IAPA's 80th Annual Convention

March 13 - 14, 2017

 

ICT Research Report
FHWA-ICT-12-009
Best Practices for Bicycle Trail Pavement Construction and Maintenance in Illinois

NAPA IS-129 
A Guideline for the Design and Construction of HMA Pavements for Trails and Paths

Bike, Pedestrian, and Golf Cart Paths


ACKNOWLEDGEMENTS

This document is a product excerpt from the Colorado Asphalt Pavement Association. The primary author was Mr. Eric West, P.E., WesTest, Inc. We appreciate and acknowledge the effort by Mr. West in its development. This document is not intended to replace or supersede any established standard or specification requirement. It is intended to be a resource for the proper design and construction of asphalt trails and paths.

OVERVIEW

The current emphasis in America on health, conservation and the environment has drastically increased the use of, and need for, trails and paths. Once used by only a few enthusiastic cyclists, paths are now needed for joggers, walkers and rollerbladers. Asphalt pavement provides a smooth, flexible, long lasting surface preferred by the outdoor enthusiast for recreational purposes and for use as a means of transportation and links to public transportation centers.

This report is intended to provide guidelines for design and construction of asphalt pavements for trails and paths. A cost comparison of asphalt versus concrete is included as well as information on the advantages of using asphalt pavement. The report concludes with a summary of key factors contributing to quality asphalt pavement paths and trails.

In order to properly design and construct asphalt pavements, several factors must be considered. Pavements need to be designed to fit the needs of the people. The existing terrain, environment, climate, drainage and, depending on use, pavement loading need to be addressed in the design phase. These factors, in conjunction with sub-grade characteristics, will affect the design thickness of the pavement as well as the design of the asphalt mixture.

Properly designed asphalt pavements provide user friendly, cost effective, long lasting bike paths and trails which enable the public to use a surface which is smooth, quiet and safe.

DESIGN CONSIDERATIONS

The selection of surface material for trails and paths is primarily based on anticipated type and intensity of trail use. Other considerations of surface material include, terrain, climate, design life, maintenance, cost, and availability. Soft surface materials include earth, grass, bark and wood decking. Hard surface materials include stone, brick, concrete and asphalt. Hard surface materials are preferred for multi-use trails where usage is high.

Each surface material type has advantages and disadvantages. Soft surface materials are low cost, but require substantial maintenance and are not suitable for many of the recreational activities today's trails and paths are used for. Hard surface materials, specifically concrete and asphalt, provide years of service with low maintenance. The designer must consider all of the selection variables and stay within budget constraints

SELECTING THE PROPER PAVEMENT TYPE

Both the asphalt pavement industry and ready mix concrete industry are well represented. Each product can provide excellent quality for specific applications. There are several factors that should be considered prior to selecting the appropriate pavement type.

USER PREFERENCE

Asphalt pavement provides a continuous, smooth, joint-free travel lane. This flexible pavement alternative is quieter with improved rideability preferred for cycling and rollerblading. Joggers and walkers also prefer the softer surface asphalt pavement delivers.

AESTHETICS

Paving techniques allow asphalt pavement to be placed on minor slopes, over undulating topography, and blended into the existing landscape. The free flow lines of asphalt pavement do not detract from the natural environment. In addition, asphalt pavement can be colored to preserve the natural setting. Color may be accomplished using available polymer pigments, or by specifying colored aggregate which will provide a base color more visible through time.

COST

An analysis of typical construction costs for both pavement types indicates a significant savings can be realized by using asphalt pavement. However, actual construction costs will vary depending on project specifics, grading requirements, location and local pricing differences, and distance from concrete or asphalt supplier plants.

CONSTRUCTIBILITY

Asphalt pavements can be constructed where space is limited and topography is rugged. In addition to the direct savings outlined above, indirect savings also occur when using asphalt pavement. Construction time is significantly shorter for asphalt pavement. This shorter construction time provides additional savings to the agency by reducing field inspection and management costs. The public also has access to the paths or trails sooner. In some mountain locations, where the construction season is short, this reduced construction time can be a determining factor in the type of pavement selected.

MAINTENANCE

Asphalt pavement maintenance is kept to a minimum through proper design and construction. A significant advantage over concrete pavement is asphalt's ability to be repaired quickly and inexpensively.

