Study of the influence of mix design and technological factors on the properties of self-compacting concrete mixtures

Abstract

The transition of cast-in-place construction to self-compacting mixtures is constrained by a narrow technological window: a composition that meets the required flowability during laboratory proportioning loses it on site as the mixture temperature and the placing time change. The classification given in GOST R 59714-2021 sets class boundaries for flowability, viscosity, passing ability and segregation resistance, leaving open the question of which governing parameter drives the mixture across these boundaries fastest. The aim of the work was a quantitative separation of the contributions of five factors – the water-to-powder ratio, the polycarboxylate superplasticiser dosage, the microsilica share in the binder, the volumetric sand fraction in the aggregate blend and the mixing duration – to the formation of five standardised properties of the mixture. An author's computational model was built on a fractional factorial design with 16 model batches. The coefficients of the linear relationships were obtained by least squares in coded variables, and the factor contributions were estimated through the shares of the sums of squares. The water-to-powder ratio accounts for 57,4% of the slump flow variance, 56,2% of the segregation variance and 78,2% of the compressive strength variance. The microsilica share yields 18,2% of the segregation variance against 9,7% of the slump flow variance. Passing ability in the L-box proved to be the only response where the contribution of the sand fraction (22,4%) is comparable with that of the admixture dosage (21,8%). Nine of the sixteen model compositions satisfy all requirements of the standard simultaneously. The opposite directions of the rheological and strength responses to the same change in the water-to-powder ratio indicate that the domain of admissible compositions is formed by the intersection of two opposing requirements – the lower threshold of passing ability and the upper threshold of segregation. The width of this intersection governs the robustness of the mixture to deviations in temperature and delivery time.