens under study were made of an ordinary concrete substrate and sand concrete (repair material). The substrate was prepared from ordinary concrete of two different classes of strength, i.e. the first one contains a superplasticizer (OCS) and the other has no superplasticizer (OC). To this purpose, four different surfaces used as substrates, namely low roughness (LRG), high roughness (HRG), drilled holes (DH), and combination of high roughness with drilled holes (HRGDH) surfaces were prepared. For the repair materials, two mixes of sand concrete were selected, namely sand concrete with 100% of limestone filler (SCL), on one hand, and sand concrete with 50% of limestone filler and 50% of glass powder (SCG), on the other. Two adhesion tests (flexure test, and splitting test) were performed in order to evaluate the response of the composite test specimens under loading, by determining the mode of failure produced after the test. The results of splitting strength obtained showed that the sand concrete gives remarkable bond strength of 2.54 MPa for cylindrical specimens and 317 MPa for cubic specimens. These values present a good to excellent bond quality, depending on the strength level of ordinary concrete, reflecting a good adhesion to the substrate, which manifested a positive monolithic response. These performances allowed us to consider the sand concrete as a good cementitious repair material.
Concrete interfacial roughness plays a great role in contributing to the new-to-old concrete bonding. In this research, artificial roughness was created on the surface of 180-day concrete with iron combs of different-shaped saw-teeth; the morphology data of concrete interface was captured using self-created analysis equipment of laser triangulation ranging; then the morphology data was processed and reconstructed into an interface in three dimensional form with MATLAB software; the fractal dimension of the three-dimensional reconstruction interface was calculated based on the above reconstruction. Mechanical properties of new-to-old concrete bond were tested. The influence of interface roughness on new-to-old concrete bond was studied. The effects of interface adhesion agents on new-to-old concrete bond were also studied. Results show that the concrete interfacial fractal dimension is in good relevance with new-to-old concrete mechanical properties, including splitting tensile strength, flexural strength and bonding strength. If concrete interfacial fractal dimension is higher, new-to-old concrete can obtain higher mechanical strengths. The interface adhesion agents greatly improve the mechanical strength of new-to-old concrete, and result in higher bonding strengths. Different kinds of adhesion agents may have different effectiveness, and they may contribute to a different degree of increase in mechanical strength. The three zone-two layer model was established to explain the mechanism of new-to-old concrete bonding, which indicates that the transition zone between old and new concrete is the key region of the new-to-old concrete bonding. While the interfacial roughness affects the permeable layer and the interface adhesion agents improves the reaction layer, the transition zone between old and new concrete is properly improved, which contributes to a better new-to-old concrete bonding.
The influence of substrate moisture preparation on the direct shear bond strength of composite substrate-overlay specimens was evaluated. The substrate surface was exposed to four different moisture conditions prior to overlay application. A quantitative analysis of backscattered electron images of the microstructure of the overlay transition zone was carried out to quantify its properties along the interface and help analyse the results of the shear bond testing. The results show that pre-wetting the substrate surface prior to application of the overlay provides no added benefit towards increasing the bond strength and may in some cases reduce bond strength significantly. The microstructural investigations confirmed that pre-saturated substrates increase the w/c ratio and the porosity in the overlay transition zone (OTZ), which was found to have a thickness of about 100 μm. The OTZ in overlays cast on dry substrate surfaces had lower porosity and an increased amount of anhydrous cement.
Under the Mises yield condition, the failure mechanism of the adhesive layer between self-compacting concrete and old concrete is presented in this paper. After the plastic limit analysis of bonding strength, the theoretical formula of upper bound theorem (τ) for adhesive shear strength is deduced. Based on the test result, the main parameter in the formula is also determine, which can help the retrofit design of concrete structures by self-compacting concrete.
