curing cement/concrete

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2 questions really

Sometimes it has to be kept wet, other times not. Does the atmospheric temperature & humidity affect how long it takes to cure, and how strong it will be?

I remember needing to wet down a new patio regularly as it set up, but this isn’t done with streets, sidewalks, or poured buildings.

In: Chemistry
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The reaction that “sets” concrete is called hydration…the portland cement is literally bonding with water molecules. Without water, it will not react. If it dries out before all the cement has bonded, it will be weak.

Temperature & humidity effect how fast water evaporates from the surface. This doesn’t usually matter if the mix is really wet (lots of excess water) but if it’s pretty dry then you could lose water too fast, before it fully hydrates, and then you get weaker concrete.

Temperature also impacts how fast the hydration reaction happens (hotter = faster) so, regardless of humidity, higher temperature will lead to faster cure. The chemical reaction is also exothermic (generates heat) so large volumes of curing concrete can self-heat. If they get hot enough to boil the water before it can hydrate, that’s also a problem. Large concrete pours may need cooling for this reason.

Whoever designed the concrete should take all this into account. If you don’t need the strength hit you get from not keeping it properly moist during cure, like a sidewalk, you don’t need to bother. Poured buildings are usually pouring into a form where they can’t lose water from most of the surface (and will often pool water or plastic over the top to minimize evaporation).

Very high strength concrete, like skyscrapers or dams, will have carefully design temperature, protection, and moisture profiles, backed up by testing, to make sure that the concrete actually gets to the strength it needs to. High strength concrete cures slooooooooowly.

Concrete never stops curing, and constantly uses water in a chemical reaction to strengthen and become harder. Normally you wet cure concrete when it needs to reach a specific hardness by a specific time and even more so when it’s hot out, as the concrete needs water to strengthen and the last thing you want is evaporation to take place.

Wet curing can be done on anything, as I’ve done it on foundations, entire floor slabs, bridge decks, silos etc. It’s all dependent on the conditions and the specs set from engineers

You can either keep the surface wet through misting it, flooding it or spraying it down regularly.

You can also stop water in the concrete from evaporating. This can be achieved by putting a burlap covering over it and wetting it, covering it with plastic sheets or spraying it with an impermeable membrane that wears off later on. Things like columns and walls are cured because they’re enclosed in formwork that stops evaporation.

Almost all concrete is cured in some way like this, even sidewalks – if you don’t it’ll experience shrinkage and drying cracks, well… more severe shrinkage and drying cracks than usual.

Temperature, humidity, and wind affect evaporation. The cement in the concrete needs to keep moisture in so that a chemical reaction can take place for it to become strong. If the water on the surface evaporates before the concrete “cures” it will result in surface cracking.

It is always better for concrete to be cured properly so that the cracking is minimized. Many organizations like your state or countries department of transportation require concrete to be covered, watered, or sprayed with compounds that block evaporation. If freshly placed concrete appears very white in color and is not covered, that usually means it has one of those compounds sprayed on the surface that are supposed to seal moisture in. These are used so that it does not need to be covered or watered.

Large flat areas like bridge decks or big slabs have wide areas where evaporation and cracking are more likely to happen and these absolutely need to be cured properly. The compounds only work so well so these wide areas are better sprayed with water (sometimes they even do both). If you poured a column it usually has forms around the sides and is only exposed at the top. The forms should hold in most of the moisture and it’s really only the top where evaporation would happen, so the evaporation is reduced.

It’s also important to remember that all concrete will crack. The best thing you can do is try to control where it cracks. This is why sidewalk has that groove about every 4-5 feet. This is a weak spot made on purpose so that the concrete’s more likely to crack along that groove rather than somewhere else.