Advanced shrinkage compensation of concretes by a combined admixture system

Abstract

Advanced shrinkage compensation of concretes by a combined admixture system Katalin Szilágyi, research fellow, BME István Zsigovics, senior lecturer, BME Csaba Szautner, product manager, Mapei Hungary IntroductionCracking of concrete and particularly the formation of shrinkage cracking is one of the major concerns in the durability of concrete structures. Appropriate concrete technology can help to avoid cracking and to provide improved durability. Smaller permeability, better water tightness, higher freeze-thaw resistance and higher resistance against aggressive chemical attack can be resulted.To avoid shrinkage cracking several possible solutions are available mainly concerning the mix design, such as: use of expanding cements, use of shrinkage compensating admixtures, mixing concretes with low cement-paste content, mixing concretes of reduced sand content, and with effective curing of concrete.Volume changes (i.e. shrinkage) due to loss of water are possible both in the fresh concrete and in the hardened concrete [1,2,3]. In the fresh concrete, loss of water by evaporation causes plastic shrinkage and can lead to surface cracking. Withdrawal of water from concrete stored in unsaturated air causes drying shrinkage [4,5,6]. Autogenous shrinkage (volume changes due to chemical reactions during hydration) is also discussed in technical literature, however, for practical purposes these relatively small changes need not to be distinguished from drying shrinkage [7]. Factors affecting shrinkage are the followings:− volume of the cement stone, − stiffness of the aggregate, − possible water absorbing capacity of the aggregate, − relative humidity of the ambience, − climatic conditions (temperature, wind speed, etc.), − volume-to-surface ratio of the concrete structure.Prolonged moist curing of concrete delays the initiation of shrinkage, but the effect of curing on the magnitude of shrinkage is small [6]. On the other hand, prolonged moist curing can help to avoid crack formation due to higher tensile strength of concrete developed.Drying shrinkage of concrete stored at unsaturated air can not be avoided as the equilibrium water content in the capillary pores is balanced at a relatively low water content [8]. Zero shrinkage of concrete is possible only at ~94RH% of the ambient that is not a usual case in civil engineering. Advanced shrinkage compensationCompensation of the shrinkage phenomenon and elimination of the shrinkage cracking need the following principal considerations: 1) a concrete mix of reduced cement paste content, 2) expansive cements or admixtures, 3) effective curing of concrete. Reduced cement paste content can be reached either by decreased cement content or low water-to-cement ratio (i.e. low water content). Water-to-cement ratio can be reduced by superplasticizer admixtures.Expansive cements or admixtures are developed to increase the volume of concrete after setting and during early age hardening. When concrete is properly restrained e.g. by reinforcement, expansion will induce tension in the re...