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.     

Experimental study and code predictions of fibre reinforced polymer reinforced concrete (FRP RC) tensile members

Due to their different mechanical properties, cracking and deformability behaviour of FRP reinforced concrete (FRP RC) members is quite different from traditional steel reinforced concrete (SRC) having great incidence on their serviceability design. This paper presents and discusses the results of an experimental programme concerning concrete tension members reinforced with glass fibre reinforced polymer (GFRP) bars. The main aim of the study is to evaluate the response of GFRP reinforced concrete (GFRP RC) tension members in terms of cracking and deformations. The results show the dependence of load-deformation response and crack spacing on the reinforcement ratio. The experimental results are compared to prediction models from codes and guidelines (ACI and Eurocode 2) and the suitability of the different approaches for predicting the behaviour of tensile members is analysed and discussed.     

An experimental study on the effect of environmental exposures and corrosion on RC columns with FRP composite jackets

The objective of this study was to evaluate the effects of various environmental conditions on the long-term behavior of reinforced concrete (RC) columns strengthened with carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) sheets. Small-scale RC columns were manufactured in the laboratory and conditioned under accelerated environmental cycling and accelerated corrosion process of reinforcing bars. Then, uni-axial compressive failure tests were conducted in order to evaluate the change of mechanical properties of the test columns due to the environmental effects. The results revealed that the mechanical properties of RC column system (RC + FRP) were altered due to the environmental conditioning and the corrosion of steel reinforcement, and each type of environmental conditions had its unique effects and features.     

Structural response of hyperstatic concrete beams reinforced with GFRP bars: Effect of increasing concrete confinement

The design of concrete structures reinforced with glass fibre reinforced polymer (GFRP) bars is influenced by their reduced stiffness and brittleness. In hyperstatic structures, the methodology used in force analysis depends on the ductility of the structural systems, which in this case, being essentially provided by the concrete, can be potentially increased by confining concrete in critical zones. This paper presents experimental and numerical investigations about the flexural behaviour of continuous beams reinforced with GFRP bars, namely of their service and failure responses, and the effect of increasing concrete confinement in critical cross-sections. A calculation procedure to quantify the confinement effect in beams due to the reduction of the spacing between shear stirrups is first presented. The experimental investigations comprised a comparative study in which two-span concrete beams reinforced with either GFRP or steel bars were tested in bending. In the former, the effect of reducing the shear stirrups spacing was analyzed together with the under- and over-reinforcement at the central support and midspan cross-sections, respectively. The development of a crack hinge in the continuity support zone highlighted the better performance of beams under-reinforced on the top layer with GFRP bars compared to “equivalent” beams reinforced with steel, namely at the resistance level. In addition, the confinement at critical zones increased significantly the strength and ductility. The numerical investigations included the development of non-linear finite element models for all beams tested - numerical results are in good agreement with test data and seem to confirm the confinement effect observed in the experiments.     

Evaluation of performance and design of GFRP dowels in jointed plain concrete pavement – part 2: numerical simulation and design considerations

The article presents a numerical investigation using three-dimensional finite element analysis (FEA) to compare the performance of glass fibre reinforced polymer (GFRP) and steel dowel bars as load transfer devices across the transverse joints of jointed plain concrete pavements (JPCP). The FEA model used concrete damaged plasticity formulation to characterise the concrete pavement; elastic, transversely isotropic material characteristics were assumed for the GFRP dowels and classical metal plasticity formulation was used for the steel dowels. The numerical results were validated with the experimental results, and a good agreement was achieved. The results showed less stress concentration in the concrete underneath the GFRP dowels (38 mm diameter) compared with the steel dowels (25 mm diameter) of similar flexural rigidity. Finally, on the basis of a detailed parametric study, design considerations for the GFRP dowels in JPCP are suggested.     

Short and long-term cracking behaviour of GFRP reinforced concrete beams

A total of ten simply supported beams reinforced with different amounts of GFRP and steel bars were subjected to two consecutive test phases in order to evaluate their short and long-term cracking behaviour. The beams were initially tested up to service load and subjected to two additional load cycles. Subsequently, the specimens were subjected to two different levels of sustained load for 250 days. The effect of cyclic load during short-term tests resulted in an increase in crack width up to 25% more than the initial value. The sustained load led to an increase in crack width up to 2.9 times larger than that measured under the corresponding short-term load. A similar cracking behaviour was observed when reinforcing solutions with similar stiffness (GFRP or steel bars) were used.Existing models to estimate crack spacing and crack width for FRP and steel reinforced concrete elements, including ACI 440.1R-06, Eurocode 2 and Model Code 2010 are discussed and their performance is assessed against the experimental results. Model Code 2010 was found to yield more accurate predictions of the cracking behaviour of the test specimens under both short-term and long-term loading.     

