Mode I fracture toughness of fibre reinforced concrete Carpinteri, Andrea; Fortese, Giovanni; Ronchei, Camilla ...
Theoretical and applied fracture mechanics,
October 2017, 2017-10-00, 20171001, Volume:
91
Journal Article
Peer reviewed
Open access
•The fracture behaviour of fibre reinforced concrete (FRC) specimens is examined.•A new method to compute the fracture toughness for quasi-brittle material is employed.•The new method can take into ...account the possible crack deflection (kinked crack).•Fracture toughness is improved by adding fibres and increasing the fibre volume fraction.
Low fracture toughness of concrete represents a serious shortcoming. An effective way to improve the concrete toughness is represented by the dispersion (during mixing) of discontinuous fibres into the concrete mix. The principal beneficial effect of fibres is the crack bridging in the cementitious matrix, providing resistance to crack propagation before fibre debonding and/or pulling out or failure. In the present paper, the fracture behaviour of FRC (fibre reinforced concrete) specimens is examined, with micro-synthetic polypropylene fibrillated fibres being randomly distributed in concrete. The modified two-parameter model, proposed by the authors to calculate Mode I plain-strain fracture toughness for quasi-brittle material, is able to take into account the possible crack deflection (kinked crack) during stable crack propagation.
•The effect of global stiffness of beam on impact force profile is revealed.•A 2DOF analytical model is developed for RC beams under drop weight impact.•Stress wave propagation is considered in the ...analytical model.•A simplified impact force profile model is proposed for RC beam under impact.
Impact force and structural response of reinforced concrete (RC) beams under drop weight impact have been intensively studied and reported in the literature. The prediction of the peak impact force has been well investigated while the study of the overall impact force profile governing the structural response is limited. The impact force parameters acting on a structure including the peak impact force, plateau force, and impact duration all affect the structural responses. This study investigates the impact force profiles from drop weight impact on RC beams. The effects of global stiffness (by changing the beam span and boundary condition) on the impact force profile are numerically studied by using LS-DYNA. A two-degree-of-freedom (2DOF) analytical model is developed for RC beams under drop weight impact by using fiber beam section analysis method to predict the impact force. The developed 2DOF analytical model is validated by drop weight impact test results. With the validated analytical model, a simplified model of impact force profile is then proposed by determining six characteristic points, which are expressed by empirical equations fitted by the analytical parametric study results. The simplified impact force profile predicted by the empirical equations shows a good agreement with the test data.
•New shear test method for RC beams.•Effect of span-to-depth ratio on shear strength components.•Variation of shear strength components.•Evaluation of existing shear strength models.
The additive ...model of shear strength of reinforced concrete (RC) members, i.e. shear strength (V) is equal to the sum of the contribution from concrete (Vc) and that from transverse reinforcement (Vs), has been widely accepted in the literature and engineering practice. Shear span-to-depth ratio (a/d) is known to be a significant factor affecting V of RC members. However, very few quantitative studies on the influence of a/d on Vc and Vs have been reported in the literature. Another issue is related to the controversial relationship between the shear force at first diagonal cracking (Vcr) and Vc, for which different guidelines are given in ACI, AASHTO LRFD and CSA codes. Through direct measurement of Vc and Vs from 11 RC beam tests, this work provides experimental evidence for these issues. The experimental results show that Vc can be very different from Vcr. At small shear span-to-depth ratio (a/d), Vc is much larger than Vcr, while it is the opposite for beams with a large a/d value. Not all stirrups crossing the critical shear crack yield at ultimate shear strength, and Vc as well as Vs are not constant under increasing member deformation. Although design codes give a conservative prediction of V, they predict an un-conservative value of Vc at large a/d.
Despite intensive research on material properties of non-metallic technical textiles for internal reinforcement in concrete, the long-term durability is not yet fully understood. In this work, ...results of preloaded long-term durability tensile tests on carbon-reinforced concrete specimens under environmental factors of stress, temperature, moisture and alkalinity are presented. Based on investigations of non-metallic glass fiber reinforcements with polymer matrices, where strength losses occur over time, it was planned to derive a time to failure curve and to determine a reduction factor for the tensile strength of the carbon textile reinforcement. However, no loss of strength was discovered in residual capacity tests due to the high material resistance and therefore no reduction factor due to the environmental factors could be derived. After more than 5000 h of testing, the residual capacity tests showed an increase in the ultimate failure stress in comparison with the short-term tests. In addition to the long term-durability tests, the influence of the preloading was investigated. The preload was applied to the long-term tests and led to a straighter alignment and loading of the filaments and thus to an increase in the ultimate capacity.
