Concrete tips: why effective curing practices are important

by arslan_ahmed | October 10, 2022 11:52 am

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By Dean E. Craft, DBA, CSI, CDT, CCCA, ASTM, ACI, LtCol USMCR (ret.)

Proper concrete curing methods are critical for achieving the overall strength, durability, performance, and aesthetics of concrete. It is directly related to reducing maintenance costs, extending service life, and enhancing the sustainable attributes of concrete. Unfortunately, despite the relatively low cost of effective curing practices, this step is often overlooked or minimized. Failing to give curing the required level of attention can lead to failed expectations, expensive repairs, and delayed project timelines.

One might wonder, if the importance of proper curing is so well-documented, why is the process marginalized or even skipped? Part of the reason is confusion around the definition of “curing.” Few terms associated with concrete have been so misused, misinterpreted, or misquoted as the term “curing.” Many associate this term with the overall strength gain of concrete and believe the process takes 28 days. Others believe 28 days is the amount of time needed before a moisture-sensitive flooring, roofing material, or coating can be applied to a concrete substrate surface. Unfortunately, neither of these are completely accurate and this misconception routinely leads to confusion and frustration on projects. To accurately understand what curing means, it is necessary to turn to published literature. A quick review gives two different descriptions.

According to the American Society for Testing and Materials (ASTM) international, ASTM C125, Standard Terminology Relating to Concrete and Concrete Aggregates and the American Concrete Institute (ACI), CT-13: ACI Concrete Terminology, curing is defined as an “action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop.”

ACI 308R-16: Guide to External Curing of Concrete incorporates the above definition and states, “a mixture is properly proportioned and adequately cured when the properties of the in-place concrete equal or exceed the design properties of the concrete.” Further, the document points out the term “curing” has been “used in a more general sense to describe the process by which hydraulic cementitious concrete matures and develops hardened properties over time as a result of the continued hydration of the cementitious materials in the presence of sufficient water and heat.”

Though seemingly similar, there are some key differences between the two. The definition of curing says “curing” is an “action taken,” meaning curing can, and should, be specified, scheduled, planned for, observed, and evaluated. Also, it uses the term “freshly placed cementitious mixture,” shifting focus to when curing should take place, which is from the time of placement until the desired concrete properties develop.

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In contrast, the second description of curing can be interpreted as the long-term development of concrete, often expressed as “concrete never stops curing,” and nowhere in this description is there a call to proactive action.

Based on the evolution of the literature, the first definition above aligns best with the perspective of the concrete industry, which is, curing is something to be specified and done.

Specifying proper curing

When done right, curing reduces cracking, improves durability, and increases strength. Conversely, when concrete is improperly cured, it can lead to inadequate strength gain, surface dusting, surface crazing, plastic shrinkage cracks, excessive slab curl, reduced resistance to freeze-thaw, scaling, etc.

Curing requires two conditions: suitable moisture and temperature. Water is a chemical necessity for hydration and pozzolanic reactions to occur, and these reactions turn wet cementitious material and aggregate mix into a hardened mass. As soon as the wet concrete leaves the mixer, ambient conditions immediately begin to reduce the water available for hydration through evaporation. Elevated ambient air temperature, low ambient relative humidity, high concrete mix temperature (from heat of hydration, hot aggregate which has been in the sun, and high temperature for extended periods), wind, and any combination of these, rapidly remove necessary water from the mix. The need to protect freshly placed concrete from excess evaporation is not solely related to warm weather conditions as it can potentially occur at any ambient temperature.

Recall the second necessity for proper curing is suitable temperature. Curing is not just about keeping sufficient moisture in the freshly placed concrete; it is also about maintaining suitable temperature in and for it. Ambient temperature, mix temperature, and the temperature of the base on which the concrete will be placed on needs to be discussed, monitored, planned for, and addressed.

It may seem daunting to consider all the variables, but there are many resources to aid in properly specifying “curing” in project documents. Some ACI standards to follow are:

• ACI 308, Specification for Curing Concrete
• ACI 305, Specification for Hot Weather Concreting
• ACI 306R, Specification for Cold Weather Concreting
• ACI 301, Specifications for Structural Concrete

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Methods of curing

ACI 308 discusses a few general systems for maintaining adequate moisture content for freshly placed concrete to achieve the desired design properties:

• Continuous or frequent application of water through ponding, fogging, steam, or saturated cover materials, such as burlap or cotton mats, rugs, sand, and straw or hay. None of these are recommended for slabs to receive a moisture-sensitive covering.
• Minimization of water loss from the concrete by use of plastic sheets or other moisture-retaining materials placed over the exposed surface for three to seven days after concrete placement. This is highly impractical on most jobsites, especially with elevated slabs.
• Application of a membrane-forming compound (commonly referred to as a curing compound) meeting the requirements of ASTM C309 or ASTM C1315.

For concrete slabs to receive moisture-sensitive flooring, adhered roofing, or coatings, most of the above methods are impractical, unfeasible, costly, or time consuming. Further, the duration of curing required to achieve the desired levels
of strength, durability, or both, depends on a complex set of factors, making it difficult to confidently state the minimum curing time. Since most curing methods cannot be used with slabs given the cost, unknown duration, wind, disruption to other trades, or overall project timelines, among other factors, concrete slabs are most often cured with curing compounds.