PAVEMENT WIDTH

Trail width selection is based on the intended use of the path. Multi-use trails must be wide enough to accommodate fast-moving bicyclists and skaters along with slower moving pedestrians and joggers. Unless rigorous enforcement is anticipated, trails and paths must accommodate two way traffic. The minimum recommended width for two way multi-use paths is ten feet, with twelve feet recommended for heavy use areas. Sight distance also affects the choice of pavement width for multi-use paths. Adequate pavement width should be provided to allow passing of slower moving users. If possible, trails and paths should be designed with a ten foot wide, hard surfaced primary lane for bicyclists and skaters, and a separate five foot wide soft surfaced trail for pedestrians and equestrians. In order to design for cost effective construction, the designer needs to consider construction equipment size. Typical paver width is ten feet, with eight feet available in some locations. Design guidelines for path width, sight distance and other safety and user-friendly features are outlined in the AASHTO Guide for the Development of Bicycle Facilities. Trail design should also meet the Americans with Disabilities Act, including maximum slope and cross pitch requirements.

PAVEMENT THICKNESS

The first step in analyzing pavement thickness is determining the loading the pavement will be subjected to. Pavements need to be designed to support wheel loads from vehicles that will have access to them. These may include emergency, patrol, snow removal, maintenance and other motor vehicles.

The next step is to determine the load carrying characteristics of the native soil. A soils investigation should be performed to determine the sub-grade strength, load support capabilities, and ground water conditions. In some areas, the swell potential of the native soils must also be addressed. The soil investigation should be performed with test hole locations at appropriate intervals to account for the varying soil conditions that may be encountered.

Pavement thickness is dependent on the loading that will be applied to the pavement, the asphalt mix design and the ability of the underlying soil to support the loads. Full depth asphalt pavement is the overwhelming choice to distribute loads to the sub-grade. However, depending on the existing soil's ability to support the loads, an aggregate base course and/or geo-textile may be used to improve the stability and/or load carrying capability of the native soil.

The geo-technical engineer performing the soils investigation should recommend design thicknesses for the pavement based on the anticipated loading conditions provided to him by the owner, and the results of strength testing performed on the native soils. As soil conditions vary across the site, recommended design thickness may change. The standards for determining the supporting capabilities of the native soil vary depending on location.

The most common test performed is the R-value, AASHTO T190 and T99, ASTM D2844. This test provides a relative soil strength to be applied to nomographs, or design equations, which include environmental and loading criteria for determination of a required structural number for the pavement. The required structural number must be achieved by providing an adequate thickness of pavement. Each pavement layer is assigned a strength coefficient. These strength coefficients are based on the type of material used. A dense graded hot mix asphalt is assigned a strength coefficient of between 0.34 and 0.44, based on research done by AASHTO, and the properties of the mix.

In an area with reasonably good soil (R-value > 20), occasional maintenance vehicle use, and good drainage, a required structural number of approximately 1.6 is determined from design nomographs. To determine the necessary thickness of hot mix asphalt, divide this structural number by the strength coefficient of the material. For a typical hot mix asphalt, we will assume a strength coefficient of 0.40. The calculation of 1.6/0.4 provides a recommended pavement section of 4 inches of hot mix asphalt.

The above example is typical of the method used by geo-technical engineers to provide recommended pavement sections. This example is based on numerous assumptions and should not be used for actual construction. Your geo-technical engineer or landscape architect will provide site specific information for your project.

In general, it is recommended a minimum 3-inches of hot mix asphalt be used for bike paths and trails where loading from vehicles will be negligible. As soil conditions deteriorate and loading increases, the pavement thickness should be increased.

HOT MIX ASPHALT MIX DESIGN

Not all hot mix asphalt is the same, and the type of mix used for a state highway may not be the appropriate mix for a trail or bike path. Specific mixes are designed for specific applications.

The hot mix asphalt specified for your project should provide adequate strength and durability. The overall objective for the design of asphalt paving mixtures is to determine a cost-effective blend of aggregates and asphalt that yields a mix having:

  • sufficient asphalt to provide:
  • durability;
  • adequate stability to resist distortion and displacement;
  • sufficient voids to provide for expansion and contraction due to temperature fluctuations;
  • sufficient workability to allow proper field compaction to resist moisture damage and minimize segregation; and
  • proper aggregate texture and hardness to provide sufficient skid resistance.

Specifications for trail and bike path hot mix asphalt should address the specific application and use, and allow the use of locally available aggregate, where its quality is adequate for the project. The gradation specification should be consistent with local specifications, meeting the IL Department of Transportation criteria.

Bike paths and trails are not subjected to heavy loading. Many of these paths are also constructed in terrain difficult for large construction equipment to access. Thus the hot mix asphalt should have a reasonably high asphalt cement content. This "rich" mix will provide excellent durability and allow for ease of placement and compaction. In addition, high asphalt binder content mixes reduce segregation potential and improve the surface texture of the mix for this type of application. The choice of asphalt cement, or asphalt binder, to be specified will depend on the climactic conditions of the region.

