Flooring
How to Test for Curing Compound on a Concrete Slab
Drop water on the slab in several spots. If it soaks in within seconds, the surface is open. If it beads or sits for a full minute, something is sealing it, often curing compound or a cure and seal. Confirm with a bond test using the adhesive, thinset or coating you plan to install.
You usually cannot see curing compound once it has dried, and the concrete sub may be long gone when the flooring or coating crew arrives. Three field checks tell you what you are dealing with: a water drop test, an acid fizz check and a bond test. Run them before you commit to a prep method or an install date.
What is the quickest test for curing compound?
The water drop test. Put drops of clean water on the slab and watch. Sherwin-Williams says in its concrete coating tips that “a dime-sized bead of water dropped on the surface should be absorbed within five or 10 seconds.” If it is not, the concrete has likely been treated with a cure and seal.
How do you run the water drop test?
Test the whole floor, not one spot. Application rates vary across a slab, and the heavy areas near walls, columns and the edges of each pour are where flooring lets go first.
- Sweep the test areas and let any damp spots dry.
- Grid the floor and mark test spots on a plan, including edges, corners and areas under the roof that stayed shaded.
- Put a few drops of clean tap water on each spot.
- Time each drop for up to one minute and note whether it darkens and soaks in or stays on top.
- Mark failing spots on the slab with a crayon or paint stick and on the plan.
ASTM F3191 formalizes this. As described in The Construction Specifier, it uses “a single drop of potable water (approximately 0.05 mL).” A drop absorbed within one minute means porous. A drop still sitting after a full minute means non-porous. The test is qualitative, not a pass or fail on bond.
How do you read the results?
Fast absorption means an open surface, and a drop that sits means something is sealing it. Use the table to pick the next step for each area you marked.
| What you see | What it likely means | Next step |
|---|---|---|
| Drop darkens and soaks in within about 10 seconds | Open surface, little or no film | Bond test with your product |
| Drop soaks in between 10 seconds and 1 minute | Partly sealed, or a dense troweled surface | Bond test, and check the product’s porosity limits |
| Drop beads or sits past 1 minute | Curing compound, cure and seal, sealer or hardener | Mechanical prep, then retest |
| Mixed results across the floor | Uneven application or partial dissipation | Map it and prep the failing areas |
LATICRETE’s TDS 154 uses the same logic: “If the water beads or does not readily absorb,” mechanical abrasion or scarification is required. Custom Building Products says the same in TB92: if the water beads up, prep the surface first.
Can a slab pass the water test and still have curing compound?
Yes. A thin or partly broken down film can let water through and still weaken adhesion. Silicate hardeners and densifiers can also tighten the surface without leaving a visible film. That is why the water test is a screen, not a sign off.
A dissipating curing compound is the common trap. It may be gone where sun hit the slab and still there in the shaded bays. Test each area on its own.
How do you use an acid test for curing compound?
Diamond Vogel describes a muriatic acid check in its surface preparation guide: “If the solution does not react (bubble vigorously),” a curing compound is indicated. Bare concrete fizzes. A film keeps the acid off the paste.
Wear eye protection and gloves, use a small spot, and rinse it well. Treat this as a quick indicator only. Diamond Vogel also notes that acid etching generally does not remove chemical curing compounds, so do not count on it as prep.
How do you do a bond test before flooring?
Install a small test area with the exact adhesive, thinset or coating and the exact flooring, on prepped and unprepped areas. AHF’s Floor Expert guidance says to run a bond test to decide whether removal is needed, and if the bond fails after 72 hours, the compound must be removed.
For tile and underlayment, Custom Building Products describes a mock-up with tensile pull tests that should cure “a minimum of 7 days and ideally 28 days.” On a tight schedule, that wait is often why crews skip to shot blasting.
If the water beads on your slab
Stop and plan prep before any adhesive goes down. Map the failing areas, then remove curing compound mechanically and retest with water until drops soak in. If the product was a cure and seal, see how to remove cure and seal for the extra steps.
Then run the bond test on the prepped surface. For what each flooring and adhesive maker requires, see curing compound and flooring adhesion. For coatings and polished floors, see prep before epoxy, tile or polishing.
Frequently asked questions
How can you tell if concrete is sealed?
Put a few drops of water on it. Sealed or cured concrete makes the water bead or sit on top. Open concrete darkens and absorbs the drop within seconds to a minute.
How long should a water drop take to soak into concrete?
Sherwin-Williams says a dime-sized bead should be absorbed within five or 10 seconds on concrete ready to coat. ASTM F3191 calls the surface porous if a 0.05 mL drop absorbs within one minute.
Does the water drop test prove flooring will bond?
No. It is a qualitative porosity check. A slab can absorb water and still have residue that hurts adhesion, so follow it with a bond test using the actual product.
Can you test for curing compound with muriatic acid?
Diamond Vogel describes it: diluted muriatic acid that does not bubble vigorously on the slab points to a curing compound. Use it as a quick indicator only, and rinse the spot.