Control Joints in Slab for Water Tank 1

[pricklyPete]

I have a 12" thick slab with #5 @12" o.c top and bottom. I want the steel to be continuous through the control joints (and construction joints). I read somewhere that the control joint should be cut to a depth of 1/3 the slab thickness. If I have top steel with 2" of cover, they will not be able to cut the joint any deeper than 2".

The slab is for a water tank that will probably be leak tested. There will be no membrane on the tank floor during this test.

Does anyone have any suggestions? The tank is 70'x70'

 

[SlideRuleEra]

Unless the concrete will be subjected to large temperature swings, perhaps you could consider not using any control joints. Of course the rebar can be continuous through construction joints - be sure to use waterstop in the construction joints. For a slab with those dimension you should be ok with no control joints.

Another option would be to use a little more rebar to create "continuous reinforcement" (rebar = 0.5% to 0.7% of the concrete cross-sectional area).

Whichever way you go the most important action is to immediately and continuously wet cure the concrete (at least 7 days) to minimize shrinkage and improve watertightness.

 

[pricklyPete]

I agree that they probably will not need any construction joints for this slab.

What do you mean by the term "continuous reinforcement". Is this a technical term. Do you have a reference for the 0.7% reinforcing ratio or are you just basing this on the chart in ACI350R

 

[SlideRuleEra]

Continuous reinforcement is normally used with pavement for the logitudinal rebar. There are no joints, any cracks that develope are held together tightly by the rebar. It does work for pavement - the oldest slab (6 inches thick) that I designed with this technique was 18 years ago (60 foot long approach ramps to an industrial truck scale). The scale is used many times everyday, and the ramps still look like new.
For a two-dimensional slab, like yours - this general concept (large, heavily reinforced concrete slabs with minimal joints) is used for the (watertight) basins of electric power generating station cooling towers.

My reference for the pavement is (and was 18 years ago):
Page 16-59
"Standard Handbook for Civil Engineers, Third Edition"
Frederick S. Merritt, Editor
McGraw-Hill, Publisher

Best Wishes

 

[pricklyPete]

Thanks, SlideRuleEra. I went and looked at one of the reference books we have and was able to verify your recommendations and the definition of a 'continuous reinforcement' for pavements. Thanks for the direction.