•Applicability of damage plasticity model to simulation UHP-FRC is investigated.•Strain rate effects on UHP-FRC are experimentally determined.•Significance of different UHP-FRC properties on the ...numerical results is addressed.
This paper presents a numerical investigation on assessing whether the concrete damage plasticity (CDP) constitutive model can be used to simulate new ultra-high performance fibre reinforced concrete (UHP-FRC) material under impact loading rates at different damage stages. The performance of the numerical models is verified by comparing numerical results to the experimental data that were previously tested by the authors. This paper also presents experimental tests that aimed to characterize the strain rate effect on UHP-FRC. The numerical simulations have been performed using ABAQUS/Explicit. CDP parameters are identified based on impact test results of a control specimen. Subsequently, the predictive capability of calibrated model has been investigated by simulating two UHP-FRC plates with varied steel reinforcement ratios tested under repeated drop-weight impact loads.
It has been found that compressive and tensile strength enhancement predicted using CEB-FIP Model Code (1990) fits well with test results of the strain rate effect on UHP-FRC material. The numerical results demonstrate the feasibility of the CDP constitutive model for analyzing UHP-FRC under dynamic loading rates. Computed responses are sensitive to CDP parameters related to the tension, fracture energy, and plastic volumetric change. The effect of tensile strain hardening response could be ignored in the nonlinear finite element (FE) analysis of UHP-FRC materials with low strain-hardening behaviour.
The flexural behaviour of steel fibre-reinforced concrete beams is discussed in the framework of Fracture Mechanics. The Bridged Crack Model is updated by means of a cohesive softening constitutive ...law of the reinforcement layers, taking into account the fibre slippage within the cementitious matrix. In this way, the model is able to describe the global response of the composite, including the local instability phenomena characterizing the different post-cracking regimes. The structural response is found to be governed by two dimensionless numbers: the reinforcement brittleness number, NP, which defines the minimum reinforcement condition of the composite beam, and the pull-out brittleness number, Nw, which describes its plastic rotation capacity. The former, NP = (Vfσ¯sh1/2)/KIC, depends on the fibre volume fraction, Vf, on the fibre slippage strength, σ¯s, on the concrete fracture toughness, KIC, and on the beam depth, h. The latter, Nw = (wcE)/(KICh1/2), depends on the fibre embedment length, wc, on the concrete Young’s Modulus, E, on the concrete fracture toughness, KIC, and on the beam depth, h. These two parameters make it possible to identify a softening post-cracking response of the fibre-reinforced concrete element that is scale-dependent. Finally, the numerical analyses are compared to several experimental results reported in the scientific literature, proving the effectiveness of the described model.
•A new direct tension test method using side-glued steel plates is developed.•The new test method is free of end effects due to stress concentration.•Method used to test FRC specimens made with 4 ...different types of steel fibres.•Effects of steel fibres on cracking strengths and strain capacities studied.•Fibre factor for strain hardening and formulas for strain capacities obtained.
A new direct tension test method of applying tension to a dumbbell-shaped specimen through side-glued steel plates at the end blocks for measuring the tensile stress-strain characteristics of fibre reinforced concrete (FRC) was developed. Finite element analysis of the test setup revealed substantially reduced stress concentration in the specimen. The new method was applied to test FRC specimens made with four different types of hooked steel fibres. It was found that both the first cracking strength and post cracking strength increased with the fibre volume but at different rates for the different types of steel fibres. From the stress-strain curve, two different values of tensile strain capacity, one defined as the tensile strain at maximum tensile stress and the other defined as the tensile strain at which the tensile stress dropped to 80% of cracking stress, were obtained. Regression analysis of the test results yielded good correlations to the fibre factor and empirical formulas for estimating the minimum fibre factor for strain hardening and the tensile strain capacities.
