Structural Response of Reinforced UHPFRC and RC Composite Members

Ultra High-Performance Fibre-Reinforced Concrete (UHPFRC) combines high strength, deformation capacity and low permeability. This makes UHPFRC particularly suitable as a strengthening and protective layer for application to existing reinforced concrete structures (RC-UHPFRC composite members). The structural behaviour of UHPFRC layers in composite members is investigated with a comprehensive experimental program. In a systematic approach the behaviour of plain UHPFRC and reinforced UHPFRC (R-UHPFRC), the bond behaviour of reinforcement in UHPFRC and the flexural behaviour of composite beams are analysed. The study is limited to thin UHPFRC layers (≤ 50 mm) and small diameter bars. Special attention is given to the representativeness of the experimental specimens. In order to avoid preferential fibre orientations, all UHPFRC and composite specimens are cut from larger elements. The high scatter of the tensile behaviour of plain UHPFRC, quantified by the testing of a high number of specimens, confirms the reasonableness and necessity of additional bar reinforcement. The additional reinforcement increases not only the resistance but also improves the deformation capacity and strain hardening behaviour of UHPFRC. The use of reinforcing bars with different steel grades and surface characteristics, namely ribbed and smooth bars, improves the current knowledge base on the bond behaviour. It is shown that the pre-peak behaviour is independent of the bond strength and that the crack spacing is dominated by the fibre reinforcement. The reinforcing bars with low bond strength allow for large post-peak deformations and avoid the localised stress concentration in the UHPFRC macro-crack. The use of high yield strength steel is beneficial in view of the increased deformation capacity of R-UHPFRC. The apparent strain hardening of UHPFRC is significantly increased by the reinforcement. The strengthening of reinforced concrete beams with a R-UHPFRC layer is characterised by the significant increase of the resistance and stiffness (depending on the reinforcement) combined with an important reduction of the rotation capacity. With a good balance between resistance increase and loss of rotation capacity the smooth reinforcing bars of the UHPFRC layer represent an interesting alternative to conventional reinforcement. The use of high yield strength steel proves to be advantageous due to the higher deformation capacity. Through the use of an analytical model for plain UHPFRC the origin of the scatter of the tensile behaviour and the conditions for the occurrence of strain hardening are identified. The modelling of R-UHPFRC gives an insight into the interaction of fibres and reinforcing bars and the apparent UHPFRC tensile behaviour. Further, it is shown that the characteristic structural behaviour of composite sections can be described with the extended sectional analysis. Finally, the main experimental and analytical results are summarised in recommendations for the application of R-UHPFRC to existing structures. This thesis contributes to the understanding of the behaviour of R-UHPFRC and composite members and emphasises the necessity of additional reinforcing bars in UHPFRC in order to achieve a reliable structural behaviour.

Structural Response of R-UHPFRC - RC Composite Members Subjected to Combined Bending and Shear

