Hairline cracks in shed base

[Roof_problems]

***reposting to building as no replies in the garden forum***

Hi
I’m looking for some advice on dealing with hairline cracks in a concrete base that I mixed myself. It’s laid on compacted MOT and the concrete is between 100mm and 120mm deep. The slab measures 3300 x 3600. It seemed to be going really well until a few hours into curing when I noticed several hairline cracks appearing as per the photos. Ultimately it only needs to support a wooden shed (2500 x 3000) so I’m hoping these don’t pose too much of an issue. Any advice on what I should / shouldn’t do with them would be appreciated e.g. should I be filling them and if so, with what?
Many thanks

 

[Wayners]

It's been very warm and dry.
Not my area of knowledge but maybe adding a plasticiser and dampening down the concrete for a few days may have helped.

 

[Roof_problems]

Yes, I wish I’d thought of the plasticiser beforehand and we did have a particularly hot couple of hours in the early afternoon which I suspect is the cause. Just wondering whether there’s any value in trying to do something now such as filling the cracks or if it’ll likely be ok left well alone

 

[accelerator]

I'm no expert but if it was me I would just fill them in with one of the various concrete filler products available, and this is just to avoid the risk of water getting in and then freezing during winter, which could cause the gaps to get bigger. If the gaps are all very shallow, they are unlikely to ever cause problems, but for the sake of a few quid I'd probably do it anyway.

 

[Alec_t]

..... and when you've put the carpet down you'll forget all about the cracks.

 

[stevie888]

Concrete probably laid too wet and almost cdertainly not cured would be my guess.

How old is it?

 

[Roof_problems]

It was laid on Sunday and it was made quite wet in an attempt to keep it workable for as long as possible

 

[noseall]

Yes, I wish I’d thought of the plasticiser beforehand and we did have a particularly hot couple of hours in the early afternoon which I suspect is the cause. Just wondering whether there’s any value in trying to do something now such as filling the cracks or if it’ll likely be ok left well alone

You don’t generally plasticise concrete as strength is more about ridding concrete of air rather than introducing it.
It can dry out just as rapidly regardless.
It looks like surface shrinkage to me and is unlikely to affect performance.

 

[stevie888]

It was laid on Sunday and it was made quite wet in an attempt to keep it workable for as long as possible

Probably too late to make a scrap of difference now but hosing it down won't hurt.

People make concrete too wet because it is easier to level out, but it's detrimental to the final strength. There will be a tendancy for the coarse agregate to settle and the excess water to rise ("bleeding") bringing cement up and leaving a dusty surface. Final finishing is harder too. Allowing the surface to dry out too quickly can easily result in incomplete hydration of the top of the concrete.

FWIW Plasticisers are commonly used in concrete as they have the effect of reducing the amount of water required to achieve workability and hence increasing the strength. In particular structures exposed to salts (so marine or roadside) are usually specc'd with air-entrained concrete where typically 4-5% of air is deliberately introduced into the mix in the form of tiny bubbles which act as crack arrestors. The same chemical is used to form the bubbles (stength reduced) and plasticise the mix (stength increased)