How to fight the effect of weather during concreting

Extreme hot or cold weather usually cause concrete surface to crack which is a defect

In very hot weather

1. The workability of the concrete drops within a short time. Even though people usually add more water to the concrete, it is not the best option. It is more ideal to use a set-retarding admixture in the concrete when the weather is too hot.
2. If the option (1) is not available, one can dampen the subgrade (in road construction) and formwork (in building and other structures) but don’t allow excess water around. Also ensure that the materials for the concrete are kept out of direct sunlight.
3. After mixing the concrete ensure that casting progresses fast without delay.
4. After casting concrete under very hot weather, one can spray the concrete with aliphatic alcohol after the initial finishing to reduce evaporation and cracking.
5. If there is risk of cold unbound joint (joint usually found between two concrete parts cast at different times due to quick stiffening as a result of hot weather), one may need to provide construction joint.
6. It is wrong to apply cold water over hot concrete surface as thermal cracking may develop as a result of sudden cooling

In very cold weather

1. Extended setting time may cause costly delays. One can use warm water in the mix and also try to keep all materials warm.
2. Ensure that formwork and subgrade are covered to keep them free of frost and ice.
3. Make use of set-accelerating admixture.
4. Concrete should never be laid on frozen ground and effort should be made to keep its temperature above 10⁰C as possible for first few days.
5. During curing, effort should be made to use an insulating layer or other method to ensure that heat is retained in the concrete.

States of concrete

Concrete have three (3) different states

1. Plastic state: This is when concrete is first mixed. At this state, concrete can be worked or moulded into different shapes. Concrete is plastic during placing and compaction.
2. Setting state: This is when concrete begins to stiffen. It comes after compaction and during finishing.
3. Hardened state: At this state, a concrete that has set begins to gain strength and harden. The properties of hardened concrete are strength and durability.

Good practice in using concrete mixer

1. Add the aggregate first, then sand and cement in measured amounts.
2. Mix materials until there is no visible sand.
3. Do not overload the mixer.
4. Check the capacity of the mixer before starting work.
5. Avoid delays between batches to get maximum output.

Good practice in using poker vibrator to compact concrete

1. Do not vibrate the concrete randomly with poker vibrator but use definite pattern to ensure that the radius of action of the vibrations overlap. The radius of action is the radius over which air bubbles rise to the surface during vibration
2. After vibration, remove the vibrator slowly to avoid ‘poker holes’.
3. Don’t touch formwork face with poker to avoid damage.
4. Don’t touch reinforcement with poker to avoid poor bond with concrete.
5. Don’t move concrete sideways with poker as it may cause segregation. Use shovel to do that instead.
6. When poker is not in use, switch it off.
7. Vibration can be done 5 – 15 seconds.
8. Do not under vibrate. When you do not see air bubbles coming up again, then the vibration should be done.

Curing of concrete

Curing means keeping concrete moist for some time (about 3 – 7 days). Curing should commence immediately after finishing the concrete surface. Well cured concrete has surface that wears better, lasts longer and better protects steel reinforcement. The longer the concrete is cured, the better its strength.

Ways concrete can be cured

1. Application of water: A thin misty spray of water on the surface of the concrete should be done regularly to cure the concrete. Do not use splash as splash water can damage concrete surface.

2. Use of sheeting: A plastic sheet can be used to cure concrete by reducing water loss from the concrete. The sheet should be used to cover the concrete surface. To avoid damaging the concrete surface, the sheet should be moist.