July/August
2002
Taking
Concrete to the Next Level
by Marcia J. Simon and
Michael P. Dallaire
Are you
building a long-lasting, effective road? Two elements contribute to
the performance of concrete and the lifetime of pavements the concrete
mixture itself and the construction practices engineers use to mix,
place, and cure the concrete. The "perfect" mixture combined
with bad practices can produce an inferior result with problems such
as cracking, poor concrete durability, and premature deterioration.
Likewise, a poor mixture coupled with good practices also can produce
a substandard result. In either case, the outcome will be additional
repair or replacement costs, user delays, and more congestion.
|
Turner-Fairbank
Highway Research Center (above). |
The Federal
Highway Administration's (FHWA) Portland Cement Concrete Pavement
(PCCP) team at the Turner-Fairbank Highway Research Center in McLean,
VA, is studying various aspects of concrete performance, including
fresh (before hardening) and hardened concrete properties. Questions
they are addressing in the lab include:
- Can
statistical optimization methods be used to identify the appropriate
concrete mixture for projects with multiple performance requirements?
- Do
the standard "rules of thumb" for an adequate air void
system apply to all concretes exposed to freezing and thawing conditions?
- Is
there a fast and reliable way to evaluate the susceptibility of
concrete mixtures to expansion caused by alkali-silica reaction?
- How
can a design engineer accurately measure the coefficient of thermal
expansion for concrete?
- What
better test in the field might be used in place of slump for testing
workability of low-slump paving concrete?
These
studies have a common emphasis—a focus on improving concrete,
through better material selection and proportioning, and improving
lab and field test performance predictions.
Mixture
Optimization Using Statistical Methods
The advent
of high-performance concrete made the process of proportioning concrete
mixtures more complex. Using the right mix proportions for each project
is extremely important, and designers must deal with several constraints,
such as:
- Component
material availability and properties varying widely throughout the
country
- Climate
and design constraints leading to different performance requirements
for each project
- Material
prices continually increasing
- Multiple
engineering performance requirements (in addition to strength) needing
to be met simultaneously
A means
of optimizing concrete mixtures (meeting several performance requirements
while minimizing costs) could result in material cost savings and
more confidence that a concrete mixture will meet specifications.
For example, reducing the cost of concrete by $10 per yard could yield
savings of $10,000 per 765 cubic meters (1,000 cubic yards) of concrete,
or about 0.8 lane-kilometer (0.5 lane-mile) of 30.5-centimeter (12-inch)
concrete pavement.
|
Impact
hammer hits the concrete to measure the internal damage in the
concrete due to freezing and thawing. |
Although
the American Concrete Institute's (ACI) 211 Guide for Proportioning
Concrete Mixtures and other procedures are good starting points
for concrete proportioning, they do not provide information on the
optimal proportions for meeting several performance criteria at the
same time. As a result, engineers normally consider one factor at
a time using a trial-and-error approach, which can be inefficient
and costly, and may not produce the best combination of materials.
To replace
the inefficient trial-and-error method, FHWA looked outside the transportation
industry for successful optimization approaches. Response surface
methods (RSMs) are a set of statistical tools that other industries
use for optimizing products such as gasoline, foods, and detergents.
Since these products, like concrete, are blends of various component
materials, the PCCP team and researchers and statisticians from the
National Institute of Standards and Technology investigated whether
RSM also might be applicable to concrete. The objectives of this study
were to determine whether statistical optimization methods are applicable
to concrete mixture proportioning and, if so, to develop an interactive
Web site tool to enable prospective users to learn about and try this
approach.
