Thermal cracking due to early age thermal contraction can occur in large concrete sections. This where the smallest dimension is of the order of 0,5 m or larger, and the binder content is high enough to generate a significant amount of heat during hydration.
There are other contributing factors as well and this tip discusses the phenomenon.
Heat Of Hydration
Cement hydration reactions are exothermic and heat is therefore generated during hydration. The hydration reactions of fly ash, ground granulated slag and silica fume are also exothermic to a greater or lesser degree.
The critical factor is the rate of heat generation in the first 24 to 30 hours after placing. During this period the rate of heat generation exceeds that of heat lost to the environment and the temperature of the concrete rises.
As a rough rule of thumb one can expect a temperature increase of 10 to 12 degrees centigrade per 100 kg of binder in the mix with the peak temperature being reached between 24 and 48 hours after placing. The timing and magnitude of the peak temperature depends on the binder characteristics and the section thickness.
It is undesirable for the core temperature of the concrete to exceed 60 degrees centigrade or the temperature differential between the interior and exterior of the concrete to exceed 20 to 25 degrees centigrade.
Section Size and Geometry
The minimum section dimension determines the rate of heat loss of the concrete to the surrounding environment.
Generally speaking, heat of hydration only starts to become a problem once the minimum dimension exceeds 0,5 m. Once the least dimension exceeds about 2 metres, the interior of the concrete will not get any hotter, but the section will take longer to cool down.
Initial Concrete Temperature
The initial concrete temperature affects the rate of heat generation at early ages, but not the amount of heat generated and the ultimate rise in temperature.
The main benefit of cooling the plastic concrete, for example with ice or liquid nitrogen, is to depress the peak temperature and hence reduce the temperature differential between the concrete and its immediate environment.
While it might be expected that high ambient temperatures increase the probability of early thermal cracking, this is not necessarily the case.
Moderate daytime temperatures coupled with low night-time temperatures can be a more severe condition in practice.
For example, if concrete with a binder content of 350 kg/m3 is placed during the day at 20 degrees centigrade, the core temperature could reach nearly 60 degrees after 24 hours at the time of stripping. If the night-time temperature were to fall to only 10 degrees, the temperature differential could be as high as 50 degrees which would cause rapid cooling of the exposed surface of the concrete and thermal cracking caused by thermal contraction would be highly likely.
The probability of the thermal cracking of concrete is, however, a function of the restraint on the member.
If concrete is free to contract then thermal cracking will not occur. In reality however, there is always some internal and external restraint.
Examples of external restraint are if the concrete is cast onto a previously hardened base, or between or adjacent to similar elements without the provision of a movement joint.
In this case, the internal restraint is the still warm interior of the concrete, which will only cool and contract later. Differential thermal strains will cause cracking, the extent being dependent on the temperature difference and strain capacity of the concrete.
Reinforcement can be used to control the cracking of concrete to some degree. Using small diameter bars at close centres reduces crack widths, as does the use of the minimum allowable cover.
Common Examples of Early Thermal Contraction Cracks
According to the Technical Report, the most common occurrence of this type of thermal cracking is in cantilever walls in, for example, reservoirs, retaining walls, bridge abutments and basements. They are also common in other large pours where some member dimensions may exceed 2 metres.
In either case, if the temperature differential cannot be kept below 20° C, the preferred approach is to insulate the concrete surfaces which are exposed. This has little effect on the core temperature but keeps the surfaces warm and reduces the temperature differential. Temperatures can be monitored with thermocouples and insulation removed when appropriate.
Carefully co-ordinated planning between the designer and the contractor will reduce cracking:
Design and Specification:
- Restraint – size of pour, joints
- Distribution steel – size, spacing, cover
- Heat development - section thickness, cement type and content
- Restraint – sequence and timing of pours, additional joints
- Heat development – choice of concrete materials and formwork type
- Cooling – striking of formwork, curing, insulation