Slab Finishing Timing and Surface Defects
Read when a slab is really ready, adjust for vapor retarders, air and wind, time the saw and the cure, and diagnose dusting, scaling, crazing, blisters, delamination and curl.
Table of contents
Nearly every finishing failure comes down to timing. The surface looked ready, so the crew went at it, and the concrete below was still moving. A few weeks later there is a floor that dusts, a driveway that peels, or a sheet of hollow concrete that lifts off in plates.
You know the order: screed, bull float, wait, float, trowel, cure. This guide is about the judgment inside that order. It covers how to read a slab that is lying to you, what shifts the timing, when to saw and cure, and how to look at a damaged surface and work out what happened.
Most of the figures here come from the Concrete in Practice sheets of the National Ready Mixed Concrete Association (NRMCA) and American Concrete Institute (ACI) guides. Your specification controls, and floors with a tight flatness requirement have their own rules.
What You Will Be Able to Do
- Tell a truly ready surface from a surface that has only crusted over.
- Adjust timing for vapor retarders, air, cold ground, wind and sun.
- Choose the right time to saw joints and start curing.
- Look at a surface defect and name its most likely cause.
- Decide when a problem needs repair, and when it needs a bigger call.
The Rule Behind All of It
Fresh concrete settles. The heavy solids sink, and water and air rise. That is bleeding. Finishing closes up the surface. If you close it before bleeding has ended, water and air get trapped just under a thin dense skin, and you have built a weak layer into the slab.
So the real question is never "does the top look ready?" It is "has the concrete stopped bleeding all the way down?"
How to Read a Slab
Use more than one check, because each can fool you.
- Look for sheen. Do nothing but level the slab until the water sheen has gone.
- Cover a small patch. Lay a small piece of plastic on one spot for a few minutes. If water collects under it, the slab is still bleeding, even if the open surface looks dry.
- Step test. Before final finishing, a boot print should sink about 1/8 to 1/4 inch.
- Probe it. Push a sharp object through the top. If the layer beneath is still soft and plastic, the top has crusted and the slab is not ready.
- Watch the trowel. If the surface tears, it may not be ready.
The Crust Trap
Wind, sun and dry air can stiffen the top while the concrete below is still plastic. The sheen disappears early, the surface takes a boot print, and it looks ready. It is the most common way that good finishers cause delamination. A cold base does the same thing, because the slab stiffens from the top down. On a day like that, protect the surface so it cannot crust, with a cover or evaporation retarder, and keep the final passes as late as you can.
What Shifts the Timing
| Condition | What it does | What you change |
|---|---|---|
| Slab directly on vapor retarder | More bleed water, for longer | Wait longer, watch for a crust |
| Entrained air | Slower bleeding, sheen leaves early | Do not hard trowel it |
| Fly ash or slag | Slower set and longer bleeding | Plan a longer day, keep checking |
| Cool concrete or cold base | Slow set, stiffens top down | Warmer mix, accelerator |
| Sun, wind, low humidity | Surface stiffens first | Shade, windbreak, covers |
| High slump from water | More bleeding | Stop adding water |
| Over-vibrating | Thick mortar layer on top | Keep vibrating screeds to 3 inches of slump or less, and move them quickly |
Tools and Passes
- Bull float early. Finish this step before bleed water shows, and keep the blade flat. A tilted blade seals the surface.
- Pick the right float. Use magnesium or aluminum on air-entrained concrete. A wood float tends to drag on it.
- Keep the first machine passes flat. Float blades or pans go on first. Raise the pitch in stages as the slab firms up.
- Delay the final passes as long as the slab allows. Each pass packs the surface tighter.
- Know your exterior finish. A broom or light float texture leaves the surface open. Repeated steel troweling closes it, and it is the wrong finish for an exterior air-entrained slab.
Hard troweled floors should not use air-entrained concrete. NRMCA recommends no more than 3 percent air on them. The risk of delamination is greater than the risk from freezing during construction. If you see air on the ticket for a slab that is getting a burnished finish, stop and ask.
Never sprinkle water or dust dry cement on a slab to help it along. Both weaken the top layer and cause dusting or scaling.
Joints: Timing and Layout
You saw too early and the blade rips aggregate from the surface, which is called raveling. You saw too late and the slab cracks on its own first.
| Method | When | Depth |
|---|---|---|
| Hand tool | Early in finishing, and rerun later | At least 1/4 of the slab |
| Early-entry dry saw | 1 to 4 hours after finishing | At least 1 inch, often less than a conventional cut |
| Conventional saw | 4 to 12 hours after finishing | At least 1/4 of the slab, and 1 inch minimum |
Those are guides. Heat shortens every window and cold stretches it. Test a short cut first. If the edges ravel, wait a little and try again. Plan for sawing before the pour, including who stays late.
Worked example. You have a 6-inch slab, 28 by 42 feet.
- Spacing runs 24 to 36 times the thickness: 144 to 216 inches, or 12 to 18 feet.
- NRMCA recommends no more than 15 feet, so 12 to 15 feet is your range.
- Depth: 6 / 4 = 1-1/2 inches.
