Prediction of crack initiation for notched concrete beam from FPZ at maximum load

The initial cracking load ( Pi ) of a notched concrete beam, difficult to measure, has been predicted from the maximum fracture load ( Pmax ) together with the notch-tip fracture process zone (FPZ) at Pmax. A simple analytical model is used to link Pi and Pmax together through the FPZ. The corresponding initiation and unstable fracture toughness, Ki and Kun, have also been determined. The model assumes that the lower limit of Pmax from a group of identical notched concrete beams corresponds to the lower limit of the FPZ length FPZ L, so that the lowest limit Pmax  =  Pi is established for FPZ L  = 0. Detailed FPZ measurements before and at Pmax using a digital image correlation technique were analysed and used to predict the initial cracking load Pi. Six different specimens with various sizes ( W  = 40, 60 and 80 mm), initial notches ( a0  = 12 to 48 mm) and FPZ L (7 to 13 mm) at Pmax showed the average ratio of FPZ L /( W – a0 ) was around 0.25, indicating the Pi / Pmax ratio was around 0.67 based on the analytical model. The crack growth resistance KR -curve between the crack initiation toughness Ki at Pi and the unstable fracture toughness Kun at Pmax was also established approximately by the simple model. Estimated intrinsic fracture toughness KIC was compared with Ki and Kun. The influence of average aggregate size dav on Pi and FPZ L at Pmax was also discussed.