structural concrete slump

huangyhg

structural concrete slump
do you generally specify what range of slump is required for your structural concrete?
if so, what are the ranges you use for different types of construction (i.e. slabs, beams, columns, etc) and what is the reasoning?
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aci 301 provides some pretty generic guidance with a 4" slump unless otherwise specified.  once you apply the aci 117 tolerance of +/- 1", you have a concrete that will work fine in most conditions.  i usually caution against being too restrictive on the slump.  if you specify a 3" max slump, you are placing the responsibility of the quality of the concrete on one of the lowest paid laborers on the site.  the vibrator operator.
also keep in mind, the slump test is really only a measure of the slump of the concrete.  it is primarily a tool for measuring the batch to batch consistency and alerting you to the probability that there is something wrong with a batch where the slump is inordinately high or low.
greg
with reasonable concrete made according to the specified mix designs, the problems usually occur with the placement procedures or finishing of concrete. excessive slump makes the visual appearance better, but may not prevent other/future problems. on site water addition is one of the major points since it can be arbitrary unless approved.
dick
the standard is 100mm in australia unless specified.
our general notes specify 80mm for suspended slabs and beams, however 100mm flows better. there can be a loss of strength and increased shrinkage due to higher water content at the higher slumps (which is why we leave it at 80 on our notes), however superplasticizer is often used to increase the 80mm to 120mm (+/-20mm). you can then get good compaction and workability without sacrificing strength.
proper slump depends on a lot of factors, and contrary to most other's opinions, i beleive more slump is better than less slump (with in limits) some of the factors include rebar conestion, method of delivery, use of super p, need for strength/ durability size of aggergate and amount of sand and cement.
personally, i belive that a 4 inch slump is an absolute minimum except for certian situations. for most strucures six inch will give you a a better finished product than 4 inches. there was a very lively debate on this issue recently aci concrete code issues "slump test after adding super p" thread 167-22742