By Dean E. Craft, principal of ISE Logik, DBA, CSI, CDT, CCCA, ASTM, ACI Lt Col USMCR (ret.)
The CURE to successful concrete slab placement and covering
Proper concrete curing is critical to achieving the overall strength, durability, performance, and in many aspects, aesthetics everyone on a project expects of concrete. Further, proper curing is directly related to reduced maintenance costs and extended service life, therefore enhancing the overall sustainable design attributes of using concrete. Unfortunately, despite the relatively low cost of effective curing practices, this step in the process of concrete construction is often overlooked or minimized; and, failing to give curing its proper place can rapidly lead to failed expectations, expensive repairs, and delayed project timelines.
But why does this occur? If the importance of proper curing is so well-documented, why is it so often marginalized or even skipped? This may seem simplistic but perhaps part of the reason lies in confusion amongst project team members on just what “curing” means. Few terms associated with concrete have been misused, misinterpreted, or misquoted as much as the term “curing”, with many associating this term with the overall strength gain of concrete and going so far as to interpret, even state, that it takes concrete 28 days to “fully cure”; and with others further extrapolating that “28-days cure time” as being the magic amount of time needed before a moisture sensitive flooring, roofing material, or coating can be applied to a concrete substrate surface. Unfortunately, none of these are completely accurate and routinely lead to confusion and frustration on projects. In support of the supposition that there may exist confusion on what “curing” means across project team members, it is necessary to turn to the published literature. A quick review gives two distinct but substantially different descriptions.
According to ACI and ASTM, 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 (SEE: ACI 308R-16; ACI CT-18, ASTM C125) 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 (SEE: ACI 308R-16).
Yet, also within ACI, it is pointed out that curing is a term:
frequently used to describe the process by which hydraulic-cement concrete matures and develops hardened properties over time as a result of the continued hydration of the cement in the presence of sufficient water and heat (SEE: ACI 308R-16).
Though seemingly similar, there are some very important differences between the two. The definition of curing clearly points out that “curing” is an “action taken”. That means curing can, and should, be specified, scheduled, planned for, observed, and evaluated. Also, the definition of curing focuses on “freshly-placed cementitious mixture”, thus focusing project team members as to “when” curing should be affected; from time of placement until desired concrete properties are developed. In contrast, the second description of curing can easily be interpreted as the long-term development of concrete, often expressed as “concrete continues to cure throughout its life”. And note that nowhere in the description of curing is there any call to proactive action.
But why do the Evaluation in the first place? Certainly, the adhesive manufacturers know if their products are compatible with the floor covering and subfloor surfaces; glance at any adhesive product datasheet, and the list is spelled out. But there are Jobsite conditions that necessitate the Evaluation be conducted. Let’s look at a few more common issues that may interfere with or skew an Evaluation result.
Specify Proper Curing on ALL Projects
Proper curing of concrete 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, and on and on. Recall from the above definition that curing requires two conditions: suitable moisture and temperature. This is because water is a chemical necessity so that hydration and pozzolanic reactions can occur; these reactions are what turns the wet cementitious material and aggregate mix into a hardened mass. And, as soon as that wet mix leaves the ready-mix concrete truck, ambient conditions immediately begin to take some of that water away through evaporation. Elevated ambient air temperature, low ambient relative humidity, high concrete mix temperature (from heat of hydration, hot aggregate that has been in the sun and high temperature for extended periods, etc.), wind, and any combination of these all work to rapidly remove necessary water from the mix; and this can potentially occur at any ambient temperature. The need to protect against excess evaporation is not solely related to warm weather conditions.
Recall the second necessity as well for proper curing; suitable temperature. Curing is not just about keeping sufficient moisture in the freshly placed concrete, it is also about maintaining suitable temperature for and in the concrete; so, ambient temperature, mix temperature, and the temperature of the base on which the concrete is to be placed all have to be discussed, monitored, planned for, and addressed.
It seems daunting to have to consider all the variables but don’t despair; you are not alone. There are many excellent resources to aid in properly specifying “curing” in project documents. For example:
• ACI 308.1 – Specification for Curing Concrete
• ACI 305.1 – Specification for Hot Weather Concreting
• ACI 306.1 – Specification for Cold Weather Concreting
• ACI 301 – Specifications for Structural Concrete
And remember to ensure all project team members, to include the owner, understand what “curing” really means, and the absolute importance curing has on a quality concrete structure.
Methods of curing
ACI 308 discusses a few general systems for maintaining adequate moisture content so that freshly placed concrete can achieve the desired design properties:
1. the 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 which are recommended for slabs to receive a moisture sensitive covering
2. the minimization of water loss from the concrete by use of plastic sheets or other moisture-retaining materials placed over the exposed surfaces; for three to seven days after concrete placement which is highly impractical on most jobsites, especially with elevated slabs; or
3. by the application of a membrane-forming compound (commonly referred to as a curing compound) meeting the requirements of ASTM C 309 or C 1315
For concrete slabs to receive moisture sensitive flooring, adhered roofing, or coatings, most of the above methods are completely impractical, unfeasible, too 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 that makes it difficult to confidently state the minimum curing time (SEE: ACI 308). Therefore, since most of the recommended curing methods simply cannot be used with slabs given the cost, unknown duration, wind, disruption to other trades, overall project timelines, and so on, in most cases, project concrete slabs are most often cured with curing compounds.
