Construction joints in slab-on-grade

[ajk1]

Are keyed (tongue and groove configuration) construction joints ok in slab-on-grade? It is a large maintenance facility for municipal vehicles. The area where they jack the vehicles up has quite large concentrated loads (70 kN ±). The remainder of the slab which is the much larger area, has only the loads from unloaded vehicles. Since corrosive road salts are tracked into the garage, I would like to avoid the use of steel dowels which can corrode and spall the concrete, but am concerned that the tongue of the joint will lose contact with the groove of the joint when the concrete shrinks longitudinally, since the groove has a sloped surface. Can closer spacing of construction joints get around this?

 

[Teguci]

Keyed joints at construction joints are effective and probably better than loosely spaced dowels. Worst case volume change strains for normally unstressed elements tops out at 750 millionths. For a thick SOG with a very low surface to volume ratio, you should get less than 300 millionths. If this is still an issue, then closer spaced CJs will help. Another thing to consider is staging the pours for the SOG. If you checkerboard your pour sequence with a weekend in between, you can get a good 10 to 20% of the shrinkage out of one side of each joint.

 

[KootK]

For these kinds of joints, I believe that the shear transfer capacity for keyways is quite limited. I'd be looking at something else.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Teguci]

http://www.fhwa.dot.gov/pavement/t504030.cfm

I was wrong. For heavy loads, do not use keys. They spall the top surface of the joint.

 

[ajk1]

ok, that is what I thought.

I am also very skeptical that 10% to 20% of the shrinkage occurs over a weekend. Not what any time-shrinkage curves that I have looked at would indicate, nor does that accord with my experience. Slab-on-grade shrinkage is a much slower process in an unheated environment. We have found that when the building's winter heating system is turned on, that is when the shrinkage rate really accelerates for slab-on-grade.

If we are forced into steel dowels or other steel devices, I suppose it should be stainless steel, but that is very expensive. About 5 times the cost of carbon steel. Another possibility might be glass fibre or carbon fibre dowels. Maybe epoxy coated plain bars, if such exist (probably do not exist), but epoxy coatings are not great corrosion protection. Or maybe we use carbon steel and just take our chances on rusting and resultant concrete spalling...

Anyone know what the acceptable shear transfer load for keyways might be, approximately?

Any other suggestions?

 

[BUGGAR]

At least thicken the edge of the concrete at the joint. Have you explored those diamond shaped plate shear keys?

 

[KootK]

Believe it or not, this is all I know of: Link

Maybe a galvanized version of the diamond dowel system?

The latest ACI standard is not too supportive of the checkerboarding either. Not much bang for effort buck it seems.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ajk1]

To Kootk: Yes I agree that checkerboarding has its problems. I read an article about 20 years ago explaining why checker boarding was not a good thing, and could actually make things worse. I cannot recall off the top of my head how the reasoning went, but it seemed convincing when I read it.

 

[ajk1]

To Teguci: The slab is 200 mm (8" thick). I have had spalling the top of the joint occur in a much thinner slab. I suppose it may also occur in an 8" slab. Have you seen this occur in an 8" slab?

 

[Teguci]

ajk1:
I have limited experience with heavy loads on an SOG. I'd think no keyway, best practice subsurface preparation and tooled joints would be the way to go. With an 8" slab and consequently a low surface to volume ratio, shrinkage will be low and long in coming.

 

[KootK]

The document that Teguci linked mentioned that they considered 10" to be the lower limit on shear key usage assuming that you follow their proportioning guidelines.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

The document which Teguci linked discusses dowels, size, spacing, and corrosion protection. I would think stainless steel would be worthwhile in corrosive environments.

 

[Ron]

ajk1....I would dowel the joints...either with conventional dowels or plate dowels. A keyed joint in an 8" thick slab will work but the key should be shallow and the transition angles high.

More importantly, make sure the subgrade is well compacted and FLAT! Variations in subgrade surface level have a significant effect on restraint and cracking. I would not thicken the edges at the joints. This creates additional restrain and promotes more cracking.

Keep the control joints close. The more control joints you have, the smaller the gap will be in all of them, thus making them a bit easier to maintain and provide aggregate interlock load transfer.

Last....Use the largest coarse aggregate you can use in the mix. This increases load transfer and decreases shrinkage.