How does this "spoiler" condition appear? There may be any number of variables involved. I'll identify eight common situations.
1. It may be due to a new (green) slab that has not speedily discharged the excess free water that's essential for blending and pouring. In this case, the culprit may be a 3-inch layer of sand that was placed over a vapor retarder. The combination of the sand and retarder may be preventing water loss at the bottom of the slab, thus forcing most if not all of the free water through the top surface of the concrete.
2. Another possibility is excessive irrigation after the home is occupied. This can cause lateral moisture penetration above the vapor retarder.
3. The problem may stem from poor soil drainage. Natural drainage can be blocked or eliminated entirely by a homeowner who's filled in swales on the property with soil and/or plants.
4. An off-site source of water may be making its way onto the property either from a neighbor's residence or as a result of construction of neighboring facilities with large parking lots.
5. It could also be due to a lack of gutters or blockage within existing gutters, downspouts or carry-aways at the bottom. Lateral moisture intrusion, as detailed in items 2, 3 and 4, is the likely root cause.
6. The situation also may be precipitated by a rising water table, poor soil capillary action and/or a punctured vapor retarder.
7. Another cause might be due to underground streams changing their course.
8. Finally, the moisture intrusion may be the result of broken pipes beneath the slab.
Before going further, I must emphasize that it is an established fact that a concrete slab can be dry today, wet next month, and then dry again at some point after that. It is important to remember that most moisture intrusion occurs long after the floor covering installation, except in cases characterized by green concrete.
What can you conclude from all this? Well, you do want a dry slab on the day of your installation, particularly to allow the adhesive to cure to the point of being water resistant if not completely waterproof. There is a difference between "dry" adhesive and "cured" adhesive. Generally speaking, an adhesive requires three days to cure.
Because of the need for moisture testing, and concerns regarding the maximum emissions levels set by manufacturers, a number of geotechnical specialists and civil engineers are actively investigating the overall problem.
An example of such investigations is the extensive research conducted by Bruce A. Suprenant and Ward R. Malisch, as published in the January 2000 edition of Concrete Construction. They enlisted the aid of six independent laboratories to participate in a testing program. Based on the results of laboratory and field tests, Suprenant and Malisch concluded that the generally accepted 3 to 5 lb. maximum emissions limits established for flooring installations are unreasonable.
Another revealing report is detailed in an article by Robert W. Gaul that appeared in the October/November issue of Concrete Repair Digest. In the article, Gaul states that the bond strength of coatings (including adhesives, I assume) applied to concrete substrates falls within a range of 100 to 200 pounds per square inch (psi). He also points out a hydrostatic head of 20 feet will result in a bond strength of only 9 psi. The force of water rising by capillary action alone ranges from 15 to 22 psi. These water-developed pressures are not sufficient to cause dislodgement.
In Gaul's opinion, the moisture present in the concrete creates an upward pressure on the uncured coating (or adhesive) that prevents the intimate contact with the slab that's vital to establishing a strong bond. He also states that this moisture may prevent proper curing. Prevention of this intimate bond results in a failure where the product has a hard underside and is easily removed from the concrete.
A study by a wholesale wood floor distributor revealed that among the results of 600 separate calcium chloride tests, only 5% showed moisture emissions of less than 3 lbs. In fact, the mean reading was 7.5 lbs.
It is readily apparent that moisture testing should be performed prior to the flooring installation. The big question relates to when and by whom. As an installer, you need to know immediately if the job is to proceed.
Consider ASTM F1869-98 Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloors Using Anhydrous Calcium Chloride. Under point 4, "Significance and Use," it states that the moisture vapor emission rate only reflects the condition of the concrete floor at the time of testing. As I mentioned previously, moisture in a slab can be present at one point in time, disappear later and again reappear. What is needed is immediate, on-time checking (i.e. instant gratification).
Also consider points raised in ASTM E1907-97 Standard Practice for Determining Moisture-Related Acceptability of Concrete Floors to Receive Moisture Sensitive Finishes. Under item 5, "Significance and Use," it states that these practices are intended to be used by applicators of floor finish products. Under section 5.3 it says that, although these practices are called "tests" for conformity, they are intended to be used only in concert with the judgment and experience of the user.
What is significant in these ASTM documents is what I call an "early warning system" that may be used by the contractor/installer. If you use a moisture meter to test the slab and get a "dry" or acceptable reading, you may proceed with the installation.