大直径钢绞线预应力先张梁的粘结机制与性能研究

Undesirable grouting and rusted strands severely threaten the durability of post-tensioned concrete girders. In contrast, pretensioned concrete girder is attractive due to its low maintenance cost and reliable quality. In recent years, larger strands were used in engineering. Using large strand may significantly increase span length when the strands number is the same, while application of large strand leads to desirable strands layout and efficient tension; thus, application of large strand will contribute to decrease of cost and time. However, inappropriate design of pretensioned concrete girder using large strand may result in concrete crushing and splitting, and anchorage failure. Bond between prestressed strand and concrete refers to prestress transfer and development. Transfer length and development length are two important bond parameters, and they are critical sections for strength calculation. Inaccurate prediction results in irrational and unsafe design of pretensioned concrete girder. No equation for calculation of transfer length and development length of large strand is provided in current specifications. Moreover, no quantitative analysis of bond mechanism is available. This project will aim at bond of pretensioned concrete girder using 17.8mm-diameter large strand. Transfer bond and flexural bond within transfer length and development length will be studied. The effect of strand diameter and prestress level on transfer length and development length will be investigated. Through pull-out test of specimens using untensioned strand and specimens using prestressing strand with only one Hoyer effect, beam flexural test, and finite element simulation, qualitative and quantitative analysis of bond mechanism and behavior will be conducted.

预应力后张梁因压浆不密实与孔道内预应力筋锈蚀而影响耐久性，而先张梁维护费用较低，易于保证工程质量。采用同等数量大直径钢绞线的先张梁跨径将得到大幅提高，相同跨径时采用大直径钢绞线则更利于布束和张拉，从而减少造价，缩短工期。然而，采用大直径钢绞线先张梁的不恰当设计可能导致混凝土的压碎、劈裂及锚固失效。预应力筋与混凝土间的粘结涉及梁内预应力的传递与发展。传递长度和锚固长度是反映粘结性能的重要指标，是承载力计算的关键截面，不准确的计算会导致先张梁设计的不合理和不安全。但目前国内外规范并未给出大直径钢绞线传递长度和锚固长度计算公式，更缺乏对粘结机制的定量分析。本课题将研究17.8mm大直径钢绞线传递长度和锚固长度内的传递粘结与弯曲粘结，分析钢绞线直径、预应力水平的影响，并基于非张拉钢绞线的拔出试验、仅含一端Hoyer效应的预应力筋拔出试验以及梁弯曲试验，结合有限元模拟，开展粘结性能和粘结机制研究。

课题基于有限元分析、传递长度和锚固长度试验、拔出试验分析，完成了大直径钢绞线预应力先张梁的粘结机制与性能研究。18mm钢绞线的拔出试件考虑了钢绞线埋置长度与预应力水平两个主要参数的影响。通过对比埋置长度相同的未预张拉和预张拉试件的试验结果，分析了两种拔出试件的粘结性能差异，包括失效模式、钢绞线自由端滑移与拔出力的关系、初始的P-S曲线斜率、混凝土表面应变分布以及拔出功等，从而反映了两种试件中粘结机制的本质区别。试验结果证明钢绞线传递长度与有效预应力水平有关，有效预应力水平更高，其传递长度更长。基于提出的D值比这一参数，首次定量分析传递长度范围内Hoyer效应的贡献度达到了90%，从而证实Hoyer效应是先张梁梁端的主要粘结机制。