Parking Lot - Joint Spacing 1

[privateer]

I'm designing a parking lot - overall dim. 118ft x 116ft. Considering the slab thickness, my general joint spacing will be about 19'-8" x 19"-4". My question is, how should the construction joints be spaced? I'm thinking 19'-8" wide "strips" for each pour, with control joints every 19'-4"? What is a practical spacing for construction joints? Do I need construction joints at all in this case - should I make all the joints control joints?

Thanks.

 

[Ron]

You don't give the concrete thickness, but assuming it is 6" or so, your joint spacing is too wide. You need to keep the joints to around 24x to 36x the thickness, which would be (36x6)/12 or 18 feet max, preferably more in the range of 12 to 15 feet.

With your dimensions, you could logically set your joints for 11'-10" and 11'-8", which will provide you will very good pattern and will help to keep the joint widths smaller so that you get better load transfer.

 

[privateer]

Ron,

My slab thickness is 8". According to the references I've seen, 20ft joint spacings are ok, but please let me know if you suggest something different.

Also, I need to make a distinction between "construction joint" and "control joint", as stated in my first message: a control joint being used to control cracking, and the construction joint which is a type of keyed form, used to facilitate construction. So my question is how far should my "construction joints" be spaced? Or could this slab be constructed using only "control joints." I'm thinking placing the construction joints at 19'-8" intervals, creating 6 "strips" to make the entire slab. Each strip is segment at 19'-4" intervals with control joints. Like this......

construction joint
--------------------------------------
l l l l
l l l l <- control joint
--------------------------------------
construction joint

Thanks.

 

[Ron]

Privateer...I would still reduce the spacing to 15 to 18 feet max.

Depending on your crew sizes, this is a lot of concrete to work without a construction joint. You might want to divide into two placements.

Other than ease of placement, all of your joints could be control joints. No necessity for construction joints unless your crews just can't get it done in reasonable time.

Make sure the joints are cut at least 2 inches deep and that they are cut as soon as they can start cutting without raveling the aggregate. Should be able to start cutting joints in 6 to 8 hours after placement.

Good luck.
Ron

 

[privateer]

Ron,

Will reduce spacing as suggested. I thought my use of construction joints was a little excessive (that's why I sought advice). Thinking now of quartering the entire slab with construction joints, so construction will be done in 4 pours. The rest of the joints will be control joints.

Do you believe in thickening the edges? I've seen details of slabs with thickened edges about 1.5 times the thickness of the slab, about 3 feet in from the edge. Also, do you like re-bar or wire mesh for slabs on grade?

Thanks

 

[jheidt2543]

A few suggestions regarding your floor pours:

1. Regarding saw cutting of the control joints, look into the relatively new SoftCut saws. They are small, lightweight saws with 4&quot;-6&quot; blades that can cut the concrete while it is still at initial set. They work great and prevent the early cracking that Ron warned about above.
They are expensive and so are the blades, but it is worth it in quality concrete floors.

2. The pour size depends on your crew size. With the SoftCut saw the days of checkerboard pouring are OVER. I've seen super flat floors poured 12'wide and 300'long and warehouse floors poured in 10,000 sf chunks, but we had the crew to do it! How you pour makes a difference too. Do you plan to truck dump the concrete, buggy it or pump it?
Figure roughly 1,500 to 2,000 sf of floor per cement finisher. Are you using walk behind finishing machines or riders? How flat do you have to finish the floor? You should also take into account overtime for the finishers depending on what admixtures you are allowed or required to use.

Food for thought, good luck.

 

[Ron]

Privateer...don't thicken the edges! This creates a restraint condition that promotes cracking. For your slab thickness, no need to treat the edges any differently.

Wire mesh is fine. Just make sure it ends up in the right place (middle 1/3 of thickness).

Ron

 

[privateer]

Ron,

Do I need dowels in my contraction joints if I'm using wire mesh in the slab?

 

[Ron]

Not typically. One of the reasons for keeping the joint spacing small is so that each joint is very narrow, thus giving better load transfer from aggegrate interlock.

Unless you have inordinately high loads or high frequency of loading, dowels are not necessary.

 

[privateer]

Ron,

OK, I understand about the dowels - been reading PCA's &quot;Concrete Floors on Ground.&quot; They make a distinction between dowels and tie-bars. Are tie bars needed across the contraction/control joints? They also present a formulae to predict the width of crack spreading (pg. 55). Do you use this formulae? If so, could you please explain some of the terms, such as &quot;concrete cooling.&quot; I find the explaination of this formula very vague. Does this formulae mean that if the crack is too wide, you have to use dowels/tie-bars because there is not enough aggregate interlock? And what is the range for &quot;too wide.&quot;

Thanks for all your help

 

[2civil]

If you are only go to have regular car traffic in the parking area you might consider forgetting about wire and mesh. Remember the wire is only there to keep the concrete together. It provides no structural support.

We succesfully poured a 14.5 acre parking lot with no mesh or fiber our thickness was 6&quot;. I also highly recommend the SoftCut saws. The contractor was able to start cutting that same evening after a morning pour. The site has been successfully used now for 2 years with no problems.

Recommended Reading: &quot;Guide for Design and Construction of Concrete Parking Lots&quot; - ACI Committee 330

Don't worry about tiebars unless the perimeter of your slabs are unconfined (i.e: no curb).

 

[Ron]

privateer...my older version of that publication does not have that formula. Please post and we'll take a shot at it, but I'm not a fan of predictive equations for cracks in concrete.

 

[privateer]

Ron,

A data point that maybe I should have mentioned - this is a parking area for 18-wheelers, for loading and unloading cargo. They will park, and immediately be loaded or unloaded. Cargo will not be stored or placed next to trucks for extended periods. The forklift may temporarity set the cargo down next the truck, maybe to reposition the load, but then the cargo will be immediately hauled inside. But the trucks may stay parked for a few days at a time - either loaded or unloaded.

The equation is stated in the publication like this:

Joint spacing = 15 ft = 180 in
Concrete cooling = 20 degrees F from initial hardening to facility condition in winter
Temperature coefficients = 5 x 10 ^ -6 / degrees F
Drying shrinkage coefficient = 100 x 10 ^ -6
Cooling contraction = 20 x (5 x 10 ^ -6) x 180 in = 0.0180 in
Drying shrinkage = (100 x 10 ^ -6) x 180 = 0.0180 in
Total crack width = 0.036 in

Obviously, according to this equation crack width is a function of cooling contraction and drying shrinkage - so it passes the sanity check in that respect. But what does the &quot;concrete cooling&quot; part mean. I wrote it here exactly as it appears in the book. And where the hell did the coefficients come from? Do they change with temperature? The book does little to explain this equation. It does say that &quot;...this example shows a crack width that is slightly exceeding the aggregate interlock effectiveness limit for slabs up to 7 in thick....&quot; But it gives no information to compare the total crack width to acceptable ranges for different slab thicknesses.

But book lends some importance to this equation with respect to the relation between aggregate interlock and tiebars. Meaning, if agg. interlock is insufficient at a control joint due to predicted crack width, then tiebars may be needed to keep the joint from spreading, therefore maintaining agg. interlock.

Like to know what ya'll think.

Thanks.