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This paper intends to analyze the microstructure of concrete with recycled aggregates (RA) from construction and demolition waste from various Portuguese recycling plants. To that effect, several scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were performed. Various concrete mixes were evaluated in order...
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... mix produced had a 125 ± 15 mm slump, in order to compare mixes with the same application range, A preliminary stage was needed to adjust, whenever necessary, the water content of each concrete mix in order to fulfil this requirement. Table 1 shows that the effective w/c ratio had to be raised as the RA content increased. This increase was not the same in all the families of RA mixes. ...
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... Mix M-CB, made with crushed clay bricks, had a compressive strength of 68.7 MPa and was well-hydrated with good mechanical properties, as indicated by the presence of C-S-H and ITZ as shown inFig. 9[36,37]. ...
Concrete, the backbone of modern infrastructure, exhibits varying mechanical behaviour that depends on its components, with aggregates playing a crucial role in its strength and durability. This study aimed to investigate the influence of elevated temperatures on the compressive strength of concrete made with different types of aggregate. A comprehensive evaluation of concrete mixes was conducted using quartz, crushed clay bricks, crushed andesite, expanded clay and expanded glass coarse aggregates. For each coarse aggregate type, concrete mixtures were made with the same amount of Portland cement, water-cement ratio, natural sand, and superplasticizer. The effective water-cement ratio and cement content were kept constant in each concrete mixture. The grading and maximum particle size were the same in all concrete mixtures. The results revealed that the type of aggregate has a significant impact on the compressive strength and thermal resistance of concrete, with andesite-containing concrete exhibiting the highest residual strength after heating up to 800 °C and clay brick-concrete displaying the highest strength under elevated temperatures up to 1000 °C. The study also found that the age of concrete affects its strength at elevated temperatures, as concrete in the early stages of hydration is more susceptible to thermal cracking than concrete that has had more time to cure. Generally, the compressive strength of normal-weight aggregates depends on the strength of the parent rock. But in the case of fire (elevated temperature), the cement paste matrix loses its strength, and the aggregates effects become more significant.
... The study concluded that use of coarse RA reduces the durability of concrete and water absorption tends to increase linearly with increase in percentage of coarse aggregate replacement. Bravo et al. [7] performed an analysis on the microstructure of concrete with recycled aggregates from construction and demolition waste from different sources in Portugal. Study concluded that the bonding capacity of the aggregate to cement paste is greatly influenced by the nature of RA. ...
Paper presents experimental and Finite Element Method (FEM) analysis of fracture behavior of concrete made using Recycled Aggregate (RA). Concrete mixes were prepared using Construction and Demolition Waste (CDW) as replacement of natural coarse aggregates. To study fracture performance, concrete mixes were prepared with water to cementitious content (w/b) ratios between 0.4 and 0.5.
Beam specimens of size 100 mm x 100 mm x 500 mm were cast and tested as per method of Three-point bend test on notched beam proposed by RILEM. Fracture parameters like fracture energy, stress intensity factor, energy release rate and characteristic length were evaluated using Load-CMOD (Crack Mouth Opening Displacement) and load deformation curves. Mechanical properties of concrete such as compressive and flexural strength, modulus of elasticity and split tensile strength were also evaluated. The performance of concrete using RA has been compared with concrete using Natural Aggregate (NA) from literature. Results suggest slightly better fracture performance in case of concrete made using RA in comparison to conventional concrete in spite of having similar strength
and w/b ratio. Fracture energy parameter in terms of stress intensity factor obtained from FEM analysis were similar to experimental results wherein no significant variation in stress intensity factor for concrete mixes with recycled and natural aggregate were observed. However, it can be stated that values of stress intensity factor of 0.47_NA was lowest and 0.5_RA was highest. There was no significant difference in average fracture energy of mixes and it lies in range of 180 N/m to 300 N/m.
... Construction and demolition waste (CDW) has been extensively used in the construction industry since the second world war considering the massive amount of debris and the desire to meet the increasing demand for suitable construction materials [138,139]. In EU countries, roughly 50% of the total volume of CDW is recycled, and about 70% of the total CDW amount is expected to be reused by 2020 [140]. In the US, nearly 136.1 Mt of waste was produced by construction and demolition activities, but merely 28% were recycled [141]. ...
