纤维增强复合材料加固钢筋混凝土单向板受弯性能研究

通过纤维增强复合材料（FRP）布加固钢筋混凝土单向板的受弯性能试验,研究了FRP布种类、层数、宽度、粘贴方式和锚固措施等因素对加固效果的影响。结果表明：在板底粘贴CFRP布和高强GFRP布均可明显提高钢筋混凝土单向板的受弯承载力和纵筋屈服后刚度。当采用横向粘贴1层U形CFRP布条作为端部锚固措施时,所有加固单向板试件的破坏模式均为跨中弯曲裂缝引起的剥离破坏。当FRP布宽度和层数相同时,采用CFRP布加固效果优于高强GFRP布加固。与对比试件相比,FRP布加固钢筋混凝土单向板试件的位移延性略有降低。高强GFRP布加固钢筋混凝土单向板的位移延性优于CFRP布加固钢筋混凝土单向板。FRP布的粘贴方式对FRP加固钢筋混凝土单向板的位移延性也有影响,单层FRP布加固单向板试件的延性较好。通过对试验结果的分析,提出了FRP布加固钢筋混凝土单向板构件受弯破坏模式的判别方法,验证了已有文献和GB 50608-2010《纤维增强复合材料建设工程应用技术规范》中给出的跨中弯曲裂缝引起的FRP剥离应变计算公式对FRP布加固混凝土单向板的适用性,建立了FRP布加固钢筋混凝土单向板受弯承载力计算方法。     

Correlations among mechanical properties of steel fiber reinforced concrete

In this paper, applicability of previously published empirical relations among compressive strength, splitting tensile strength and flexural strength of normal concrete, polypropylene fiber reinforced concrete (PFRC) and glass fiber reinforced concrete (GFRC) to steel fiber reinforced concrete (SFRC) was evaluated; moreover, correlations among these mechanical properties of SFRC were analyzed. For the investigation, a large number of experimental data were collected from published literature, where water/binder ratio (w/b), steel fiber aspect ratio and volume fraction were reported in the general range of 0.25–0.5, 55–80 and 0.5–2.0%, respectively, and specimens were cylinders with size of Φ 150 × 300 mm and prisms with size of 150 × 150 × 500 mm. Results of evaluation on these published empirical relations indicate the inapplicability to SFRC, also confirm the necessity of determination on correlations among mechanical properties of SFRC. Through the regression analysis on the experimental data collected, power relations with coefficients of determination of 0.94 and 0.90 are obtained for SFRC between compressive strength and splitting tensile strength, and between splitting tensile strength and flexural strength, respectively.     

Post-crack (or post-peak) flexural response and toughness of fiber reinforced concrete after exposure to high temperature

Several studies have already reported on the various effects of high temperature on the mechanical properties of fiber reinforced concrete (FRC). Some of these effects include changes in; compressive strength, compression toughness and splitting tensile strength. None of the previous studies have investigated the changes that might occur on the post-crack flexural response and flexural toughness. Post-crack (or peak) response and toughness is considered one of FRC’s key beneficial characteristics – as the purpose of adding fibers is to increase the energy absorption and load carrying capacity after an initial crack. In this study, the flexural toughness test according to ASTM C1018 was carried out on two types of concrete: plain concrete and fiber reinforced concrete with three different types of fiber (steel, polypropylene, and polyethylene) at 0.5% and 1.0% by volume fractions. Prior to the flexural test, the specimens were put in an oven chamber and subjected to high temperatures using the ISO/TR834 standards of: 400 °C, 600 °C and 800 °C. The results showed the typical load–deflection response of FRC was a double-peak response. The first peak represented the properties of concrete matrix and the second peak represented the properties of the fibers used. Under flexural load, instead of dropping (or remaining unchanged), the post-peak load and the toughness were found to increase at lower temperatures (400 °C) and later, decreased as the temperature increased (600 °C and 800 °C). Fiber type and content also played an important role. At a temperature of 400 °C, all FRCs exhibited higher flexural strength and increased post-peak response and toughness. A significant decrease in strength, toughness and load–deflection response was observed with synthetic or plastic FRC (PFRC) when the temperature approached 800 °C. When steel FRC (SFRC) was used, those effects were relatively small. It appears, SFRC has better heat resistance than the PFRC. The density (measured by ultrasonic pulse velocity) was found to decrease more in the PFRC than in the SFRC.     

