Abstract: Eight PP-ECC beams and two RC beams were fabricated in order to investigate the crack development law of PP-ECC beams under bending load． Firstly，the crack development pattern，crack width and crack exten-sion height of PP-ECC beams were studied by four-point bending load，and compared with those of RC beams．Then the average crack spacing model，the maximum crack width model and the maximum crack extension height model were established based on the bond-slip theory and plane cross-section assumptions，considering the tensile capacity of the ECC beam when the beam was flexed． And the rationality and accuracy of each model were verified by comparing the model calculation results with the test results． The results show that PP-ECC has obvious strain-hardening characteristics under uniaxial tension，showing a mode of multiple micro-cracks development． The num-ber of cracks in PP-ECC beams under bending load is significantly more than that in ＲC beams，and cracks develop through the whole test process． The width and extension height of main cracks in PP-ECC beams are smaller than
that in RC beams，and cracks show more uniformity with the increase of load． The accuracy coefficient of PP-ECC beam average crack spacing model based on bond-slip theory is between 0. 95 and 1. 05，the accuracy coefficient of the maximum crack width model is between 0. 85 and 1. 2，and the accuracy coefficient of the maximum crack extension height model established based on plane cross-section assumption is between 0. 95 and 1. 1．

Key words: PP-ECC beam, strain-hardening, crack morphology, maximum crack width, maximum crack ex-tension height, calculation model