In the
initial portion of the project, the team performed laboratory experiments
to evaluate two possible approaches: a classical mixture design and
a central composite design (CCD). The materials used in both cases
were Type I cement, silica fume, high-range water reducer, natural
sand, and crushed limestone. The team optimized the following properties:
1-day and 28-day strength, slump, and resistance to chloride penetration
as indicated by the American Association of State Highway and Transportation
Officials' (AASHTO) T277 test (rapid chloride test). The experiment
design (classical mixture or CCD) defined the mixture proportions
for the lab experiments. Upon test completion, researchers analyzed
the data, estimated models using linear regression, and employed graphical
techniques, such as means plots, scatter plots, and contour plots
to interpret the results.
Based
on the experimental results, both classical mixture and CCD can be
used in concrete mixture proportioning. However, because the CCD approach
is more straightforward, the research team chose it for Web site development.
The aim
of the interactive Web site, Concrete Optimization Software Tool (COST),
is to introduce the procedures for using the statistically based (CCD)
approach and to guide the user step-by-step through the planning and
execution of a set of trial batches, and analysis of the results.
The optimization procedure in COST involves six steps:
- Steps
1 and 2: The user inputs information about materials and performance
requirements, and COST generates mixture proportions for a set of
trial batches (according to the CCD design).
- Step
3: The user batches the concrete, fabricates specimens, and
tests them.
- Step
4: The user enters the results into COST.
- Step
5: COST performs several analyses.
- Step
6: COST provides a summary of the analysis and optimal settings.
Because
potential COST users may not have a statistical background, COST depicts
many of the analysis results graphically and provides some guidance
on interpretation. A user's guide is available online and will be
published shortly. The final report for the project is slated for
publication in September 2002.
Freeze-Thaw
Resistance Of Concrete with Marginal Air Contents
The concrete
industry has known the benefits of air entrainment (intentionally
incorporating microscopic air bubbles into a concrete mixture) for
more than 60 years. An adequate entrained air void system is considered
necessary to prevent damage from freezing and thawing cycles, which
ultimately affects the durability of concrete. The commonly accepted
rules of thumb for a good air void system include: air content of
6 percent generated by a chemical admixture during mixing, specific
surface greater than 600 in2/in3, and a spacing
factor between air voids less than 0.02 centimeters (0.008 inches).
However, some studies show that concretes with marginal air void systems
(not meeting the accepted criteria) may still be resistant to freezing
and thawing. Also, there is some debate in the industry about the
need for air entrainment in concretes with sufficiently low water-cement
ratios. The objectives of this study are to investigate the freeze-thaw
durability of concrete with marginal air void systems and to suggest
possible improvements to freeze-thaw testing procedures.
|
This
screenshot shows the COST homepage at http://ciks.cbt.nist.gov/cost.
COST is an online design-analysis system to assist concrete
producers, engineers, and researchers in determining optimal
mixture proportions for concrete. |
Freeze-thaw
testing by AASHTO T-161 and the American Society for Testing and Materials
(ASTM) C666 exposes concrete to a temperature cycle of 54 to 32 to
54 degrees Celsius (40 to 0 to 40 degrees Fahrenheit) in 2 to 5 hours.
Researchers remove specimens from the freeze-thaw machine periodically
(every 10 to 30 cycles depending on condition) and assess them for
visible changes, internal damage, and changes in mass. Internal damage
is evaluated nondestructively using the impact method (ASTM C 215)
to determine the resonant frequency of the test beam. Relative dynamic
modulus (a measure of internal damage) and durability factor are calculated
from the resonant frequency.
There
are presently two accepted variations of the freezing and thawing
procedure. Procedure A submerges the test beams in water during freezing
and thawing. This procedure is not representative of field conditions
due to confinement of containers used and is considered more severe
than field conditions, where periodic drying can take place. Procedure
B exposes test beams to air during freezing and submerges the beams
in water during the thawing cycle. This method is more realistic,
but concerns were raised about the possibility of excessive specimen
drying during the freezing cycle. In response to these concerns, the
Strategic Highway Research Program (SHRP) proposed a variation researchers
wrapped test specimens in removable terry cloth covers and tested
them according to Procedure B. The terry cloth protects the concrete
surface from excessive drying.
