Optimal Mix
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New Concretes are Designed to Deliver
A tailored approach helps designers max out the benefits, not the
materials
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| The Helena, a LEED-registered residential development
in New York, incorporated 40 percent slag cement in its concrete mix.
Courtesy of Fox & Fowle Architects, New York. |
Some of the greatest concrete innovations today are focused on creating
new mixes. Supplementary cementitious materials (SCMs) are part of the picture,
redirecting industrial byproducts from landfills and supplying concrete
with greater durability, strength, and finishability. Name a design challenge,
and there’s a concrete mix with SCMs that can solve it.
There are four main kinds of SCMs: fly ash, ground granulated blast furnace
slag (also known as slag cement), silica fume and natural pozzolans. Of
the four types, fly ash and slag cement are the most widely used: in 2002,
the cement industry used 11.4 million metric tons of fly ash; shipments
of slag cement in 2003 topped three million metric tons.
Fly Ash and Slag Cement
Fly ash is a byproduct of the electrical power industry.
The material consists of captured particles that are left over after coal
is burned in power plants. Depending on the type of coal burned and other
factors, the fly ash produced will be classified as either Type C or Type
F. When proportioned properly, fly ash in concrete can increase workability,
easing pumping and finishing. It also increases strength and durability
by increasing density. Fly ash content can also guard against sulfate
attack, alkali-silica reactivity and efflorescence.
Slag cement is a byproduct of the steel industry. It
is created when iron ore is reduced to iron in a blast furnace. Molten
slag from a blast furnace is quenched with cold water, granulated, dried
and ground into a fine powder. It acts as a hydraulic cement, and when
proportioned correctly, slag cement in concrete will increase strength,
improve durability by reducing permeability, protect against chemical
attack and reduce instances of rebar corrosion.
Used individually or together with other admixtures, fly ash, slag cement
and other SCMs can create optimal concrete mixes for nearly every application.
Supplementary Cementitious Materials in the Spotlight
So why all the fuss about SCMs? Engineers and architects are specifying
concrete mixes with fly ash, slag cement and other SCMs to not only increase
the benefits for structures, but also to qualify for points toward LEED
Recycled Content credits. Use of specific proportions of SCMs can also
aid in earning a credit for Innovation in Design, Use of High Volume Supplementary
Cementitious Materials. And as LEED evolves and branches into more facets
of construction and design, there are even more opportunities to gain
from using SCMs.
But a few industry experts are concerned that some designers are specifying
excessive amounts of SCMs in order to achieve LEED credits, when in fact
a more optimized mix—designed with one or more SCMs and other components
for a specific application—would perform better.
“In the past year, I’ve participated in several green building
forums where a speaker recalls his use of 40 to 60 percent fly ash in
concrete on a green building project. Naturally, other architects want
to follow that example,” says David Shepherd, AIA, LEED-AP and PCA
director of sustainable development. “What many architects assume
is that fly ash is a filler material, but it actually has a significant
chemical influence on concrete. And because fly ash is a byproduct, performance
characteristics can vary from plant to plant. It’s not as simple
as picking an amount like 40 percent and throwing it into the mix. Within
one job site, there may be applications where no SCMs are appropriate,
while in others, high volume replacement is an ideal solution,”
Shepherd continues.
To that end, he says, design professionals should consider SCMs for application
optimization. “There is a trend of SCM maximization that’s
being driven by the green building movement, because projects get points
for meeting a minimum recycled content level,” he says. The key
is to work with local ready-mixed concrete suppliers to optimize mix design
with SCMs for the construction use.
Emphasis on Optimization
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| More than 60 percent of the 1400 5th Avenue project
in New York (aiming for LEED gold) is constructed with materials that
were recycled or are recyclable, and slag cement was used at a substitution
rate of 10 percent, Photo by Harold E. Rhynie, courtesy of Full
Spectrum of NY, LLC |
Nearly all facets of the concrete industry recognize that for concrete
mixes to evolve and continue to contribute to sustainable design and construction,
the focus on SCMs must move from maximization to optimization. “Designers
have an extraordinary number of tools with which to create really good
concrete mixtures for particular applications,” says Jan Prusinski,
executive director of the Slag Cement Association (SCA). “The challenge
for the designer is to recognize what he can use to achieve his objectives,
and do it properly. With the advent of high-performance concrete and self-consolidating
mixes, I see the industry moving toward more optimization.”
