Cracks in finished floor?

jasgrif11

My builder just completed my first floor finished floor on Friday. I decided to put in the finished floor first and then continue the blockwork upto wall plate. The thing is the builder stated that as Friday was a really hot day the concrete started to dry out very quick and he had to keep wetting it while power floating. When I checked it this morning there is about 5/6 cracks in different places but they are hair line cracks. The builder has a assured me that these are just heat cracks and not structural. is this so? I should have mentioned that I have pre-cast floor with sub floor on top then insulation-underfloor heating and then my finished floor.

Many Thanks

muffler

Any idea of the depth of screed that was put down

jasgrif11

75mm Screed on top of 75mm Insulation (aeroboard Platinum)

Johnniep

If the screed is sitting on insulation then there is no way that this is a structural screed!

sesswhat

I put down a similar 75mm screed over underfloor heating pipes. There was an upstand of 25mm of insulation all around each room to allow for expansion as well as insulation. There should be no continuous runs of screed through doorways and definitely not under walls as Johnniep points out.

The only crack I had was in a big 6m by 5m area which cracked straight down the middle, so maybe I should have divided this up as well. The whole area was tiled and now hairline cracks are starting to appear in the tiles over the crack in the screed. This is despite the fact that the tile adhesive is supposed to a flexible type for use with underfloor heating.

jasgrif11

I should have said that he laid out all the boundary walls upstairs with soap blocks these were cemented to the sub floor and stuck out 75mm over the top of the insulation. When pouring in the screed he levelled the floor off the soap blocks. The concrete is also 30N 10, does this make a difference?

Would it be best to scrap out the cracks and fill them with cement. They are only hairline but I wouldn't like for them to cause cracks on bathroom tiles afterwards.

Also I didn't put any perimeter insulation(25mm) upstairs just downstairs

Johnniep

Just clarify something for me......your first floor construction consists of:

75mm Non-structural screed (Underfloor heating)
Insulation
Structural Screed
Precast Unit

Am I correct?

The provision of 25mm insulation around the perimeter allows for expansion of the concrete, however your cracking was caused by shrinkage cracking of the concrete.

Here comes the science bit.......

For concrete to hydrate (gain strength) it goes through an exothermic reaction generating heat. As the heat dissipates, the concrete cools, shrinks and because concrete is poor in tension, shrinkage cracks develop. All concrete experiences this and as a result, the grade of concrete (in your case 30N10, 30 representing the crushing strength of the concrete in N/mm^2, and 10 representing the maximum aggrigate size) doesn't matter.

If the concrete element is non-structural, any cracking cannot have structural implications!

Hope this helps......

jasgrif11

Thanks Johnniep.

One thing though should I try repairing cracks before tiling. Is there a risk of the cracks extending?

Cheers

Johnniep

The rate of shrinkage will reduce dramatically after say 7 days. Shrinkage will continue, but the rate will be negligable. I would not bother repairing the cracks, as they are non-structural and should not increase in length or width.

Before tiling though, I would ensure that the moisture content of the screed is at a low level. Check the tech spec's of any grout etc you are using and measure the moisture content of the screed. As a rough rule of thumb, the drying rate of the screed is appox 25mm per month so take your time or install an isolation membrane. Your tiler will be able to give you better info.

shakeydude

Johnniep wrote:

For concrete to hydrate (gain strength) it goes through an exothermic reaction generating heat. As the heat dissipates, the concrete cools, shrinks and because concrete is poor in tension, shrinkage cracks develop. All concrete experiences this and as a result, the grade of concrete (in your case 30N10, 30 representing the crushing strength of the concrete in N/mm^2, and 10 representing the maximum aggrigate size) doesn't matter.

Hey Johnniep, I thought that the higher the crushing strength the greater the heat of hydration? Is that correct?

Johnniep

Hey shakeydude,

You are correct in what you say, as to get a higher crushing strength requires a greater cement content in the concrete. The greater the volume of cement the greater the heat generated. However, the effect in this case would be negligable.

sesswhat

Johnniep wrote:

If the concrete element is non-structural, any cracking cannot have structural implications!

As you say Johhniep, the initial cracking is due to shrinkage. There are no structural implications and you won't lose pets or small children down the crevices.

I tiled over the crack in the screed, which had been drying out for over 3 months but my problem only started when the underfloor heating had been operating for a few months. The tiles are now starting to crack over the original crack in the screed. I can only imagine that the two sides of the crack are undergoing some sort of differential movement with the expansion and contraction of the floor.

Having said that I don't know if there is any remedy I could have used once the crack was in place, apart from leaving a joint in the tiling itself.

shakeydude

Sess,

Did you leave the heating on for an extended period before you put on the tiles????