Researchers
in the SHRP project also identified the quality factor as a
potential parameter for assessing damage to concrete specimens during
testing. They found that using the quality factor as a damage measure
might enable researchers to predict the freeze-thaw test result in
significantly less time (100 to 150 cycles). In this study, the PCCP
team is looking at the validity of the quality factor approach, which
could reduce the cost and time required for freeze-thaw testing significantly.
|
Freeze-thaw
specimens in the testing machine are labeled according to the
procedure they are undergoing (Procedure A specimens are in
metal containers, Procedure B no container or cover,
and SHRP variation terry cloth covers). |
Concretes
mixtures with marginal air contents are being tested using standard
AASHTO-ASTM procedures and the terry cloth procedure. Researchers
are calculating the quality factor in some of the experiments to evaluate
its potential for predicting the durability of concrete before 300
cycles. The experimental program has several phases.
Test
Phases Procedure
A: Test beams submerged in water during freezing and thawing
Procedure B: Test beams submerged in water during freezing and
air during thawing
Terry Cloth Variation: Test beams are wrapped in removable terry
cloth covers and tested according to Procedure B |
The first
phase of research tested concretes with fresh air content levels of
2.7, 2.9, and 3.1 percent using the A, B, and terry cloth test procedures.
For each air content level and test procedure, researchers tested
five beams. The findings from the terry cloth procedure in most cases
indicate a severity level equal to or greater than the cases tested
using procedure A. There appeared to be less variability between results
from procedures A and B. Mass loss was considerably greater in procedure
A due to scaling.
Currently
in progress, the second phase involves testing concretes with a wider
air content range from 2.5 to 4.5 percent. Other variables in this
phase include water-cement ratio (ranging from 0.40 to 0.50) and two
different types of air entraining admixtures (AEA)vinsol resin and
a synthetic AEA. In addition to performing freeze-thaw testing, the
researchers will determine hardened air void system parameters for
each mix and calculate and evaluate the quality factors.
Mixture-Specific
Method For Prediction of ASR Expansion
Alkali-silica
reactivity (ASR) occurs when alkalies in cement react with certain
types of silica in concrete aggregates to form an alkali-silica gel.
This expansive gel can absorb water and expand, causing pavements
to crack and eventually fail. Currently, there is no rapid test method
claiming to evaluate the ASR susceptibility of concrete mixtures.
The ASTM C 1260 (mortar bar) test method is specified as a test to
assess aggregates and not combinations of aggregates and cementitious
materials (although some researchers have investigated its use for
that purpose). The concrete prism test developed in Canada (ASTM C1293)
is more reliable, in that it tests concrete rather than mortar, but
the mixture proportions for the concrete are prescribed. Another drawback
of the prism test is that it takes a year or more to perform. Other
methods have been suggested or tried, but are not recommended because
of limited data. The objective of this study is to identify a fast,
dependable test method for assessing the ASR potential of a concrete
mixture.
The approach
in this study is to perform multiple prism tests varying the water-cement
ratios (w/c), cement content, fly ash content, and lithium dosage
for a given aggregate. A factorial experiment design will be used.
Concrete prism tests will be performed at 38 degrees Celsius (100.4
degrees Fahrenheit) for 1 year (standard ASTM C1293 conditions) and
at 60 degrees Celsius (140 degrees Fahrenheit) for 3 months. Tests
also will be performed at 60 degrees Celsius using modified prisms
with small longitudinal holes developed at the University of New Hampshire.
The test results will be used to estimate a predictive model that
can be used to predict expected ASR expansion anywhere within the
ranges used for w/c, cement, fly ash, and lithium. Testing began in
July 2002.
Thermal
Coefficient of Concrete Cores from LTPP Sections
Researchers
at FHWA have developed a precise method for determining concrete's
coefficient of thermal expansion (CTE), a property that affects pavement
design and subsequent performance. The CTE is an important element
of FHWA's Long-Term Pavement Performance (LTPP) database and a critical
element in the new AASHTO 2002 Design Guide for concrete pavements.
