Monday, November 7, 2011

RE: Concrete Pedistal large

Sorry for not responding sooner.  I have been lecturing and vacationing in southwestern Montana and northwestern Wyoming.  It is a great time of year.  No crowds, a lot of wildlife, good hospitality and rates at the old hotels, and had a great dinner in the ghost town of Marysville, MT. 
Now back to business. 
Your contractor wants to save the forming cost, because the added concrete is less in cost than the labor for forming.  That is understandable.  What you have is not normally considered mass concrete, but it is mass concrete for thermal considerations.  There are many variables and there are many methods of mitigation of the differential temperatures and the ensuing cracks that can result. 
PCA has a good article on the subject by John Gajda of CTL.  He presents the causes and cures in pretty good depth.
As a minimum, I would strongly urge that the exterior be covered with insulating blankets of some form.  The thing that you want to do is equalize the temperature and reduce the thermal differential.  It is the thermal differential that causes the cracks.  This should be part of the curing process.  I am not sure how easy you can get concrete insulating blankets in your area, but just using batt insulation would sure be effective.  The insulation needs to be placed as soon as possible.  I would suggest having the insulation on the forms in advance and then putting a blanket over the top after the concrete placement. 
Other than that, you have covered the most important aspects.  Adding ash will slow the rate of hydration and the heat of hydration.  Additional steel predicated on the mass of the concrete would help, but it will not eliminate cracks.  I have seen 48" diameter concrete columns reinforced with 8 bundled #18's crack with 8,000 psi concrete.  You can not get much more rebar than that.  The cracks were longitudinal but the ties were pretty significant but had little effect on the cracks. 
The other thing is to use as cool of a concrete mix that you can get.  Ice is often added as is nitrogen injection.  But nitrogen may be overkill.  Just reduce the water temperature as much as you possibly can and use aggregate not at the surface to have cooler aggregate.  Talk to the contractor and the mix supplier to get the initial temperature down.
I can't think of any more admixtures that I would suggest.   

Regards, Harold Sprague

Subject: Concrete Pedistal large
Date: Thu, 3 Nov 2011 11:33:26 -0400

Hello all


I am designing an entry toward structure for a large box store…like use to be a Circuit City store.  We have arch pedestals that are 6'-4" x 4'-4" x 4'-0" high.  We designed this shape as a 12" thk concrete walls around the outside perimeter.  I have #4's @ 18" each face for the walls.  I have braced frames sitting on top of 2 sides. Below the frame columns are 18"sq columns w/4-#8's & #3 ties @ 8"oc.


The contractor wants to pour the whole volume as solid conc. in one pour and only use the rebar as designed for the walls around the perimeter.   I have not designed any concrete above ground like this before.


Any comments or area to watch out for would be much appreciated.  Admixtures  for conc.  Additional steel etc.


Harold I need to tap into vast knowledge and experience relating to concrete.



Joe Venuti

Joven Engineering

P.O. Box 5098

La Quinta,  CA

760 408-6441