FRP筋加固的混凝土结构

纤维增强聚合物(FRP)筋具有强度比高、电中性、重量轻、操作方便、耐腐蚀等优点，在土木工程中被广泛用于代替钢筋。随着生产技术的日益成熟和产业化，FRP已成为一种经济、有竞争力的结构材料。本文在总结国内外研究成果的基础上，重点介绍了FRP筋加固混凝土结构的研究进展。本文的研究内容包括FRP筋在混凝土中的粘结性能、抗压性能、抗弯性能以及近年来国际上FRP筋加固混凝土结构的延性。关键词:FRP筋、混凝土结构、粘结性能、拉拔性能、压缩性能、弯曲性能和延性。

因钢筋锈蚀引起的基础设施锈蚀已成为全球[1]面临的重大挑战。使用FRP筋作为混凝土构件的加固，似乎是克服传统钢筋混凝土结构因钢筋锈蚀而产生耐久性问题的有效方法。因此，用FRP筋代替钢在世界范围内越来越受欢迎。它具有强度重量比高、电磁中性、重量轻、操作方便、无腐蚀、重量强度比低(钢密度的1/5 ~ 1/4倍)、纵向抗拉强度高、无磁性等优点。虽然FRP加固的初始成本比钢筋高，但FRP加固的结构或构件的全寿命周期成本较低，因为FRP加固的结构或构件的维修成本要低得多。FRP筋在土木工程中的应用可分为两类。一是在混凝土结构中替代钢筋，二是对旧结构进行维护加固。近年来，随着FRP材料技术的发展，越来越多的学者开始关注FRP的应用研究工作。本文主要介绍了FRP筋加固混凝土结构的研究进展。本文的研究内容主要包括FRP筋在混凝土中的粘结性能、抗剪性能、抗弯性能、抗压性能和延性- FRP筋与混凝土之间的粘结强度及其影响因素:- FRP筋与混凝土之间的粘结应力传递机理已被广泛研究。粘结应力是与FRP筋与混凝土界面平面平行的剪应力。埋入钢筋的粘结，无论材料如何，通过三种主要的机制来抵抗拔出。首先是两种材料在界面上的化学粘附。第二种是摩擦键，它是由于棒材表面的粗糙造成的。第三种作用于粘结的机制是机械承重，如周围混凝土[4]上钢筋的凸耳所产生的机械承重。通过对FRP筋加固混凝土的研究，将影响粘结强度的因素分为以下几类:FRP筋在混凝土中的粘结性能是影响其力学性能、破坏形式、使用性能、裂缝宽度、变形和结构分析设计的主要因素，是FRP筋在混凝土中的基本力学性能。

FRP筋加固混凝土梁的抗弯性能对FRP筋混凝土梁的抗弯性能进行了试验研究，并与普通钢筋混凝土梁的抗弯性能进行了比较。Rafi等人研究了碳纤维增强聚合物(CFRP)钢筋混凝土梁和普通钢筋混凝土梁的抗弯性能，并对两者的结果进行了比较。试验结果表明，碳纤维布加固混凝土在许多方面与普通混凝土相似。CFRP混凝土梁与钢筋混凝土之间的粘结良好，没有粘结破坏或滑移的迹象。另一方面，钢筋混凝土梁在几乎两倍的荷载下由于混凝土破碎而破坏，而钢筋混凝土梁则由于钢筋屈服而破坏的长期弯曲行为的混合动力系统是由连续纤维增强复合材料(FRP)钢筋和纤维-钢筋混泥土(FRC)被王调查和Belarbi[10],结果表明,极限抗弯强度经验的轻微减少暴露在环境条件相结合,包括冻融循环、高温(60°C),和除冰盐的解决方案。混凝土的劣化可能是混凝土抗弯强度劣化的主要原因。GFRP的行为(玻璃纤维增强聚合物)RC梁与不同的配筋率和混凝土强度的百分比在静态和冲击荷载下的调查Goldston等。[3],结果显示,6个GFRP RC梁在静载荷测试,其余六GFRP RC梁在冲击荷载测试(使用落锤)。研究其破坏模式和相关的能量吸收能力。研究结果表明，与静弯矩相比，钢纤维混凝土梁的动弯矩提高了15-20%，配筋率和混凝土强度对钢纤维混凝土梁的受力性能有较大影响。他们提出了一个修复方案，用GFRP筋代替钢筋混凝土梁的受拉钢筋，GFRP筋是一种不受腐蚀的材料。按照作者的观察,恢复与GFRP梁酒吧表现出双线性行为,直到失败的混凝土简如预期自参考钢筋梁的延性性能是不可能复制由于GFRP材料线性弹性性质,直到失败。Hosen et. al.[11]进行了一项试验研究，研究了用SNSM(侧向近表面安装)-GFRP(玻璃纤维增强聚合物)加固钢筋混凝土梁的性能。研究结果表明，采用侧近表面贴装的SNSM -GFRP筋加固后，开裂和极限荷载分别提高了4.38倍和1.55倍。使用GFRP作为SNSM加固，在荷载-挠度行为中表现出三线性响应，并降低了试件在任何荷载水平下的挠度，这将解决正常使用问题。

Fiber reinforced polymer (FRP) bars have been widely used in civil engineering used as a substitute for steel reinforcement because it has many advantages such as high strength-to-weight ratio, electromagnetic neutrality, light weight, ease of handling and no corrosion. Moreover, the productive technology becomes more and more mature and industrialized so that FRP has become one economic and competitive structure material. Based on the recent researches, this paper mainly introduces progress in the studies on concrete structures reinforced with FRP bars. These contents in this paper includes the bond performance of FRP bars in concrete, Compression Behavior, flexural behavior, and ductility of concrete structure reinforced with FRP bars in the past few years in the world.

