Concrete Curing Explained: Why Concrete Cracks, Scales and Fails Years Later, and the First 72 Hours That Decide It

Concrete is a reaction, not a drying

When cement meets water, crystals start growing and interlocking, and that growth is strength. Drying removes the water the reaction needs. Concrete that loses its moisture early stops hydrating in those areas, and it does not pick up where it left off when it gets wet later: lost hydration is lost strength and lost durability, permanently. That is why a slab can look perfect on day one and be dusty, weak and scaling in year five; the damage happened quietly in the first few days.

The 28-day myth

The 28-day number comes from testing, not chemistry: mixes are specified and cylinders are broken at 28 days because it is a consistent benchmark at which most of the long-term strength has arrived. Hydration continues for months and years if moisture is there. The period that decides the concrete is the first two or three days, when the reaction is fastest and moisture loss does the most harm. Waiting 28 days to call it cured fixes nothing that went wrong on day one.

Water, slump and strength

In an ICF wall we pour 25 to 30 MPa with a plasticizer for flow, and nobody touches the hose. Concrete pour day and curing is the day-of checklist.

Shrinkage, steel and joints

Concrete shrinks as it loses water, as it cools, and as it keeps hydrating, and because it is restrained by soil, walls and steel, that shrinkage becomes stress and stress becomes cracks. Crack-free concrete is not a goal; controlled cracks are. Rebar and mesh do not stop cracks; they hold them tight and spread the stress, and mesh only works if it is in the middle of the slab, not lying on the gravel. Fibres reduce early plastic cracking. Control joints are the planned cracks: at re-entrant corners, door openings, changes of thickness, and on a grid roughly 24 to 36 times the slab thickness; saw-cut at the right hour, which is a judgment call on temperature and mix, not a clock. Cut too early and the edges ravel; too late and the slab has already chosen its own crack. Loads are the engineer’s business; cracks are the finisher’s.

Placement and consolidation

Concrete does not flow like water; it has to be guided. Dropping it from height segregates it, pouring a wall too fast blows out the form or traps air, pouring too slow makes cold joints, and congested rebar leaves honeycomb if nobody vibrates. In an ICF wall the rules are lift height (3 to 4 feet), placement rate, and consolidation with an internal vibrator used with restraint or external vibration on the form; under-vibration leaves voids and over-vibration separates the paste. The crew that has poured a hundred ICF walls knows the sound a full form makes. What ICF is describes the pour inside the system.

Finishing: where good concrete goes to die

• Finishing too early traps bleed water under the surface and the top layer delaminates.
• Over-trowelling brings paste and water up and leaves a weak, dusty skin.
• Steel-trowelling air-entrained exterior concrete seals the air out of the surface and it scales the first winter; a broom or float finish is correct outside.
• Finishing by the clock instead of by the concrete; temperature and wind change the timing every day.

Curing: the step everyone skips

Curing is keeping the concrete moist long enough for hydration to continue: wet burlap or blankets kept wet, a curing compound sprayed on, plastic sheeting sealed down, or ponding on a slab, for at least three days and ideally seven. “It rained once” and “the forms were still on” are not curing. Concrete that dries early is weaker, more porous and loses its surface. In an ICF wall the foam does the curing for you: the concrete is sealed on both faces and cures slowly and evenly, which is one of the quiet reasons ICF walls are so consistent.

Ontario weather

Cold. Below about 5 degrees C hydration slows; if the concrete freezes before it reaches roughly 3.5 MPa the damage is permanent. Winter pours need a warm mix, accelerators or heated water, blankets or heated enclosures, and patience before the protection comes off. The Code and the CSA standard set the protection rules; the builder sets whether anyone follows them at 4 p.m. on a Friday in January. The foam of an ICF form insulates the pour on both faces, which is why ICF walls go in through most Ontario winters with far less drama than open-form concrete.

Hot and windy. When evaporation outruns bleed water, the surface cracks within hours (plastic shrinkage). Pour early, fog the surface, use evaporation retarders, start curing at once.

Exterior slabs. Garage floors and driveways live in salt, meltwater and freeze-thaw. Air entrainment, a broom finish, proper curing and a year before de-icing salt touches them. Exterior concrete almost always fails from the surface down.

Code minimum versus long life

Passing the footing and foundation inspection means the requirements were met that day. It does not mean the concrete will last. Mix selection, placement, finishing, curing and protection are the builder’s decisions, and concrete failures do not blame the weather or the supplier; they blame the builder. Handled with respect, concrete lasts generations. Handled carelessly, it fails patiently, and by the time it shows, the decisions are long past. Why new basements leak and building inspections are the two pages to read next.

Related guides on this site

Sources and further reading

• The 2024 Ontario Building Code
• CMHC
• National Building Code of Canada 2020

Questions people ask