For the most part, a slump test only checks water content. The psi is a
function of both water and cement content. A concrete supplier has much
more to gain by cheating you out of the required cement than by adding
too much water. Commercial construction sites often make test blocks
(sometimes required by insurance even if the concrete supplier is
trustworthy) that are subsequently (after curing) pressure tested to
check the actual psi. It is desirable to wait 30 days to do that, but
not always necessary, since the cure curve is fairly predictable versus
time and temperature. Of course, by the time you'd be able to do a psi
test your bolts or tower base would be already permanently encased.
Even if the mix is an honest 3500 psi when the truck leaves the concrete
supplier's yard, you might want to make sure that the driver didn't add
water along the way to make it pour easier, especially if it is a long
trip to your lot. The slump test can help you there, but if the stuff
pours like soup you didn't get what you paid for. A 3500 psi mix is
going to be fairly stiff.
You can improve the strength of your foundation by "rodding" the
concrete well as it goes into the hole. Rodding is best done with a
vibrating rod built just for that task, but you can achieve just about
the same results using a long wooden pole or an old broom handle. Work
the rod up and down continuously into the wet concrete in the hole.
This accomplishes two things ... it works out voids and air bubbles that
would otherwise compromise strength, but just as importantly it helps
"float" excess water to the surface. Use smooth strokes --- long slow
strokes to fill voids, and short faster strokes to coax out the water.
If you use a shovel or hoe to keep the concrete slihgtly mounded up
toward the center, the water you float out will drain toward the side to
be absorbed by the soil. Rod steadily but not so fast that you
reintroduce bubbles. I'm a firm believer that rodding is really
important but it will just plain wear you out if you do it right, so
recruit some friends to help. Start when the pour first starts and
don't quit until it is done.
There are other ways of cutting corners on a good concrete mix ... the
best results come from a proper mix of graded aggregate (from roughly 1
inch stones to pea gravel to sharp sand) and not every supplier bothers
to do that. The truth is, as an individual you're probably dependent on
the honesty of the concrete supplier, and the most effective thing you
can do might be to ask several concrete subcontractors in the area for
recommendations on the best supplier. They will have a pretty good idea
regarding who supplies good stuff and who doesn't.
Don't forget to keep the surface of the concrete damp while it cures.
Dave AB7E
jp@ezoom.net wrote:
> Hi Pat,
>
> You need to do what's called a slump test when the mix is delivered. To be
> 100%
> sure, you need to pour a cylinder of the mix you used and then have it tested
> (crushed) after 30 days to determine the psi of the mix actually in the
> ground.
>
> J.P.
>
> Quoting Pat Masterson <bat@grumman.com>:
>
>
>> If I order 3500 LB concrete from the supplier, how do I know that when
>> the truckload arrives, it doesn't contain a weaker mix? What are the
>> tests that can be performed before the stuff is poured into the hole?
>> Should I hire a professional inspecter to do the tests? -pat
>>
Anything worth doing is worth doing to excess.
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