预应力超高性能混凝土梁抗剪破坏机理及承载力研究

The formulas in design codes to calculate the shear bearing capacity for prestressed ultra-high performance concrete (UHPC) beams generally yield much lower results than those measured values in experiments. There is no popularly accepted and accurate formulas in present studies. The purpose of this study is to establish a scientific and applicable analytical method and simplified formulas. Based on our previous research on UHPC material and its application in bridge structures, this study will firstly establish the constitutive models for several types of UHPC materials including the high strain strengthening, low strain strengthening and strain softening ones through material tests. The modeling methods for discrete macro cracks and smeared micro cracks during the failure process are the key techniques to be carefully investigated, thereby a refined FEM analytical method for the whole failure process of prestressed UHPC beams under the shear force is to be established. Thereafter, the shear failure mechanism and failure modes are to be theoretically clarified for prestressed UHPC beams. As a step forward, the applicability of various kinds of simplified methods for shear analysis are to be verified and utilized to realize a fast and accurate analysis of the shear bearing capacity after proper optimization. The effects of several parameters including the section dimension, the prestressing level, the reinforcement ratio and so forth are to be investigated through a large number of numerical calculations, thus their influence law on the bearing capacity can be clarified and the key parameters will stand out. Several prestressed UHPC testing beams with rectangular cross sections will be fabricated. Therefore, the actual failure modes are to be observed and the measured values of shear bearing capacity are to be obtained through the shear failure experiments. The refined and simplified analytical methods can thus be improved through the comparison between the calculated and measured values. Finally, a set of design formulas for the shear bearing capacity compromising the key parameters with a moderate safety level can be eventually established. The research results are of important scientific value for the development of the structural design theory and the codes modification for prestressed UHPC beams, which is also of great significance to the popularization of this promising structure in the engineering practice.

目前预应力超高性能混凝土（UHPC）梁抗剪承载力按规范计算结果严重偏低，已有研究缺少公认准确的计算公式。本研究旨在建立科学适用的计算方法和简化公式。基于课题组前期的材料研发及试验研究，首先建立适用于高应变强化、低应变强化、应变软化等多种UHPC材料的本构模型，研发离散型主裂缝和弥散型微裂缝的模拟技术，建立抗剪破坏全过程精细化分析方法，通过典型结构分析成果澄清其破坏机理和失效模式。进而考察现有多种简化分析方法的适用性，优化后实现抗剪承载力的快速、准确分析。据此开展大量数值计算探明结构尺寸、预应力度、配筋率等因素对抗剪承载力的影响规律。通过破坏试验探明实际抗剪失效模式、得到承载力实测值，据此修正精细化和简化分析方法。最后建立一套计入关键参数、具有适当安全度的抗剪承载力计算公式。研究成果对发展预应力UHPC梁的结构设计理论、制定设计规范具有重要的科学价值，对推广其工程应用具有重要意义。

本研究首先通过材料性能试验，建立了UHPC受拉和受压本构模型，进而对25根大尺寸UHPC矩形梁开展了抗剪试验，主要关注UHPC梁的内部受力特点、裂缝发展情况、破坏形态、极限挠度和极限承载力，并通过控制变量法对剪跨比、配箍率、预加力值、纵筋配筋率、UHPC抗拉强度及钢纤维掺量、类型和尺寸共8个关键因素对UHPC梁抗剪性能的影响规律进行了研究。其次，建立了精细化UHPC矩形梁抗剪过程数值模拟分析方法，采用混凝土塑性损伤模型（CDP）计算数值梁拉压损伤变化情况，采用扩展有限元方法（XFEM）计算数值梁中裂缝的产生和开展。该数值模型能模拟UHPC梁中主裂缝的发展和弥散性裂缝的分布，能充分考虑钢纤维和预应力钢筋的作用，与试验现象和主要数据吻合良好。进一步地，对国内外UHPC相关规范抗剪承载力计算方法和基于极限平衡法的抗剪承载力计算方法进行了分析和比较，比较了各方法的计算假定、计算模式和考虑的抗剪贡献因素，总结得出极限平衡法考虑因素全面、计算结果精度好。最后建立了一套UHPC梁抗剪承载力计算公式，该公式能全面考虑UHPC梁各部分抗剪贡献、充分计入钢纤维影响且具有一定富余度，计算结果与本研究试验、文献及相关试验数据吻合良好。成果直接应用于实际工程，取得了良好社会、经济和环保效益。