恶劣环境下纤维增强聚合物片材拉伸性能

通过拉伸试验,研究了恶劣环境作用后纤维增强聚合物(FRP)片材的拉伸性能。试验参数包括恶劣环境类别和作用方式、FRP片材种类和层数。试验结果表明,常温环境下、冻融和干湿循环作用后,碳纤维增强聚合物(CFRP)片材和玻璃纤维增强聚合物(GFRP)片材的拉伸应力-应变关系近似为直线;常温环境下,CFRP片材和GFRP片材的拉伸强度和延伸率几乎不受片材层数的影响;冻融循环对GFRP片材的影响大于CFRP片材,冻融循环75次时,CFRP片材和GFRP片材的拉伸强度分别是未冻融的0.978倍和0.898倍,并且随着循环次数的增加,CFRP片材和GFRP片材拉伸强度逐渐下降;干湿循环作用对GFRP片材拉伸性能没有明显的影响。基于对有关文献及本文试验结果的分析,提出了恶劣环境下FRP片材拉伸强度的计算方法。     

Creep and durability of sand-coated glass FRP bars in concrete elements under freeze/thaw cycling and sustained loads

This paper presents the test results of 21 concrete beams (1800 × 130 × 180 mm) reinforced with sand-coated glass FRP composite bars. The individual and coupled effects of freeze/thaw cycles and sustained bending stresses on the long-term behaviour of concrete beams reinforced with GFRP composite bars were investigated. The beams were exposed to 100, 200 and 360 freeze/thaw cycles (−20 °C to +20 °C) either in an unstressed state or loaded in bending to cause a tensile stress equals to 27% of the ultimate tensile strength of the GFRP bar. The conditioned beams were tested up to failure in a four-point bending set-up over a clear span of 1500 mm. The test results showed that the single or coupled action of freeze/thaw cycles and sustained bending stresses has no significant effect on the behaviour of the tested beams in terms of deflections, strains, and ultimate capacity. It was also concluded that the long-term deflections and the creep strain limits specified by ACI 440.1R-06 are conservative.     

Effect of accelerated freeze–thaw cycling on mechanical properties of hybrid PVA and PE fiber-reinforced strain-hardening cement-based composites (SHCCs)

In this study, the influence of rapid freezing and thawing actions on mechanical properties of hybrid fibers reinforced strain-hardening cement-based composites (SHCCs), which exhibit multiple cracking and strain-hardening behavior in direct tension, were investigated. Four SHCC mixtures with different water-to-binder (W/B) ratios and hybrid fiber combinations were assessed experimentally. The SHCC mixtures incorporating hybrid polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (PE) fibers at the 1.5% volume fraction were exposed to freezing and thawing according to ASTM C 666 (Procedure B). The freeze–thaw tests continued until the specimens achieved 300 freeze–thaw cycles. The results of these tests indicate that rapid freeze–thaw cycles in the laboratory have little effect on the compressive and tensile strength characteristics of the SHCC mixtures prepared in this study, whereas multiple cracking behavior and deformation capacity of SHCC specimens under direct tensile and flexural loadings indicate that freeze–thaw cycles have a negative effect on the these characteristics of the SHCC mixtures. A tendency toward reduced ductility is prominent for SHCC materials with higher W/B ratio and more hydrophilic PVA fiber.     

Bond–slip on CFRP/GFRP-to-concrete joints subjected to moisture,salt fog and temperature cycles

Research on bond between composite and concrete on beams externally reinforced with fiber reinforced polymers (ERBs) has generated many publications, but uncertainties remain. The issue of the long term behavior of those joints, especially the effect that severe and prolonged environmental actions may induce, justifies the search for additional data and recommendations to avoid premature debonding and failures.The present paper devotes attention to the effects of environmental aging on the constitutive bond-slip curves. Aging was imposed on an accelerated manner and several actions used to degrade the behavior of the joints evaluated by mechanical testing.Prismatic blocks of reinforced concrete linked on the upper side by a stainless steel hinge and externally bonded by a continuous strip of carbon or glass fiber reinforced polymer (CFRP or GFRP) adherent to the soffit were made so as to fit in commercially available laboratory climatic chambers. After aging, they were tested till failure under four point loading.Aging conditions imposed on the CFRP beams were (i) cycles of high-low relative humidity, (ii) salt fog cycles, and, on the GFRP beams, (iii) salt fog cycles, (iv) dry/wet cycles (water with 5.0% weight of NaCl), (v) total immersion in salt water, and (vi) freeze/thaw cycles.The results of the experimental program enabled the proposal of bond-slip laws that take into account the aging of the beams. They also showed that salt fog cycles were more severe in the case of CFRP, while freeze/thaw cycles were more degrading on bond of GFRP-to-concrete. The salt water effects on the GFRP beams appeared to be beneficial, most likely by improving the tensile strength of concrete. Numerical modeling certified by the obtained experimental data is presented that allows more general estimates of the environmental effects.     

