The comment was meant to make you think about what you purport to say is
true when it is not, in the context you use.
Concrete does indeed have pores and they are very small in relationship to
the volume of the sample you consider and do not necessarily line up to form
a conduit for moisture intrusion. Concrete is also alkaline which tends to
inhibit acidic attack and sulfate ion corrosion. The full reason that there
is a requirement for the spacing of the reinforcement is to help with the
moisture intrusion from micro cracking which takes place when the concrete
cures and resists loads. This is also the reason that the code clear cover
differs from concrete cast against soil (3"), cast in form exposed to
atmosphere (2"), and cast in form and indoor exposure or top of slab
(varying from 1 1/2" to 1" depending on bar size).
The moisture does not travel through pores, it will travel through micro
cracks. These cover values were determined over the years by examining the
in place behavior of real work concrete in various climates, locations, and
types of structures or purposes. Curious thing about concrete is for it to
work as a design intends it has to crack (very small and not always in
organized pattern), this means that the load is being resisted and the
forces ( stresses) are be resisted. It is these cracks that the cover
requirements are meant to mitigate so that free air and moisture do not
reach the reinforcing which would tend to corrode. When the bars corrode
they swell and further crack the concrete exposing more and more surface to
the free air and moisture and so on.
Not pores (or porosity as you allude to).
Your treated wood example is also not quite accurate, treated wood for floor
plates is due to the fact that the inside of stud walls have varying
humidity due to the moisture content of the wood framing members when it is
originally constructed. The inside wall covering and the more modern used of
a vapor barrier (TyWrap) on the outside over the base exterior siding under
the outside finish effectively seal the wall cavity and there is (due to
temperature differentials) a tendency for the moisture to collect at the
base of the wall. The treated plate helps the wall maintain is integrity at
the base by resisting rot (dry and otherwise) and/or softening so the wall
does not pull away from the required hold down anchors or bolts when loaded
I see that you have your beliefs and concepts about why things are required
and I have mine and it is probably best that this discussion be taken off
this reflector so a more tower specific topic can be addressed.
Lonberg Design Group, Inc.
H. Lonberg, P.E.,S.E.
From: TowerTalk [mailto:firstname.lastname@example.org] On Behalf Of David
Sent: Sunday, September 16, 2012 8:17 PM
Subject: Re: [TowerTalk] AN Wireless tower
That's kind of a silly extrapolation, don't you think? We're talking
"moisture" here, not volumes of water. What was the point of that
comment??? As a P.E., do you seriously think that concrete is indeed not
porous to moisture? Why do you think that all building codes require
treated wood for wood that is in permanent contact with concrete floors
(i.e., plates at the bottom of frame walls)?? In case someone spills water
on the floor? Nope ... it's because moisture can propagate through a
concrete slab from the ground beneath it.
On 9/16/2012 6:45 PM, KR7X wrote:
> In that case you had better tell all those concrete dams that have
> been built that they should not be holding back those reservoirs as they
> Lonberg Design Group, Ltd.
> H. Lonberg, P.E.,S.E.
> -----Original Message-----
> From: TowerTalk [mailto:email@example.com] On Behalf Of
> David Gilbert
> Sent: Sunday, September 16, 2012 4:20 PM
> To: firstname.lastname@example.org
> Subject: Re: [TowerTalk] AN Wireless tower
> Sorry, but that's an old wives tale. Concrete itself is porous as
> hell and will pass moisture almost as easily as any cinder block.
> There is no way on this earth that concrete will keep moisture away
> from the iron, but the alkalinity of the concrete will indeed keep the
> iron from rusting and that's why our bridges hold together and our towers
don't fall over.
> Dave AB7E
> p.s. The specs for my tower called for 16 yards of concrete (9' by 9'
> by 5' deep), but it was impossible to carve a hole with straight sides
> into this rocky hillside so I ended up with over 20 yards myself.
> On 9/16/2012 4:02 PM, Edwin Karl wrote:
>> Do NOT use cinder blocks or red brick to support iron for the pour
>> Use concrete material, block pieces, curb stop etc. The purpose is to
>> keep moisture away from the iron. Cinder and red brick pass the
>> moisture through.
>> At least that's what we were told in the before time when I worked as
>> an iron worker.
>> I have an HD-90, it's a tough tower. The base got away from me and we
>> used 22 yds of concrete. It ain't going nowhere.
>> ed K0KL
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