Watering New Gunite and Concrete - Further Reading

Why Should You Water New Gunite or Concrete?

Spraying water on concrete is one of the oldest and best ways to cure concrete.

The chemical reaction that binds the sand and gravel together to form hardened concrete can take up to 28 days to fully complete.

Concrete curing and hardening is a chemical process that is exothermic that generates heat. You want concrete to be cured slowly, uniformly, and evenly from top to bottom.

Strength of 28 Day Moist Concrete.png

Shotcrete / concrete is ready for water as soon as the initial set is complete, which is about 4-6 hrs after placement. Water is applied to aid in hydration/curing, a process which decays exponentially, so watering is most critical in the first hours/days after placement.[1]

Concrete will continue to cure/harden forever, but the strength used for design work is the strength after curing for 28 days. This is primarily because it's close to an inflection point in the strength curve with much lower strength gains afterwards (and 4 weeks is an easy timeline to use for construction purposes). As the chart shows, it's really the first 7 days that are critical for keeping the concrete moist if you want to achieve full 28 day strength.

During the summer months, the outside temperature can become hot. By keeping the surface wet, you are keeping the concrete temperature low. Be sure to start watering the concrete in the morning and keep watering throughout the hottest part of the day. You can use sprinklers on a timer to help with this. Do not start watering during the hottest part of the day because it could shock the concrete into developing surface crazing (similar to a hot glass breaking when filled with cold water).

Spraying water on the surface prevents the surface of the concrete slab from drying faster than the bottom.

Watering a Finished Surface

With a finished surface, i.e. final decking with no stone going over top, stay off it the day it is poured. By the next morning, stay off if you can, or walk gingerly (no twisting or boots), and keep it wet by misting, or if you need to leave, cover with wet burlap, old sheets, towels, etc. You only need to do that for a day or two, after which occasional misting is good for around a week.

Be careful not to cool a finished surface really fast by flooding it if it is dried out and hot from the sun - just mist it to cool it slowly, after it stays wet for an hour or so, you can flood it if that's helpful for keeping it moist. Gentle temperature change allows the cracks to develop more slowly, and more likely follow the intended control joints.

Rapid cooling induces cracks. This is the worst risk when people are advised to mist twice a day. That's way too long between drinks in the sun (for a finished surface), and ineffective if it is concrete that will be surfaced with something else.

Poured concrete (i.e. decking) does need different care, because much more water is in it initially. Don't walk on until about 1.5 days and be more careful watering it.

Watering a Pool Shell

A pool shell (that will be plastered) and surrounds that will be finished with stone or other material, can't be hurt by walking on it the next day. Do not walk down steps or ledges on the same day it was put in as the pressure of having all of your weight on one foot could cause an imprint.

Gunite/shotcrete is applied much drier than poured concrete. You can literally walk on it with in a few hours of it being applied. The drier application also gives it much better compressive strength than poured concrete for otherwise the same mix. That makes it a little more critical on the watering for the first few days.

You can place sprinklers in the gunite shell and let it run for an hour or more a few times a day. You can get timers to help out with this.

How to Water

As soon as the concrete is set enough to walk on (which with shotcrete, that's quick...) you can start watering it. Do it several times a day 3-4 times or more. If it starts to rain you don't need to add to that. If you are unsure start early the morning of the day after it is poured.[2]

Avoid shooting a jet stream of water at the concrete at first. It won't matter by the second day after finishing, but contractors don't know how well their customers absorb instructions, so they perhaps find it safer to just say 1.5 days.

How long to Water Concrete?

The most critical time is in the first 3-4 days. There really isn't a great reason to go more than 14 days with this process.

The benefit of keeping concrete from drying too quickly mostly happens at the surface and in the early stages of curing. After the first few days the surface has settled in, and the rest of the curing is going on deeper, and misting has little to no effect. Misting for longer doesn't hurt anything, and can give you something to do while you're anxiously waiting for the next steps