Composite tubes as an alternative to steel spirals for concrete members in bending and shear

Glass fibre-reinforced polymer (GFRP) tubes are compared to steel spiral reinforcement in circular concrete members with longitudinal reinforcement and prestressing, using six beam tests. Two 324 mm diameter and 4.2 m long prestressed specimens were tested in bending. Four 219 mm diameter reinforced specimens were also tested, including two 2.43 m long beams tested in bending and two 0.6 m long beams tested in shear. In each set, one specimen was essentially a concrete-filled GFRP tube, while the other control specimen included steel spiral reinforcement of comparable hoop stiffness to that of GFRP tube. The strength of control specimens was governed by crushing and spalling of concrete cover. Unlike spiral reinforcement, GFRP tubes confined larger concrete areas and also contributed as longitudinal reinforcement, leading to increases in flexural and shear strengths, up to 113% and 69%, respectively.     

Experimental and finite element analysis on the steel fiber-reinforced concrete (SFRC) beams ultimate behavior

Steel fiber-added reinforced concrete (SFRC) applications have become widespread in areas such as higher upper layers, tunnel shells, concrete sewer pipes, and slabs of large industrial buildings. Usage of SFRC in load-carrying members of buildings having conventional reinforced concrete (RC) frames is also gaining popularity recently because of its positive contribution to both energy absorption capacity and concrete strength.This paper presents experimental and finite element analysis of three SFRC beams. For this purpose, three SFRC beams with 250 × 350 × 2000 mm dimensions are produced using a concrete class of C20 with 30 kg/m³ dosage of steel fibers and steel class S420 with shear stirrups. SFRC beams are subjected to bending by a four-point loading setup in certified beam-loading frame, exactly after having been moist-cured for 28 days. The tests are with control of loads. The beams are loaded until they are broken and the loadings are stopped when the tensile steel bars are broken into two pieces. Applied loads and mid-section deflections are carefully recorded at every 5 kN load increment from the beginning till the ultimate failure.One of the SFRC beams modeled by using nonlinear material properties adopted from experimental study is analyzed till the ultimate failure cracks by ANSYS. Eight-noded solid brick elements are used to model the concrete. Internal reinforcement is modeled by using 3D spar elements. A quarter of the full beam is taken into account in the modeling process.The results obtained from the finite element and experimental analyses are compared to each other. It is seen from the results that the finite element failure behavior indicates a good agreement with the experimental failure behavior.     

FRP加固RC双向板受力全过程的非线性分析

本文通过有限元法模拟和模型试验,对外贴玻璃纤维条带加固钢筋混凝土双向板的受弯性能进行了非线性全过程分析,研究了加固板的挠度以及纤维应变随荷载变化规律,对玻璃纤维条带加固RC双向板的受力机理进行了分析。分析结果表明,通过合理选择有限元数值模型,可以较好地模拟外贴玻璃纤维条带加固RC双向板的受弯性能。分析结果和试验结果均表明外贴玻璃纤维条带可以有效地提高双向板的抗弯刚度、开裂荷载和极限承载力。     

Flexural strengthening of RC beams with prestressed NSM CFRP rods – Experimental and analytical investigation

The effectiveness of strengthening reinforced concrete (RC) beams with prestressed near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) rods was investigated. Four RC beams (254 mm deep by 152 mm wide by 3500 mm long) were tested under monotonic loading. One beam was kept un-strengthened as a control beam. One beam was strengthened with a non-prestressed NSM CFRP rod. Two beams were strengthened with prestressed NSM CFRP rods stressed to 40% and 60% of the rod’s ultimate strength. The test results showed that strengthening with non-prestressed NSM CFRP rod enhanced the flexural response of the beam compared to that of the control beam. A remarkable improvement in the response was obtained when the RC beams were strengthened with prestressed (40% and 60%) NSM CFRP rods. An increase up to 90% in the yield load and a 79% in the ultimate load compared to those of the control beam were obtained. An analytical model was developed using sectional analysis method to predict the flexural response of RC beams strengthened with prestressed NSM CFRP rods. The proposed model showed excellent agreement with the experimental results.     