Talayeh Noshiravani

The addition of a thin overlay of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) to Reinforced Concrete (RC) members is an emerging technique to strengthen and protect existing structures and to design durable new structures. Combining UHPFRC with closely spaced, small-diameter steel rebars in Reinforced UHPFRC (R-UHPFRC) layers improves the UHPFRC's strain hardening behaviour. For reasons of practicality, R-UHPFRC layers are cast or glued (in the case of prefabricated elements) on top of RC members, thus changing the latter into R-UHPFRC - RC composite members. The high strength and deformation capacity of R-UHPFRC elements make them a suitable external flexural reinforcement for RC members over intermediate supports, e.g., bridge decks and slabs or beams in buildings. Over reinforcement of RC beams and slabs with tensile flexural reinforcement can result in their shear failure at either a lower resistance or deformation than the associated values for member failure in flexure. A comprehensive experimental program was conducted to study the flexure-shear behaviour of R-UHPFRC - RC composite beams. The program comprises two test series on cantilever beams and continuous beams. The test parameters include shear span-depth ratio (a/d), the amount of transverse reinforcement ( ρν), the amount of longitudinal reinforcement, and the strength and bond condition of the R-UHPFRC rebars. The experimental results reveal the different failure modes of R-UHPFRC - RC composite members and the contribution of the R-UHPFRC elements to the member resistance, ductility and capacity to redistribute the internal stress. It was shown that in R-UHPFRC - RC beams with ribbed rebars and a shear span to depth ratio greater than 2.5 the stresses are carried by beam action. Depending on the degree of longitudinal reinforcement, all but two of the beams with 3.0≤a/d≤3.4 and ρν≤0.17 had a flexure-shear failure; the rest failed in flexure. The flexure-shear failure of the composite beams was at an approximately equal rotation level as their RC reference beam but at a resistance 2.3 times that of the RC beam. This is due to (1) the debonding interface zone between the elements that allows the R-UHPFRC - RC beams to rotate more freely and (2) the out-of-plane resistance of the R-UHPFRC element that contributes to the shear resistance. The internal flow of forces and the structural response of composite members strongly depend on the bond condition between the R-UHPFRC and RC, the UHPFRC and its rebars, as well as the concrete and its rebars. Cracking of the concrete along the interface zone causes bond reduction, i.e., softening of the shear connection, between the two elements. In presence of high shear stresses and diagonal flexure-shear cracks, interface zone softening is observed between the elements prior to the maximum resistance, while UHPFRC is strain hardening. The cause of this softening behaviour is the prying action due to the relative rotational movement of the RC rigid bodies separated by the flexure-shear cracks. Static and kinematic solutions of the theory of plasticity for RC beams are extended to predict the collapse load of R-UHPFRC - RC composite beams at the ultimate limit state. A mechanical model for predicting the structural response of composite beams is proposed. In combination with truss models, the concept of an R-UHPFRC - RC plastic hinge is introduced to calculate the force-displacement response of composite beams. The failure criterion based on the collapse mechanisms (kinematic solutions) sets the limit of the force-displacement response. The model is corroborated by the experimental results. This model provides a tool for analysis of RC members reinforced with an added tensile R-UHPFRC element.

EPFL2012

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

Zum Tragverhalten von zugbeanspruchten Bauteilen aus Ultra-Hochleistungs-Faserbeton

Jörg Jungwirth

Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) can be considered, due to its high strength and improved durability, as one of the leading recent innovations in civil engineering. Material scientists have developed a very efficient, high-tech material by optimizing the concrete composition and improving the density of the cement matrix. Its structural behavior and in particular its relatively high ductility, despite the brittle behavior of the cement matrix, is influenced significantly by the crack-bridging behavior of the fibers. This behavior affects not only the tensile behavior, but also the global material behavior, i.e.: bending (tension by bending), compression (transversal tension, confinement), shear, punching, concentrated load introduction, etc. In the presented study, the tensile behavior of UHPFRC is analyzed as a basis for its structural behavior. The investigations extend from the 'micro-scale' behavior of the crack-bridging fibers in one crack to the 'macro-scale' behavior of tension members composed of UHPFRC. To obtain efficient tensile performance, the use of the UHPFRC in combination with reinforcing bars is expedient. The bond between the reinforcing bars and the UHPFRC is investigated, in order to analyze and understand the structural response of reinforced UHPFRC tension members. At each level of investigation, emphasis is placed on comprehension of the mechanical process. The influence of the fiber orientation, creep, shrinkage, and pre-stressing is discussed. Based on the studied mechanical processes, micro- and macro-scale modeling of the structural behavior is carried out. The cracking mechanisms of UHPFRC are examined in the research work. For the structural response of reinforced UHPFRC tension members, the influence of the double multi-cracking behavior is particularly relevant. First, multiple meso-cracks form controlled by the fibers and then multiple macro-cracks form controlled by the bond between the UHPFRC and the reinforcing steel. With a model based on these mechanisms, the structural response of tension members made from different types of UHPFRC can be described. Depending on the configuration of the tension member (i.e.: type of UHPFRC, reinforcement ratio, steel type, pre-stressing), five different failure regimes result for reinforced UHPFRC tension members. For practical applications the following conclusions can be drawn: Tension members composed of UHPFRC show very efficient structural behavior. The resistance of the UHPFRC supplements the steel resistance according to its strain-respective cracking state. The well distributed cracking of the UHPFRC leads to a considerable increase in the rigidity of the tension member (high tension stiffening effect). To obtain an efficient tension member with a large deformation capacity, a relatively high reinforcement ratio is necessary. Due to the associated continuous hardening behavior and significant elastic deformation, the use of high-strength reinforcing steel without a yielding plateau is favorable.

EPFL2006