The problem of the equivalent time to start the internal frost resistance test on self-compacting concrete with supplementary cementitious materials

The article presents the results of tests on frost resistance of self-compacting concretes made with the use of cement containing mineral additives after 150 freezing and thawing cycles. Concrete with types of cement containing 30 and 50% blast furnace slag, cement with 30% silica fly ash and with types of cement containing both slag and ash with a combined proportion of 60% (30% both mineral components) and 80% (45% slag and 35% ash). The relative dynamic modulus of elasticity determined based on the ultrasonic pulse transit time RDMUPTT was used as the basis for the measure of frost resistance. The tests were carried out on cubic specimens with a side length of 150 mm, which are standard for testing the compressive strength of concrete. The relative dynamic modulus of elasticity RDMUPTT has proved to be a qualitatively correct measure of concrete's frost resistance. The RDMUPTT values corresponded to the values of the auxiliary measure, which in this case was the relative decrease in specimen mass. It was confirmed that tests on frost resistance of concretes with mineral additives should be started in the so-called "equivalent time" depending on the cement composition. Due to the varied increases in concrete strength over time, the consideration of "equivalent time" is intended to improve the reliability of testing methods, including the ultrasonic method. Tests have also shown that the use of an additive in the form of silica fly ash can weaken concrete frost resistance, especially when the fly ash to clinker ratio is high. A positive effect of the addition of granulated blast furnace slag on the frost resistance of concrete was also noticed.