Adding water to concrete in heat extremes
It is a common practice to add water to the mix and re-temper the concrete once the truck arrives. Although you may improve the workability of the concrete, you will impact the water-cement ratio and change the slump.
- Every inch you lose in slump, you also will lose 200 lbs of compressive strength. A rule of thumb: 1 gallon of water per yard = 1 inch of slump.
- Don't re-temper the concrete by adding water more than once while it is on the truck.
- Water the sub-grade and forms prior to the pour. This ensures that moisture will not be drawn out of the concrete from below. Make sure you do not leave puddles of water.
- Mist or fog concrete during the finishing process on a hot or windy day. Make sure you are not making puddles. Also make sure you are not working this water into the surface. Stop misting or fogging 5-10 minutes before each finishing operation. Add a surface retardant after each process.
- Try water curing the concrete as soon as you have completed finishing it. Water curing is an excellent way to cure hot weather concrete. Remember that water curing will require you to keep the surface wet for at least seven days.
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Pouring concrete in hot weather takes special care. The key is allowing the cement in the concrete to properly hydrate and cure. If the water in the concrete evaporates too early, you will likely have plastic shrinkage cracking and scaling problems. Follow these guidelines.
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During hot weather, we use several techniques to ensure proper hydration of the cement paste. This includes lowering the temperature of the concrete mix ingredients at our plants and adding retardants. On the job site, there are several keys that will help you finish concrete in hot weather.
Hot and dry conditions
Watch the relative humidity and the dew point. The higher the humidity, the slower water will evaporate from the concrete surface. With low humidity, evaporation speeds up. The dew point is the temperature when water vapor turns to liquid, or dew. A low dew point means that moisture from any exposed material will quickly turn into vapor. On hot and dry days, make sure you are spraying cold water on your forms and your aggregate sub-base before pouring concrete. This keeps the moisture from being pulled into the sub-base. Make sure to get the concrete off of the truck as soon as you can. Shade any trucks in waiting.
Wind
Gusty winds will rob the surface of the concrete of its water. This leads to uneven water to cement ratios and, eventual scaling. You should spray the surface of the concrete with evaporation retardants and repeat after each finishing process. You can also combat the wind with a wind barrier. If you notice cracks appearing that are perpendicular to the wind, you are dealing with an evaporation problem. Mist the surface.
Direct sunlight
On a cloudless, dry day, try to pour slabs on the west side of a building in the morning and slabs on the east side in the late afternoon or evening hours. If the sun dries out the surface quicker than the rest of the slab, bleed water can get trapped just under the surface and cause crazing. If you are placing concrete in an area where there is partial direct sunlight and shade, take extra precautions to block the sunlight from the solar exposed areas. Keep your tools in the shade. Mist the sun-exposed areas, but do not over-water the shaded areas. Ask your IMI rep about the use of retardants in your concrete batch. This will slow down the hydration process and buy your crews time on a hot, sunny day.
Quick temperature drops
Watch for quick temperature shifts in the forecast. In hot conditions, pop up storms can occur and greatly lower the air temperature. You will need to cover your concrete to ensure consistent curing. Do not add accelerators to the mix in the summer.
Rain, hail or anything else that falls from the sky
If cold precipitation falls on concrete that has just been finished, it may damage the surface. Cover curing concrete with plastic sheeting if there is a threat of storms. Never finish concrete in the rain.
