A New and Improved High-Performance Concrete
Move over, high-performance concrete (HPC). The next generation of
HPC, known as ultra-high-performance concrete (UHPC), is currently
being evaluated at the Federal Highway Administration's (FHWA) Turner-Fairbank
Highway Research Center in McLean, Virginia. The Center's Structures
Laboratory is testing two American Association of State Highway and
Transportation Officials (AASHTO) Type II prestressed concrete girders
fabricated from UHPC. The tests are intended to characterize the girder's
structural behavior and determine how well the current AASHTO design
provisions represent that behavior.
UHPC is a steel fiber-reinforced reactive powder concrete that typically
displays twice the compressive strength of any HPC used in United
States bridge construction to date. The French firm Bouygues SA developed
the reactive powder concrete, which is engineered to be a highly compacted
concrete with a small, disconnected pore structure that helps to minimize
many of the limitations of typical HPC. These advancements are achieved
through a combination of finely ground powders and the elimination
of coarse aggregates.
The addition of small steel fibers to the mix is responsible for
much of the tensile strength and toughness of the material. These
fibers eliminate the need for mild reinforcing steel in the girders.
The placement and curing of UHPC can be performed using procedures
similar to those already established for use with some HPCs. The fluid
mix is virtually self-placing and requires no internal vibration.
If required, external form vibration causes the mix to smoothly flow
into place. Following an initial set of 24 hours, the curing process
requires at least an additional 48 hours, including a vapor bath at
a constant 88 °C (190 °F).
The structural test of the first girder delivered some impressive
results. Just prior to failure, the 24-m (80-ft) long girder displayed
a mid-span deflection of more than 485 mm (19 in). Even more impressive
was the girder's ability to sustain a large load and associated deflection
without creep, relaxation, or any visual sign of distress. For example,
the flexural test was suspended for more than 12 hours with 300 mm
(12 in) of mid-span deflection locked into the girder. During that
time, the girder was unchanged, and even with the aid of a 3x magnifying
glass there were no detectable cracks.
To date, three additional structural tests designed to determine
the shear capacity of the UHPC girders have been completed. These
tests revealed that the shear capacity of a UHPC girder without shear
reinforcement is approximately two to three times that of a conventionally
reinforced prestressed concrete girder.
FHWA's testing of UHPC has revealed it to be a promising addition
to the HPC currently being used by the bridge construction industry.
FHWA plans to conduct additional tests that will fully characterize
the material, and once the behavior of UHPC is understood, its use
in bridge construction will be further investigated. Joey Hartmann
of FHWA says, "UHPC is a very promising material that will have
multiple applications, but there will need to be further application
development to make it efficient."
Numerous other countries are already employing UHPC. Canada and South
Korea have used UHPC for pedestrian bridges, Portugal has employed
it for seawall anchors, Australia has committed to its use in a vehicular
bridge, and France has used it in building power plants. In all of
these cases, the material was chosen for its ability to stand up to
high stress, both environmental and load related. The increasing deployment
of UHPC worldwide and FHWA's initial testing results for the product
bode well for its future use.
|
FHWA's testing of UHPC has revealed
it to be a promising material for use in the bridge construction
industry. FHWA plans to conduct additional tests that will fully
characterize the material.
|
For more information on UHPC, contact Joey Hartmann at FHWA, 202-493-3059
(fax: 202-493-3442; email: joey.hartmann@fhwa.dot.gov).
Back
to top
..................................................
Articles in this issue:
Pavement Preservation Forum II: Investing in
the Future
A New and Improved High-Performance Concrete
Welcome to the Word of Nondestructive Testing
Fulfilling the LTPP Promise
Highway Technology Calendar
FHWCA Announces New Source of Bridge Preservation
Funding
