Hot Mix Asphalt Technology

VOLUME 15, NUMBER 3

MAY/JUNE 2010

PUBLISHED BY THE NATIONAL ASPHALT PAVEMENT ASSOCIATION

WWW.HOTMIX.ORG

Going Green There’s More to Being a Green Producer Than Warm Mix and Recycling

Eliminating Delamination in Asphalt Overlays The Evolution of Perpetual Pavements Tack Coats: Setting the Record Straight Warm Mix: Where We Are, Where We’re Going

Troxler Model 5850 Gyratory Compactor

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molds for Asphalt Mix Performance Testing and a Shear Measurement option.

Trust Troxler to make testing easier with a keypad adjustable angle, the most ergonomic design on the market and USB and serial ports for data storage and downloading.

Patents Pending

Trust Troxler

Trust TROXLER The Leader in Construction Testing Equipment Troxler Electronic Laboratories, Inc. - Troxler International, Ltd. 3008 Cornwallis Road, P.O. Box 12057, Research Triangle Park, NC 27709 - USA Telephone: 1.919.549.8661 Telefax: 1.919.549.0761 Web: troxlerlabs.com

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The RAPwranglerTM is designed to be powerful, efficient and easy to maintain. It can shred football sized asphalt chunks, down to more suitable gradations for introduction into both stationery and portable drum mix and batch plants. It eliminates off-site pre-crushing of asphalt materials to be recycled. It is powered by either a 30HP or a 50HP TEFC motor, and can be shive belt or direct driven.

Shaker decks are a very economical way to filter out oversized aggregate or RAP from getting into your mix. Shaker decks are economical, durable, dependable, rugged and affordable. As a result of their simple construction, there is less to maintain and break down. Shaker decks are constructed to use common size screen cloths, thereby making it easy to locate the various sizes you need to get your mix done.

The 20" rotor is equipped with 312 replaceable tungstentipped milling teeth that shred chunks of asphalt down to 1" at a rate of 250 tons per hour. The rotor spins at a constant 127rpm undermining the material in the 30" x 40" hopper directly above it.

Shaker decks can be custom made - such as mounting the motor on a certain side, should you be tight for room. A custom stand can also be manufactured for your shaker deck. Alternatively, you can have a conveyor line in either stationery or portable configurations manufactured. Shaker decks come in various sizes. Single decks range from 3' x 5' up to 5' x 10'. Screen cloths are also available.

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United States of Amercia Office Trinidad Asphalt Corporation of America (TACA) One Gateway Center, Suite #2600 Newark, NJ 07012 Tel: 973.622.3423

TLA-X is a warm mix additive providing grade bump to the next higher level with load bearing enhancement and environmental benefits.

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MAY/JUNE 2010 – VOLUME 15, NUMBER 3

THE VOICE OF THE ASPHALT PAVEMENT INDUSTRY

C O N T E N T S 2010 National Officers CHAIRMAN Larry H. Lemon Haskell Lemon Construction Co. Oklahoma City, Oklahoma FIRST VICE CHAIRMAN Kim W. Snyder Eastern Industries, Inc. Center Valley, Pennsylvania SECOND VICE CHAIRMAN Kurt Bechthold Payne & Dolan, Inc. Waukesha, Wisconsin THIRD VICE CHAIRMAN John J. Keating Oldcastle Materials Grp.-East Leominster, Massachusetts SECRETARY William C. Ensor, III Maryland Paving, Inc. Timonium, Maryland TREASURER Michael Cote The Lane Construction Corporation Cheshire, Connecticut NAPA Executive Staff

Features 13 Book Review: Third Edition of Hot-Mix Asphalt Materials, Mixture Design and Construction 16 Eliminating Delamination in Asphalt Overlays 18 The Evolution of Perpetual Pavements 22 Going All Green: A Contractor’s Guide to Environmental Stewardship

