Effect of thermal treatment on properties of high strength concrete

High Performance Concrete (HPC) and Ultra-High Performance Concrete (UHPC) are modern building materials with advanced mechanical properties and durability compared with traditional concrete. Increasing of their mechanical properties allows reducing cross section of construction element. As a result, fewer raw materials are consumed. Obtaining of concrete with high mechanical strength is related to a low W/C ratio, superplastificators and use of specific mixing and hardening technologies. One of the factors that influence properties of concrete is thermal treatment during hardening process. For traditional concrete steam treatment is used, but in the case of specific modern materials like HPC and UHPC increasing of mechanical properties can be achieved with the thermal treatment at temperature over 100°C. The effect of thermal treatment on concrete compressive strength was investigated on cube-shaped specimens of 5cm. The heat treatment temperature varied from 50°C to 200°C with a 50°C increment. The specimens were heated under the same conditions for each temperature level; speed of increasing the temperature was 2°C per minute, thermal treatment at maximal temperature lasted 4 hours. Thermal treatment and compressive strength tests were carried out on the 3rd and 28th days. Tests were performed with specimens cooled down slowly to room temperature after heating. Additional tests were made for water absorption and permeability, including SEM analysis comparing concrete specimens with and without specific thermal treatment applied. The results showed that, despite the possible dehydration, compressive strength of thermally treated HPC specimens has improved. Maximal compressive strength was achieved for specimens thermally treated at 200°C. Compressive strength of non-heated specimens was about 80 MPa (on 3rd day) and 130 MPa (on 28th day), and thermally treated specimens showed a strength about 155 MPa (on 3rd and 28th day). Results for SEM, water absorption and permeability, compressive strength of specimens at different thermal treatment conditions are summarized it this paper.