Steel-fibre-reinforcement and hydration coupled effects on concrete tensile behaviour

This paper presents the effect of hydration development on the tensile behaviour of steel-fibre-reinforced concrete. Tensile tests were performed on plain and fibre-reinforced concretes at 2, 7 and 28 days in order to determine the response of the composites, in particular to establish the post-peak behaviour and the evolution of the residual post-peak strength with hydration development and the associated improvement in the fibre–matrix bond.From the laws governing the tensile behaviour of plain concrete on the one hand and the residual strength capacity due to fibre reinforcement on the other, parameters fitting an analytical model of the uniaxial tensile response of fibre-reinforced concrete were determined. The proposed model takes into account the tensile damage to the concrete and the development of hydration. An original aspect of the model is that it also integrates the damage to the fibre–matrix bond.     

Energy dissipation capacity of fibre reinforced concrete under biaxial tension–compression load. Part I: Test equipment and work of fracture

The objective of this research was to analyse the differences in the dissipated energy under uniaxial tension and biaxial tension–compression load of fibre reinforced concretes using the Wedge Splitting Test. Under biaxial load the specimens were subjected to compressive stress ratios from 10% to 50% of the concrete compressive strength perpendicular to the direction of the tensile load.Under biaxial tension–compression load the energy dissipation capacity of the specimens decreases compared to the uniaxial tension load case on average 20–30%. It is believed that the decrease is a result of the damage mechanism of the concrete matrix and deterioration of the fibre–matrix and/or aggregate–cement paste interfaces in case the section is additionally loaded with compression stresses. This indicates that dimensioning of concrete elements under biaxial stress states using material parameters obtained from tests conducted on specimens under uniaxial tensile load is unsafe and could potentially lead to a non-conservative design.In the second part of this paper the extent of the fracture process zone under uniaxial tension and biaxial tension–compression load will be examined with the Acoustic Emission technique and the reasons for decrease of the energy dissipation capacity under biaxial load will be further discussed.     

Tensile strength of axially loaded unidirectional Nextel 610™ reinforced aluminium: A case study in local load sharing between randomly distributed fibres

The tensile failure of unidirectional alumina fibre reinforced aluminium is studied in uniaxial loading along the fibre axis. The tensile strength is measured as a function of matrix yield strength, which is varied by varying the testing temperature, from RT to 600 °C. Over the range of matrix yield strength (i.e., of temperature) examined, the fracture mode remains brittle. Batdorf’s (J Reinforced Plastics Compos 1982;1:153-164) simple ideal local load-sharing model describes well the observed behaviour, under the condition that it be adapted to account for the actual number of nearest neighbours characteristic of the fibre distribution in the composite. This is shown to be close to three, i.e., at variance with the usually assumed idealized hexagonal or square fibre arrangement patterns.     

Factors contributing to early age shrinkage cracking of slag concretes subjected to 7-days moist curing

The objective of this study was to investigate all the factors contributing to early age shrinkage cracking in concrete, namely, shrinkage, tensile creep, tensile elastic modulus, tensile strength of concretes, and to study the effect of slag as a binder on these factors. The above-mentioned factors were measured in early age concretes made with 0, 35, 50 and 65% level replacement of ordinary Portland cement by slag. All the concretes studied were moist cured for 7-days. It was found that, at lower slag replacement levels (0, 35 and 50%), the tensile strength decreased with increasing slag replacement. However, this is more than compensated by decreasing tensile elastic modulus and shrinkage. There was no significant change found in tensile creep with the changing slag levels. The study shows that the influence of the tensile elastic modulus is a major consideration for early age cracking of slag concretes.     

Mechanical properties of high strength concrete reinforced with metallic and non-metallic fibres

This paper focuses on the experimental investigation carried out on high strength concrete reinforced with hybrid fibres (combination of hooked steel and a non-metallic fibre) up to a volume fraction of 0.5%. The mechanical properties, namely, compressive strength, split tensile strength, flexural strength and flexural toughness were studied for concrete prepared using different hybrid fibre combinations – steel–polypropylene, steel–polyester and steel–glass. The flexural properties were studied using four point bending tests on beam specimens as per Japanese Concrete Institute (JCI) recommendations. Fibre addition was seen to enhance the pre-peak as well as post-peak region of the load–deflection curve, causing an increase in flexural strength and toughness, respectively. Addition of steel fibres generally contributed towards the energy absorbing mechanism (bridging action) whereas, the non-metallic fibres resulted in delaying the formation of micro-cracks. Compared to other hybrid fibre reinforced concretes, the flexural toughness of steel–polypropylene hybrid fibre concretes was comparable to steel fibre concrete. Increased fibre availability in the hybrid fibre systems (due to the lower densities of non-metallic fibres), in addition to the ability of non-metallic fibres to bridge smaller micro cracks, are suggested as the reasons for the enhancement in mechanical properties.     

