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The Selfridge Air National
Guard Base Project
Soil and Materials Engineers, Inc.
(SME) has been formulating concrete
mixtures for more than 35
years. During that time there have
been numerous new admixtures, cements,
and aggregates introduced to improve the
Portland cement concrete used in construction.
As a CCRL-certified laboratory, SME
performs concrete-mix designs for all types
of construction.
In the past few years, SME has discovered
different ways to proportion the concrete
paving mixtures for Michigan Highways.
These have included varying the percentage
of coarse aggregate based on the dry,
loose, or rodded unit weight of the coarse
aggregate; the blending of larger particles,
4AA, with smaller particles, 6AAA; and
now the dense-graded or Shilstone method
of blending all the aggregates to meet a
grading band. This article is about the experiences
SME has had with the dense-graded
mixtures at the Selfridge Air National
Guard Base.
Supplemental Specification
The specification for this project
employed a U.S. Air Force supplemental
specification that called for blending the
aggregates to meet the 8-18 percent retention
guidelines in the Shilstone design
method, which produces a dense-graded
aggregate blend. In addition to the dense
grading, the specification had very stringent
aggregate quality standards and a
maximum water cement ratio of 0.45.
Based on the aggregate quality standards,
Port Inland Limestone was chosen for the
coarse aggregate and a local 2NS was
selected as the fine aggregate supplier.
Once the sources were established, SME
received gradations of the various products
from the aggregate suppliers. The gradations
were used to design a blend to meet
the 8-18 design curve.
Avoiding ASR
To comply with the criteria, it was necessary
to blend a fine limestone, Ohio #8,
with a MDOT 6A material. Blended
aggregate samples were then submitted
for ASTM D-1260 testing to determine
ASR (Alkaline Silicate Reaction) potential.
The test results indicated that there
was a potential for ASR using what was
considered a local low-alkaline cement.
The ASR potential meant either the sand
source had to be changed or the cement
needed to be blended with Type F fly ash
or ground granulated blast-furnace slag
cement to reduce the reaction potential.
Changing the sand source would have
meant the importing of sand, because
all of the local sources have similar
characteristics.
An Advantageous Solution
Ajax Paving Industries, Inc. chose to use
the slag cement. To further reduce the
potential for ASR, with the consent of the
design engineer the cement content was
lowered from a minimum of 565 pounds
of total cement to 480 pounds per cubic
yard. Another D-1260 test with 40 percent
of the Type I cement replaced with
slag cement was performed. The ASR
potential was low, and the trial-batch test
results indicated good flexural strengths.
Using a mix of slag and Portland cement,
the flexural strengths of the trial batches
achieved the design strength of 700-psi
flexural in 28 days after seven days of
laboratory curing. The 28-day flexural
strength results for this mixture averaged
more than 1,100 pounds. The entrained
air content was 6.0 percent, the slump
was 1.5 inches, the water cement ratio was 0.45, and with a yield determination
of 27.04 cubic feet per cubic yard. At that
point, Ajax Paving had a design mix to
use on the project, and the work of producing
the mixture on-site had just begun.
Precise Conditions
The specification had several special
requirements for quality control that
are not normally enforced on a paving
project. These included the testing of the
coarse and fine aggregates every 200 tons
initially, and then every 500 tons only if
the first 2,000 tons were uniform; evaluating
the concrete every 500 cubic yards;
plotting all test results on wall graphs;
and producing daily reports signed by all
parties involved prior to allowing the
next day’s placement.
The on-site testing gradations varied
slightly from the gradations used for the
mix design. As a result, the proportions
of the aggregates were adjusted to keep
the end-result aggregate grading within
the 8-18 guidelines. Subsequent testing of
the gradations revealed additional variations
that required changes in the proportions
to keep the blended gradation in the
specified grading bands. These changes
were relatively easy to predict with the
constant monitoring of the gradations.
Monitoring Gradations
SME personnel would observe the gradations
drifting either to the fine or coarse
side of the original gradation on the wall
charts. Once the difference began to
affect the blended gradation, the proportions
were adjusted to bring the blend
back within the guidelines. This was
achievable with the three aggregate blend.
If Ajax Paving had chosen to use a single
blended coarse aggregate, instead of two
sizes of coarse aggregate, the project
would have had to be halted to blend
a new stockpile of coarse aggregate.
The plastic concrete testing consisted of
air content, slump, temperature, and
unit weight for yield tests, along with the
molding of six beams for flexural strength
analysis every 500 cubic yards. With the
aggregates being tightly controlled for gradation,
there were only minor variations
in the slump, air content, and yield tests.
This uniformity was also reflected in the
flexural strengths, which produced the design
strength within four days of casting.
These tasks involved a lot of work to
keep the concrete within the stringent
specification guidelines. Ajax Paving personnel
worked closely with SME’s on-site
staff to produce a concrete that was uniform,
paved exceptionally well, and able
to achieve design strength in an average
of four days cure time. Based on this partnership
in quality, there were no concrete-related
failures during the project.
For more information contact SME at (734)
454-9900 or visit www.sme-usa.com |
