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concrete slabs on grade

concrete slabs on grade

concrete slabs on grade

(OP)
I'm designing 9"thick x 250'long x 36'wide slab on grade for a bus station. The slab has expansion joints all around due to buildings on one side, a bridge on the other and a bus entrance and exit. My question is related to the reinforcement. If I used contraction joints following ACI they would be spaced 24 to 36 times the thickness of the slab (18' to 27' respectively) is that spacing considering I use no reinforcement? Also, while reading ACI it says to not use expansion joints due to settlement and bearing issueses at the ends due to the wide openingand water infiltration. If that's the case than the contraction joints serve two purposes one to control cracks at the contraction joints and two to help with temperature effects. For a 250' slab my expansion will be around 1.5". So in essence i need the contraction joints to crack so that on hot days when the sun is bearing down on the slab it can expand without over stressing the expansion joints at the edges of the slab. So my question is that if I use to much steel will my shrinkage be to minimal compared to my expansion and thus making the contraction joints not open up wide enough to allow for my full expansion? Will my expansion joints fail? Does steel reduce the amount of shrinkage or does the steel just distribute the shrinkage to the contraction joints?

RE: concrete slabs on grade

Firstly, shrinkage will happen over about 30 years, with about 50-70% in the first 12 months. So if you are considering requiring full shrinkage to occur to counter the temperature expansion, you are going to have to keep it cool for the next 30 years!

The steel in the concrete will reduce the overall slab shrinkage. It will not catch up at the joint. The steel goes into compression and the concrete into tension due to the presence of the steel. You can calculate the amount of tension/compression and from this estimate the reduction in the overall slab shrinkage.

Subgrade friction will also reduce the amount of overall slab shrinkage.

Remember the temperature might fall and add to the shortening from shrinkage. It is unlikely that you will be pouring on the coldest day in the next 50 years!

Personally, I would never do 27' length unreinforced!

RE: concrete slabs on grade

(OP)
Thank you for the quick response. So for really long lengths of concrete slabs on grade (for instance concrete pavements) there would be a need for expansion joints which will take the movement of the slabs. In my case my movement is to little and the expansion joints will accommodate the movement.

RE: concrete slabs on grade

Quote (rjp1121)

So my question is that if I use to much steel...

For applications like this one, there is an optimum amount of reinforcing steel so that no (intentional) joints are required. It is 0.6 to 0.7 percent reinforcing steel. Design the slab as "continuously reinforced concrete pavement". I've used it (30 years ago), it works for the long term. Here is what the Concrete Reinforcing Steel Institute (CRSI) and the U.S. Federal Highway Administration (FHWA) have to say about it:
Continuously Reinforced Concrete Pavement

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: concrete slabs on grade

Can you break the pour into 80', 90' and 80' with an actual control joint and sawcut it at 20' intervals, with a longitudinal sawcut in the middle?

Dik

RE: concrete slabs on grade

(OP)
We could actually do that but I think the contactor wants to do one continuous pour.

SlideRuleEra:
We have manholes and drainage inlets in the slab. I was going to put isolation joints around these facilities. Can you still put isolation joints even though its one continuous slab?

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