Traditional waste management schemes raise various environmental, social, and economic concerns; thus, require productive replacements. A large amount of natural resource is utilized to meet the demand of concrete production, which causes environmental issues; also, enriched properties of concrete with low density are demanded. Consequently, the idea of lightweight concrete (LWC) has gained popularity. Practically, there is a vast potential of using waste byproducts of the various sectors in LWC; thus, it could also be taken as a sustainable waste management solution. Extensive studies have been carried out on the use of various waste byproducts in LWC as an aggregate, cementing agent, admixture, and their combinations. The current paper reviews the research status, gaps, challenges and solutions of the use of waste byproducts of various sectors in LWC. Various governing factors are identified to use waste byproducts in different roles in LWC, i.e., material, admixture, or combination of both; based on which, the suitability of the use of any waste material in LWC can be assessed. Despite a large number of conducted studies, characterization of more waste material for use in LWC, assessment, and mitigation of hazardous impacts of waste byproducts in LWC, durability, and life cycle assessment of waste-based LWC are identified as the research dimensions to be further focused. The current paper aims to benefit the associated researchers and practitioners working in the fields of construction, waste management, sustainable development, resources conservations, and recycling.
... Regarding the drd and the dssd, three homogeneous groups without statistically significant differences can be observed (Figure 4a,b): the group of natural aggregates (CNA, Ru), Rb and masonry mortar (Rc2), and concrete particles (Rc1). It has been proven that cementitious particles (Rc1 and Rc2) present lower values than natural aggregates in drd and dssd [21][22][23][24][25][26][27]. However, natural aggregates, Rb, and Rc1 had similar dap, while Rc2 had the lowest value ( Figure 4c) and presented significant differences. ...
... Apparent particle density (d ap ) 9.32 1 × 10 −4 Oven-dried particle density (d rd ) 62.05 1 × 10 −4 Saturated and surface dried particle density (d ssd ) 43 Regarding the d rd and the d ssd , three homogeneous groups without statistically significant differences can be observed (Figure 4a,b): the group of natural aggregates (CNA, Ru), Rb and masonry mortar (Rc2), and concrete particles (Rc1). It has been proven that cementitious particles (Rc1 and Rc2) present lower values than natural aggregates in d rd and d ssd [21][22][23][24][25][26][27]. However, natural aggregates, Rb, and Rc1 had similar d ap , while Rc2 had the lowest value ( Figure 4c) and presented significant differences. ...
Recycled aggregates (RA) from construction and demolition waste are an alternative to natural aggregates in the construction sector. They are usually classified according to their composition. The main constituent materials are separated into the following categories: unbound natural aggregates, ceramic particles, cementitious particles, bituminous materials, and other materials considered impurities, such as glass, plastic, wood, or gypsum. In this research, a large number of samples of RA were collected from three different recycling plants and their properties were studied. After that, 35 samples were selected randomly, and their RA constituents were separated under laboratory conditions. Cementitious particles were differentiated into two subcategories: masonry mortar and concrete particles. Subsequently, their physical–mechanical properties were measured. The statistical analysis carried out exhibited that the constituents had a statistically significant influence on the physical–mechanical properties studied. Specifically, masonry mortar particles had higher water absorption and worse mechanical properties than concrete and ceramic particles. Secondly, multiple regression models were performed to predict the physical–mechanical properties of RA from their composition since mean absolute percentage error (MAPE) ranged between 0.9% and 8.6%. The differentiation in the subcategories of concrete and masonry mortar particles in compositional testing is useful for predicting the physical–mechanical properties of RA.
... Bacterial suspension at a concentration of 10 9 CFU mL −1 was mixed with cement and water before adding into the mold. Ordinary Portland Cement was mixed with tap water at a water/cement ratio of 0.5, which is the efficient ratio for all concrete mix, [35] and then casted into a 70×70×70 mm 3 mold. The cement was left to harden at ambient temperature for 24 h prior to demolding. ...
Background:
Biocement, calcifying bacteria-incorporated cement, offers an environmentally-friendly way to increase the cement lifespan. This work aimed to investigate the potential use of Lysinibacillus sp. strain YL towards biocement application in both theoretical and experimental ways.
