An Evaluation of the Effects of Tread Depth, Pavement Texture, and Water Film Thickness on Skid Number-Speed Gradients

About the report:

Thirty-one sites representing the types of pavement surfaces on the highways of Virginia were tested by the Virginia Highway and Transportation Research Council skid trailer in an effort: (1) to determine the influence that tire tread depth, pavement texture, and water film thickness have on the deterioration of skid numbers with increased test speeds, and (2) to develop curves which will provide a means of predicting skid numbers for given combinations of these factors other than those employed during testing. The tests which employed six tire conditions, four water conditions, and five test speeds for each site were conducted between September 21, 1972 and June 6, 1974. Based on the data gathered in this study, the following conclusions regarding Virginia's pavement surfaces are warranted: 1. High skid number-speed gradients are common to pavements that do not contain a relatively high degree of macrotexture. This finding, of course, only substantiates the observations of many others. However, it should be pointed out that if the surface has a sharp microtexture, its skid resistance can be excellent at all legal speeds with legal tires. Bald tires on such surfaces can provide extremely low skid numbers. 2. The converse of number 1 is also true, i.e., the slope of the skid number-speed gradient curve decreases with increased macrotexture. However, the degree of this decrease is strongly influenced by the characteristics of the microtexture. Pavements such as grooved portland cement concrete that provide ample means of water escape might have quite steep skid number-speed gradients if the microtexture is not harsh. It follows that the same is true for open-graded bituminous mixes containing polish susceptible aggregates. 3. Pavements that have essentially the same skid number-speed gradients can have quite different relationships between treaded and bald tires. This difference is due to the difference in macrotexture, i.e., as the macrotexture increases, the divergence between the treaded and bald tire skid numbers decreases until the values for the two tires are the same; in fact, in some cases the values for the bald tire become higher than those for the treaded tires. 4. The highest skid numbers. recorded in the study were at low speeds in the steep speed gradient group, which means that a low macrotexture- high microtexture surface provides the best skid resistance at low speeds (40 mph (17.9 m/s) and below). 5. Grooving does not greatly influence the skid resistance or the skid number-speed gradient slope for treaded tires but does manifestly increase the skid resistance for bald tires. The latter fact may account for some of the reduction in wet pavement accidents where concrete pavements have been grooved. 6.The skid number-speed gradient curves developed in this study can be used to predict skid numbers for speeds other than those at which tests are run on Virginia pavements. 7. The skid number decreases as the test tire tread decreases. This decrease averages about 1 1/4 SN per 2/32 in. (0.16 cm) increment in tread depth for a total loss of 5 SN from 11/32 in. (0.87 cm) to 3/32 in. (0.24 cm). A correction for this change in skid number can be made on a straight-line basis. 8. After a tire has worn beyond 3/32 in. (0.24 cm) tread depth, it has a high reaction to pavement macrotexture and should, therefore, not be used for routine testing. 9. Because of the small change in skid number with a change in water film, the normal fluctuation in water output by the test trailer should not be a matter of concern.

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