Elevated Concrete Slab Construction Joint Placement

[wayniac3]

I am designing an elevated, one-way reinforced concrete slab. The slab thickness is 9 inches. The primary reinforcment is #9 @ 11" oc top and bottom with cover of 1.5" top and bottom. The temperature steel is #5 @ 16" oc. The slab is for a 20,000 sf building. I am determining where to place the keyed construction joints. My initial thought is to place them parallel to the primary reinforcing along intermediate grid lines (no columns) in order to maintain continuity in the main reinforcing direction and splice the temperature steel. However, I can find no documentation in ACI saying this is acceptable. Instead I am directed to place the construction joints in the middle third of the slab span. Does anyone have any thoughts on the placement of the construction joints for an elevated one-way slab?

 

[hokie66]

What supports the one way slab? Beams? Walls?

You could do it either way. Placing construction joints parallel to the slab span means having construction joints in the beams, if any.

I wouldn't use keys in slab joints. If you need shear transfer, you should use dowel bars.

 

[wayniac3]

It is a large pier and grade beam foundation. The slab and beams are poured seperately and doweled together. The one-way slab spans continuously over 24" square reinforced concrete beams which in turn bear on 24" diameter reinforced concrete piers. Typical spans are approximatley 17 feet. Why would I need construction joints in the beams? What is the reason for using dowels instead of the key? Could the slab reinforcing handle the shear or are the dowels in addition to the rebar?

 

[hokie66]

You wouldn't need construction joints in the beams, as the beam and slab are not cast monolithically. I thought they might be until you explained. So all things being equal, and if it just a choice between the two options, I would make the joints parallel to the span. If you do it the other way, you probably don't require much shear transfer if the joints are at or near centre span. The joint can just be clean and laitance free. Keys in thin concrete elements have a habit of breaking at the top or bottom, so I avoid them.

 

[TXStructural]

I would put tight, keyed cold joints in either direction as required for the placement. Keys are not required where there is little shear, but it's good practice to make all cold joints the same to avoid errors. Joints across the main reinforcement would best be placed in a region of low shear (hence the middle third, per ACI 318 6.4.4). Joints parallel to main reinforcement should be away from parallel beams, supports, or other discontinuities, to reduce shear across the joint. Do not break reinforcement.

 

[hokie66]

With this specific arrangement, i.e. beam and slab cast separately, another option for the slab construction joints would be along the beam centreline. Then we wouldn't have to argue about shear transfer.

With the beam cast first, there is more potential for restraint cracking of the slab perpendicular to the beam. Depending on whether this type cracking is undesirable, you may want to rethink the amount and arrangement of the crack control steel.