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Subject: Re[2]: Water Retaining Concrete
Author:  James F Fulton at CCMCED01
Date:    10/6/98 4:20 PM


It sounds like your design nearly meets the minimum rebar required to control 
shrinkage and temperature cracks specified in ACI 350. For control joints 
spaced at 25', the min rebar required is .28% xbxh. BUT this is the total 
rebar for the section, or .14% xbxh on each face. For b=12" and h=24", this 
works out to be 0.40 sq in /ft. Since ACI 350 limits the max rebar spacing to 
12", the economical choice would be to provide #5@9" (0.41). You have 
provided somewhat less than this at #5@12. Note also that ACI 350 allows for 
walls 24" and thicker for the "minimum shrinkage and temperature 
reinforcement at each face (to be) based on a 12" thickness". Thus, the min 
required at each face is .0028x12x12=.40 sq in/ft as before.

With this amount of rebar you provided, even though somewhat less than ACI 
350, it is hard to believe that the 35 mil crack you mentioned is due to 
uncontrolled shrinkage and temperature. To me this is more like a structural 
crack. The "z" calc and limits provide a quantitive basis for determing 
minimum rebar size and spacing for loaded structures, where by a cracked 
section investigation the rebar stress can be calculated. An equivalent 
procedure for concrete shrinkage and temperature is needed.        

Jim Fulton


______________________________ Reply Separator 
_________________________________ Subject: RE: Water Retaining Concrete
Author:  seaint(--nospam--at)seaint.org at Internet Date:    9/30/98 3:38 PM


First, you should be using 0.25% of the gross wall area to determine your 
shrinkage steel.  This would indicate that you are a little light in your 
horizontal rebar.

It would be interesting to see what your control joints look like. Generally, 
I like to form a joint with a formed V joint on each side of the wall and cut 
every other or each horizontal rebar that would otherwise run through the 
joint to insure that the crack will occur in the control joint. Place a water 
stop in the joint.  It is important to form a weakened vertical plane.

Now for the bad news.  Your concrete will continue to shrink for about 5 
years.  At 28 days only about 40% of the shrinkage has occurred.  At 1 year 
80% of the shrinkage has occurred.  If you seal the cracks with an epoxy, the 
wall will probably crack somewhere else.  You might want to consider a 
material like Sika Fix which seals the crack with a material that reacts with 
water, foams up, and remains flexible.

Harold Sprague
The Neenan Company
harold.sprague(--nospam--at)neenan.com


-----Original Message-----
From: Chance, Acie P. [mailto:acie_chance(--nospam--at)wdi.disney.com] Sent: Wednesday, 
September 30, 1998 2:46 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: Water Retaining Concrete


I am designing retaining walls which will form the edges of a lagoon. The 
first two walls we poured have cracks which go completely through the wall. 
The cracks appear to be shrinkage cracks. The mix design uses 4000 psi 
concrete with flyash, super P and well graded aggregate. The walls are 
between 10 and 20 ft. high and 2 ft. thick. Vertical steel is # 7 @ 12 back 
side and  #5 at 12 water side. Horz. steel is # 5 @ 12 both faces. Crack 
control joints are located at 25 ft. O/C. This follows ACI 350 
recommendations. I am worried about the damage to the rebar from the 
chlorinated water moving through the cracks. We do not intend to waterproof 
the lagoon but we do need to insure the structural integrity of the wall. The 
existing cracks are from .002" wide to .035" wide and will be injected but I 
have a lot of wall left to pour. If any one knows of a good authority on cast 
in place concrete for water retention I would be interested.