What is Heat of Hydration?
When water is added to cement, it initiates a series of chemical reactions collectively known as hydration. The hydration process generates heat as a result of the chemical reactions that occur between cement and water. This phenomenon, known as the exothermic reaction, is a critical aspect of concrete curing. The primary compounds in cement react with water to form calcium silicate hydrate (C-S-H) and calcium hydroxide (CH). These reactions generate heat as a byproduct — known as “heat of hydration.”
The Stages of Heat Generation
The heat generation during hydration occurs in stages. First is the Initial Stage: Upon mixing, there’s a rapid but brief generation of heat, known as the initial heat burst, which lasts for a few minutes.
This is followed by a Dormant Period where heat generation slows down. During this phase, the concrete remains workable.
After the dormant period is the Acceleration Phase. The main hydration reactions begin, leading to a significant increase in heat generation. This is when the concrete starts to harden and gain strength.
Deceleration and Steady State: As hydration continues, the rate of heat generation gradually decreases until it reaches a steady state as the reactions near completion.
Heat generated during hydration is beneficial in many cases because it accelerates the curing process and increases the early strength of concrete. However, in mass concrete pours or large structures, excessive heat generation can create temperature differentials between the interior and exterior of the concrete, leading to cracking. Managing the heat of hydration through mix design, cooling methods, and proper curing practices is essential to ensure the structural integrity and durability of the concrete.
A Temperature Differential Occurs as Heat Dissipates
The temperature throughout freshly poured concrete is not uniform. Heat dissipates more easily, (and therefore more rapidly), along the outer surfaces of the concrete. As the outer surfaces cool and harden, the concrete in these areas begins to contract.
However, the temperature in the core of the concrete remains heated for longer. This heat causes an outward expansion of the concrete.
These opposing forces cause internal stresses in the concrete. If the temperature differential becomes too great, cracking may result.
What Role Does Temperature Play in Concrete Cracking?
There are various ways that temperature plays a role in the tendency for new concrete to crack.
- Thermal Expansion and Contraction: High temperatures cause concrete to expand, while low temperatures cause it to contract. If concrete is subjected to significant temperature fluctuations, these expansions and contractions can induce stress within the material, leading to cracks.
- Temperature Differential: If different parts of the concrete structure experience different temperatures, this can create internal stresses due to uneven expansion or contraction, leading to thermal cracking.
- Curing Conditions: The temperature during the curing process is vital. Excessively high temperatures during curing can cause the concrete to set too quickly, reducing its final strength and increasing the likelihood of cracks. Conversely, very low temperatures can slow down the curing process, leading to delayed setting times and potential freezing of the mix, which can also cause cracks.
- Plastic Shrinkage Cracking: If the surface of freshly poured concrete dries out too quickly due to high temperatures (especially in combination with wind or low humidity), it can shrink rapidly, causing shrinkage cracks. This is more common in hot weather.
- Thermal Gradient: During the early stages of curing, if there’s a significant temperature gradient between the interior and exterior of the concrete (like in mass concrete pours), the differential cooling rates can cause internal stresses that lead to cracks.
Managing Concrete Temperature
Effective management of concrete temperature during a pour is crucial to ensuring the quality, strength, and durability of the finished product. Temperature control is particularly important in large or mass concrete pours, where excessive heat generation can lead to cracking, delayed setting, and reduced strength. Here are some effective strategies:
- Pre-Cooling the Ingredients:
- Chilled Water: Use chilled water for mixing to lower the initial temperature of the concrete.
- Ice: Replace part of the mix water with crushed ice, which melts and absorbs heat as it mixes, lowering the concrete temperature.
- Cool Aggregates: Pre-cool the aggregates by shading them or spraying them with cold water before mixing. This reduces the temperature of the overall mix.
- Controlling the Mix Design:
- Low Heat Cement: Use a cement type that produces less heat during hydration, such as Type IV cement, which is specifically designed for large pours.
- Supplementary Cementitious Materials: Incorporate materials like fly ash or slag, which generate less heat during the hydration process.
- Lower Cement Content: Reduce the cement content in the mix, as less cement means less heat generation. This option is particularly effective if the concrete producer is using a precision inline aggregate blending batch plant. This method of mixing concrete is proven to produce concrete of exceptional strength and quality over traditional methods when mixing with reduced cement content.
- Temperature Monitoring:
- Thermocouples: Embed thermocouples in the concrete to monitor internal temperatures at different depths. This allows for real-time adjustments if temperatures approach critical limits.
- Thermal Control Plan: Develop and implement a thermal control plan that sets temperature thresholds, identifies cooling methods, and outlines actions to be taken if temperatures exceed safe limits.
- Environmental Controls:
- Insulating Blankets: Cover the concrete with insulating blankets or thermal mats to reduce heat loss during cold weather or to slow down the temperature rise in hot weather.
- Windbreaks and Shading: Set up windbreaks and provide shading to reduce the effects of wind and direct sunlight, which can accelerate surface drying and create temperature gradients.
- Staggered Pours: Pour the concrete in layers or segments. Allow each layer to cool and cure before pouring the next.
- Curing Methods:
- Moist Curing: Keep the concrete surface moist by applying water, using wet burlap, or applying curing compounds. This slows down the evaporation process, helps maintain a more consistent temperature, and reduces the risk of cracking.
- Extended Curing: In some cases, extending the curing period with moist curing or by maintaining a more controlled environment can help manage temperature and improve the overall quality of the concrete.
By combining these methods, concrete temperature can be effectively managed during a pour, minimizing the risk of thermal cracking and ensuring the long-term durability of the structure.