Shrinkage in concrete

[CivilEngineerUK]

I have a concrete slab, which are restrained in both ends.

When designing for cracks in SLS, do I have to consider stresses from shrinkage and stresses from static load separately. Or shall I add the stresses together?

 

[MotorCity]

Not sure what "SLS" is, but for flexural members, you don't design for stresses in concrete per se. Shrinkage stresses are considered to be resisted by providing shrinkage reinforcement per the code (this is different than flexural reinforcement)

 

[CivilEngineerUK]

SLS = Serviceability limit state (term used in Eurocode). According to Eurocode you shall design flexural concrete members for stresses/cracks.

 

[hokie66]

Flexural reinforcement resists stress due to shrinkage. But if the shrinkage reinforcement requirement exceeds the flexural requirement, you have to add the difference.

 

[CivilEngineerUK]

What if shrinkage reinforcement doesn't exceed flexural reinforcement? In that case, can I "ignore" shrinkage reinforcement?

 

[Trenno]

Correct. The reo provided will serve dual purposes. Keeping the cracks under control for SLS and then when needed provide enough strength capacity for ULS.

 

[hokie66]

You don't "ignore" shrinkage reinforcement. But if your flexural reinforcement is sufficient for shrinkage crack control, you don't have to add more. A caveat...sometimes flexural reinforcement is not in the right places to control cracks, so in those cases, additional reinforcement is needed.

 

[rapt]

If the slab is restrained against shrinkage shortening, you should do a cracked section analysis allowing for both the flexural stress effects and the shrinkage restraint effects. This will give the stress in the reinforcement for the overall stresses and strains in the section.

Do not calculate them separately and add.

You may also need to allow for restraint to temperature change depending on the situation.

 

[CivilEngineerUK]

Thanks a lot for all answers

 

[molibden]

I don't do a lot of concrete design but for small residential houses it is not common to include shrinkage effects. In which cases do you do such an analysis and when you do it, do you use FEM software? I can imagine that with complex geometry it would be too hard to do a hand calculation.
Do you use a temperature differential load for modelling shrinkage effects?

 

[southard2]

For slabs on grade down here in Florida we use turned down slabs a lot. Typically you install saw cut control joints at a distance spacing of 4 x the slab depth. If I'm dealing with a monolithic turned down slab now I put in the first joints from each corner at half this distance sine the turned down slabs will constrain the slab.

So for example if a house has a 4" slab the typical sawcut control joint spacing is 12 feet on center. I'll start the first control joints 6feet from each edge of the building just to be safe. And I'll do the same thing to relieve any internal turned down slabs....

I learned this the hard way building my own house. And honestly any more I don't hesitate to space joints on a 4" slab at 10 feet on center. Adding joints in the right locations makes a huge difference in the performance of you slab. Heck look at your average cracked driveway and they simply spaced the joints too far apart. It is amazing to me that this happens all the time and this is something that anyone can understand using a simple rule of thumb.

Sorry I know this is a bit off topic versus a structural slab. But restraint from turned down slabs is something I'm more aware of now and I thought everyone else should consider it also when locating control joins on their plans. Even a simple driveway if you turn down the edges of each side of the driveway can make things worse. Food for thought.

John Southard, M.S., P.E.

https://www.pdhlibrary.com/