|FHWA Policy Memorandums - Office of Engineering|
Superseded on 06/17/2005 by Technical Advisory T 5040.36 Surface Texture for Asphalt and Concrete Pavements
|INFORMATION : Surface Finishing of Portland Cement Concrete Pavements - Final Report FHWA-SA-96-068, Tire Pavement Noise and Safety Performance, May 1996||November 12, 1996|
Director, Office of Engineering
Director, Office of Technology Applications
Federal Lands Highway Program Administrator
Mr. William A. Weseman's November 1, 1995, memorandum on the above subject re-emphasized FHWA's policy that pavement surfaces provided on federally-aided highway projects have a skid resistant surface. A preliminary summary of the joint State, industry, and FHWA Portland cement concrete (PCC) Surface Texture Technical Working Group's (TWG) findings was attached.
Attached are three copies of the TWO's final report. A direct distribution of three copies will also be made to each division office. The Executive Summary (pages v through ix of the final report) updates the preliminary guidance previously furnished.
It is suggested that the State highway agencies immediately update transverse fining specifications for projects with design speeds of 80 km/h or higher to those suggested in Section 4 of the Executive Summary. Specifying a random versus a uniform transverse tine texture will greatly reduce the probability of developing the annoying tire/pavement whine that has occurred on some PCC pavements.
Appendices A and C include listings of references providing guidance on surface textures that provide friction to reduce wet weather accidents while minimizing tire/pavement noise. In 1995, a significant number of FHWA guidelines were updated and additional technical reports on noise related issues became available which are summarized in the Final Report. Additional evaluation studies are being planned in 1996 and 1997 in cooperation with the Wisconsin researchers to refine and verify the recommendations made in this report. The FHWA Technical Advisory T 5140.10, Texturing and Skid Resistance of Concrete Pavement and Bridge Decks, September 18, 1979, will be updated when this additional evaluation is completed. It is expected that the bridge deck texturing guidelines (which now usually require uniform 13mm transverse saw grooves) will be revised to a random pattern similar to that which has been found to avoid the tire whine on highway pavements.
Available information indicates total highway fatalities in the United States increased from 39,235 in 1992 to approximately 41,700 in l995, reversing a long downward trend. The increasing number of highway fatalities, plus the significant increases in the legal speed limits in many States as a result of the repeal of the national maximum speed limit by Section 205(d) of the National Highway System Designation Act of l995, re-emphasize the need for providing safe, durable pavement surfaces.
The States' pavement management and highway safety management systems should be organized to periodically determine the wet weather accident rates for the various pavement types and mixes used. This information should help demonstrate that safe, durable pavement surfaces are being provided and/or support the State's request to change pavement texturing related specifications.
The Pavement Division is available to discuss application of these guidelines to meet the needs of the various highway agencies. Please contact Messrs. Paul Teng, John Hallin, or Roger Larson at (202) 366-1324 for assistance.
Joseph S. Toole Gerald L. Eller
PCC SURFACE TEXTURE TECHNICAL WORKING GROUP FINDINGS
1. Basis for Surface Texture SelectionWell designed and constructed Portland Cement Concrete (PCC) pavements can provide safe, durable surfaces with low-noise characteristics. The selection of the surface texture to be provided at specific locations in a State should be based on existing conditions at that site, such as climate, amount of traffic including vehicle type distribution, speed limit, percent grade, conflicting movements (intersections or frequent side approaches), materials quality and cost, and the presence of noise-sensitive receptors. The primary purpose of the surface texture is to help reduce the number and severity of wet weather accidents.
Where the State has data to show that a specific texturing method results in similar wet weather accident rates and/or has similar friction characteristics [when tested with an ASTM skid trailer (Method E 274), preferably with a blank tire (E 524)] to another texturing method or to alternate materials used, the States proposed method should be allowed. The States pavement management and highway safety management systems should be organized to develop these comparisons periodically. Due to widely varying conditions at individual project locations, it is unlikely that one surface texturing method will always be the best choice in any given State.
2. Safety ConsiderationsThe pavement surface on high-speed (80 km/h or greater) facilities must have sufficient microtexture (usually provided by siliceous sand) and macrotexture [provided by transverse or longitudinal tining (or plastic brushingSpanish technique) preceded by a longitudinal artificial carpet or burlap drag; or an exposed aggregate surface treatment] to provide good friction characteristics during wet weather conditions. Current research and past experience have shown that longitudinal texturing with only an artificial carpet or burlap drag will generally not provide a safe, durable surface on high-speed facilities. They will usually provide adequate friction on roadways with speeds less than 80 km/h. Both FHWA guidelines and the 1993 AASHTO Guide on the Evaluation and Abatement of Traffic Noise recommend that the designer should never jeopardize safety to obtain noise reduction. Longitudinal tining or grooving should follow the 1976 AASHTO Guidelines for Skid Resistant Pavement Design (1976 AASHTO Guide) due to the concerns expressed by drivers of vehicles with smaller tires.
