Concrete Cracking Part 2: Shrinkage & Thermal Behaviour
Thermal cracks can occur in concrete as temperatures increase or decrease. This includes both rapid changes from a day to day basis when the concrete has been poured recently, but mostly occur over long periods of times with the change of seasons.
How does Thermal Behaviour cause cracks?
As temperatures increase, concrete will expand as the atoms are more active at greater temperatures, which in turn causes them to take up more space and ultimately crease the size of the material. Meanwhile, during cooler temperatures, the concrete will contract as the atoms are less active.
Daily Temperature Changes
Daily changes in temperature will cause a slab of concrete to have varying temperatures throughout the thickness of the slab. While the surface temperature is that of the air around it, inside may be cooler or warmer depending on the previous day. This means that on overly hot days the concrete might be expanding near the surface, but underneath the slab, it remains the same size. This causes the slab to crack on the surface. Likewise, if it is a cold day and the surface is cooler than that below the top will contract, causing a curling shape on the surface. This can cause cracking; however, this is far more common on freshly poured concrete and daily temperatures will seldom affect concrete once it has completely set.
The change in length of a concrete slab is due to seasonal temperatures changes. Most places will have at least a 10°C change due to seasons changing. If concrete was to be poured in summer, its length will decrease during the cooler months, as the temperature drops. This can cause cracking due to shrinkage and contraction. Alternatively, if the concrete is poured in winter, the concrete will grow during the warmer months, causing expansion cracking. As such expansion and contraction joints are installed in slabs to ensure that concrete does not buckle and remains strong.
Heat of Hydration
The term ‘heat of hydration’ is used for the heat that concrete generates as cement goes through hydration. This is due to an exothermic chemical reaction process while cement is setting into concrete slabs. Without proper hydration concrete will not properly form, because of this concrete curing methods are used to help ensure that the concrete remains hydrated enough. When the concrete is about 12 hours old, the amount of heat it generates can cause the concrete to be between 35°C and 50°C. It is hardening at a high, expanded temperature, and cools down the concrete will naturally contract. However, as the concrete is already stiffened, this contraction will cause cracks. Because of this, it is vital that installation of any and all expansion/contraction joints be made within 4-12 hours. More information about these joints can be found in part 3.
Much like soils, concrete can shrink and expand due to moisture content. When cement is mixed with water and poured, the cement particles harden around the water, these are known as capillary void spaces. Another type of void space that occurs is between the concrete fibres as it hardens and is called gel void space. Combined the void spaces make up approximately 20% of the total volume in concrete, which is a rather large amount. Throughout the concrete water can move very slowly through the capillary voids. When water starts to evaporate from capillary voids the remaining water in the concrete goes into a state of tension, pulling the solid material fibres closer together, causing the concrete to shrink and cracks to occur.
We already know that some cracking is going to occur regardless of our actions, it simply can’t be avoided completely. However, there are ways to lower the amount and avoid detrimental cracking that will affect the overall concrete strength.