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RE: Limits of Concrete Construction Joints

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In addition to the aforementioned, the thickness of the concrete in a slab
or wall is a significant factor that should be taken into consideration when
locating control or construction joints.  PCA (Portland Concrete Assoc.)
provides formulas on location and depth of joints relative to the thickness
of the slab or wall.  


> -----Original Message-----
> From:	Nicholas Blackburn [SMTP:nblackburn(--nospam--at)fdgoak.com]
> Sent:	Monday, March 12, 2001 8:44 PM
> To:	'seaint(--nospam--at)seaint.org'
> Subject:	Re: Limits of Concrete Construction Joints
> 
> 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
> typically do.
> 
> 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
> corners etc.
> 
> 
> 6. Any other limits on CJ's?
> 
> Nick Blackburn, PE
>