Abstract: The double-K fracture parameters of three low strength concretes are investigated using three point bending notched beams. In the tests, half-bridge strain gauges with lengths of 5 mm and 10 mm are used to determine the initial cracking load of the pre-cast cracks. Comparison of their workability results show that strain gauges are sensitive to material strengths and short ones are more suitable for low strength concrete. Four other half-bridge strain gauges are arranged along the fracture ligament. When the tensile strain of the gauge decreases, the factual crack has propagated to the measuring point. With the load and the crack mouth opening displacement at this moment, the corresponding equivalent crack length can be calculated and then compared with the actual one. Results show that double-K fracture model can be well applied to predict crack lengths in concrete.