Concrete shrinkage cracking over time

[palves]

I have a project where the architect is applying a waterproofing membrane over the top of some concrete work we are doing. The architect does not want the waterproofing to be damaged when the concrete cracks and would like to know how long he should wait after placement before applying the waterproofing. This is a nonstructural topping slab so I am basically trying to determine how long it takes for the majority of shrinkage cracks to occur. Any ideas out there?

Also, I know there is a graph showing the percentage of shrinkage cracking vs time but cannot for the life of me track it down. Can anybody point me in the right direction to finding that?

 

[rapt]

It would probably be in ACI209 if you are looking for a USA document. AS3600 for Australia has the curves as would the different CED-FIB documents for Europe.

 

[Ron]

palves....shrinkage cracking in concrete depends on a lot a variables....aggregate size, cement content, design slump, admixtures and most importantly, the water-cement ratio. If the concrete has appropriate control joints, the cracking can be mitigated so as not to affect an overlying membrane. If you are using a fully adhered membrane on the concrete, make sure the control joints are more closely spaced than would ordinarily be done.

Another point to consider is that when you put a membrane on the concrete, it will help hold in the water in the concrete, thus reducing the overall drying shrinkage and the width of shrinkage cracks.

 

[oldestguy]

I'd investigate the shrinkage potential as related to the water content of the mix as placed, probably tied to slump. There also might be a different coef. of expansion-shrinkage with temperature change. Also, percentage of reinforcement enters the picture. Better be careful about any coating. Coatings on gymnasium floors have been known to come loose due to the salts in the concrete ending up with higher concentration at the surface as water evaporated from there leaving dissolved salts there. Once the coating is on the remaining water content in the concrete migrates to the top surface to dilute the salts, causing pressure, and (for gym floors) results in bubbles before it comes loose. I'd be whole lot more concerned with the coating coming loose due to this factor. One way to minimize this effect is to wit as long as possible (months) before applying coating. Also a washing of the surface hoping to reduce the salt concentration there has helped. Why does concrete need to be waterproofed anyhow? I think it is asking for trouble.

 

[hokie66]

In this case of a topping, cracks will form often and quickly because of restraint from the underlying concrete. The width of such cracks will be dependent on factors such as listed by Ron, and also dependent on reinforcement in the topping, if any. I am not an expert on waterproofing, but think in this case a sheet membrane may be more appropriate than a coating. Suggest you and/or the architect get advice from someone who knows, like Sika.

oldestguy, maybe you are being facetious about the need to waterproof concrete, but you waterproof it because it is not impervious.

 

[mjr6550]

I had that information, but cannot put my hands on it. I think it is typically around 80% within one to two years and close to 100% at three years.

 

[Retrograde]

I understand it depends a LOT on the section thickness. In a very thin section you could get 80% of the shrinkage occurring in a month whereas a very thick section could take several years.

 

[ABA102]

Palves..there is an available bituminous paint that can be applied before waterproofing membrane that serves as curing agent as well and can prevent shrinkage cracks. And which can be easily applied at required thickness just after the concrete dried up.(check manufacturer's recommendation). Try to use Nitoproof 30 by Fosroc.