fly ash (HVFA) Concrete, the
ash replacement is more than
and in many cases, exceeds a 50% weight replacement for cement.
technically, there is a higher than normal volume of ash within the mix
and because of the specific gravity difference between cement (3.15)
standard fly ash (2.54 to 2.65), the volume of total cementitious
concrete (i.e. High Volume Fly Ash Concrete) can be used in a variety
structural applications. It has been proven as a technology that can
accommodate sustainable development and improve the lifespan and
structures. With advances in admixture technology and proper
HVFA mix designs can have characteristics identical to typical concrete.
1981, long before using HVFA concrete in structural applications was an
accepted practice, architects used concrete with 40% fly ash in the
holding tanks in the Monterrey Bay Aquarium (U.S.A) for durability
considerations. Since then, concrete throughout U.S.A has regularly
20% to 35% fly ash.
desire from the environmentally conscious community to mitigate
emissions has resulted in a movement to allow higher fly ash contents
would have been thought possible a few short years ago. In fact,
now regularly asking for higher fly ash content for performance and
environmental reasons. This movement is forcing the concrete industry
research the nature and performance of local materials that allow HVFA
to perform as needed.
of HVFA concrete is directly related to performance of local materials.
of this, proportions of HVFA concrete vary from location to location.
all concrete utilized in construction, field verification testing and
placements should be used to make sure HVFA concrete performance meets
early projects with HVFA Concrete: Initially,
HVFA projects utilized concrete with very low water contents achieved
use of high dosages of high-range water reducing admixtures. A few of
projects are listed below:
research into proportioning HVFA concrete mixtures revealed that higher
content could be used while maintaining acceptable performance. This
door for the use of mid-range water reducing admixtures and more
“user-friendly” concrete. The following HVFA concrete products
were built with
the new mix ideology:
other structures are either underway or in planning as the sustainable
proportioning techniques that take advantage of the workability offered
generate a least voids condition in concrete, which complements the
action of fly ash.
with the demand for higher fly ash contents, mid-range and high-range
admixtures have been developed to further enhance water-reducing action
causing an unacceptable delay in setting time. Non-chloride set
admixtures have proven useful for controlling set time within
for finishing time.
concrete can be used in a variety of structural applications. Fly ash
only been used regularly in massive concrete applications, but has been
columns, shear walls, and floor slabs as well. Even though technology
improved HVFA concrete performance, some mixtures may be inappropriate
certain applications requiring very early age performance. Mixtures
with 25% to
55% fly ash may be acceptable, depending upon conditions, for mass
higher percentages of ash are used, the age of strength acceptance may
be extended to 56 or 90 days. Column and shear wall work
can generally accept
30% to 35% fly ash, especially when time of form-stripping is of
Higher cement content (20% to 30% depending on circumstances) may be
to provide appropriate finishing time for slab on metal deck exposed to
temperatures and for achieving strength of 3000 psi (i.e. 21 N/mm2) at
days of age for post-tensioned concrete applications.