Punching Shear Response of High-Performance Fiber-Reinforced Cementitious Composite Slabs

The response of high-performance fiber-reinforced cementitious composite (HPFRCC) slab panels to punching shear loading was investigated. The slabs were square, 180 mm (7 in.) thick, simply supported at their periphery, and concentrically loaded. Conventional distributed steel reinforcement, if provided, consisted of one or two layers of bottom reinforcement (one- or two-way reinforcement), or four layers of reinforcing bars (two on top and two on bottom, one in each principal direction). The four layers simulated a typical bridge deck designed according to the AASHTO LRFD specifications. Moreover, the effect on punching shear behavior of three different fibers (polyvinyl alcohol [PVA]; ultra-high molecular weight polyethylene identified as SPE; and twisted steel identified as Torex) was evaluated. Test results showed that the punching shear resistance, the energy absorption capacity, and the resistance to spalling of HPFRCC slabs having only two bottom layers of reinforcing bars (one in each direction) were significantly better than for the control specimen with four layers of reinforcing bars and regular concrete. Analysis of the results suggests that the punching shear resistance of HPFRCC slabs with geometric properties similar to those of the test specimens can be safely taken as twice that recommended for design by the ACI code.

Keywords: bridge deck; diagonal tension; polyethylene; punching shear; steel.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

INTRODUCTION

In a prior investigation,1,2 the feasibility of a new bridge deck system constructed with a ductile fiber-reinforced cementitious composite and reduced steel reinforcement was evaluated. To investigate the flexural behavior of that new deck system, a reference reinforced concrete deck, designed according to the AASHTO LRFD Bridge Specification,3 was used for comparison purposes. This reference deck (slab) was reinforced with four layers of reinforcing bars (two top layers and two bottom layers, one in each principal direction) supported by beams running along the bridge longitudinal direction, such as in a composite bridge where the beams are made of either steel or precast prestressed concrete (Fig. 1(a)).

In the new bridge deck system proposed by Chandrangsu and Naaman1,2 the concrete matrix is replaced by a tensile strain-hardening, high-performance fiber-reinforced cement composite (HPFRCC) and three layers of reinforcement are eliminated, keeping only one layer normal to the supporting beams, as shown in Fig. 1(b). The reinforcing bars were straight with a constant bottom cover of approximately 1/3 the depth of the slab. This was also the case at the supports, where negative moments occur. Results from bending tests showed that the proposed deck system provides an overall performance superior to that of a conventional deck designed according to the AASHTO LRFD bridge specification in terms of load-deflection and moment-rotation response, and in particular ductility, and plastic hinge rotation capacity.2,3 Moreover, cracking behavior, in terms of crack width and spacing and their implication on penetration of corrosive agents, was significantly improved. The advantage of the new deck system is that it does not require top reinforcement, thus reducing the influence of corrosive agents, eliminating spalling of the top cover due to bar corrosion, reducing repair needs, and increasing the service life of the deck.

Although the study reported by Chandrangsu3 provided information on the bending behavior and one-way shear resistance, no information was available on the punching shear resistance of the proposed deck system. Indeed, the influence of dowel bar action on shear resistance of slabs is known to be important, and thus there was a need to evaluate the effect on punching shear resistance of eliminating several layers of reinforcing bars. Evaluation of punching shear resistance of the proposed HPFRCC bridge deck system was then the main objective of this study.

Punching shear failure in reinforced concrete slabs subjected to concentrated load, such as that at joints with columns, is brittle and difficult to repair. Several researchers have investigated this type of failure and have proposed empirical4,5 or theoretical6-12 models and alternative strengthening solutions to allow practicing engineers to address it in the design of slabs. These models generally lead to conservative approximation of the punching shear strength of slabs.

Since the introduction of modern fiber-reinforced concrete (FRC), there was a sentiment among researchers that adding fibers should improve the shear resistance of concrete and thus reduce or eliminate the need for transverse shear reinforcement (stirrups) in beams. Subsequently, numerous studies were carried out to evaluate the contribution of fibers to shear resistance and how to account for it in design.5,13-17 To the best knowledge of the authors, however, no study to date has specifically focused on the punching shear response of highperformance (tensile strain-hardening) fiber-reinforced cement composites (HPFRCCs),18 as defined in the following. This was needed to ascertain that the new deck system described previously (Fig. 1(b)) is not only superior in bending to conventionally reinforced concrete decks, but also provides equal or superior performance under punching shear.