Effect of the curing conditions of concrete on the behaviour under freeze–thaw cycles*

The aim of this work is to relate the curing conditions of concrete and the addition of an air‐entraining admixture with the damage caused by freeze–thaw cycles. In countries with a continental climate, the curing of concrete in summer is performed under climatic conditions of high temperature and low humidity, and during the winter the concrete suffers conditions of freeze–thaw, often accompanied by the use of de‐icing salts. This paper shows the experimental results of the behaviour of concrete specimens cured under climatic summer conditions (high temperature and low humidity) and then subjected to freeze–thaw cycles. Curing of the specimens includes conditions of good and bad practice in relation to wetting and protection of the concrete. It also examines the effectiveness of using an air‐entraining admixture in both cases. The experimental programme includes an evaluation of the mechanical properties of the concrete, the study of the cement hydration and the measurement of the volume and pore sizes of the concrete. These tests were performed before and after the application of the freeze–thaw cycles. The results obtained showed that the specimens without air‐entraining admixture show a deterioration of mechanical properties after the freeze–thaw test. However, the inclusion of air bubbles benefits the behaviour of concrete against freeze–thaw cycles so even better mechanical properties after the test were observed. This anomalous behaviour is because the cement hydration process continues over the freeze–thaw tests, closing the pore structure. This aspect has been confirmed with the DTA and TG tests performed.     

Experimental characterization of fracture of glass fiber reinforced composites laminates subjected to freeze‐thaw cycles

The simultaneous effect of moisture and freeze‐thaw cycle on the mechanical behavior of glass/epoxy composites laminates is experimentally investigated. The study is planned in order to simulate the detrimental presence of humidity due to rainfall in surface damage of composite structures operating in cold weather. Different mechanisms governing the monotonic response of specimens subjected to freeze‐thaw cycles are pointed out by taking advantages of SEM images. Comparing SEM images taken from dry and wet specimens shows that the failure mechanisms such as matrix cracking and delamination are vastly activated around the notched region when the material is exposed to humidity and freeze‐thaw cycle. The load‐displacement response of examined specimens, namely the linear response, is remarkably altered under these conditions. A reduction of approximately 40% in ultimate load and 30% in slope of tangent line of load‐displacement curves is identified after 100 cycles of freeze‐thaw as well as more than 20% decrease of strain energy release rate.     

Strength and durability performance of concrete axially loaded members confined with AFRP composite sheets

The performance of concrete columns externally wrapped with aramid fiber reinforced polymer composite sheets is presented in this paper. The confined and unconfined (control) specimens were loaded in uniaxial compression. Axial load and axial and hoop strains were measured in order to evaluate stress–strain behavior, ultimate strength, stiffness, and ductility of the wrapped specimens. Results show that external confinement of concrete by fiber reinforced polymer (FRP) composite sheets can significantly enhance strength, ductility and energy absorption capacity. An analytical model developed earlier by the author to predict the entire stress–strain response of concrete specimens wrapped with FRP composite sheets was applied. Comparison between the experimental and analytical results indicates that the model provides satisfactory predictions of the stress–strain response. The paper also presents the performance of the wrapped concrete specimens subjected to severe environmental conditions such as wet–dry and freeze–thaw cycles. The specimens were exposed to 300 cycles of wetting and drying using salt water. Results show that specimens wrapped with aramid fibers experienced no reduction in strength due to wet/dry exposure, but some reduction was observed due to freeze/thaw exposure.     

