Over 8,500 cubic yards of structural lightweight concrete, produced with
TXI expanded shale from their facility in Boulder, Colorado, was used to
help construct an 860,000 square-foot office tower for the city and
county of Denver. This 12-story addition to Denver’s Civic Center
increased the complex’s office space by 65%, and allowed Denver to
consolidate most of the city and county offices from various locations
throughout the city.
The building was constructed using lightweight concrete over metal deck.
This format provided the most economical building solution to match new
floor heights with the existing structure. By making it easier to pump,
the vacuum-saturation of TXI’s expanded lightweight aggregate
product from their Boulder facility enhanced the placement of the
concrete for this project. It also offered significant dead load
reduction, reduced foundation size, and created more usable space per
floor.
"One of the many benefits of using lightweight concrete on the floor
decks of the Civic Center is that it helped save 1 1/4 inches per floor
in the height of the building, which equates to a savings in the curtain
wall alone of $55,000, and allowed us to align the new building floor to
floor with the existing 1950’s structure. It also provided savings
in the foundation, shaftwalls, and lateral system."
Michael McAffrey, a principal at Martin and Martin Consulting Engineers.
Advantages
Martin and Martin designs and specifies Structural
Lightweight Concrete projects across the nation. The benefits of
specifying Structural Lightweight Aggregate Concrete include the
following:
Dead Load Reduction – A reduced in-place density equates to
smaller foundations, or the ability to use a site with poor or marginal
soil conditions.
More Leasable Space – Reduced dead load often results in the
ability to utilize smaller columns. Smaller columns and/or potentially
greater spans create more gross leasable space.
Ability to Design to a Specified Density – Applying
“specified density” technology saves time and money. In the
precast/prestress market, manufacturers and specifiers have the benefit
of using concrete ranging from 100 to 135 lbs/ft3. Since more precast
elements can be shipped on a truck, this flexibility can result in
substantial transportation cost savings even with a modest density (unit
weight) reduction.
In the ready mix market, specified density concrete offers the designer
the maximum design flexibility in meeting project requirements and
accommodating artistic expression.
Structural Lightweight Concrete’s Superior Fire Ratings Reduce
Spray-On Fire Proofing – Lightweight aggregate concrete is more
fire resistant than normalweight concrete. A 3.75-inch thick lightweight
concrete panel will supply two-hour fire endurance. It would take 5
inches of normalweight concrete to supply equal endurance.
High Performance Seismic Design – Reducing the mass of a structure
reduces the seismic inertial forces.
Internal Curing – The porous nature of the lightweight aggregate
allows it to absorb water. This absorbed water contributes to the
continuous hydration of the cementitious materials. The absorbed water
is not part of the mixing water and does not effect the
water/cementitious ratio.
Construction
The Expanded Shale for the project was Vacuum Saturated by
TXI's Boulder, Colorado facility. This contributed to insuring the
construction schedule was met. “The contractor had implemented a
vigorous placement rate for the Structural Lightweight Concrete and the
Vacuum Saturation process for the aggregate greatly enhanced the
placeability of the concrete,” reported Miles Dee, Sales Manager
Colorado, TXI-Western Aggregates.
The concrete mixture was specified at 3500 psi with 15% Class F flyash
allowed in the total cementitious content. The slump tolerance was 4" to
6" at point of placement, with an equilibrium density of 110 lb/ft3 (+/-
3 lb/ft3).