Curing compounds

There are substantial misconceptions surrounding curing compounds as well. Namely, many believe they are all the same. This misconception has led design and project teams to not specify or use these products. The truth of curing compounds, however, remains; they are not the same.

On a broader scale, curing compounds can be separated into two major classifications: one for only curing freshly placed concrete, and the other is formulated to cure and seal it. To aid in better understanding these products, there are two ASTM standards for each classification:

• ASTM C309-19, Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete
• ASTM C1315-19, Standard Specification for Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete

Although they share similar traits and properties, one can easily misinterpret the products to be relatively equal from their descriptions. For example, both ASTM C309 and ASTM C1315 describe “liquid membrane-forming curing compounds” to not negatively react with the concrete surface, and both have water loss properties and drying time listed as requirements. However, ASTM C1315 products have a much more stringent water loss requirement than ASTM C309 products, and ASTM C1315 requires a pull-off test for adhesive applied over the cure-and-seal compound.

It is critical to understand the adhesion pull-off requirement of an ASTM C1315 product. The standard states, a cure-and-seal product must be independently tested for sufficient bond with a ceramic tile adhesive, but this does not cover every type of adhesive. When discussing  specifying an ASTM C1315 product, it is noted, “there is an extremely wide range of adhesive types and formulations, and it’s inappropriate to extrapolate the performance of other adhesives from a test of only one.”¹² This remains relevant to this day.

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However, ASTM C1315 also states, “adhesives for other systems shall be of a type recommended for the installation of materials of interest over concrete.” Today, almost every flooring adhesive has added very specific instructions for application to a porous or non-porous substrate surface. Since 2021, ASTM F710, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring, requires all concrete slab substrate surfaces be tested for porosity, either following the manufacturer’s written instructions, or in the absence thereof, ASTM F3191, Standard Practice for Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring. An ASTM C1315 cure-and-seal product is going to render a non-porous substrate surface where a slab is finished by power troweling; this should be accounted for by ensuring proper bond tests are specified in the relevant 07 and 09 sections.

Conclusion

Proper curing means suitable temperature and moisture conditions within the freshly placed concrete, and there are a few factors which can damage the concrete more than improper curing. Most curing methods are impractical, too expensive, or too disruptive to other project trades and timelines to be properly carried out on slabs which will receive flooring, roofing, or a coating. The simple and direct “cure” is to specify an ASTM C1315 curing compound and to ensure all bond tests and subsequent installations first test the substrate surface for porosity. If the adhesive is not suitable for a non-porous substrate, there are alternatives which do not require expensive and time-consuming profiling. Knowing the standards, knowing the products, and specifying sustainably is the prescription for properly cured concrete.

Notes

¹ Refer to ACI 301, Specifications for Structural Concrete, American Concrete Institute (ACI), Farmington Hills, Michigan.

² Refer to ACI 305.1, Specification for Hot Weather Concreting, American Concrete Institute (ACI), Farmington Hills, Michigan.

³ Refer to ACI 306.1, Specification for Cold Weather Concreting, American Concrete Institute (ACI), Farmington Hills, Michigan.

⁴ Read the ACI 308R-16, “Guide to External Curing of Concrete,” American Concrete Institute (ACI), Farmington Hills, Michigan.

⁵ Refer to ACI 308.1, Specification for Curing Concrete, American Concrete Institute (ACI), Farmington Hills, Michigan.

⁶ Read the ACI CT-18, “ACI Concrete Terminology,” American Concrete Institute (ACI), Farmington Hills, Michigan.

⁷ Refer to ASTM C125-19, Standard Terminology Relating to Concrete and Concrete Aggregates.

⁸ Refer to ASTM C309, Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete, ASTM International, West Conshohocken, Pennsylvania, 2007.

⁹ Refer to ASTM C1315, Standard Specification for Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete, ASTM International, West Conshohocken, Pennsylvania, 2007.

¹⁰ Refer to ASTM F710-21, Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring, ASTM International, West Conshohocken, Pennsylvania, 2007.

¹¹ Refer to ASTM F3191-16, Standard Practice for Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring, ASTM International, West Conshohocken, Pennsylvania, 2007.

¹² Learn more by reading Hukey, John C (2008). “Specifying Conformance with ASTM C1315: What Assurance does it Really Provide?” Concrete International, V. 30, No. 3, Mar., pp. 51–53.

Author

Principal of ISE Logik Industries, manufacturer of concrete moisture products, Dean E. Craft has presented more than a thousand times on how to proactively address concrete moisture in the design phase. He is the principal author and technical chair of ASTM F319–16 Standard Practice for Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring. Craft completed his doctoral work in 2017 with a dissertation entitled, “Fallacy of Current Industry Approach to Assessing Concrete Moisture Before Flooring Installation” and is a participating member of the ASTM Committee D08 on Roofing and Waterproofing, ASTM Committee F06 on Resilient Floor Coverings, American Concrete Institute (ACI), Single Ply Roofing Systems (SPRI), National Ready-Mix Concrete Association Research (NRMCA), and Engineering & Standards Committee (ESC). He is a retired, U.S. Marine Corps Lieutenant Colonel with 23-years of total service; and graduate of the United States Naval Academy (BS), the Naval Postgraduate School (MS), and California Intercontinental University (DBA; Doctorate in Global Leadership).

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