Repair and strengthening of concrete structures often include the application of a concrete overlay to the existing substrate. Sufficient bond strength between substrate and overlay is a prerequisite for the durability and serviceability of the repaired or strengthened structure. Factors considered most important for good bond strength between substrate and overlay are cleanliness and texture of the substrate concrete surface, as well as overlay placement and curing techniques. In terms of substrate moisture condition prior to overlay application, a saturated, surfacedry substrate is generally considered best practice to achieve high bond strength. In some cases, bonding agents are specified, which may consist of commercial products or site-made cement slurries. Despite the extensive application of bonded concrete overlays, the usefulness of bonding agents is still the subject of much debate among researchers, specifiers and practitioners. Conflicting opinions also still exist as to whether pre-wetting the substrate concrete can be expected to have a positive influence on bond strength. The laboratory-based research described in this paper identified that a saturated, surface-dry substrate concrete has generally no beneficial influence on overlay bond strength. In many instances, the use of substrate surfaces prepared to what is commonly considered the optimum moisture condition resulted in significantly lower bond strength, compared with non-preconditioned substrates. This phenomenon was ascribed to better mechanical interlock, which exists when the fresh overlay material can flow into the unsaturated cavities and pores of the substrate. Bonding agents were found to enhance bond strength only when an overlay of low workability was applied, both commercial and self-made bonding agents performing equally well. For overlays with workability characteristics of conventional concretes (slump values between 70 and 110 mm), bonding agents were found to have no noticeable influence on bond strength. This relates to the circumstance that overlays of sufficient fluidity can fill the pores and cavities of the substrate without the help of a bonding agent.
Ultrahigh-performance concrete (UHPC) exhibits several properties that make it appropriate for the rehabilitation of concrete structures. In this investigation, the application is focused on bridge deck overlays, but the study is equally applicable to other rehabilitation applications. Its negligible permeability makes this material suitable as a protective barrier that prevents any water or chemical penetration into the substrate. In addition, its ultra-high compressive strength and post-cracking tensile capacity could provide an improvement to the bearing capacity. However, for extensive acceptance, it has to be demonstrated that the bond between UHPC and normal strength concrete (NSC) offers a good long-term performance under a variety of operating conditions. The UHPC-NSC interface can experience high tensile, shear, and compressive stresses at both early and later life stages and the environmental conditions inherent to the operating environment. The success of the rehabilitation will depend on whether the bond interface can withstand the stress combinations subjected throughout its service-life owing to material incompatibilities or applied loads. This paper explores the bond characteristics between UHPC and NSC under varying stress configurations and environmental conditions. Variables, such as roughness degree of the concrete substrates, age of bond, exposure to freeze-thaw cycles and wetting conditions of the concrete substrate, were included in this study. The combination of splitting tensile test with 0, 300, 600, and 900 freeze-thaw cycles was carried out to assess the bond performance under severe environmental conditions. The slant-shear test was conducted with different interface angles to provide a broader understanding of the bond performance under several combinations of compression and shear stresses. In addition, measurements of the bond tensile strength, using the pull-off test, were used to provide data that can be correlated in the future with the other tests that only can be used in the laboratory. The experimental program showed that the bond performance between UHPC and NSC is adequate for bridge overlay applications, regardless of the degree of roughness of the concrete substrate, the age of the composite specimens, the exposure to freeze-thaw cycles, and the different loading configurations. The controlling factor was the strength gain of the UHPC at early ages relative to the strengths of the substrate. (C) 2014 American Society of Civil Engineers.
One of the main processes for repairing concrete structures is patch repair. Efficiency and durability of a repaired system depends on the bond between concrete substrate and repair material. By increasing the surface roughness, the surface treatment of concrete substrate can promote mechanical interlocking that is one of the basic mechanisms of adhesion. Nevertheless, some problems may arise from “co-lateral” effects of the treatment, especially due to the development of microcracks inside the substrate. In the presented paper, the effect of concrete substrate surface preparation has been characterized by roughness measurement, description of microcracking in the near-to-surface layer and a pull-off cohesion test. After repair, pull-off bond strength has been evaluated. It is concluded that selection of a suitable surface treatment technique should be preceded by the analysis of its aggressiveness in relation to the concrete substrate strength. A procedure for bond strength estimation using multiple regression approach, based on parameters describing surface quality really generated from various roughening techniques, is then proposed.
Based on the analysis of the early shrinkage mechanism of concrete, shrinkage experiment of new concrete restrained by old concrete were conducted to analyze the impact of the different interface roughness on shrinkage stress of new concrete and bonding surface. It is shown that the inner tensile stress of new concrete decreases with increasing of interface roughness, and increase restrained thickness of the new concrete, but it is not significant effect of interface roughness on the shrinkage at bonding surface; the shrinkage strain of section reduces with reduction of the distance of section to bonding surface and reaches minimum on bonding surface, which reflecte significant nonuniformity and increases gradually with age.
