Early-age fracture tests are conducted on concrete beams with discrete steel, macro polypropylene and blends of macro and micro polypropylene fibers. The fracture behavior with aging of the concrete matrix containing 0.66% volume fraction of fibers is investigated at 1, 3, 7, and 28 days of age. The age-dependent multi-linear cohesive stress crack-separation (σ-w) relationship and the modulus of elasticity of the fiber reinforced composite material are obtained from the measured fractured responses. The generic multi-linear form of the σ-w relationship captures the age dependent variations in the stress with crack opening associated with initial softening and the hardening produced by fibers. The σ-w relationships determined from the fracture responses indicate that all fibers types significantly increase the tensile strength of concrete at 1 day. By 28 days, the tensile strength of the fiber reinforced concrete is comparable to the plain concrete. The steel fibers significantly increase the Young’s modulus of the fiber reinforced concrete in the first 3 days. At any age, steel fibers provide the largest σ as a function of w, when compared to the polypropylene fibers. The differences between σ-w responses of the polypropylene and steel fiber reinforced concretes increase with age and there is a steady increase in the fracture energy with age. The improvement in the post-cracking response of steel fibers with age is due to increases in the resistances to crack propagation and pullout of fibers. Blends of micro and macro polypropylene fibers exhibit higher resistance to crack propagation and to pullout of fibers when compared with the macro polypropylene fibers alone.

早期断裂试验是在混凝土梁上使用不连续的钢，大尺寸聚丙烯以及大尺寸和微型聚丙烯纤维的混合物进行的。在1、3、7和28天龄时研究了含有0.66％体积分数纤维的混凝土基体随老化的断裂行为。从测量的断裂响应中获得了随年龄变化的多线性内聚应力裂纹分离（σ-w）关系和纤维增强复合材料的弹性模量。σ-w关系的通用多线性形式捕获了随年龄变化的应力变化，其中裂纹的开裂与纤维的初始软化和硬化有关。该σ-W由断裂响应确定的关系表明，所有类型的纤维在1天时都会显着提高混凝土的抗拉强度。到28天时，纤维增强混凝土的抗拉强度与普通混凝土相当。在开始的三天内，钢纤维显着增加了纤维增强混凝土的杨氏模量。与聚丙烯纤维相比，在任何年龄段，钢纤维都具有作为w的函数的最大σ。σ-w之间的差异聚丙烯和钢纤维增强混凝土的响应随着年龄的增长而增加，并且断裂能随着年龄的增长而稳定增加。随着时间的流逝，钢纤维开裂后响应的改善是由于增加了对纤维的裂纹扩展和拔出的抵抗力。与单独的大聚丙烯纤维相比，微和大聚丙烯纤维的混合物显示出更高的抗裂纹扩展性和抗拔出性。