Strain measurement in a concrete beam by use of the Brillouin-scattering-based distributed fiber sensor with single-mode fibers embedded in glass fiber reinforced polymer rods and bonded to steel reinforcing bars

The strain measurement of a 1.65-m reinforced concrete beam by use of a distributed fiber strain sensor with a 50-cm spatial resolution and 5-cm readout resolution is reported. The strain-measurement accuracy is +/-15 microepsilon (microm/m) according to the system calibration in the laboratory environment with non-uniform-distributed strain and +/-5 microepsilon with uniform strain distribution. The strain distribution has been measured for one-point and two-point loading patterns for optical fibers embedded in pultruded glass fiber reinforced polymer (GFRP) rods and those bonded to steel reinforcing bars. In the one-point loading case, the strain deviations are +/-7 and +/-15 microepsilon for fibers embedded in the GFRP rods and fibers bonded to steel reinforcing bars, respectively, whereas the strain deviation is +/-20 microepsilon for the two-point loading case.     

Effect of steel and synthetic fibers on flexural behavior of high-strength concrete beams reinforced with FRP bars

Six high-strength concrete beam specimens reinforced with fiber-reinforced polymer (FRP) bars were constructed and tested. Three of the beams were reinforced with carbon FRP (CFRP) bars and the other three beams were reinforced with glass FRP (GFRP) bars as flexural reinforcements. Steel fibers and polyolefin synthetic fibers were used as reinforcing discrete fibers. An investigation was performed on the influence of the addition of fibers on load-carrying capacity, cracking response, and ductility. In addition, the test results were compared with the predictions for the ultimate flexural moment. The addition of fibers increased the first-cracking load, ultimate flexural strength, and ductility, and also mitigated the large crack width of the FRP bar-reinforced concrete beams.     

混合配筋钢纤维增强混凝土梁受弯承载力试验及理论计算

为研究玻璃纤维增强聚合物复合材料(GFRP)筋与普通钢筋混合配筋钢纤维增强混凝土(SF/混凝土)梁的受弯性能及其受弯承载力计算方法,在考虑受拉区混凝土抗拉强度的基础上,给出混合配筋SF/混凝土梁的界限配筋率及受弯承载力计算公式;在此基础上设计制作了三种配筋方式的SF/混凝土梁,重点探讨了混合配筋率及筋材面积比(A_f/A_s)对试验梁失效模式和受弯承载力的影响;同时,借助已有相关试验结果,对比分析了混凝土强度对混合配筋SF/混凝土梁受弯性能的影响。试验和对比分析结果表明:混合配筋SF/混凝土梁正截面应变仍符合平截面假定;相同配筋形式下,混合配筋SF/混凝土梁的受弯承载力和跨中挠度随筋材面积比A_f/A_s的增加而增大;单层配筋梁的受弯承载力比双层配筋梁大;合理提高混凝土强度可在充分发挥GFRP筋抗拉作用的同时进一步提高混合配筋SF/混凝土梁的受弯承载力;采用本文给出的界限配筋率公式能有效预测混合配筋SF/混凝土梁的失效模式;梁受弯承载力建议公式的预测值与试验值吻合较好,具有良好的适用性。      

全玻璃纤维增强树脂筋混凝土电缆排管的抗弯试验

提出了采用全玻璃纤维增强树脂基复合材料（GFRP）筋混凝土电缆排管代替传统的钢筋混凝土电缆排管,该结构形式具有减少能耗的优点。通过对小尺寸和足尺GFRP筋混凝土电缆排管试件进行抗弯性能试验,研究其抗弯能力、变形及破坏特征等。试验结果表明,GFRP筋混凝土电缆排管具有与普通钢筋混凝土梁相似的力学特征,以混凝土开裂为分界点,位移-荷载曲线表现为双线性,排管侧面拉应力分布不均匀,部分区域出现了较高拉应力。提出了GFRP筋混凝土电缆排管的抗弯设计计算方法,理论计算结果与试验测试结果较为吻合。     

Numerical calibration of bond law for GFRP bars embedded in steel fibre-reinforced self-compacting concrete