- Layout: joints at 14 feet make 2 panels across and 3 panels along. Each is 14 by 14, which is square.
- Keep panels square or close to it, with the long side no more than 1.5 times the short side, and avoid L-shaped panels.
Warning: Dry sawing concrete makes silica dust. Use water or a vacuum on the saw, follow your employer's silica plan, and wear the respirator that plan calls for. Keep the saw guard on.
Curing: Timing Is Part of the Finish
A perfect finish that dries out will still fail. Concrete in a dry environment can lose as much as half of its potential strength compared with moist-cured concrete, and the top is the first to suffer.
| Method | When | Watch for |
|---|---|---|
| Curing compound | Right after sheen leaves the final finish | Too late, and no film forms. Two coats at right angles give even coverage |
| Plastic sheet | As soon as it will not mark the slab | At least 4 mils, lapped, taped, weighted. Wrinkles leave dark streaks |
| Wet burlap | After the surface can take it | It must stay wet. Wet and dry cycles are not curing |
| Water or ponding | Above freezing only | Needs a steady water supply |
Use a wax-free curing compound if the slab will get paint or a bonded floor covering, because some compounds will stop those from sticking. Sheets should run past the slab edge by at least twice the slab thickness. Keep curing for the time your specification sets, often 3 to 7 days, and let it end slowly.
Reading the Damage
Start with where the damage is, how deep, and when it showed up. Then look for what happened on pour day.
| Defect | What it looks like | Usual cause |
|---|---|---|
| Dusting | Powder under traffic, scratches with a nail | Finished in bleed water, sprinkled water or cement, poor curing, heater exhaust, frost |
| Scaling | Surface flakes after freeze-thaw, starts in patches | No or low air, weak surface, poor curing, early deicers |
| Crazing | Fine map cracks, cells up to about 1-1/2 inches, rarely over 1/8 inch deep | Rich paste on top, finishing in bleed water, late curing |
| Blisters | Hollow bumps 1/4 to 4 inches wide | Surface sealed early, air and water trapped |
| Delamination | Thin layer 1/8 to 1/4 inch comes off, sounds hollow | Sealed while bleeding continued, often a crust |
| Plastic shrinkage | Parallel surface cracks in first hours | Surface dried before set |
| Curling | Slab edges lift at joints, rock under load | Top dries and shrinks faster than bottom |
A few details help you sort them.
- Crazing looks bad but does not affect structural strength.
- Blisters and delamination are the same failure at different sizes. Blisters break under traffic in dim light, so you may not see them at first.
- Delamination is found by tapping with a hammer or dragging a chain. Sound concrete rings. A delaminated spot sounds hollow. ASTM D4580 describes the method.
- Curling happens most on interior slabs over moist ground, more so on a vapor retarder, and more with wet mixes.
Thicker separation with rust stains is a different problem. That points to steel corroding too close to the surface, and it goes to the engineer.
What Happens Next
Delaminated areas can be patched once the loose layer is removed and the concrete below is clean, and wide areas may need grinding and an overlay. Blisters are hard to repair after the concrete hardens. Crazing is usually left alone. Light dusting may respond to a surface hardener. Treat these as options for the foreman to weigh.
Wide or offset cracks, and any crack in a structural slab, are not finishing problems. Do not patch those. Report them.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Trusting the sheen alone | Delamination, because the crust hid live bleeding |
| Steel troweling exterior air-entrained concrete | Blisters and scaling |
| Putting the same crew and clock on a slab over vapor retarder | Sealing in water that was still rising |
| Going home before sawing | Random cracks that no one can fix |
| Spraying curing compound the next morning | No film, weak surface |
Check Your Understanding
1. A wind is blowing across an interior slab with no roof on yet. The sheen went away in 40 minutes and it takes a boot print. Your probe finds soft concrete an inch down. What do you do?
Do not close the surface. It has crusted while the concrete below is still working. Protect it, stop the wind if you can, and hold final passes until the concrete underneath has caught up.
2. You saw a joint and the edges chip out and aggregate pulls loose. What does that tell you?
The concrete is not hard enough yet. Wait a short time, test again, and do not let the window close, or the slab will crack on its own.
3. A warehouse floor sounds hollow in a 6-foot patch. Thin pieces lift off. What likely happened?
Delamination. The surface was troweled closed while the concrete below was still bleeding. The usual fix is to remove the loose layer and patch, or grind and overlay a large area.
4. Basement slab, heated with an open propane heater, and a powdery surface a month later. Name two likely causes.
Carbonation from unvented heater exhaust, and water condensing on the cold slab and getting troweled in. Poor curing could add to it.
5. How deep and how far apart should joints go in a 5-inch slab?
At least 1-1/4 inches deep. Spacing of 24 to 36 times 5 inches is 10 to 15 feet.
Key Takeaways
- Bleeding must end all the way down before you close the surface.
- Cover test, boot print and a probe together beat judging by eye.
- Wind, sun and cold bases make a crust that looks ready and is not.
- Do not hard trowel air-entrained concrete.
- Conventional saw cuts go in 4 to 12 hours after finishing, or sooner in heat. Test first.
- Curing starts right after final finish, and late curing compound does nothing.