[TowerTalk] Concrete Strength

[David Gilbert]

That's probably true for large volumes of concrete like a tower 
foundation, and I stand corrected.

It is NOT necessarily true for smaller volumes of concrete, especially 
here in Arizona, where the concrete can indeed dry out before it has 
cured.   I've seen it happen more than once.

In any case, damp ground does not make concrete cure more slowly.

73,
Dave   AB7E




On 10/27/2010 11:47 AM, kr7x1 at frontier.com wrote:
> List readers:
>
> Please go to the PCA (Portland Cement Association) or ACI (American Concrete Institute) web sites and read the FAQs on concrete.
>
> Concrete once mixed with water does not need additional water to assist the exothermic reaction (generates heat) that allows the mixture to harden. The reason you cure the exposed surfaces with water is to help to draw away the heat and assisted in the limiting of shrinkage cracking at the exposed surfaces.
>
> A mix designed for 2500 psi is a specification that it should develop 2500 psi compressive stress strength in 28 days. It actually continues to cure to a higher strength after that point but at an asymptotic rate where it would take many years to get to some significantly higher strength value.
>
> I typically specify 2500 psi concrete strength when designing amateur tower foundations. I do so to avoid special inspection requirements by some jurisdictions, as this is in the building codes. This allows for just about any method of placement by any yahoo.
>
> The "rule of thumb" is that after 1 week the concrete mix typically cures to about 2/3's of its design strength, value whatever that is.
>
> Keep your facts straight by doing some research. If you can post here, you should be able to use "google" or another search engine.
>
> Regards
> Lonberg Design Group, Ltd.
> H.Lonberg, P.E.,S.E. / KR7X
> President
>
>
> ----- Original Message -----
> From: "David Gilbert"<xdavid at cis-broadband.com>
> To: towertalk at contesting.com
> Sent: Wednesday, October 27, 2010 10:07:25 AM
> Subject: Re: [TowerTalk] Concrete Strength
>
>
> Concrete actually>cures<  faster when damp.  Concrete needs water to
> form the chemical reaction that gives it strength, and without moisture
> present it merely dries out.
>
> 73,
> Dave   AB7E
>
>
> On 10/27/2010 6:36 AM, Mike wrote:
>> It cures more slowly in the damp ground which is good, wait the recommended
>> month.
>>
>> Mike, K6BR
>>
>> -----Original Message-----
>> From: towertalk-bounces at contesting.com
>> [mailto:towertalk-bounces at contesting.com] On Behalf Of Dan Bookwalter
>> Sent: Wednesday, October 27, 2010 5:18 AM
>> To: towertalk at contesting.com
>> Subject: [TowerTalk] Concrete Strength
>>
>> I seem to recall from about 8 years ago that concrete is up to 75% of its
>> strength in 72 hours or something like that. UST calls out 2500 PSI concrete
>>
>> after 28 days , so that i don't have to wait a month i was going to use 4000
>> PSI
>> concrete and put the tower on it after a week or so , thoughts ?
>>
>> Dan N8DCJ
>>
>>
>>
>>
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