Investigation of the Residual Mechanical and Porosity Properties of Cement Mortar under Axial Stress during Heating

Abstract

The preload load on concrete during heating is considered to cause a ‘densification’ of cement mortar which led to the increased compressive strength. In order to assess the influence of coupled load and heating effects on porosity characteristics of concrete, the porosity of mortar after mechanical and thermal loading was measured by X-ray computed tomography (X-ray CT). The preload at pre-stress ratios of 0, 0.2, 0.4, and 0.6 (ratio of stress applied to the specimen to its compressive strength at room temperature) were applied on mortar specimens during heating. The residual compressive strengths of the heated and stressed mortar specimens were tested after cooling to room temperature. Combined analyses of the residual compressive strength test results and porosity test results, it shows that the porosity of the specimens under the coupled stressing and heating conditions were slightly lower than that under the unstressed conditions; however, the conclusion that the increase of compressive strength of stressed mortar was caused by the ‘densification’ of cement paste was insufficient. The preload reduced the cracks in the mortar, especially the crack induced due to the thermal mismatch in aggregates and hardened cement paste (HCP), and this may account for the increased compressive strength of stressed mortar.

Effect of Different Temperatures on the Hydration Kinetics of Urea-Doped Cement Pastes

“…The UC increases by 8% and the porosity increases by 7% under the condition of 20 °C at after 1 day. This is because urea itself has strong hygroscopicity and can absorb water molecules around the environment at the early stage of hydration, so that, if there is excess water in the paste, this excess water does not participate in the preliminary hydration reaction, thus resulting in a larger porosity value at the early stage [ 43 , 44 , 45 ]. In addition, urea inhibits the hydration rate, resulting in the hydration products not being able to fill the pores in time and thus inhibiting the further reduction of the pores.…”