For Fibre Reinforced Concrete Research Articles

Destructive and Non-Destructive Evaluation of Fibre-Reinforced Concrete: A Comprehensive Study of Mechanical Properties.

Ultrasonic pulse velocity (UPV) and rebound hammer tests are accepted as alternatives to destructive testing to determine the compressive strength, dynamic modulus of elasticity, and Poisson’s ratio, which are needed for structural design. Although much work has been conducted for plain concrete, the research data for fibre-reinforced concrete (FRC) is insufficient. In this regard, this study explains the correlations between compressive strength, rebound hammer, and UPV tests for plain concrete and FRC contains 0.25%, 0.50%, and 1.00% of 30 mm and 50 mm long steel fibres. A total of 78 concrete cube and beam specimens were tested by direct, semi-direct, and indirect UPV and rebound hammer test methods. The study found that the rebound hammer test is more suitable for measuring the compressive strength of matured FRC than young concrete. The UPV test revealed that the volume fraction does not, but the length of steel fibres does affect the UPV results by the direct test method. The UPV direct method has the highest velocity, approximately two times the indirect velocity in FRC. UPV measurements can be effectively used to determine the dynamic modulus of elasticity and Poisson’s ratio of FRC. The dynamic elastic modulus increases while the Poisson’s ratio decreases for the same steel fibre length when at increasing FRC fibre content. The results of this study will be significant for non-destructive evaluations of FRC, while additional recommendations for future studies are presented at the end of the paper.

A multiscale model for optimizing the flexural capacity of FRC structural elements

In the present work, a multiscale model for fibre reinforced concrete (FRC) beams failing in bending is presented. At the microstructural level, the fibre is modelled as a one-dimensional continuum with axial, shear and bending deformability, with cohesive-like interfaces to simulate the interaction with the surrounding concrete. At the macroscopic level, the response of the beam is simulated by discretising the cross-section into layers and by enforcing the proper compatibility conditions between the layers. In the post-cracking stage, the tensile capacity is assured by the fracture energy of the concrete and the fibre resisting mechanisms simulated by the fibre pull-out constitutive laws determined at the microstructural level. The model can account for fibre distribution and orientation, controlled by the casting conditions and geometry of the mould. By using experimental data available from the open literature, it is proved that such an integrated approach is able to derive, by inverse analysis, the stress-crack width relationship of FRC, which is the fracture mode I information in the material nonlinear analysis of FRC structures with approaches based on the finite element method.

Constitutive model for fibre reinforced concrete by coupling the fibre and aggregate interlock resisting mechanisms

This paper describes the development of a constitutive model for Fibre Reinforced Concrete (FRC) by combining the fibre and aggregate interlock resisting mechanisms. The proposed model predicts the normal and shear stresses in function of the crack opening and sliding displacements. For this purpose, two well established aggregate interlock models and three fibre pull-out models reported in the literature are used, and their predictive performance is assessed in the context of the present study. In order to understand the impact of the variables involved in the model, a parametric study, targeting different maximum aggregate diameters, concrete compressive strengths and fibre volume ratios, is conducted. The results show that higher concrete compressive strengths and fibre volume ratios lead to higher normal and shear stresses. Finally, the predictive performance of the model is assessed by comparing the numerical predictions with experimental results collected from the literature. Results show that the model is capable of predicting the shear peak-stress with good accuracy, however the post-peak response is generally overestimated. A modification to the aggregate interlock models is proposed in order to adapt the original formulations for FRC. The results of the modified version show an overall much better agreement between the numerical and experimental results.

Design-oriented approach to determine FRC constitutive law parameters considering the size effect

Tensile strength constitutive laws for fibre reinforced concrete (FRC) are commonly defined through the parameters of flexural tests conducted on standard prismatic specimens. However, there are no specific criteria to determine such parameters using small specimens that could simplify the testing procedure and provide more representative results of slender structural FRC elements. In this line, the influence of size effect becomes an issue particularly relevant during the characterisation stage given that the residual strength decreases while increasing the size of the element. The objective of this document is to propose a methodology to obtain the parameters of the constitutive law using small specimens. For this, FRC residual strength was determined through three-point bending tests on prismatic notched beams of 40 × 40 × 160, 100 × 100 × 400 and 150 × 150 × 600 mm. An analytical model based on sectional analyses aimed at reproducing the flexural strength of FRC was used to assess the results of the alternative methodology to determine the parameters for the constitutive law. The results show that an approach based on the rotation instead of the crack opening as the reference parameter to estimate the stresses for the constitutive law leads to results less influenced by the size effect when designing small elements.