The influence of freeze–thaw cycles on the unconfined compressive strength of fiber-reinforced clay

Freeze–thaw cycling is a weathering process that frequently occurs in cold climates. In the freeze state, thermodynamic conditions at temperatures just below 0 °C result in the translocation of water and ice. Consequently, the engineering properties of soils such as permeability, water content, stress–strain behavior, failure strength, elastic modulus, cohesion, and friction angle may be changed. Former studies have been focused on changes in physical and mechanical properties of soil due to freeze–thaw cycles. In this paper, the effect of freeze–thaw cycles on the compressive strength of fiber-reinforced clay is investigated. For this purpose, kaolinite clay reinforced by steel and polypropylene fibers is compacted in a laboratory and exposed to a maximum of 10 closed-system freezing and thawing cycles. The unconfined compressive strength of reinforced and unreinforced specimens is then determined. The results of the study show that for the soil investigated, the increase in the number of freeze–thaw cycles results in the decrease of unconfined compressive strength of clay samples by 20–25%. Moreover, inclusion of fiber in clay samples increases the unconfined compressive strength of soil and decreases the frost heave. Furthermore, the results of the study indicate that fiber addition does not decrease the soil strength against freeze–thaw cycles. Moreover, the study shows that the addition of 3% polypropylene fibers results in the increase of unconfined compressive strength of the soil before and after applying freeze–thaw cycles by 60% to 160% and decrease of frost heave by 70%.     

海水浸泡对FRP筋-珊瑚混凝土粘结性能的影响

开展了30℃海水浸泡条件下玻璃纤维增强树脂基复合材料（GFRP）筋、碳纤维增强树脂基复合材料（CFRP）筋与珊瑚混凝土粘结性能的试验研究,分析了纤维增强树脂基复合材料（FRP）筋-珊瑚混凝土粘结滑移曲线特征、破坏形态及粘结强度变化。试验结果表明,海水浸泡后FRP筋力学性能和粘结性能均表现为不同程度的降低。随浸泡时间增加,GFRP筋表层树脂与纤维间的孔隙率明显增大,并逐渐出现脱粘现象,纤维本身遭受到侵蚀,而CFRP筋仅表面基体有少许损伤,其耐久性明显优于GFRP筋;FRP筋-珊瑚混凝土粘结强度呈现出先增加后减小的趋势,且后期下降速率逐渐变小,部分GFRP筋-珊瑚混凝土试件的破坏模式逐渐由筋被拔出转变为筋材断裂;增加保护层厚度能有效地减缓海水对GFRP筋的侵蚀,有利于保持GFRP筋-珊瑚混凝土间的粘结性能。     

Harsh environments effects on the axial behaviour of circular concrete-filled fibre reinforced-polymer (FRP) tubes

Concrete-filled fibre-reinforced polymer (FRP) tubes (CFFTs) are becoming an attractive system for structural elements proposed to harsh environments. FRP tube provides a corrosion resistant element, reinforcement, confinement for the concrete core, and a stay-in-place formwork. Harsh environments may affect the mechanical performance of the FRP tube, which consequently affect the structural response of the CFFT members. This project investigates the environmental degradation and the durability of concrete cylinders unconfined and confined by filament-wound glass-FRP tubes. Standard plain concrete cylinders and CFFT cylinders were immersed in pure water, salt and alkaline solutions, and exposed to 200 freeze–thaw cycles, between −40 °C and +40 °C. Then, the cylinders were tested under uniaxial compression test to evaluate their performance by comparing the stress–strain behaviour and their ultimate load capacities. Test results indicated that the FRP tube, in CFFTs, is significantly qualified as a sustainable coating material to resist the harsh environments attacks. Theoretical predictions using long term confinement models from CSA and ACI codes are presented.     