The fly ash industry recognizes the importance of optimization as well,
says Russell Hill, vice president of technology development for Boral
Material Technology, which markets fly ash. Success with fly ash “comes
from a combination of knowing the characteristics of the materials you’re
working with, but also doing some field testing, making changes in aggregate
proportioning and admixtures,” he says. “Concrete is a complex
system; it’s the sum of the performance of each component as they
work together,” says Hill.
Simply replacing a prescribed percentage of cement with an SCM just isn’t
good enough. Designers must become familiar with all the available options,
and get comfortable with the idea that the best results might not come
from a typically prescribed mix. “In some cases, that might require
a ternary mixture that combines silica fume, slag cement and portland
cement to achieve excellent alkali-silica reactivity mitigation, high
early strength, and workability,” says Prusinski.
Dave Goss, executive director of the American Coal Ash Association (ACAA),
agrees that ternary mixes are a solution in many applications: “It’s
a direction that many DOTs are going in, reducing costs and improving
quality,” says Goss. “It reduces the volume of individual
components, but opens up more opportunities when people…recognize
there’s some value in using two waste streams along with portland
cement. Everybody benefits.”
Architects aren’t the only people designing concrete; engineers
have a responsibility, too. “A LEED Accredited Professional is tasked
with minimizing resources and the impact of a structure,” says Ed
Alsamsam, PhD, PE, SE, LEED-AP, and manager of PCA’s buildings group.
“Then the engineer’s role is not necessarily to use less cement
in the mix, but to use less concrete in the structure—to come up
with the leanest, meanest building that uses concrete efficiently and
economically. That chain of thinking on economical design and efficient
materials goes to the heart of sustainable development,” says Alsamsam.
Eight Parameters for Optimization
Specifying the best concrete mix for a structure requires consideration
of a wide variety of factors, says Ed Alsamsam, manager of PCA’s
buildings group. Here is a quick look at eight parameters to consider
when optimizing concrete mixes using supplementary cementitious materials
(SCMs):
- Raw Materials – Examine the types and quality of sand,
aggregates, water, admixtures and cement.
- Application – Consider the resulting member or structure:
is it a beam, column, wall or slab?
- Climate – Address regional conditions (hot or mild, humid
or dry) and seasonal variations (winter or summer).
- Construction Requirements – Examine the process: will
the structure be pre-stressed or post-tensioned? Will forms be
stripped in 24 hours or 14 days?
- Location – Consider factors affecting the site itself,
including proximity to ready-mix concrete operations or traffic
patterns that can slow down trucks.
- Availability of SCMs – Find out which materials are readily
available in your area; transporting SCMs over great distances
will cancel out their sustainable benefit.
- Quality and Grade of SCMs – Evaluate the SCMs to ensure
they will work well with other mix components.
- Economics – Consider the costs related to each of the
other seven parameters to create an overall cost benefit analysis.
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Prescription to Performance: the P2P Initiative
One way the industry is tackling the optimization issue is through a program
organized by the National Ready Mixed Concrete Association (NRMCA) called
Prescription to Performance: the P2P Initiative. P2P proposes a shift
from the design industry’s traditional method of specifying concrete
by quantifying the ingredients within a mix design, to specifying the
performance required of the product in its application.
NRMCA’s senior director of applied engineering, Lionel Lemay, PE,
SE, says the program recognizes the need to balance LEED certification
requirements with other environmental concerns. “We want quality
concrete that is environmentally friendly,” says Lemay. “LEED
is always going to be prescriptive in nature, but it does leave a lot
of initiative and flexibility to the designer.” Designers don’t
have to feel that they must max out SCMs to achieve enough credits to
have their building LEED certified.
Lemay says mix design is only one small component of the big concrete
picture: “There are many other opportunities within LEED to gain
points and contribute in a positive way to the environment: light colored
pavements to reduce heat islands, green roofs and pervious pavements to
reduce storm water runoff, high-performance wall systems that incorporate
thermal mass insulation to improve energy efficiency,” says Lemay.
“Recycled content within materials is just one step on the path
to minimizing environmental impact.”
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