Use of the Iosipescu shear test for measurement of shear properties of unidirectional laminae has been studied both analytically and experimentally. The intralaminar shear strength and shear stiffness of glass-reinforced polyester material have been measured using specimens with two different fibre orientations. Acoustic emission has been monitored and a fractographic study carried out. A finite element analysis has been conducted to evaluate the stress distribution within the specimen, assuming isotropic and orthotropic elastic properties of the material. There is a complicated stress distribution in the specimen, particularly in the vicinity of each notch root, depending on the elastic properties of the composite. The shear stress region in the specimen gauge section is almost uniform, though small normal compressive stresses exist. The experimental results have shown that the measured shear modulus does not depend on reinforcement orientation. However, it has been observed that different failure modes occur in each case. This results in a change in apparent shear strength of the composite with fibre orientation. Some explanations of these differences have been found in a detailed analysis of the local stresses at the roots of the notches. It is considered that the presence of tensile stresses in this area is primarily responsible for the apparent reduction in the shear strength.
Contour Crafting is a mega scale layered fabrication process which builds large scale three-dimensional parts by depositing paste materials layer by layer at unprecedented speed and with superior surface quality. This paper presents an overview of related research activities and the progress aimed at extending the technology to construction of residential housing units and civil structures.
A new upper bound formulation of limit analysis of two- and three-dimensional solids is presented. In contrast to most discrete upper bound methods the present one is formulated in terms of stresses rather than velocities and plastic multipliers. However, by means of duality theory it is shown that the formulation does indeed result in rigorous upper bound solutions. Also, kinematically admissible discontinuities, which have previously been shown to be very efficient, are given an interpretation in terms of stresses. This allows for a much simpler implementation and, in contrast to existing formulations, extension to arbitrary yield criteria in two and three dimensions is straightforward. Finally, the capabilities of the new method are demonstrated through a number of examples. Copyright © 2005 John Wiley & Sons, Ltd.
The present investigation focuses on a new approach for the construction of durable concrete structures. Using Pseudo-ductile
Cementitious Composites (PDCC) of relatively low water/binder ratio, permanent formworks are first fabricated. Normal concrete
is then cast to make structural components. With low permeability and high crack resistance, the permanent formwork acts as
effective surface cover to prevent the corrosion of steel reinforcements. The formwork can be made with PDCC alone, or with
the incorporation of Glass Fiber Reinforced Plastics (GFRP) rods. In some structural components, the GFRP reinforcements will
be sufficient to provide the necessary load-carrying capacity. When higher loads are to be carried, steel reinforcements can
be added to produce a component with very high durability (due to the thick cover to steel) as well as ductile behavior. This
paper focuses on mechanical aspects of this construction concept. The development of PDCC for formwork fabrication is first
described. The bond between PDCC and concrete, in relation to various surface treatment methods, will be investigated with
beam specimens. Test results on concrete beams made with GFRP reinforced PDCC formwork are then presented and compared to
theoretical predictions. A design example is performed to demonstrate the use of GFRP/PDCC permanent formwork for constructing
the deck of a footbridge. The results of this investigation show promise of the technology for practical applications.
KeywordsPseudo-ductile Cementitious Composites-Permanent formwork-GFRP reinforcement-Durability
A history of the losipescu shear test as applied to composite materials is presented along with a description of the test fixture and specimen design. Iosipescu's shear test is compared to similar test techniques, including the asymmetrical four-point bending (AFPB) test. Finally, in-plane and through-the-thickness shear properties measured using the losipescu shear tests are presented for a variety of materials, including a unidirectional graphite/epoxy, random and continuous-fiber sheet molding compounds, and two polymer materials.
The effects of the addition of silica powder, silica fume, fly ash, and hemihydrated gypsum to the interface of new-to-old paste on the enhancement of bond strength were investigated experimentally. Seven-day bond strengths for the specimens with pozzolanic materials added to the interfacial zone, except for silica powder addition, were slightly smaller than that for control without any additives, whereas the strength of all specimens with added pozzolanic materials were higher than that of the control at 28 days. It was suggested that increased effect of the bond strength at interfacial zone depended on the SiO2 and CaO contents in the additives; higher SiO2 and/or lower CaO contents were preferred. On the other hand, when a high CaO content fly ash was coated to the interface with some amount of hemihydrated gypsum, the bond strength enhanced significantly at 7 days, as well as at 28 days. This result suggests that the structure of the interfacial zone can be modified sufficiently by controlling the chemical components of the additives; even the use of fly ashes consisted of relatively high CaO and low SiO2.