BEHAVIOUR OF FIBRE-REINFORCED CONCRETES WITH REFERENCE TO FRACTURE RESISTANCE

This paper examines the effect of beam size, fibre volume fraction and various fibres on the flexural behaviour of concretes, and their fracture resistance. The ratio of flexural strength to tensile strength is used as a measure of brittleness; a low value indicating a more brittle material. Two flexural toughness indices are used which provide a basis for analysing load–deflection curves. The relation between these structural indices and the material fracture resistance is assessed by adopting parameters which involve flexural and tensile strengths alone and fibre length to reflect fracture resistance.     

Ageing effect on tensile and shrinkage behaviour of new green hybrid fibre-reinforced cementitious composites

Ageing effects on both uniaxial tensile and shrinkage behaviour of new green cementitious composites reinforced with bagasse fibre and steel fibre with ultra-high volume of fly ash are investigated in this paper. The tensile behaviour of the composites is investigated at the age of 28 days, 3 months, 6 months and 10 months after curing in weather conditions. Restrained shrinkage behaviour of the composites after curing for 3 months is tested on ring specimens. SEM tests are also conducted to study the influence of the ageing on the microstructure of the new composites. The test results show that the tensile strength of the composites at all ages increases with the decrease of the content of the fly ash and the bagasse fibre, and that the increasing application of fly ash and bagasse fibre decreases the steel ring strain and restrains the development of crack. It is also found that the tensile strength and the shrinkage of the composites such as crack width ascend greatly with time.     

Experimental investigation on uniaxial tensile strength of hybrid fibre concrete

In an experimental study on fracture properties of hybrid fibre concrete, specimens with varying fibre content (mixtures of short and long fibres) were loaded in uniaxial tension. Dog-bone shaped specimens of four different sizes in a size range of 1:8 were tested. Focus of the study was the determination of the size effect on nominal strength and fracture processes. A vacuum impregnation technique was used to investigate the fracture process. Experiments showed that multiple cracks, which formed before the peak, localised into one major crack beyond peak. Multiple cracking could be obtained by increasing the amount of thin short fibres whereas the large fibres can enhance the bridging of localised macrocracks. With decreasing strength, the size effect on the strength appears to increase. It is observed that the size effect on nominal tensile strength decreases with increasing material ductility. Preliminary analysis of the results showed that the observed size effect can be considered as a combination of statistical and structural size effects.     

Enhanced fracture strain of polypropylene by incorporation of thermoplastic fibres

The incorporation of continuous, uniaxial oriented, nylon and polyethylene terephthalate fibres into isotactic polypropylene leads to the expected increase in tensile strength as predicted by the rule of mixtures. The elongation to break, however, is found to be significantly in excess of those of the fibres and the matrix and increases with fibre volume fraction. The increase in ductility of the composites is attributed to decrease in spherulite size of the polypropylene and to restrained necking of the fibres due to transcrystalline growth at the fibre surface.     

Assessing the effect of fibre extraction processes on the strength of flax fibre reinforcement

A number of factors impede the direct translation of fibre properties from plant crop species to natural fibre composites. Commercially available fibre extraction processes introduce defects and degrade the mechanical properties of fibres. This study reports on a novel image based approach for investigating the effect of fibre extraction processes on flax fibre bundle strength. X-ray micro Computed Tomography (μCT) was coupled with uniaxial tensile testing to measure the in-situ fibre bundle cross-section area and tensile strength in flax plant stems. The mean tensile strength result was 50% higher than that of the fibres extracted through the standard commercial process. To minimize fibre damage during fibre extraction, a pre-treatment was proposed via saturating flax plant stems in 35% aqueous ammonia solution. By environmental scanning electron microscopy (ESEM), it was evident that ammonia treatment significantly reduced the extent of damage in flax fibre knots and the optimum treatment parameter was identified.     

