One of the ways to enhance the mechanical properties of the concrete is to compress the fresh concrete and remove the excess water and air from within it; hence knowing the stress-strain behavior of the compressed concrete is essential for the linear as well as nonlinear analysis and design purposes. In order to investigate the stress-strain behavior of the compressed and uncompressed concrete, an extensive experimental study was conducted and then, based on the experimental results, empirical formulas for the stress-strain curve of the compressed and uncompressed concrete are developed. In addition, the effect of compression on the amount of energy absorption as well as the extent to which the stress-strain curve is linear is investigated. In doing so, a total of 90 concrete cylindrical specimens divided into 10 series with the reference compressive strength ranging from 17.9 to 52.6 MPa were produced. The specimens in each series were grouped into three main categories: (1) short-term pressure-compressed concrete (SPCC), (2) long-term pressure-compressed concrete (LPCC) and (3) reference concrete (RC). The obtained results show that the proposed relationship for the actual stress-strain curve of the compressed and uncompressed concrete is not only able to predict the experimental results with high accuracy, but also it is applicable to a wide range of strength values. Furthermore, compressing the fresh concrete increases the amount of energy absorption and the linear stage of the concrete stress-strain curve, although the energy absorbed by the compressed concrete is lower compared to that of the uncompressed concrete with the same strength.
