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RE: Removing structural plaster from shotcrete

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Thank's Steven,

The crack is nearer the surface of the pool. After nine years it would
be difficult to pinpoint the cause but the crack is diagonal and
slightly more than hairline (I am judging by pictures). The pool has
been drained and the plaster removal completed. As you noted, the steel
is exposed in regions.

The owner choose to fix the pool by gunniting another shell within the
existing and then finishing again with a structural plaster. From my
perspective, it seems that the method of crack repair done in the past
was incorrect as it looks like it was done by the original contractor
who used a silicone caulking to attempt the repair. 

I have not finished reviewing all of the information provided but the
strength of the concrete was tested between 1976 psi and 2800 psi (and
specified at the time as 2000 psi). The concrete appears to be
sufficient in strength and the crack is approximately 12-18 inches long
from what I can see on the drawings. The crack penetrated the plaster
and I assume that the crack was existing before the plaster was applied
- transfering in time through the plaster.  

I don't believe the crack was caused by any earth movement. It was more
likely caused by a heavy surcharge placed next to the pool wall shortly
after gunniting. The site is flat and the house sufficiently far from
the pool to no cause a surcharge. There has been no significan seismic
events in this area for he last ten years and the location of the crack
at the surface suggests either temperature shrinking or some localized
surcharge.

I will contact Chem-Rex to see what they might suggest, but I think the
Epoxy-Gel is an excellent solution that will minimize the loss of
material behind the wall if done in steps to insure sealing the backside
of the crack first.

Thank you for the advice.

Dennis

> -----Original Message-----
> From: Steven A. [mailto:cratylus(--nospam--at)earthlink.net] 
> Sent: Thursday, June 28, 2001 11:15 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Removing structural plaster from shotcrete
> 
> 
> Original pool plaster is structurally bonded to the gunite 
> shell. It's impossible to remove this layer without pulling 
> out some gunite. It seams to me you have a large concrete 
> repair project on your hands.
> 
> I would venture to say that the removal of the dense cement 
> plaster cover will result in corrosion of the rebar, now that 
> it is exposed to the elements. Ultimately, you may be 
> applying a corrosion inhibitor over the rebar.
> 
> As for the crack, or cracks, the method of repair would 
> probably depend on whether it forms at the 'hinge' 
> (transition from shallow to deep end). More importantly, the 
> appropriate repair method may be dictated by the cause; for 
> example, if it's settlement related and the pool exists at 
> the top of a non-conforming slope.
> 
> If the cracks are injectable, this would be 'blind-side' 
> injection into a more porous (probably moist to wet) 
> concrete, or rather gunite. Not an easy thing to spec out in 
> terms of structural resins.
> 
> I would lean towards a moisture insensitive epoxy gel, using 
> pressures that would accommodate the particular crack widths. 
> I would use a cementatious (perhaps hydraulic and preferably 
> white) cap seal, because I would grind it down later and 
> would feel uncomfortable with the bonding of new plaster onto 
> the surface area of repair if it has been smeared/coated with 
> resins. Getting a cementatios cap seal to handle significant 
> pressures would be your challange! Further, I would take 
> quality control very seriously as un-catalyzed resin in a 
> swimming pool shell would be disastrous.
> 
> Good luck.
> 
> Steven A.
> Los Angeles
> 
> 
> Structuralist wrote:
> 
> > I have been asked to review a problem with a local swimming 
> pool. The 
> > pool was constructed of reinforced Shotcrete (pneumatically injected
> > concrete) that ranges from 4" thick to 5-1/2" thick. The 
> rebar appears 
> > to be within one inch of "air" and three inches to earth. A 
> 1" plaster 
> > was applied over the shotcrete and tile was added around 
> the perimeter 
> > at the area above the water line.
> >
> > There was a crack in the pool and the ower decided to remove the 
> > plaster and create a new shotcrete shell within the existing. His 
> > contractor removed the plaster using a jackhammer and took 
> off enough 
> > of the concrete below the plaster to expose the steel in parts.
> >
> > Here are some questons I have:
> >
> > 1. What other methods can be used to remove plaster without 
> doing so 
> > much damage to the concrete below.
> >
> > 2. If there was a crack in the concrete, is there a 
> standard for the 
> > crack repair in a pool and what is it? Is it less 
> destructive and less 
> > costly than the method that the contractor took to strip the entire 
> > interior of the pool which the owner is claiming cost him 
> $35,000.00 
> > including the new shell.
> >
> > 3. It seems to me that minor cracks (which this appears to be) are 
> > repairable by injection epoxies or injection grouting and then 
> > resealing the surface with either plaster or another epoxy or 
> > elastomeric type finish. Any suggestions?
> >
> > I am trying to get an idea to justify what was done or to find out 
> > what options for pool repair is available that will correct a minor 
> > crack. I was intending on contacting my representative from 
> Chem-Rex 
> > to see which of their products would do the trick. Assuming 
> the owner 
> > wants the same finish as he had, what are the alternatives to such 
> > drastic demolition and repair as he had chosen?
> >
> > Thanks
> >
> > Dennis
> >
> > Regards,
> > Dennis S. Wish, PE
> > Structural Engineering Consultant mailto:structures(--nospam--at)engineer.com
> > (208) 361-5447 E-Fax
> >
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