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7: "L-shape" panel test: experimental setup (dimension in mm) and crack pattern (WINKLER ET AL. 2004).
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Aiming at the model-based analysis, design and optimization of engineering structures such as segmental tunnel linings made of fiber-reinforced concrete (FRC), a multilevel modeling framework consisting of a series of model components is developed, which facilitates the investigation of the effect of various design parameters (concrete class, fiber...
Citations
... Fig. 5.12Model validation for the pullout of hooked-end fibers embedded in high-strength concrete: Model results vs. experimental results of the load-displacement relation for different inclination angles[101] ...
In this chapter, important research results for the development of a robust and damage-tolerant multimaterial tunnel lining are presented. This includes the production, design and optimization of fiber-reinforced hybrid segmental lining systems based on numerical models and experimental investigations under tunneling loads. In addition, novel tail void grouting materials are developed and optimized regarding their infiltration and hardening behavior while taking the interaction with the surrounding ground into account. In order to expand the applicability of mechanized tunneling regarding soils characterized by significant swelling potential due to water uptake by clay minerals, a deformable segmental lining system is presented. The risk of damage due to high localized loads is reduced by the integration of additional radial protective layers on the lining segments and a compressible annular gap grout, which protect the tunnel structure by undergoing high deformations after reaching a certain yielding load. However, the deformability of such support systems affects the distribution of the stresses around the tunnel which governs the magnitude and buildup of the swelling pressure in the soil. Therefore, the development of damage tolerant lining systems requires a material and structural design which ensures an optimal soil-structure interaction through a synergy of computational and experimental techniques.
... En l'absence de données, l'orientation est le plus souvent prise comme parfaitement aléatoire, la force moyenne pour ce type d'orientations s'obtient donc en intégrant le produit de la densité de probabilité et des forces fonctions de l'inclinaison des fibres [50]. Zhan [91] propose de représenter l'orientation des fibres par un ellipsoïde caractérisée par trois valeurs principales. Il détermine par la suite une densité de probabilité d'orientation en supposant que les repères de fissuration et de l'ellipsoïde sont identiques. ...
Un modèle de comportement mécanique des bétons fibrés est développé et implanté dans le logiciel élément fini Cast3m. Il vient compléter le modèle de fissuration orthotrope du béton Fluendo3D, en ajoutant la capacité de traiter des matériaux à fibres courtes, cylindriques et rectilignes. Ce modèle, qui s'appuie sur des essais et modèles de la littérature, permet d'apporter des éléments de compréhension du comportement de ce type de matériau, et notamment concernant le phénomène de multi-fissuration. La caractérisation des phénomènes mis en jeu durant l'extraction des fibres est le point de départ de cette étude. Les effets de l'inclinaison des fibres par rapport à la direction d'extraction sont pris en compte et interviennent dans le comportement du modèle qui présente la capacité d'utiliser des orientations préférentielles de fibres. Le phénomène de multi-fissuration est représenté grâce à une loi d'effet d'échelle de Weibull qui permet de tenir compte de la dispersion des résistances à la traction du béton et d'expliquer le développement de la multi-fissuration. Cette représentation permet d'obtenir des distributions d'ouvertures de fissures dans des macro-éléments finis, et apporte donc une nouvelle précision dans le calcul des ouvertures de fissures dans des structures de grandes dimensions.
... However, it is challenging to develop a numerical model that accounts for all possible configurations. Therefore, uneven supports are simulated as suggested by Cavalaro et al. (2011) and Zhan (2016). With this purpose, one segment of the previously assembled ring is displaced by e L , as shown in Fig. 15. ...
... It should be emphasized that the cantilever effect was not too significant comparing the tunnels with D i of 3.5 m and 6 m. Modeling TBM force considering circumferential imperfection, adapted from (Cavalaro et al., 2011) and (Zhan, 2016). ...
In the past twenty years, several precast tunnels were constructed using Fiber-Reinforced Concrete (FRC). Also, it has been reported that concrete segments experience damage during TBM operations due to factors as load eccentricity and uneven supports. Simulating in a laboratory such conditions together with interactions between elements, as in a field, is challenging. So numerical models can provide essential guidance for the design. Therefore, the current research presents a broad investigation considering three usual tunnel diameters, different number of TBM shoes by segments, shoe dimensions, and configurations (German and French). Also, FRC classes, determined according to the fib Model Code, are tested. It was found that the French distributions of shoes presented the best performance for all studied cases when compared to the German one. Tunnels with an internal diameter of Di = 6 m were the only ones sensitive to the increase from two to three shoes by segment. In the case of four pads, all tunnel diameters increased the load capacity. The FRC class change from 1.5a to 3a significantly enhanced the load capacity relative to the crack opening of tunnels with Di of 6 m and 10 m. For changing class from 1.5a to 4a, all tunnels had enhanced performance. For Di = 10 m, the increases in load capacity were 2.23, 2.18, and 2.60 for segments with 2, 3, and 4 shoes, respectively. Also, the best aspect ratios found for the shoes’ geometry are reported in the paper.
