[TowerTalk] Vibrator for 2x2x4 hole?

[David Gilbert]

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 at 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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