•A HPFRCC material is proposed to repair RC columns damaged in earthquake.•Cyclic responses of RC columns are evaluated before and after repair.•Effects of repair height are investigated.•Effects of ...axial load during repairing are evaluated.•Repair with the HPFRCC improves the cyclic performance of RC columns.
A high-performance fiber-reinforced cementitious composite (HPFRCC) prepared with high-volume fly ash is proposed to repair damaged reinforced concrete (RC) columns. This study aims at developing an effective and easy-to-apply repairing technique for RC columns damaged in earthquake. Four columns with 200mm×200mm cross section and 900mm height were prepared and tested to 85% of the load-carrying capacity under amplitude-increasing lateral loads and a constant axial load. The damaged columns were repaired using the HPFRCC: two repair heights (300 and 500mm) and two repairing processes (with and without axial loads). The effectiveness of the repairing schemes was evaluated by comparing load-carrying capacities, displacement ductility, stiffness, and energy dissipation of the columns. The results indicated that the load-carrying capacity and ductility of the repaired columns could be respectively 14% and 29% higher than those of the original columns. With axial loads during repairing, the repaired columns displayed better cyclic performance. Increasing the repair height beyond the plastic hinge zone slightly improved the load-carrying capacity and ductility. Considering the performance-to-cost ratio, it is recommended that the repair height of HPFRCC be 1.5 times the depth or width of the damaged column.
•Tests on six FRC panels subjected to in plane pure shear monotonic stress conditions.•Evaluation of influence of fibers in lightly reinforced panels.•Design of a test rig specifically developed to ...perform pure shear tests under displacement control.•Proposal of a new analytical formulation for the prediction of crack spacing in FRC panels under pure shear.
The load-deformation response of Fiber Reinforced Concrete (FRC) elements subjected to pure shear is still matter of strong debate within the scientific community. In this paper, the tests on six fiber reinforced concrete panels under pure shear are presented and discussed. The tests were conducted under displacement control and a peculiar loading frame was designed to ensure that a pure shear state of stress was established. Steel fibers were added in relatively low amounts (20 and 50 kg/m3), and two steel reinforcements (0.21% and 0.74%) were selected, aiming at simulating lightly reinforced elements. A critical discussion on the influence of fibers on both global and local behavior (tension stiffening, cracking formation and propagation, post-cracking stiffness and residual strength) is presented. Finally, a novel crack spacing formulation, extended to FRC, is proposed and compared against available experimental data.
Recent developments in the area of ultra-high-performance steel fiber reinforced concrete (UHP-SFRC) has enabled a reduction in the use of steel reinforcement and has led to enhanced ductility and ...toughness of structural components owing to its resilient tensile behaviour. This paper presents the results of an experimental study that was conducted to investigate the tensile behaviour of UHP-SFRC. Four commercial mixes and two in-house mixes were evaluated using the procedures prescribed in the 2019 edition of Annex U of CSA-A23.1 and in Annex 8.1 of CSA-S6 2018. The tensile strength of UHP-SFRC was quantified and correlated through the direct tension test, splitting test, inverse analysis of four-point bending tests using either code expressions or nonlinear finite element analysis, and a calibrated empirical expression that links this property to the cylinder compressive strength. In addition, the effect of important parameters on flexural strength including casting methodology, volumetric ratio of steel fibers, and aspect ratio (shear span to depth ratio) of bending prisms have been assessed. The casting methodology affected the fiber distribution, an attribute that directly relates to flexural strength. Prisms containing 1% steel fibers by unit volume failed in a relatively brittle manner and possessed less flexural strength than the prisms containing 2% steel fibers. Prisms with an aspect ratio of 1 tended to develop greater flexural strength than the prisms tested with an aspect ratio of 2. The tensile strength obtained from inverse analysis is generally greater than the strength obtained from the direct tension test, the finite element analysis, and the calibrated empirical expression. Furthermore, it was determined that tensile strength obtained from the splitting test should be multiplied by a correction factor of approximately equal to1/pi, to match the strength obtained from the direct tension test. The majority of the mixes tested exhibited a tension hardening behaviour with a hardening ratio greater than 1.1 and an ultimate tensile strain greater than 0.001.
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