为探明超高性能混凝土(UHPC)的断裂性能与机理，以合成纤维替代率为变量，通过三点弯曲断裂试验，以聚乙烯醇纤维(PVA)、聚丙烯纤维(PP)替代部分钢纤维对UHPC断裂性能的影响进行研究，并使用修正后的双 K 断裂韧度与断裂能的计算公式，分别计算了不同纤维含量的 UHPC 断裂性能指标。研究结果表明： 合成纤维(PVA或PP纤维)替代部分钢纤维后，荷载-裂缝口张开位移曲线会出现“二次硬化”现象，荷载-位移曲线的下降段会出现明显波动；PVA纤维替代部分钢纤维后，可有效提高 UHPC的起裂韧度与延性，同时亦会造成黏聚韧度与纤维桥接韧度的下降，以致失稳韧度的下降；PP纤维替代部分钢纤维后，会导致UHPC起裂韧度小幅下降，失稳韧度与断裂能明显下降；PVA纤维与PP纤维各半，共同替代30%的钢纤维后，能综合提高 UHPC的断裂力学性能，是最理想的钢纤维替代方案。
In order to explore the fracture performance and mechanism of ultra-high performance concrete(UHPC)，the effect of polyvinyl alcohol fiber(PVA)and polypropylene fiber(PP)replacing part of steel fibers on the fracture performance of UHPC was studied by three-point bending fracture test with the substitution rate of synthetic fiber as the variable. The modified double-K fracture toughness formula and fracture energy formula were used to calculate the fracture performance indexes of UHPC with different fiber contents. The results show that，the phenomenon of "secondary hardening" occurs in the load-CMOD curve and obvious fluctuation can be found in the descending section of the load-deflection curves after replacing part of steel fiber with synthetic fiber(PVA or PP). The initiation toughness and ductility of UHPC could be effectively improved by replacing part of steel fiber with PVA fiber，and the cohesive toughness and bridge connection toughness are decreased，as so to reduce the unstable toughness. When PP fiber is used to replace part of steel fiber，the fracture initiation toughness of UHPC is slightly decreased，but the unstable toughness and fracture energy are reduced significantly. The fracture mechanical properties of UHPC could be comprehensively improved when 30% of steel fiber is replaced by mean proportion of PVA fiber and PP fiber，which is the most ideal solution for replacing steel fiber.