Structural Behavior of R-UHPFRC - RC Composite Slabs

The application on existing Reinforced Concrete (RC) slabs of cast-on-site Ultra-High Performance Fiber Reinforced cement-based Composite (UHPFRC) layers is an efficient reinforcement technique, currently spreading. The thin layer of UHPFRC, with or without steel rebars, serves as a tensile reinforcement for the RC slab, creating a composite element. This thesis combines material and structural engineering to study the behavior and resistance of two-way spanning composite slabs, with a certain focus on punching shear resistance. When analysing the behavior of composite elements, the effect of fiber orientation on the in-plane tensile response of the UHPFRC layer needs to be accounted for. Theoretical tools are derived herein to analyze the complementarity of fiber orientation in perpendicular directions and determine the average effect of fiber orientation on fiber efficiency at pull-out. A comprehensive material testing campaign on a strain-hardening UHPFRC is carried out on specimens with various thicknesses and casting processes. The most likely fiber orientation in a layer of UHPFRC for the casting method considered herein is finally estimated with the results of the theoretical and experimental work and a representative tensile response is scaled accordingly. An experimental campaign is carried out on six composite slabs without transverse reinforcement. The parameters of the tests include the thickness of the UHPFRC layer and the amount of reinforcement in it. The punching shear resistance of all composite slabs is higher than the resistance of the reference RC slab. The layer of UHPFRC increases the rigidity of the slab and provides added shear resistance to the cracked RC section by out-of-plane bending accompanied by limited or inexistent Near Interface Cracking (NIC) in the concrete section prior to failure. By doing so, it allows more deformation to take place in the RC section before punching shear failure. This results in rotations at maximum resistance close to what is observed for the reference RC slab. An analytical model is then developed to predict the global bending behavior of the composite slab and the punching shear resistance. A multilinear moment-curvature relation for composite sections is used to calculate the force-rotation curve of a slab. The intersection between this curve and a deformation based composite failure criterion predicts the punching shear resistance. This criterion combines the concrete and the UHPFRC layer contributions. The latter resists to punching shear by out-of-plane bending over a limited length. This mechanism induces tensile stresses perpendicularly to the interface with the concrete. The contribution of the UHPFRC layer to the punching shear resistance thus depends on the tensile strength of concrete. In the final section of the work, a description of the parameters influencing the shear resistance of composite elements is done. With the tools developed in this work, the effects of fiber orientation in a layer can be mastered and the analytical models allow to simply verify the resistance of a composite section.

Comportement et modélisation des éléments de structure en béton fibré à ultra-hautes performances avec armatures passives

Dario Redaelli

The present research improves the experimental and theoretical knowledge on the behaviour of structural elements made of ultra high performance fibre reinforced concrete (UHPFRC) with ordinary reinforcement. UHPFRC is a recently developed material with much higher mechanical properties and durability than ordinary concrete. However, it has been used so far in a limited number of structural applications. A better knowledge on the behaviour of UHPFRC in structural members is needed to be able to take advantage of its outstanding properties in structural design. One of the ways in which the structural efficiency of UHPFRC can be studied is to investigate the improvement in structural performance that is gained by using UHPFRC instead of ordinary concrete in common structural members. As an alternative, new structural shapes and structural concepts can be explored, more adapted to the specific material properties. The work presented in this thesis focuses on the first approach by considering the behaviour and modelling of UHPFRC structural members loaded in tension and in bending with axial load, and reinforced with ordinary steel reinforcement. The experimental and theoretical study carried out on tensile members contributes to the understanding of the fundamental mechanisms that control cracking of a tensile member reinforced with both fibres and bars, in service and at ultimate. A numerical model has been developed to solve the differential problem of bond in the presence of strong mechanical non linearity. This model has been used to simulate the main aspects controlling bond in a UHPFRC member. The results obtained with the model help in clarifying the differences between cracking in ordinary concrete and in UHPFRC ties. The model allows identifying the parameters that influence the strength and ductility of the tie and was used to validate simplified assumptions to model the behaviour of UHPFRC ties in service. The behaviour of columns has been studied with a theoretical and experimental approach. The results of two test series on UHPFRC and HPFRC columns are reported in the thesis. An analytical model has been developed to study the behaviour of a confined concrete member in compression. A numerical model has been implemented to simulate the non linear behaviour of ordinary concrete and UHPFRC columns. The models have been used to simulate test results reported in this thesis or taken from the literature. The numerical model has been used to evaluate the effect of variations of the mechanical behaviour of concrete on the structural behaviour of a column, with or without reinforcement. The results of those parametrical studies help understanding the influence of mechanical properties on structural response. This thesis contributes to a better understanding of the behaviour of UHPFRC members with ordinary reinforcement. The comparative approach used in the research makes it possible to show the differences between the structural behaviour of ordinary, high strength and fibre reinforced concrete. Indications are also given on the possible ways in which an effective use of UHPFRC in structural members can be achieved.

