What is Curing in Concrete Construction?
Curing is the process of maintaining an adequate water cement ratio and temperature in concrete from the time the concrete is poured, and until the concrete has met the specified properties of the concrete mix design.
Why is Curing Necessary?
The water cement ratio in concrete is vital for a proper concrete mix. When hydration occurs, this reaction between the cement and water causes tiny crystals to form. If there is a lack or surplus of water in the mix, these crystals do not properly form, and the concrete is incapable of reaching strength.
A lack of water is the issue most frequently encountered due to evaporation of bleed water from the surface, especially in hot and arid climates. Imagine squeezing a sponge full of water. How do you keep the concrete from losing its water like this? Curing. Inadequate curing will ultimately lead to weak concrete that is more susceptible to cracking, abrasion, and adverse effects.
What are the Different Methods to Cure a Slab?
Overall, there are two points at which a concrete cure process must be considered: during the finishing process (initial curing) and after the finishing process (final curing).
Initial Curing – Both fogging and evaporation retarders can be applied during the placing and finishing process. This curing method also assists the finishers by increasing the “workability” of the concrete to achieve a better finish.
Fogging
Fogging is the act of creating a fog-like mist that is not applied directly to the slab but above the slab. This initial curing method creates a humid atmosphere for the concrete in dry climates and lowers the rate of evaporation.
Evaporation Retarder (liquid)
Evaporation retarder does exactly as the name implies; it stops the water from evaporating from the concrete. The evaporation retarder is sprayed onto the concrete being placed and creates a film that reduces the rate of evaporation as the concrete is being finished.
Final Curing – The method of curing a concrete slab after placing and finishing the concrete.
Sprinkling the Surface with Water
The sprinkling of water to cure a concrete slab is just as it sounds; the surface of the concrete slab is kept sufficiently moist as the concrete continues to set.
Positives: Effective and simple curing method.
Negatives: Water sprinkling cannot be applied when the temperature is below freezing. In addition, the cost of water and labor may be high as water must be periodically applied to keep the concrete surface damp.
What You Will Need: Access to water, hose, sprinkler, or spray gun.
Ponding
Ponding is the act of boarding up the edges of a concrete slab and flooding the slab with a few inches of water. Ponding is rarely applied, due to the extensive requirements.
Positives: Effective curing method.
Negatives: Time & labor consuming.
What You Will Need: Access to water, forms, nails, & hose.
Concrete Covers – Burlap Mats, Sand, or Straw with Water
There are multiple methods to covering a concrete slab for curing. Each of these materials are laid out on the surface of the slab prior to finishing and aid in maintaining a high moisture content. Some of the materials used include:
- Burlap/Cotton Mat
- Sand
- Straw
Positives: Effective curing method, if not the most effective curing method
Negatives: Time & labor consuming to place and remove burlap or other material.
What You Will Need: Access to water and a burlap mat, straw, or sand.
Concrete Cure – Liquid Membrane Forming Compounds
The most common form of concrete curing today is spraying a liquid membrane forming compound commonly referred to as ‘cure’. The cure is sprayed across the surface of the slab, which creates a film that reduces the evaporation of water from the surface. For most slabs, a clear resin cure is typically used that dissipates over time or is later removed with a cleaner. Some exterior slabs use a white pigmented cure (such as sidewalks) to give it a finished white color.
Positives: Cost & Labor Effective. Can be applied immediately upon finishing. Does not require hours of attention and labor.
Negatives: Be sure to confirm cure chemicals will not interfere with final material floor finish.
What You Will Need: Cure & Spray Gun
What Variables Affect the Curing of Concrete?
Weather: Temperature, Humidity, Precipitation, Etc. In arid or dry climates, concrete is highly susceptible to evaporation and drying out!
Concrete Additives: Retarders, Water Reducers, Acceleraters, Air Entrainmers, Shrinkage Reducers, and Superplasticizers.
How Long Does Concrete Take to Cure?
Concrete typically takes seven days to reach its designed compressive strength. However, concrete never stops curing. The concrete will reach its compressive strength and the chemical reaction does significantly slow down to an extremely low rate, but concrete will often reach 500-1,000psi above it’s designed compressive strength in twenty-eight days.
