Analysis of the effect of the phased introduction of additives on maintaining the mobility of concrete mix for monolithic housing construction

Abstract

Trends in the development of modern monolithic housing construction show the need to increase the viability of the concrete mixture and determine the feasibility of using the method of «late» introduction of additives («two-step mixing method»), which consists in the introduction of additives in two stages. At the first stage, to increase the viability of the mixture and lengthen the injection period of hardening, which is especially important for laying concrete at elevated temperatures, plasticizer and hardening inhibitor additives are introduced into the concrete mixture. At the second stage, immediately before laying and compacting the concrete mixture in the structure, the secondary dosage of additives and the hardening accelerator are introduced into the concrete mixture. This method allows restoring the mobility of the concrete mixture by adding a very small amount of water included in the working solution of the additive. The effect of repeated introduction of additives on the kinetics of plastic strength is analyzed. The analysis showed, firstly, the possibility of a sharp decrease in structural strength at the time of introduction of additional additives; secondly, the possibility of lengthening the induction period, which prolongs the shelf life of the mixture; thirdly, the additional introduction of the accelerator as part of the “secondary” complex additive allowing accelerating the set of plastic strength of cement paste, which should positively affect the growth of early concrete strength in the structure. To test the effect of this additives introducing scheme on early strength and in the later stages of hardening, special experiments were performed on a 15-point D-optimal plan, in which the concentrations of superplasticizer, moderator and accelerator were varied. Based on the obtained models of physical and mechanical properties, the influence of various concentrations of additives on the early strength of concrete in the structure and in a later period is analyzed. The areas of optimal concentrations of additives were determined. The proposed method of working with concrete mixture under conditions of prolonged transportation and at increased temperatures allows the use of additives of different types and their combination separately at different stages, depending on the time of year, the organization of concrete work, the distance to the construction site, taking into account the cost of transportation and other factors.