Use of Ground Penetrating Radar (GPR) for Measuring Air Void Content in Asphalt Pavements
Posted: 2013-5-24
Source:
PR Web
Little did Resource International, Inc. (Rii) know that a study conducted almost
six years ago for quality control measurements on new asphalt overlays would
open the door for this kind of innovative study to be more widely used today.
Rii conducted a study six years ago on a residential area road in Columbus, Ohio
to investigate the use of Ground Penetrating Radar (GPR) for quality control
measurements on new asphalt overlays. Now, the use of GPR is widely used
across the country today, and Rii is one of the few firms in the country qualified
to perform these services.
In the initial study that took place, GPR data was collected using dual air-
launched antennas mounted to the back of a moving vehicle (Figure 1). The 26-
foot wide road was scanned at intervals of 2-inch longitudinally and 2-foot
transversely. A typical GPR scan is shown in Figure 2.
According to the U.S. Asphalt Institute, the air void content for compaction of
asphalt pavements should be controlled between 7 percent and 3 percent. Once
voids reach 8 percent or higher, interconnected voids can develop which allow air
and moisture to permeate the pavement which reduces the durability of the
pavement. On the other hand, if air voids fall below 3 percent, there will be
inadequate room for expansion of the asphalt binder in hot weather, and when
the void content drops to 2 percent or less, the mix becomes plastic and
unstable.
Presently, core samples are the most commonly used technique for quality
control measurement on new asphalt overlays. However this technique has its
limitations and the results have not always been acceptable. GPR can be
considered as a better alternative to coring since it is nondestructive, can be
performed without closing lanes to traffic, and principally provides continuous
information and 100 percent coverage of the entire pavement.
The technique used to measure the air void content is based on the calculation
of the dielectric values of the asphalt pavement. A dielectric constant of a
material is an electrical property which is most influenced by the density of the
material. A strong correlation exists between air void content and the surface
dielectric of the pavement. The dielectric values are computed in the field by
measuring the amplitude of GPR surface reflection and the reflection obtained
from a metal plate. An increase in void content will result in lower dielectric
values of asphalt. Instead, a decrease in air void content in the asphalt mix will
cause an increase in asphalt dielectric. The method included taking some
calibration cores and determining their void content in the lab. The data was also
used to evaluate the GPR accuracy for pavement layer thickness determination.
The graph of Figure 3 shows a strong correlation between air void content and
the dielectric constant of the asphalt overlay. Air void contents of miles of
roadways can be evaluated by simply measuring the amplitude of the reflected
GPR signal in the field and converting it to surface dielectrics. A few cores are
needed to determine a relationship between the percentage of air void and the
dielectric constants. The results are best reported on colored contour maps
which give a general view of void content values (Figure 4). The GPR survey can
take place right after paving to identify locations with low asphalt density.
About Resource International, Inc.
Resource International, Inc. (Rii) is a broad-based, multi-disciplined professional
engineering consulting firm specializing in construction management,
information technology and planning and design of building and infrastructure
projects valued in excess of $1 billion annually.
This family-run business has evolved over the years to assist public and private
clients in finding optimum solutions to improve their environment and
infrastructure. Our specialized equipment, technology and resources offer the
best solutions.
Companywide, Rii has more than 150 employees engaged in transportation
planning, design, and construction services. Rii’s Corporate Office is located in
Columbus, Ohio, with full-service branch offices in Cleveland and Cincinnati,
Ohio; Pittsburgh, Pennsylvania; and Indianapolis, Indiana.
six years ago for quality control measurements on new asphalt overlays would
open the door for this kind of innovative study to be more widely used today.
Rii conducted a study six years ago on a residential area road in Columbus, Ohio
to investigate the use of Ground Penetrating Radar (GPR) for quality control
measurements on new asphalt overlays. Now, the use of GPR is widely used
across the country today, and Rii is one of the few firms in the country qualified
to perform these services.
In the initial study that took place, GPR data was collected using dual air-
launched antennas mounted to the back of a moving vehicle (Figure 1). The 26-
foot wide road was scanned at intervals of 2-inch longitudinally and 2-foot
transversely. A typical GPR scan is shown in Figure 2.
According to the U.S. Asphalt Institute, the air void content for compaction of
asphalt pavements should be controlled between 7 percent and 3 percent. Once
voids reach 8 percent or higher, interconnected voids can develop which allow air
and moisture to permeate the pavement which reduces the durability of the
pavement. On the other hand, if air voids fall below 3 percent, there will be
inadequate room for expansion of the asphalt binder in hot weather, and when
the void content drops to 2 percent or less, the mix becomes plastic and
unstable.
Presently, core samples are the most commonly used technique for quality
control measurement on new asphalt overlays. However this technique has its
limitations and the results have not always been acceptable. GPR can be
considered as a better alternative to coring since it is nondestructive, can be
performed without closing lanes to traffic, and principally provides continuous
information and 100 percent coverage of the entire pavement.
The technique used to measure the air void content is based on the calculation
of the dielectric values of the asphalt pavement. A dielectric constant of a
material is an electrical property which is most influenced by the density of the
material. A strong correlation exists between air void content and the surface
dielectric of the pavement. The dielectric values are computed in the field by
measuring the amplitude of GPR surface reflection and the reflection obtained
from a metal plate. An increase in void content will result in lower dielectric
values of asphalt. Instead, a decrease in air void content in the asphalt mix will
cause an increase in asphalt dielectric. The method included taking some
calibration cores and determining their void content in the lab. The data was also
used to evaluate the GPR accuracy for pavement layer thickness determination.
The graph of Figure 3 shows a strong correlation between air void content and
the dielectric constant of the asphalt overlay. Air void contents of miles of
roadways can be evaluated by simply measuring the amplitude of the reflected
GPR signal in the field and converting it to surface dielectrics. A few cores are
needed to determine a relationship between the percentage of air void and the
dielectric constants. The results are best reported on colored contour maps
which give a general view of void content values (Figure 4). The GPR survey can
take place right after paving to identify locations with low asphalt density.
About Resource International, Inc.
Resource International, Inc. (Rii) is a broad-based, multi-disciplined professional
engineering consulting firm specializing in construction management,
information technology and planning and design of building and infrastructure
projects valued in excess of $1 billion annually.
This family-run business has evolved over the years to assist public and private
clients in finding optimum solutions to improve their environment and
infrastructure. Our specialized equipment, technology and resources offer the
best solutions.
Companywide, Rii has more than 150 employees engaged in transportation
planning, design, and construction services. Rii’s Corporate Office is located in
Columbus, Ohio, with full-service branch offices in Cleveland and Cincinnati,
Ohio; Pittsburgh, Pennsylvania; and Indianapolis, Indiana.