34 Unlocking the Potential of Porous Asphalt Pavement 40 Warm Mix: Where We Are, Where We’re Going

Column 9

Chairman’s Commentary

In Every Issue 46 On the Road with Pavia Systems A Closer Look at How Warm Mix is Produced 49 Tools for the Trade

Margaret Blain Cervarich Vice President, Marketing & Public Affairs

50 Calendar of Events

R. Gary Fore Vice President, Environment, Health & Safety Jay Hansen Vice President, Government Affairs Kent Hansen, P.E. Director of Engineering Mike Kvach Vice President, Product Deployment Nancy Lawler Vice President, Convention & Meetings Patricia Davitt Long Director of Communications Howard Marks, Ph.D., J.D. Director of Regulatory Affairs David Newcomb, P.E., Ph.D. Vice President, Research & Technology Carolyn E. Wilson Vice President, Finance & Operations

22

11 Industry News

Mike Acott President

Tracie Christie Associate Director of Awards & Marketing

16

30 Tack Coats: Setting the Record Straight

49

52 Index of Advertisers 54 Advertisers.com On the cover: APAC Southeast, Inc. in Kissimmee, FL has been a recipient of NAPA’s Diamond Achievement Commendation since 2000.

National Asphalt Pavement Association All contents copyright NAPA 2010, world rights reserved. – 3rd class postage paid at Fargo, ND – Subscriptions to HMAT are complimentary to qualified public works officials, pavement consulting engineers, architects, and others involved with paving materials and pavement design, construction and maintenance at U.S. addresses. To receive HMAT, send your request on your agency or company letterhead to the address below. Address all editorial inquiries and correspondence to Editor, HMAT, National Asphalt Pavement Association, 5100 Forbes Blvd., Lanham, MD 20706-4407, 301-731-4748, fax: 301-731-4621, e-mail: [email protected], or visit www.hotmix.org. Address advertising insertion orders and inquiries to Naylor, LLC, HMAT Magazine, 5950 Northwest 1st Place, Gainesville, FL 32607-2025, 800-369-6220, 352-332-1252, fax: 352-331-3525, or visit www.naylor.com. Published by Naylor, LLC., 5950 Northwest 1st Place, Gainesville, FL 32607-2025, 800-369-6220, 352-332-1252, fax: 352-331-3525, www.naylor.com

Publishers: Mike Acott, NAPA President Chris Hodges, Naylor, LLC Editor: Patricia Davitt Long, NAPA Director of Communications Managing Editor: Catherine Jones, Naylor, LLC Project Manager: Tom Schell Sales Manager: Tom Schell Publication Director: Paul Walley Advertising Sales: Ryan Griffin, Rick Sauers, Jamie Williams, Chris Zabel Contributing Writer: Mary Lou Jay Layout and Design: Emma Law Advertising Art: Reanne Dawson Published April 2010/NAP-S0310/9736

Hot Mix Asphalt Technology – MAY/JUNE 2010 • 7

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CHAIRMAN’S COMMENTARY A Legacy of Which We Can All Be Proud What you leave behind is not what is engraved in stone monuments, but what is woven into the lives of others. Pericles

by Larry Lemon

A

s I take the reins as NAPA Chairman for 2010, there is one thing of which I am certain; the greatest asset my generation of contractors can leave to the next generation is long-lasting asphalt roads that are built as Perpetual Pavement. The legacy that we, as a road building industry, can provide for future generations will be pavements that remain a permanent asset for this nation. Through research and a desire for continuous improvements, the asphalt paving industry has created the technology to produce high-quality pavements that our grandchildren and their grandchildren will be able to use, pavements that are infinitely reclaimable, reusable, and renewable. Without a doubt, asphalt perpetual pavement is the greatest thing that’s happened to the road construction industry. It is the best value for the owner/engineer, and for the American taxpayer. It has transformed the way we engineer and build our roads and has created a new generation of highways that are safer and smoother for the traveling public. Perpetual Pavements are defined as “an asphalt pavement designed and built to last longer than 50 years without requiring major structural rehabilitation or reconstruction, and needing only periodic surface renewal in response to distresses confined to the top of the pavement.” This concept was validated by the Asphalt Pavement Alliance (APA)