Preliminary information indicates total highway fatalities in the United States have increased from 39,235 in 1992 to 41,700 in 1995, reversing a long downward trend. The increasing number of highway fatalities, plus the significant increases in the legal speed limits in many States as a result of the repeal of the national maximum speed limit by section 205(d) of the National Highway System Designation Act of 1995, reemphasize the need for providing safe, durable pavement surfaces.
3. Need for Quality Mix Designs on Heavily Traveled, High-Speed HighwaysAdequate microtexture is usually most economically provided by specifying a minimum siliceous sand content of 25 to 40 percent of the fine aggregate portion of the mix. FHWA Technical Advisory T 5080.17, Portland Cement Concrete Mix Design and Field Control, dated July 14, 1994, reemphasizes good mix design practices. Fine aggregates usually containing a minimum of 25 percent siliceous sand; durable, nonpolishing coarse aggregates; a low water-cement ratio; an adequate air content; an adequate cement factor; and good curing practices are all necessary if a high-quality, durable concrete is to be provided.
Where cost effective and high-quality aggregates are not readily available, consideration should be given to two-layer construction, wet on wet, with a higher quality mix in the thinner (Michigan DOT recommends aminimum thickness of 70 mm based on recent construction experience) surface layer. This may provide a more durable, yet economical concrete pavement.
4. General PCC Surface Texture ConsiderationsTransverse tining, preceded by a longitudinal artificial carpet or burlap drag, remains the most desirable PCC surface texture method for many high-speed (80 km/h or greater) locations. With quality design and construction, it has been shown that pavements with excellent friction characteristics and low-noise levels can consistently be provided. In particular, research demonstrates that transversely tined concrete pavements with low-noise characteristics and minimal splash and spray can be constructed.
With high-quality mix design and construction practices, longitudinal tining or brushing and the exposed aggregate surface treatments will also provide sufficient macrotexture to prevent hydroplaning and reduce the number and severity of wet weather accidents on high-speed highways.
Transverse tiningWhen used, random transverse tine spacing (minimum spacing of 10 mm and a maximum spacing of 40 mm with no more than 50 percent of the spaces exceeding 25 mm) should be specified pending the results of further research. The actual tine width should be 3 mm (+/- 0.5 mm), and the tined depth should be a minimum of 3 mm and a maximum of 6 mm (provided minimum dislodging of the aggregate particles results). Narrow (less than 4 mm width), deep grooves are considered better than wider, shallow grooves for minimizing noise. The average texture depth as measured by the sand patch test (ASTM E 965) should be 0.8 mm with a minimum of 0.5 mm for individual tests. Measurements of random spacings at two locations in Wisconsin that generate low-noise levels and no tire / pavement whine are as follows:
Wisconsin researchers have now developed a noise measuring system in a vehicle and an analysis method that will identify pavement textures that generate objectionable tonal qualities. The revised noise evaluation procedure was developed when previous research reports and the current study revealed that the subjective noise ratings given by persons in test vehicles on transversely tined and certain other types of textured pavements did not correspond with the objective total noise measurements taken 7.5 m from the pavement edge. Noise measurements were taken inside of vehicles with instruments using a Fast Fourier Transform (FFT) analysis mode to observe narrow band frequencies. It was discovered that there are peak sound pressure levels up to 10 dB(A) around 1,000 Hertz that cause a pure tone that is irritating to the human ear (either a higher frequency tire / pavement whine or, in fewer cases, a lower pitched rumble). The third octave band analysis was not detecting these frequency peaks because averaging of the total sound masked them. The tones were discovered on most uniformly spaced transverse tining. This is a major discovery because surface textures creating these tonal characteristics can be identified so that they can be avoided on future projects.
Longitudinal TiningWhere longitudinal tining is desired (particularly in noise-sensitive areas or drier climates), it is recommended that the uniform tine spacing be 20 mm, actual tine width 3 mm (+/- 0.5 mm), and the individual tined depth be 3 to 6 mm (with an average surface texture depth of 0.8 mm and a minimum of 0.5 mm for individual tests as measured by the sand patch test ASTM-E 965). Wider longitudinal grooves are particularly objectionable to drivers of vehicles with small tires and must be avoided.
