High-Strength Concrete Comes of Age
New Technologies Push Strengths to New Heights
As
recently as 25 to 30 years ago, the steadily increasing availability of
concrete with compressive strengths above 6,000 psi had the industry’s rapt
attention. Back then, in many regions, strengths this high were considered
revolutionary. Some people even questioned the need.
Fast forward to
2014, and designers are specifying strengths of 10,000 psi, 12,000 psi and
higher. A new 42-story residential tower in downtown Seattle is one of the
first in the country to use 15,000 psi concrete in its columns, reflective of
the recent advances in concrete materials technology. Used effectively, this
“super concrete” allows smaller columns, shorter and thinner shear walls, and
reductions in other structural elements. This results in additional interior
real estate. And, when this concrete is paired with modern high-production
formwork and the advanced pumping technologies now available, the sky is quite
literally the limit.
Concrete
Advancements
Concrete has
been used as a building material for thousands of years and has played a role
in the construction of some of the world’s most prominent structures. For
example, the Pantheon in Rome – its unreinforced cast-in-place concrete dome
completed in 126 AD – still stands today. The icon’s longevity underscores the
superiority of its design and construction. In fact, one wonders if the
structure could be improved upon if rebuilt today. Since that time, however,
concrete’s use has been extended upward into towers of nearly 2,000 feet in
height, and it is for these structures that high-strength concrete has found a
home.
Concrete
strength started trending upward in the last century when high-range water
reducers began gaining prominence. Until the 1950s and 1960s, concrete
suppliers struggled to create stronger concrete without sacrificing
workability. It was well known that concrete’s strength could be increased
through a reduction in water content, but, without a way of preserving
workability, the side effect of excessive water reduction was an extremely
low-slump material that was nearly impossible to place. When material
scientists began developing more effective chemical water reducers, however,
the problems associated with stiff, low slump concrete began to disappear.
“Superplasticizers” such as “Mighty 150” and others hit the
market and quickly became popular, gaining mainstream use by the 1980s. These
new high-range chemical water reducers allowed the creation of concrete with
both higher strength and adequate workability. Research and
development into superplasticizers saw a steep climb during this time,
simultaneous with increased R&D associated with the use of cement
replacements such as fly ash, silica fume, and blast furnace slag. The result
was the development of the modern high-strength concretes in use today.
As concrete
strengths have increased, forming systems and pumping technology have also made
significant gains, allowing structural engineers and architects to consider
concrete for use in buildings that would previously have been designed using
other materials. Concrete forming systems that were formerly “hand set” have
largely been replaced by well engineered components that can be assembled and
disassembled quickly, allowing higher field productivity and faster
construction. Pumping technology has also advanced, eliminating the need for
bucketing on even the highest of structures. Concrete for the Burj Khalifa
(formerly the Burj Dubai) in the United Arab Emirates, in fact, was pumped to a
height of almost 2,000 feet, at which point structural steel continued upward
to complete the 163 story, 2,722 foot structure. The Burj Khalifa was completed
in January 2010.
Concrete Columns
Today,
high-strength concrete has found its niche primarily in the columns of
high-rise towers. Just a few decades ago, many designers believed that concrete
was inappropriate for tall buildings, since the then lower strength mixes
available would require column sizes of prohibitive proportions. These
excessive column sizes would interfere with floor layouts and monopolize
valuable leasable space. Now, with higher strengths more widely available,
column sizes can be reduced and often remain constant for a building’s full
height, with the concrete strength decreasing at higher levels of the
structure. With standardized column sizes and fewer changes over a building’s
height, formwork costs drop and schedule impacts are minimized.
