Static and Fatigue Shear Strength of Reinforced Concrete Slabs Under Concentrated Loads Near Linear Supports

Reinforced concrete slabs without shear reinforcement under concentrated loads near linear supports are typical cases of bridge deck slabs, transfer slabs or pile caps. Such members are often designed or assessed in shear/punching shear with code provisions calibrated on the basis of tests on beams or one-way slabs loaded over the full width, as well as tests on isolated flat slab elements supported on columns in axis-symmetric conditions. However, these tests are not representative of the actual behavior of slabs under concentrated loads near linear supports. Moreover, concentrated loads of heavy vehicles have a repetitive nature, causing loss of stiffness and potential strength reductions due to fatigue phenomena. In this thesis, two experimental campaigns are presented. The first one consists of twelve static tests on six full-scale cantilever slabs subjected to a concentrated load with a central line support, that allows tracing the linear reaction evolution. Parameters such as the location of the concentrated load, and the presence, material and injection of ducts were varied. All slabs failed in shear and significant redistributions of the linear reactions were observed prior to failure. The second campaign has a similar test setup and consists of four static tests on two full-scale cantilever slabs (reference tests) and other eleven fatigue tests on eight identical slabs. The results show that cantilever slabs are significantly less sensitive to shear-fatigue failures than beams without shear reinforcement. The static reference tests presented shear failures. Some of the fatigue tested slabs failed due to rebar fractures. They presented significant remaining life after the first rebar failure occurred and eventually failed due to shear. The shear failures exhibited by the static tests on cantilever slabs from this thesis and others from the literature can be reasonably predicted with the Critical Shear Crack Theory (CSCT), provided that the influence of direct load strutting and redistribution of internal forces is accounted for, and that no contribution due to the inclination of tapered members to shear transferring is considered. Simply supported slabs under concentrated loads near linear supports may exhibit shear or punching shear failures. Factors like the ratio between the dimension of the load parallel to the support and the slab width, or the type of loading (monolithic or not) seem to be crucial to determine the failure mode. In this thesis it is proposed to use the CSCT for both shear and non-axis-symmetric punching shear combined with certain proposals on how to determine the internal forces and punching perimeter lengths to assess tests from the literature. The proposed approaches are not fully capable of predicting the correct failure mode, but allow a safe design. A reasonable accuracy is obtained, either knowing or not a priori the correct failure mode. A consistent design approach for shear-fatigue failures of reinforced concrete members without shear reinforcement is also presented, based on FM applied to quasi-brittle materials in combination with the CSCT. This leads to simple, yet sound and rational design equation incorporating the different influences of fatigue actions (minimum and maximum load levels) and shear strength (size and strain effects, material and geometrical properties). The accuracy of the design expression is checked against available test data in terms of Wöhler (S-N) and Goodman diagram

Shear strength of reinforced concrete bridge deck slabs

Rui Vaz Rodrigues

Reinforced concrete bridge deck slabs without shear reinforcement can be subjected to concentrated or distributed loads of important magnitude. Under these loads their structural response is not always ductile. In particular under concentrated loads their deformation capacity can be limited by shear or punching shear failures, which prevent them from reaching the ultimate load predicted by pure flexural analysis. This problem has been studied in this research by means of an important experimental program and theoretical modeling. The limited ductility of bridge decks was investigated by means of full scale tests on bridge deck cantilevers under groups of concentrated loads. Six large scale laboratory tests were performed on two bridge deck cantilevers with a span of 2.8 m and a length of 10.0 m. All slabs failed in a brittle manner, in shear or punching shear. The theoretical flexural failure load estimated using the yield-line method was never attained. Despite the brittle failures, the results of tests on cantilevers have shown that some amount of yielding can occur before the shear failure and therefore reduce the shear strength. This effect was quantified on eleven full scale tests on slab strips without shear reinforcement with a length of 8.4 m. The results clearly show that the increase of plastic strains in the flexural reinforcement leads to a reduction of the shear strength. The measured rotation capacity of the plastic hinge was thus limited by a shear failure. A particular problem of bridge deck slabs is the introduction of concentrated loads applied by wheels with pneumatic pressure. Punching shear with these loads is usually treated in a manner similar to punching by a column. A punching shear test was performed with a concentrated load simulating a vehicle wheel with pneumatic pressure to investigate the differences. It appears that punching shear with a wheel with pneumatic pressure is less critical because curvatures tend to be distributed over the surface of the applied load rather than concentrated near the edges of the column. In order to investigate the experimental results on slab strips without shear reinforcement, a mechanical model is proposed to predict the shear strength and rotation capacity of plastic hinges. The shear strength is formulated as a function of the opening of the shear crack and of the strength of the concrete compression zone. The results of the mechanical model are in good agreement with the measured values, both for the shear strength and for the shear carried across the shear crack. Based on the mechanical model, a simplified equation is proposed. The model can also be used to predict the shear capacity of yield-lines. A non linear finite element model was implemented during this work and used to correctly predict the measured rotations and load-displacements curves of the tested cantilevers and other full scale tests performed by other researchers. The measured failure loads are accurately estimated by using the results of the non-linear model and the one-way shear and punching shear criteria proposed by Prof. A. Muttoni (Muttoni 2003).

EPFL2007

Applications of bent-up bars as shear and integrity reinforcement in R/C slabs

Miguel Fernández Ruiz, Stefan Lips, Aurelio Muttoni, Luca Tassinari

Bent-up bars were extensively used as shear reinforcement in beams and slabs from the first R/C developments up to the 1970’s. Their use was justified by the fact that they allowed enhanced development of the tensile reinforcement at the same time they acted as shear reinforcement. New reinforcing bars (with reduced development lengths) in combination with shear reinforcing systems led however to the abandon on their use. Recently, interest has again grown on the use of bent-up bars in flat slabs. This is explained by their efficiency both as shear reinforcement (alone or in combination with stirrups) and as integrity reinforcement. In this paper, the results of 7 full-scale specimens (3.0×3.0×0.25 m) reproducing the support region of an actual flat slab with different layout of bent-up bars are presented. The tests show different potential failure modes and allow understanding the contribution of this reinforcement to the shear-carrying capacity of the specimens. Additionally, the performance of bent-up bars as integrity reinforcement is discussed with reference to 9 half-scale specimens (1.5×1.5×0.125 m) where the influence of the development conditions of the bent-up bars is clearly assessed.

fib Symposium Prague 20112011

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.