Influence of yield stress and compressive strength on direct shear behaviour of steel fibre-reinforced concrete

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Abstract

This study aims in examining the influence of the paste yield stress and compressive strength on the behaviour of fibre-reinforced concrete (FRC) versus direct shear. The parameters studied are the steel fibre contents, the aspect ratio of fibres and the concrete strength. Prismatic specimens of dimensions 10 × 10 × 35 cm made of concrete of various yield stress reinforced with steel fibres hooked at the ends with three fibre volume fractions (i.e. 0%, 0.5% and 1%) and two aspects ratio (65 and 80) were tested to direct shear. Three types of concretes with various compressive strength and yield stress were tested, an ordinary concrete (OC), a self-compacting concrete (SCC) and a high strength concrete (HSC). The concrete strengths investigated include 30 MPa for OC, 60 MPa for SCC and 80 MPa for HSC.

The results show that the shear strength and ductility are affected and have been improved very significantly by the fibre contents, fibre aspect ratio and concrete strength. As the compressive strength and the volume fraction of fibres increase, the shear strength increases. However, yield stress of concrete has an important influence on the orientation and distribution of the fibres in the matrix. The ductility was much higher for ordinary and self-compacting concretes (concrete with good workability). The ductility in direct shear depends on the fibre orientation and is significantly improved when the fibres are perpendicular to the shear plane. On the contrary, for concrete with poor workability, an inadequate distribution and orientation of fibres occurred, leading to a weak contribution of the fibres to the direct shear behaviour.

Highlights

► We examine the behaviour of fibre-reinforced concrete (FRC) against direct shear. ► Increasing compressive strength from 30 to 80 MPa increase shear strength of 120%. ► Adding of a volume ratio of 1% of metallic fibbers able a shear strength’ increases of 65%. ► Low yield stress of fresh FRC increase the effectiveness of the fibbers. ► FRC with low yield stress has a better ductility.

Introduction

Fibre reinforced concrete (FRC) has known many developments and numerous studies have been carried out on this material during the last three decades in order to overcome the tension weakness common to all types of concretes.

The increase in strength of concrete is associated with the increase in material compressive and tensile strengths [1]. However, concretes with higher strength are known to be more brittle than normal strength concrete, representing a significant limitation for their wide-range application in innovative structural design. Furthermore, low toughness characteristics can result in sudden and catastrophic shear failure for concrete with high compressive strength. Broader use of higher strength concrete in structures requires the development of information concerning its shear behaviour. Furthermore, basic knowledge is needed on the effects of the addition of randomly oriented steel fibres on shear characteristics of concrete for various levels of strength. These fibres reinforce and compensate the brittleness of concrete by stitching the micro-cracks and the macro-cracks and taking up the subsequent tensile strains acting on concrete. This confers a relatively better ductility to the hardened concrete. The steel fibres develop a better bond with a compact cement matrix of high-strength concrete [2].

Numerous research results concerning the behaviour of fibre reinforced concrete subjected to various load conditions, including flexural, compressive, and tensile loadings have been reported [3]. The cracking and failure of structures due to in-plane shear is also considered as Mode II failure, which is of considerable importance in many civil engineering structures. Some practical cases, such as deep beams, point or concentrated loading, punching shear in foundations, penetration of projectiles into concrete, shear keys, all experience shear-induced cracking. Several investigations dealing with structural applications of steel-fibre reinforced concrete can be found in the literature. It has been shown that the replacement of vertical stirrups by steel fibres provides effective reinforcement against shear failure [4], [5]. Casanova has shown that a quantity of 1.25% of steel fibres in volume is mechanically equivalent to 1.1% of conventional steel reinforcement [6].

Recently, the use of steel fibre reinforced concrete has been reported to replace vertical stirrups in thin-webbed beams and enhances the web-flange shear transfer in girders [7]. The efforts have revealed that the steel fibres are advantageous over the vertical stirrups or bent up bars particularly in improving the strength, as well as ductility and cracking resistance. The addition of steel fibres showed a reduction of shear deformations at all stages of loading, and proved that this phenomenon was pronounced more as the fibre volume fraction increased. Fibres were found to be effective as shear reinforcement by increasing the shear strength and allowing the shear deficient beams to reach their full flexural capacity resulting in ductile flexural failure. The beams without fibre reinforcement failed by diagonal shear cracking while those with fibres failed in flexure rather than in shear. The use of hooked steel fibres in a volume fraction greater than or equal to 0.75% led to an enhanced inclined cracking pattern (multiple cracks) and improved shear strength in beams without stirrup reinforcement, greater than or equal to 0.33 fcMPa [8]. Cuenca and Serna [9] showed that failure occurrence of Z-shaped push-off specimens is better controlled thanks to the presence of fibres; the shear behaviour is more ductile.

