Curing and Protection After the Pour

Purpose

Curing is the cheapest strength the shop will ever buy and the first thing a tired crew skips. Concrete gains strength by hydration, which stops when the surface runs out of water, and the surface is exactly the part the customer walks on and the part that has to resist freeze-thaw and de-icers. A slab left bare can lose a large share of its potential surface strength while its interior tests fine, which is why cylinder breaks come back passing on a driveway that is already dusting. The second half of this procedure matters just as much: a correctly cured slab that a subcontractor parks a loaded truck on at three days is cracked, and the shop that placed it owns that crack in the customer's mind.

Scope

Covers method selection, application, maintenance, physical protection and the release decision for cast-in-place slabs, footings, walls and flatwork on residential and light commercial work, from the moment the surface will accept a cure to the removal of protection.

It does not cover finishing, which hands over at the same moment; freeze protection and heated enclosures in cold conditions, or evaporation control during placement, both owned by the cold and hot weather pour standards; formwork stripping and shoring, owned by its own SOP; or strength acceptance testing, owned by the cylinder sampling SOP, whose results this procedure reads.

Roles and responsibilities

Procedure

1. Pick the method before the pour, against what comes next on that slab. Choose between continuous wet cure, wet covering under sheet, sheet alone, or a membrane-forming curing compound, on the basis of the specified finish and any later coating, topping, adhesive or hardener. Acceptance: a method named on the work order along with the reason, and where a coating or topping is scheduled, either a dissipating compound the coating manufacturer accepts in writing or a wet cure. Wrong looks like a standard compound sprayed on a slab that is getting an epoxy coating in a month, which leaves a bond-breaker and hands the shop the cost of shot-blasting it off; stop rule, unknown follow-on work means wet cure, which forecloses nothing. Hazard: none at this step, it is a decision made at a desk.

2. Compute coverage, then buy to the number. Take the slab area, divide by the manufacturer's stated coverage rate for the specific product, and stage that quantity plus a margin, or for a wet cure stage enough burlap, sheet and hose to hold every square foot continuously. Acceptance: the arithmetic written on the work order and the containers on site counted against it. Wrong looks like ordering by feel, which reliably lands short and gets stretched over the area rather than reordered; stop rule, short material means the cure does not start until more arrives or the method changes to wet cure. Hazard: curing compounds are solvent-bearing, so read the SDS for the product in use, use the respiratory protection it specifies in any enclosed or partially enclosed space, and keep ignition sources away from solvent-based products.

3. Start the cure the moment the surface will take it, and log the time. Begin as soon as the surface will not be marred by the method, and for a compound apply in two passes at right angles to get even coverage. Acceptance: cure started inside the window the specification sets, coverage rate met by emptied containers against the step 2 figure, and no dry interval between finishing and cure. Wrong looks like a crew cleaning tools first and curing after, which loses the hour that matters most on a hot day; stop rule, cure before cleanup, and a slab that dried before the cure started is logged with the gap so the surface can be watched. Hazard: spray application puts aerosol at head height, so the operator works upwind with the eye and respiratory protection the SDS names, and the sprayer is depressurized before any nozzle is cleared.

4. Hold the cure continuously for the full duration, and check it daily. Keep wet cure wet without interruption, keep sheet sealed at laps and edges, and inspect once a day. Acceptance: no dry interval at any point across the specified duration, which for many specifications is seven days for Type I cement at moderate temperature though the specification governs, with the daily check initialled. Wrong looks like burlap that dried out over a weekend, which is worse than never wetting it because the wet-dry cycling drives its own surface cracking; stop rule, a break in the cure is logged and the duration extended to make up the lost time, not waved through. Hazard: wet sheet on a slab is a slip surface and a wind hazard, so laps are weighted, and anyone walking a wet-cured slab uses the same footwear the pour crew used.

5. Keep traffic and loads off, in writing. Barricade or tape the area, post the date and time it may take foot traffic and the date it may take vehicles, and give the same in writing to the general contractor or customer. Acceptance: barrier in place, dates posted at the slab, and a written notice acknowledged. Wrong looks like a verbal warning to whoever was standing there, followed by a loaded pickup at three days and a crack across two panels; stop rule, 29 CFR 1926.701(a) prohibits placing construction loads on a concrete structure until the employer determines from information received from a person qualified in structural design that the structure or portion of it is capable of supporting the loads, so no equipment, material stacks or vehicles go on until that determination exists. Hazard: barricades themselves must not create a trip or a blocked egress route on an active site, so they are set clear of doorways and lit if they stand overnight.