Designers should consider specifying a HMA Surface, Mix C, IL 9.5, N30 with 3.5% voids with a PG 58-28 asphalt binder. Mix designs meeting this criteria will provide an excellent, long lasting pavement for cyclists, walkers, joggers and rollerbladers. It is important to note that using mix design criteria developed for higher traffic volumes (e.g. major arterials and highways) will not provide a mix with sufficient durability and workability for bike path application. Mix design criteria must be representative of the anticipated loading. A mix developed for highway construction will generally contain less asphalt cement and be more prone to oxidation, raveling and cracking on trails and bike paths. Designs developed for low volume application, as outlined above, will compact easier, remain more flexible and provide excellent service life.

GENERAL CONSTRUCTION GUIDELINES

Proper construction of asphalt pavement will ensure a project that provides good serviceability throughout its design life. Recommendations provided by the geo-technical engineer should be followed during construction.

The following are the key elements to consider in building quality asphalt trails;

  • proper drainage
  • proper sub-grade compaction
  • adequate pavement thickness
  • adequate pavement compaction

DRAINAGE

Bike paths and trails should be constructed to match the existing topography as closely as possible, however, longitudinal slopes should not exceed five percent and a cross slope of two percent is desirable to provide adequate drainage away from the pavement surface. Proper drainage is one of the most important factors affecting pavement performance. Proper drainage entails efficient removal of excess water from the trail. Surface water runoff should be handled using swales, ditches and sheet flow. Catch basins, drain inlets, culverts and underground piping may also be necessary. These structures should be located off of the pavement structure.

SUBGRADE

Prior to construction, vegetation should be cleared and stumps and roots removed along the trail for a minimum of five feet outside the edge of the proposed pavement. This will allow construction equipment access and help prevent roots and growth from eventually encroaching on the path. If adequate access width cannot be provided, the contractor will be forced to use less efficient equipment with increased costs.

Bike paths and trails should be constructed to match the existing topography as closely as possible, however, longitudinal slopes should not exceed five percent and a cross slope of two percent is desirable to provide adequate drainage away from the pavement surface. Proper drainage is one of the most important factors affecting pavement performance. Proper drainage entails efficient removal of excess water from the trail. Surface water runoff should be handled using swales, ditches and sheet flow. Catch basins, drain inlets, culverts and underground piping may also be necessary. These structures should be located off of the pavement structure.

The asphalt should be placed on compacted sub-grade that extends a minimum of two feet beyond the edge of pavement. The edge of pavement should be feathered with native soil to avoid any sharp drops from the trail edge. The sub-grade should be prepared by removing vegetation, topsoil and unstable soil, shaping to grade, scarifying the surface to a minimum depth of six inches, moisture conditioning, and compacting. The sub-grade should be compacted to a minimum of 95% of standard Proctor density, AASHTO T 99, and the moisture should be maintained within 3% of optimum. If aggregate base course is used in the pavement section it should be compacted to a minimum of 95% of modified Proctor density, AASHTO T 180, ASTM D 1557.

Depending on the soil conditions, compaction and moisture criteria may vary. Consult your landscape architect or geo-technical engineer for site-specific information. After compaction a soil sterilant and/or root inhibitor should be applied. Application should be carefully controlled to the pavement area only. Typical shaping, grading and compaction crews consist of a motor grader or blade, landscape tractor with back box for grading, and a rubber tire roller for compaction. Additional compaction equipment and access to water may be required.

Prior to placement of the asphalt pavement it is recommended the sub-grade be proof rolled to highlight areas of uncompacted or unstable soil. This may be accomplished using a loaded single axle or tandem dump truck or a loaded rubber tire loader. Soft or unstable areas should be recompacted or removed and replaced with stable soil. It is also important that all utility installations, including sprinkler systems, be complete prior to paving.

PLACEMENT

Placement of the hot mix asphalt should be accomplished with a self-propelled paver, where possible. Where pavers cannot be used, a spreader box attached to a dump truck may be used. Minimum paver width is generally eight feet. For widths less than eight feet cutoff shoes may be placed in the screed to reduce the width of paving. The screed controls mat thickness and crown. Vibratory screeds are typical and provide a small amount of compaction prior to rolling. In general, the uncompacted mat should be 1/4 inch thicker than the final desired thickness to allow for densification during rolling operations.

The hot mix asphalt should be delivered to the paver at a temperature adequate to allow proper compaction. This depends upon the type of asphalt cement used, but generally ranges between 235 to 300 degrees Fahrenheit. The contractor's ability to achieve compaction is dependent on the mix temperature, pavement thickness, subgrade temperature, ambient temperature and wind velocity.