Damaged RC beams repaired by bonding of CFRP laminates

This paper summarizes the results of experimental studies on damaged reinforced concrete beams repaired by external bonding of carbon fiber reinforced polymer (CFRP) composite laminates to the tensile face of the beam. Two sets of beams were tested in this study: control beams (without CFRP laminates) and damaged and then repaired beams with different amounts of CFRP laminates by varying different parameters (damage degree, CFRP laminate width, concrete strength class). All beams were tested in four-point bending over a span of 1800 mm. The tests were carried out under displacement control. The most investigated parameter in this experimental study is damage degree (ratio between pre-cracked load and load capacity of control beam). Repairing damaged RC beams with externally bonded CFRP laminates were successful for different degrees of damage. The observed failure modes were peeling off and interfacial debonding. These failure modes depend only on the laminate width.The results indicate that the load capacity and the rigidity of repaired beams were significantly higher then those of control beam for all tested damage degrees. The authors remarked that for a load capacity improvement, reinforcement with a CFRP having about a half width of the beam is satisfactory. Finally, the contribution of CFRP laminates on the load capacity and rigidity of repaired RC beams is significant for any concrete strength class.     

Testing of full-scale concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars

This paper presents an experimental study investigating the behavior of FRP-reinforced concrete bridge deck slabs under concentrated loads. A total of eight full-scale deck slabs measuring 3000-mm long by 2500-mm wide were constructed. The test parameters were: (i) slab thickness (200, 175 and 150 mm); (ii) concrete compressive strength (35–65 MPa); (iii) bottom transverse reinforcement ratio (1.2–0.35%); and (iv) type of reinforcement (GFRP, CFRP, and steel). The slabs were supported on two parallel steel girders and were tested up to failure under monotonic single concentrated load acting on the center of each slab over a contact area of 600 × 250 mm to simulate the footprint of sustained truck wheel load (87.5 kN CL-625 truck). All deck slabs failed in punching shear. The punching capacity of the tested deck slabs ranged from 1.74 to 3.52 times the factored load (P_f) specified by the Canadian Highway Bridge Design Code (CHBDC) CAN/CSA S6-06. Besides, the ACI 440.1R-06 punching strength equation greatly underestimated the capacity of the tested slabs with an average experimental-to-predicted punching capacity ratio (V_exp/V_pred) of 3.17.     

Effect of fiber type and content on bleeding of steel fiber reinforced concrete

In general, the addition of fibers in concrete mix significantly improves many of the engineering properties of concrete. On the other hand, steel fibers reduce the workability of concrete. This paper presents the effect of steel-fiber length (aspect ratio) and content on bleeding of steel fiber reinforced concrete (SFRC). Two different steel fiber types (both is hooked-end) were used at a ratio of 0% (control), 0.3%, 0.64%, 1% and 1.3% by volume. Slump, Ve-Be test, air content and unit weight were determined experimentally. Specimens were poured in the standard moulds and the bleeding water content was measured 30 min, 60 min, 90 min, 120 min, 150 min and 180 min after starting the test. The results indicated that the workability of concrete significantly reduced as the fiber dosage rate increases. This was assessed through standard slump test and Ve-Be consistometer test. The bleeding water content was increased by increase of the fiber volume fraction and fiber aspect ratio according to experimental results. Also, a bleeding coefficient value for SFRC made with and without steel fiber was proposed as a result of this study.     

碳纤维增强复合材料修复震后玻璃纤维增强复合材料管钢筋混凝土桥墩柱抗震性能试验

为了研究碳纤维增强复合材料（CFRP）修复震后玻璃纤维增强复合材料（GFRP）管混凝土桥墩柱的抗震性能,对4根CFRP修复震后破坏GFRP管混凝土桥墩柱进行拟静力试验,对比修复后钢筋混凝土桥墩柱与原型桥墩柱的滞回曲线、骨架曲线、延性和耗能能力。试验结果表明,修复后的墩柱承载力和延性指标均能得到较好的恢复甚至提高。     

Creep effect on the behavior of concrete beams reinforced with GFRP bars subjected to different environments

The effect of different environmental conditions on the creep behavior of concrete beams reinforced with glass fiber reinforced polymer (GFRP) bars under sustained loads is investigated. This is achieved through testing concrete beams reinforced with GFRP bars and subjected to a stress level of about 20–25% of the ultimate stress of the GFRP bars. Reference beams were loaded in the temperature-controlled laboratory (24 ± 3 °C). Other test beams were either completely or partially immersed in different environments (tap-water and sea-water) at elevated temperature (40 ± 2 °C) to accelerate the reaction. During the exposure period, which lasted for ten months, strains in concrete and GFRP bars as well as the midspan deflections were recorded for all considered environmental conditions. The results show that the creep effect due to sustained loads was significant for all environments considered in the study and the highest effect was on beams subjected to wet/dry cycles of sea-water at 40 ± 2 °C.     

