Abstract: Strain gauges are adhered to the tip of the pre-cracks on three-point bending beams, whose maximum size is 680mm×160mm×40mm. The initial load Pini can be measured according to the sudden decrease of stain gauges’ values. Then initial fracture toughness can be obtained using Pini and initial crack length a0. Moreover, critical crack length can be obtained by means of the maximum load Pmax and the corresponding crack mouth opening displacement CMODC from test. The experimental results show that it is simple and effective to measure the initial load of concrete using strain gauges. The results also indicate that the measured double-K fracture parameters, , , based on three-point bending beams, are independent of specimens size. Hence, double-K fracture parameters can be viewed as the criterion to predict or estimate crack development.