超高性能混凝土（Ultra-high performance concrete, UHPC）是通过掺和超细掺合料显著提高混凝土的密实度和抗压强度，采用钢纤维提高其抗拉能力，使其为具备超高抗压强度、较强抗拉强度和优越耐久性能的“新型超级混凝土”。当前，世界范围内已逐步将UHPC材料从接缝、剪力连接件等局部附属部位，开始应用于主承力构件，甚至有部分结构开始全部采用UHPC。各国学者对UHPC的研究逐步深入，材料领域的研究多聚焦于采用颗粒堆积理论和加入纳观微观级颗粒优化配合比、从微细观角度揭示不同骨料及纤维对界面粘结性能和材料力学性能的影响、水化硬化机理和流变性能等；结构领域多借助传统经验方法，通过宏观尺度的构件和结构试验进行结构受弯、受剪、抗震、抗冲击和抗爆等研究，在宏观层面建立相应的设计计算理论。材料研究和结构研究存在脱节现象，突出表现在传统的结构设计理论框架下无法合理利用材料领域关于UHPC纳观、微观、细观研究成果。建立两者联系的核心纽带是各组分对结构构件力学性能贡献从细观到宏观的科学定量及界面粘结滑移问题，其中，钢纤维-UHPC基体界面粘结滑移性能是核心问题之一。本文采用多尺度手段，研究了钢纤维与UHPC基体界面粘结性能（微观-细观尺度），基于细观钢纤维-UHPC基体界面粘结性能建立了结构跨尺度（细观-宏观）受弯受剪理论计算模型，并通过试验对提出的理论模型进行了验证。主要研究内容包括：
基于细观力学分析，揭示了纤维对残余抗拉强度贡献的机理，提出了考虑界面粘结强度和纤维分布、取向以及埋深的超高性能纤维混凝土（Ultra high performance fiber reinforced concrete, UHPFRC）细观本构模型，为预测UHPFRC残余抗拉强度提供新思路，进而建立了UHPFRC梁跨尺度受弯分析模型和受弯承载力理论计算方法。采用9片受弯试验梁结果对提出的理论进行验证，并对不同作用机制的抗弯贡献比例进行了分析。
针对如何计算纤维抗剪贡献的核心难点问题，提出了全新的有效纤维抗剪区域（Effective Fiber Distributed Region, EDR）思想，即破坏斜裂缝周围一定宽度内的纤维可提供抗剪。基于概率论或纤维拔出荷载-滑移曲线手段确定EDR宽度，进而得到纤维抗剪贡献，并建立跨尺度（细观-宏观尺度）纤维-基体分离受剪承载力理论计算模型（Mesoscale Fiber-Matrix Discrete Model, MFDM），为预测纤维抗剪贡献提供新的思路。建立的模型可以考虑纤维取向、埋置深度和纤维基体间粘结强度等因素，从细观角度反应了纤维抗剪机理。
广泛搜集国内外钢纤维混凝土（Steel fiber reinforced concrete, SFRC）梁受剪试验数据，建立总数为941的SFRC梁剪切数据库（Shear database, SDB），对SFRC-SDB进行样本整体研究，分析以往试验研究重点及参数分布情况，基于SFRC-SDB详细探讨受剪承载力随各主要影响因素的变化规律，最后对现行规范和抗剪设计方法进行评价。
Ultra-high performance concrete (UHPC) is a new class of super concrete with ultra-high compressive strength, considerable tensile strength and superior durability, in which superfine active admixture is added to improve the compactness and compressive strength and the steel fibers are dispersed to enhance the tensile strength. UHPC has been gradually applied from ancillary facilities, such as joint and shear connectors, to the main structural component, and it is even totally adopted in some structures. Researches in material engineering field mainly focus on optimizing mix proportion by using particle dense packing theory and with the addition of nanoscale particles, investigating the effect of different aggregates and steel fibers on the interfacial bond behavior and material properties, hydrating and hardening mechanisms, rheological property and so on. In structure engineering field, researchers usually use conventional method to investigate the structural behavior under the bending load, shear load, seismic load, impact load and blast load via testing and to establish corresponding design methods at the macro level. It can be concluded that the material researches and structural studies are out of line, especially the nano-scale, micro-scale and meso-scale research achievements of material engineering cannot be utilized appropriately in conventional structural design theory. The core links are the quantification of the contribution of each component on the mechanical properties of structural components from meso-scale level to macro-scale level and the interfacial bond behavior. The bond behavior between fibers and matrix is one of the critical problems. This dissertation presents an experimental investigation and theoretical study on the bond behavior between steel fibers and matrix (micro-scale to meso-scale), and the cross-scale (meso-scale to macro-scale) flexural analysis model and shear strength model of UHPC beams derived from multi-scale analysis. The proposed theories are verified by the experiment results. Details of the study are summarized as follows:
(1) Experimental study on the bond behavior between steel fibers and UHPC matrix (micro-scale to meso-scale)
90 pull-out specimens are tested to investigate the bond behavior between steel fibers and UHPC matrix with the parameters of fiber orientation, fiber type and maximum particle size of sand aggregate. The failure interface is observed using scanning electron microscope at a micro-scale. Two macro-scale failure modes, including fiber rupture and fiber being pulled out are observed and three micro-scale failure modes, incorporating interfacial transition zone failure, matrix failure and fiber surface scratching are identified.
