Prerequisite knowledge:-
Fine and coarse aggregate are by themselves far more stronger that cement water paste, also we have very little control over the strength of these mineral constituents of concrete.
But it is the cement water paste which binds all the constituents of the concrete together.
So a strong cement water paste will give a stronger concrete and this is dependent on the water/cement ratio.
Too much water will cause capillary cavities which makes the concrete weaker.
If water is not enough, hydration of cement wont be completed, again making the concrete weaker.
If water/cement ratio is more than 0.7, the concrete mix will not harden, that means it will remain a pour for all eternity.
So, in short, the water in your concrete mix is the most important single factor, which effects quality of concrete.
The relation between water and cement is a very complex one , and a rainstorm definitely do not understand this relationship.
Should you concrete on a rainy day?
I wish i could just say no. But there are many factors which affect the decision making process. But, ideally, you should avoid a rainy day. Modern forecast do help a bit bit, but we all know they are not foolproof and so you might find yourself pouring concrete in heavy rains.
How do we prepare for concreting?
Always expect rain.
Be ready with plastic sheets, to cover the concreting surface.
If you are locally mixing the concrete, you should consider covering cement, fine and coarse aggregate.
Calculate percentage of bulking of sand.
Cover or block any rain water drain, which opens up into the concreting surface.
If it rains - Things you should do
Immediately try to cover the concrete surface.
If the fresh concrete is exposed to rain directly, try and brush water off the surface before it seeps into the concrete.
Check if water is seeping into the concrete through form-work.
If it rains - Things you should not do
Never rub the water into the concrete. This is what most masons would do, you cannot harm your concrete more.
Do not sprinkle dry cement over the concrete surface to balance the water content. This would cause a thick layer of cement water paste, which would flack and dust very soon.
Damage control
The surface tends to be rough and a scratch test and visual inspection will ascertain the extend of damage, tough some core samples checked under a microscope will be foolproof.
After the rain you can put a cement slurry over the concrete surface, only if you think that the surface was compromised. But even this is rarely needed, if the concrete was finished 2 hours before.
You could also remove the damaged surface concrete and re-concrete the surface. This depends on the extend of the damage caused. But this is a step needed in extremely rare cases.
Usually the concrete is good to go after about 4 hours from concreting. And after about 2 hours, the water does not seep in enough to cause any real damage.
But those plastic sheets to cover the surface really do come in handy.
Light shower after concreting is good as it will aid in the curing process of concrete.
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