Infrastructure decay due to corrosion of embedded reinforcing steel stands out as a significant challenge worldwide [1]. The use of FRP bars as reinforcement for concrete elements seems to be an effective solution for overcoming durability problems of traditional steel reinforced concrete structures due to the corrosion of metallic bars. For this reason, the replacement of steel with FRP bars is gaining popularity worldwide [2]. It has many advantages such as high strength-to-weight ratio, electromagnetic neutrality, light weight, ease of handling, no corrosion, low weight to strength ratio (1/5 to 1/4 times of the density of steel), high longitudinal tensile strength, and non-magnetic characteristics. Although the initial cost of FRP reinforcement is higher than steel reinforcement, the total life cycle cost of the structure or structural components reinforced with FRP is lower, as significantly less maintenance costs are required for structures or structural components reinforced with FRP [3]. The application of FRP bars in civil engineering can be divided into two classes. One is to substitute steel bars in concrete structures, and the other one is to maintain and strengthen old structures. In the past few years, with the development of FRP material technique, more and more scholar began to focus on the application research work on FRP. This paper mainly introduces progress in the studies on concrete structures reinforced with FRP bars. These contents in this paper include the bond performance of FRP bars in concrete, shear resistance, flexural behavior, compressive behaviour and ductility of concrete structure reinforced with FRP bars in the past few years in the world

Bond strength and its factors: – The mechanics of bond stress transfer between FRP reinforcement and concrete has been investigated extensively. Bond stress is the shearing stress whose direction is parallel to the interface plane of FRP bars and concrete. The bond of an embedded bar, regardless of material, resists pull-out via three main mechanisms. The first is chemical adhesion between the two materials at their interface. The second is the friction bond which is due to coarseness in the surface of the bar. The third mechanism contributing towards the bond is mechanical bearing, such as that generated from the lugs on reinforcing bars upon the surrounding concrete [4]. Based on the studies on concrete reinforced with FRP bars, the factors that influence the bond strength can be divided to several classes below:

The bond performance of FRP bars in concrete, which is the basic mechanical behavior, is the main factor of the mechanical performance, failure mode, serviceability, crack width, deformation and structure analyses and design.

Flexural behavior of Reinforced concrete beams with FRP bars
Several experimental studies were conducted to investigate the flexural behavior of FRP reinforced concrete beams and comparison with that of steel reinforced concrete beams. Rafi et. al. [9] investigated flexural behavior of CFRP (Carbon fiber reinforced polymer) reinforcement RC beams and normal RC beams and compared the results of both. Test results show that the structural behaviors of CFRP reinforced concrete are similar to normal RC in many aspects. The CFRP RC beams displayed good bond between the reinforcement bars and concrete, with no signs of bond failure or slip. Beams failed due to concrete crushing at almost double the loads on the other hand steel RC beams failed due to steel yielding. The long-term flexural behaviors of a hybrid system consisted of continuous fiber-reinforced-polymer (FRP) rebar and fiber- reinforced-concrete (FRC) were investigated by the Wang and Belarbi [10], and the results shows that the ultimate flexural strength experienced minor reduction when exposed to combined environmental conditioning, including freeze–thaw cycles, high temperature (60°C), and de-icing salt solution. The degradation of concrete may be the main reason for the flexural strength degradation. The behaviour of GFRP (Glass fiber reinforced polymer) RC beams with different percentages of reinforcement ratio and concrete strengths under static and impact loading were investigated by the Goldston et. al. [3], the results reveals that the six GFRP RC beams were tested under static loading and the remaining six GFRP RC beams were tested under impact loading (using a drop hammer). To examine the failure modes and associated energy absorption capacities. Author reported that the 15-20% higher dynamic moment capacities compared to static moment capacities and Reinforcement ratio and the strength of concrete influenced the behaviour of GFRP RC beams. Escorcio and Franca [18] presented a rehabilitation solution to replace the tension steel reinforcement of a RC beam with GFRP bars, which is a material immune to corrosion. As per the author observation that the rehabilitated beams with GFRP bars exhibited a bilinear behaviour until failure in terms of load-deflection as expected since the ductile performance of the reference beam with steel reinforcement is not possible to replicate due to the GFRP material linear elastic property until failure. An experimental study was conducted by Hosen et. al. [11] to investigate the performance of RC beams strengthened with SNSM (side near-surface mounted)-GFRP (Glass fiber reinforced polymer) bars. The results of this study showed that the Strengthening using SNSM (side near-surface mounted) -GFRP bars enhanced the first crack and ultimate loads up to 4.38 and 1.55 times compared with the control specimen. The use of GFRP as an SNSM reinforcement has exhibited a tri-linear response in load-deflection behavior and reduced the deflection at any load level of the specimens, which would address the serviceability concerns.