不同配比和养护条件对超高性能混凝土微观结构的影响

为了优化超高性能混凝土（Ultra High Performance Concrete，UHPC）制备方法，采用宏观力学试验与微观电镜技术相结合的方法，探讨不同配合比和养护条件对UHPC内部微观结构的影响。基于硅砂骨料的致密堆积级配，设计21个变量组，共制作了63个立方体试件，开展UHPC流动度试验、轴压试验和扫描电镜试验，分析水胶比、砂胶率、钢纤维掺量、消泡剂掺量、养护方法、龄期等因素对UHPC工作性能、抗压性能及其微观结构的影响规律以揭示UHPC的增强机制。研究结果表明：凝胶与骨料界面过渡区（ITZ）是UHPC内部的薄弱环节，提高ITZ的密实度和强度是增强UHPC的关键；UHPC的流动度随着水胶比的提高显著增大，但其抗压强度随着水胶比的提高先增大后降低；过高的砂胶率不利于UHPC工作性能，同时会造成其抗压强度下降；掺入消泡剂可以有效提高UHPC的表观质量，但可能会降低UHPC的工作性能和抗压强度；掺入2.5%的钢纤维能大幅提高UHPC的抗压强度，并明显改善其脆性特征，但会降低工作性能；高温养护能显著激发微硅粉和矿渣的火山灰效应，使UHPC的4 d抗压强度比常温养护提高约50%，有明显的早强优势，但存在后期强度下降的可能。

Effects of Mix Proportion and Curing Condition on the Microstructure of Ultra-high Performance Concrete

The aim of this study is to optimize the mix design and preparation method of ultra-high performance concrete (UHPC). By combining the macro-mechanical and scan electron microscopy (SEM) tests, the effects of mix proportion and curing condition on the microstructure of UHPC was investigated. A total of 21 groups of UHPC mixtures were developed, and 63 cubic specimens were fabricated based on the dense packing gradation of a silica sand aggregate. A series of UHPC fluidity, axial compression, and SEM tests were then conducted. The effects of water-binder ratio, sand-binder ratio, steel-fiber content, defoamer content, curing method, and curing age on the workability, compressive performance, and microstructure of UHPC were analyzed. The reinforcement mechanism of UHPC was revealed, and the results show that the interfacial transition zone (ITZ) between the gel and aggregate is a weak link in UHPC. Improving the density and strength of ITZ is the key to enhancing UHPC. With an increase in the water-binder ratio, the flowability of UHPC increases significantly, but its compressive strength increases first and then decreases. An excessively high sand-binder ratio is detrimental to the workability and compressive strength of UHPC. Adding a defoamer can effectively improve the apparent quality of UHPC, but may reduce the workability and compressive strength of UHPC. The compressive strength and brittleness of UHPC can be greatly improved by adding 2.5% steel fiber. However, the addition of steel fiber reduces the workability of UHPC. High-temperature curing can considerably stimulate the pozzolanic effect of silica fume and slag, resulting in that the four-day compressive strength of UHPC increases by approximately 50% as compared with that under normal temperature curing. This finding indicates that UHPC under high-temperature curing has the character of early strength. However, high-temperature curing may cause a reduction in the long-term strength of UHPC.