An experimental program was carried out at the Laboratory of Structural Division of the Civil Engineering Department of the University of Minho (LEST-UM) to investigate the bond behaviour of glass fibre reinforced polymer (GFRP) bars embedded in steel fibre reinforced self-compacting concrete (SFRSCC) for the development of an innovative structural system. Thirty-six pull-out-bending tests were executed to assess the influence of the bond length, concrete cover, bar diameter and surface treatment on the bond of GFRP bars embedded in SFRSCC. This paper reports the results of a numerical study aiming to identify an accurate GFRP–SFRSCC bond–slip law. Thus, the above mentioned pullout bending tests were simulated by using a nonlinear finite element (FE) constitutive model available in FEMIX, a FEM based computer program. The bond–slip relationship adopted for modelling the FE interface that simulates the interaction between bar and concrete is the key nonlinear aspect considered in the FE analyses, but the nonlinear behaviour of SFRSCC due to crack initiation and propagation was also simulated. The evaluation of the values of the relevant parameters defining such a bond–slip relationship was executed by fitting the force versus loaded end slip responses recorded in the experimental tests. Finally, correlations are proposed between the parameters identifying the bond–slip relationship and the relevant geometric and mechanical properties of the tested specimens.     

Early-age restrained shrinkage cracking of GFRP-RC bridge deck slabs: Effect of environmental conditions

Most of codes and guidelines for glass fiber reinforced polymers (GFRP) - Reinforced Concrete (RC) are based on modifying corresponding formulas, originally developed for steel bars, taking into account the differences in properties and behavior between FRP and steel. The main objective of this research is to investigate the effect of cyclic environments on early-age cracking of GFRP-RC bridge deck slabs experimentally. Two full-scale (measuring 2500-mm long × 765-mm wide × 180-mm thick) cast-in-place slabs reinforced with similar amounts of reinforcement ratio of 0.7% with GFRP and steel bars, respectively, were tested in adiabatic laboratory conditions as control specimens. In comparison, two other GFRP-RC deck slabs were tested under freezing–thawing and wetting–drying conditions. The test results are presented in terms of materials degradation, cracking pattern, crack width, and spacing, and strains in reinforcement and concrete. Test results indicate that the minimum reinforcement ratio (0.7%) recommended by the Canadian Highway Bridge Design Code 2006 (CHBDC 2006) for bridge deck slabs reinforced with GFRP bars satisfied the serviceability requirements after being subjected to the simulated cyclic exposures.     

Evaluation of performance and design of GFRP dowels in jointed plain concrete pavement – part 1: experimental investigation

Dowel bars are provided at the transverse joints of the jointed plain concrete pavement to allow for expansion and contraction of the pavement due to moisture and temperature changes. This paper presents experimental and analytical investigations for the deflection response of glass fibre-reinforced polymer (GFRP) dowels for different joint widths and concrete grades. The results were compared with those obtained from investigations into the conventional epoxy-coated steel dowel bars of similar rigidity. The experimental results showed that the 38 mm (1.5 in.) GFRP dowels perform better in terms of joint face deflection compared with 25 mm (1 in.) epoxy-coated steel dowel bars. In addition, these results showed that the deflection of the GFRP dowel was significantly affected by changing the concrete compressive strength and the joint widths.     

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.     

Investigation of structural behaviours of laterally restrained GFRP reinforced concrete slabs

The corrosion of reinforcement in concrete bridge deck has been the cause of major deterioration and of high costs in repair and maintenance. Glass fibre reinforced polymer (GFRP) reinforcement is a more durable alternative to steel reinforcement and has higher strength to weight ratio. Due to the low value of elasticity and brittle behaviour of GFRP, the service behaviour of GFRP reinforced concrete structure is critical. However, laterally restrained slabs, such as those in bridge deck slabs, exhibit arching action or compressive membrane action (CMA) which has a beneficial influence on the service behaviour such as the deflection. This paper presents the results of experimental tests and numerical analysis of laterally restrained GFRP reinforced concrete slabs with varying some structural variables. The analysis results are discussed and conclusions on the compressive membrane action in GFRP reinforced concrete slabs are presented.     

Long term durability of pultruded polymer composite rebar in concrete environment

Pultruded glass fibre reinforced polymer (GFRP) composite rebar was immersed in alkaline concrete environment for 0, 1, 2, 3, 4, 6, 14 and 24 months at 60 °C to evaluate its durability in concrete structure. Moisture absorption of the rebar was found to be only 0.76%. It was also found that both the glass transition temperature (T_g) and the short beam shear strength were retained by about 91.5%; and the above properties were remained almost unchanged during the ageing period from 1 month to 24 months. Design tensile strength and tensile elastic modulus of the rebar were retained by 100% after 24 months exposure in concrete environment. Degradation of GFRP rebar was not evident in the FT-IR results as supported by scanning electron micrographs and energy dispersive X-ray analysis. Moisture absorption was found to be a critical factor that controlled thermal and mechanical properties of GFRP rebar.     

