Safe driving depends on adequate traction for normal vehicle
maneuvering, turning, and braking. What methods are states using
to evaluate the skid resistance of hot-mix asphalt concrete
(HMAC) pavement surfaces on their highways? To learn the answer,
the Department of Civil Engineering at Texas Tech University
conducted a survey of methods for evaluating skid resistance as
part of a three-year research study sponsored through a
cooperative program of the Federal Highway Administration (FHWA)
and the Texas Department of Transportation (TxDOT).
Researchers P.W. Jayawickrama, R. Prasanna, and S.P.
Senadheera reported on the results of this study in "Survey of
State Practices to Control Skid Resistance on Hot-Mix Asphalt
Concrete Pavements." The paper appeared in the Transportation
Research Board's Transportation Research Record 1536 published in
1996.
Background
HMAC pavement should remain stable under vehicle
acceleration, deceleration, and vertical loads. Moreover,
vehicles turning and braking normally should not slide or skid on
these surfaces. The frictional resistance of a paved surface is
quantified as a skid number (SN). Defined as "the ratio between
the frictional resistance acting along the plane of sliding and
the load perpendicular to this plane," the skid number is an
important safety factor for states to consider when selecting
materials for roadway design and construction.
Good skid resistance results from controlling the
microtexture and macrotexture of HMAC pavement surfaces.
Microtexture refers to fine-scale grittiness on the pavement's
surface produced by coarse aggregates. State Departments of
Transportation (DOTs) select microstructure materials based on
their initial roughness and on their ability to retain roughness
when exposed to the polishing action of traffic. Macrotexture
refers to the large-scale roughness obtained through the
arrangement of aggregate particles. The shape, size, and
gradation of coarse aggregates determines this texture.
Properties of the mix and factors in the environment (such as
temperature) affect how the macrotexture will stand up to
traffic.
To create a safe, skid-resistant HMAC pavement surface, DOTs
must use good-quality, polish-resistant materials combined in a
way that provides appropriate aggregate gradation and mix
stability. The research described in this paper examined the
procedures used in the various states (excluding Alaska and
Hawaii) for evaluating HMAC pavement materials and for setting
standards to define acceptable skid-resistance performance in new
pavement and over the life of paved surfaces.
Survey Procedure
As part of a Texas Tech University three-year project,
researchers conducted this survey "to obtain information on the
current skid control practices used by the different state DOTs
in the contiguous United States." They began by identifying a
representative from each state directly involved with skid
control. In a telephone interview, researchers collected
information on the state's skid-control practices. A
questionnaire followed the telephone interview and obtained more
detailed information. Respondents returned with the
questionnaire any additional information they felt would help
describe their state's practices.
Seventy-four percent of the states returned a completed
questionnaire. Many attached laboratory test procedures,
research reports, and (or) design specifications and guidelines.
If representatives did not return questionnaires or the
information was unclear, researches made a second telephone
inquiry. When researchers needed still more information, they
relied on published data.
Summary of Survey Responses
Issues addressed in the survey included: (1) HMAC pavement
design procedures for good skid resistance, (2) methods and
equipment used in measuring skid resistance in the laboratory and
in the field, and (3) threshold values for acceptable levels of
skid performance.
The most common method of evaluation reported by state DOTs
was the locked-wheel skid test following ASTM (American Society
for Testing and Materials) E274. Using ASTM E274 specifications,
states described skid numbers (SNs) of 30 and above as acceptable
for low-volume roads and 35-38 as acceptable for heavily traveled
roads. Maryland, Minnesota, and Pennsylvania, however, used 40
as their acceptability cutoff. When SNs fell to 34 through 31,
states indicated they would keep those sections of pavement under
frequent surveillance. Some states also posted skid-danger
warning signs when skid resistance was marginal. If the SN fell
below 30, states reported taking measures to correct the low skid
resistance, since the potential for dangerous skidding would be
high.
Design and evaluation procedures for HMAC pavements varied
considerably from state to state. Twenty-one of the forty-eight
states had no guidelines specifically dealing with pavement skid
resistance. The states that had design procedures reported
significantly different assumptions and methods. Some DOTs felt
proper mix design would provide adequate skid resistance. Others
controlled the polishing characteristics of aggregates to obtain
good skid resistance. Different assumptions led to different
evaluation procedures. While research has associated surface
macrotexture with good skid resistance, only one state reported
using a design procedure that evaluated macrotexture. That state
was Wisconsin, which had developed a statistical regression model
to estimate the skid number of an HMAC surface after a given
number of vehicles had driven over it.