Failure mechanism of silica coated polypropylene fibres for Fibre Reinforced Concrete (FRC)

This work investigates the effect of a fast, acid-catalysed sol-gel silica nano-coating on the mechanical performance of draw-wire Polypropylene (PP) fibres used as dispersed reinforcement in Fibre Reinforced Concrete (FRC). The failure mechanism is investigated. To this aim, the role of curing time is also considered. Mechanical performance is assessed in pull-out and three-point bending tests of un-notched beams. Coating deeply affects the post-cracking behaviour of FRC, which shifts from brittle (plain concrete), to softening (uncoated) and finally to plastic-softening (coated fibres). Remarkably, 28-day curing improves over 8-day curing in terms of energy dissipation capability for coated fibres only. This suggests that fibre-to-matrix bond enhancement moves the failure mechanism from delamination at the interface to failure in the interphase zone. In the former case, failure is inconsistent and occurs independently from the curing time while in the latter failure depends on the matrix quality.

Flexural Modelling and Finite Element Analysis of FRC Beams Reinforced with PVA and Basalt Fibres and Their Validation

A flexural capacity model for fibre‐reinforced concrete (FRC) beams reinforced with PVA and basalt fibres is suggested for the rectangular beam sections. The proposed models are based on the concept of equivalent stress block parameters for both compressive and tensile stresses, similar to Eurocode and ACI code. The parameters are defined by allowing the conversion of the stress‐strain models into equivalent rectangular stress blocks, similar to Eurocode 2. The flexural model is suggested to determine the loading capacity of 21 FRC beams containing up to 3% volume fraction of PVA and basalt fibres without reinforcing bars. In order to investigate the accuracy of the proposed flexure models, finite element analysis (FEA) of the same beams was carried out using the compressive and tensile stress‐strain curves. Furthermore, 21 FRC beams subjected to three‐point bending were tested. The results of the flexural models showed good agreement with the load‐carrying capacity of the tested FRC beams, and the results of FEA of all beams showed a good correlation with the experimental results in terms of the maximum load, load versus midspan deflection patterns, and the maximum tensile strains.

Influence of fibre count on variability in post-crack performance of fibre reinforced concrete

High variability in experimental estimates of post-crack performance for fibre reinforced concrete (FRC) remains a problem that compromises progress toward a comprehensive understanding of this material and reduces the realisable performance of FRC in design. In recent years, the possible influence of many factors on variability in performance has been studied, and as a result, the number of fibres bridging and supporting tension across the crack plane has been recognised as contributing to variability in commonly used flexural performance tests. Unfortunately, no commonly used post-crack performance testing standard for FRC requires the number of fibres that protrude from the surface of crack faces to be counted. This leads to a paucity of data that obscures the significant role this parameter plays in performance variability, particularly when very high levels of performance variability are encountered. The present paper summarises the results of numerous investigations in which direct fibre counting has been conducted as part of FRC assessment, thereby providing a large body of data to facilitate an empirical means of quantifying the influence of fibre count on post-crack performance variability.

A plastic-damage constitutive model for the finite element analysis of fibre reinforced concrete

A unique constitutive model for fibre reinforced concrete (FRC) is presented, which combines a number of mechanics-based sub models for the simulation of directional cracking, rough crack contact and the crack-bridging action of fibres. The model also contains a plasticity component to simulate compressive behaviour. The plasticity component employs a frictional hardening/softening function which considers the variation of compressive strength and strain at peak stress with fibre content. Numerical results from a range of single-point and finite element simulations of experimental tests show that the model captures the characteristic behaviour of conventional fibre reinforced concrete with good accuracy.

