patrick_g at windstream.net
Sat Nov 2 08:03:40 EDT 2013
Concrete cures, not dries. As stated in prev posts minimal water in a stiff
mix is harder to work but cures to a stronger final strength. Left to their
own discretion often times concrete contractors and their workers will call
for more water to make a soupy mix. This makes their job much much easier
but your concrete much weaker in its final form.
There is an excellent alternative, "Water Reducer Plasticizer additives"
which make the concrete flow like it has extra water added but it does not
destroy the final cured strength of the concrete. It is a win-win
situation. The concrete flows easily without loosing final cure strength.
These additives are not expensive. They have a limited pot life and should
be added just enough ahead of the pour to be thoroughly mixed in the
delivery truck. Added on site not at the plant when loading the concrete
1/3 of the ground floor of my house is ICF (Insulating Concrete Forms)
construction, our master suite is a safe room. The contractor was used to
working with a stiff mix to retain strength and did things like cut holes in
the window sills to allow adding concrete to get a complete fill whereas the
stiff concrete would not flow into those areas. I requested a water reducer
plasticizer additive and told the contractor. He had no experience with it.
When the concrete was poured some of it shot up about 18 inches into the air
through the holes they cut in the window sills. "Thar she blows!"
Moral of the story? A good stiff low water mix with a water reducer
plasticizer additive will flow very easily. It can be a very handy approach
but you need to be prepared. Filling a simple hole in the ground with lots
of rebar in it is not trivial with a proper low slump stiff mix. It should
be "rodded" or vibrated to ensure air is not trapped and that you get good
coverage-contact between concrete and rebar. A water reducer plasticizer
can sure make that easier.
From: Roger (K8RI) on TT
Sent: Saturday, November 02, 2013 4:36 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] concrete
On 11/2/2013 3:27 AM, David Gilbert wrote:
> That (heat) is not why you should keep the concrete moist. Concrete is a
> hydraulic cure, meaning it requires water to cure. If concrete is allowed
> to prematurely dry out it doesn't become strong until it gets wet again,
> and may not ever reach the strength it would have if it had remained wet.
> A bit of surface water will do almost nothing to affect the internal heat
> generated by curing, but it will greatly facilitate the cure itself.
> By the way, too much water in the mix is not a good thing either, because
> after the cure is complete the excess water will eventually depart leaving
> small interstitial spaces ... and air is very poor in compression. ;)
> The strongest concrete is that which just barely has enough water
> throughout the curing cycle to complete the chemical reaction. That is
> rarely seen in practice, though, because such a mix would be very stiff
> and hard to work ... certainly difficult to pour.
My garage and shop aprons are excellent examples of that. It was one of
the hottest days of summer. They put extra water in the mix to keep it
from drying out. No amount of watering would fix that. It looked great,
but after the first winter's, surface freeze and thaws the surface
spalled off. The contractor said it was due to the salt because the
pattern followed the tires (loosely), but that was just hydraulic
pressure forcing water into the voids. I eventually cleaned it and gave
it 3 coats of epoxy, but the concrete under that eventually came loose too.
> Dave AB7E
> On 11/1/2013 12:20 PM, Tom Anderson wrote:
>> One thing you might do is what I was told when I put in my base back in
>> 1993. Lightly water the concrete base daily. I mean lightly, no
>> gushers, etc. Just a sprinkle for a couple minutes. This helps take out
>> the heat from the curing concrete. I mnust have worked because my 4x4x6
>> ft base is still in fine shape as when I put it in.
>> Tom, WW5L
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