Variation of strength and stiffness of fibre reinforced polymer reinforcing bars with temperature

This paper presents the results of tensile mechanical properties of FRP reinforcement bars, used as internal reinforcement in concrete structures, at elevated temperatures. Detailed experimental studies were conducted to determine the strength and stiffness properties of FRP bars at elevated temperatures. Two types of FRP bars namely: carbon fibre reinforced polyester bars of 9.5 mm diameter and glass fibre reinforced polyester bars of 9.5 mm and 12.7 mm diameter were considered. For comparison, conventional steel reinforcement bars of 10 mm and 15 mm diameter were also tested. Data from the experiments was used to illustrate the comparative variation of tensile strength and stiffness of different types of FRP reinforcing bars with traditional steel reinforcing bars. Also, results from the strength tests were used to show that temperatures of about 325 °C and 250 °C appear to be critical (in terms of strength) for GFRP and CFRP reinforcing bars, respectively. A case study is presented to illustrate the application of critical temperatures for evaluating the fire performance of FRP-reinforced concrete slabs.     

Shear-stress transfer across a crack in steel fibre-reinforced concrete

The ability of cracked reinforced concrete to transfer shear stresses is of major importance for concrete members designed to sustain high shear forces. Thereby, the maximum shear capacity is mainly affected by the aggregate interlock mechanism, the dowel action of longitudinal reinforcement, the restraining action of stressed reinforcement crossing the crack interface and the possible presence of stirrups. In case of steel fibre reinforced concrete (SFRC), where fibres are used to replace either completely or partially traditional stirrups, research has proven that the direct shear transfer capacity of cracked concrete is increased significantly by using fibres. By means of 69 direct shear tests, the shear-friction behaviour of SFRC with or without confining pressure has been studied further and existing empirical formulations have been checked. Since these models only provide a maximum shear strength, a more fundamental approach to model the direct shear behaviour of cracked SFRC is proposed in this paper. This model deals with the fibre-matrix interaction by means of fibre pull-out and aggregate interlock, as a function of the shear crack opening behaviour (i.e. combined opening and slipping).     

Strength and fracture properties of industrially prepared steel fibre reinforced concrete

The paper reports on a study of steel fibre reinforced concrete (SFRC) which was prepared using normal industrial mixing, compaction and curing conditions. Both strength (compressive and tensile) and fracture (toughness measurements) characteristics have been investigated with test specimens prepared from 5 m long SFRC piles. The piles contained only steel fibre reinforcement and were manufactured in exactly the same way as ordinary piles.Slight differences in the tensile strengths (determined via torsion tests) were observed due to the existence of preferential fibre orientation. Flexural tests on notched beams (to evaluate fracture characteristics) produced a much more stable, reproducible, test than that observed for un-notched beams. Hence, it is concluded that the notched beam is a better geometry in terms of test stability and reliability. The results showed that tests specimens taken from industrially prepared SFRC displayed similar characteristics compared to that observed with test specimens prepared under laboratory conditions, with regards to the strength, fracture characteristics and, in particular, the variation observed.     

Design of SFRC structural elements: post-cracking tensile strength measurement

Utilisation of steel fibre reinforced concrete (SFRC) for designing structural members requires knowledge of the post-cracking tensile response. This paper reviews the experimental characterisation tests and subsequent analysis commonly used for determining the post-cracking tensile properties of SFRC. The experimental program supporting this investigation comprised five different SFRC mixes with fibre volumes ranging from 0.75 to 1.25% used to fabricate a set of characterisation specimens for uniaxial tension tests, notched beam tests and round panel tests carried out in parallel with an extensive experimental program on large scale beams. Characterisation test results allowed a comparison between direct stress–crack opening measurements and the stress–crack openings retrieved from the inverse analysis of bending tests. Discrepancies in post-cracking tensile results obtained with the three types of tests are analyzed and related mainly to test configurations, the presence of a predefined crack, support conditions, fibre orientation, and cracked surface size. Results obtained using material characterisations are then applied to the reproduction of the structural behaviour of large scale beams, documented in a companion paper.     

