Why is my cement cracking?

[69elan+2]

Quick question about cement.

Whenever I've used it inside I've had no problems. But today I was cementing around a drain cover on the drive - did all the prep work, sealed the bricks etc.

However as it dried it started to crack, nothing major - just hairline cracks but it looks poor.

It was a new bag, mixed it as normal and it wasnt in sunlight or very warm when it was left to dry. The area I was cementing wasnt very deep either.

Any ideas what might cause this? To wet, to dry, not enough sand?

[robinhood21]

Possibly should have wet the area around where the cement was to go, so as to allow the cement to dry out slowly, thus preventing the cracks.

[mrsxllifts]

^^^As he said...next time try chucking a damp hessian sack over the top. Keeps it damp whilst drying and hopefully stops the cracks!

[69elan+2]

Was just thinking that chucking a wet towel over it might have been a better idea.

Thanks

[Chrispy Porker]

mrsxllifts said:

^^^As he said...next time try chucking a damp hessian sack over the top. Keeps it damp whilst drying and hopefully stops the cracks!

I believe that is what the pro's do.

[Landlord]

Concrete doesn't "dry", it "cures". It's the chemical reaction between cement and water that causes the concrete to harden.

The reason it's cracking could be because the mix was too dry as much as because of the ambient temperature.

That said, if the water is evaporating before the concrete fully cures, cracks can appear. The solution to which is what's been suggested, keep the cement well wetted (no need for over watering though).

IMHO, of course. IANAB(uilder) etc.

EDIT: Sand/aggregate is only added, AFAIK, to bulk out the mix. It's neither efficient nor cost effective to have a pure cement/water mix.

Edited by Landlord on Wednesday 21st May 01:06

[absolutely]

EARTHQUAKE! Maybe?

[xllifts]

mrsxllifts said:

^^^As he said...next time try chucking a damp hessian sack over the top. Keeps it damp whilst drying and hopefully stops the cracks!

Why you cheeky wench....
Go on tell em who said that from the kitchen Mmmm Mmmm and yes it was me but being a bricky by trade i kinda cheated.

[mrsxllifts]

Chrispy Porker said:

mrsxllifts said:

^^^As he said...next time try chucking a damp hessian sack over the top. Keeps it damp whilst drying and hopefully stops the cracks!

I believe that is what the pro's do.

I like to try to sound intelligant at times!! mr xl is a pro builder and told me what to say!!

[xllifts]

Landlord said:

EDIT: Sand/aggregate is only added, AFAIK, to bulk out the mix. It's neither efficient nor cost effective to have a pure cement/water mix.

Edited by Landlord on Wednesday 21st May 01:06

Adds to the elasticity and cohesive properties.

Useless bit of info for you, A concrete slab Size of a house base of say 300mm thick, sets in about 8hrs but takes upto 20 years to fully cure and reach optimum strength, unless its high alumina cement then you need to runaway as it was used in high strength beams and rotted the steel re-inforcement, you'll be glad to know they stopped using it about 30 years ago.

[thinfourth2]

The bodies are starting to rot and the build up of gas is causing stress cracks. So run away before it explodes and aunties head lands in the middle of the lawn

[phumy]

Landlord said:

Concrete doesn't "dry", it "cures". It's the chemical reaction between cement and water that causes the concrete to harden.

The reason it's cracking could be because the mix was too dry as much as because of the ambient temperature.

That said, if the water is evaporating before the concrete fully cures, cracks can appear. The solution to which is what's been suggested, keep the cement well wetted (no need for over watering though).

IMHO, of course. IANAB(uilder) etc.

EDIT: Sand/aggregate is only added, AFAIK, to bulk out the mix. It's neither efficient nor cost effective to have a pure cement/water mix.

Edited by Landlord on Wednesday 21st May 01:06

Off the top of my head and with an answer like that, i bet you run a pub!

[ALawson]

What depth did you lay it at? Also was it only sand and cement, or any larger aggregate in it?

Sounds like dry shrinkage cracking, as has been said above you need to either place wet hesian over the pour or use some form of sprayed curing membrane.

[PGM]

First of all, do not work in direct sunlight.

You need a clean and damp surface to start with.

Your mix needs to be not too cement rich (3:1 sand:cement ratio will suffice)

Mix it up quite dry (the more water you use the more prone to cracking) but obviously enough water to get a finish on it.

Do not overwork it on finishing.

Put damp hessian on to allow to cure.

You can also use a bit of external grade PVA in the mix and spray it over the patch at the end for curing.

[p1esk]

xllifts said:

Landlord said:

EDIT: Sand/aggregate is only added, AFAIK, to bulk out the mix. It's neither efficient nor cost effective to have a pure cement/water mix.

Edited by Landlord on Wednesday 21st May 01:06

Adds to the elasticity and cohesive properties.

Useless bit of info for you, A concrete slab Size of a house base of say 300mm thick, sets in about 8hrs but takes upto 20 years to fully cure and reach optimum strength, unless its high alumina cement then you need to runaway as it was used in high strength beams and rotted the steel re-inforcement, you'll be glad to know they stopped using it about 30 years ago.

20 years to reach optimum - or maximum - strength ?

Maybe, I wouldn't know about that, but when they take test cubes I thought they checked the strength with a compression test after 28 days, by which time the concrete should have reached its design strength. By that time it should be capable of safely supporting the loads to which it is to be subjected, but no doubt it will continue to gain strength for some time afterwards.

Best wishes all,
Dave.