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Fiber reinforced concrete fill

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For water and wastewater treatment basins, where concrete fillets are
required to shape the corners of a square basin or where concrete fill is
required to shape the bottom of a basin, I have been specifying a 2500 psi
concrete mix with 3/8-inch aggregate and with polypropylene fibers added at
a rate of 1.5 pounds per cubic yard. Otherwise the concrete fill has not
been reinforced with rebar (although it is placed within a reinforced
concrete structure).  I have not heard of any problems with this mix, but I
am currently looking at concrete fills of a bit larger proportions than I
have used in the past, e.g. a 12-ft high fill sloped at 45-degrees; a 16-ft
high corner fillet; and 4.5-ft thick fill above a base slab. I prefer the
fiber reinforced concrete to adding rebar as the rebar gets more difficult
with the fill configurations, but I am concerned with how large a fill
volume can be done using just fibers and no rebar, without getting excessive
cracking. The fill doesn't need to be completely watertight but we do not
want a lot of surface cracking or large cracks. 

Has anyone else had good or bad experiences with thick concrete fills using
fiber reinforced concrete? 

William C. Sherman
Camp Dresser & McKee, Inc.
Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)