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RE: Clarifier grout topping

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I have put bonded toppings on several slabs.  The parent slab will restrain
the topping slab and prevent large cracks from forming.  It will reflect any
existing cracks in the parent slab.  The topping slab will still shrink
relative to the parent, but the cracking is frequent and generally can't be
easily seen, and should not impair performance.  Unless you are using steel
fibers, you are wasting money on fibers, and making it more difficult to
finish.  I generally prefer making the topping thick enough to put
reinforcing steel in the topping.  In a clarifier, I wouldn't consider WWF.

The mix should employ the largest aggregate possible.  Another major
consideration is curing.  I like using polycarbonate super plasticizers
which reduce initial set time allowing the curing process to begin earlier.
The polycarbonate super plasticizers also make the finishing easier and
therefor you get a better finish.  And for a clarifier slab, I would
consider calcined (metacaolin) clay cements to reduce permeability.  You
might want to consider shrinkage compensating cements if cracking is big

You could consider going to an unbonded slab, but then you should add a
crusher fine base to reduce the possibility of differential top to bottom
curing and the resulting curling and cracking.  The surface should be as
smooth as possible to allow slip to occur.  

The unbonded slab just trades one set of problems with another.  I would opt
for a bonded topping.

Harold Sprague

> -----Original Message-----
> From:	Brad Hughes [SMTP:brad(--nospam--at)]
> Sent:	Thursday, March 30, 2000 4:14 PM
> Subject:	Clarifier grout topping
> I am hoping for some feedback/advice on installation of a
> fiber-reinforced grout topping in a clarifier which I designed.
> I have specified that, after water blasting of the slab, a cement slurry
> be troweled onto the concrete surface immediately prior to the grout
> topping placement.  The question has come up about a spec for the cement
> slurry, and whether a bond breaker can be used in its place.  The reason
> for the specified surface prep is my understanding that a bonding agent
> is not recommended because of the likelyhood of improper installation
> leading to it acting as a bond breaker.
> First, is there another successful method that should be used?  And, if
> a slurry is appropriate, is there a suggested mix specification.  (I
> should note that the specification was created using a standard spec
> with a long track record, but I have learned again to not put a simple
> material description in a spec without knowing exactly what is needed!)
> Thanks in advance for any help.
> Brad Hughes