Experimental study on the fracture toughness of concrete reinforced with multi-size polypropylene fibres

To study the effect of the addition of multi-size polypropylene fibres (PPFs) on the fracture characteristics of concrete, two sizes of fine PPF and one size of coarse PPF were used to fabricate eight sets of PPF-reinforced concrete notched beam specimens with different fibre mix proportions. Three-point bending tests were conducted to obtain the load–displacement curves of each group of specimens. The test results showed that the addition of mixed fibres led to significant increases in initial fracture toughness and unstable toughness of the concrete specimens. In addition it was found that the smaller the fibre diameter and the closer this diameter to the size of microcracks, the more obvious was the inhibitory effect on microcrack expansion. Specimens mixed with one type of fine PPF showed an increase in initial fracture toughness of 17%. Specimens mixed with three different types of PPF showed an increase of 68% in unstable toughness while specimens with the addition of only one type of fibre showed an increase of only 10%, indicating that the addition of fibres of different sizes produces benefits in mixing and leads to improved unstable toughness. The initial fracture toughness and unstable toughness of each group of specimens were calculated based on the double-k fracture criterion and the mechanism by which fibres improve the fracture characteristics of concrete is discussed.