Curing Methods and Cold/Hot Weather Placement

Why Curing Matters

Concrete does not harden by drying out — it hardens through a chemical reaction called hydration, in which water combines with portland cement to form the binding compounds that give concrete its strength. If water evaporates too quickly from the slab surface, hydration stops prematurely and the concrete never reaches its design strength. Curing is any method that keeps the concrete moist and at an acceptable temperature long enough for hydration to proceed. ACI 308 guidance indicates that for ordinary portland cement mixes under normal conditions, a minimum of 7 days of continuous curing is a standard benchmark for adequate early strength development.

Liquid Curing Compounds

A membrane-forming curing compound is a liquid applied to the concrete surface shortly after final finishing. It forms a thin film that seals moisture inside the slab. Compounds compliant with ASTM C309 are the standard for flatwork. Most are water-based or solvent-based resin solutions; always check the Safety Data Sheet (SDS) before use — some solvent-based products require respiratory protection in enclosed spaces.

Application procedure:

1. Finish the slab and allow bleed water to disappear completely before applying. Applying over standing bleed water dilutes the compound and causes holidays.

2. Fill a hand-pump garden sprayer and pressurize it. Wear rubber gloves and safety glasses — the compound can irritate skin and eyes.

3. Apply in overlapping passes (overlap each pass by roughly one-third) at the coverage rate stated on the product label. Coverage rate is usually expressed in square feet per gallon.

4. Inspect for holidays — missed or thin spots — by holding a raking light source at a low angle to the surface. Holidays appear as dull patches against the sheen of the film. Re-apply immediately over any holiday.

5. Log the application time, product name, lot number, coverage area, and any re-application in your lab log. Photograph the finished surface.

Do not apply curing compound on a surface that will receive bonded topping, tile, or coatings — the membrane prevents adhesion.

Wet Burlap and Polyethylene Curing

This method physically keeps water at the slab surface throughout the curing period. It is more labor-intensive than a compound but leaves no residue and is compatible with surfaces that will receive further finishes.

Procedure:

1. Soak burlap thoroughly before placing it. Dry burlap will wick moisture out of the slab rather than hold it in.

2. Lay burlap gently over the finished surface — do not drag it, which would damage the fresh concrete.

3. Cover the burlap immediately with 6-mil polyethylene sheeting, lapped at seams by at least 12 inches and weighted at all edges with boards or sandbags to prevent wind from lifting it.

4. Check the burlap daily. If it dries out, re-wet it. Never let burlap dry against fresh concrete.

5. Maintain for the full curing period — at least 7 days for standard Type I/II portland cement mixes under ordinary temperature conditions.

Hot-Weather Concrete Placement (ACI 305R Guidance)

High temperatures accelerate hydration and evaporation, reducing workability, increasing the risk of plastic shrinkage cracking, and lowering long-term strength. Precautions include:

• Use chilled mix water or ice to lower the fresh concrete temperature.
• Apply an evaporation retarder (spray-applied to the surface between finishing passes) when evaporation rates are high.
• Erect sunshades and windbreaks at the pour site.
• Schedule pours for early morning or evening when ambient temperatures are lower.
• Begin curing immediately after final finishing.

ACI 305R provides detailed guidance on hot-weather concrete production and placement.

Cold-Weather Concrete Placement (ACI 306R Guidance)

When temperatures fall below 40 °F, hydration slows dramatically; at or below freezing, water in the mix can freeze before the concrete gains adequate strength, causing permanent damage. Precautions include:

• Heat mix water and aggregates to raise the fresh concrete temperature.
• Use accelerating admixtures (such as calcium chloride where permitted, or non-chloride accelerators for reinforced work) to speed strength gain.
• Cover finished concrete immediately with insulating blankets to retain heat and maintain temperature above freezing.
• Protect against rapid temperature drops when blankets are removed (gradual removal prevents thermal shock cracking).
• Monitor concrete temperature with thermometers embedded in the slab.

ACI 306R provides detailed cold-weather concrete guidance.

Safety on the Lab Floor

• Wear safety glasses and rubber gloves when handling curing compounds and wet-curing materials.
• Review the SDS for any chemical product before use.
• Keep the spray nozzle pointed away from people and away from your face when pressurizing the sprayer.
• Wet polyethylene sheeting is extremely slippery — walk carefully around cured panels.
• Good housekeeping: roll and store burlap and sheeting promptly to prevent trip hazards.
• Dispose of unused compound and contaminated materials per the SDS instructions.