# Effects of Freeze-Thaw and Wet-Dry Cycles on Tension Stiffening Behavior of Reinforced RAC Elements
## 1 Introduction  
The continuous development of the construction and infrastructure sectors has become a global concern in regards to sustainability issues, due to the high consumption of non-renewable natural resources as raw materials and the massive production of Construction and Demolition Waste (CDW). The use of CDW to produce Recycled Concrete Aggregate (i.e., RCA identifying recycled aggregate obtained from concrete debris) can be an effective way to mitigate these negative environmental impacts [1, 2, 3]. The main difference between Recycled Concrete Aggregate (RCA) and Natural Aggregate (NA) relies on the presence of the existing Attached Mortar (AM) layer on the recycled aggregate surface, and, consequently, the presence of an Interfacial Transition Zone (ITZ) between AM and original aggregate [4, 5]. An investigation carried out by [PERSON] et al. [4] showed that AM makes the RCA a more porous material compared to NA. As a consequence, RCA presents lower value of specific gravity and higher water absorption capacity,apart from typically higher value of L.A. abrasion wear [4; 6]. Despite this, [PERSON] et al. [7] verified the feasibility of producing normal and high strength Recycled Aggregate Concrete (i.e., RAC identifying concrete mixture produced with recycled aggregates) without compromising mechanical properties by using the Compressible Packing Model (CPM) for the rational proportioning of RAC mixtures. However, the use of RCA in the construction sector is not widely accepted and it is often limited to non-structural applications [8]. Besides the mechanical performance, durability is one of the main requirements for concrete structures, as they are widely exposed to environmental attacks, but should guarantee the requested safety levels during their entire life service. In fact, degradation mechanisms are usually driven by the exposure to environmental conditions, such as temperature and relative humidity variations and the availability of ions and deleterious agents [9]. The most common attacks that compromise durability of reinforced concrete structures are corrosion of steel, freeze-thaw damage, sulfate and chloride attack, salt scaling, and alkali aggregates reactions [10]. As a common consequence, all these attacks can result in cracking and spalling of the reinforced concrete surfaces, intensifying the degradation processes. Within this context, some studies have been carried out to investigate the steel-concrete bond behavior under the action of environmental factors and, in this general context, this paper focused on the degradation processes of reinforced Recycled Aggregate Concrete (RAC) due to the effects of freeze-thaw and wet-dry cycles. Since freeze-thaw cycles reduce concrete strength, change the internal pore structure and cause the appearance of cracks, they also influence the bond between steel and concrete, which can affect structural performance [11; 12]. The damage caused to the structure by repeated freeze-thaw cycles varies from the surface fragmentation to complete concrete disintegration. However, the damage caused by this phenomenon can be minimized by reducing the capillary pores volume in the concrete with, for example, a lower _w_/c ratio or using suitable admixtures [13; 14; 15; 16]. [PERSON] et al. [17] investigated durability of both normal- (C35) and high-strength (C60) RACs produced with RCAs derived from different sources and subjected to different numbers of freeze-thaw cycles (0, 150, and 300). The results showed that the degradation mainly affected the concrete mortar. For the high-strength RAC, the attached mortar was the fragile phase, while on the normal-strength RAC, the new mortar was the one which did not resist the internal efforts generated by the cycles. The degradation cycles had a higher impact on the mechanical and physical performances of normal strength concretes than the high strength mixtures. Nevertheless, all concrete mixtures showed a reduction in the compressive and tensile strength, as well as in the concrete mass.