Moisture Testing a New Orleans Slab Before Epoxy

Why New Orleans slabs get tested first

Much of New Orleans sits below sea level, behind levees, with pumping stations moving storm water out. In much of the metro, groundwater can sit close to the ground surface. A slab poured on that ground can release moisture vapor from its surface — which is exactly what one of the standard tests measures.

Moisture in a slab can make a floor coating fail after it goes down, which is why coating makers generally require a moisture test first. The ASTM standard for in-slab testing says it plainly: excess moisture in a slab after a floor covering goes down can cause the floor system to fail, and manufacturers generally require moisture testing before installation on concrete.

The three tests

In-slab relative humidity — ASTM F2170

This is the test that looks inside the concrete. Small holes are drilled, probes go in, and the reading is taken once the probes have settled. Following the standard means three details that are easy to skip:

• Depth. The hole goes to 40% of the slab's thickness for a slab that dries from one side only — which describes a slab on the ground, like a garage floor.
• Time. Each sensor has to equilibrate for at least 24 hours before a reading counts.
• Conditions. The slab and the air above it should be at service conditions for at least 48 hours before testing.

Calcium chloride — ASTM F1869

A dish of anhydrous calcium chloride is sealed under a dome on the concrete, where it takes up the moisture vapor the surface gives off. The result is a rate, reported in pounds of moisture per 1,000 square feet over 24 hours. It measures what is coming out of the top of the slab rather than what is deep inside it.

Plastic sheet — ASTM D4263

A square of plastic is taped down, and after a set time you look for droplets underneath or darkened concrete. ASTM describes this as a way to indicate the presence of capillary moisture before coating. One big-box kit builds a version of it into its instructions: a 2-foot square of plastic, taped down for 24 hours. It is a quick yes-or-no check, not a measurement.

How many tests a garage needs

For the in-slab method, the published guidance is three test holes for the first 1,000 square feet and at least one more for each additional 1,000 square feet. A standard two-car garage is under 1,000 square feet, so three readings, spread across the floor — including near the door and along any wall where water has come in.

Reading the result

The test methods tell you how to measure; they do not set a pass mark. That comes from the coating maker, for its own product. Two installers can look at the same reading and both be right if they plan to use different primers. What you want in writing is the reading, the product, and the limit on that product's data sheet.

When the slab fails

A high reading changes the system, not the plan. Coating makers sell moisture-tolerant epoxy primers designed to reduce vapor transmission through the slab. One maker's data sheet rates its primer for readings up to 25 pounds on the calcium chloride test and 100% relative humidity on the in-slab test.

The same data sheet is candid about limits: it calls the product a vapor reduction system only, and says it does not prevent failures caused by osmotic blistering on its own. That is the kind of detail worth asking about when a slab reads high. If water is actively coming in at a wall or the door, fix that source and retest before coating.

Test close to coating day

A moisture reading describes the slab only at the time it was taken. A test from a dry week does not speak for the floor after a stretch of rain. Coating makers also ask installers to measure the slab's moisture, the air's humidity, the temperatures, and the dew point right before application, and one data sheet asks for those values to be recorded at least every three hours while the work is under way.

Humidity on coating day

South Louisiana has a hot, humid climate, and humidity can matter twice for a coating: moisture in the slab, and condensation on a cool slab during application. The rule coating makers publish for the second risk is that the slab should be at least 5°F above the dew point when coating goes on.

Where the test shows up on a quote

A written quote worth comparing lists four things as separate lines: prep, repair, the coating system, and the finish. The moisture test belongs on the first of those lines, together with any moisture-tolerant primer it calls for.

1. 1PrepThe method (diamond grinding or shot blasting), the moisture test, and any moisture-tolerant primer that test calls for.
2. 2RepairCrack and spall repair, removal of an old coating or sealer, and whether that number is fixed or an allowance the crew settles once the floor is opened up.
3. 3Coating systemThe chemistry, the number of coats, and the total mil thickness, not just a product name.
4. 4FinishSolid color, flake, or metallic; the topcoat; and the return-to-service windows for foot traffic and for vehicles.

Questions to ask

• Which test will you run, and where on the floor?
• For the in-slab test: how deep are the holes, and how long do the probes sit?
• Which primer do you plan to use, and what reading does its data sheet allow?
• Will I get the readings in writing?

Common testing mistakes

• Testing weeks before the job. The reading describes the slab only at the time of the test.
• Shallow holes. For a slab drying from one side, the probe goes to 40% of the slab's depth; a shallow reading sees only the drier top.
• Reading too soon. Probes need at least 24 hours to equilibrate before a reading counts.
• One test for a whole floor. The guidance is three for the first 1,000 square feet, spread out.
• No product named. A reading means nothing without the limit from the data sheet of the primer that will go down.