Assessment of the effectiveness of steel fibre reinforcement for the punching resistance of flat slabs by experimental research and design approach

The present paper deals with the experimental assessment of the effectiveness of steel fibre reinforcement in terms of punching resistance of centrically loaded flat slabs, and to the development of an analytical model capable of predicting the punching behaviour of this type of structures. For this purpose, eight slabs of 2550 × 2550 × 150 mm³ dimensions were tested up to failure, by investigating the influence of the content of steel fibres (0, 60, 75 and 90 kg/m³) and concrete strength class (50 and 70 MPa). Two reference slabs without fibre reinforcement, one for each concrete strength class, and one slab for each fibre content and each strength class compose the experimental program. All slabs were flexurally reinforced with a grid of ribbed steel bars in a percentage to assure punching failure mode for the reference slabs. Hooked ends steel fibres provided the unique shear reinforcement. The results have revealed that steel fibres are very effective in converting brittle punching failure into ductile flexural failure, by increasing both the ultimate load and deflection, as long as adequate fibre reinforcement is assured. An analytical model was developed based on the most recent concepts proposed by the fib Mode Code 2010 for predicting the punching resistance of flat slabs and for the characterization of the behaviour of fibre reinforced concrete. The most refined version of this model was capable of predicting the punching resistance of the tested slabs with excellent accuracy and coefficient of variation of about 5%.     

An experimental study on the tensile strength of steel fiber reinforced concrete

This paper deals with steel fiber reinforced concrete mechanical static behaviour and with its classification with respect to fibers content and mix-design variations. A number of experimental tests were conducted to investigate uniaxial compressive strength and tensile strength. Different mixtures were prepared varying both mix-design and fiber length. Fibers content in volume was of 1% and 2%. Mechanical characterization was performed by means of uniaxial compression tests with the aim of deriving the ultimate compressive strength of fiber concrete. Four-point bending tests on notched specimens were carried out to derive the first crack strength and the ductility indexes. The tensile strength of steel fiber reinforced concrete (SFRC) was obtained both from an experimental procedure and by using an analytical modelling. The experimental tests showed the different behaviour of SFRC with respect of the different fiber content and length. Based on the experimental results, an analytical model, reported in literature and used for the theoretical determination of direct tensile strength, was applied with the aim of making a comparison with experimental results. The comparison showed good overall agreement.     

Fracture characterisation of short bamboo fibre reinforced polyester composites

In the study, fracture behaviour of short bamboo fibre reinforced polyester composites is investigated. The matrix is reinforced with fibres ranging from 10 to 50, 30 to 50 and 30 to 60 vol.% at increments of 10 vol.% for bamboo fibres at 4, 7 and 10 mm lengths respectively. The results reveal that at 4 mm of fibre length, the increment in fibre content deteriorates the fracture toughness. As for 7 and 10 mm fibre lengths, positive effect of fibre reinforcement is observed. The optimum fibre content is found to be at 40 vol.% for 7 mm fibre and 50 vol.% for 10 mm fibre. The highest fracture toughness is achieved at 10 mm/50 vol.% fibre reinforced composite, with 340% of improvement compared to neat polyester. Fractured surfaces investigated through the Scanning Electron Microscopy (SEM) describing different failure mechanisms are also reported.     

Experimental study of bond-slip of GFRP bars in concrete under sustained loads

The structural behaviour of reinforced concrete (RC) elements depends heavily on the bond performance between the concrete and the reinforcing material. Bond behaviour under short-term testing has been extensively analysed for steel reinforcement and many studies have been carried out for fibre reinforced polymer (FRP) reinforcement. However, there has only been limited investigation of the long-term effects of this interaction. Several factors can affect the long-term bond behaviour of these elements, the most important being bond length and the immediate and time-dependent properties of reinforcement and concrete (concrete grade, creep, shrinkage and stiffness). This time-dependent behaviour is likely to cause changes and redistributions in bond stresses not properly considered in the limited existing literature. In this experimental study, the bond performance of GFRP RC under sustained load is investigated through pull-out tests. A total of 12 pull-out specimens were tested for a period of between 90 and 130 days. Two concrete strengths (35 MPa and 50 MPa), two bond lengths (5 and 10 times the diameter of the reinforcing bar) and two reinforcing materials (glass fibre reinforced polymer (GFRP) and steel) were used. Experimental results regarding immediate and time-dependent slip are presented and analysed here. In addition, some specimens were instrumented, with internal strain gauges in the reinforcing bar to provide data on the reinforcement strain, thus allowing the distribution of bond stresses and their evolution during sustained loading to be also presented and analysed.     

