DEF of high-performance concrete with rapid, non-standard heat-treatment

High-performance concrete is often heat treated to increase early strength. Standardized temperature conditions for at least 24 hours and sulphate-resistant cement thereby preventing delayed ettringite formation (DEF). In contrast, this paper investigates the risk of DEF for rapidly heat-treated HPC with a minimum temperature duration of 1 hour to achieve a rapid early strength for stripping. In addition, a binder is used that does not exhibit increased sulphate resistance. For this purpose, standard prisms (L x W x H = 16 ×4 x 4 [cm]) are heat treated at 80 °C between 1 and 6 hours without a pre-storage time. Samples with a temperature duration of 24 hours serve as reference. First, the pore structure is analyzed using Mercury porosimetry to identify damage to the concrete matrix due to the rapid heat treatment. The investigation of DEF is achieved by two common approaches. The sulphate resistance is determined by means of wet-dry cycles on specimens for 90 days, whereby the changes in mass and strain as well as the cracking pattern on the concrete surface serves as a qualitative evaluation criterion. In addition, the SVA method on specimens directly after heat treatment, which determines the change in mechanical properties and the swelling behavior of the concrete, leads to a quantitative evaluation of sulfate resistance. The measured pore distribution of the samples shows a significant increase of the capillary pores of up to 109% for decreasing temperature durations. However, the porosity does not affect the sulphate resistance of the concrete. The wet-dry cycles cause no damage due to DEF, since the change in mass and strains decreases with increasing cycles from up to 62.7% to -2.5%. Furthermore, no visual differences were detected on the concrete surface between specimens with and without sulphate attack. As a result of the SVA test, maximum absolute strains of about 0.1 mm/m occur that are below the limit value of 0.152 mm/m. Also, the mechanical properties do not show a reduction in strength, but even an increase in tensile strength. In conclusion, the high-performance concrete formulation used in combination with rapid heat treatment investigated here did not result in detectable damage due to DEF.