Vibrating with a machine or rodding with a stick does more than just
fill voids. Properly done, it will float excess water to the surface
where it can drain off or be skimmed off. Most of the water in the
concrete mix is there to make it flow easier (especially if it is going
to be pumped) but it takes very little water to effect the chemical
reaction that turns cement back into stone. Any water that is not
actually needed for the reaction eventually dries out, and the resulting
porosity reduces the final strength. If you look at the charts of
ultimate concrete strength versus water content in the mix you will see
what I mean. Here are some relevant links:
http://matse1.mse.uiuc.edu/concrete/prin.html
http://tinyurl.com/2gunno (a nice chart of predicted strength versus
water/cement ratio)
http://www.cement.org/basics/concretebasics_concretebasics.asp
Lots of people order 3000 or 4000 psi concrete, but sometimes somebody
along the line adds water to it to make it work easier and the customer
ends up with a lower strength than they paid for. Good rodding or
vibrating helps to float some of that water out of the mix by
consolidating the varying sizes of aggregates in the mix (rock, sand,
and cement particles), essentially juggling them so they "nest" together
better and squeezing out excess water. As-poured concrete will almost
always be weaker than concrete which has been properly consolidated ...
i.e., vibrated or rodded.
You still want to try to keep the concrete damp for the length of the
cure, though. Concrete doesn't need very much water to cure but it
needs it for an extended amount of time (30 days is a typical spec).
73,
Dave AB7E
gdaught6@stanford.edu wrote:
> On 16 Aug 2007 at 11:56, Gene Smar wrote:
>
>
>> Dana:
>>
>> For a small hole like that you probably can get away with poking at the
>> concrete with a 2x4 piece of wood as it's being poured. The goal is to
>> ensure there are no voids or air pockets in the concrete after it sets up,
>> thereby reducing the ultimate strength of the pour.
>>
>> My tower's foundation is made of 10.5 cuyds of concrete so I rented an
>> electric vibrator for the project. The 2x4 idea would not have been as
>> effective for my project because of the volume of concrete, plus the
>> foundation was undercut around the bottom edges and I wanted to make sure
>> that that space was filled with mud.
>>
>
> When my base was poured (by a concrete professional) it was actually
> pumped. The guy said that if you have the concrete mix correct and
> the flow rate right, you can get a "standing wave" of the mix going
> in that virtually assured that there would be no voids. No vibrating
> necessary. I watched as he did it, and it was slick! Once he got
> the hose flowing, it took 90 seconds to fill the hole it took me over
> a week to dig! [2.3 cu. yds.]
>
> 73,
>
> George T. Daughters, K6GT
>
>
>
>
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