How Do You Test a Riverside Garage Slab for Moisture Before Coating

Twenty-four hours under a taped square of plastic sheet is long enough to show whether a garage slab is giving off moisture. Every teardown riverside epoxy flooring ruts n guts has logged shows the same thing: the coatings that blistered had a damp slab under them before the first coat went down. Epoxy bonds to dry concrete, and a garage floor in Riverside, CA is no exception just because the air is dry for most of the year.

Why does a dry climate not make the slab dry?

Concrete pulls water up from the soil beneath it, and the amount depends on what sits under the slab, not on the weather above it. Many garage slabs poured across Riverside County decades ago went down on bare compacted fill with no vapor barrier at all. Irrigation on a neighbouring lawn, a hillside draining toward the foundation near the Box Springs Mountains, or a high water table close to the Santa Ana River can all keep a slab damp from below.

What does the plastic sheet test show?

A square of clear plastic about two feet across is taped down on all four edges and left for a full day. Fogging or droplets on the underside mean vapor is rising through the concrete, and a dark patch on the slab after the sheet comes off means the same. The test is crude, but it catches the worst slabs before any grinding begins.

How do calcium chloride and probe tests differ?

The calcium chloride kit seals a dish of salt under a dome on the slab for three days and weighs the water the salt absorbs. A probe test drills a small hole, lets the hole stabilise, then reads the relative humidity inside the concrete. The probe reads deeper in the slab and is less affected by a surface that was recently washed. Both give a number that is compared against what the primer maker allows, and a reading over the limit means a moisture-tolerant primer or a vapor-blocking base coat goes down first.

What happens when the reading is high?

A high reading changes the system rather than ending the job. A penetrating epoxy primer with a high solids content can seal the pores and hold vapor back, and it is applied only after diamond grinding has opened the surface so the resin can soak in. Polyaspartic and polyurea topcoats are usually kept off a damp slab, because a fast-curing film traps vapor and lifts at the first weak spot. Waiting for a dry spell and testing again is also an option when the cause is a recent wet season.

Where on the slab should the tests go?

A garage slab is rarely uniform, so the corner nearest the downspout and the strip along the main door each get a test, plus one in the centre.