The heat produced by concrete during concrete curing is called heat of hydration. This exothermic reaction occurs when water and cement react. The amount of heat produced during the reaction is mostly related to the composition and fineness of the cement.
THE FIVE PHASES OF HEAT EVOLUTION IN CONCRETE
Heat evolution in concrete is a very complex and extensively researched topic. To simplify this process, the heat evolution over time can be separated into five distinguished phases. The heat profile can change depending on the type of cement. Typical hydration for Type I cement is graphically represented in the figure below.
PHASE i: PRE-INDUCTION
A short time after the water comes into contact with the cement, there is a sharp increase in temperature, which happens very quickly (within a couple minutes). During this period, the primary reactive phases of the concrete are the aluminate phases (C3A and C4AF). The aluminate and ferrite phases react with the calcium and sulphate ions to produce ettringite, which precipitates on the surface of the cement particles. During this phase, at a lesser extent, the silicate phases (mainly C3S) will also react in very small fractions compared to their total volume and form a very thin layer of calcium-silicate-hydrate (C-S-H).
PHASE ii: DORMANT PERIOD
This phase is also known as the induction phase. During this period, the rate of hydration is significantly slowed down. Traditionally, this is believed to be due to the precipitation of the aforementioned compounds on the surface of the cement particles, which leads to a diffusion barrier between cement particles and water. Nevertheless, there is significant debate on the physical and chemical reasons behind the occurrence of this stage and the methods to predict it. This is the period at which the fresh concrete is being transported and placed since it has not yet hardened and is still workable (plastic and fluid). The length of the dormant period has been shown to vary depending on multiple factors (cement type, admixtures, w/cm). The end of the dormant period is typically characterised by the initial set.
PHASE iii and iV:
STRENGTH GAIN In this phase, the concrete starts to harden and gain strength. The heat generated during this phase can last for multiple hours and is caused mostly by the reaction of the calcium silicates (mainly C3S and to a lesser extent C2S). The reaction of the calcium silicate creates “second-stage” calcium silicate hydrate (C-S-H), which is the main reaction product that provides strength to the cement paste. Depending on the type of cement, it is also possible to observe a third, lower heat peak from the renewed activity of C3A.
PHASE V: STEADY STATE
The temperature stabilises with the ambient temperature. The hydration process will significantly slow down but will not completely stop. Hydration can continue for months, years, or even decades provided that there is sufficient water and free silicates to hydrate, but the strength gain will be minimal during such period of time.
Why Monitor Concrete Temperature?
In Phase II, the temperature of concrete can be measured as the concrete is poured. The temperature measurement is typically done to make sure the concrete is in compliance with certain specifications that define a certain allowable temperature range. Typical specifications require the temperature of the concrete during placement to be within a range of 10°C to 32°C. However, different specified limits are provided depending on the element size and ambient conditions (ACI 301, 207). The temperature the concrete exhibits during placement affects the temperature of concrete during the next hydration phase. Monitoring the temperature of the concrete during phase III and IV is a quality control component that is regularly being performed. The main reason behind this measurement is to ensure the concrete does not reach temperatures that are too high or too low to allow proper strength development and durability of the concrete. Another reason for monitoring concrete temperature during this phase is to evaluate the in-place strength, where the rate of hydration is the principal behind the maturity method (ASTM C 1074).
Generally, a limit of 70°C is specified for the concrete temperature during hydration. If the temperature of the concrete during hydration is too high, it will cause the concrete to have high early strength but consequently gain less strength in the later stage and exhibit lower durability. Furthermore, it has been observed that such temperatures interfere with the formation of ettringite in the initial stage and subsequently its formation in the later stages is promoted; which causes an expansive reaction and subsequent cracking. Additionally, high temperature issues are of concern, especially in mass concrete pours, where the core temperature can be very high due to the mass effect, while the surface temperature is lower. This causes a temperature gradient between the surface and the core, if the differential in temperature is too large it causes thermal cracking.
COLD WEATHER CONCRETING
If the ambient temperature is too low, the hydration of the cement will significantly slow down or will completely stop until the temperature increases again. In other words, there will be a significant reduction or an end to the strength development. If the concrete temperature reaches freezing before reaching a certain strength (3.5 MPa) (ACI 306), the concrete will have a reduced overall strength. This will also cause cracking as the concrete does not have sufficient strength to resist the expansion of water due to the formation of ice. To ensure proper strength development and avoid cracking of the concrete, the general guidelines suggest that the concrete temperature must be maintained higher than a certain temperature for a specific amount of time (>5°C for 48hrs) (ACI 306).