Proper curing of concrete is critical for the cement to fully hydrate, prevent cracking, and develop the desired engineering properties of the concrete.

What Is Curing?

The action is taken immediately following finishing to maintain moisture and temperature conditions (for a sufficient period of time) in freshly mixed concrete to allow hydration and pozzolanic reactions to proceed.

• Proper curing prevents rapid water loss from the mixture and allows more thorough cement hydration.
• Temperature and moisture directly influence both early and long-term concrete properties such as strength and durability.

Properties Impacted by Curing

• Durability - the resistance to freezing/thawing and deicing chemicals.
• Strength - reductions up to 50% can occur.
• Permeability - the ease at which water enters the concrete.
• Abrasion Resistance - dusting or flaking.
• Volume Stability - an increase in the number of cracks.

Note: Exposed slab surfaces are particularly sensitive to curing as strength development and durability of the top surface of a slab can be significantly reduced when curing is neglected.

Curing Requirements

• Duration: As long as possible, but a minimum of 3 days. Begin immediately the following finishing!
• Temperature: 50-100°F (Do not confuse with curing cylinders!)
• Moisture: Keep the concrete saturated at all times. (If curing is interrupted then resumed, hydration will be reactivated but the original strength potential will not be achieved.)

Note: Strength development (hydration) ceases after the internal relative humidity in the concrete drops below 80%.

Cure System

• Too often it is the last thing on our minds.
• More critical with admixtures in use today.
• Less bleed water.
• More critical with low water-cement ratio.
• Concrete can dry out and not hydrate.
• We need to keep the moisture in the concrete to reduce curl effects.
• Cool air will have lower humidity than warm air and tend to increase the evaporation rate.
• The cure system should be on-site and tested prior to concrete placement.

Curing Tips

• Begin curing as soon as possible (within 30 minutes or immediately the following finishing).
• In the past, waiting until bleeding subsides was recommended.
• Today most mixes have low or no bleed rates and any delay in curing is not advisable.
• If the surface looks dry – it is & you are behind!
• Hairline cracks may develop which will allow water to penetrate and hurt the surface in the near future.

Curing Methods

Concrete can be kept moist (and in some cases at a favorable temperature) through the following curing methods:

1. Supplying additional moisture:
   Ponding, immersion, spraying, fogging, or wet coverings.
2. Sealing in the mix water:
   Membrane-forming curing compounds (white or clear), plastic sheeting (poly), impervious paper.
3. Accelerated curing:
   Live steam, heating coils, or heated forms.

SUMMARY

Strength and durability of concrete increase with age as long as moisture is present and temperatures remain favorable.
Lack of curing or poor curing practices can cause a loss of up to 50% of the potential strength and significantly decrease durability.