warehouse concrete slab on grade contract joint

[EIT01]

For a warehouse slab foundation (200'x 90'), the columns are on the perimeter. The foundation will be 6" slab with 24"x36" perimeter beam.

The slab will be reinforced with #6 @ 12" oc both way with no contact joint. Is this a good option? or it would be better to design contact joint with less reinforcement?

 

[EIT01]

Sorry, there is a typo. Contraction joints - not contact joints.

 

[lkjh345]

Are there storage racks sitting on the floor that will deliver point loads to the slab? If so, you will need to check the slab for this loading.

If there are no storage racks, and assuming proper sub-base prepartation, #6@12" o.c.e.w., properly chaired to approximately 2" below the finish floor elevations, should ensure that any cracks (and there will be some) will remain 'tight'.

As a general rule, we find that if we keep As to at least 0.5% of the area of concrete, concrete joints will stay tight.

If you have a contractor on board, you may want to discuss if he is comfortable with pouring this size of floor without joints. This is a decent size area, and it will take coordination to get the concrete trucks to coordinate their arrivals properly. Not impossible, larger pours are done all the time, but still needs to be thought out before hand.

 

[Teguci]

if you are OK with cracking every 15 ft or so in the slab then don't worry about contraction joints. Usually, people like to locate where they would like the cracks to go and place tooled joints there.

If desired, do a volume change calculation on your slab. For 200' you will probably have around 2" of crack widths from one end to the other.

 

[jheidt2543]

One product you don't hear too much about any more is the use of expansive cement in concrete floor mixes. They were "popular" back in the mid 70's to mid 80's. I worked on a food plant where the warehouse slab was poured with "Chem-Comp" additive and there where construction joints only at the pour stops and no diamonds around the columns. The concrete was poured right up around the columns, 10,000 sf at a time, which at that time was about four times the "normal" floor pour size. The one draw back is the higher percentage of steel required in the floor slab.

Just a thought!

 

[EIT01]

Thanks for all your answer.

The owner asked me not to worry about the storage stack force or forklift. He want to save the cost of construction. That's the reason I try to eliminate the joint. I understand without the contract joints, even with continuous reinforcement, the minor random cracks will still develop.

There are interior columns, but are located inside the mezzanine area. For this area, I use 4" slab with thicken slab for the load bearing from columns. Is a construction joint need for the connection between 4" and 6" concrete slab?

 

[MiketheEngineer]

It will crack. Have the owner sign off that he acknowledges that this will happen.

 

[jheidt2543]

Storage racks and forktruck traffic subject the floor slab to negative moments and high concentrated loadings. Without reinforcing, the slab will crack. Get a good floor slab design reference, like the book from Aberdeen Publishers by Ringo and do it right the first time!

 

[EIT01]

The owner told me that this is a light duty warehouse. But I still check the slab for 250 psf live load with Staad/Pro. The reinforcement has plenty capacity. Actually most area only required minimum reinf requirement.

 

[JStructsteel]

your owner is an idiot if he thinks that elimiating the joints is cheaper than labor and cost to place the #6@12 o.c.

If its light duty, save your money on reinforcing, add control(crack) joints, and be done. We have done many a slab on grade with no reinforing, and not had problems. We did 6" all the time. Im sure that 18,000 sq. ft. of buying and placing rebar is far more expensive than a finisher with a saw.

Also, without the rebar, you can get a laser screed in the building, and not have to leave strips out for it, giving you a superior finish.

 

[civilperson]

Construction joints at intervals can alleviate the shrinkage cracks and act as contraction joints. Place concrete in alternate strips, from 0 - 20', 40' - 60', 80' - 100', and so on in a first placement. Come back and fill in the gaps days later to minimize the shrinkage overall.

 

[ronster]

If the slab is tied into the perimeter wall then a slab of that size with no joints will crack. If your slab edge detail allows for lateral movement then a reinforced slab should not crack. I would place the slab in two 90'x100' seperate pours as a minimum. I would not place the entire slab in one pour. I would also you lighter bars at a tighter spacing (#5 @ 9" o/c each way). Make sure the slab is cured properly (curing compound and covered) and not to much water is used in the mix.
What is the purpose of the perimeter 24"x36" beam? Why not a frost wall and footing?
BTW, the correct term for the joints would be control joints or construction joints.

 

[JAE]

Reinforcing doesn't prohibit cracking - just minimizes the size of the cracks.

 

[miecz]

If the owner intends to use steel wheeled forklifts in the warehouse, then avoid the use of control joints. With time, the slab will shrink and the joints will grow. Steel wheels travelling over the jopint will impact the corners of the slab adjacent to the joint and chip it. An expandable joint filler/sealant is too soft to prevent this. A rigid joint filler/sealant won't expand to fill the gap when the slab shrinks, leaving a gap in the joints, so the slab will will chip anyway. Better to have a thousand tiny cracks that won't chip.