Constitutive model for fibre reinforced concrete based on the Barcelona test

Several constitutive models for fibre reinforced concrete (FRC) have been reported in the past years based on the flexural performance obtained in a bending test. The Barcelona test was presented as an alternative to characterise the tensile properties of FRC; however, no constitutive model was derived from it. In this article, a formulation to predict the tensile behaviour of FRC is developed based on the results of the Barcelona test. The constitutive model proposed is validated by simulating the results of an experimental program involving different types of fibres and fibre contents by means of finite element software. Moreover, the simplified formulation proposed is compared with constitutive models from European codes and guidelines.

Application of FRC constitutive models to modelling of slabs

Fibre reinforcement technology has advanced significantly in recent years and, as a result, expanded into different applications. In particular, steel fibre-reinforced concrete has been successfully employed in flat slabs of several buildings in Europe with fibres as the only reinforcement. However, design methods for fibre-reinforced concrete (FRC) structures do not consider differences that may result from material characterisation tests that are not representative of the structural behaviour and fibre orientation of real-scale elements. In this regard, this paper presents a numerical study in which two constitutive models for FRC based on the bending test are applied to estimate the flexural behaviour of full-scale slabs of different dimensions. Likewise, a parametric study is conducted to analyse how the parameters of these constitutive models affect the flexural response of the slabs, and a numerical fit of the experimental data is performed. Finite element simulations using the constitutive models overestimate the experimental results. The parametric study also reveals that the parameter σ 2 (stress after cracking) has a particularly importance influence on the response. Furthermore, enhanced sectional behaviour of the slabs was observed as their width was increased.

Effect of nylon fibres on mechanical and thermal properties of hardened concrete for energy storage systems

The use of recycled materials in new concrete production is very appealing because of the low cost of the raw materials and the landfill space saving, as well as the enhancement of the resulting concrete’s properties. This paper focuses on the production of concrete for thermal energy storage systems using polyamide fibres from post-consumer textile carpet waste. We investigated the influence of a fibre content of 5kg/m3 on the mechanical and thermal properties of fresh and hardened concrete with a w/c ratio of 0.35, comparing it with plain concrete. The nylon fibre-reinforced concrete was slightly more ductile and tougher than the plain concrete because the fibres have an important role after failure. The tensile strength, maximum load-bearing capacity and modulus of elasticity decreased slightly with the addition of nylon fibres, but drying shrinkage was lower than for plain concrete. Given that heat capacity and thermal conductivity are the most important parameters for thermal energy storage purposes, we measured these parameters for all the samples at 25°C, and after a thermal treatment at 300°C. The heat capacity values were 0.63 and 0.81Jg−1K−1 for fibre-reinforced concrete (FC) and plain concrete (C), respectively, and the thermal conductivity values were 1.16 and 1.02Wm−1 K-1.

Performance evaluation of a polypropylene-based draw-wired fibre for concrete structures

A polypropylene-based draw-wired fibre suitable for fibre reinforced concrete (FRC) is studied and its effect on flexural behaviour, toughness and shrinkage cracking is shown. In order to assess the performance offered by such a fibre over commercially available synthetic and metallic ones, comparative tests are carried out with respect to standard four-point bending and constrained shrinkage behaviour. The experimental evidence shows that FRC reinforced with these fibres possesses interesting structural features; in particular, fibres significantly improve crack resistance while enhancing toughness and durability of FRC elements. Equally important, such behaviour is gained without significantly affecting the workability of the mixture.

Properties of fibre reinforced concrete using recycled fibres from carpet industrial waste

A study was carried out to evaluate the use of recycled fibres from carpet industrial waste for reinforcement of concrete at 1 and 2 vol% fractions. Compressive, flexural, splitting tensile and shrinkage tests were performed. Significant increases in shatter resistance, energy absorption and ductility were observed. This paper reports on the experimental programme and compares the effectiveness of such recycled fibres with that of virgin polypropylene fibres specially made for fibre reinforced concrete (FRC). The paper also discusses the benefits of using such FRC for construction applications and possible ways to further enhance the performance of such FRC.