To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: Re: Limits of Concrete Construction Joints
From: Nicholas Blackburn <nblackburn(--nospam--at)fdgoak.com>
Date: Mon, 12 Mar 2001 17:43:48 -0800
I'm interested in getting other's input or rules of thumb on practical
limits for concrete construction joints (CJ's):
I'll take a stab at it. Can't really give you rules of thumb, just what we
Limits are hard to set directly since they are dependant on the concrete
quality, location/type of restraints to shortening, use of the structure,
location of batch plant etc.
I highly recommend ACI 224.3 "Joints in Concrete Construction" for the
latest information on joints.
See also ACI 224 "Control of Cracking" and ACI 224.1 "Causes, Evaluation and
Repair of Cracks."
1. Maximum practical concrete placement in cubic yards for one shift, i.e.
locate slab or wall CJ's to limit overall required concrete placement.
Can't really answer the volume question but we frequently have pours of
around 350yds. The ability of a contractor to place concrete should not
necessarily control joint location.
The quality and shrinkage characteristics of the concrete and location and
type of restraint to shortening are very important. Higher shrinkage
concrete or concrete with high w/c+p ratios will crack more frequently
requiring a tighter joint spacing.
We use SOG contraction/construction joints at 20'oc EW max. Ideally the
contraction joints would be formed in during the placing/finishing process,
not cut in. Saw cutting tends to do little as the concrete has already
shrunk and cracked where it wants to by the time the saw cutting commences.
Wall construction the joint spacing along the length of the wall should be 1
to 3 times the wall height for tall to short walls respectively.
For elevated slabs, construction joints/pour strips should be placed where
they will break the slabs up into areas 1:1.5 max. This can be pushed
somewhat if you are using post tensioning.
2. Maximum straight length of wall (continuously reinforced) between CJ's.
We use 60' with contraction joints at 6'-15'oc. All joints should be cast
in. Cutting the contraction joints after the fact does nothing as the
concrete has already cracked where it wants to.
3. Maximum concrete box shape to be placed with continuous concrete walls
(without vertical CJ's).
4. Maximum vertical wall height between horizontal CJ's.
This is partially a function of formwork. ACI 224.3 mentions 30' as an upper
limit but doesn't recommend it. For the types of buildings we usually
design, we limit the vertical pour to the story height(10'-15').
5. Maximum elevated concrete slab placement or distance between CJ's.
This is partially a function of the concrete quality and restraints to
shortening. We try to keep our post tensioned deck pours to about 100' -
120' in length with a maximum of 190' wide. Construction joints/pour strips
should not be randomly placed to suit the contractor without special
attention by the engineer. Frequently the contractor's choice is not the
best structurally due to restraint locations, large openings/re-entrant
6. Any other limits on CJ's?
Nick Blackburn, PE