A big construction project has always a long completion period, so it is definite to experience the season change and obvious rain too. When rain season comes, every firm hesitates to concrete. Even a sudden downpour of rain is enough to compel to think twice before going for concrete, even for an experienced contractor. So should we do the concrete pour during rain? Let’s discuss each aspect in this article.
Before diving into this topic, first, we need to understand how concrete is made and how it behaves?
Table of Contents
What is concrete?
Concrete is a homogeneous mixture of cement, aggregate, sand, and water. The water-cement ratio controls the concrete’s strength. The rainwater is unwantedly added to the concrete at the time of mixing, transportation, placing or finishing. During rain, sand and aggregate absorbed moisture, which raises the proportion of water in concrete.
These factors increase the water-cement ratio. Although it increases the workability of the concrete but reduces the strength to a great extent. On average, 0.1 increase in water-cement ratio reduce strength by 5%.
Hence, we should try our best to avoid excess water on concrete. But it does not mean we cannot pour concrete on the concrete during rain.
Impact of rain on concrete
What will happen to our concrete during the rain? It will depend on the below-mentioned conditions.
Case 1: Rain occurs within 30 minutes, i.e. initial setting time of concrete
The rain severely affects the concrete if it occurs before the initial setting. It means concrete is in fresh condition. Concrete made of OPC (Ordinary Portland Cement) has an initial setting time of 30 minutes.
The rain increases the water content of fresh concrete, which ultimately results in the bleeding of concrete. This bleeding water also carries cement particles with it, which produce a porous layer of cement and weaken the top surface in the concrete.
Case 2:Rain between initial and final setting time i.e. after 30 minutes and before 600 minutes
When fresh concrete surpasses the initial setting time (30 minutes), it comes into a gel condition. In this condition, your finger leaves only an impression on the concrete surface but does not dip into the concrete when you press your finger.
If rain occurs and your concrete is in gel condition, then rain does not affect the concrete mix very much. The rain only leaves an impression of rain droplets on the concrete top surface. This top layer may look ugly as rain impact the surface finish but it does not affect the strength of concrete.
Note: If the rain intensity is high and concrete has high internal voids (Lean concrete) then rain may affect strength of the top layer of concrete.
Case 3: If rain occurs after final setting time, i.e. 600 minutes,
The rainwater does not affect the pouring concrete if it occurs after the final setting time of concrete which is 600 minutes. At this stage concrete become stiff, i.e. your finger cannot leave an impression when you touch it.
Note: Even the light rain helps concrete to gain the strength as rain work like curing water.
Precautions while pouring concrete in rainy season
We all know that delays caused by weather lead to over cost of the project. For that, we need to have some alternate plans. Some areas in any country have a long rainy season, then we have no other choice but to pour the concrete in the rainy season. With some tricks and precautions, we can do the concrete even in the rainy season.
1. Choose the right timing to pour the concrete
Even though it is a rainy season, it would not rain every day. When we are preparing to pour concrete, we need to make sure that the weather is not extremely wet or windy. We can delay the concrete pour for some hours or a day for dry weather with the help of some weather tracking apps. If conditions are not allowing to delay, then we will proceed with the below-mentioned tips.
2. Control the water use in the mixture
During the rainy season, humidity in the air is increased to its highest level, so aggregates and sand absorb the moisture. This extra moisture increases the water-cement ration hence concrete start bleeding. To control the wetness, moisture content of aggregates and sand should be checked and this extra water should be compensated according to the design mix. Admixture can also be used to reduce the water-cement ratio however, it must be consulted prior with the client.
3. Use waterproofing admixtures
Using hydrophilic admixtures when pouring concrete during rain provides it resistance against water infiltration. These admixtures react with water and cement particles to form calcium silicates that reduce the pores and micro-cracks in concrete.
Note: This reaction last for the entire life of concrete and seals all initial shrinkage cracks along with the cracks those occur over the time.
4. Arrange covers before concrete
It is a common practice to keep the concrete pouring area dry and rainproof. This can be done by covering the concrete with tarpaulin sheets. Precautions must be taken by covering with a tarpaulin sheet but that sheet should not be in contact with concrete otherwise it may prolong the setting time. The sheet should cover the concrete in such a way that proper air circulation is maintained which helps the concrete set timely.
5. Combat heavy winds with nets
Strong shuttering must be ensured before concrete. Heavy rain may cause the settlement of the nearby soil and may dismantle the shuttering arrangements. Heavy wind due to rain may also cause bleeding in concrete, which leads to shrinkage cracks. Construction nets are often used to minimize the impacts of heavy rain and wind on concrete surfaces.
6. Plan transportation routes of RMC Trucks
Mostly Construction sites are not equipped with well-maintained roads. Generally, construction sites have earthen roads at least in the initial phase of construction. A slight rain creates mud and causes traffic jams and congestion on roads. So during the planning stage, we should include the best possible access to the RMC trucks or concrete mixer trucks. As all RMC trucks have open-top, any delay in the delivery may allow rainwater into the truck through the top opening. This will again increase the water content, hence the quality of concrete.
7.Make instant repairs when there are damages
Light rain may not harm the concrete, but heavy rain will definitely do. As soon as the rain stops or slows down, it is necessary to assess the damage caused to the concrete surface. A simple scratch test with a screwdriver or piece of reinforcement bar may give the idea of damage by comparing the questionable surface with a good quality surface. If it is found that concrete has been compromised, then we should do immediately minor repairs.
Minor repairs can be done by spreading the cement on the concrete surface. If detected damage is high and the concrete section is thin, then it is advisable to remove that damaged concrete and cast it again.
Sudden rain during concrete pouring
In any case, the pouring of concrete is in progress, and rain started suddenly. It is best to cover the recently poured concrete with tarpaulin sheets and wait to rain pass. As the rain stops, remove the water from the concrete’s surface and spread the cement on the top surface to compensate for the washout cement due to rain and complete the finishing.
Conclusion
Damage caused due to delay by rain to the construction project can be big and costly. Important measures to minimise the damages to the concrete are proper planning, accurate prediction and good timing.
Read more:
Bleeding of Concrete – Causes, Disadvantages, and ways to reduce it