How to Control Moisture Loss and Temperature in Freshly Placed Concrete

Curing is one of the most critical steps in concrete construction. As defined by ACI 308, curing is the action taken by contractors to maintain adequate moisture and temperature of freshly placed concrete, promoting the chemical hydration of the cementitious materials within the mix. Without proper moisture and temperature control, even a well-designed concrete mix can fail to achieve its intended strength, durability, and service life. Understanding how to manage moisture loss and temperature in freshly placed concrete is essential for every contractor and builder. For additional insight on how environmental factors affect fresh concrete, see our article on What Are the Effects of Rain On Freshly Placed Concrete.

Curing directly influences hardened concrete properties including strength, permeability, abrasion resistance, shrinkage, and resistance to freezing and thawing cycles. The curing-affected zone, which is the near-surface region of the concrete, is particularly vulnerable to inadequate curing practices. This article covers the key methods for managing moisture loss and temperature during the three stages of curing: initial, intermediate, and final.

Understanding Moisture Loss in Freshly Placed Concrete

When concrete is first placed, it contains more water than is needed for hydration. This excess water, known as bleed water, rises to the surface. If this water evaporates too quickly, the surface can dry out before hydration is complete, leading to plastic shrinkage cracking, surface crusting, and reduced durability. Managing the rate of evaporation is therefore the primary objective during initial curing.

The Science of Hydration and Curing

Hydration is the chemical reaction between cement and water that produces the compounds responsible for concrete strength and durability. This reaction requires a continuous supply of moisture. When the relative humidity inside the concrete drops below 80 percent, hydration slows significantly and can stop altogether. The purpose of curing is to keep the concrete saturated or as close to saturation as possible until the cement paste has achieved the desired properties.

The curing process is divided into three stages:

• Initial curing occurs between placing and final finishing. The goal is to reduce moisture loss from the surface while the concrete is still being worked.
• Intermediate curing occurs between final finishing and the start of final curing. This stage is needed when the surface is too soft for final curing methods to begin.
• Final curing lasts from the end of finishing through the end of the curing period. This stage reduces moisture loss and controls concrete temperature.

Why Moisture Retention Matters for Durability

Horizontal surfaces such as floors and exterior flatwork must be hard and durable to resist wear from pedestrian and vehicular traffic, as well as damage from exposure to freezing and thawing and deicing chemicals. Unlike vertical surfaces, which are free draining and less prone to environmental attack, horizontal slabs depend heavily on proper curing for long-term performance. For more on this topic, see Moisture Concrete Floors.

The table below summarizes the three curing stages, their timing, and primary objectives.

Initial Curing Strategies to Prevent Surface Drying

The period immediately after concrete placement is when it is most vulnerable to moisture loss. Several strategies can be employed during initial curing to prevent the surface from drying out before final finishing is complete.

Sunshades and Windbreaks

Erecting sunshades and windbreaks is the most direct way to protect freshly placed concrete from rapid evaporation. However, for most projects this is easier said than done. Sunshades and windbreaks are often not practical or efficient for large areas. The preferred alternative is to place concrete indoors or under a roof where ambient conditions can be controlled. It is considerably easier and more economical to manage moisture loss and maintain proper concrete temperatures when working under cover. In general, installing floors under a roof produces a higher quality product with fewer curing-related defects.

Fogging Techniques

Fogging is an effective method for increasing relative humidity above a freshly placed slab. By forcing water through special nozzles that atomize the water into a fine mist, the humidity directly above the concrete can be raised significantly. As the humidity increases, the evaporation rate from the concrete decreases.

Key points for effective fogging:

• Stand upwind and direct the fog spray above the slab so the mist settles onto the surface.
• Aim for a reflective or wet appearance on the concrete surface.
• On windy days, frequent or continuous fogging may be required.
• If water accumulates on the surface, treat it like bleed water. Do not finish accumulated water into the concrete.
• Allow accumulated water to evaporate before finishing operations resume.

Adding water to the surface by fogging effectively replaces the bleed water that has prematurely evaporated. It is acceptable for water to accumulate as long as it does not mar, penetrate, or erode the surface.

Evaporation Reducers

When sunshades, windbreaks, and fogging are not practical, evaporation reducers (also called evaporation retarders) can be applied to the concrete surface after strike-off and between finishing operations. These products are water-based mixtures containing chemical compounds that form an oil-like film over the concrete surface, reducing the evaporation rate of bleed water.

Important considerations when using evaporation reducers:

• The effective time of these products is limited. Repeat applications may be required depending on drying conditions.
• Because evaporation reducers are about 90 percent water, they should not be worked into the concrete surface to restore workability. Doing so increases the water-to-cement ratio at the surface, reducing quality and durability.
• These products can minimize the risk of plastic shrinkage cracking and surface crusting when applied correctly.

Final Curing Methods for Horizontal Surfaces

After final finishing, the concrete must be protected for the duration of the curing period. Final curing methods either add water to the surface or prevent moisture from escaping. Choosing the right method depends on project size, access to water, surface appearance requirements, and budget. Also read about How to Protect Freshly Placed Concrete During Curing for additional guidance.

Water-Based Curing Methods

Applying clean water to the concrete surface after final set is one of the oldest and most effective curing methods. Common techniques include sprinkling with garden sprinklers or soaker hoses, and ponding, where dikes or dams are built along slab edges to hold standing water.