Hybrid concrete and pultruded-plank slabs for highway and pedestrian bridges

Studies on two novel uses of hybrid structural members consisting of commercially produced glass reinforced pultruded ribbed fiber reinforced polymer (FRP) planks and concrete are discussed in this paper. Pultruded planks are produced by all the major pultruders in the world and are utilized primarily as decking for platforms. These highly optimized panels have the potential to be used in many other infrastructure applications, but their flexural stiffnesses have generally been too low to be used in highway and pedestrian bridges due to current span requirements. However, when used “compositely” with concrete or cementitious materials in a hybrid form they have the potential to be much more widely used. Two research studies conducted on two possible hybrid systems of different structural depths are discussed in this paper. The first study describes the use of pultruded planks as permanent formwork in highway bridge decks where the plank is used with concrete to produce a solid slab of 200 mm depth that is typical of slabs seen in highway bridge decks. The second study describes the use of pultruded planks in pedestrian bridge decks where the pultruded plank is used with a cement-board or a cast-in-place concrete panel to produce a hollow slab of 75 mm depth that is typical of timber decking used in FRP pedestrian bridges. Tests were conducted on beam-type specimens of the hybrid slabs to investigate the load transfer mechanisms between the pultruded plank and the cementitious “overlays” for both the 75 mm and 200 mm depths. From analysis of the load-carrying capacity and failure mechanisms of the hybrid slabs it was concluded that such hybrid slabs are viable systems for both highway and pedestrian bridge decks. A bridge deck using the 200 mm deep hybrid slab system was recently constructed on a highway in Wisconsin, USA.     

Creep of concrete masonry walls strengthened with FRP composites

The objective of the research was to examine the creep behavior of masonry walls strengthened with FRP composites compared to that of conventional reinforcement. Eight full-scale (40 in wide by 96 in tall [1.02 m × 2.44 m]) unreinforced concrete masonry walls were constructed for testing long-term deflections out-of-plane. The walls were strengthened with externally bonded CFRP or GFRP composites. Two additional walls were constructed with mild steel reinforcement grouted in the center cell of the specimens. Long-term deflections due to creep in FRP reinforced walls were shown to be ≈22–56% higher than those of steel reinforced walls.     

CFRP strengthened openings in two-way concrete slabs – An experimental and numerical study

Rehabilitation and strengthening of concrete structures with externally bonded fibre reinforced polymers (FRPs) has been a viable technique for at least a decade. An interesting and useful application is strengthening of slabs or walls where openings are introduced. In these situations, FRP sheets are very suitable; not only because of their strength, but also due to that they are easy to apply in comparison to traditional steel girders or other lintel systems. Even though many benefits have been shown by strengthening openings with FRPs not much research have been presented in the literature.In this paper, laboratory tests on 11 slabs with openings, loaded with a distributed load are presented together with analytical and numerical evaluations. Six slabs with openings have been strengthened with carbon fibre reinforced polymers (CFRPs) sheets. These slabs are compared with traditionally steel reinforced slabs, both with (four slabs) and without openings (one slab). The slabs are quadratic with a side length of 2.6 m and a thickness of 100 mm. Two different sizes of openings are used, 0.85 × 0.85 m and 1.2 × 1.2 m.The results from the tests show that slabs with openings can be strengthened with externally bonded CFRP sheets. The performance is even better than for traditionally steel reinforced slabs. The numerical and analytical evaluations show good agreement with the experimental results.     

