The industry is rapidly changing to using RH (in-situ) probes for testing for moisture in the slab. Photo of Rapid RH 4.0 product courtesy of Wagner Electronics.


I received many interesting questions from viewers during my recent “Troubleshooting Installations over Concrete Subfloors” webinar, which drew a crowd of over 900 industry professionals. I could not respond to all of the questions, but wanted to cover some of them here. Readers who would like to listen to the entire webinar can register and view it by going to www.ntlfloortrends.com and clicking on the webinar button at the far right of the home page.

Q: What is the percent of RH vs. Calcium Chloride testing being done? (Al Ellis, Alpha Professional Tools)

A:  I would guess it is about 50/50, but it is rapidly changing to the RH (in-situ) probe. The real change started with the published results of “Drying of Construction Water in Concrete,” by Goran Hedenblad of the Moisture Research Group at the Lund Institute of Technology, in July 1996.   

Q: When testing for RH, what is the purpose of the interior slab temperature, and does that have any relation to the slab’s fitness for floor coverings? (Kent Edel, Certified Carpet & Surfaces Inspection Co., Baltimore)

A:The slab temperature is important in determining how close the slab is to being stabilized. The ambient temperature can be erroneous if the heat has recently been turned on, but the temperature of the slab will not lie. Too many times have I gone to a jobsite start, only to find the slab cold because the heat had recently been turned on and the slab not acclimated. This is a major cause of concrete movement problems.

Q: How soon can you seal a concrete floor after it has been poured and is there a standard cure time before doing this? (Veronica Love, Vandams Abbey Carpet and Floor, Marysville, Wash.)

A:  The curing compound and/or sealer is applied almost immediately after the concrete has set.

Q: Will muriatic acid remove curing compounds on smaller tile projects? (Brad Bjorklund, Huscraft LLC, Bellevue, Wash.)

A:Muriatic acid will only affect the Portland cement in the concrete and will not impact the curing compound.

Q: Will the use of gas heat rather than A/C to bring down the concrete moisture be harmful to the slab [with ventilation]? (Paul Styka, AFS, Naperville, Ill.)

A:Any type of dry heat will work. Avoid wet heat such as propane, heating oil, diesel or kerosene, as that will add additional moisture into the air and increase the amount of carbon dioxide, which creates excessive carbonization of the slab. Ventilation and air movement is imperative.

Q: How do moisture and alkalinity each contribute to adhesives failure? (Aaron Abbott, Laticrete)

A:Moisture conditions prevent the adhesive from totally curing, keeping them soft and gummy. Alkalinity will break the adhesive down over a period of time.

Q: If the adhesive darkens when checking under flooring, what does it tell you? (Terry Price, NM Flooring Inspection and Consulting Service, Albuquerque)

A:It is an indication of the adhesive drying and a possible moisture condition.

Q: What is the solution for installing flooring over control joints? (Edward Dueppen, BLDD Architects, Inc., Champaign, Ill.)

A:The object is to get the slab acclimated so that there is as little movement as possible once the flooring material is installed. The true equilibration does not occur until sometime after the flooring material is applied.

Q: Will shot blasting help remove the curing layer and improve the performance of adhesives? (Brian Mackenzie, Samling Global, Greer, SC)

A:The removal of the curing compound will allow the adhesive to obtain a better mechanical bond to the substrate. The presence of a curing compound will fill the pores of the concrete, preventing a good bond.

Q: What is your opinion about European Carbide Method (CM) for moisture measurement? Is it more accurate than the electronic one? (Gleb Gavrylov, Tarkett)

A:I place this type test in the same category as the calcium chloride method, as the test measures only the surface of the concrete and will not determine the moisture deeper in the slab where it is most likely lurking. However, it is definitely more accurate than the surface moisture meter.

Q: Where does the moisture go when caught between the vapor retarder and the floor covering or mitigation process? (Kent Edel, Certified Carpet & Surfaces Inspection Co., Baltimore)

A:There is always moisture in concrete, and concrete loves moisture. Once a slab has the moisture encapsulated in it, the retained moisture will gradually dissipate out because the barriers are permeable and allow the moisture to migrate out.

Q: What are the drawbacks of Calcium Chloride tests compared to R.H. tests? Are adhesive manufacturers revising warranties to reflect a preferred method? (Sonny Webster, All Surfaces, Houston)

A:The measurement of moisture at 40 percent the slab thickness has been proven to be very accurate at that depth. The surface measurements are too susceptible to too many variables. Almost all flooring manufacturers are making the change; however, they are doing it slowly for legal reasons.

Q: What are the concerns with concrete sealers and floor leveling products when a cementitious floor leveling material is used? (Corey Zussman, Berglund Construction, Chicago)

A:Like adhesives, the mechanical bond of a cementitious leveler is also limited by attempting to go over a concrete curing/sealer compound. Leveling compounds need to get a mechanical bond into the pours of the concrete.

Q: Is there anything the industry can do to better educate architects and contractors or to hold curing agent salespeople accountable for the presence of curing agents at the time of installation? (Coleen Heslin, Morales Group, Inver Grove Heights, Minn.)

A:True curing agents are either UV or oxygen degraded. The amount of time for degradation is dependent on time, exposure and thickness. Even when degraded, the pours of the concrete are generally filled by a curing agent that has not degraded. The only way to get the attention of the general contractor and architects is to hit them in the wallet by charging them for removal of the substance.

Q: How does the type of aggregate affect the flooring installation? (Nate Hall, The Enchanted Floorist, LLC, Grafton, Wis.)

A:Aggregate size matters to the curling and the cracking of the slab. The type of aggregate, the amount of absorption and reactivity of the aggregate all matter. The absorption controls the drying rate and the reactivity controls the dimensional stability.    

Q: Why do so many installers refuse to perform moisture tests? (Phil Meyer, Carpet One, Concord, Calif.)

A:The WFCA white paper states, “It is unreasonable to expect a contractor or installer to have sufficient expertise to anticipate and ask the proper questions for evaluation of potential concrete/flooring problems…It is therefore our recommendation that concrete moisture vapor emissions testing be performed by qualified, independent agencies.” Plus, installers are often not educated in these scientific endeavors.


Thank you, everyone, for making the webinar such a success and for asking such great questions. If you have any specific questions that were not answered here, please feel free to contact me via email at rthompson@cablespeed.com.