For a 4" concrete slab, how long of a run before an expansion joint is
necessary? 50', 100'?
At the expansion joints, what is the best way to maintain the two separate
slabs on the same level over time. Dowel and sleeve or keyway (tongue and
For cutting of the control joints, I hear within 24 hours (green) and also
10 days ("cured") after the pour. So what is the best time to cut the
control joints and why is this timing so important?
Depends on what you are pouring. If its a sidewalk, we usually
do an expansion joint every 12-15 ft. If it is a slab for a
house, we don't generally put expansion joints, if it is a
commercial building, the engineer will spec when and where to put
the EJs, but it is usually about every 200-400 sf. Different
situations need different remedies.
Sleeved smooth dowels. Sometimes sleeved on both sides,
sometimes only one. Sometimes standard rebar dowels without
sleeves. It is difficult if not impossible to put a keyway into
a slab when you are pouring both sides of the joint at the same
Within 24 hours. Cutting at this time is easy, plus if the slab
is going to crack, it usually does it pretty quickly after the
first few days. If you wait 10 days, the cracks you are trying
to control may have already happened.
But, different soils, different situations, require different
things. In my history with concrete, I have seen so many
different specifications that there IS no general rule you can go
by. Engineers do wonders in deciding those kinds of things.
Expansion joints every 60 feet or less in my experience. Also
needed at change of dimension, use, and to prevent locking a slab.
Construction joints are for your convenience to give yourself
manageable pours. They can be directed by architect or dictated
by the equipment being used. Greased or sleeved slick dowels,
load transfer plates, and keyway will all work. The plastic
sleeves fastened to the forms sure beat drilling and fighting
dowels. None of them will make up for poor subsoil preparation
and compaction. All will require high concrete strength (14 or
more days) before loading
Saw joints (contraction joints) need to be cut as soon as you can
get on the concrete without raveling. Depending on temperature
and humidity conditions, you might chance waiting to the next
morning, but concrete will usually have decided where to crack by
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