Concrete Pavement Technology Update

Pavement Texture Research Enhances Safety

CPTP pavement texture efforts lead to enhanced pavement safety as brought forth in FHWA Technical Advisory T 5040.36 (Surface Texture for Asphalt and Concrete Pavements) issued in June 2005. The technical advisory provides state-of-the-practice information on pavement surface texture/friction and guidance on selecting techniques that will provide adequate wet pavement friction and low tire-surface noise. The advisory is consistent with FHWA pavement policy contained in 23 Code of Federal Regulations 626.3 stating, "Pavement shall be designed to accommodate current and predicted traffic needs in a safe, durable, and cost-effective manner."

With tining, friction and tire-pavement noise in the finished pavement are significantly influenced by spacing, depth, width, and orientation of the tines. Longitudinal tining is the preferred choice for tined surface in many States.

In 1987, the Permanent International Association of Road Congresses (now called the World Road Association) defined three classes of pavement surface texture. Two classes of surface texture affect wet pavement friction: microtexture, with wavelengths of 1 mm to 0.5 mm (0.00004 to 0.020 in.), and macrotexture, with wavelengths of 0.5 to 50 mm (0.02 to 2.00 in.). The first is generally considered to be a function of the fine aggregate used in a concrete pavement mixture, while the second is related to tining or other concrete finishing operations. A third class, megatexture, with wavelengths of 50 to 500 mm (2 to 20 in.), plays a role in tire-pavement noise.

While the advisory points out that safety is paramount, it notes that tire-surface noise also should be considered and that a safe and low-noise surface is achievable with the proper design considerations. The effect of texture on tire-pavement noise is complex. Macrotexture wavelengths of 2 to 10 mm (0.08 to 0.40 in.) tend to decrease the exterior noise generated at the tire-pavement interface, while increased megatexture of 50 to 500 mm (2 to 20 in.) has been shown to increase interior noise in vehicles. The interaction of the three classes of texture raises the possibility that an optimized texture might provide both a safe and quiet pavement surface. Much of the current research is directed at achieving that optimized condition.

While this article focuses primarily on safety issues, a later one will deal more indepth with tire-pavement noise. In fact, Larson, Scofield, and Sorenson (2004) note that because texture has such an effect on both noise and friction, the issues must be considered together to fully address highway users' concerns.

New Construction Texture

Several construction techniques have been found to provide good concrete pavement texture under various conditions:

1. Tining preceded by a burlap drag is an effective method of providing macrotexture to new portland cement concrete pavements. However, spacing, depth, width, and orientation of the tines have a significant influence on both the friction and tire-pavement noise of the completed pavement. In general, narrow, deep grooves rather than wide, shallow grooves, within the limitations given below, are better for minimizing noise.

   General recommendations on transverse tine configuration are a width of 3.0±0.5 mm (0.125±0.020 in.) and a maximum depth of 3 mm (0.125 in.) with an average random spacing of 13 mm (0.5 in.) or 26 mm (1.0 in.). The advisory provides typical random spacings that will yield the suggested averages.

   For longitudinal tining the same width and depth recommendations apply (3 mm [0.125 in.]) wide by 3 mm [0.125 in.] deep) while a straight, uniform spacing of 19 mm (0.75 in.) has been shown to provide adequate handling characteristics for small vehicles and motorcycles. Longitudinal tining is the preferred choice for tined surface in many States.

2. Exposed coarse aggregate in the pavement surface is sometimes an effective means of providing an acceptable, lower noise texture. In this case, texture depth, aggregate size, and aggregate durability greatly affect the friction, noise, and longevity of the pavement surface. Exposed aggregate surfaces typically are constructed in two layers with the top layer having fine and coarse aggregates that are wear-and-polish resistant with a minimum of 25 percent of the fine aggregate being siliceous material. This is the preferred method for surface texture in many European countries.
3. An alternative for surfaces where the design speed is 80 km/h (50 mi/h) or greater may be a broom or artificial turf finish of the fresh concrete surface, provided adequate safety performance has been demonstrated. Either method typically results in striations of 1.5 to 3 mm (0.063 to 0.125 in.) in depth.
4. Diamond grooving is also an effective method of imparting macrotexture to a new concrete surface. Again, spacing, depth, width, and orientation of the grooves have a significant influence on the friction and noise characteristics of the surface. Suggested configurations are the same as those given above for transverse and longitudinal tining.
5. Diamond grinding may be an effective means of imparting macrotexture to a newly constructed pavement, especially when nonpolishing, large aggregate is used. The FHWA advisory points out that adequate safety performance should be demonstrated. Diamond grinding typically provides grooves that are approximately 3 mm (0.125 in.) wide spaced at 5 to 6 mm (0.20 to 0.25 in.) intervals. Specific groove depth and spacing are dependant on hardness of the aggregate.
6. A burlap drag alone as a means of imparting texture is recommended only where design speeds are less than 81 km/h (50 mi/h). Again, striations of 1.5 to 3 mm (0.063 to 0.125 in.) in depth are typical.
7. Additional methods, sometimes including thin, asphalt-based surface treatments (microsurfacing) or thin, epoxy-bonded layers, are used by various agencies where research has shown those methods to be effective.

Restoration of Texture

Restoration techniques identified to restore lost texture on an aging concrete pavement are diamond grinding, transverse or longitudinal grooving, and the application of thin overlays of either epoxy- or asphalt-based materials. The life of both grinding and grooving techniques is heavily dependent on the polishing resistance of the coarse aggregate used in the original pavement surface. Agencies are encouraged to do their own research on the types of rehabilitation approaches yielding the most benefit.

Other Considerations

Other site-specific factors to consider when evaluating a pavement texturing approach are splash and spray, climate, traffic volume, traffic speed, roadway geometry, the likelihood of conflicting traffic maneuvers, cost, and the presence of noise-sensitive receptors. The FHWA advisory points out that no one treatment will always be the best choice for a wide range of projects and conditions. They encourage local research and the evaluation of the available literature on setting texture requirements and friction thresholds.

In addition, Larson, Scofield, and Sorenson (2005) point out evidence that suggests increased emphasis on texture/friction at sites with higher accident risk could significantly reduce fatalities, injuries, and resulting traffic delays. Such sites include two-lane roads, intersections, curves, work zones, freeway ramps, and sites where frequent erratic traffic movements occur.

Finally, Larson (2005) notes the importance of comparing the friction demand assumed during pavement design with that actually provided on the pavement surface throughout the pavement's service life, especially at known critical sites. He further points out that, although numerous American Association of State Highway and Transportation Officials Guides address various aspects of this issue, the amount of information is so voluminous that computer-supported tools are being developed to help apply the results.

Sources

FHWA Technical Advisory T 5040.36, Surface Texture for Asphalt and Concrete Pavements, June 2005

Larson, Roger M., Larry Scofield, and James B. Sorenson. 2004. "Pavement Surface Functional Characteristics." Presented at Fifth Symposium on Pavement Surface Characteristics, Toronto, Canada, June 2004.

Larson, Roger M., Larry Scofield, and James B. Sorenson. 2005. "Providing Durable, Safe, and Quiet Highways." Proceedings of the 8th International Conference on Concrete Pavements, Vol. II, pp 500-522, Colorado Springs, CO, August 2005. International Society for Concrete Pavements, Bridgeville, PA.

Larson, Roger M. 2005. "Using Friction and Texture Data to Reduce Traffic Fatalities, Serious Injuries, and Traffic Delays." Presented at International Conference on Surface Friction Christchurch, New Zealand, May 2005 .

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