Abstract: Based on the characteristics that the flow of fresh-mixed UHPC changes the orientation of inner fibers, the method of channel flow combining with the vibration was used to realize the fiber aligning casting of UHPC. The effect of fiber aligning casting on the inner fiber orientation of UHPC was measured by image analysis, and the effect of fiber orientation on tensile properties of UHPC was measured by bending test and axial tensile test. Finally, the index for quantitative evaluation of the influence of fiber orientation on the tensile strength of UHPC was proposed. The research results show that the fiber orientation coefficient of UHPC under the conventional casting is 0.487, which is close to 0.5; while the fiber orientation coefficient of UHPC under fiber aligning casting reaches 0.731. With the effect of fiber aligning, the cracking strength of UHPC is increased by 24%, and the tensile strength and flexural strength of that are increased by about 66% and 71% respectively. About 69% increment for UHPCs tensile strength by fiber aligning is calculated by aforementioned quantitative evaluation index, which is close to the measured value and can be considered as a reliable index to quantitatively evaluate the influence of fiber orientation on the tensile strength of UHPC.

Key words: bridge engineering, ultra-high performance concrete (UHPC), fiber aligning, image analysis, assessment model, tensile performance

摘要： 基于新拌UHPC流动改变内部纤维取向的特性，采用通道流动结合振动的方法，实现UHPC的纤维定向浇筑，并采用图像分析技术测量了纤维定向浇筑对UHPC内部纤维取向的影响，利用抗折试验和轴拉试验测量了纤维取向对UHPC抗拉性能的影响，最后提出了用于定量评估纤维取向对UHPC抗拉强度影响的指标。研究结果表明：常规浇筑下UHPC的纤维取向系数为0.487，接近0.5，纤维定向浇筑的UHPC的纤维取向系数达到了0.731；具有纤维定向效果的UHPC开裂强度提升24%，抗拉强度和抗折强度分别提升66%、71%；评估UHPC抗拉性能的指标计算出的强度提升约为69%左右，与实测值相近，可认为是一个可靠的用以定量评估纤维取向对UHPC抗拉强度影响的指标。

关键词: 桥梁工程, 超高性能混凝土（UHPC）, 纤维定向, 图像分析, 评估模型, 受拉性能