in 2001 with structural engineering of current asphalt materials and designs. However, our industry has been building long-lasting asphalt pavements since the 1960s, and more than 69 pavements across the nation have already received awards for being built as “Perpetual Pavements,” since they have been giving their owners long life, with minimum maintenance. The APA recognized that many well-built, thick asphalt pavements that were categorized as either “fulldepth” or “deep-strength” have been in service for decades, with only periodic surface rehabilitation to remove defects and improve ride quality. Using these pavements as a model, the APA designed the methodology for a pavement that resists bottom-up fatigue cracking and deep structural rutting. When the structure of a roadway can support the millions of cars and trucks that will be using it, our only needed maintenance will be to renew the surface every 15 or 20 years. What a great benefit our families receive when we are able to renew the driving surface with the latest, safest, anti-skid materials of the day and give the roadway a completely new life. And even better, we can do this in offpeak hours when most of America is asleep, to minimize the inconvenience of our construction. There’s no need for the entire highway to be removed and replaced from the ground up, as with other pavement types. Perpetual pavements are more environmentally

friendly than other pavements because they reduce the amount of material resources over the life of the pavement and recycle any materials removed for the old pavement surface. This issue of HMAT includes a perpetual pavement synthesis overview that lays out the history of perpetual pavements and the design philosophies that have been shown to provide adequate strength over extended life cycles. Incredibly, these asphalt pavements have endured an unprecedented amount of traffic growth – a 580 percent increase in the average daily ton-miles of freight – from 1970 to 1998. And, that freight loading continues to increase 2.7 percent per year. To my mind, the story of perpetual pavements is an example of true leadership in action. It reflects vision and execution and results in a greater good for our society and for generations to come. It is just one of the reasons why I’m proud to be an active member of the asphalt industry, and the Chairman of the National Asphalt Pavement Association. My children, and their children, will be better off, both economically and environmentally, because of the asphalt roads we are building today. That’s a legacy of which we can all be proud. HMAT Larry Lemon is the president of Haskell Lemon Construction Co. He is the 2010 Chairman of the National Asphalt Pavement Association.

Hot Mix Asphalt Technology – MAY/JUNE 2010 • 9

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INDUSTRY NEWS Global Alliance Fosters New Asphalt Initiatives The Global Asphalt Pavement Alliance (GAPA), an international group of asphalt pavement associations organized to foster greater international coordination within the industry, held their second meeting in January, immediately following the NAPA Annual Meeting. Represented were Europe (EAPA), Southern Africa (SABITA), Australia (AAPA), Japan (JRCA), and the United States (NAPA). Each association gave presentations on the challenges and opportunities in its country and the group formulated some key agreements with regard to industry initiatives. The Alliance agreed to release a position statement extolling the benefits of warm-mix asphalt, including support for an international warm-mix conference in 2011. The group also agreed to evaluate the UK’s carbon calculator, and create a statement on the differences between “re-use” and “recycling” to highlight the benefits of asphalt in this regard. The Alliance elected NAPA President Mike Acott as Chairman of GAPA for 2010-2011.

Roads Fail in Engineers’ 2009 Infrastructure Report Card The condition of America’s roads continues to worsen, according to the American Society of Civil Engineers (ASCE). The group’s fouryear Infrastructure Report Card, issued earlier this year, gave roads a grade of D-. Roads, transit and aviation were cited by ASCE as having worsened since its 2005 report card, while bridges and most other types of infrastructure were thought to have stayed the same. Joining roads at the D- grade level, the lowest on the 2009 report card, were drinking water, inland waterways, levees and wastewater.

