By Ray Thompson, Jr.
There are many ways that water vapor can be a problem. The following are solutions to help keep moisture problems away from your flooring installations. Perform the outlined eight steps and you’ll reduce the possibility of floor failures on your jobs.
#1 Placing a Capillary Break
A capillary break will stop moisture as a liquid as deep as 20 feet down, depending upon the fine content of the soil. A layer of coarse crushed rock 6″ deep will stop the water as a liquid from getting to the slab, but it will not stop water vapor, which can travel as deep as 80 feet into the soil.
#2 Placing a Vapor Barrier
Use a 15-mil vapor barrier, which is recommended by ASTM E-1745 Class A standards, to stop moisture vapor. Don’t use just any plastic cover. The vapor retarder should be designed for a vapor retarder and is made of virgin polyolefin, which keeps the perm rating at a minimum. It should not be a plastic, which is made from recycled plastic such as Visquene, a polypropylene. Don’t skimp and use a 6- or 8-mil thickness product as these are easily damaged. Polypropylene lets moisture through them in amounts greater than some floor covering allows. All vapor retarders should be overlapped a minimum of 6″ and secured with a special tape made for that purpose. Also, all penetrations should be taped and wrapped to prevent leakage of water as a liquid or vapor from below.
It makes no difference to what area, wet or dry, a vapor retarder needs to be used. As a matter of fact, the arid desert country has a greater moisture problem than rainy or humid areas. This misconception is why vapor retarders are not used in the desert areas. Water vapor is definitely prevalent in desert areas.
The process: The concrete goes directly on the vapor retarder when a floor covering material is applied to the concrete’s surface. A blotter layer (a layer of sand or granular fill) is used over the vapor retarder when there is no flooring material applied to the surface. The reason? All of the mix water hides in the granular fill and is hard to get out for flooring purposes.
#3 Proper Pour of Concrete Slab
Get the mix just right. The ASTM F-710 states the water-to-cement ratio should be 0.40 to 0.45, but the flooring industry says the water-to-cement ratio is supposed to be less than 0.50, yet the national average is 0.58. This means the concrete is more porous and will take longer to dry, but it is easier to work.
Next, determine the grade of the mix. You have well-graded and gap-graded. The well-graded has a course, intermediate and fine grade of aggregate, about a third of each, which plugs the holes between the coarse and fine aggregates. Each particle is coated with Portland cement and the more Portland cement is used, the more it shrinks and creates cracking.
Mid-range, water-reducing agents is a good idea instead of adding more water. A water-reducing agent thins the existing water without adding more water to the mix; you get the advantage of workability during placement without changing the water-to-cement ratio.
The use of more Portland cement should be kept at a minimum of 4.5 to 5.5 sack mix. Any more than a 5.5 sack mix (a sack is 94 lbs. of Portland cement), then you’ll have to worry about higher alkalinity, which comes from the lime in the Portland cement. Many of the alkalinity problems come from high sack mixes in the mix design.
The use of type “F” fly ash is a good idea as long as you do not get too much. Fly ash allows for easier placement and a substitute for Portland cement. Around 20% is the maximum amount to be used.
#4 Allow Proper Slab Curing
Because of the vapor retarder, the slab can only dry from the top downward. You have to be careful not to dry too fast because the slab will curl and crack. The use of a wet cure or curing blankets is necessary to prevent these phenomena from occurring. Remember, it takes 28 days for a concrete slab to hydrate before it starts to dry.
The use of sealers, curing compounds and parting compounds are not recommended, but are used regardless. If they are used, they must be removed as they are a potential cause for flooring failures.
#5 Vapor Testing the Slab
There has been a lot said on moisture testing, but I am convinced that the ASTM F-2170 is the way to go for moisture testing. The testing needs to be down at the 40% thickness of the slab (drying from one side) and not the entire hole. If you measure the entire hole, then you’ll get a diluted measurement, which can be the difference between proceeding or waiting to complete the flooring installation.
#6 Alkalinity Testing the Slab
Alkalinity testing needs to be done along with the moisture testing. Usually the pH is high along with the moisture, but sometimes the moisture is ok and the pH is still high. That should be a red flag that something else is wrong.
#7 Shot Blasting the Slab
If the concrete has a sealer, curing compound, parting compound or adhesive of some type, it must be removed in its entirety. The best way to remove it down into the pores is to shot blast the surface of the concrete. The depth of the shot blasting depends on the porosity of the concrete and the substance on the concrete. A shot blast must get the entire residue off the concrete and down into the pores, or you risk a failure. Therefore, you must choose a Concrete Surface Profile (CSP) 1-2 for sealers and curing compounds, and 3-4 for adhesives, although there are exceptions to this rule depending on the concrete.
#8 Placing a Moisture Coating on the Slab
I have never been an advocate of a penetrant. I have seen too many failures due to something being done or something not being in the concrete. A penetrant may be cheaper than coating, but it does not work as well. I have seen coating fail because of failure to prepare the surface of the concrete or not coating properly (two coats is best, rolled in opposite directions) but if those things are done, well, they work most of the time.