Wet burlap, sheet materials, and surface-applied compounds are seldom used to cure decorative concrete because they can cause discoloration and unwanted changes in surface appearance. This is a problem for decorative concrete, where the cure-affected zone is the value-added product.
Decorative contractors have used non-traditional methods and materials, such as dry release powders and sodium silicates, as curing agents, to achieve durable surfaces while minimizing imperfections. Anecdotally, these approaches produce desirable results. However, they don’t meet the definition of curing in American Concrete Institute (ACI) guides or specifications, ASTM International requirements, and some contract documents. There’s also little or no data comparing traditional to non-traditional methods, making it impossible to evaluate conditions under which non-traditional methods and materials may be acceptable.
ACI committees 308 (Curing) and 310 (Decorative Concrete) have formed a task force to resolve these issues. ACI 308/310TG2, Curing Decorative Concrete Joint Task Group is proposing an experimental research study to develop objective data for evaluating non-traditional methods. The American Society of Concrete Contractors pledged $25,000; the Portland Cement Association pledged $20,000; and additional industry funding is expected to exceed $10,000.
Five bottom-line questions
The study’s problem statement is, Under what conditions do these materials perform well and is there a limit to their applicability? The research will address five contributing factors.
Concrete. Water-cement ratio varies from about 0.40 for decorative concrete exposed to weather and deicing salts to about 0.50 for interior applications. Are all non-traditional curing methods equally effective across this range?
Surface finish. Traditional curing tests are conducted on concrete with a floated finish. Decorative concrete, however, is often trowel-finished. A June 2014 technical bulletin written by Nox-Crete Products Group Technical Sales & Marketing Vice President Craig Coppersmith, PE reports that troweled concrete transmits 80% of the moisture of floated concrete when the ASTM C309 test method is used. In another test sponsored by Concrete Construction, a hard-troweled warehouse floor transmitted less moisture than one treated with a curing compound.
Are non-traditional curing methods effective only when used on troweled surfaces?
Environment. Curing methods and duration depend on ambient environmental conditions. For example, curing isn’t necessary when high relative humidity will prevent excessive moisture loss.
It’s possible that the apparent success of non-traditional methods is based on favorable conditions. Are all methods equally successful in all ambient conditions?
Differences in materials. There are many manufacturers of dry release powders and sodium silicates used as curing agents, but no ASTM standard for these products. Do products from different manufacturers perform equally?
Differences in application rates. Each manufacturer provides recommended dosages and/or application rates for their materials. This is an important consideration. Results with a dry powdered release agent may be unacceptable at 3 pounds per 100 square feet but fine at 4 pounds per 100 square feet. A sodium silicate at 200 square feet per gallon may affect hydration differently than one at 300 square feet per gallon. How do these products perform at different dosages?
Test method: ASTM C1151-91
To answer these questions, the task group will compare traditional and non-traditional curing methods using three variables: concrete water-cement ratio, surface finish, and evaporation rate due to ambient conditions. Testing these variables should determine when some non-traditional curing methods are appropriate.
Researchers will use ASTM C1151-91, Test Method for Evaluating the Effectiveness of Materials for Curing Concrete, which measures test specimen property rather than behavior of the applied material. By comparing the absorptivity of a near-surface layer to that of an internal layer the relative degree of hydration is determined which indicates the effectiveness of the curing.
The method’s used to analyze concrete of all ages in the field, but isn’t accepted as a routine test for liquid membrane-forming curing compounds because of its complexity. However, although it was withdrawn by ASTM in 2000, it’s still used to conduct research because:
- It was developed using an ASTM consensus process
- The results correlate to durability and strength testing
- Traditional curing methods can be compared to non-traditional curing methods
- It provides background on acceptable and unacceptable values
- It has a precision statement.
Eight tests on 13 samples
The study’s design is based on the problem statement and the five research questions. First, a range of water-to-cement ratios typically used for decorative concrete (0.40 to 0.50) will be tested. One surface will be have a float finish as prescribed by the C1511 test method. A second surface will receive a steel trowel finish because that’s how decorative concrete is usually finished.
ASTM C1151 requires a combination of temperature, relative humidity, and wind velocity to provide a minimum evaporation rate of 0.4 kilogram per square meter per hour (kg/m2/hr). Depending on the capabilities of the testing agency chosen to perform the research, one or more of these would be changed to provide less severe evaporation. This is reflected in the test program as a change from 40% to 60% relative humidity.
There will be 13 slab samples in each of eight different series of tests. Each series will be run and the collected data will be analyzed to determine if changes are needed for the next series of tests. Statistical analysis will determine if there is a significant variation as a result of the product or test procedure. The research is fluid, though, and may be adjusted as participants provide their input and knowledge. This testing program should provide enough evidence to indicate when non-traditional curing methods are appropriate.