Cracks in concrete roof slab 1

[Snatch]

I was asked by owner of a newly constructed building to inspect cracks in the top of a concrete roof. The building was constructed in April this year. The cracks were noticed because after heavy rainfall water would leak through the slab and into the building. The crack widths are about 3mm thick but there are quite a number of them on the roof slab, however none appearing on the soffit. The concrete supplier is blaming the contractor for inadequte curing, whilst the contractor is suggesting the concrete mix was not correct.

Question:
Can anyone recommend any products that will seal the cracks or any solutions to this cracking problem. After sealing i would also advise that the slab be rendered with slope to facilitate faster runoff

Thanks for any input

SNaTCH

 

[dik]

3mm is a pretty wide crack and you may want to determine the cause independently rather than listen to the supplier and the contractor. I've encountered this type of width a few times ( a couple of unusual ones); one was due to a column support not being constructed (they missed it, but there was an end bearing caisson where the column should have been. Another was caused by flexural rotation at a column capital (slab was 8" deep and cap was 9&quot. The entire slab part between the capitals was in tension, and you could pass a piece of paper through the slab.

 

[structuresguy]

Sika Corporation is probably your best concrete repair product manufacturer out there. There are many others who also make great specialty products, but Sika makes great products that cover all aspects of the repair.

They have injection epoxies to structurally repair the cracks. They have thin set mortars to resurface your roof slab. They have surface coatings to waterproof the slab. And they make a range a carbon fiber composites to strengthen your damaged slab, if necesary.

Their website is

http://www.sika.com,

and they are available pretty much world wide.

I have been using Sika products for about 6 years now, and have no complaints.

I agree with dik, you should figure out why the slab cracked first though. That will dictate what type of repair is necessary.

 

[JAE]

I guess I'm surprised that a roof is left as exposed concrete. Concrete is really not a waterproof material and always cracks to some degree anyway. Why isn't there a roof membrane system over it?

Simply filling a few cracks won't make it waterproof - eventually more water will penetrate.

 

[Matrix]

3 mm thick? I think it is most than maximum acceptable.

You need to check the cause of this cracks. If it was cause by evaporation of the water or retraction. Others causes should be checked like stiffness, load and the structural slab detaling.

Even well adensed and correct aditived concrete can´t be "water proof". So roof slabs should be protected from rain and high insolation that causes excessive movimentations and craks.

Matrix

 

[ishvaaag]

More than anything summarizing:

A building has structure, and there 3 mm wide cracks are nontolerable. Then, a roof needs be made damp-proof and both things are requirements of sound construction. And if "caulking" the thing is to be the solution, Sika is a great election.

 

[structuresguy]

I just want to clarify something. I agree with those above that concrete by itself is no good as a waterproof roofing. However, if the cause of the cracking has been determined and presumably alleviated, then repairing the existing cracks, in combination with some form of waterproofing for the concrete, is an option. Sika (and many others) make flexible roll-on membranes that could be applied over the concrete to waterproof it, after the major cracks were repaired.

Exposed concrete may be the desired surface, say if there are high traffic loads on the roof. Maybe from rolling equipment. If that would be the case, then a typical roofing sheet membrane might not be acceptable. However, I think leaving bare concrete was a mistake on the designer's part if the concrete was intended to provide the waterproofing layer. Definitely, the original reason for leaving exposed concrete should be considered when selecting the repair method.