Categories of Practices to Control Skid Resistance
Surveys revealed that states fell into five general
classifications according to their guidelines for evaluating HMAC
pavement skid resistance. The authors developed the following
category titles:
| |
I. |
No specific guidelines to address skid resistance |
| |
II. |
Skid resistance is accounted for through mix design |
| |
III. |
General aggregate classification procedures are used |
| |
IV. |
Evaluate aggregate frictional properties using laboratory test procedures |
| |
V. |
Incorporates field performance in aggregate qualification |
The table below is from the research report and shows the
category of design procedure used by each state DOT.
|
State DOT
|
Category I
|
Category II
|
Category III
|
Category IV
|
Category V
|
|
Alabama
|
X
|
X
|
X
|
X
|
X
|
|
Arizona
|
X
|
X
|
X
|
X
|
X
|
|
Arkansas
|
X
|
X
|
X
|
X
|
X
|
|
California
|
X
|
X
|
X
|
X
|
X
|
|
Colorado
|
X
|
X
|
X
|
X
|
X
|
|
Connecticut
|
X
|
X
|
X
|
X
|
X
|
|
Delaware
|
X
|
X
|
X
|
X
|
X
|
|
Florida
|
X
|
X
|
X
|
X
|
X
|
|
Georgia
|
X
|
X
|
X
|
X
|
X
|
|
Idaho
|
X
|
X
|
X
|
X
|
X
|
|
Illinois
|
X
|
X
|
X
|
X
|
X
|
|
Indiana
|
X
|
X
|
X
|
X
|
X
|
|
Iowa
|
X
|
X
|
X
|
X
|
X
|
|
Kansas
|
X
|
X
|
X
|
X
|
X
|
|
Kentucky
|
X
|
X
|
X
|
X
|
X
|
|
Louisiana
|
X
|
X
|
X
|
X
|
X
|
|
Maine
|
X
|
X
|
X
|
X
|
X
|
|
Maryland
|
X
|
X
|
X
|
X
|
X
|
|
Massachusetts
|
X
|
X
|
X
|
X
|
X
|
|
Michigan
|
X
|
X
|
X
|
X
|
X
|
|
Minnesota
|
X
|
X
|
X
|
X
|
X
|
|
Mississippi
|
X
|
X
|
X
|
X
|
X
|
|
Missouri
|
X
|
X
|
X
|
X
|
X
|
|
Montana
|
X
|
X
|
X
|
X
|
X
|
|
Nebraska
|
X
|
X
|
X
|
X
|
X
|
|
Nevada
|
X
|
X
|
X
|
X
|
X
|
|
New Hampshire
|
X
|
X
|
X
|
X
|
X
|
|
New Jersey
|
X
|
X
|
X
|
X
|
X
|
|
New Mexico
|
X
|
X
|
X
|
X
|
X
|
|
New York
|
X
|
X
|
X
|
X
|
X
|
|
North Carolina
|
X
|
X
|
X
|
X
|
X
|
|
North Dakota
|
X
|
X
|
X
|
X
|
X
|
|
Ohio
|
X
|
X
|
X
|
X
|
X
|
|
Oklahoma
|
X
|
X
|
X
|
X
|
X
|
|
Oregon
|
X
|
X
|
X
|
X
|
X
|
|
Pennsylvania
|
X
|
X
|
X
|
X
|
X
|
|
Rhode Island
|
X
|
X
|
X
|
X
|
X
|
|
South Carolina
|
X
|
X
|
X
|
X
|
X
|
|
South Dakota
|
X
|
X
|
X
|
X
|
X
|
|
Tennessee
|
X
|
X
|
X
|
X
|
X
|
|
Texas
|
X
|
X
|
X
|
X
|
X
|
|
Utah
|
X
|
X
|
X
|
X
|
X
|
|
Vermont
|
X
|
X
|
X
|
X
|
X
|
|
Virginia
|
X
|
X
|
X
|
X
|
X
|
|
Washington
|
X
|
X
|
X
|
X
|
X
|
|
West Virginia
|
X
|
X
|
X
|
X
|
X
|
|
Wisconsin
|
X
|
X
|
X
|
X
|
X
|
|
Wyoming
|
X
|
X
|
X
|
X
|
X
|
In general, design guidelines relied on proper
identification of good-quality coarse aggregates to provide good
skid resistance. The procedures to do this varied greatly from
state to state, and the differences followed no identifiable
geographic pattern. The authors described the rationale behind
the procedures characteristic of each of the five categories.
Category I: No specific guidelines used during the design
Since these DOTs judged from experience that their HMAC
pavement surfaces performed well and no laboratory evaluation of
aggregates was necessary, the states did not assess skid
resistance of the pavement surface when designing new pavements.
Respondents cited the availability of good quality aggregates as
the main reason they did not need evaluation guidelines.
While these states had no special procedure for evaluating
the skid resistance of a pavement surface before highway
construction, they did frequent field testing to assure that skid
numbers remained above acceptable levels. These states took
corrective measures when SNs fell too low.
Category II: Skid resistance control through proper mix design
Rather than evaluate the skid-resistance properties of their
aggregates, these states controlled the mix design of aggregates.
As in Category I, these states had past experience revealing no
major problem with HMAC pavement skid resistance. Controlling
the mix design and performing frequent skid field testing assured
adequate skid resistance.