Methods and results:
Strain YL was grown using calcium acetate (Ca(C2 H3 O2 )2 ), calcium chloride (CaCl2 ) and calcium nitrate (Ca(NO3 )2 ). Maximum bacterial growth of ~0.09 hr-1 and the highest amount of CaCO3 precipitation of ~8.0 g/L were obtained when using Ca(C2 H3 O2 )2 . The SEM and XRD results confirmed that biogenic CaCO3 were calcites. The bulk, Young's and shear moduli of biogenic CaCO3 calculated via the VRH approximation were ~1.5-2.3 times larger than those of ordinary Portland cement. The Poisson's ratio was 0.382 and negative in some directions, suggesting its ductility and auxetic behaviors. The new model was developed to explain the growth kinetic of strain YL in the presence of Ca(C2 H3 O2 )2 , whose concentration was optimized for biocement experiments. Strain YL could increase the compressive strength of cement up to ~50% higher than that of the uninoculated cement.
Conclusion:
Strain YL is a promising candidate for biocement applications. This work represents the trials of experiments and models allowing quantitatively comparison with large-scale production in the future.
... [26] When the RAC is stressed, the cracks start from the fragile area near the ITZ and gradually spread to the aggregate and cementitious materials, resulting in the destruction of the entire concrete [24,28]. It has been found that ITZ is composed of pores, Ca(OH) 2 , C-S-H gel, ettringite and cement particles [29]. At the same time, it can be found that the volume ratio of C-S-H in the ITZ between the aggregate and the new cementitious material is greater than that of the old cementitious material, as shown in Figure 2 [29]. ...
... It has been found that ITZ is composed of pores, Ca(OH) 2 , C-S-H gel, ettringite and cement particles [29]. At the same time, it can be found that the volume ratio of C-S-H in the ITZ between the aggregate and the new cementitious material is greater than that of the old cementitious material, as shown in Figure 2 [29]. Studies have shown that the characteristics of ITZ in RAC are related to the quality of the attached mortar and have nothing to do with the quantity of mortar [30]. ...
... Because RCA is attached to more residual gelling materials, the thickness of ITZ in NAC is usually 9-51 µm, and the thickness of ITZ in RAC is 30-60 µm [31,32]. [29] On this basis, some scholars continue to deepen the study of the microstructure of RAC and found that the strength of ITZ is more affected by the water-to-binder ratio. In the lower water-binder ratio concrete, its strength is mainly affected by the ITZ between the residual cementitious materials, and the new ITZ mainly affects the high water-binder ratio concrete components. ...
In the global construction industry, with the growth of population, two important construction problems have to be faced, namely, the excessive consumption of non-renewable resources and the massive accumulation of construction waste. In order to solve these problems, geopolymer recycled aggregate concrete (GRAC) arises at the historic moment. On the one hand, using geopolymers produced by industrial wastes to completely or partially replace cement can reduce cement consumption, thus reducing CO2 emissions. On the other hand, recycled concrete made of recycled aggregate can consume accumulated construction waste and save non-renewable materials and land resources. The combination of the two materials can protect the environment to the greatest extent and save resources. This article reviews the current research on the mechanical properties of GRAC, makes a systematic analysis of GRAC materials, reaction mechanisms, and evaluation indicators, and also discusses the application prospects of GRAC, and strives to make contributions to the field and industry.
... For 100% RA incorporations, the 28-day chloride migration coefficients increased by 59.9%, 37.1%, 27.7%, 26.9% and 27.9% for five c groups 300-500 kg/m 3 , respectively. These very significant increases (especially the groups with lower c) are probably caused by RA's porosity, which is closer to the concrete surface provides pathways for chloride penetration, besides the micro cracks have a very important influence on the permeation transport mechanism of concrete [59]. In higher cement content mixes this effect is not as significant which can be explained by the denser cement matrix and lower involving volumes of cementitious paste for chloride ingress [60]. ...
This paper puts forward a strength-based mix design method for recycled aggregate concrete using a modified empirical formula. A total of 30 mixes, the amount of cement varies from 300 kg/m³ to 500 kg/m³ and the proportion of recycled coarse aggregate ranges from 0% to 100%, were prepared for the regression analysis of relationships between multiple parameters. Then for a defined workability and specified value of recycled coarse aggregate replacement ratio, and known natural water absorption of recycled coarse aggregate to be used in a mixture, two mixture design parameters, cement–water ratio and cement content, are generated from the proposed method after determining the water consumption. Moreover, the confirmatory experiments show that the test results of 26 validation mixes, designed strength ranges from 30 MPa to 50 MPa and recycled coarse aggregate replacement ratio varied from 20% to 100%, were in satisfactory agreement with the target compressive strength. Recycled aggregate concrete durability performances corresponding to this method were also evaluated, and results show that the increasing of recycled coarse aggregate, degrades the freeze–thaw resistance, drying shrinkage, chloride penetration and carbonization.