Experiments on concrete under uniaxial impact tensile loading

A problem of practical importance for designing of structural elements is discussed in this paper—the behaviour of concrete subjected to uniaxial impact tensile loading. The “Split Hopkinson Bar” technique was adopted for testing concrete in uniaxial tension at stress rates between 2 and 60 N/mm²/ms. A remarkable increase in tensile strength was observed due to high stress rate. The ratio of impact and static tensile strength varied between 1.33 and 2.34 for various concrete mixes. The influence of maximum aggregate size, water-cement ratio, cement content, cement type and quality, specimen humidity, static compressive strength and loading/casting direction upon the uniaxial impact tensile strength was studied. The high stress rate resulted in an increase of the modulus of elasticity of concrete in uniaxial tension. An explanation for the observed phenomena is suggested.     

虚拟裂缝尖端拉应力大小的确定

针对单边切口的混凝土轴心受拉构件,基于虚拟裂缝模型提出一种计算极限承载力的解析模型,并在此基础上确立了虚拟裂缝尖端拉应力与混凝土轴心抗拉强度之间的关系。结果表明:二者的比值随初始缝高比的增大呈线性增加,但对混凝土强度等级的变化不敏感。其原因是由于所有的混凝土试件都存在初始缺陷,导致截面上存在明显的应力梯度,因而得到的混凝土轴心抗拉强度值是截面应力的平均值,而虚拟裂缝尖端拉应力为截面上的最大应力。很显然,轴心受拉构件的初始缺陷越长,截面的应力梯度越大,虚拟裂缝尖端拉应力与平均应力的比就越大。通常情况下,虚拟裂缝尖端的拉应力大小约为混凝土轴心抗拉强度值的1.22倍,约等于混凝土的抗折强度。     

基于水泥水化度的混凝土早期力学性能发展预测

实验测量了3个强度等级混凝土(28d抗压强度分别为30MPa,50MPa和80MPa左右)在不同养护条件下典型龄期的P-CMOD曲线,借助断裂力学方法,获得了相应的混凝土开裂强度、抗拉强度和抗弯强度及其随龄期的发展规律,分析了养护条件对混凝土力学性能发展的影响。考虑混凝土内部湿度对水泥水化度的影响,对干燥环境下水泥水化度进行了修正,建立了基于水泥水化度的混凝土开裂强度,抗拉强度和抗弯强度的预测模型。     

Mechanical properties of sisal fibre at elevated temperatures

Sisal fibres extracted from the leaves of Agava sisalana plants 3, 5, 7 and 9 years old were tested at different temperatures for tensile strength, elongation, toughness and modulus. The tensile strength, modulus and toughness values of sisal fibre decreased with increase in temperature. The effect of plant age on tensile strength, tensile modulus and toughness of sisal fibre became very much less at 100 °C as compared to 30 °C. Fractured fibres were observed by using a scanning electron microscope. The ends of fibres fractured at elevated temperature showed a failure similar to that of inorganic fibres. Elongation values at all temperatures increased with age. Elongated capillaries were observed in fibres fractured at 80 and 100 °C, due to the removal of moisture and volatiles originally present in the fibres. The fibrils are clearly observed in the form of hollow cylinders. Fractured surfaces are composed of brittle as well as ductile phases. The ductile portion increased with the increase of temperature.     

Mechanical properties of randomly oriented short Sansevieria cylindrica fibre/polyester composites

The tensile, flexural and impact properties of randomly oriented short Sansevieria cylindrica fibre/polyester (SCFP) composites are described for the first time in this work. Composites were fabricated using raw S. cylindrica fibres (SCFs) with varying fibre lengths and weight percents of fibre. When the length of the SCFs was increased, the tensile, flexural and impact properties of the composite were increased up to a 30-mm fibre length, and then a curtailment in properties occurred for higher fibre length composites. SCFP composites showed a regular trend of an increase in properties with fibre weight percent until 40% and afterwards a decrease in properties for composites with greater fibre weight percent. Tensile tests revealed that the tensile strength was about 76 MPa, the Young’s modulus was 1.1 GPa and the elongation at break was between 7% and 8.3%. The flexural strength and modulus were estimated to be around 84 MPa and 3 GPa, respectively. Impact tests exhibited a strength of approximately 9.5 J/cm². The analysis of the tensile, flexural and impact properties of short SCFP composites displayed a critical fibre length and optimum fibre weight percent of 30 mm and 40%, respectively. Scanning electron microscope (SEM) studies were carried out to evaluate the fibre/matrix interactions. The experimental tensile strengths were compared with the theoretical predictions and found to be in good agreement with Hirsch’s model. An X-ray diffraction (XRD) analysis of the composites exposed the presence of cellulose IV with a crystallinity index of 60% and crystallite size of 68 nm.     