Long term properties under marine exposure of steel fibre reinforced concrete containing pfa

The paper presents results on the long-term mechanical properties and durability under marine exposure of a steel fibre reinforced concrete (SFRC) mix containing pulverized fuel ash (PFA) which was developed for marine applications. The mix was of proportions by weight of PFA:OPC:fine aggregate:coarse aggregate of 0.26:0.74:1.51:0.84 with a water/(OPC+PFA) ratio of 0.4. The resulting cement content of the mix was 435 kg m⁻³. Theconcrete was reinforced with low-carbon steel, corrosion-resistant (galvanized) or melt-extract (stainless) steel fibres. Prism specimens were cured in the tidal zone at Aberdeen beach, under wet-dry cycles of sea-water spray in the laboratory, in a water-tank in the laboratory and in the laboratory air. The specimens were cured for up to 1200 marine cycles of exposure (640 days) and were tested at regular intervals of age. The paper presents results on long-term compressive strength, flexural strength and energy absorption capacity as measured from the load-deflection curves. The state of corrosion of fibres is also described. The results indicate that fibres embedded within concrete remain free from corrosion under marine exposure. In the case of fibres exposed at the concrete surface during casting, extensive corrosion occurs in low-carbon steel fibres, isolated rust spots appear in corrosion-resistant fibres and no corrosion is evident in melt-extract fibres. This corrosion, however, remains a surface phenomenon and does not penetrate the concrete. The long-term mechanical properties indicate no deterioration due to possible corrosion. In general the compressive strength of concrete increases significantly with fibre reinforcement.Increases in flexural strength and post-cracking ductility due to fibre reinforcement are of the order normally expected of SFRC.     

Fatigue strength of steel fibre reinforced concrete in flexure

The paper presents a study on the fatigue strength of steel fibre reinforced concrete (SFRC). An experimental programme was conducted to obtain the fatigue-lives of SFRC at various stress levels and stress ratios. Sixty seven SFRC beam specimens of size 500×100×100 mm were tested under four-point flexural fatigue loading. Fifty four static flexural tests were also conducted to determine the static flexural strength of SFRC prior to fatigue testing. The specimens incorporated 1.5% volume fraction of corrugated steel fibres of size 0.6×2.0×30 mm. Concept of equivalent fatigue-life, reported for plain concrete in literature, is applied to SFRC to incorporate the effects of stress level S, stress ratio R and survival probability L_R into the fatigue equation. The results indicate that the statistical distribution of equivalent fatigue-life of SFRC is in agreement with the two-parameter Weibull distribution. The coefficients of the fatigue equation have been determined corresponding to different survival probabilities so as to predict the flexural fatigue strength of SFRC for the desired level of survival probability.     

Tensile behaviour of FRC under high strain-rate

This paper presents experimental results on two types of concrete reinforced with steel and polyvinyl-alcohol (PVA) fibres subjected to dynamic tensile loading. The tests were carried out by using a Modified Hopkinson Bar apparatus on fibre reinforced concrete notched-specimens under three different strain-rates (50, 100, and 200 s⁻¹). From the experiments it was found that there is a significant enhancement in tensile strength with increasing strain-rates. The dynamic tests on steel FRC with the smaller loading rate (50 s⁻¹) showed a strength similar to the one measured from static tests; however, for increasing loading rates, a remarkable decrease of post-peak strength and ductility occurs. In specimens with PVA fibres, an enhancement of the tensile strength was also observed and a significant reduction of fracture energy and ultimate deformation occurred. Some experimental aspects are also discussed as the specimen shape, its dimension, the loading rate as well as the different post-peak behaviour from static and dynamic tests.     

Influence of pinning on static strength of co-cured metal-GFRP hybrid single lap joints

This study presents an experimental investigation into the effects of through-thickness pinning reinforcement on the static strength and damage tolerance of hybrid mild steel–glass fibre prepreg co-cured composite single lap joints (SLJ). Stainless steel pins of 0.3 mm in diameter were inserted as mechanical fastening, in addition to adhesive bonding, to form hybrid joints between metal and glass fibre reinforced polymer substrates. Using the hybrid SLJ tensile testing, the failure modes and static strength were experimentally determined for mild steel–glass fibre prepreg co-cured composites. It is revealed that pinning can improve the static failure load via bridging mechanism by as much as 58% depending on the number and location of pins and the presence of clamping due to bent-ends.     

