2.5" line pump for placing slab on grade concrete?

[gte447f]

I have a project with a 5" slab on grad. The concrete spec'd was 4500 psi, max w/c ratio 0.45, max coarse aggregate size 1" (but 3/4" is commonly used in mix design around here), air entrainment 4.5% or 5% plus/minus 1.5% for 1" or 3/4" max aggregate size respectively, and max slump of 5". The contractor wants to use a 2.5" diameter line pump to place the concrete, and has submitted an alternate mix design for the line pump with 3/8" max coarse aggregate, 5% plus/minus 1.5% air entrainment, and 4-7" slump range. The f'c and w/c ratio meets the original spec, but I am worried that the smaller aggregate and higher slump could result in increased shrinkage and cracking. Also, the smaller aggregate size should come with an increase in air entrainment to 6% plus/minus 1.5% according to ACI. Are my concerns about shrinkage valid or is this proposed alternate mix design OK? Could my original spec work OK for pumping through a 2.5" line pump (I suspect not with 3/4" to 1" max aggregate size)? Is it even OK to place concrete with a line pump like this, or should they be reserved got pumping grout? This is the second project recently where I have been asked to accept an alternate mix design to use a small diameter line pump. I think the contractors are just trying to be cheap. FYI, this slab is about 100'x36'.

 

[Ron]

gte...presumably yes; however, concrete placement guys are fairly adept at putting down concrete in the easiest manner possible...which means soupy. Have the inspector measure the slump at the back of the transit mixer and again at the end of the pump hose. The one for record should be the one from the transit mixer. If they are slipping water in at the pump (watch for water hose hanging over side of the pump), the slump at the end of the hose will not have changed much and could actually increase. It should decrease through the hose, depending on the length of hose from the pump.

Also, if the inspector is busy doing slump, air content and other plastic concrete tests, he might miss something.