COMPACTION AND JOINT CONSTRUCTION

Compaction should be accomplished immediately after placement by the paver. Steel wheel vibratory rollers are generally used for initial breakdown rolling behind the paver, followed by a steel wheel finish roller. Depending on the compactibility of the mix, a pneumatic tired roller may also be used. Minimize their tendency to pick up fine aggregate from the surface with proper tire temperature or the use of a release agent. It is recommended the hot mix asphalt be compacted to 92%-96% of the Theoretical Maximum Specific Gravity, AASHTO designation T209, ASTM designation D2041.

Joint construction should be carefully done to ensure a uniform mat. Longitudinal joints, which occur where mats are laid side to side, should be constructed with a vertical face or a step taper. The step taper should have a 1.5" vertical face at the surface, tapered at a 3:1 slope from this point to the subgrade. Prior to placing the adjoining mat the joint should be tack-coated. Asphalt placed against a longitudinal joint should overlap the existing asphalt by 1". Compacting longitudinal joints should be accomplished by rolling from the hot side of the asphalt. The steel wheel roller is placed with the majority of the drum on the hot, newly placed asphalt, with approximately 6" of the drum extending over the cold asphalt.

Transverse joints occur at any point the paver ends work and then resumes at a subsequent time. The end of the paving mat should be cut off vertically to allow the full lift thickness to be placed against it. Lumber is used as a bulkhead, paving over the lumber and leaving a taper that is removed along with the bulkhead prior to resumption of paving. Another method is to form a papered transverse joint where heavy wrapping paper is placed along the entire face of the vertical edge of the pavement. The paper extends approximately three to four feet onto the subgrade. The paver resumes paving over the paper to form a taper. Prior to resumption of paving, the paper and material on top of it is removed forming a vertical edge.

When paving resumes the vertical edge is tack-coated, heated and the paver backed over the existing asphalt with the screed resting on the previously placed mat. The shims should have a height equal to the expected compacted thickness, i.e. 1/4" per inch of material. Mix is delivered to the paver and the paver starts forward slowly. Excess mix left by the paver is bumped back to the joint location and/or removed. The joint is then rolled transversely from the cold side beginning with the roller approximately six inches on the newly placed mat and continuing across in six to twelve inch increments. Timbers should be placed along the outside edges of the mat to support the roller and minimize distortion of the outside edges.

MAINTENANCE

Properly constructed asphalt pavement using an appropriate mix design requires minimal maintenance. Providing proper drainage is also a key to reducing maintenance costs. Maintenance is generally divided into two categories, preventative maintenance and corrective maintenance. Preventive maintenance is performed on a regularly scheduled basis to improve the life of the pavement and decrease the rate of deterioration. Corrective maintenance is performed to correct a specific pavement failure or distress area. Normal periodic maintenance, depending on path location, drainage and climate, should include sweeping the trail of debris.

The path or trail should be inspected on an annual basis to determine the overall condition of the drainage, asphalt pavement, signage, pavement markings and vegetation growth. Drainage areas should be improved or repaired where problems are noted. Vegetation should be removed from the pavement and surrounding areas where it will affect use of the path. Signage should be repaired, replaced or upgraded. The asphalt pavement should be inspected for cracks, raveling, disintegration, and premature signs of failure. Cracks which are wide enough (generally 1/4 inch to 1/2 inch) should be thoroughly cleaned, dried and filled with a sealant. The best method is to rout the cracks, clean the crack with compressed air, and pour hot crack filling material into the crack. The crack fill should be left 1/4 inch below the surface of the pavement.

Preventive maintenance should include sealing the surface of the asphalt pavement. Surface seals are used to retard oxidation of the asphalt, restore skid resistance, seal small cracks, provide additional moisture protection to the pavement, and retard raveling of aggregate from the surface. Common surface seals include fog seals, rejuvenators, chip seals and slurry seals. The type of seal used will depend on the age and condition of your pavement. In general, a fog seal will improve the moisture resistance of the pavement, reduce future oxidation and fill small cracks. Chip seals and slurry seals will provide the benefits of fog seals and improve the surface texture and skid resistance of the pavement. Caution should be used on the application of chip seals to trails and paths. Chip seals generally consist of an open graded "coarse" surface which may not be desirable to rollerbladers and road cyclists.

INSPECTOR'S ROLE

The inspector's role is a vital one during asphalt pavement construction. It is the inspector's job to verify that the requirements of the plans and specifications are met in a safe manner. Anomalies from the plans are the rule rather than the exception, and the inspector must be able to exercise judgement and make decisions that ensure the construction of a quality product that will perform as designed.

