Post-cracking Tensile Strength Research Articles

A semi-analytical model to predict the pull-out behaviour of inclined hooked-end steel fibres

The residual post-cracking tensile strength of conventional steel fibre reinforced concrete is directly related to both the amount of fibres crossing a crack and the individual pull-out responses of all activated fibres. Therefore, the knowledge of the single pull-out behaviour is essential to understand the uni-axial or bending behaviour of SFRC when it is considered as a full-fledged composite. Since hooked-end steel fibres are considered to be the most suitable fibre type for structural purposes, the need to accurately predict the pull-out response of these type of fibres, is of great practical importance. In this paper, an experimental investigation of the pull-out response of both straight and hooked-end steel fibres is discussed. Based on the obtained experimental data, a semi-analytical model is developed to predict the fibre pull-out behaviour. The ability of the model to deal with different geometrical and mechanical fibre characteristics as well as the influence of orientation, embedded length and matrix compressive strengths, reflects its overall quality.

Development of a pedestrian bridge with GFRP profiles and fiber reinforced self-compacting concrete deck

In recent years, the number of pedestrian bridges built from composites materials has notably increased. The combination of fiber reinforced polymers (FRP) profiles with fiber reinforced concrete (FRC) elements is being adopted in this type of structures, since the ductility, high post-cracking tensile strength, compressive stiffness and strength of FRC can be combined with the benefits derived from the use of FRP’s profiles to obtain high performance structural systems. In the context of the present work a 12 m long single span pedestrian bridge with two composite I-profiles was designed. In terms of deflection requirements imposed by serviceability limit states, the influence of the height and thickness of glass fiber reinforced polymer (GFRP) profiles, as well as the addition of a thin layer of prestressed carbon fiber sheet in the bottom flange of the GFRP profile was evaluated. Using software based on the finite element method, the structural behavior of the developed structural systems was analyzed. Furthermore, two prototypes of this structural system were built and monitored in order to assess their long-term deformational behavior when subjected to a loading configuration correspondent to the load combination for the deflection serviceability limit states. The main results obtained are presented and discussed.

Analysis on the Moment-Curvature Relationship for Prestressed UHPFRC Beams without Stirrup

To simplify the analysis, an elastic perfectly plastic stress-strain law was presented for UHPFRC. The post-cracking behavior was described by the average constant post-crack tensile strength. A strain parameter μ is proposed to evaluate the performance and efficiency of steel fibre reinforcement. 8 rectangular beams were tested in this investigation. Based on the proposed constitutive model, the full history of their flexural moment-curvature relationship for UHPFRC beams was calculated and compared with experimental data on prestressed UHPFRC beams. Good agreement between calculated strengths and experimental data was obtained.

Shear strength of pseudo strain hardening cementitious composite coupling beam

Experimental and theoretical studies were conducted to propose an equation for predicting the shear strength of a pseudo strain hardening cementitious composite (PSH2C) coupling beam with steel fibers, polyethylene and embedded plates. The application of these fibers in structural member has been known to improve the post-cracking tensile strength of concrete, and significantly enhances the shear strength of RC beams. It is very important to quantify the post-cracking tensile strength of fibers and the contribution of embedded plate in the PSH2C coupling beams before discussing the proposed equation to predict the shear strength of the member. In particular, the post-cracking tensile strength rely on the shape of fibers, volume fraction, aspect ratio, bond stress between fibers and matrix, and the properties of the concrete matrix. For PSH2C coupling beams with a shear span-to-depth ratio (a/d) less than 2.5, the effect of shear transfer by arch action is also taken into account. The results of 71 tests of PSH2C beams were used to evaluate existing and proposed empirical equation for shear strength. The predicted values by proposed equation were in good agreement with both test results and other test data from the literature. Finally, this paper provides background for the simplified design equation to predict the shear strength of a PSH2C coupling beam with steel fibers and polyethylene.

Simple Plastic Model for Shear Critical SFRC Beams

A simple physical model, for prediction of ultimate shear strength of steel fiber-reinforced concrete (SFRC) beams is developed on the basis of a plastic approach originally proposed for reinforced concrete beams without stirrups. It is founded on the hypothesis that cracks can be transformed into yield lines, and thus is know as crack sliding model (CSM). First, the CSM is improved to take into account the shear strength increase for deep beams, due to the arch effect. Then, the effectiveness factors for fibrous concrete under biaxial stresses are evaluated, taking into account the postcracking tensile strength of SFRC and its ability to control slippage along shear cracks. With the aim of providing a handy and fast tool for design of SFRC beams without stirrups, a simplified design model is also derived. Finally, the proposed models are validated by the results of a large set of experimental tests taken from literature.

Steel-fibre-reinforced concrete coupling beams subjected to monotonic loading

An experimental programme on the monotonic tests of large-scale, steel-fibre-reinforced concrete (SFRC) coupling beams is reported in this paper. The ultimate shear strength and post-peak load behaviour of the beams are investigated with the span-to-depth ratio, shear reinforcement ratio, and steel fibre content in concrete as major parameters. The experimental results indicate that the inclusion of steel fibres in the concrete matrix helps to enhance the post-cracking tensile strength of concrete, thus increasing the shear capacity, and to improve significantly the post-peak load behaviour of the coupling beams, where large deformation can be attained without a substantial loss of shear strength, thus providing high damage tolerance that is required in the cases of seismic actions. The ultimate strengths and post-peak load performance of all SFRC specimens are evaluated and compared with the predictions of the design codes BS 8110 and Eurocode 2.

