# The Effect of Curing Conditions on Selected Properties of Recycled Aggregate Concrete
## 1 Introduction
In [24] it was stated that the use of aggregate soaked in water significantly reduces the water absorbability and sorption of concrete in relation to composites, where superficially dried aggregate was used (without pre-soaking). The latter type of aggregate contributes to a higher porosity of the contact zone of the aggregate-cement paste. Furthermore, the presence of water in aggregate grains is an additional source (as well as classical curing) of water during hydration processes. For this reason the two-stage mixing approach (TSMA) is recommended in [25]; this is an effective way of ensuring stability at the time that the concrete mix is made, and achieving a higher compressive strength and durability of the concrete. Thus, TSMA seems to be the most optimal method, taking into account the conclusions of [23], with a pre-soaking time of about 30 min. In the context of the mentioned RCA defects and RAC defects, it is particularly important to obtain the firm and tight microstructure of concrete, which is, among other factors, determined by the conditions of its curing. Improperly cured concrete achieves lower quality. Hence, the appropriate curing conditions of RAC seem to be more important than in the case of ordinary concrete, especially because of the different characteristics of their interfacial transition zones [26; 27; 28]. However, this does not mean that studies on the influence of curing conditions on the properties of concrete made of natural aggregates have been neglected. One of the most recent studies concerned the effect of curing conditions on the properties of such concretes but produced from alkali-activated cements as binders which enable the reduction of the carbon emissions footprint in comparison with plain Portland cement. Special attention has been focused on the influence of curing conditions on shrinkage, which to some extent determines the durability of concrete [29; 30]. The data in the literature concerning the influence of curing conditions on the properties of concrete with waste aggregates, including recycled concrete aggregate, are not numerous, but the results of research conducted with different material assumptions as well as curing conditions may contribute to the knowledge on this subject. In [31], the effects of curing conditions of four types were studied: laboratory curing at 100% relative humidity (RH) and 20 \({}^{\circ}\)C, outdoor natural curing (with a variable temperature and air RH from 25% to 88%), indoor storage (RH = 45-65%), and tap water storage. After using recycled concrete aggregate at 20%, 50%, and 100% as a substitute for natural aggregate, the 56-day relative (relative to natural aggregate samples) compressive strength decreased slightly with an increase in the amount of recycled aggregate for all curing conditions, except for the case of the curing of concrete samples stored in a chamber with RH equal to 100%. In the case of 100% RCA, only the external natural conditions of curing contributed to a decrease in strength (by 7%). In the case of the modulus of elasticity, its reduction was more related to the increase in the amount of concrete recycled aggregate than to the curing method, although slightly better results were obtained for samples stored in tap water and in a climate chamber with RH = 100%. Interesting insights have been provided into the curing of recycled aggregate concrete in steam curing, which has proved to have an adverse effect on the compressive strength of concrete. According to [32], steam treatment for 4 h after concrete mixes were made, combined with subsequent curing in a chamber with a high level of RH (>95%), resulted in a reduction in the 90-day compressive strength of concrete. Although one-day compressive strength proved to be higher by approx. 20%, no difference was observed after 28 days. In turn, thanks to steam curing, the modulus of elasticity of recycled aggregate concrete slightly increased. Such a trend was also observed in the case of splitting tensile strength.