Preliminary information indicates that longitudinal tining at 20 mm spacing, preceded by a burlap or artificial turf drag, will provide a safe, durable pavement if a high-quality surface mixture with adequate microtexture is used that includes a minimum of 25 percent siliceous sand. CALTRANS specifies a minimum siliceous sand content of 30 percent of the fine aggregate portion and a minimum friction coefficient of 0.30 per itsstandard test procedure.
When considering the use of longitudinal texturing, the disadvantages of slightly slower surface drainage and more splash and spray compared to transverse tining should be considered especially in wetter climates subject to freezing conditions. Where very high speeds are expected (130 km/h or greater), British research indicates that longitudinal textures may not provide satisfactory friction characteristics. The New South Wales, Australia, Concrete Pavement Manual also states that longitudinal grooving treatment is unsatisfactory for both stopping distance and for rotational stability of a braked vehicle at high speeds (see appendix H).
Longitudinal plastic brushingSpain has reported the successful use of a combined texture consisting of a longitudinal burlap drag followed by a plastic brush to provide high-friction characteristics while minimizing tire / pavement noise. A minimum of 30 percent siliceous sand is required to assure satisfactory microtexture. An average texture depth of 0.7 to 1.0 mm is required. Skid resistance is reported similar to porous asphalt with an average noise level of 0 to 2 dB(A) or higher (this noise level difference is not discernable by the human ear). Two PIARC study locations in Spain had .29 and .21 Friction Numbers with mean texture depths averaging 1.1 mm and 0.8 mm, respectively, when tested in 1992 at 90 to 95 km/h (55 mph) by an ASTM skid trailer with a blank tire. The SN40 ribbed tired friction numbers were .53 and .41 and the SN40 blank tire friction numbers were .48 and .27 respectively. The wet weather accident rate for this texture compared to other textures or surface types is not currently available.
Exposed Aggregate SurfaceThe exposed aggregate surface treatment technique is normally constructed on a pavement composed of two layers (wet on wet): top layer 40 to 70 mm thick, 30 percent siliceous sand 0 to 1 mm and 70 percent high-quality chips 4 to 8 mm (Los Angeles abrasion less than 19, polished stone value greater than 50); bottom layer maximum aggregate size 32 mm with lower quality yet durable aggregates. A high-quality concrete, with a maximum water-cement ratio of 0.38, minimum 450 kg/m3 cement content, a plasticizer, and an air entrainment agent, is used in the top layer to ensure durability and to keep the modulus of elasticity as high and shrinkage as low as the bottom layer. Recommended texture depth is 0.9 mm as measured by the sand patch test ASTM E 965 (less depth is reported to result in higher noise). If 60 or more chips per 2,500 mm2 are exposed, the noise will be low. Overfinishing of the surface must be avoided, and special vibrators must be used for the top layer. Advantages include noise similar to porous asphalt, excellent high-speed skidding resistance, and low splash and spray.
Other options that have been used include:
5. Profile ConsiderationsSome deep transverse tining has resulted in dislodging the aggregate particles with a resulting decrease in pavement ride quality and an increase in objectionable tire / pavement noise. However, a 1974 Belgian study concluded that randomly spaced (15 to 30 mm), deeper transverse tining (5 to 7 mm depth) did not affect the evenness (roughness) of the pavement surface and thus did not reduce the riding quality as compared to untined roads.
The International Roughness Index (IRI) is considered the best current indicator of pavement surface smoothness. European research indicates that the pavement smoothness also affects the noise level and an additional or an improved longitudinal smoother is being used to reduce total PCC tire / pavement noise to a level similar to that of a dense graded asphalt.
6. Alternative Surface Treatments to Improve Friction Properties of Existing Portland Cement Concrete SurfacesThere are many tradeoffs necessary in design, construction, materials quality, and maintenance procedures primarily due to the limited funding available. Therefore, it may be necessary to restore friction properties on sections of roadways with a high ratio of wet weather accidents or possibly on pavements having undesirable noise characteristics.
Possible alternatives to restore a skid resistant surface on Portland Cement Concrete (PCCP) pavements include:
It is normally more cost effective to retexture the existing concrete surface than provide a thin surface treatment or overlay unless polishing aggregates are involved.
7. Research NeedsA review of the State-of-the-Practice and the State-of-the-Art revealed the following areas where additional research is needed: