Construction joints in slab-on-grade
Construction joints in slab-on-grade
(OP)
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?






RE: Construction joints in slab-on-grade
RE: Construction joints in slab-on-grade
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.
RE: Construction joints in slab-on-grade
I was wrong. For heavy loads, do not use keys. They spall the top surface of the joint.
RE: Construction joints in slab-on-grade
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?
RE: Construction joints in slab-on-grade
RE: Construction joints in slab-on-grade
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.
RE: Construction joints in slab-on-grade
RE: Construction joints in slab-on-grade
RE: Construction joints in slab-on-grade
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.
RE: Construction joints in slab-on-grade
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.
RE: Construction joints in slab-on-grade
RE: Construction joints in slab-on-grade
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.