(2) Cross-scale flexural model of ultra-high performance fiber reinforced concrete (UHPFRC) beams based on a mesoscale constitutive model (meso-scale to macro-scale)
The resistance mechanism of fibers on the residual tensile strength is investigated based on the mesomechanics analysis. A mesoscale constitutive model is proposed, taking into consideration uniform distribution, embedment length and orientation of fibers. The proposed model provides an alternative solution for the determination of UHPFRC residual tensile strength. A flexural strength model is subsequently derived on the basis of the proposed mesoscale constitutive model. The proposed model is verified by 9 bending test results and various machanisms on flexural strength are detailed discussed.
(3) Experiment verification on the cross-scale flexural model of UHPFRC beams
An experimental study is executed to verify the proposed cross-scale flexural model and to assess the flexural performance of UHPFRC beams with the test parameters of the fiber volume fraction, fiber type, longitudinal reinforcement ratio and concrete strength. The failure of the fiber bridging is one by one fiber and the cracks exibit more and fine distribution characteristics. A new ductility index, expressed in the form of dividing the ultimate deflection by flexural cracking deflection, is introduced to characterize the post-cracking ductility capacity. Calculations with the proposed model show good agreement with the test results and the complete loading response of the test beams can be predicted.
(4) Cross-scale shear strength of UHPFRC beams: mesoscale fiber-matrix discrete model (meso-scale to macro-scale)
To solve the key problem of determining the shear contribution of fibers, a new concept of an effective fiber distributed region (EDR) along the critical diagonal shear crack, where fibers are efficient at providing shear resistance, is proposed. Two methods to determine the width of EDR are proposed based on probability theory and the pullout load slip relationship. Combining the number of efficient fibers and the bond strength of a single fiber, the shear contribution of fibers is derived. A mesoscale fiber-matrix discrete shear strength model is proposed, which offers an alternative perspective in understanding the shear behavior of UHPFRC beams, especially the shear resistance of fibers at a mesoscale level. The proposed model takes into account the fiber orientation, embedment length and bond strength between the fibers and matrix and can reflect the shear force resistance mechanism of fibers.
(5) Experiment verification on the cross-scale shear strength model of UHPFRC beams
Eleven T-beams, reinforced with high strength steel, are tested to failure to verify the proposed cross-scale shear strength model and to investigate the effect of shear span to depth ratio, fiber ratio, fiber type, concrete strength and stirrup ratio on the shear behaviour, especially post-cracking shear strength and deformability, of UHPFRC beams. The shear failure of UHPFRC beams is firstly defined as semi-ductile, semi-brittle failure. To characterize the post-cracking shear behavior and stiffness degradation of UHPFRC beams, a new ductility index, which is expressed in the form of dividing ultimate deflection by shear cracking deflection, is introduced. The proposed shear strength model could accurately predict the shear strength of UHPFRC beams.
(6) Evaluation of shear strength model based on the shear database for SFRC beams
The shear test data of SFRC beams are widely collected from literatures, a shear database for SFRC beams (SFRC-SDB) is established with the total number of 941. Based on the analysis of the samples in the SFRC-SDB, the influence of main factors on shear behavior is investigated and the shear strength models are evaluated.