Flexural Design of Fiber-Reinforced Concrete

The authors present a set of closed form equations for fiber-reinforced concrete flexural design. Based on simplified tensile and compressive constitutive response, these equations may be used in a serviceability-based criterion or limit state approach that limits the effective capacity of tensile strain. Flexural moment-curvature response generation is allowed by the equations in regard to a rectangular beam section for use in calculating structural analysis, in addition to strain softening fiber-reinforced concrete design charts. Equations are also proposed for minimum post-crack tensile strength to prevent sudden post flexural cracking failure and control crack width. The average crack width of structural members is limited by the analytical tensile strain equations proposed for serviceability. Additionally short-term deflections of structural members are determined through use of the bilinear moment-curvature model in conjunction with the geometrical relationship between deflection and curvature. The calculation steps are demonstrated through an example of a one-way slab.

Characteristics of Tension Softening of Polypropylene Fiber-Reinforced Lightweight Concrete

The purpose of this study is to determine the tension softening diagram and post-crack tensile strength of polypropylene fiber-reinforced lightweight concrete from bending tests and pull-out tests. Lightweight concrete has been actively applied in civil engineering applications, because the lightweight concrete is effective to reduce the dead load of structures and to improve the earthquake response of substructures. The addition of discontinuous fibers to cementitious materials drastically extends the toughness and strength of concrete. The tension softening diagram and fracture energy are important factors to analyze the mechanical behavior of concrete structures. And also, the post-crack tensile strength has been applied to predict the shear strength of fiber-reinforced concrete beams. In this study, the tension softening diagram and post-crack tensile strength of polypropylene fiber-reinforced lightweight concrete was determined by the bending test and the method based on the poly-linear approximation analysis. Furthermore, the post-crack tensile strength was clarified by the fiber-matrix interfacial bond strength obtained from the pull-out test.

Punching shear strength of interior concrete slab–column connections reinforced with steel fibers

Abstract A theoretical study was performed to investigate the punching shear strength of interior slab–column connections made of steel fiber reinforced concrete (FRC). In the steel FRC slab–column connection, the shear force applied to the critical section is resisted by both the compression zone and the tension zone at the critical section. The shear capacity of the compression zone was defined by considering the interaction between the shear and the normal stresses developed at the critical section. The shear capacity of the tension zone was defined by considering the post-cracking tensile strength of FRC. By using the shear capacity, a new strength model for the punching shear strength of steel FRC slab–column connections was developed. The proposed strength model was verified using existing test results and showed very good accuracy. For convenience in design, a simplified design equation was also developed.

섬유보강 콘크리트 보를 위한 변형 기반 전단강도모델

A theoretical study was performed to investigate the behavioral chracteristics and shear strength of fiber reinforced concrete slender beams. In the fiber reinforced concrete beam, the shear force applied to a cross section of the beam was resisted by both compressive zone and tensile zone. The shear capacity of the compressive zone was defined addressing the interaction with the normal stresses developed by the flexural moment in the cross section. The shear capacity of the tensile zone was defined addressing the post-cracking tensile strength of fiber reinforced concrete. Since the magnitude and distribution of the normal stresses vary according to the flexural deformation of the beam, the shear capacity of the beam was defined as a function of the flexural deformation of the beam. The shear strength of the beam and the location of the critical section were determined at the intersection between the shear capacity and shear demand curves. The proposed method was developed as a unified shear design method which is applicable to conventional reinforced concrete as well as fiber reinforced concrete.

Can Steel Fibers Replace Transverse Reinforcements in Reinforced Concrete Beams?

Steel fiber reinforced concretes currently used do not have enough post-cracking tensile strength to be used alone, without main reinforcement (rebars), in a beam. On the other hand, replacement of the secondary reinforcement (stirrups) seems promising. The use of fibers would save considerable time and would facilitate placement in highly reinforced structures. A large experimental program was carried out to quantify the mechanical contribution of fibers in beams. In a first step, five large scale double-T-girders were tested, with the transverse reinforcement varied - concrete only, stirrups, or fibers. A global optical method, stereophotogrammetry, was used to monitor cracking. It showed a mainly mode I crack opening process. Furthermore, these tests suggested a scale effect linked to the height of the beam: the higher the height of the beam, the wider the crack opening at failure, i.e. the smaller the residual stress carried by the fibers. These findings lead to optimize both material and structure in regard to shear behavior. A high-strength steel fiber reinforced concrete was used in small rectangular beams, 0.25-m high, to provide high residual stress of the material and small crack opening at failure. For both types of beams--T-girders and rectangular beams--the mix was optimized using the Baron-Lesage method and the mechanical characterization of the material was conducted using a uniaxial tensile test monitored by the rate of the crack opening. A method of analysis based on a block mechanism is proposed and compares well with experiments.

Postcracking Tensile Strength of Fiber Reinforced Concrete

A probalilistic model for predicting the expected value and the variance of the post-cracking tensile strength of fiber reinforced concrete is presented. Fibers are assumed to occur as a spatial Poisson process. Orientation of the fibers is assumed to be of 3 types: (1) randomly oriented in space; (2) randomly oriented within a prescribed plane; and (3) fibers all aligned along an axis. Analytical strength predictions are compared with existing strength data and the model assumptions are critically discussed.