无粘结预应力U形CFRP加固混凝土梁抗剪试验

针对外贴纤维布抗剪加固钢筋混凝土梁极易发生纤维剥离破坏、纤维利用率低且被动受力等问题,开发了一套U形纤维带预应力系统,包括侧面端部自锁锚固装置、角钢和底部连接及张拉装置.为验证该系统的可行性及探讨碳纤维带预应力大小对抗剪加固效果的影响,完成了5个较大截面矩形梁段试件的单点加载抗剪试验,其中1个未加固,4个采用无粘结预应力U形碳纤维带进行抗剪加固.试验结果表明:该系统能给U形纤维带提供可靠预应力,并把常见的纤维剥离破坏转变为拉断破坏,大大提高纤维利用率;施加预应力的纤维带对梁裂缝的产生与发展及开裂后梁刚度的退化均有明显的抑制作用;预应力大小对主斜裂缝倾角及直接参与抗剪的纤维带数量影响较大,但对这些纤维带的有效应变影响不大;并非预应力越大,抗剪加固效果越好,本试验中,仅将纤维带预应变张拉至0.001就能最大幅度提高梁的抗剪承载力.基于试验结果分析,从考虑竖向平均预应力对主斜裂缝倾角的影响入手,提出了无粘结预应力U形碳纤维带加固混凝土梁抗剪承载力公式,其计算值与试验值符合良好.

Shear tests of RC beams strengthened with unbounded prestressed U-straps of CFRP

A novel set of prestressing system for U-straps of FRP was developed for shear strengthening of RC beam to restrain debonding failure, realize active reinforcement and achieve efficient utilization of FRP.It included self-locking anchorage, angle steel and device for connecting and tensioning. The single point loading tests of 5 RC beams with full-sized rectangular section were carried out, among which one remained unstrengthened, and the other four were strengthened in shear using unbonded prestressed U-straps of CFRP.The feasibility of the technique was verified and the influence of prestress on the behavior of RC beams shear-strengthened with unbounded prestressed CFRP U-straps was discussed.The experiments show that the system can provide reliable prestress for U-straps of FRP and transform the conventional debonding failure of FRP to the tensile failure.Prestressing can obviously inhibit the occurrence and development of main diagonal crack and the stiffness degradation after cracking. The level of prestress has a significant influence on the inclination angle of the main diagonal crack and the number of U-straps of FRP directly involved in shear resistance,but it has a little effect on the effective strain of these fibers.But the shear-strengthening effect cannot keep becoming better with the increase of the prestress.In this experiment, the maximum shear capacity of the strengthened beams is reached with a prestrain of 0.001.Based on the test data, a new expression was proposed for shear capacity of the strengthened beams using unbounded prestressed U-straps of CFRP, in which the influence of the vertical average prestress on the inclination of the main diagonal crack was considered.The predicted capacities are in accordance with the test results.