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Re: Lightweight Concrete

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We have had a lot of experience with all concrete types deflecting more than ACI code rules indicate. The kcs approach for long-term deflection calculations becomes less accurate (unconservatively) as the concrete section thickness reduces. Also, the compression reinforcement component of this method follows the same trend but more quickly.  So, for thinner slabs and beams, the calculations would normally underestimate the deflections significantly.

A further problem is introduced by restraint to shortening which needs to be considered in the cracking moment calculation. A large degree of restraint would completely nullify the benificial effects of "tensile strength"of concrete on these calculations.

At 03:25 27/07/99 -0500, you wrote:

anyone have experience with lightweight concrete deflecting more than calculations indicate?

conventionally reinforced.
got the reduced E due to lightweight.
got the reduced fr in the cracking moment calc.
still get more measured deflection at 150 days than calculated delta dead plus 1/2 of total creep.

any common experiences with such in lightweight (spec'd as <115pcf, 4000psi)

actual 28 day breaks exceed 4500psi.



robert d. rollo, PE
Corporate Associate
Parkhill, Smith & Cooper, Inc.
Lubbock, Texas
(806) 761-7539vox
(806) 747-7146 fax

Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd.
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422
email:  gil(--nospam--at)