# Limitations on the Use of Recycled Asphalt Pavement in Structural Concrete
## 4 Mechanical Behaviour of Concrete Containing RAP
### Durability  
The implementation of RAP in concrete needs to be investigated from another perspective, which is its intended use in buildings: is RAP concrete suitable for structural applications or does the strength reduction limit it to non-structural applications? Recent research [5] suggests that RAP concrete may retain sufficient strength for structural applications if high strength concrete mixture designs are used. For high strength concrete applications, durability must be proven, and an evaluation of Alkali Silica Reactivity (ASR), chloride permeability, freeze-thaw durability, and coefficient of thermal expansion is essential. In terms of concrete durability, ASR is considered a deleterious reaction between the alkaline pore solution of concrete and various metastable forms of silica contained in aggregates [101]. In particular, ASR of RAP aggregate has been investigated by [PERSON] et al. [49] by using the methods reported in ASTM C1260 [102]. In this standard, ASR is studied in terms of the expansion of mortar bars at 80 \({}^{\circ}\)C at 14 days and an expansion of more than 0.20 percent is considered potentially deleterious [49]. From the results reported in [49], the expansion of the bars was around 0.23 percent, therefore exceeding the limit posed by the standard. However, it must be said that during the testing phase, the elevated temperature affected the bituminous material on the RAP aggregates by causing its stripping from the exterior of the mortar bars and the formation of a slick on the top of the solution [49]. For this reason, ASTM C1260 [102] results are a poor indicator of ASR vulnerability for RAP aggregates. Similarly, [PERSON] et al. [48] reported that the expansion is due to the elevated temperature in the NAOH solution and in order to mitigate this expansion, multiple solutions, such as the use of supplementary cementitious materials and/or a low-alkali cement, should be considered [48]. However, according to [PERSON] et al. [48], the test for ASR revealed that RAP aggregates were non-reactive. Furthermore, in [33; 39], it was reported that thanks to an increase in the RAP binder content, which causes an increase in viscosity, it was possible to reduce the alkali-silica reactions in RAP aggregates. Chloride permeability is a function of porosity and pore connectivity, and, specifically, low porosity and limited pore connectivity reduces the diffusion of chlorides into the concrete matrix. The permeability of asphalt is much higher than that of even low-quality concrete [5]. This could be problematic if the concrete obtained with the utilization of RAP is destined to be coupled with steel reinforcement, since the inclusion of RAP could limit the resistance to chloride permeability [5]. However, results reported by [5] indicate that the chloride permeability of high strength concrete, estimated by surface resistivity using a Wenner array probe in accordance with the specifications of AASHTO TP 95 [103], is unaffected by the replacement of up to 50% of natural coarse aggregate with RAP [5]. Unlike chloride permeability, freeze-thaw durability in concrete is a function of the pore structure [5]; specifically, good freeze-thaw durability requires pores of sufficient size and connectivity to allow pore water to expand as it freezes without exerting pressure on the matrix. Air voids facilitate the expansion of pore water, which provides a great resistance of concrete to freezing and thawing. The presence of RAP aggregates improves freeze and thawing resistance to a limited degree, because RAP is not distributed in a uniform way and spacing remains too large for any significant improvement in freeze-thaw durability [5]. [PERSON] et al. [48] reported that an excellent freeze-thaw durability is exhibited for normal concrete with 0 and 20% RAP coarse aggregate, while [PERSON] et al. [49] showed a reduced freeze-thaw durability when the percentage of RAP was increased from 50% to 100%.