Cracked new concrete

[winco2]

Just put a large concrete slab down (see Previous topic)

On finishing the mamoth task of moving 8.5 m3 into place and finishining it off, i have noticed that there are a few hairline cracks in the surface over varios parts of the slab. (not happy)

Can anyone advise why this has happened, and if the cracks will get larger and cause me problems?
or are they a few mm deep and common at my level.

The slab is 11m x 3.6m in dimesion, 8" thick around the edges and 6" across the middle with A252 steel in it.

The concrete was quite wet when put down and not knocked down too much, i ran a timber across the shuttering from front to back an a scissor action to remove the excess and get a level.
The sun was on it but it wasnt too hot.

A friend has told me to when the garage is built to put a leveling comound and hardener on the floor.
Will this help.

[Tony McC]

Hairline surface cracks are a feature of concrete that has been allowed to 'dry' to fast on the surface. As the water evaporates, it results in shrinkage cracks, but these rarely go more than a few millimetres intoi the surface and should not compromise structural integrity. Deeper in the concrete slab, the water is unable to evaporate and so the cement content is more fully hydrated and the necessary strength will be achieved.

There are a few techniques that can be used to prevent or reduce the incidence of these surface cracks, but, in all honesty, I wouldn't worry about them on this type of project. They'll fill up with dust and all sorts of other rubbish during the rest of your build, and by the time you've got the roof on, you'll have a job trying to find them again.

[winco2]

Thanks again, thats a relief

[Spanish Pat]

Whilst it is true that these plastic shrinkage cracks will probably be harmless and not be noticed inthe long run, there is a very simple way to avoid these altogether by the addition of a small bag (600g) of very fine monofilament polypropylene fibres. I know as we manufacture this product (surprise, surprise) and it is the easiest and most reliable technology to use as it is just added to the concrete during mixing time. The fibres not only stop plastic cracking, but also improve surface toughness by keeping the mix homogenous with sufficient aggregate and cement paste interlock thoughout the slab. There is also a marked lack of loose and dusty cement past on the finished surface which easily cracks off and leaves exposed areas which are subject to subsequent damage and present an open door to water ingress, which is the main cause of concrete damage inthe UK.

If you are interested to know more, please drop me a line.

Cheers

Spanish Pat.