Response of steel fiber reinforced high strength concrete beams: Experiments and code predictions

The shear-flexure response of steel fiber reinforced concrete (SFRC) beams was investigated.Thirty-six reinforced concrete beams with and without conventional shear reinforcement (stirrups) were tested under a four-point bending configuration to study the effectiveness of steel fibers on shear and flexural strengths, failure mechanisms, crack control, and ductility.The major factors considered were compressive strength (normal strength and high strength concrete up to 100 MPa), shear span-effective depth ratio (a/d = 1.5, 2.5, 3.5), and web reinforcement (none, stirrups and/or steel fibers).The response of RC beams was evaluated based on the results of crack patterns, load at first cracking, ultimate shear capacity, and failure modes.The experimental evidence showed that the addition of steel fibers improves the mechanical response, i.e., flexural and shear strengths and the ductility of the flexural members.Finally, the most recent code-based shear resistance predictions for SFRC beams were considered to discuss their reliability with respect to the experimental findings. The crack pattern predictions are also reviewed based on the major factors that affect the results.     

Observations on the electrical resistivity of steel fibre reinforced concrete

Steel fibre reinforced concrete (SFRC) is in many ways a well-known construction material, and its use has gradually increased over the last decades. The mechanical properties of SFRC are well described based on the theories of fracture mechanics. However, knowledge on other material properties, including the electrical resistivity, is sparse. Among others, the electrical resistivity of concrete has an effect on the corrosion process of possible embedded bar reinforcement and transfer of stray current. The present paper provides experimental results concerning the influence of the fibre volume fraction and the moisture content of the SFRC on its electrical resistivity. The electrical resistivity was measured by alternating current (AC) at 126 Hz. Moreover, an analytical model for the prediction of the electrical resistivity of SFRC is presented. The analytical model is capable of predicting the observed correlation between the fibre volume fraction and the electrical resistivity of the composite (the SFRC) for conductive fibres and moisture saturated concrete. This indicates that the steel fibres were conducting when measuring the electrical resistivity by AC at 126 Hz. For partly saturated concrete the model underestimated the influence of the addition of fibres. The results indicate that the addition of steel fibres reduce the electrical resistivity of concrete if the fibres are conductive. This represents a hypothetical case where all fibres are depassivated (corroding) which was created to obtain a conservative estimate on the influence of fibres on the electrical resistivity of concrete. It was observed that within typical ranges of variation the influence of the moisture content on the electrical resistivity was larger than the effect of addition of conductive steel fibres, but also that the relative impact on the electrical resistivity due to conductive steel fibres increased when the moisture content of the concrete was reduced.     

Flexural behaviour of small steel fibre reinforced concrete slabs

Influence of length and volumetric percentage of steel fibres on energy absorption of concrete slabs with various concrete strengths is investigated by testing 28 small steel fibre reinforced concrete (SFRC) slabs under flexure. Variables included; fibre length, volumetric percentage of fibres and concrete strength. Test results indicate that generally longer fibres and higher fibre content provide higher energy absorption. The results are compared with a theoretical prediction based on random distribution of fibres. The theoretical method resulted in higher energy absorption than that obtained in experiment. A design method according to allowable deflection is proposed for SFRC slabs within the range of fibre volumetric percentages used in the study. The method predicts resisting moment–deflection curve satisfactorily.     

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.     

Experimental Investigation of Bond Stress and Deformations in LWAC Ties Reinforced with GFRP Bars