Curing Compounds
As with the term curing, there exists some substantial misconceptions surrounding curing compounds; namely, many believe that they are all the same. And, from this misconception, some design and project teams have been influenced to not specify nor use these products. The truth of curing compounds, however, is that they are not all the same.
In a broad sense, curing compounds can be separated into to two major classifications; those that are only for curing freshly placed concrete, and those that are formulated to cure and seal freshly placed concrete. To aid in better understanding these products, there are two ASTMs that cover each classification:
• ASTM C309, “Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete”
• ASTM C1315, “Standard Specification for Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete”
When laid side by side, there are enough similar paragraphs, and properties across both ASTM documents that if one did not read carefully, it could be easy to misinterpret that the products they discuss are relatively equal. As an example, both ASTM C309 and ASTM C1315 describe “liquid membrane-forming curing compounds” that will not negatively react with the concrete surface, and both have water loss properties and drying time 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 curing compound.
It is critical to understand the adhesion pull off requirement of an ASTM C1315 product. The standard states that 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 a project may come across. Hukey (2008), when discussing the appropriateness of specifying an ASTM C1315, recognized this and noted that “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”. This remains relevant to this day. However, ASTM C1315 also states “adhesives for other systems shall be of a type recommended for the installation of materials of interest over concrete.” The difference between today and 2008 is that since the 2008 observation, almost every flooring adhesive has added very specific instructions for application to a porous or non-porous substrate surface; and, since 2021, ASTM F710 has required all concrete slab substrate surfaces be tested for porosity either following the manufacturer’s written instructions, or in the absence thereof, ASTM F3191. An ASTM C1315 cure and seal product is going to render a non-porous substrate surface in almost all cases where a slab has also been finished by power troweling; this should be accounted for by ensuring proper bond tests are specified in relevant 07 and 09 sections.
Proper curing means suitable temperature and moisture conditions “within” the freshly placed concrete, and there are few things that can damage freshly-placed concrete more than improper curing. Most all curing methods are simply impractical, too expensive, or too disruptive to other project trades and timelines to be properly carried out on slabs that 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 concrete substrate surface for porosity. If the adhesive is not suitable for a non-porous substrate, there are many alternatives that are that do not require expensive and time-consuming substrate surface profiling.
Know the standards, know the products, and specify sustainably.
References:
1. ACI 301, “Specifications for Structural Concrete”, American Concrete Institute, Farmington Hills, MI.
2. ACI 305.1, “Specification for Hot Weather Concreting”, American Concrete Institute, Farmington Hills, MI.
3. ACI 306.1, “Specification for Cold Weather Concreting”, American Concrete Institute, Farmington Hills, MI.
4. ACI 308R-16, “Guide to External Curing of Concrete”, American Concrete Institute, Farmington Hills, MI.
5. ACI 308.1, “Specification for Curing Concrete”, American Concrete Institute, Farmington Hills, MI.
6. ACI CT-18, “ACI Concrete Terminology”, American Concrete Institute, Farmington Hills, MI.
7. ASTM C125-19, “Standard Terminology Relating to Concrete and Concrete Aggregates”
8. ASTM C309, “Standard Specification for Liquid Membrane-Forming Compounds for Curing Concrete”, ASTM International, West Conshohocken, PA, 2007.
9. ASTM C1315, “Standard Specification for Liquid Membrane-Forming Compounds Having Special Properties for Curing and Sealing Concrete”, ASTM International, West Conshohocken, PA, 2007.
10. ASTM F710-21, “Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring”, ASTM International, West Conshohocken, PA, 2007.
11. ASTM F3191-16, “Standard Practice for Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring”, ASTM International, West Conshohocken, PA, 2007
12. 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.
DEAN E. CRAFT
DBA, CSI, CDT, CCCA, ASTM, ACI Lt Col USMCR (ret.)
Principal of ISE Logik Industries, manufacturer of concrete moisture products, Dean has presented more than 1000 times on how to proactively address concrete moisture in the design phase. Dean is the principal author and technical chair of ASTM F3191 – 16: “Standard Practice for Field Determination of Substrate Water Absorption (Porosity) for Substrates to Receive Resilient Flooring”, 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 voting or participating member of the ASTM Committee D08 on Roofing and Waterproofing, ASTM Committee F06 on Resilient Floor Coverings, American Concrete Institute, Single Ply Roofing Systems (SPRI), and National Ready-Mix Concrete Association Research, Engineering & Standards Committee. Dean 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).