初始损伤对CFRP加固混凝土梁受弯性能的影响

通过9根碳纤维布加固的具有初始损伤钢筋混凝土梁、6根碳纤维布直接加固钢筋混凝土梁及3根对比钢筋混凝土梁的抗弯性能试验，分析了初始荷载、初始裂缝及加载历史等对加固梁裂缝发展、屈服荷载、刚度、极限荷载的影响。试验结果表明，采用碳纤维布加固钢筋混凝土梁可以有效的提高其抗弯性能。初始荷载、初始裂缝及加载历史等对加固梁的裂缝发展、刚度、屈服荷载和极限荷载均有不同程度的影响。     

Evaluation of the transfer length of prestressed near surface mounted CFRP rods in concrete

The transfer length of a prestressed near surface mounted (NSM) fiber reinforced polymer (FRP) rod is the distance over which the rod must be bonded to the epoxy to develop the prestressing force in the rod. The transfer length is intended to provide bond integrity for the strengthened concrete member. This paper presents experimental results and an empirical equation to estimate the transfer length of prestressed NSM Carbon FRP (CFRP) rod in concrete beams. Twenty-two reinforced concrete specimens were strengthened with NSM CFRP rods. Two types of CFRP rods were used: spirally wound and sand blasted rods. Four prestressing levels were used: 40%, 45%, 50% and 60% of the tensile strength of the CFRP rod. The strain behavior in the CFRP rod was monitored by gauges mounted on the CFRP rod along the length of the beam. The test results showed that the transfer length of the prestressed NSM CFRP rod was about 35 times the diameter of the CFRP rod. The maximum bond stress of the CFRP rod in epoxy was found to range from 11 to 16 MPa for the sand blasted rods and from 12 to 23 MPa for the spirally wound rods. An empirical expression based on curve fitting of the measured data was proposed to predict the prestressing stress in the CFRP rod along the length of the beam.     

Durability performance of RC beams strengthened with epoxy injection and CFRP fabrics

Cracks in reinforced concrete (RC) should be repaired if they present the potential for durability related problems such as corrosion of reinforcing steel. One way to repair extensive cracks is the use of epoxy injection. Another repair technique to enhance shear or flexural strength in deficient RC members is the utilization of externally bonded carbon fiber reinforced polymer (CFRP) fabrics. The effect of environmental conditioning on crack injection with or without CFRP strengthening is of interest in this investigation. Test results showed that the crack injection provided an increase in initial stiffness for un-strengthened RC beams. An increase in initial stiffness and ultimate strength was achieved in CFRP strengthened RC beams. Surface roughness combined with crack injection significantly increased the flexural capacity of the specimens. Environmental conditioning significantly affected the bond performance of the epoxy injection. The presence of sustained load during environmental conditioning resulted in reduced section capacity and ductility.     

外贴FRP条带加固RC双向板的试验研究及承载力计算

对3组9个简支外贴纤维条带加固RC双向板进行试验研究。外贴纤维条带采用均匀布置和跨中板带加密布置两种粘贴方式。试验中考察了加固板的抗裂性能、变形能力、刚度、裂缝分布等受力性能,对外贴玻璃纤维条带加固RC双向板极限状态时的破坏特征和极限承载能力进行了重点研究。研究结果表明:外贴纤维条带加固能有效延缓裂缝的出现、抑制裂缝的开展、改善结构的抗裂性能及提高结构的极限承载力,加固板的刚度也有一定的提高,但延性略有降低。通过对试验数据的回归,提出双向受力作用下,纤维剥离应变计算公式,分析普通板和纤维加固板板单元截面抵抗弯矩的区别,探讨外贴纤维条带对双向板极限承载力的贡献以及纤维加固双向板板单元截面抵抗弯矩的取值。采用屈服线理论,建立纤维加固板的极限承载力计算公式,计算结果与试验值吻合较好。研究成果可供实际加固改造工程参考。     

基于改性磷酸盐水泥的CFRP网格加固钢筋混凝土板的受弯试验

采用碳纤维(Carbon Fiber Reinforced Polymer,CFRP)网格增强无机改性磷酸盐水泥基材料(Magnesium Phosphate Cementitious,简称MPC)加固钢筋混凝土板,MPC是对磷酸盐水泥在配合比和矿物掺合料两个方面进行改性优化,该加固方法充分利用了MPC具有早期强度高、低收缩、与旧混凝土黏结力强、良好的耐久性、防火防老化等优点。主要开展了CFRP网格增强MPC水泥基体加固混凝土板的抗弯试验,试验结果表明,破坏模式为混凝土板与加固层界面的剥离破坏;加固后板的开裂荷载提高了133%,极限荷载提高了108%。CFRP网格增强无机磷酸盐水泥基体复合材料加固技术不仅能够有效地提高混凝土板的抗弯承载力,还可以提高构件的刚度和抗裂性能。