FRP-钢管-混凝土构件抗震性能试验研究

为了研究低周循环往复荷载作用下GFRP(glass fiber reinforced polymer)-钢管-混凝构件的力学性能以及对比CFRP(carbon fiber reinforced polymer)-钢管-混凝土构件的性能差异, 对尺寸相同而加固方式不同的圆形截面FRP(GFRP、CFRP)-钢管-混凝土试件进行了拟静力试验, 荷载采用轴压、双向弯曲的组合来模拟地震动力。结果表明: FRP(GFRP、CFRP)的加固可以有效地提高构件抗动态弯曲的能力; GFRP-钢管-混凝土构件延性高于相同情况下CFRP-钢管-混凝土构件; 与普通钢管-混凝土相比, 环向、纵向和双向包裹的GFRP-钢管-混凝土构件的耗能系数分别提高2.0%、7.0%和12.7%, 而CFRP-钢管-混凝土分别提高2.0%、5.8%和6.7%。     

侵蚀环境下FRP条带加固锈蚀钢筋混凝土圆柱轴心受压试验

通过侵蚀环境下碳纤维增强聚合物(CFRP)复合材料条带和玻璃纤维增强聚合物(GFRP)复合材料条带加固锈蚀钢筋混凝土圆柱试验,分析了侵蚀环境对混凝土强度、纤维增强聚合物基复合材料加固锈蚀柱的极限荷载和荷载-轴向位移曲线的影响。结果表明,混凝土强度受冻融环境影响较大,受干湿环境影响较小;纤维增强聚合物(FRP)复合材料加固锈蚀柱的轴向极限荷载与冻融循环次数、钢筋锈蚀率及FRP复合材料种类有关,随冻融循环次数分别增加到25次、50次、75次,GFRP复合材料条带和CFRP复合材料条带加固锈蚀钢筋混凝土圆柱的轴向极限荷载分别降低了10.97%、13.37%、16.04%和5.95%、4.66%、4.33%;FRP复合材料加固锈蚀柱的刚度和耗能受侵蚀环境种类、侵蚀环境作用次数、锈蚀率及FRP复合材料种类的影响。在试验研究的基础上,通过理论分析侵蚀环境下混凝土强度损伤系数和锈蚀钢筋强度退化方程,提出了侵蚀环境下FRP复合材料条带加固锈蚀钢筋混凝土圆柱轴心受压承载力计算方法。      

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.     

Deflections of concrete beams reinforced with FRP bars

The aim of this study is to experimentally and theoretically investigate the flexural behavior of concrete beams reinforced with fiber reinforced polymer (FRP) bars. In this research, three types of experiments were made. First, the tensile properties of FRP and steel bars were tested, then the bond-slip behavior between bars and concrete was tested on standard specimens and, in the end, three series of concrete beams reinforced with GFRP, CFRP and steel bars were tested up to failure. The theoretical model for calculating deflections was developed, which included bond-slip behavior of FRP bars. The theoretical results were compared to the test results of beam deflections, as well to deflection results obtained by theoretical models developed by other authors.     

冲击载荷下CFRP及GFRP层板断裂韧性的研究

利用Hopkinson杆加载装置, 对带有单边切口的炭纤维增强复合材料(CFRP)及玻璃纤维增强复合材料(GFRP)层板试件进行冲击拉伸加载实验。根据一维应力波理论求得作用于试件上的载荷P(t)和试件加载点的位移δ(t)。 根据试样中应力随时间的变化历史σ(t), 并基于断裂韧性测试原理, 建立了动态应力强度因子K_Ⅰ (t)响应曲线。利用柔度变化率方法确定起裂时间, 分别得到在两种加载速率下CFRP、 GFRP层板的动态断裂韧性。结果表明, 随着加载速率的提高, 这两种复合材料的断裂韧性降低。      

Investigation of freeze–thaw effects on mechanical properties of fiber reinforced cement mortars

Fibers are used for improving some properties of conventional concrete (which is a brittle material) such as tensile strength, abrasion resistance, absorption and crack control. This study investigates the usability of fibers against the harmful effects of freeze–thaw cycles on cement mortars. For this objective, five different types of fibers, i.e., Polypropylene (PP), Carbon (CF), Aramid (AR), Glass (GF) and Poly vinyl alcohol (PVA) in four different ratios (0.0%, 0.4%, 0.8% and 1.2%) were added to cement mortars along with an amount of air agent. These samples were then subjected to five different freeze–thaw cycles (0, 25, 50, 75 and 100). Thus, mechanical behaviors were investigated under freeze–thaw effects.The most important results of the study are summarized; the fibers increase flexural strength and deflection ability of the samples while decreasing compressive strength, dynamic modulus of elasticity and specific mass. The highest flexural strength was obtained with a 1.2% addition of CF fiber for the samples in normal conditions. The mechanical properties of the samples subjected to repetitive freeze–thaw cycles were also investigated; the best flexural strength was provided with 1.2% CF addition, while the highest dynamic modulus of elasticity was obtained with a 1.2% PP addition.     

