THE EFFECT OF SHRINKAGE CONTACT STRESSES ON THE ADHESION STRENGTH OF THE WATERPROOFING COMPOSITION AND THE «OLD» CONCRETE

Abstract

The paper is devoted to the study of the effect of shrinkage contact stress on the bonding strength of the waterproofing composition of fine-grained cement concrete and «old" concrete. In order to obtain a strong contact when adhering old and new concrete, it is necessary to minimize the amount of internal stress in it. The cause of concrete shrinkage during the initial curing period is the reduction of the cement stone gel, the amount of free water that is consumed by evaporation and hydration of the cement. The semi-bound water (film water) surrounding the gel particles is then consumed. This causes the gel particles to come closer together and shrink further. In addition, the capillary pressure in the pores is very significant and increases with decreasing pore size. Since the micropores are scattered in the cement stone in different directions, the pressure, balancing each other, acts as a comprehensive compression, which also causes volumetric deformation. With further drying of the concrete, the moisture gradient decreases, the growing crystal growths provide more and more resistance to internal pressure and the shrinkage deformation gradually subsides. Thus, the more intensive the hydration processes in the waterproofing coating and the more crystalline hydrates are formed at earlier curing times, the earlier the resistance to comprehensive compression in the coating will be, and as a result, the crack resistance of the coating and the strength of its adhesion to the «old» concrete will increase. As a result of the research carried out in this work, it was found that the compaction of the waterproofing composition, which helps to reduce internal stresses during curing and thus increases the amount of its adhesion to old concrete, is achieved by introducing a finely ground calcium carbonate filler and a complex of chemical water-soluble additives of the second class. The mineral filler and chemical additives allow for the additional synthesis of etringite, calcium hydrocarbonate and calcium hydrochloraluminate, low-base calcium hydrosilicates, and calcite during cement hydration in its pores and capillaries. The developed permeable waterproofing composition in the form of a mortar has high adhesion strength to old concrete and can be used in repair, restoration and protection of building structures, buildings and structures from external influences related to the water environment.