Turkey is frequently exposed to destructive earthquakes causing loss of lives and property. There are many factors that affect the seismic vulnerability of existing buildings. Poor concrete quality is a major and widespread problem for existing buildings in Turkey. The number of applications on the risk assessment of deficient buildings has been increased significantly after the enforcement of a recent code namely "Determination of Seismically Vulnerable Buildings (DSVB)" in the context of " Urban Transformation Law". Modulus of elasticity is an important parameter used for the linear elastic analysis procedure in the corresponding code. Hence, the assumptions in the calculation of elasticity modulus of concrete are vitally important. It is known that the existing code equations for elasticity modulus are based on experimental results, which are obtained for normal strength concrete and it may not be valid for low strength concrete. In this study, an experimental study was conducted to investigate the elasticity modulus of low strength concrete by performing uniaxial compression tests with 36 samples prepared in the laboratory conditions. A new elasticity modulus formulation is proposed for low strength concrete and compared with the ones suggested by the existing codes. Moreover, the risk assessment of an existing RC building is performed by considering various elasticity modulus formulations. Then, the risk assessment results were compared and the differences between them were investigated in detail.