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.     

Cryogenic through-thickness tensile characterization of plain woven glass/epoxy composite laminates using cross specimens: Experimental test and finite element analysis

This paper investigates the through-thickness tensile behavior of woven glass fiber reinforced polymer (GFRP) composite laminates at cryogenic temperatures. Tensile tests were carried out with cross specimens at room temperature and liquid nitrogen temperature (77 K), and the through-thickness elastic and strength properties of the woven GFRP laminates were evaluated. The failure characteristics of the woven GFRP laminates were also studied by optical and laser scanning microscopy observations. A three-dimensional finite element analysis was performed to calculate the stress distributions in the cross specimens, and the failure conditions of the specimens were examined. It is found that the cross specimen is suitable for the cryogenic through-thickness tensile characterization of laminated composite materials. In addition, the through-thickness Young's modulus of the woven GFRP composite laminates is dominated by the properties of the matrix polymer in the given temperature, while the tensile strength is characterized by both, the fiber to matrix interface energy and the cohesion energy of the matrix polymer.     

Effect of waste tire cord reinforcement on unconfined compressive strength of lime stabilized clayey soil under freeze–thaw condition

Mechanical properties of fine grained soils, which are subjected to freeze–thaw condition, often change considerably, so when these soils are used as a part of a structure or as an infrastructure, determining a proper solution is necessary. In this paper, stabilization and fiber reinforcement are simultaneously examined as a soil modification method. A series of unconfined compression tests was carried out to investigate the effects of tire cord waste products on mechanical characteristics of a lime stabilized and unstabilized clayey soil subjected to freezing and thawing cycles. Several specimens were prepared at three percentages of lime (i.e. 0%, 4%, and 8%) and four percentages of discrete short nylon fiber (i.e. 0%, 0.5%, 1%, and 1.5%) by weight of dry soil. The samples were saturated and exposed to one up to three freeze–thaw cycles before testing. The results indicated that the compressive strength and stress–strain behavior of specimens depend considerably on the amounts of both fiber and lime. For stabilized specimens, the reinforcement effect of fiber was more than unstabilized ones and also, by inclusion of fiber, 4% lime stabilized specimens indicated more strength in comparison to the untreated and 8% lime stabilized specimens. Furthermore, the contribution of fiber in the strength of samples increased as the number of freeze–thaw cycles was increased.     

Experimental and Analytical Studies on Concrete Cylinders Wrapped with Fiber Reinforced Polymer

Fibre-reinforced polymers (FRPs) are being introduced into a wide variety of civil engineering applications. These materials have been found to be particularly attractive for applications involving the strengthening and rehabilitation of existing reinforced concrete structures. In this paper, experimental investigations and analytical studies on four series of the concrete cylinders wrapped with FRP are presented. First series consist of concrete cylinders wrapped with one layer carbon fiber reinforced polymer (CFRP), second series concrete cylinders wrapped with two layers CFRP, in third series, concrete cylinders were wrapped with one layer glass fiber reinforced polymer (GFRP) and the fourth series consist of concrete cylinders wrapped with two layers of GFRP.The results show that external confinement significantly improves the ultimate strength and ductility of the specimens.Coupon tests have also been carried out to determine the mechanical properties of the FRP. Further, review of three analytical models for confined concrete from the literature is presented in detail. The stress-strain curve of confined concrete in these models consists of a parabolic first portion and a straightline second portion. Predicted stress-strain curve of these models are compared with authors's experimental curves. In predicting the second portion of the stress-strain curve considerable deviation was observed. An analytical model is also proposed for determining the stress-strain relationship of confined concrete.The model is validated by comparing with experimental values. It is observed that the proposed model well predicts the ultimate axial strains and stresses and reproduce finely the stress-strain response of confined concrete with carbon or glass FRP.