骨料级配对二维模型混凝土单轴抗拉强度影响的理论研究

混凝土尺寸效应及其宏观力学非线性根源于其材料细观组成的非均质性。结合混凝土细观结构形式,将混凝土看作由骨料颗粒、砂浆基质及界面过渡区组成的复合材料。采用双线性弹性损伤模型来描述砂浆基质及界面过渡区的力学行为,假定骨料颗粒为弹性体而不发生破坏,进而推导并获得了单轴拉伸条件下不同骨料颗粒级配混凝土断裂裂缝扩展路径长度及其抗拉强度的理论解。最后,对比了建立的理论公式结果与细观尺度数值模拟结果,验证了构建的关于裂缝长度及抗拉强度理论解的准确性和合理性。     

Shrinkage-cracking behavior of OPC-fiber concrete at early-age

The current paper presents the results of early-age restrained shrinkage (RS) tests on Ordinary Portland Cement (OPC) concretes incorporating admixed polypropylene fiber (PP). Four concrete mixtures made with OPC containing various volume fractions of PP fiber were tested. Two identical specimens of each mixture were tested: one subjected to fully restrained conditions and the other allowed to shrink freely, both under the drying conditions of 23°C and 50% relative humidity at the age of 24 h. Direct and indirect tensile tests were also performed in the same concretes to monitor the tensile strength development. With increasing fiber contents in mixture, the tensile strength, creep and elastic modulus characteristics have not significantly changed during the first week of age. Increasing the volume fractions of PP fiber significantly delayed the time of cracking owing to the delayed onset of RS, which is beneficial to crack resistance.     

On the estimation of the tensile strength of carbon fibres at short lengths

Generally, to determine the fibre-matrix interfacial properties in fibre reinforced plastics, it is necessary to know the tensile strength of the fibre at very short lengths, for which direct measurements are not possible. Accordingly, in this study, the determination of the tensile strength of high strength carbon fibres and their gauge length dependence are analysed by means of the Weibull model. The influence of the estimator chosen and of the sample size on the calculated value of the tensile strength of the fibre are first determined. Secondly, the accuracy of the three- and the two-parameter Weibull distributions is examined. Finally, it is shown that the most appropriate extrapolation at short length is performed by means of a linear logarithmic dependence on gauge length of the tensile strength. This method seems to be valid for untreated as well as for surface-treated high strength carbon fibres.     

Tensile fatigue behaviour of ultra-high performance fibre reinforced concrete (UHPFRC)

The tensile fatigue behaviour of ultra-high performance fibre reinforced concrete (UHPFRC) under constant amplitude fatigue cycles is presented. Three series of uniaxial tensile fatigue tests up to a maximum of 10 million cycles were conducted with the objective to determine the endurance limit of UHPFRC that was supposed to exist for this material. The fatigue tests reveal that an endurance limit exists in all three domains of UHPFRC tensile behaviour at S-ratios ranging from 0.70 to 0.45 with S being the ratio of the maximum fatigue stress to the elastic limit strength of UHPFRC. Rather large variation in local specimen deformations indicates significant stress and deformation redistribution capacity of the UHPFRC bulk material enhancing the fatigue behaviour. The fatigue fracture surface of UHPFRC shows features of the fatigue fracture surfaces of steel, i.e. fatigue crack propagation is identified by a smooth surface while final fracture leads to rather rough surface. Various fatigue damaging mechanisms due to fretting and grinding as well as tribocorrosion are identified.     

Mechanical behaviour of circular and triangular glass fibres and their composites

Single fibre testing of circular (CircGF) and triangular (TriGF) glass fibres of equivalent cross-section has shown the TriGF to have a 25% higher average tensile strength compared to CircGF. Micro-composite compression testing (using resin bonded tows of 12–15 filaments) has revealed the TriGF to have a compression strength 60% greater than CircGF. Some of the increase can be attributed to an effective increase in second moment of area for the TriGF specimens due to imperfect packing. However, allowing for this effect there still appears to be an underlying significant improvement in compressive strength performance attributable to the inherent fibre shape. Mechanical testing under tensile load has shown that the triangular glass fibre reinforced plastic (TriGFRP) performs marginally better (20%) than that manufactured using circular fibre (CircGFRP) for equivalent fibre volume fractions. Similarly, under compressive loading the TriGFRP outperforms CircGFRP by a significant margin of 40%. Interlaminar shear testing has also indicated that TriGFRP may offer a performance advantage of approximately 5%, although this needs further verification to be conclusive.