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RE: Concrete slab problem

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John,

I have seen this on many (too many) occasions.  The problem is generally a
high w/c ratio in the top layer of concrete, often caused by a premature
finish.  I would presume that this is a steel trowled finish.  If it is a
steel trowled finish, the finisher had to race with the weather and set time
to get it covered.  I would be interested in what accelerator was used.
Accelerators need to be tested in the specific mix including the specific
cement that you are using.  Proper dosage is a crap shoot in the best of
circumstances.  And accelerators (except for the banned calcium chloride)
are expensive.  Recently I have been using a combination of Adva and Polar
Set to get sets in the range of 4 hours.

With 12 trucks all on site at the same time, you should have had 6 laser
screeds and a crew of about 50.  If they placed all of the trucks, and they
had a set accelerator, you can bet that they sold a lot of Pepsi and Coke
(the retarder of choice for the desperate).  It was probably the later
trucks that had the delamination problems.

If the concrete froze, you should know it by the powdery consistency and the
thud it makes when you strike it with a hammer.

The entire batch might have an O.K. w/c ratio, but if the air entraining
agent ranged much above 5%, the tendency is to finish too soon, trapping
water in the upper layer of the slab.  

I guess I don't know what you are going to epoxy inject if you have surface
delaminations.

If the concrete below the delaminations is sound, I would:
1.  Review the petrographics for a proper diagnosis of the problem. 
2.  Chain drag the entire surface
3.  Mark out the delaminations
4.  Remove the unsound concrete
5.  Scarify the surface
6.  Apply a new concrete surface, and cure, cure, cure.

I read one of the responses suggested polypropylene fibers.  Please read the
archives for my opinion on polypropylene fibers.  I am not a fan for the
vast majority of industry suggested applications, and would not consider it
for secondary reinforcing for a slab on grade.

Regards,
Harold Sprague


> -----Original Message-----
> From:	John Schwarz [SMTP:jschwarz(--nospam--at)theschneidercorp.com]
> Sent:	Thursday, December 23, 1999 2:53 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Concrete slab problem
> 
> Seasons Greetings.
> 
> There once was a concrete slab, 8 inch thick, unreinforced, placed on
> November 4, 1999 which has irregular areas of delamination ranging from
> 1/4 to approx. ½" below the top of slab surface.
> 
> I'm in the process of trying to determine two things.  First, due to
> project schedule, I need to find an appropriate repair.  I think the three
> options are slab removal/replacement, delam removal/patching, and epoxy
> injection.  I'm leaning toward epoxy injection because it looks like it
> would provide an adequate repair and have good post-repair aesthetics
> (what?? I'm an engineer.. did I type aesthetics or just think it?) without
> the expense of a tearout.
> 
> Question 1:  Any thoughts?
> 
> The second determination to make is the cause.  The problem seems to be
> somewhat rare.  The mix, as modified by the supplier without my input or
> knowledge, as far as I have been able to learn so far, is 4500 psi, 4 %
> air, accelerating admixure, mid-range (although the testing agency log did
> not indicate this). 
> 
> Evidently, the concrete installer had scheduled 5 trucks to show up at 5
> am ready to place.  But, around a dozen trucks showed up before 5 am (some
> were actually eventually dumped before the concrete began to set).  The
> contractor also had to remove placed concrete because it was chuted too
> late.  (Keeps getting worse and worse, doesn't it?)
> 
> The air temperature in the morning was above freezing.  I am having
> trouble finding historical temperature info on the night time temp(any
> ideas).  The subgrade was at least cold, maybe frozen.. I don't know yet.
> 
> My thinking is that it is probably mix and temp related.  We are having a
> petrographic analysis performed on a core, above and below the delam..
> should give a pretty good picture of the material placed. ( the concrete
> suppliers rep said ,"Shoot, if you want the w/c ratio, I can give you that
> from the batch tickets".. no really, he said that, I'm not making that
> up).
> 
> Question 2:  Any thoughts?
> 
> Sorry for the lengthy message.. thought a few of you would be interested
> in this one.
> 
> John Schwarz, P.E.
> 
>