Concrete Curing and Striking Times: Strength, Not Schedule, Decides When Formwork Comes Off

The Calendar Is Not a Test Method

"We always strike walls next day" is one of those site truths that is right until it is expensively wrong. Concrete does not gain strength by calendar; it gains strength by chemistry, and the chemistry runs at a speed set by mix design and temperature. The same wall pour that is comfortably ready to strike after 18 hours in July can be nowhere near it after 48 in January. Striking decisions are strength decisions: the question is never "how long has it been?" but "what strength has this concrete actually reached, and what strength does this element need before its support is removed?" Get that order right and striking is routine. Get it backwards and the results range from surface damage and grout loss, through deflection and cracking that lives in the structure forever, to the falsework collapses that put striking procedures into British Standards in the first place.

What Curing Is Actually Doing

Cement hydration needs two things: water and warmth. Curing is simply the discipline of keeping both available while the concrete is young. Let the surface dry out early — sun, wind, low humidity — and hydration stops in the very zone that becomes your wearing surface and your cover to reinforcement: weaker, dustier, more permeable, and prone to plastic shrinkage cracking within hours of the pour. The methods are old and unglamorous because they work: leave formwork in place (the best cure a vertical face can get), cover slabs with polythene or wet hessian, apply spray-on curing membranes where covering is impractical, and in hot drying weather protect the surface from the moment finishing allows. In cold weather the priority flips to warmth: insulated formwork and frost blankets to hold the heat of hydration in, never pouring onto frozen ground, and remembering that fresh concrete that freezes before reaching a few newtons can be permanently damaged. The specification will state a curing period and method for a reason — treat it as part of the pour, priced and planned, not an afterthought for whoever is left on Friday.

Temperature and the Rate of Strength Gain

The single most useful mental model on site: strength gain roughly follows the concrete's temperature history — its maturity — not elapsed time. Warm concrete hydrates fast; cold concrete hydrates slowly; very cold concrete effectively pauses. This is why in-situ strength on a winter pour lags the cube in the 20-degree curing tank, and why a thin slab exposed to wind behaves differently from a thick wall cooking on its own hydration heat. Mix choices move the curve too: higher cement contents and finer grinds gain early strength faster, while high-replacement GGBS and PFA mixes — increasingly standard for carbon reasons — gain more slowly early and need their striking expectations adjusted accordingly. None of this needs a laboratory to respect: it needs the striking decision to reference the actual pour, its mix and its weather, rather than a habit formed on a different mix in a different month.

Proving Strength Before You Strike

For low-risk vertical faces, many sites operate to tabulated minimum periods appropriate to the mix and ambient temperature — legitimate, provided the table's assumptions match reality and someone is honest when they do not. As consequence rises, so should the evidence. Site-cured cubes, stored next to the element rather than in the tank, tested at the striking age, tell you what the element's concrete has done in the element's weather. Maturity methods — temperature sensors cast into the pour, converting the recorded temperature history into predicted strength against a calibrated curve for that mix — give a continuous, element-specific answer and have moved from novelty to normal on programme-critical pours. For soffits, falsework and anything whose early loading is structural, the striking strength comes from the designer and the decision runs through the temporary works process: a formal permit to strike, issued only when the evidence says the required strength exists. The pour that hits programme is the one where striking was planned like this from the start — mix chosen for the season, curing protected, evidence arranged before the pour, and the strike falling out of data rather than debate.

The Records That Close the Loop

A pour record worth the name captures what the striking decision relied on: mix and ticket references, placing times, weather and concrete temperatures, curing method and duration, cube or sensor results, and who authorised the strike with what evidence in front of them. Partly that is protection when a surface defect or a crack raises questions months later. Mostly it is how a site gets better: a season of honest pour records tells you exactly how your mixes behave in your conditions — and turns next winter's striking times from an argument into a lookup.