预应力U形碳纤维带加固持载混凝土梁的抗剪机理及计算模式

In order to overcome the shortcomings of the conventional way for structural strengthening by externally bonding sheets of carbon fiber reinforced polymer (CFRP), a kind of interlocking device for the sheets has been invented and patented. It mechanically anchors the straps simply and effectively by wrapping around paralleled bars without relying on the bond and becoming an eyesore. Such novel devices have been successfully used to strengthen reinforced concrete continuous T-shaped beams with shear deficiency, in which cases debonding is hardly avoidable even when the U-straps of CFRP are at quite low stresses if no reliable measures have been taken for end anchorage. However, it has been found that the strengthening effect and fiber strain at shear failure would decline when there exist pre-loads, especially pre-cracks. Meanwhile a lot of inclined cracks have been observed in the webs of many box-girder bridges in service due to insufficiency of vertical prestresses. In this proposal, the U-straps of CFRP will be prestressed and anchored at ends with the interlockers as well. The prestressing equipment has been devised and the results of preliminary tests are inspiring. Tests and analyses will be carried out on a number of T- beams to study variation of their mechanical behavior with such parameters as the levels of both pro-load and pre-stress. Prestress loss, fiber strain distribution, shear contribution from different parts, and ultimate behavior will be investigated for the strengthened beams to discuss the influence of pre-load, explain shear failure mechanism and establish strength prediction model. Numerical simulation and model experiment will also be done for box-girders in which the operation system is not allowed to penetrate up to the flanges as in actual bridges. The outputs will be the experimental results, analytical solutions and design method to evaluate the load-carrying capacity of the RC beams shear-strengthened with the innovative technique of prestressing and anchoring U-straps of CFRP. The achievements of this project will be beneficial to the efficient utilization and healthy development of the FRP strengthening technique.

对粘贴纤维片材加固法扬长避短，已开发出可不靠胶粘、也不碍观瞻、却能简捷高效锚固纤维布的绕结加固专利技术，并成功应用于原本极易发生剥离的混凝土楼盖连续T梁抗剪加固。却发现持载加固时承载力有下降趋势，且对持载已裂梁的加固效果明显降低，梁剪坏时纤维应变往往不高，同时注意到许多在用箱梁桥因竖向预应力不足产生大量腹板斜裂缝，结构安全堪忧。故拟对加固用绕结U形碳纤维条带施加预应力，现已摸索出实施方案，其探索性试验结果令人鼓舞。本项目先对T梁开展较大规模加固试验和参数分析，探讨加固效果随持载水平等原梁条件及预应力度等加固指标的变化规律，研究预应力损失、纤维应变分布、剪力分配及极限性能，以深入分析裂缝影响、合理解释抗剪机理、提出加固计算模式；再通过数值模拟和验证试验，研究确定预应力绕结U带加固箱梁的技术手段和设计方法。该项目方法兼具原创性和实用性，其应用基础研究成果将促进纤维加固技术的高效应用。

采用纤维增强片材按常规粘贴工艺对钢筋混凝土梁进行抗剪加固时，极易发生界面剥离，持载加固时承载力更低，因而加固后结构安全堪忧。针对这些问题，项目组基于申请人发明的柔性带材绕结自锁法，自主研发出与其配套的U形纤维条带预应力抗剪加固技术。.在应用基础研究过程中，先后以T梁、矩形梁和箱梁为研究对象，采用上述技术手段，通过系统的模型试验、深入的理论分析和/或必要的数值模拟，考察了预应力绕结U形碳纤维（CFRP）条带抗剪加固梁的力学性能，分析了破坏模式和加固机理，研究了持载水平和预应力度等原梁参数或加固指标对加固效果的影响。研究结果表明：相比纯粘贴抗剪加固而言，条带端部带绕结的无粘结和有粘结预应力加固在抑制混凝土梁主斜裂缝形成及发展、抑制梁刚度退化、延缓箍筋屈服和提高箍筋塑性利用率等多个方面都有明显改善。预应力绕结加固能使纤维带得到可靠锚固，避免了端部剥离，中部剥离后拉断破坏仍能发生，大幅度提高了CFRP强度利用率和梁的抗剪承载力。试验梁破坏模式主要有纤维带拉断破坏和混凝土剪压破坏两种。持载对预应力抗剪加固效果有不利影响；配纤率和锚固高度较大时，抗剪承载力提高较多；但并非预应力越大，抗剪加固效果越好。通过综合分析，最终提出了预应力绕结U形CFRP条带抗剪加固混凝土梁的承载力计算公式，其中首次考虑了预应力大小的影响。所提计算模型能够较好预测预应力CFRP的有效应变和加固梁抗剪承载力。.该研究成果能够合理解决有关工程需求，充分发挥碳纤维布的高性能，保证加固效果的可靠性和经济性，为桥梁和建筑结构中混凝土梁的抗剪加固提供了高效实用的新方法，具有重要的推广应用价值。