Challenges with water-based curing include obtaining a sufficient supply of clean water, managing runoff, and constructing water-retaining barriers. Water must be free of harmful impurities that could attack or stain the concrete. The concrete must also be hard enough that the surface is not damaged by premature water application.

Wet burlap and cotton mats offer a more practical approach to water-based curing. These absorbent materials hold water against both horizontal and vertical surfaces. Best practices include:

• Thoroughly rinse burlap with clean water before placement to avoid concrete staining.
• Keep the material saturated throughout the curing period.
• Cover burlap or mats with plastic sheets or tarps to reduce evaporation.
• Do not let burlap dry out, or it will absorb moisture from the concrete instead of supplying it.
• Always dry burlap before storage to prevent mildew and rot.

Synthetic Curing Covers and Plastic Sheeting

For commercial and industrial slabs, curing covers made from synthetic fabrics and bonded white-pigmented polyethylene films have become extremely popular. These covers offer easy application, excellent moisture retention, and efficient curing. Due to the polyethylene film layer, no additional water is typically required after the initial wetting, even for a seven-day cure period.

Plastic sheeting (polyethylene) works by entrapping moisture within the concrete. When using plastic sheeting, follow these guidelines:

• Use a minimum thickness of 0.004 inches.
• Use white sheeting during hot weather to minimize heat gain from solar radiation.
• Use black sheeting for cold weather concreting to maximize heat gain.
• Note that plastic sheeting can cause patch discolorations or a mottled appearance due to variations in moisture and temperature beneath the sheeting.
• Periodically flooding the concrete under the sheeting can help minimize surface discolorations.
• For exterior applications, anchor sheeting securely to prevent wind from tunneling underneath or displacing it.

Waterproof reinforced paper is installed similarly to plastic sheeting. Holes and tears should be repaired promptly, and the concrete should be re-wetted under the sheeting if needed.

Liquid Membrane-Forming Compounds

Spray-on curing compounds, also known as liquid membrane-forming compounds, work by forming a surface membrane that seals moisture into the concrete. These are the preferred curing method for large surface areas or areas that are difficult to cure using other methods. Typical compounds consist of waxes or resins emulsified in water or solvents. Water-based compounds have become more common due to VOC emission regulations.

Proper application of curing compounds requires attention to several factors:

• Apply uniformly at the manufacturer’s recommended coverage rate.
• For small areas, use a wide, soft-bristle brush or paint roller.
• For large areas, use a power sprayer with the appropriate wand and nozzles.
• Apply immediately after the disappearance of the surface water sheen following final finishing.
• Delayed application allows surface drying and increases the likelihood the compound will be absorbed instead of forming a water-retention membrane.
• If needed, re-wet the surface and allow it to achieve a uniform damp appearance before applying the compound.

There are many different types and classes of curing compounds formulated for different applications. Contractors should review specifications to determine which type and class has been specified for the job.

Curing Vertical Surfaces and Best Practices for All Concrete

While horizontal surfaces require the most attention during curing, vertical concrete elements such as walls, columns, and beams also need protection. Curing vertical surfaces is less critical than curing horizontal surfaces because vertical elements are free draining and less prone to freeze-thaw damage. However, neglecting vertical surface curing can still lead to soft, chalky, and discolored finishes. For related information, see Carbonation in Freshly Placed Concrete Slabs Causes Risks.

Form Retention and Non-Absorbent Facing Materials

Leaving forms in place is the most common approach to limit moisture loss from walls, columns, and beam sides. However, keeping forms on for an extended period of three to seven days ties up formwork, causing construction delays and requiring an increased inventory of forms. Both outcomes can increase costs.

When forms are used, facing materials must be non-absorbent. Metal forms or sealed plywood surfaces minimize moisture loss from the concrete. Absorbent facing materials draw moisture from the concrete, creating weak, chalky surface layers and discoloration. If additional curing is needed after form removal, contractors can:

• Sprinkle vertical surfaces with water.
• Apply wet burlap against the surface.
• Use plastic sheeting secured against the concrete.
• Apply liquid membrane-forming compounds by sprayer or roller.

Common Curing Mistakes to Avoid

Regardless of the curing method selected, several universal principles apply. Avoiding these common mistakes will improve curing outcomes:

1. Allowing concrete to dry during the curing period. If surface drying occurs, immediately re-wet the surface. Dry concrete will not absorb sufficient external water to restore internal moisture loss.
2. Delaying the application of curing compounds. Late application allows the surface to dry, and the compound gets absorbed rather than forming a protective membrane.
3. Using absorbent form facing materials. Untreated plywood and other absorbent materials draw moisture from the concrete, weakening the surface.
4. Finishing accumulated fogging water into the surface. This increases the water-to-cement ratio at the surface, reducing durability.
5. Letting wet burlap dry out. Dry burlap reverses its function and starts pulling moisture from the concrete.
6. Using plastic sheeting when appearance matters. Plastic can cause uneven discolorations and a mottled finish.

Proper curing is not an optional step in concrete construction. It is a critical process that directly determines the strength, durability, and appearance of the finished concrete. By understanding the three stages of curing and selecting the appropriate methods for each stage, contractors can ensure their concrete performs as intended throughout its service life. Whether using fogging and evaporation reducers for initial protection, water-based methods for final curing, or membrane-forming compounds for large areas, the key is to maintain adequate moisture and temperature control from placement through the end of the curing period.