Failure modes and shear design of prestressed hollow core slabs made of fiber-reinforced concrete

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Abstract

Hollow core slabs (HCS) are usually precast by extrusion and it is not easy to place stirrups; thus, it is difficult to guarantee shear resistance in some cases. This paper describes an experience using fiber-reinforced concrete (FRC) to produce HCS by extrusion to gain shear reinforcement.

An experimental program consisting of 26 HCS was developed. Elements were produced and tested in shear according to the following variables: amount of steel fibers (0, 50 and 70 kg/m3) and a shear span/depth (a/d) ratio of 2.3–4.4 and 8.6.

Different failure modes took place. Some of the main conclusions drawn were that fibers improve quality of the material for shear, HCS with fibers achieved greater loads than HCS without fiber reinforcement and with a more ductile behavior.

Introduction

Hollow core slabs (HCS) were developed in the 1950s when long-line prestressing techniques evolved [1]. HCS are advanced products in the prestressed precast concrete industry, especially in terms of efficient use of material (low self-weight), and given their high quality due to the efficient production line manufacturing process [2], [3]. The extrusion method has a very widespread use and has been utilized to produce inexpensive and easy-to-handle HCS [2], [4], [5], [6], [7], [8], [9], [10], [11]. Nevertheless, the extrusion method is not without its drawbacks because it does not allow shear reinforcement incorporation, and anchorage reinforcement by bond is produced. Shear on HCS and its failure modes have been studied by different authors [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], who analyzed various failure modes: flexure, anchorage and shear (by web shear tension or by shear-flexure).

The parameters which mainly affect the shear strength of prestressed precast HCS are amount of prestress force, concrete strength (compressive and tensile strengths), loading conditions, and interlocking forces in the shear crack.

Given the circumstances, introducing fibers into concrete could improve the shear capacity [15], [16], [17] of the HCS. Paine et al. [8], [9], [10], [11], [14] experimentally studied shear on HCS made with fiber-reinforced concrete (FRC), using 48 kg/m3 of hooked-end steel fibers, 30 mm in length, with an aspect ratio of 60 by testing them with an a/d of 2, 3 and 4.5. The conclusions they drew were: manufacture of extruded HCS reinforced with steel fibers has been shown to be practicable; addition of steel fibers to HCS increases the first crack and ultimate shear capacity; the nature of improvement was dependent on the a/d ratio; the post-cracking ductility of fiber-reinforced slabs also substantially improved in comparison with plain slabs; a safer, controlled failure was observed.

Nowadays, the relevant method to calculate HCS is given in European Standard EN 1168 + A2 [18]. This Standard deals with the requirements and the basic performance criteria and specifies minimum values where appropriate for precast hollow core slabs made of prestressed or reinforced normal weight concrete according to EN 1992-1-1:2004. This European Standard covers terminology, performance criteria, tolerances, relevant physical properties, special test methods, and special aspects of transport and erection. However, it is not contemplated the contribution of steel fibers to the HCS resistance, in those cases MC2010 [19], [20] and RILEM [21] approach will be used to determine the contribution of steel fibers to shear resistance.

Section snippets

Research significance

Given the impossibility of placing stirrups in the HCS produced by extrusion, introduction of steel fibers into concrete can prove to be a system that helps improve shear resistance. However, the literature includes very few works on the shear behavior of HCS made with FRC and does not offer characterization tests in relation to HCS made with FRC which can be adapted to current Design Codes.

In other words, in the last 10 years very few papers have been published related to shear behavior of HCS

Experimental investigation

Twenty-six HCS were tested and classified into two different series, mainly differenced by the tension in the prestressing strands and the different design failure modes expected in them.

In Series I, HCS and their test disposition were designed so that failures were produced by shear-flexure. For Series II, a program with greater variability of predictable failure modes was developed, and was based on a design for more heavily prestressed HCS which contemplates a wider range of fiber

Failure modes

Different failure modes were observed. The following notation was used:

  • Shear Failure Modes:

    • SF: Shear-flexure failure, Fig. 7. On the shear-flexure failures, flexure cracks initially developed, but eventually one of them caused the failure. The failure crack was always situated in the shear span close to one of the load points. Firstly, the crack grew vertically to finally turn in direction by taking a shear slope near the top.

    • S: Web shear tension failure of concrete; Fig. 5. In most cases,

Conclusions

According to the tests, the following conclusions can be drawn:

  • It is possible to produce fiber-reinforced concrete hollow core slabs (HCS) without encountering technical problems.

  • An extensive experimental program has been developed to analyze the shear strength and failure behavior of HCS with different failure modes. The effect of fiber amount and shear span on behavior has been analyzed.

  • HCS with fibers reached higher shear capacities than those without fiber reinforcement, and obtain a more

Acknowledgments

The authors of this work wish to thank the research bureau of the Spanish Ministry of Science and Innovation and the Plan E, for the funding of the project “BIA 2009-12722”. The collaboration of the precast industry “PREVALESA S.L.” is also acknowledged.

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