There has been significant interest in determining the response of FRC under direct shear failure and analyzing the behaviour in a local failure plane. Several types of specimens have been used for this purpose [9], [10], [11]. The general objective of performing these tests has been to produce shear failure along a prescribed plane (normally defined by cutting notches in the specimen), employing compression and bending loads. One exception is the Japanese JSCE-SF6 Standard [12] where a double-shear test is performed on unnotched prisms.

The orientation of the fibres inside the matrix is affected by a number of parameters, essentially the geometry of fibres and their interaction effects (fibres-aggregates-formwork), the flowability of concrete, the means of pouring and compacting the concrete, the geometry of the concrete shafts (free surface, two or four boundaries) and their dimensions. When rigid particles such as sand and gravel particles are mixed in a fluid suspension such as cement paste, the type of particle/particle or fluid/particle interactions may vary according to the packing of the particles in the system [13], [14], [15], [16], [17]. Barthos and Hoy [18] noted that packing density is reduced with fibre addition in a slightly larger way with coarse aggregate than with sand, because the sand is able to pack tightly around the fibre, whereas, coarse aggregates are pushed apart by the fibres’ presence.

From our point of view, the rheology parameters of the flowing material, particularly the yield stress and the wall effect generated by the geometry of the formwork, are the greatest influences on the orientation of the fibres within the fresh concrete. The distribution and orientation of fibres is, in turn, the parameter which most influences the ductility of fibre concretes. The contribution of the fibres to the ductility depends on their distribution within the concrete element and especially their orientation [19], [20], [21], [22].

Concrete is a concentrated suspension of solid particles (sand and gravel) in a yield stress fluid (cement paste). Fresh cementitious materials, as many materials, behave as a fluid with a yield stress, which is the minimum stress needed to initiate flow. From a practical point of view, yield stress may be the most interesting value where filling or passing ability is concerned, whereas plastic viscosity may be associated to the velocity at which a given concrete will flow once the material begins to flow. It has been reported that low yield stress provides good placeability of concrete [23], [24], [25], [26], [27]. The ability of the mixture to guarantee a uniform dispersion of fibres during casting has also been recognized as a peculiarity of the FRC [28]. It has been demonstrated that the addition of fibres to a concrete matrix affects its fresh state performance, due to the both the larger surface area of fibres, which requires more fluid phase to be properly enveloped and lubricated, and the significant interparticule friction and interlocking between fibres and between fibres and aggregates [17], [28]. In practice, yield stress is the most important parameter for formwork filling, the viscosity of the material will only play a role on the time needed to obtain a horizontal surface.

The present work aims, through experimental investigation, to provide some information on the direct shear capacity of steel fibre-reinforced concrete with various levels of yield stress and strengths of concretes, various volume and aspect ratios of fibres. Correlation between yield stress, fibre orientation, shear strength and ductility in shear is then developed.

Section snippets

Experimental work

The main objective of this research work is to determine the influence of the yield stress of the fresh mix on the distribution and orientation of the fibres inside the fresh concrete and then to study their effects on the shear behaviour of the hardened fibre-reinforced concrete material.

Rheology of concretes

The rheology of concrete is a major concern for many users. Indeed, the ease of placing of fibred concrete has a direct impact on the orientation of fibres inside the material’s structure and consequently on its structural efficiency once the hardened concrete is loaded.

As previously seen, the distribution of fibres is modified according to the rheology of the cement matrix. A tendency toward balling is a serious problem in fibre-reinforced concrete. Balling reduces workability and increases

Conclusions

The main scope of the paper was to examine the influence of the paste yield stress and compressive strength on behaviour of fibre-reinforced concrete (FRC) against direct shear. From the experimental results reported in this study, the following conclusions can be drawn:

  • 1.

    In the study cases, adding of a volume fraction of steel fibres ranging from 0.5% to 1% increases slightly the yield stress of concrete, confirming the prediction model presented in [16]. However, the yield stress of the mixture

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