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RE: Construction Joint for massive Foundation Slab

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You might try spraying 70 degree water on a slab that's poured in 90
degree heat.  

-----Original Message-----
From: Harold Sprague [mailto:harold.sprague(--nospam--at)neenan.com]
Sent: Thursday, May 27, 1999 2:39 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: RE: Construction Joint for massive Foundation Slab


Correct me if I'm wrong, but the issue is cracking.  That is why we are
concerned about the thermal gradient.  

As far as spraying water on young concrete goes, that is the best way to
cure slabs.  So it is an issue of the mass of the concrete and the heat
of
hydration which creates the thermal gradient that should give us
concern.

Tensile stresses and the tendency to crack can be calculated.  I would
suggest you reference Section 6.7.4 in "Handbook of Concrete
Engineering",
by Fintel.  It is a very good article.

Harold Sprague
The Neenan Company


-----Original Message-----
From: Kho Poh Teck [mailto:ptkho(--nospam--at)pc.jaring.my]
Sent: Tuesday, May 25, 1999 6:17 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Construction Joint for massive Foundation Slab


The main thrust of my question is not so much of the measures to control
the
heat of hydration but the minimum period elapsed between the peak
temperature of
concrete in a section of the work and removing the forms. By doing so, I
presume
the concrete may be subjected to thermal shock. Think of it as baking a
cake.

Spraying cold water on warm and young concrete in the preparation of
construction joint as you have mentioned is probably a wrong practice
though
it
is still commonly done. Also the cooling of the first pour will cause
thermal
cracks on the young conrete of the second pour because of the restraint
at
the
construction joint.

I have never investigated the formation of micro cracks in the
construction
joint and its effects on the final tensile strength of the hardened
concrete.
Comments please.

Regards
KHO pt
CEng

Harold Sprague wrote:

> This is an interesting question.  I don't know a lot about dam
construction,
> but an adjacent placement is cast after the forms are stripped and the
> surface is prepared by wet sand blasting and water cleaning.  The heat of
> hydration is more of an issue in controlling the thermal gradient.
>
> You can control the heat of hydration by minimizing the cement, lowering
the
> initial mix temperature, and using fly ash as a partial cement
replacement.
>
> A well graded aggregate mix and the use of as large a maximum aggregate as
> possible will also drop the amount of cement required and help minimize
> shrinkage.
>
> Harold Sprague
> The Neenan Company
>
> -----Original Message-----
> From: Kho Poh Teck [mailto:ptkho(--nospam--at)pc.jaring.my]
> Sent: Friday, May 21, 1999 9:26 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: Construction Joint for massive Foundation Slab
>
> Can anyone give some advice on the appropriate time lapse prior to
> casting a 2nd half pour of a massive foundation slab? Size 50m x 50m x
> 3m. Is it acceptable to cast the adjoining piece at the peak temperature
> of hydration of the first slab provided adequate reinforcement is
> provided to prevent thermal cracks. Personally I don't think the young
> concrete is able give sufficient bond strength for making tthe rebars
> effective to limit the crack width. What is the normal practice in Dam
> construction?
>
> Regards
> KHO Poh Teck
> CEng
>
>