Concrete drying out and cracking

Kashkai

I filled in some trenches in my floor today that were dug out to fix some leaking pipework. I was careful mixing the concrete so it wouldn't be too dry/wet and I added Polybond to it for extra measure to help it bond.

However, the surface is now cracking all along the edges and accross the fresh concrete. I've now moistened the surface and covered it with plastic sheeting to stop it drying out too fast but is the damage done? Will it be ok and how long should I keep the plastic there?

mathepac

The most common problem when joining old to new is not wetting the old generously prior to applying the new; that and drying out too fast are the most common causes of cracking IME

Kashkai

I painted the old concrete with a 3:1 water/polybond mixture prior to applying the new concrete to help them bond.

Prenderb

How wide is the trench, and what sort of concrete mix did you use? Was it well compacted?

Kashkai

The trench was about 6 inches wide x 4 inches deep. I used a 3 parts sand to one part cement mix with the polybond added for extra strength. I compacted the cement as well. Thought I did everything right, with the exception of not covering it up to prevent it drying out too fast.

How long should it take this to dry? My own guess is 3-5 days. I have to relay some wooden floor when its dried so I want to make sure the cement is well dried out first.

Buford T Justice

3 - 5 days is plenty. If you're covering the floor over then the I wouldn't worry about the cracks. I mixed 4:1 sand to concrete to do my floors and it was sort of the same with the cracking.

CBYR1983

I take it you found the leak?

Kashkai

The pressure in teh central heating system has been holding steady at 0.9 bar for the past 2 months but when I checked it this evening, it was 0.8 bar. The wife ran the heating last Friday for a few hours and teh pressure went to 1.3 bar and gradually fell back to 0.9 bar as the system cooled. However, today was the first time it had dropped below this. I'm hoping that its because the heating being on caused some air to come out of the water in the pipework (it was refilled with fresh water when the leak was fixed) and so this has caused the pressure drop.

If not, then I'll be more than a little annoyed seeing as how I left the floors open for months waiting to see if the leak reappeared and once I fill in the holes, hey presto, water pressure starts to drop again. Its just not fair:mad:

Martron

ideally you would want some aggregate in it. as the water "dries" your mixture will shrink. a small aggregate in it will help it maintain its size.

keeping it damp will also help as concrete cures rather than dries. this is a chemical reaction rather than a physical one.