Closure Strips in Suspened Slab

[ajk1]

I am interested in views on "closure strips" in suspended slabs. These are about 1 m wide strips left out of the suspended slab placements, for at least 28 days after the concrete is placed in tne floor each side of the closure strip, to allow some of the shrinkage to occur before the concrete is placed in the closure strip.

We figure that about 30% of the long term shrinkage will have occurred at 28 days after completion of the 7 day curing perod (i.e 35 days after placing the slab).

However contractos don't like the 28 day period and want to reduce it to 7 days, but at 7 days after the 7 day cure we figure that only perhaps 10% of the long term shrinkage will have occurred, which would not accomplish our goal of minimizing shrinkage cracking, This is particularly important where the slab soffit forms the finished ceiling of the apartments.

I am interested in knowing if closure strips as described above are generally being used?

If so, for how long do you keep them open?
Any other way that you are dealing with shrinkage, or do you just ignore it?

 

[ron9876]

Have used them before and yes the contractors complain. We left them open around two months. One week almost isn't worth the effort in my opinion. I assume you are doing this to avoid adding an expansion joint right? If it was clear on the plans then you have to stick to your guns.

Maybe ask the contractor if they will take the liability if they pour sooner and there any problems.

 

[DST148]

The shrinkage (closure) strips are generally used in concrete structures with larger lengths and widths. We had used it for one of the facilities where the length was about 350 feet and the owner did not want an expansion joint. The strips were closed after 90 days from the day of the pour. The shrinkage strip and the related details formed part of the bid / contract documents.
I have not used, but some times shrinkage compensating concrete may also be specified.
Do we ignore it? Absolutely not. It has to be catered for, either in the design or through good construction practices.

 

[dik]

How big is your building? I've only used them a couple of times in the last 30 years... and only for post tensioning issues...

Dik

 

[hokie66]

Have used delayed pour strips many times. There is always a battle between the engineer and the builder about timing. The longer left open, the better. 28 days helps, but 58 or 90 days is much better. 7 days is not of much benefit.

 

[rapt]

I will throw in an extra dimension here. Temperature shortening can about equivalent to shrinkage shortening in many buildings depending on the climate and it is not a long term effect.

So close them at the coldest time possible and you will be far better off. Coldest time of the day is normally late night, and coldest time of the year. So, if waiting 3 months pushes you to a 20C higher temperature you might be better off doing it earlier!

And get it done at the coldest time of the day!