The information generated by this precise method will lead to longer-lasting,
smoother roads and will improve the pavement design process by better
matching pavement to its environment. Before FHWA's new testing procedure,
pavement designers usually assumed an average value of the concrete
CTE in their designs.
A literature
review revealed that several methods were developed over the years,
but none were widely used within the transportation industry. Turner-Fairbank
Highway Research Center (TFHRC) researchers were asked to develop
a standard test for measuring CTE and to use that test to measure
the CTE on cores from LTPP pavement test sites. Desired characteristics
of the test method were accuracy, repeatability, ease of use, and
relatively low cost.
Calculation
of CTE The
CTE is calculated using the above equation: where DDL = length
change (cm), L0 = initial specimen length (cm), and
DT = temperature change (°C). Of the three required measurements,
the most difficult to obtain is specimen length change, because
it is very small (on the order of hundred-thousandths of inches).
Linear variable differential transformers (LVDTs) were identified
as appropriate devices for measuring this length change. |
Another
issue involved the moisture condition of test specimens during testing.
The thermal coefficient of concrete varies with internal relative
humidity (RH), with the maximum value usually occurring at 60 to 70
percent RH. The value at 100 percent RH is 20 to 25 percent less than
the maximum. However, the fully saturated condition is the most practical
from a testing standpoint. Furthermore, pavements in the field have
an internal RH of 80 percent or more, except the region near the surface.
The test
frame developed by FHWA features vertical support rods made of Invar,
a very low-CTE steel, to minimize frame expansion during heating.
The LVDT is centered above the specimen, which is supported by three
hemispherical points. The frame is placed in a water bath to submerge
the test specimen completely (but not the LVDT). The frames are calibrated
(to correct for any frame expansion) using 10-centimeter diameter
by 18-centimeter long (4-inch diameter by 7-inch long) A304 stainless
steel cylinders.
The final
test method includes the following steps: sawing the cores to a standard
length (178 mm/7 in.), grinding the ends parallel, soaking the cores
to reach SSD condition, mounting the core in a measuring frame with
an LVDT, placing the frame in a controlled temperature water bath,
varying the temperature of the bath between 10 and 50 degrees Celsius
(50122 degrees Fahrenheit), and reading the length change over
that temperature range.
AASHTO
recently approved the new test method as a provisional test method
TP60-00, Standard Test Method for the Coefficient of Thermal Expansion
of Hydraulic Cement Concrete. It is included in the 2000 edition of
the AASHTO Provisional Standards. The pavement team researchers are
using the test in-house to measure the thermal coefficient of concrete
pavement cores from around the country. The LTPP program provided
about 2,500 cores for CTE testing, and tested about 600 cores on a
continuing basis, with an average of 40 to 50 tests a month.
Workability
of Paving Concretes
The slump
test is the most widely used test for assessing concrete workability.
However, slump is, at best, a tool for monitoring production variation
from batch to batch. Several other test methods and devices were developed
to characterize the rheological properties of concrete, but these
devices will not work with the low-slump mixtures typical of paving
concrete. Under an interagency agreement, the U.S. Army Corps of Engineers
recently developed a workability-measuring device for FHWA. This device,
called the vibrating slope apparatus (VSA), quantifies workability
from measurements of mass loss as a function of time from an inclined
chute at a given level of vibrator energy.
|
The
thermal coefficient test method developed at TFHRC uses an Invar
frame, an LVDT, and a 10-centimeter by 18-centimeter (4-inch
by 7-inch) concrete specimen. The frame is submerged in a water
bath during the test. |
FHWA
is performing a follow-up study on the VSA with the following objectives:
- To
evaluate the operation of the VSA and the validity and interpretation
of the workability parameter it provides
- To
modify the device and procedure as needed, based on the results
of the evaluation
- To
use the VSA on a range of concrete paving mixtures to determine
which factors have a significant influence on workability of paving
concrete
PCCP
team researchers evaluated the prototype VSA and tested it for operational
characteristics, ruggedness, and ease-of-use. They also performed
an initial assessment of the validity of the test result and its interpretation.