Category III: Use of general aggregate classification procedures
When designing new pavement surfaces, these DOTs followed
procedures to control the quality of aggregates used and,
thereby, provide adequate skid resistance. State guidelines
specified the types of aggregates that might be used and the
percentage of certain aggregates considered acceptable. For
example, since limestone aggregates have a poor record for
resisting polishing from traffic movement, state guidelines might
classify them as poor quality and limit their use. Again, skid
field testing served as backup to guidelines and determined if
skid resistance had remained adequate over time.
Category IV: Aggregate screening based on laboratory evaluation
These states primarily employed the acid insoluble residue
(AIR) and polish value (PV) tests to evaluate the frictional
properties of aggregates used in HMAC pavements. Tests followed
procedures in ASTM D 3042-86 and ASTM D 3319-90 respectively.
Using diluted hydrochloric acid, the AIR test helps DOTs identify
and eliminate carbonate aggregates in their mixtures. Carbonate
aggregates tend to polish excessively. The PV test uses a
British wheel to expose aggregate samples to nine hours of
intense polishing. Simulating the effects of extended exposure
to traffic, the test allows states to estimate in the laboratory
how a mixture will perform in the field.
Various states reported using other testing procedures. One
test, petrographic analysis, follows the procedures of ASTM C
296-90 to identify the mineral composition of aggregates and
allows evaluation of predicted overall behavior. Another, Moh's
hardness test, yields a number that identifies harder aggregates,
which are considered to have better skid-resistance performance
in the field. Mississippi and South Carolina reported
mechanically crushing aggregates and predicting performance based
on an evaluation of freshly fractured particles. Indiana's test
looked for the presence of dolomite aggregates to produce skid-
resistant surfaces. And Michigan had developed an aggregate wear
index (AWI) to rate the polish resistance of aggregate samples.
Category IV states used evaluation procedures that rated the
performance of HMAC pavement materials in the laboratory. Only
aggregates that met established specifications during laboratory
testing qualified for roadway construction use.
Category V: Aggregate screening based on field performance
A significant problem with laboratory testing of aggregates
is that laboratory test results often correlate poorly with
performance in the field. While many states followed laboratory
testing with field surveillance, Category V states used both
laboratory and field-test results to decide the classification of
an aggregate and its appropriateness for use in new construction.
Florida, for example, reported using AIR laboratory testing
and then building a trial pavement section using aggregates that
passed AIR screening. Test methods from the ASTM E 274 procedure
determined the frictional characteristics of the trial section.
Given satisfactory results, the aggregate was used on a stretch
of roadway with a minimum speed limit of 50 miles per hour (80
kilometers per hour) and a traffic count of at least 14,000 per
day. A control section meeting the same standards as the test
section but constructed of a previously approved aggregate
connected to the test section. Crews did field skid tests
immediately, then monthly for two months, and finally every two
months until six million vehicles had passed or the skid
resistance stabilized. When needed, an additional test section
evaluated performance at 60 miles per hour (96 kilometers per
hour). If the test section maintained skid numbers over 30 and
performed as well as the control section, the aggregate received
approval for use.
Kentucky, Pennsylvania, and Texas also had comprehensive
aggregate-testing procedures that combined laboratory analysis
with performance history and (or) skid field tests. States found
that sometimes aggregates that met laboratory specifications
performed poorly in the field, while sometimes aggregates that
failed laboratory test guidelines exhibited adequate performance
in the field.
Conclusions
The authors reiterated that their survey revealed state DOT
procedures for evaluating skid resistance of HMAC pavement
materials varied from no guidelines to elaborate laboratory and
field testing. An important research finding was that "none of
the state DOTs rely on skid field testing as their primary
mechanism for aggregate evaluation."
Arguably, field testing is technically superior to
laboratory testing. Why is it not the primary method used by the
states? The authors offered four considerations:
- States must collect field skid data for several years
to develop a performance history that will adequately
predict the success of an aggregate. Waiting for
field-test results would put new or recent aggregate
sources on hold for a long time.
- Aggregates from a given source change over time, and
the aggregate present in a field-tested sample may not
be the same as that currently available from the same
source.
- Field skid numbers are influenced by many factors such
as rainfall, ambient temperatures, test speed
variations, test position on the roadway, and
differences in road surfaces or in equipment operators.
Different skid numbers resulting from these extraneous
factors affect the reliability of test results.
- Field testing is labor intense and, therefore, creates
heavy demands on money, personnel, and equipment.
While current practices emphasized controlling aggregate
quality and, therefore, controlling the microtexture, research
shows that pavement macrotexture greatly influences skid
resistance. In closing, the authors described developing
technology that will allow testing of macrotexture and
microtexture. Analysis of macrostructure has been difficult;
however, promising methods using laser beams or a strobed band of
light with high infrared content would give the states the
ability to gather skid resistance data using a vehicle traveling
at normal highway speeds. Such developments may make analysis of
macrotexture feasible and change the way state DOTs evaluate HMAC
pavement materials.
