Crack initiation, crack growth and time-dependency of the crack in unreinforced and reinforced concrete have been an interesting topic for decades. Parallel to the research on the ‘shear capacity of concrete beams under sustained loading’, in which the shear capacity of large-scale beams with longitudinal reinforcement under high sustained loading is being investigated, this research is done to investigate the behaviour of crack in unreinforced concrete. A series of short-term and long-term loading has been performed to acquire the ultimate capacity of the beams (in case of short-term loading), displacements, and time of failure with different load ratios (in case of long-term loading). Subsequently, a finite element model is proposed to develop the acquired results into a general finite element analysis. The aim of this research is to investigate the time-dependent growth of single crack and multiple cracks in plain concrete beams subjected to sustained loading.