People more often confuse drying time with setting time of concrete. The setting time directly describes the rate of hardening process in the concrete while drying time relates to the evaporation rate. Water mixed in the concrete contributes to two things. One is hydration. 23% of cement content will spend on the hydration. The rest will produce capillary pores.
It’s important to understand the core concept of setting time, curing time, and drying time to get the maximum out from the construction.
Setting time of concrete
As mentioned, setting time is defined using the level of hardening. Hydration starts when cement mix with the water. This reduces the workability of the mix. The time taken to reduce the workability up to a certain level is identified as initial and final setting times of the concrete. ASTM C403 / C403M is the standards test for initial and final setting times.
Initial setting time
We need to have certain flowability for transporting, pumping, and placing works. This is the time where the mix starts to lose its plasticity. Transporting and placing activities are allowed until you reach the initial setting time. Not just the materials, but also the mix ratios and environmental conditions can affect the initial setting time.
Typical setting time varies between 3-6 hrs. It can adjust by changing the mix properties, exposing environment, temperature, or adding different admixtures.
Final setting time.
At this point, concrete will lose its plasticity completely. Final setting time is the limit to stop consolidating the concrete. Any means of vibration hereafter will disturb the bond between particles.
Curing time of concrete
Cement hardening needs water for its reaction. During hydration, water will react with cement to produce stiffer tricalcium silicate hydrates. Hydrating is significant in the first 28 days for general concrete. Therefore it is mandatory to retain some excess water in the concrete at least one month’s time. During the curing, we ensure that hydration water present in the concrete.
Curing time varies with the structure type and exposure condition. General members like slabs, beams, and columns at least 14 days curing is essential. Improving the curing condition will result in good hydration. Select the most effective and economical method for the work.
Different ways to cure the concrete
- Using curing agents
- Covering with vapor-proof sheets
- Arranging water sprinklers
- Using admixtures
Curing during cold weather is a special situation to handle. See the recommendations for cold weather concreting & curing.
Drying time of concrete
Drying time in concrete is more often consider when you need moisture-sensitive special floor finishes. It is important to dry the concrete when you need wooden floors, vinyl composition tile, linoleum, or epoxy floors. Evaporating moisture and its pressure can harm these floors.
Typical 4-inch concrete slab with water to cement ratio of 0.5 will take 3-4 months to dry. Drying time will depend on the following parameters.
- Concrete thickness: It will take at least one month to dry out one inch of the concrete layer.
- Density: denser the mix, your capillary water paths are thinner. This will make drying much longer to bleed out the excess water
- Initial slump: The present water content at the beginning is too much; you will have to wait more time for evaporation. However, increasing slump by other means can be beneficial to reduce drying time.
- Placing & environmental temperature: hotter climate and warm concrete will harden at a fast rate. Even though it doesn’t affect drying time, this will reduce the curing time. The overall waiting period will be shortened according to that.
- Exposing weather: lower dew point and windy condition can dry out the concrete fast. Still, a high rate of water evaporation can cause shrinkage cracks. Follow proper guidelines when you are exposing concrete to hot weather.
How to improve the concrete drying rate
It is obvious that drying will start after you allow the curing water to evaporate. It is important to reach the target strength to let go of the curing water. Unused water for hydration will keep in the capillary pores and start to evaporate during drying. Having high water content in the concrete will leave to much evaporable water in the concrete. This needs more drying time.
On the other hand, mixes with low water content will take less time to dry out. See the different ways to cut the drying time.
- Use water-reducing admixture if possible. This will help to maintain the desired workability while reducing water content.
- Try to use different placing methods that need less workability. Chutes and belts require less slump than pumping.
- Try to optimize the mix design for low water to cement ratio. This way, the mix will be economical too.
- Try adding accelerators to the concrete. It will speed up hardening and drying can start quickly.
- Try to schedule concreting in warm weather and use warm water during mixing. This will improve the hardening rate
- Power troweling can seal the capillary ways on the surface. Entrapping water in the concrete will cost a longer drying time.
- Use dehumidifiers, heating, ventilating, and air conditioning system (HVAC): These machines will artificially dry out the concrete using half of the natural drying time.
- Always use the bottom polythene layer with foundation constructions. Otherwise, groundwater will saturate the concrete forever
- Make sure the necessary wind flow is available to the job site.
Always try to start drying the concrete when you reach the necessary strength in the concrete. Aggressive drying can cause shrinkage cracks in the concrete.