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- To: seaint(--nospam--at)seaint.org
- Subject: Water Retaining Structure-reply
- From: James_F_Fulton(--nospam--at)RohmHaas.Com (James F Fulton)
- Date: Thu, 15 Oct 1998 15:10:04 -0400
Subject: Re: 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.
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