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[TowerTalk] Concrete from on-site materials

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Subject: [TowerTalk] Concrete from on-site materials
From: Bill Otten" < (Bill Otten)
Date: Tue, 19 Feb 2002 22:36:59 -0500
Remarks inserted between comments..........

Subject: [TowerTalk] Concrete from on-site materials

> I shall be installing a base for 100' of Rohn 45 by hauling in
> Portland Cement and using mostly materials found on site.  This
> will essentially be a CB2, a BPC45G base, and a pier pin.  I am
> designing do-it-yourself (DIY) concrete and will use this project
> as hands-on experience from which I wish to write an article on
> the subject.  For reference, two other towers here were done with
> ready mix.
> The DIY design will be based on available materials to the extent
> that I can meet or exceed standard designs.  I would like to be
> able to discuss what results can be expected by various
> deviations from standard designs.  What I'm looking for here via
> TT is to build a bibliography of reference books, web sites, and
> messages as a basis to write this research paper.
This was a site I got a bunch of early info from when I did my  pour......

> Here are some of my early thoughts.
> 1.  The sand will come from the hole and elsewhere in the yard.
> (I live on a historic sand bar!).  A county septic system
> inspector tested sand by squeezing a hand full and if a ball
> results it's mud and if it falls apart it is sand.  Hmmm.  I'll
> need to build a sluiceway to wash the sand and use a REAL test
> method.

My biggest concern of all you've written....I'd shy away from the septic
system inspector's methodology and IF you're hellbent on using the local
sand a thorough washing and sieving will be absolutely necessary. Sieving
can also 'size' the sand to some level of uniformity.
> 2.  The stone aggregate will be of various sizes.  Concrete from
> old highways is pulverized and reused so I'm looking at using
> pieces of crushed building block and a sidewalk along several
> sizes of gravel left over other construction projects.

Typically, a one inch aggregate size is considered the maximum.

> 3.  I have plenty of 8" building block.  Precast concrete is a
> big buzz phrase in construction.  Perhaps I could build about
> three courses of block, mortar them together, and build a cage in
> and around them.  When the mortar has cured, I could pour in and
> around the block.  I may need to soak the block before the final
> pour so it won't blot moisture from the mix before it hydrates.

Absolutely. Maintainance of moisture content in the concrete AS it cures is
important...having the porous block material leach away moisture at varying
rates cannot be beneficial toward a consistent, even cure.

> 4.  More about the rebar.  I have more iron that Rohn specifies
> consisting of rebar and iron construction frames that I can cut.
> I may be able install an impressive and expensive (using free
> material) metal reinforcement.  I am looking for literature about
> the contribution reinforcing iron to the strength and durability
> of concrete.

See: for info on rebar.

> 5.  The Roan catalog specifies the concrete to be four feet deep
> regardless of location.  The county inspector says my code
> freeze depth here is 2'6".  Of course, I need to get down into
> undisturbed soil.  Are there other depth considerations?
> 6.  What PSI rating should the concrete have?  The vertical
> vector of the guys pulling the tower into the base plus the less
> significant weight of the tower, equipment, and climbers come to
> some 5,000 pounds.  What safety multiplying factor should I
> apply?

Concrete strength is a factor of the ratios of the mix......routinely trucks
will deliver onsite 3000 psi mix, but mix strengths can go as high as
7000-14,000 psi ( at a premium of course). Somewhere in the above citations
there is info on concrete strengths.

> 7.  On a prior pour, the driver of the ready mix truck added
> water based on his experience.  The DIY designer lacks that
> experience, so I plan on describing how to build a slump tester.
> (A slump test is the measurement of the time for a cone of
> concrete to slump down to a specified height.)
> 8.  I need to find a reference about the impact, if any, of
> thermo coefficients of the various materials.  Will the heat from
> the hydration be a problem with strange metal stuff in the
> concrete?  How much will prestressing help?
> 9.  Finally, I need to learn more about keeping the concrete
> moist after it has been poured for so that it can cure before it
> drys.  I can erect a tent over the pour to protect it from sun
> and rain and allow me to mist it on schedule, whatever that
> schedule is.

Consider a wind shield as well. Burlap or straw will hold and release
moisture well, polyethylene sheeting can be used as well. Watch the weather
to avoid any freezes to the pour within the first few's akin to
rapidly losing all the moisture at once, exerting forces on the mix's
adhesive properties, separates the cement 'glue' from the aggregate, and
then when it thaws releases a bunch of moisture back into a ruined pour.
Misting schedule? Often, keep the pour moist for the first few days....a
slow cure means a stronger concrete. Consider too that the moisture isn't
only 'skin-deep'.

> I look forward to receive comments from TT readers with or
> without a reference source in the literature or on Internet.  I'm
> finding lots of civil engineering stuff on dot-EDU sites.
> I'm really after a learning experience so I know that I should
> hand mix.  But, it's not a perfect world; I'll use a powered
> mixer.

One time hand mixing a 2+ yard pour is all it will take to convince you of
the merits of powered mixing. You can't mix to consistency well enough or
fast enough by hand.

Bill KC9CS     'I poured far too many home, apartment, patio and driveway
slabs in Western North Carolina's mountains to EVER want to do it again."

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