Concrete shrinkage cracking depends on several factors including free shrinkage, age dependent material properties, creep relaxation, shrinkage rate, and degree of restraint (Shah et al., 1998). Some other factors include, cement (Shoya 1979), water content (Neville 1996), cement fineness (Mehta 1994), length of curing (Powers 1959), aggregate (Neville 1996), and drying (Mehta 1994) amongst others. These factors are outlined and summarised in the following subsections.
20 2.3.1 Cement
Cement paste (i.e. water added with cementitious materials) is described as the portion of concrete that most commonly experiences volume changes (Tritseh et al., 2005). Therefore, the quantities of water and cementitious materials, and hence the water-cement ratio, have become important factors that influence the shrinkage behaviour. It is also known, that shrinkage increases with increasing water-cement ratio. In this light then, the water-cement ratio controls the evaporable water content per unit volume of paste and the rate at which water can reach the surface (Tritseh et al, 2005). However, for mixes with the same water-cement ratio, shrinkage increases with increment in cement content because the volume of hydrated cement or paste also increases (Shoya 1976).
2.3.2 Water Content
Water content on the other hand may not be as influential on shrinkage as cement content does. With constant water content, increasing the cement content may have no real effect or may even decrease shrinkage due to the reduced permeability caused by the now reduced water-cement ratio (Shoya 1979). However, the water content is significant, because of the fact that it affects the volume of the aggregate mix (Neville 1996). Similarly, according to Ödman (1968), shrinkage increases at a much greater rate with decreasing aggregate volumes than it does with increasing water-cement ratio. Similarly, Troxell (1996) backed the aforesaid observation by stating that, for a given w/c ratio, concretes of wet consistency have higher paste content and have a greater amount of shrinkage than stiffer mixture
2.3.3 Cement Fineness
Cement fineness can affect the drying shrinkage of concrete (Tritseh et al., 2005). Larger cement particles that do not undergo full hydration can provide a restraining effect similar to those provided by aggregates. For this reason, shrinkage values tend to be greater for finer cement (Mehta 1994). Following, Chariton and Weiss (2002) observed that mortar made with finer cement experienced lower weight loss due to drying than mortar made with coarser cement. They believed that the increased surface area of the finer cement increased the amount of pore water that was hydrated, and hence decreased the amount of evaporable water. However, they concluded that the finer cement resulted in a finer pore structure which caused capillary stresses and increased shrinkage.
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2.3.4 Curing Duration
According to Powers (1959), an argument was made, that the length of curing is relatively unimportant with regards to the overall concrete shrinkage. He believed that longer curing times reduce the amount of unhydrated cement particles, which previously restrained the paste from shrinkage. Curing also increases the modulus of elasticity and reduces the rate of creep of the paste. These effects lead to a greater cracking potential when the paste is severely restrained. Microcracking of the paste around the aggregates however, can diminish the total shrinkage in the concrete.
2.3.5 Aggregates
Neville (1996) pointed out that, the most important influence on shrinkage is the aggregate. The aggregate is said to retain shrinkage of the cement paste, consequently, the use of more aggregate allows for a mix with less paste. Similarly, aggregates provide restraint because they do not undergo volume changes due to changes in moisture conditions. The amount, size, and stiffness of an aggregate determine how much restraint it provides (Mindess, Young, and Darwin 2003). On the other hand, Pickett (1956) stated that shrinkage was reduced by 20% for mixes with the same water-cement ratio in which the aggregate content was increased from 71% to 74%. The amount of restraint provided by the aggregate depends on its elastic properties. Similarly, Reichard (1964) observed that concrete shrinkage was directly related to the modulus of elasticity and compressibility of the aggregate. However, lightweight aggregates with low moduli of elasticity exhibit higher shrinkage (Mindess, Young, and Darwin 2003).
Several other researchers (Karagular and Shah (1990), Shah, Karaguler, and Sarigaphuti (1992), Folliard and Berke (1997), Shah, Weiss and Yang (1998), Weiss and Shah (2002), See, Attiogbe, and Miltenberger (2003)) observed improved shrinkage resistance and cracking behaviour by using shrinkage-reducing admixture (SRA) in concrete. It is well known that SRAs works by reducing the surface tension of the mix water, which in turn reduces the stresses in the capillary pores (Shah, Weiss and Yang 1998). Similarly, Shah et al. (1992) found that free shrinkage decreased with increasing amounts of SRA and that crack widths were reduced compared to mixes of plain concrete.
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2.4 Heat of Hydration
The heat of hydration produces the first thermal load on a concrete member (i.e. a concrete deck). As the fresh concrete hydrates and gains strength, the exothermic chemical reaction produces heat within the concrete placement. The temperature of the concrete slowly drops to match ambient conditions as hydration proceeds. (Folliard K et al., 2004). This process is proportional to the size of the concrete member; larger members take longer to cool to ambient temperatures. The plastic concrete can accommodate thermal loads without developing thermal stresses; after hardening, any thermal load restrained against length change will induce stresses. Thermal stresses will be the highest if the concrete hardens when it is at its highest temperature by forcing the stress-free state to be at an elevated temperature. As a result, the average temperature that the deck experiences will be lower than the environment in which the deck hardened, causing a volume contraction throughout the life of the deck (Folliard. K et al., 2004).