One of the
tallest new residential towers in downtown Seattle, Premiere on Pine, is a
42-story concrete structure that uses 15,000 psi column concrete at the base, reducing
to 8,000 psi at the upper levels. Structural designers decided to amp up the
column strength to take advantage of concrete’s efficiencies and maximize the
available leasable space. Scheduled for completion in April of 2015, Premiere
on Pine took maximum advantage of the high strength concrete available in the
Seattle market. Columns sizes of 24 inches x 30 inches and 20 inches x 42
inches are constant nearly to the structure’s top, which maximized formwork
reuse and helped maintain the rapid construction pace. Column sizes for
buildings of this height are commonly 20% to 40% larger. Premiere on Pine’s
smaller columns increased net rentable square footage when compared to
competing towers.
Seattle isn’t the only city breaking
records for its construction of high-rise concrete buildings. Two 400-foot,
side-by-side residential towers are expected to break ground in Los Angeles by
the end of this year, becoming the tallest reinforced concrete structures ever
built in that city. High-strength concrete is expected to be used in these
building’s columns as well.
While high-strength concrete is an obvious choice for columns in
tall towers, it also serves a purpose in shearwall and rigid frame construction
in these same buildings. Both the strength and the stiffness of shearwalls and frames
are increased; and, since lateral systems are often drift controlled, the added
stiffness is highly beneficial. On the 450-foot tall expansion of Lincoln
Square in Bellevue, Washington, for example, designers specified 14,000 psi for
strength, but also specified an elastic modulus of 5,700 ksi to control drift,
a major challenge in the design of tall buildings.
Different Markets, Different Concrete
The upper strength
limit for concrete in a given region is largely aggregate dependent. Some
regions have access to better aggregate than others and, as a result, are able
to produce higher strength concrete. Chicago and Seattle, for example, have
local access to quarries that produce granite aggregate, a relatively
non-porous igneous rock that is exceptionally suitable for concrete. Granite is
hard and dimensionally stable. It absorbs minimal water, unlike soft limestone
or other sedimentary rock found in some other markets, like Los Angeles, Texas
and Florida. Such aggregate also tends to be more porous and angular,
increasing the required water quantity and making the production of higher
strengths more challenging.
Historically,
geography has limited the type of concrete used in a given location since
transporting aggregate great distances tends to be too costly to make financial
sense. The volume of column concrete used on a high-rise project is relatively
small, however, compared to the overall project’s concrete needs. Thus, some
structural engineers are specifying high-strength column concrete even if the
necessary aggregate isn’t locally available, opting to sacrifice material
savings for gains in productivity and additional interior square footage.
While
high-strength concrete’s use is growing, lower-strength concrete still does the
lion’s share of the work in concrete construction. The majority of buildings
constructed in the U.S. are small enough that high-strength concrete isn’t
warranted. Further, many structural elements such as floor slabs and
foundations don’t benefit significantly from the higher-strength material.
Nevertheless, as the use of concrete increases and buildings go ever taller,
the demand for high-strength concrete is likely to continue growing.▪
The
Challenges of High-Strength Concrete
The use of
high-strength column concrete comes with its share of challenges. Since it
doesn’t make financial sense to pour floor slabs with a similarly high strength
mix – 4,000 to 6,000 psi is typically sufficient for slabs and other flexural
elements – transferring column loads through lower strength floor plates can be
difficult.
If the slab confinement requirements of
ACI 318 are met, column concrete strength can exceed the slab concrete by as
much as 2.5 times, and a blended strength is used for the column’s design. If
not, however, there are several ways to retain a high-strength column’s
integrity as it passes through a floor plate.
One is to
increase the slab strength at the slab/column intersection by “puddling”
high-strength concrete into the column area. The high-strength concrete is
poured immediately before the lower-strength floor slab so that the two
concretes can be intermixed and the possibility of a cold joint eliminated.
While this technique is workable in theory, it can be difficult to execute in
the field. It requires precise timing of concrete deliveries and a skilled
field crew to ensure that cold joints do not occur.
A less common,
but often more effective approach, is to hold back all but several inches of
slab concrete from the column perimeter when the slab is poured, leaving an
opening for the high-strength column concrete to pass through from above. This
eliminates the need to puddle concrete and ensures the integrity of the column.
With this approach, shear-friction rebar is typically added through the joint
to supplement the strength of the connection.
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