... Both global -G -and fine -F (binder enriched) -fraction analysis was conducted, and proportions of the main minerals detected were reported. are unfilled spaces between the cement grains [35]. ...
The combination of treatments may improve the extraction of toxic and critical elements from secondary resources, produce changes in materials’ physico-chemical properties and improvements on the sustainability of strategies. Mining residues were treated with an electrodialytic process alone or coupled with a thermal procedure, and reused in cement-based mortars by replacing 10, 25 and 50% of the cement binder content. This study demonstrated the technical and economic feasibility of mortars produced with treated mining residues, namely in applications where flexural strength is key factor. The mechanical properties of the mortars corroborated their compatibility with bedding mortars and fired brick masonry walls.
... As for CRA, the highest decrease was obtained when 100% of aggregates from Valnor were used (22.1%), and the difference between families of CRA was much lower than that for FRA. Besides some factors mentioned above (such as a high amount of clay materials, strength of RA and higher w/c ratio needed to maintain workability), the microscopic and durability test performed on FRA from Vimajas [8,42] allowed us to conclude that these were highly porous, thus also leading to a highly porous concrete. Kim and Yun [43] evaluated the bond-slip between steel and concrete with CRA and pointed out that the quality of concrete near the rebars, especially its porosity, highly influences the bond strength obtained. ...
This works intends to analyze, experimentally and numerically, the bond-slip behavior between steel and concrete made with recycled aggregates (RA) from construction and demolition waste (CDW) from several recycling plants in Portugal. Pull-out tests performed in concrete mixes with RA from CDW are described and the main results (bond strength, bond-slip curves and failure modes) are shown and discussed. Additionally, a comparison between experimental and analytical (using equations from the literature) results is made. Afterwards, finite element (FE) models using Abaqus are developed and compared with the experimental results. The interface behavior between steel and concrete follows that prescribed by the CEB-FIP Model Code and uses as input experimental results. After validation, the models show good results when extended to predict the bond-slip behavior of the remaining concrete mixes studied. It is generally concluded that, per each 10% of natural aggregates replaced with RA from CDW, the bond strength decreases in circa 3% (numerically) to 5% (experimentally). CEB-FIP Model Code equations are shown to be able to predict bond strength and, when used in FE models, to lead to accurate simulation of the bond-slip response of steel and concrete with RA from CDW: the average ratio between numerical and experimental bond strengths is 0.95. In the scope of the FE models developed, a simple equation to be used along with those prescribed by CEB-FIP, which accounts for fraction and ratio of aggregates replaced, is put forward, as a first approach, showing good results.
... Yet the SEM has a relatively higher cost (28). Despite its, most of the studies on RAC microstructure investigations used images from SEM to characterize the morphological features of the ITZ, or even other much more expensive techniques (24,29,30). Considering that the resolution of the imaging determines the range of pore sizes accessible, OM may be appropriate for studying the pore formation and the distribution of their components on RC microstructure, given that it is widely used in petrographic and metallographic studies; therefore, it can also be valid for use in both laboratory and industrial-scale applications (31)(32)(33). ...
Recycled concrete has a microstructure more complex than natural concrete, as it includes new interfacial transition zones, the quality of which is conditioned by the state of humidity of the aggregates used, which in turn will affect the final properties of the concrete. Bearing in mind the greater absorption capacity of recycled aggregates, it is important to improve its properties by means of a treatment method that is capable of reducing the negative effects that this may produce in the new concrete. Therefore, the influence of the pre-soaking method of recycled aggregates on the formation of the microstructure of concretes manufactured with these aggregates is analysed, to determine which treatment is the most effective for the production of concretes for non-structural use. The results show that the microstructure of the evaluated concretes differs according to the treatment method used, the most optimal method being one that uses aggregates without pre-soaking.
