Non-destructive evaluation of early strength of concrete with slag thermally activated using electrical cable

High levels of clinker substitution by supplementary cementitious materials, such as slag, are necessary to decrease the CO₂ emissions of concrete manufacturing. However, this leads to low early concrete strength, delaying construction schedules. In cold regions, heating is an essential part of accelerating the early hydration, thus, the strength of concrete. Therefore, this study analysed the effect of thermal activation of the concrete with 45% and 70% slag in the binder using internal electrical cable heating utilised in construction by evaluating compressive strength and heat development. In addition, non-destructive test methods, such as rebound hammer and ultrasound pulse velocity meter, were introduced to widen their application in estimating early strength development using the SonReb approach. The heating with the power of 20 W and 30 W during the first 10–15 h (11.1–16.7 kWh/m³) effectively improved the early strength of concrete with 45% and 70% slag up to 14% and 60%, respectively during the first 24 h without any strength losses at the later ages. However, considering the initially low strength values (2.5–10 MPa) of concrete with 70% slag, the heat curing resulted in about a 1 – 4 MPa increase, with a maximum rise of 7 MPa, requiring additional strategies for slag activation, for example, alkali activation, to fulfil construction requirements.