[TowerTalk] Vibrating Concrete...

[Dino Darling]

In reference to the message and website below, I want to point something
out about "concrete vibrating".

It is HIGHLY recommended that you do this!  But as with anything else,
"too much of a good thing can be BAD!"  You can actually OVER VIBRATE
your concrete!  Let me explain...

When you vibrate concrete, you are percolating the air pockets out of
it.  These air pockets (voids) weaken the base.  As you vibrate the
pour, you are actually causing the aggregate (rock) to move inside the
slurry.  Gravity, being what it is, will drop the aggregate down and
bring the "cream" to the top.  If you vibrate the pour too long, the
aggregate (STRENGTH) will drop to the bottom of the pour!  So much for
a strong base especially since your anchor bolts are only 27" or so
down!

You want to run the vibrator in and out of the pour quickly and don't
linger in one place too long!  Once you see no more bubble coming out,
move along!  Its best to vibrate in "lifts" as the concrete is being
added.  Once you have vibrated a lift, don't penetrate that lift again!
 Go 2' at a time and never penetrate deeper than 2' as you go up.  Don't
try to make air bubbles work their way up 6'!  Again, a few jabs into
the concrete and you should be good to go!  When it comes to vibrating,
more is NOT better!  (Is anyone else getting a little excited here or is
it just me? HA HA!)

While pouring basement footings and walls, I've actually seen forms
"blow out" from too much vibrating!  All the rock went to the bottom
and the forms (which were probably not set up correctly anyway)
couldn't hold back the water/slurry.  What a mess!

I have no idea if the person in the link below did it right or wrong. 
This is NOT the point of this e-mail!  The point is, DON'T OVER VIBRATE
YOUR BASE!!!  If you don't know what you are doing, pay a few more $$
and get someone in there who does!

As far as the re-bar cage looks, it looks good from my house!  Seriously
though, if the person followed the instructions as far as bar size,
placement, and nothing moves, then all is well.

Dino  -  K6RIX
dino at k6rix.com


> -------- Original Message --------
>
> E.g. http://bellsouthpwp.net/k/4/k4zzr/tower1.html
>
> (although I don't like the look of his rebar cage)