... The segment model is constructed using a mesh in which non-zero thickness interface elements placed between standard geometrically linear finite elements (i.e. bulk elements) in order to account for cracking in the concrete segment, as per Zhan (2016). The geometry of the segment is explicitly modeled, and therefore the correct stress distribution and cracking response in the radial and circumferential directions are captured. ...
... The softening behavior of interface is determined based on the fracture energy of concrete (Zhan, 2016): ...
... For this reason the elastic properties of the ISE must be chosen in such a manner as to match those of the bulk elements. More information concerning the ISE element formulation can be found in Zhan (2016) and Zhan and Meschke (2016). ...
Design methods for segmental tunnel linings used in mechanized tunnel constructions typically employ numerical bedded beam models and/or classical analytical solutions for the determination of structural forces (i.e. moments and shear and axial forces) and simple load spreading assumptions for the design of the reinforcement in joint areas. However efficient such methods may be, many physical details are often overlooked and/or oversimplified in the process of reducing the actual structure to a structural beam model, e.g. analytically derived loadings are employed, the grouting and ground reactions are reduced to a spring bedding, and the confinement due to grouting at the longitudinal joint is largely not considered in reinforcement design. Such a design process is not able to account for, or predict, the susceptibility of tunnel linings to often observed damages that, although they may not be structurally relevant, lead to serviceability or durability issues, such as crack development or chipping at the segment corners. Numerical methods, such as the Finite Element Method, provide an opportunity to model the segmental tunnel lining and its response to the entire TBM construction process and to explicitly model the crack development within individual segments using modern methods to model the discontinuities in structures. In this contribution, a holistic modeling procedure for the representation of the tunnel lining within the tunneling process is proposed and compared to traditional lining models. A 3D process oriented Finite Element model is used to calculate the predicted forces on the tunnel lining and the obtained results are compared with those generated by traditional methods. Subsequently, the predicted deformations are then transferred to a detailed segment model in which the nonlinear response of the segment at the longitudinal joint is modeled using an interface element based approach to simulate concrete cracking.
... where f t f f * represents the tensile strength of the fiber reinforced concrete composite and the remaining parameters are appropriately determined according to numerical results of crack bridging response obtained for a specific fiber cocktail. A detailed explanation of the material model is given in (Zhan 2016). ...
... It is observed that the computed force-displacement relation, as well as the crack pattern agrees, despite the relative coarse discretization, well with the experimental data. Nevertheless, as shown in Figure 15, the quality of the crack path is improved when using particularly unstructured refined meshes [54]. (a) (b) 0.4 Figure 15. ...
The effective analysis of the nonlinear behavior of cement-based engineering structures not only demands physically-reliable models, but also computationally-efficient algorithms. Based on a continuum interface element formulation that is suitable to capture complex cracking phenomena in concrete materials and structures, an adaptive mesh processing technique is proposed for computational simulations of plain and fiber-reinforced concrete structures to progressively disintegrate the initial finite element mesh and to add degenerated solid elements into the interfacial gaps. In comparison with the implementation where the entire mesh is processed prior to the computation, the proposed adaptive cracking model allows simulating the failure behavior of plain and fiber-reinforced concrete structures with remarkably reduced computational expense.
This study investigated the mechanical properties and failure patterns of the steel fiber reinforced concrete (SFRC) lining segments with different fiber parameters using a three-dimensional mesoscale model. The research highlights a sophisticated mesoscale model of the SFRC tunnel segment with randomly distributed steel fibers and considers the nonlinear mechanical behaviors of steel fiber, concrete and the bond-slip effect. The entire paper is organized as follows: firstly, a generation algorithm to achieve the discrete distribution of steel fibers and the nonlinear constitutive models of steel fiber and concrete were introduced. The bond-slip effect between steel fiber and concrete was converted equivalently. Secondly, the numerical model was further validated by conducting a series of simulations, including the single steel fiber pullout, uniaxial compression, splitting tension and flexural testing. Finally, the mesoscale model was employed to investigate the influences of steel fiber parameters on the mechanical properties and failure patterns of SFRC tunnel segments. It revealed that the typical features of the cracked SFRC segments in the numerical simulation are highly consistent with the experimental results. The randomly distributed steel fibers can well bear and transfer the tensile stress, which effectively improves the crack resistance of the tunnel segment. The load-bearing capacity of SFRC segments linearly increases with the steel fiber content and length while maintains a negative relationship with the steel fiber diameter. Moreover, the addition of steel fiber improves the toughness of the segment more significantly than the maximum load. The proposed model provides an applicable method for further analysis, design and optimization of SFRC segments in real tunneling engineering.
