Monitoring the effect of admixtures on early-age concrete behaviour by ultrasonic, calorimetric, strength and rheometer measurements

The effect of mineral additions on concrete setting has already been investigated thoroughly using ultrasonic methods. However, growing interest in self-compacting and high-performance concrete has also increased the use of admixtures such as superplasticisers and air entrainers, which are less documented regarding their effect on ultrasound transmission. Self-compacting mixtures with different cement types and superplasticisers were therefore tested by means of ultrasonic, calorimetric, rheometer and strength measurements. The use of air entrainer was also investigated. Setting time was prolonged by the addition of PCE (polycarboxylic ethers) and PNS (polycondensates of naphthalene sulphonate) by 1·3–2·4 h and 2·4–4·4 h respectively, as determined by both traditional and ultrasonic methods. Postponed setting can be caused by hydration retardation (1–5 h) and cement particle dispersion. More air was entrained by PNS than by PCE, leading to lower initial ultrasonic wave velocities and lower compressive strengths. As for the air entrainer, a slower increase in velocity and energy ratio was noted, together with a lower value reached after 48 h, attributed to a lower stiffness of the samples and wave scattering on air inclusions.