Thermal cracks are one of the hazardous cracks that can appear in mass or large concrete pours. These cracks can appear at 1 day to a few weeks making early age category cracks. The main reason for thermal cracks is the heat of hydration. The chemical reaction between cement and water produces heat. The temperature of the concrete increases due to this accumulation of heat energy. Until the heat generation rate matches the rate of heat loss to the surrounding, concrete keeps increasing its core temperature.
Having a hot core is not a direct reason for the thermal cracks to appear. It’s the temperature difference between core and surface, known as temperature gradient cause the cracking. During the temperature rise, the expansion is bearable because concrete is in the plastic stage. However, the cooling stage is completely different. The outer surface starts to cool down well before the core. When cooling, concrete starts to shrink while the heated core keeps expanding. This difference movement between layers makes the concrete crack.
The concrete internal temperature depends on various factors. It is important to understand how these factors affecting the hydration temperature in order to predict it.
The initial temperature of the concrete
Placement or the initial temperature of concrete is vital for peak hydration temperature. When the placement temperature is high, peak hydration temperature increases according to that. Initial concrete temperatures mainly depend on the material proportions and their temperatures.
You can estimate the fresh concrete temperature easily using the following equation.
The most effective method for adjusting the fresh concrete temperature is adding ice. The maximum ice content you can add is the water content allocated by mix design. Another method is reducing the temperature of the material. The use of chill water and cooling the aggregates are some effective ways of reducing fresh concrete temperature. Add the following spreadsheet to your library and calculate the fresh concrete temperature of various mixes.
Thermal capacity and thermal conductivity of concrete
The specific heat of the concrete varies around 1000 J/kg°C. Thermal conductivity varies between 1.5 to 3.5 J/m2S. According to the values concrete is highly effective to the heat. On the other hand, concrete take more time to dissipate the internal heat energy to the environment. This makes the internal temperature to rise up during hydration.
Cementitious material quantity and quality
The hydration rate is highly dependent on the cementitious material you are using in the mix proportion. Ultrafine cement hydrates at a high rate and fly ash mixed concrete hydrates at a low rate. This will affect the peak temperature as well as the peak time. When the hydration rate is low, the concrete has enough time to dissipate its energy into the environment. This will reduce the peak internal temperature. However, extending the hydration time will cost in curing and construction progressing rate.
Shape and size of the concrete
The size of the pour is directly involved with the total heat energy due to hydration. Large concrete pours produce high heat energy during hydration and keep up the internal heat for a long time. These situations should be addressed separately. An external cooling method such as post-cooling systems and air conditioners help to maintain the internal temperatures at low levels.
The shape of the concrete pour is another parameter that influences the internal temperature. Shapes having high surface area will cool down quickly if they are exposed.
Properties of formwork material and dismantling time
Different formworks have different insulating properties. Timber, plywood, and fiber forms are quite slow in transferring the heat. They maintain temperature buffer between concrete and the environment. So the overall temperature difference between surface and the core is less with these forms. Dismantling time of forms is critical with timber and plastic-type forms because they have insulating properties. Removing these forms change the exposure conditions completely.
Steel forms on the other hand heated and cooled down quickly to the ambient temperature. Therefore steel formworks are better heat dissipaters. Peak temperature and heated time are less with steel formworks.
Ambient temperature, wind velocity, and humidity conditions have a clear effect on concrete peak temperature. When the day and night environmental temperature variation is significant, the risk of cracking will increase at night time.
The different thermal properties of foundation and any connecting structures can develop unbalanced stress during heating as well as cooling periods. When the temperature changes, concrete try to expand or shrink. Available restraint conditions try to hold these movements and then develop stresses. Foundations are a common example of these kinds of situations.
Peak hydration temperature
The heat of hydration of cement can be measured by an adiabatic calorimeter measurement. Semi adiabatic calorimeters test is easier to perform but less precise. There are few computer programs available for peak temperature prediction.
ConcreteWorks by Texas transportation department is such a handy tool to predict the internal temperature and calculate the risk of thermal cracking. The program incorporates all the important parameters and calculates the hydration temperature.
Thermal cracks appear due to swelling and shrinking under thermal stress in the concrete. Sometimes external restraints also affect the thermal expansion of the concrete causing cracks. The widely used thumb rule to avoid thermal cracks is to maintain less than a 36°F (20°C) temperature difference between in and out the concrete. Keep in mind this is the total temperature different in and out of the concrete.