parking garage thermal and concrete movements

[ajk1]

Does anyone know of a published procedure/worked example for calculating the movement of a garage roof parking slab due to concrete shrinkage and diurnal thermal changes, for an unheated open garage? I need to do this calculation for the selection of the expansion joint seal so that it can accommodate the movement without tearing or being subject to excessive force. There is I believe a procedure in the CPCI Handbook (I will look next time I am in the office) but that is for precast structures and not all of that procedure would apply to cast-in-place structures. The procedure should, I would expect, include some accounting for creep effects, the maximum summer temperature of the internal portion of the concrete slab, and minimum winter temperature of the slab that would account for radiation out to the sky, etc. It would also need to account for the concrete shrinkage at various ages so that the shrinkage that would have occurred when the expansion joint seal is installed can be calcuated.

 

[KootK]

You should be alright with the PCI/CPCI methods. I believe that they include a post installation shrinkage term for the precast that can be altered to suit CIP.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[asixth]

How did you go with the joint spacing?

The prelim estimates that I have always worked to have been:

Shrinkage -0.25mm/m
Temperature -0.25mm/m to +0.15mm/m
Elastic Shortening and creep -0.2mm/m

I did have some reference material somewhere for this but have no chance of finding it. Anyway shrinkage and temperature are direct strains and linear with temp variations and thermal expansion co-efficient.

The initial shortening is P*L/(A*E) and the creep shortening depends on the creep function model your using. But would double to triple the initial elastic shortening.

 

[ajk1]

To Kootk ok, thanks. I will have a look at CPCI Hnaddbook when I get into the office on Monday.

To asixth - not sure that I follow what you say. Your suggested shrinkage of 0.25 mm/m equates to 250 microstrain, or 0.025%. CSA Standard A23.1 says if you want to specify a concrete mix with reduced shrinkage, to specify 0.04% at 28 days. A normal mix 28 day shrinkage would be about 0.05% as I understand it. So I would reason that the long term shrinkage would be much more. So I assume that your 0.025% (0.25 mm/m) must be assuming that the joint seal is not installed till quite some time after the concrete is placed and has undergone most of its shrinkage. Similarly you must be assuming soe temperature range for the thermal movement, but you don't say how much...what geographic region are you assuming when giving the value for temperature movement?

 

[asixth]

Geographical region is AU.

Shrinkage doesn't make sense. It should be 0.5mm/m. The Australian practice is to normally specify a shrinkage strain of 600 microstrain and most test reports I have seen have come in around that mark and we test the free shrinkage at 56 days.

Thermal co-efficinet in AU standards is given as 12*10^-6 per degree C so that would be a temperature range of -20degC to +15degC. Initial shortening is based on an average prestress of 2.0MPa and creep co-efficient of 2.

The numbers did come from this resource from VSL that suggest the -0.25mm/m for shrinkage but also suggest reducing the shrinkage by using pour strips.

https://www.dropbox.com/s/n2wiwkq3rwcvvdy/VSL Post Tensioned Slabs.pdf?dl=0

https://www.dropbox.com/s/bf0fdpn9c3szd5m/Typical Concrete Shrinkage Test Report (BNE-AU).pdf?dl=0