Effect Of Aggregate And Cement Matrix Strength On The Fracture Properties Of Concrete

The fracture of concrete structure is related to the initiation and propagation of cracks in the materials. Therefore it is critically important to have a better understanding on the crack initiation and propagation in cement composite materials to optimize concrete structure and improve the structure performance. Aggregate and cement matrix strength significantly influence the fracture properties of concrete. In the present thesis, the effect of sand ratio, aggregate volume content, aggregate size and cement matrix strength on the stress-crack width ((-w) relation of concrete and some related parameters such as compressive strength, cracking strength, flexural strength, tensile strength and fracture energy are analyzed qualitatively and quantitatively based on fictitious crack model. In the analyses, the three-point bending tests on pre-notched beams were used as inputs.Based on the results, the following conclusion had obtained: (1) The effect of sand ratio(0.30~0.67)and aggregate size (5~25mm) on concrete compressive strength is not significant, but as the aggregate volume content (45%~70%) is increased, the concrete compressive strength decreases gradually.(2)The Load-CMOD curve can be divided into three sections: Firstly, elastic stage up to the stress at the notch tip achieves the cracking strength (fc with a constant stiffness. Secondly, crack developing stage one: load increases with a gradually reduced stiffness until the peak load. Thirdly, crack developing stage two: load decreases with a negative stiffness. After peak load, high strength concrete may rupture suddenly. However, significant differences in both the maximum bending load and the stiffness in each stage. (3) The quart-linear function is accurate enough to reflect the characteristics of the stress-crack opening relationship of concrete. High tensile stress can be achieved for the same cracking width as sand ratio decreases and aggregate content increases. Small size of aggregate leads to a high tensile strength and a sharp stress dropping after the peak stress. The effect of aggregate on the shape of ((w curve almost vanishes in high strength concrete. (4) Cracking strength is almost not influenced by the aggregate, but by the cement matrix strength. The higher the cement matrix strength, the higher the cracking strength. (5) The tensile strength, flexural strength and fracture energy are all significantly influenced by the cement matrix strength. For concrete with the same workability, the tensile strength, flexural strength and fracture energy increase with the decrease of sand ratio and the increase of aggregate content. For the same aggregate content, the larger the aggregate size, the lower the tensile and flexural strength. But the fracture energy and characteristic length increase with the increasing aggregate size. As the cement matrix strength increases, the effect of aggregate on the tensile strength, flexural strength and fracture energy decreases gradually.