The current society is aware about the relevance of guaranteeing the safety of structures under extreme loading cases, which might be caused by natural catastrophes or human action. Those have a significant media and social impact due to their sudden, destructive and arbitrary character. Among extreme loads, impacts of rigid bodies at moderated speeds could result especially harmful. Impact events can be due to rockfall, vehicles impact, and collision of debris, caused by explosions or hurricanes, among other causes.
At the design stage it is common to consider dynamic actions as quasi-static equivalent loads. However, impacts are characterized by high peak loads, considerable strain rates and large energy release suddenly. Analysis based on equivalent loads might underestimate the dynamic effect of impacts, such as the development of inertia forces (that modify substantively the internal forces distribution), the propagation of their effects or that the impacts demand an energy dissipation capacity, being the impact force a consequence of the structure-projectile contact properties and of the inertia of both. Additionally, the mechanical properties of the material are sensible to the strain-rate (typically characterized by the dynamic increase factors - DIF - that could be different for each failure mechanism involved in the global failure).
The sum of these effects can be especially critical in concrete structures, which trend to develop brittle failure by shear or punching, with a limited capacity of energy absorption. This has been observed even in structures designed with flexure-governed failure and with transverse reinforcement. An interesting option to improve the impact performance of concrete structures is to enhance its energy capacity absorption by fiber addition. Thanks to its capacity to bridge tensions across crack surfaces, steel fiber-reinforced concrete (SFRC) reduces the tendency to fail in a brittle mode in dynamic range. Nevertheless, the rate sensibility of SFRC has not been completely characterized, so far.
The research presented in this Ph.D. Thesis is addressed at various levels (material and structural) from different perspectives (experimental and analytical). The proposed study is focused on the impact behavior of longitudinally reinforced SFRC beams, without stirrups, and to evaluate fiber addition role avoiding the development of brittle shear failure in dynamic range. For that purpose various mixes of SFRC are analyzed, including three types of fibers (prismatic, hooked-end and straight) in various amounts (0.5 and 1.0%), besides a reference series with conventional concrete.
The experimental campaign has consisted of material tests, on unnotched prismatic specimens, and structural tests, performed on 2000x250x125 mm beams reinforced longitudinally, both in a three-point bending configuration. For each of the seven mixes, various impact tests were performed as well as companion static tests, in both the material and structural tests. The experimental results showed an enhancement of SFRC properties with the strain-rate, with dependence on the type of fibers and its volumetric fraction. For instance, fracture energy DIF showed a tendency to decrease in mixes with fiber ruptures observed in case of prismatic and hooked-end fibers. In structural tests, certain SFRC mixes avoided shear failure, such as those with straight fibers or 1.0% of fiber amount, which were able to stitch thin diagonal cracks in the shear span.
Tests results have been discussed with theoretical models developed ad-hoc. At the material level, a model for tensile behavior of SFRC mixes in the dynamic range is presented. At the sectional level two models are proposed, one that studies the bending response as function of the curvature rate and a dynamic flexure-shear interaction model for reinforced SFRC beams without stirrups. Lastly, two structural models are presented, a finite element model with elasto-plastic beam elements and a two-degree-of-freedom model, which considers the contact non linearities, the effective span propagation and the formation of plastic hinges.
The interplay of these models has enabled the author to analyze and discuss the experimental observations in detail. Besides that, the experimental results have allowed to verify the models. The results are discussed with emphasis on the mechanical characterization of various mixes of SFRC in the dynamic range, the description of impact force, the propagation of the effective span and the rate sensibility of each mode of failure, among others. The evolution of internal forces distribution and the use of the flexure-shear interaction model has clarified the development of shear failure in SFRC beams. In essence, the results have shown that fiber addition effectiveness avoiding shear failure depends of the fiber amount and type, and especially, of its tendency to rupture in the dynamic pull-out.
For an extended and comprehensive description of the research presented in this Ph.D. Thesis please refer to Chapter 8. In that Chapter the reader will find an extended summary of this document together with the conclusions, both written in English.