6. Coordinate sawcutting without breaking the cure. The saw crew works inside the cure period, so plan the sequence: pull back only the covering they need, cut, and restore the cure behind them the same shift. Acceptance: cure restored over every cut panel before the crew leaves, and the covering off any panel for the shortest practical time. Wrong looks like an entire slab uncovered at hour six for a two-hour sawcut, which turns a continuous cure into a broken one across the whole area; stop rule, uncovered longer than planned means the duration is extended for those panels. Hazard: sawing concrete generates respirable crystalline silica, so the cut runs under the engineering control and, where required, the respiratory protection that 29 CFR 1926.1153 sets for that equipment and duration, per the silica dust control SOP.

7. Release against evidence, then remove protection and close the record. Release the slab to its service loads on strength evidence rather than on the calendar, using cylinder breaks or a maturity method where the job has them, or the age the specification names where it does not. Acceptance: a documented result or a specification age, the barrier removed, the slab walked, and the cure log closed. Wrong looks like a release because the crew needed the barricades on another job, which is how a slab gets loaded on day four; stop rule, no evidence means no release, and the PM makes the call, not the crew on site. Hazard: removing sheet and burlap lifts dried cement film, so it is rolled damp rather than dragged dry, and the walk is done in eye protection since a cured slab in full sun is a glare surface.

The record this produces

One cure log per pour. Fields: job and pour identifier, method with the reason and any coating or topping noted, computed coverage and containers actually emptied, cure start time against finishing complete, the specified duration, a daily initialled check with any break and its length, the posted traffic dates and who acknowledged them in writing, sawcut coordination times, and the release decision with the evidence behind it.

It sits with the finishing log. The crew leader reads yesterday's entry each morning to know what needs re-wetting. The PM reads it the day a coating fails to bond or a customer reports dusting, and the two most useful fields are the containers-emptied figure and any logged break. The office needs the written traffic notice and the acknowledgement the first time someone claims nobody told them.

One pass, with a step that failed

A 1,760 sq ft commercial slab, broom finish, no coating scheduled, so step 1 selected a membrane-forming curing compound and the work order said so. The product's stated coverage was 200 sq ft per gallon, giving 1,760 divided by 200, which is 8.8 gallons required, and step 2 wrote that on the work order.

Step 3 failed. The crew sprayed the slab in two passes and reported it done, and the crew leader counted 6 gallons emptied against the 8.8 required, which is 68 percent of the rate, a shortfall of about 32 percent. Nothing about the slab looked wrong, because a thin membrane looks like a membrane.

The stop rule ran. Placement had finished at 11:10 and the shortfall was found at 12:25, so the surface had not yet dried past the window and the balance was usable: the crew mixed and applied the remaining 2.8 gallons in a third pass across the two earlier directions, and the crew leader logged 8.8 gallons total with the sequence and the times. Had the gap been found the next morning instead, the stop rule sends the job the other way, to a wet cure held for the balance of the specified duration, because a second compound coat onto a surface that has already lost its bleed does not recover the water that left overnight.

Step 5 posted foot traffic from the following morning and vehicles at the age the specification named, both on a sign at the slab and in an email the site superintendent acknowledged. Step 6 ran the sawcut that evening with covering pulled back four panels at a time and restored behind the saw. Step 7 released on a cylinder break at the specified age.

Under-applying by that margin does not produce a visible defect at handover. It produces a surface that hydrated less than the concrete under it, and the customer meets it as fine dust on the floor under a pallet jack in the first year, at which point the shop has no evidence except a cure log saying 6 gallons went onto a slab that needed 8.8.

When the slab will not cooperate

Rain arrives shortly after a compound is applied: check the product's rain-free period on its data sheet, and where the film was still soft, re-apply the affected area rather than assuming it held. The customer needs the surface in service before the specified age: the answer is a strength result, so pull an extra cylinder set or use a maturity meter, and the shop does not shorten a duration on request. A sealer or hardener is specified as well as a cure: confirm compatibility with both manufacturers in writing before either goes down, because the sequence and the wait between them are product-specific. The cure has to run through a hard freeze: this leaves this procedure and routes to the cold weather pour standard, which owns protection temperature, the enclosure and the carbon monoxide hazard that comes with heating one.

References

• ACI 308.1, specification for curing concrete, and ACI 308R for method selection and duration, in the edition your specification names.
• 29 CFR 1926.701(a), construction loads, prohibiting loads on a concrete structure until a qualified determination that it can support them.
• 29 CFR 1926.1153, respirable crystalline silica in construction, for sawing controls during the cure period.
• The curing compound manufacturer's data sheet and SDS for coverage rate, rain-free period, dissipation behaviour and coating compatibility.
• See related: the slab pour and finishing standard, the joint layout and sawcut timing SOP, the cold and hot weather pour standards, and the cylinder sampling SOP.