## 6 Factors Affecting the CTE of Concrete  
### Aggregate Type and Properties  
The CTE of the coarse aggregate has the greatest influence on the CTE of the concrete since it comprises most of the volume of the material [9; 59]. With rising temperatures, aggregates expand almost linearly; the extent of this expansion is determined by the aggregate composition and characteristics while cement paste only expands up to 120 to 200 \({}^{\circ}\)C; after that, it contracts due to evaporation of water within the cement paste [60].  
One of the properties desirable for aggregate use in concrete is low thermal expansion, which enhances the cement paste's thermal compatibility [61]. When comparing concrete containing igneous and sedimentary aggregates, the former showed a lower average CTE and lower variability in the observed CTE [5] with quartz having the highest CTE followed by sandstone, granite, basalt, and limestones [62]. The CTE of other frequently used coarse aggregate types is mostly determined by their quartz concentration, with quartz having the highest CTE of all the coarse aggregate types frequently used in the building of concrete pavements [63]. While aggregates with high CTE, like quartz, contribute to increased thermal expansion, igneous materials with low thermal expansion, like granite and basalt, are preferred for enhancing the thermal compatibility of concrete. The CTE values of different aggregate types and concrete by aggregate type is presented in Table 2.  
A study discovered that concrete with siliceous river gravel (SRG) has a greater CTE than concrete with crushed limestone (LS) where gravel had a CTE value of 12.6 \(\times\) 10\({}^{-6}\)/\({}^{\circ}\)C, whereas limestone concrete had a value of 6.1 \(\times\) 10\({}^{-6}\)/\({}^{\circ}\)C and river gravel concrete had a value of 8.9 \(\times\) 10\({}^{-6}\)/\({}^{\circ}\)C [40]. The kind of coarse aggregate has a considerable impact on fracture width, with SRG portions consistently having greater cracks than LS sections in similar conditions [64; 65]. This was also observed in a study carried out in continuously reinforced concrete pavement test section in Houston where greater crack widths were observed while using SRG as opposed to LS [66]. This is due to SRG's thermal coefficient being around 60% higher than LS, producing more movement from temperature differentials and resulting in more severe cracking [67; 68; 69]. According to [PERSON] et al. [70], the aggregate's silica content has a major impact on the concrete's CTE. According to these investigations, the concrete's CTE increases with its silica content.  
Seven concrete mixes with various coarse aggregate mineralogy were tested in the lab for a study to examine the CTE, and the results indicated that limestone has the lowest CTE values. Furthermore, a study concluded that over time, CTE is impacted by the age of the concrete, increasing from 6.4% to 12.6% between 28 and 360 days [15].  
The authors of [68] advised the use of coarse aggregates with low thermal coefficients; however, if a coarse aggregate with a high thermal coefficient is to be used, or if construction is carried out in a hotter climate, measures should be taken to keep the concrete set temperature at the minimum to minimize thermally induced stresses. Since CTE of aggregates determines certain construction and curing practices, the authors suggest that aggregates with thermal coefficients exceeding 4.4 \(\times\) 10\({}^{-6}\)/\({}^{\circ}\)C may require additional measures to prevent cracking when exposed to temperatures above 32 \({}^{\circ}\)C [71]. Since aggregates are an integral part of the concrete, to reduce the thermal stress due to aggregates CTE, additional measures could be taken; e.g., [PERSON] et al. [72] demonstrated that blending low-CTE aggregates with high-CTE aggregates is a potential method to reduce the CTE of concrete. They also suggested that the percentage of replacement and the change in blended-aggregate CTE in concrete have a linear relation.