Hello out there in towerland:
I've been reading with interest the latest thread on rebar and
concrete. I thought I would try to explain some of the questions. I am
doing this in the light of helping everyone get a better understanding
of reinforced concrete and how it works.
First off, dobie blocks are not uncommon. I see them at Home Depot and
Lumber yards all the time. Call your local ready mix supplier and ask
where to get them. You only want to use a few to space the bars and
keep them out of the dirt. This means both the sides and the bottom of
the hole. The use of pavers is ok but do not make a solid mat as this
defeats the whole reinforced concrete process. The reinforcing steel
must be surrounded by the concrete mix to be effective, this is the
idea behind bond strength.
The UBC, SBC, BOCA all reference ACI 318-95 for reinforced concrete
design. This states that for concrete cast against soil there must be
3 inches of concrete cover to the reinforcing steel. Do not place the
bars directly in the soil for two reasons; first, you will not get the
complete encasement of the bar so less strength and also the bar will
corrode quite quickly. Reinforcing steel when it starts to corrode
acts like a wick or straw and corrodes along its length. When it
corrodes it expands and cracks the concrete causing further exposure
and so on.
Rebar doesn't rust when encased in concrete because it has no direct
air moisture interface. Also concrete when cured is not a sponge like
structure. It has pores but they do not form channels through the
concrete matrix. The only place where I have witnessed moisture
migration through sound concrete is at high head (>100 feet) Dams
where the water pressure on the upstream face of the dam actually over
the years causes water to seep through the concrete. In some cases
over 200 feet of solid concrete.
Do not dump mixed concrete in the hole. This causes the wired cage of
rebar to deform and shift and more importantly causes the concrete mix
to separate with the heavy aggregrate and the concrete paste forming
layers. You want as homogenous a mix of paste and aggregrate as
possible. For inaccessable areas either pump or use a chute made of
wood to place the concrete.
Concrete can be placed in cold weather. Your local ready mix supplier
has details. The concrete actually gives off heat when it cures. You
need to insulate the forms or hole and cover after pouring to prevent
surface freezing of the excess water and the formation of ice crystals
in the concrete matrix. Ask the ready mix supplier to use heated
aggregrate and hot water when they mix your batch.
As far as giving engineering answers to vague inquires I not will to
do that. I suggest you seek the help of a local structural/civil
engineer to answer questions concerning specific installations.
Remember, the rebar must not be against the soil. It should be tied in
a designed and effective arrangement to do any good. If properly
placed and compacted will outlast all of us, if not it will
deteriorate quite quickly in the presence of moisture. I know that for
a fact as one half of my present business is in the renovation and
repair of 15+ year old industrial plants that were built without
regard to these common sense facts.
73
Hank Lonberg / KR7X/ IH9P/ P.E.
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