Based on this evaluation, several design modifications were recommended,
and three new VSAs were built using the revised design. Two of these
VSAs will be taken to the field with the FHWA Mobile Concrete Laboratory
for use on actual paving projects. The remaining VSA will stay at
TFHRC for the next phase of laboratory research beginning in July
2002. This follow-up study will include assessment of test factors
(concrete slump, chute angle, and vibration force) and evaluation
of aggregate effects on workability.
|
Prototype
version of the Vibrating Slope Apparatus (VSA) in operation. |
References
1. Myers,
R.H. & D.C. Montgomery, Response Surface Methodology: Process
and Product Optimization Using Designed Experiments. New York:
Wiley, 1995.
2. ACI Committee 211, "Standard Practice for Selecting Proportions
for Normal, Heavyweight, and Mass Concrete." In ACI Manual
of Concrete Practice, Volume 1. Detroit: American Concrete Institute,
1995.
3. Simon, M.J., Lagergren, E.S. and K.A. Snyder, "Concrete Mixture
Optimization Using Statistical Mixture Design Methods." In Proceedings
of the PCI/FHWA International Symposium on High Performance Concrete,
New Orleans, October, 1997, pp. 230-244.
4. Simon, M.J., Lagergren, E.S. and L.G. Wathne. "Optimizing
HPC Mixtures Using Statistical Response Surface Methods." In
Proceedings of the 5th International Symposium on Utilization of High
Strength/High-Performance Concrete. Norwegian Concrete Association,
Oslo, Norway, June, 1999, pp. 1311-1321.
5. Janssen, Donald J. and M.B. Snyder, Resistance of Concrete to
Freezing and Thawing. Strategic Highway Research Program Report #SHRP-C-391.
National Research Council, Washington, D.C. 1994.
6. Touma, W.E., Fowler, D.W., and R. L. Carrasquillo, Alkali-Silica
Reaction in Portland Cement Concrete: Testing Methods and Mitigation
Alternatives. Research Report ICAR 301-1F, International Center
for Aggregates Research, Austin, Texas, 2001.
7. AASHTO TP-60-00, "Proposed Standard Test Method for the Coefficient
of Thermal Expansion of Hydraulic Cement Concrete." American
Association of State Highway and Transportation Officials, 2001.
8. Wong, G.S, et al, Portland Cement Concrete Rheology and Workability:
Final Report. FHWA-RD-00-025, Federal Highway Administration,
McLean, Virginia, 2001.
Marcia
Simon is a research materials engineer in the Office of Infrastructure
R&D at the TFHRC. For the past 12 years, she has been involved
in research on concrete materials and recycling. Her primary research
areas are concrete mixture optimization and concrete performance,
especially durability. She received a B.S. from MIT and an M.S. from
Cornell University, and she is a registered professional engineer
in Virginia.
Michael
P. Dallaire is currently a concrete materials engineer with SaLUT
onsite at TFHRC and oversees concrete laboratory operations. He is
a graduate of the University of New Hampshire with a master's degree
in civil engineering. His interests include the durability of high-performance
concrete for pavements and structures with specific interests in cracking
and shrinkage performance.
Other
Articles in this issue:
Taking
Concrete to the Next Level
Getting It Together
Fine-Tuning Innovative Technologies
On the Road Testing Roads
Paving the Way
Making Roads Better and Better
Texas Tests Precast for Speed and Usability
The Biggest Bang for Your Buck
New
Software Promises to Put Whitetopping on the Map
Road
Map to the Future