Rain and Its Effect on Fresh Concrete

Rain and Its Effect on Fresh Concrete

One massive concern several property owners have throughout this period of the year is how rain can affect the pouring of concrete. The frequent question usually on the lips of those constructing their dream home during this rainy days, is “should concrete pouring activity be done during rain?” or “can I lay concrete in the rain?”.

It is no secret that concrete pours are generally rescheduled for a day without rain, as concrete will generally need a period of twenty four to forty eight hours to cure so as to reach its maximum strength.

The quality of concrete is directly associated with its strength which depends on the water – cement ratio. Rain water will increase the proportion of water within the concrete mix at the time of mixing, transporting, placing, compacting and finishing. Although water-cement ratio improves the workability of concrete, it ultimately reduces the strength to a great extent.

Concrete pouring during rain depends on

1. whether or not it rains throughout concreting or if it rains within the initial setting time of concrete.

2. If it rains between the initial setting time and final setting time of cement

3. If it rains once final setting time

01. Rain affects concrete pouring when it occurs before the initial setting i.e. once concrete is in fresh condition. In addition, rain affects the concrete while mixing or transporting.

Rain will increase the water-cement ratio of the fresh mix and results to bleeding in concrete. The bleeding of water carries the cement particle with it. Hence, water cement ratio will get reduced, which further decreases the strength of concrete.

If the formwork is fixed properly, the layer of cement slurry will be formed on the top due to the settlement of solid particles by gravity action. This top layer becomes porous and weak in nature due to rain, which results in the poor strength of concrete. Sometimes plastic shrinkage cracks may also appear on the surface of the concrete.

02. Laying concrete is not easy during the rain. The rain affects the concrete pouring when it occurs after the initial setting time (30 min) and before final setting time (600 min). It means, it is not in fresh condition or it is initially set.  

In this case, the rain does not affect the mix very much. The impression of rain droplets will be seen on the surface of the concrete. It looks ugly as rain impact the surface finish, but it will not affect the strength of concrete. Sometimes, If the intensity of rainfall or the internal voids are higher in concrete, then rain may affect the top layer of concrete.

03. The rainwater does not affect the pouring concrete when it occurs after the final setting time (> 600 min) of concrete. It means concrete is in stiff condition (your finger do not leave an impression on the surface when you touch it). On the contrary, the light rain helps to cure the concrete and ultimately helps to achieve a good strength.

Precaution while Laying Concrete in the Rainy Season:

  • Always see weather forecast and probability of rain
  • The provision of the adequate good quality plastic sheet should be made in advance at the time of placing the concrete.
  • Simply reduce water cement ratio as the rain water is already present on the surface of aggregate due to raining or increase the quality of cement, which is worthy.
  • The water-cement ratio should be reduced by mixing the appropriate admixture after getting the permission of consultant.
  • The staging should be provided to cover the pouring area with the plastic sheet. Every precaution should be taken to reduce the interruption due to rain during the pouring activity. The bit of help should be taken from the weather department to plan the pouring of concrete.
  • Spray rich cement slurry on the surface, where it has rained immediately after you laid concrete.
  • If possible cover aggregate also, as when they become wet; they increase the water cement ratio.

In conclusion, the answer to your question “Can I lay Concrete in the Rain?” is generally you can, provided you do exercise all above precautions. You can pour concrete in the rain. But of course not during very heavy rain as it will wash the cement slurry in laid concrete.

You can lay concrete in the light rain like sprinkling or spraying of water with the provision of the thick plastic sheet. However, It is advisable, not to start concrete pouring in the heavy rain. It is also highly advisable to suspend the concrete pouring work while its raining heavily.

If it is raining during pouring concrete or within the first 30 minutes i.e. initial setting time of concrete then reduce the water cement ratio up to the required slump of concrete, and lay a plastic sheet over concrete laid.

If it rains between after setting time and before final setting time i.e. after 30 minute and before 600 minute (10 Hour) then cover the concreting or concrete casting area with tarpaulin and spread the rich cement slurry on the surface of concrete where it has rained.

If it rain after final setting time i.e. 600 minute (10 Hour) then no need to worry as rain infect improve curing of concrete.