Prior to beginning construction, the inspector should familiarize himself with all aspects of the planned construction, the plans and specifications. Preconstruction meetings are critical to the success of a project. A mandatory preconstruction meeting should be held several weeks prior to beginning work to review the plans and specifications, verify the contractor's schedule, receipt of submittals such as mix designs and product certifications, and discuss the overall construction techniques and equipment planned to be used to accomplish the construction. This is an excellent opportunity for the contractor to ask questions and discuss potential problems he sees and receive feedback from the contracting agency on how potential situations may be handled. The contracting agency, project engineer, construction inspector, contractor, significant subcontractors, and the geotechnical engineer or testing laboratory, including their field representative, should attend the preconstruction meetings. Minutes of the meeting should be distributed to all participants.

The rapport between the inspector and contractor is critical. The inspector's main concern is quality; the contractor's main concern is quantity. Reductions in quality should not be allowed in the interests of quantity. The inspector will obtain better results by diplomatically working with the contractor to obtain the highest quality possible.

The inspector should oversee construction and be available to answer questions or know who to contact to get answers to questions which may arise. The inspector's role is to verify that the plans and specifications are being adhered to, and makes the contractor aware of any deficiencies immediately. The inspector must generally obtain approval from the project engineer for any changes to the design. The contractor's responsibility is to determine how to provide construction in accordance with the plans and specifications. The inspector may work with the surveyors, testing laboratory and traffic control. if necessary, to assist in interpreting the plans and specifications. In general, contractors should provide quality control for their workmanship and materials. The contracting agency provides quality assurance to verify the contractor's quality control results.

The inspector, through daily communication with the contractor, should verify that minimum test frequencies are being adhered to, and should obtain copies of the test results in a timely manner. In general, the inspector should verify that the contractor has the proper lines and grades, subgrade compaction, mix temperature of the delivered hot mix asphalt, quantity of material delivered, compaction of the asphalt pavement, thickness, smoothness and proper joint construction. The inspector should also verify that the mix delivered to the project is within the production tolerances of the mix design submitted for the project. As a minimum, the mix design should be field verified prior to or during the first day's production. This verification provides an assurance that the mix can be produced as designed, or allows the contractor the ability to make adjustments, as necessary. Normal mix production testing should consist of at least one asphalt content test and one gradation test per day.

Proper inspection is vital to ensuring a quality asphalt pavement is constructed. According to Carter Page, an engineer with Jerome Gamba Associates in Glenwood Springs, inspection keys include "adequate sub-grade temperature at time of paving, level sub-grade that has been properly compacted and passed the proof-roll, and the use of the right paving equipment with trained operators to obtain a smooth, properly compacted asphalt pavement. We have provided a list of top tips for the inspector, below.

SUMMARY

Problems associated with the performance of asphalt pavement paths and trails include the growth of weeds and emergence of tree roots, subgrade failure, raveling and high maintenance costs. Properly designed and constructed asphalt pavements together with the proper thickness and proper preparation will help ensure a high quality product.

It is the responsibility of the pathway design team, comprised of the owner, landscape architect, civil engineer, geo-technical engineer and other professionals, along with the contractor, to ensure the benefits of asphalt pavement are realized. Outlined below are some keys to a successful, high quality path or trail.

KEYS TO QUALITY

Design to meet the needs of the anticipated users.

  • Follow guidelines in AASHTO Guide for the Development of Bicycle Facilities;
  • Determine pavement loading;
  • Determine load carrying characteristics of native soil;
  • Design pavement section to meet soil loading and environmental conditions;
  • Provide good drainage;
  • Design asphalt mixture to meet loading conditions;
  • Properly compact asphalt pavement; and
  • Plan preventive maintenance.

This guide has been prepared to aid in the proper design and construction of asphalt pavement for paths and trails. Specifying asphalt pavement trails and paths provides the agency and public with numerous benefits.

BENEFITS OF ASPHALT PAVEMENT

  • Cost effective construction providing the consumer with more miles of usable paths.
  • A user friendly surface providing a smooth, quiet, continuous surface with no joints for more enjoyable cycling or rollerblading.
  • A flexible surface more forgiving for walkers and loggers.
  • An aesthetically pleasing pavement surface which is constructed to blend with existing contours.
  • A low maintenance surface providing ease of repair if springtime flooding or soil heaving occurs.
  • Reduced construction time allowing more efficient use of personnel and increased trail use by the public.

Additional information is available on the American Trails website: www.AmericanTrails.org.