The structures' durability is an engineering concern for a long time but has been increased in the last years. Lightweight aggregate concrete (LWAC) combined with glass fibre reinforced polymer bars allows to create structures with high performance in terms of durability. The glass fibre reinforced polymer (GFRP) bars have different ribs from those of steel bars, and consequently, its bond to concrete is affected. Moreover, the Young's modulus of GFRP is much below compared with that of steel, and this influences significantly the behaviour of structural elements reinforced with this material. This paper presents an experimental study focused on bond between LWAC and reinforcing bars of GFRP. Thirty‐six pull‐out tests were carried out using steel and GFRP bars. These reinforcements were combined with three types of concrete, all with the same design density 1900 kg m^?3 but with different values of compressive strength: 35, 55 and 70 MPa. Furthermore, 12 reinforced ties were tested, combining different types of bars (steel and GFRP), two different diameters (12 and 16 mm) and the three types of LWAC. Based on experimental results, several relations were established to understand the behaviour of LWAC structures reinforced with GFRP bars, mainly in the serviceability conditions. These results point out that ties deformation and crack width are very affected by the reduced Young's modulus of GFRP: deformations and crack width of ties reinforced with GFRP are significantly higher, approximately three times greater, compared with those of ties reinforced with steel. The tension stiffening effect was also analysed in detail, and it was found that it is slightly influenced by the concrete compressive strength but is highly dependent of the Young's modulus of the reinforcing material.     

Mechanical behaviour of plain-knit reinforced injected composites: Effect of inlay yarns and fibre type

This paper investigates the effect of inlay yarns and fibre type (E-glass, basalt, carbon) to improve the mechanical behaviour of plain knit reinforced composites (epoxy matrix). The tensile behaviour of the dry reinforcement was investigated in the wale and course-directions and has shown that the course-wise deformation is drastically reduced whereas the strength is strongly increased when inlay yarns are included. Fibre type has a weak effect on deformation and strength. For the composite material, processed by Liquid Composite Moulding, the carbon fibre reinforcement gives always the best results whatever the testing direction. Inlay yarns decrease the 0° mechanical properties, whereas 45° and 90° are increased. Moreover a quasi-isotropic behaviour is obtained when using two inlay yarns, irrespective of the fibre types.     

Bond strength of reinforcing steel embedded in fly ash-based geopolymer concrete

Geopolymer concrete (GPC) is an emerging construction material that uses a by-product material such as fly ash as a complete substitute for cement. This paper evaluates the bond strength of fly ash based geopolymer concrete with reinforcing steel. Pull-out test in accordance with the ASTM A944 Standard was carried out on 24 geopolymer concrete and 24 ordinary Portland cement (OPC) concrete beam-end specimens, and the bond strengths of the two types of concrete were compared. The compressive strength of geopolymer concrete varied from 25 to 39 MPa. The other test parameters were concrete cover and bar diameter. The reinforcing steel was 20 mm and 24 mm diameter 500 MPa steel deformed bars. The concrete cover to bar diameter ratio varied from 1.71 to 3.62. Failure occurred with the splitting of concrete in the region bonded with the steel bar, in both geopolymer and OPC concrete specimens. Comparison of the test results shows that geopolymer concrete has higher bond strength than OPC concrete. This is because of the higher splitting tensile strength of geopolymer concrete than of OPC concrete of the same compressive strength. A comparison between the splitting tensile strengths of OPC and geopolymer concrete of compressive strengths ranging from 25 to 89 MPa shows that geopolymer concrete has higher splitting tensile strength than OPC concrete. This suggests that the existing analytical expressions for bond strength of OPC concrete can be conservatively used for calculation of bond strength of geopolymer concrete with reinforcing steel.     

Fracture Energy of Steel Fiber-Reinforced Concrete

Steel fiber-reinforced concrete (SFRC) is a cementitious material reinforced with discrete fibers. The energy absorption capacity is the main material property benefited by fiber reinforcement. Closed-loop servo-controlled equipment should be used to evaluate this property. The tests should be carried out using displacement control in order to obtain the postpeak force- displacement relationship (tensile strain-softening branch). To assess the fracture energy of SFRC, three-point bending tests were carried out using displacement control. Series of notched beams reinforced with 30, 60, and 90 kg/m3 of hooked-end steel fibers were tested. Besides the energy dissipated in fracturing the concrete, the energy determined from the force-displacement relationship can also include the energy absorbed during nonlinear behavior of concrete in compression. Ductile materials, such as concrete reinforced with a high content of fibers, develop large deflections before exhausting their energy absorption capacity. In these cases, the "fixed" points of the bar supporting the displacement transducer may not remain fixed, adding an extra deflection into the control displacement transducer and thus leading to incorrect evaluation of the fracture energy. These factors are analyzed in the present work in order to assess the suitability of the specimen dimensions and the test procedures for evaluating the fracture energy of SFRC.