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Re: Free Falling Concrete

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> >Can anybody direct me to information regarding the free falling of concrete
> >inside formed walls,is segregation an issue?
> >Tarek Mokhtar, SE
> >
> Tarek, if you have access to American Concrete Institute publication ACI-304
> "Guide for Measuring, Mixing, Transporting, and Placing Concrete" refer to
> two pages of drawings showing correct and incorrect methods of placing
> concrete, Fig. 5.4(a-h).
>
> At the top of this drawing is the statement:  "CONCRETE WILL SEPARATE
> SERIOUSLY UNLESS INTRODUCED INTO FORMS PROPERLY."
>
> On sketch a. PLACING CONCRETE IN TOP OF NARROW FORM are 2 paragraphs:
> "Discharge concrete into light hopper feeding into light flexible drop chute.
> Separation is avoided. Forms and steel are clean until concrete covers them."
>  And: "To permit concrete from chute or buggy to strike against form and
> richochet on bars and forms faces causing separation and honeycomb at the
> bottom."

In theory this is great, but in practice, most RC walls I have seen have
thickness of about 25cm. If you deduct the cover and the thickness of the
vertical and horizontal rebars, you will need a flexible chute hose of 10cm
diameter to be plunged inside the wall which the ready mix suppliers don't have
and, I think, can't use because of the high pressure it would generate in the
hose. So you end up with concrete free falling from the top of the wall causing
some segregation and air bubbles along the forms. This can be partially resolved
by good vibration and a cohesive mix with a high percentage of fines.
At least this it what I've experienced down here in Lebanon, I don't know if in
the US they have ready mix pump hoses with 10cm diameters or if most RC walls are

40cm thick and more in which case there would be no problem.


> I assure you that if good concrete is just dumped into a tall reinforced wall
> form and allowed to rattle its way to the bottom much of the cement paste
> will coat the rebar and forms and the first few inches of concrete at the
> bottom will be nothing but rocks (large aggregate) and honeycomb.
>
> Ralph Hueston Kratz, S.E.
> Richmond CA USA

Moni Serhal
MSCE UT Austin
Serhal Engineering, Lebanon