Friction coefficient between FRP pultruded profiles and concrete

This paper presents the experimental results of a direct shear test to determine the friction coefficient between fibre reinforced polymer (FRP) pultruded profiles and concrete. The FRP pultruded profile used in this study was glass fibre reinforced polymer (GFRP) I-section. The specimens were composed of a concrete block and a coupon of the GFRP pultruded profile. The experiment was conducted by using the direct shear test method. The parameters investigated included the type of concrete (self-compacting concrete and normal concrete) and the compressive strength of the concrete, as well as the different components (web and flange) of the I-section. The test results verify that the bond behaviour between the concrete and the GFRP pultruded profiles mainly depends on two factors, the friction stress and the adhesion stress. The friction coefficient between the FRP pultruded profiles and the concrete was between 0.5 and 0.6 when the normal stress fluctuated between 0.5 and 2 MPa, and the adhesion stress was about 0.2 MPa. The compressive strength of the concrete and the different components of the I-section have little effect on the friction coefficient, however, the type of the concrete significantly affects this coefficient.     

玻璃纤维增强树脂复合材料管-钢筋/混凝土空心构件抗弯性能

为研究玻璃纤维增强树脂复合材料(GFRP)管-钢筋/混凝土空心构件的抗弯性能,编制了受弯构件的非线性分析程序,系统地分析了空心率、配筋率、GFRP管管壁厚度及混凝土强度等级等主要参数对其抗弯性能的影响,并通过试验对所编制的程序进行验证,最后建立适用于GFRP管-钢筋/混凝土空心构件的抗弯承载力计算公式。结果表明:利用编制的受弯构件非线性分析程序与建立的抗弯承载力公式,计算结果与试验结果均吻合较好,抗弯承载力随空心率的减小、配筋率的提高、GFRP管管壁厚度的增加及混凝土强度的增大而增加,空心率对构件抗弯承载力影响最大,其次是配筋率和GFRP管管壁厚度,最后是混凝土强度等级,空心部分半径比在0.25~0.5为宜,可以适当提高配筋率、GFRP管管壁厚度或混凝土强度等级来弥补该空心构件抗弯承载力,研究结论可为该结构在实际应用中提供参考依据。      

Fatigue analysis of concrete bridge deck slabs reinforced with E-glass/vinyl ester FRP reinforcing bars

FRP composites have been widely used as internal reinforcement for concrete bridge deck slabs. However, experimental researches on the behavior of such FRP-reinforced elements in general have been limited, especially those on fatigue performance. This research is designed to investigate the fatigue behavior of concrete bridge deck slabs reinforced with GFRP bars. A total of six full-size deck slabs were constructed and tested under concentrated cyclic loading conditions. Different reinforcement types, ratios, and configurations were used. Also, different schemes of cyclic loading were applied till failure. Finite element modeling was used to investigate the effect of different parameters on the ultimate static capacity. The results showed the superior fatigue performance and longer fatigue life of concrete bridge deck slabs reinforced with GFRP composite bars compared to the steel reinforced ones.     

Performance of glass fiber reinforced polymer bars under elevated temperatures

This paper investigates the residual tensile properties of newly developed glass fiber reinforced polymer (GFRP) bars after being subjected to elevated temperatures for different periods. A total of 120 GFRP specimens were tested in this study. Half of the samples were covered with concrete while the other half were bare bars. The specimens were subjected to three different controlled temperatures (100, 200 and 300 °C) for three different periods (1, 2, and 3 h). Test results showed that almost no losses were observed in the tensile modulus after all exposure periods and temperatures. Losses in the tensile strength, proportional to the level of temperature and exposure period, were recorded. The bars with concrete cover showed higher residual tensile strength compared to their counterparts without coating. The concrete cover was more effective at the lowest temperature level (100 °C) and at the shortest time period (1 h). Scanning Electronic Microscopy (SEM) technique was also used to investigate the effect of elevated temperature on the degradation mechanism of the GFRP bars. The results showed that increasing the temperature level affected the resin matrix surrounding the glass fibers and consequently affected the bond between the fibers and the matrix.