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Road Engineering Journal
Copyright © 1997 by TranSafety, Inc.
June 1, 1997
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Forty-Eight State Survey Showed Pavement Skid-Resistance Evaluation Varied Considerably

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Copyright © 1997 by TranSafety, Inc.



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