轻质低收缩超高性能混凝土的制备及其微结构形成机制

The prefabricated bridge is the development trend of city bridge construction technology. Due to its excellent mechanical properties and durability, the application of Ultra-high performance concrete (UHPC) material in the prefabricated bridge can efficiently decrease the size of bridge cross-section and the consumption of rebar, as well as the self-weight of bridge. However, this material is also faced with some problems, such as high density, large early-age shrinkage and susceptibility to cracking, which limits its application in prefabricated bridges. In order to modify the abovementioned shortcomings, this project are going to prepare UHPC using lightweight aggregate (LWA) instead of traditional aggregate of quartz sand. By utilizing the “internal curing” and “arch-shell structure” effects of super-absorbent spherical high-strength LWA, the lightweight, low-shrinkage, high-ductility and autoclaving-free UHPC (performance density < 2100kg/m3, compressive strength > 100MPa) is prepared. This material are composed of high-strength lightweight aggregate and superfine steel fiber that act as the framework, and densely accumulated Portland cement (PC), sulfoaluminate cement (SAC), fly ash (FA) and silica fume (SA) that act as the matrix. In addition, the collaborative design method that allows the compatibility between the low-density and high-strength, high-elastic modulus, and compatibility between the high-strength and the low-shrinkage of lightweight UHPC are proposed; The hydration-hardening he microstructure formation mechanisms in the as-prepared concrete are ascertained. The physical-, mechanical-, working-performance and durability of lightweight UHPC are studied; The modification induced by “internal curing” and “arch-shell structure” of high-strength LWA on this material’s performance are analyzed. Finally, after laboratory simulation and field exposure experiments, the damage deterioration mechanism of UHPC served in hash environment like offshore platform and underground space, are elucidated. The implementation of this project can provide a reference for the design and development of lightweight prefabricated bridge components.

预制拼装桥梁是城市桥梁建造技术的发展趋势。具有优异力学和耐久性能的超高性能混凝土（UHPC）用于预制桥梁，可减小截面尺寸和钢筋用量，降低结构自重。但UHPC也存在密度大和早期收缩大、易开裂的问题，限制了其在预制桥梁中的应用。针对以上问题，本项目依据高吸水球型轻集料的内养护和拱壳效应机理，在UHPC中取代石英砂，形成以细轻集料与超细钢纤维为骨架，硅酸盐水泥、硫铝酸盐水泥、硅灰、膨胀剂、粉煤灰微珠等密实堆积的表观密度＜2100kg/m3、强度等级≥100MPa的轻质低收缩UHPC；提出其低密度与超高强度、高弹模及超高强度与低收缩的协同设计方法；探明其水化硬化及微观结构形成过程；研究其物理、工作、力学和耐久性能，分析轻集料内养护和拱壳效应对其性能的改善效果；最后，通过加速模拟和现场暴露实验，揭示严酷服役环境下轻质UHPC的损伤劣化机理。项目的实施可为轻质预制桥梁构件设计和开发提供参考依据。

超高性能混凝土（UHPC）是一种具备超高力学性能和优异耐久性能的新型水泥基材料。UHPC用作结构材料，不仅可有效提升结构承载能力、减小构件截面尺寸、降低配筋率和构件自重，且可延长结构服役寿命。然而，由于UHPC较高胶凝材料用量、较低水胶比及密实设计原理，导致其存在密度大（2600-2800kg/m3）和收缩大（可达8.0×10-4以上）的问题，需要蒸养来改善构件的体积稳定性和抗裂性能，导致能耗和成本显著增加，限制了UHPC的推广应用。基于上述背景，本项目围绕UHPC自重大和收缩大的问题，研究了轻集料颗粒特性对UHPC性能及其微细观结构的影响，探明了轻集料的吸释水特性及预湿轻集料对UHPC流变性能、细观力学性能、水化产物组成、界面区结构等的影响规律，创建了轻集料的吸释水模型，揭示了预湿轻集料在UHPC内部的释水、内养护及拱壳状界面区形成机制；研究了轻质低收缩超高性能混凝土（LUHPC）的协同设计与制备技术，探明了轻集料颗粒特性和其他配合比参数对UHPC工作性能、力学性能、体积稳定性能、耐久性能等的影响规律，提出了UHPC超高强度、高弹性模量、低表观密度、低收缩的协同设计方法，制备出了表观密度低至2040 kg/m3、28 d抗压/抗折强度可达120/18 MPa、弯曲韧性指数I20可至21.8、56 d收缩率低至2.62×10-4的LUHPC，并形成了其制备技术和施工质量控制措施；研究了LUHPC的微结构形成机制，探明了标准养护、蒸汽养护和蒸压养护制度下LUHPC的水化产物组成、界面过渡区结构和孔隙结构特征，揭示了LUHPC水化产物形成与演变机理、内部球形轻集料的“内养护”和“拱壳”效应、低缩高韧等性能形成机理；研究了荷载与侵蚀介质作用下LUHPC的微结构损伤与性能退化机理，选用高精度、高耐久的四点弯加载装置，设计出了适用于LUHPC的应力侵蚀系统，探明了LUHPC在弯曲荷载和侵蚀离子作用下的损伤劣化规律，建立了能够反映应力侵蚀作用的损伤演化方程。项目研究工作对丰富UHPC设计制备理论、提升UHPC综合性能和推动UHPC在建筑结构中的应用等方面具有重要意义。