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Slump Loss When Pumping?

Slump Loss When Pumping?

Slump Loss When Pumping?

(OP)
A concrete contractor stated that he "loses 1/2 an inch of slump" when concrete is placed by pumping.  I just don't see how that can happen.  What mechanism would decrease the slump from what comes out of the truck to what gets placed?  Heat?  I know it's only half an inch, but the guys here in the office want to hear from some others.

RE: Slump Loss When Pumping?

Well it depends on the length of the piping system but I can assure you that in pumping you will lose slump.  I have personally seen concrete pumped over 200' with a loss of 1-2".  And that with a grout injection at the beginning and a concrete with flyash to aid in reducing the heat.  At this location it was agree by all parties that the concrete would be tested at each end and the results monitored.  We wound up with a 5" slump going in and a 3 coming out.  It worked to everyone's benefit.

Now if your talking about a normal concrete pumper truck then I might be a little more wary of the actual amount of loss.  But I certainly wouldn't be surprised.  And unless there are problems in moving the concrete (other than laziness!) I wouldn't be concerned enough to do much about it.  If the contractor or operator is concerned about the pumper I might even ask to have another one that can pump the proper slump.  Its been done before.

RE: Slump Loss When Pumping?

Pumping means also severe mechanical treatment of the concrete, perfecting the mix. A Google search quickly gives...

http://www.norliteagg.com/maps/pumping.htm#gpd

from search

http://www.google.com/search?hl=es&q=Pumping+Concrete+Loss+Slump&btnG=B%FAsqueda+en+Google&lr=

I quote some words from the norlite site...

"Presoaking is often accomplished by using an ordinary spray system. A minimum of 24 hours should be allowed with 72 hours or more preferred. This presaturation process aides in preventing the aggregate from absorbing water during the pumping process. It therefore minimizes slump loss as well as increasing the pumpability of the concrete."

RE: Slump Loss When Pumping?

Panama Red...
Good comments from Qshake and ishvaaag.  Both are relevant to the loss of slump from pumping.

The most common reason for slump loss from pumping is that the aggregates were not in a saturated, surface dry (SSD) condition when the concrete was batched.  This results from lack of attention to the stockpile moisture as noted in ishvaaag's comments.  Most coarse aggregates can absorb between about 1-1/2 and 5 percent moisture by weight.  Most fine aggregates at about 1 percent or less.  The pressure caused by the pumping action forces the water added at the time of batching into the deficiently saturated aggregate, thus removing some of the mix water available for "viscosity" of the mix, resulting in a slump loss.  This condition is exacerbated by aggregates that are excessively porous (limerock and lightweight aggregates, particularly).  A second common reason for slump loss is the reduction of entrained or entrapped air in the mix resulting from pumping action.  There are several other, less prominent reasons for slump loss, all of which can be controlled.

Qshake makes an excellent point about taking slump tests at two locations.  For legal purposes, the slump must be taken at the discharge chute of the transit mixer (concrete truck).  The reason for this is that this is where the ready-mix producer's delivery specification responsibility ends, so you must assess if you got what you paid for.  Taking an additional slump test at the end of the pump hose is also advisable, since this is the slump you have to work with (though it is not the slump that was specified!!)  Further, there are several ways to compensate for slump loss at the end of the hose, the worst of which is to simply add water!

Your 1/2-inch slump loss is pretty good!  In my area of the US, we would be pleased with this result as we have several of the reasons for slump loss which tend to compound on us, particularly in the summer (aggregate saturation, air loss, hot weather!)

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