EPFL2009

Structural behaviour of elements combining ultra-high performance fibre reinforced concretes (UHPFRC) and reinforced concrete

Katrin Habel

Ultra-High Performance Fibre-Reinforced Concretes (UHPFRC) have high mechanical strengths (fU,c > 150 MPa, fU,t > 6 MPa) and exhibit quasi-strain hardening in tension. Their very low permeability prevents the ingress of detrimental substances. In composite structural elements formed of normal strength reinforced concrete and Advanced Cementitious Materials (ACM), UHPFRC offer a high potential in view of the load carrying and protection function of the ACM layer. The objectives of the study described in this thesis are to determine the performance and structural behaviour of composite "UHPFRC-concrete" elements in bending. Towards this end, the current knowledge of UHPFRC properties is to be extended and modelling tools are to be developed in order to predict the structural behaviour of such composite elements and to make recommendations for their design. The experimental program is performed in order to characterize the UHPFRC and determine the structural behaviour of composite "UHPFRC-concrete" elements through 15 full-scale beam bending tests. The material tests focus on UHPFRC early age behaviour and on the determination of its outstanding tensile properties with an original uniaxial tensile test. They show that the UHPFRC properties become virtually constant after 90 days. The time-dependent behaviour of the composite beams is investigated during 11 weeks starting from the casting of the UHPFRC layer. After these long-term tests, the beams are subjected to bending until failure with the UHPFRC layer in tension. The parameters are the thickness of the UHPFRC layer, the presence of rebar in the UHPFRC and the static system. The time-dependent behaviour of composite "UHPFRC-concrete" members is investigated focusing on early age by means of test results and an existing numerical model. Using the results of the material tests as input, the numerical model is able to predict the behaviour of composite "UHPFRC-concrete" elements with the exception of self-desiccation of the UHPFRC. However, the deformations of composite "UHPFRC-concrete" members are correctly modelled by introducing autogenous shrinkage directly as volumetric deformation in the model. The structural response of the composite members under bending with the UHPFRC layer in tension is investigated with an original analytical model, which is an extension of the classical bending model for reinforced concrete. The influences of cross-section geometry, the UHPFRC tensile properties, the compressive strength of the substrate and the type of reinforcement are studied. This study demonstrates that the use of UHPFRC enhances the performance of composite "UHPFRC- concrete" elements in terms of resistance and stiffness. Furthermore, durability is extended due to the low permeability and tensile strain hardening properties of UHPFRC. The incorporation of rebar in the UHPFRC layer leads to a further increase in resistance and stiffness of the composite element and to a higher apparent magnitude of hardening in the UHPFRC. The investigated composite elements show monolithic behaviour under service conditions. The time-dependent behaviour is mainly controlled by autogenous shrinkage of the UHPFRC, which may induce a few evenly distributed small-width macrocracks in the case of statically indeterminate systems, thin UHPFRC layers (≤ 1 cm) or high magnitudes of autogenous shrinkage (≥ 750 µm/m at 28 days). The results show that the magnitude of autogenous shrinkage should not exceed 1000 µm/m (at 28 days) in order to avoid debonding and extensive formation of distributed macrocracks. Finally, three basic composite "UHPFRC-concrete" element configurations are proposed: Configuration P is designed for the protection function and consists of a thin UHPFRC layer. Configuration PR is proposed for existing elements with strongly deteriorated rebar and for new construction. It consists of an UHPFRC layer with reinforcement, and assumes no reinforcement in the concrete layer near the interface zone. Configuration R is proposed for existing structures requiring an enhancement of the structural behaviour. It is made of an UHPFRC layer with reinforcement, and assumes that there is reinforcement in the concrete layer near the interface zone.

EPFL2004

Analytical modelling of R-UHPFRC - RC composite members subjected to combined bending and shear

Maléna Bastien Masse, Eugen Brühwiler, Tohru Makita

The addition of a thin overlay of reinforced UHPFRC (R-UHPFRC) to a reinforced concrete (RC) element creates a composite member which can be used for the strengthening of an existing structure and in the design of new structures. With its strain hardening and softening behavior and high resistance in tension, the UHPFRC layer serves as an external tensile reinforcement contributing to both flexural and shear resistance of the RC element. The main failure modes of a composite section were identified during previous experimental campaigns. In this paper, experimental results on a composite slab strip are presented to demonstrate how the layer contributes to the shear resistance. Using this example, an analytical model to predict the behavior and calculate the resistance of a composite member is presented and applied.