The highest grade on the 2009 report card, a C+, went to solid waste infrastructure. ASCE said the 2009 report card reflected the need for a fiveyear investment of $2.2 trillion from all levels of government and the private sector. The group also noted that the cost of needed improvements has increased by more than a half trillion dollars since the 2005 report card.

Biodiesel Passes the Mining Test Advocates of biodiesel fuels are touting a new 206-page report by the federal government’s Mine Safety Health Administration that gives the alternative fuels high marks for air quality in underground mining tests. After testing multiple biodiesel blends, MSHA concluded that biodiesel reduces emissions, especially when combined with the use of after-treatment devices on diesel engines. “The result is a cleaner and healthier working environment for miners,” MSHA states. Biodiesel is touted as a cleaner burning, renewable fuel, and may be part of a future certification program for green highway contractors. It is made from agricultural co-products, such as soybean oil, other vegetable oils, fats, and recycled cooking oil.

Road Usage Increased in 2009 The Federal Highway Administration’s estimate of Vehicle Miles Traveled increased by 1.4 percent in November 2009 compared to November 2008, the latest month reported at press time. Travel for the month was estimated at 236.4 billion vehicle miles, up 3.2 billion vehicle miles from the same period in 2008.

Highway and street usage peaked at 2,789 billion vehicle miles traveled in 2007, then declined to 2,687 VMT in 2008. Road usage has increased since the end of the first quarter of 2009, though VMT values dropped slightly in October before rising again in November. FHWA projects that VMT will increase 0.3 percent for the year.

DOT Report Targets Construction Inflation Spending by all levels of government on the nation’s highways and transit lines has increased substantially in recent years but steep increases in construction costs have eroded the purchasing power of this investment, according to a U.S. Department of Transportation report released earlier this year. The report, 2008 Status of the Nation’s Highways, Bridges, and Transit: Conditions and Performance, focused on changes in various indicators from 1997 through 2006. “In nominal dollar terms, combined investment by all levels of government in highway and bridge infrastructure has increased sharply since TEA21 was enacted,” according to the report, AASHTO News reported. Those expenditures jumped 58 percent between 1997 and 2006. However, capital spending dropped 4.4 percent in constant dollar terms during that timeframe because of construction inflation. The Federal Highway Administration’s Composite Bid Price Index shot up 43 percent between 2004 and 2006 due to notable increases in the prices of such materials as steel, asphalt, and cement. HMAT

Hot Mix Asphalt Technology – MAY/JUNE 2010 • 11

AGGRESSIVE WHEN YOU WANT IT, GENTLE WHEN YOU NEED IT. Versatile Cat® Asphalt Compactors use proven technology that is easy to set up for any mix or application. Not every mix and application is the same, so your asphalt compactor should be versatile and easy to set up to match performance to ever-changing conditions. If it isn’t easy to use, how can it be productive? Cat® Asphalt Compactors get the work done on the breakdown pass, when the mat temperatures are higher and conditions are optimal for compaction. They are comfortable and simple to operate: high amplitude for aggressive compaction, low amplitude for gentle massaging. They match high paving speeds with high frequency, and low paving speeds with low frequency. Simple and reliable, with predictable density every time in minimal passes. No complexity and constant tweaking, as there is using “non-aggressive” compaction technologies; no hoping your density numbers will catch up on the intermediate or final pass. Caterpillar is the market leader because we offer proven technology that makes turning a profit easier. Isn’t that what technology is supposed to do? To get the real story on asphalt compaction, visit your Cat Dealer today. www.cat.com SAFELY HOME. EVERYONE. EVERYDAY. SAFETY.CAT.COM™

CAT, CATERPILLAR, their respective logos, “Caterpillar Yellow” and the POWER EDGE trade dress, as well as corporate and product identity used herein, are trademarks of Caterpillar and may not be used without permission. ©2010 Caterpillar All Rights Reserved

BookThird Edition Review of Hot-Mix Asphalt Materials, Mixture Design and Construction

(E. R. Brown and N. Tran)

Reviewed by: Monte Symons, National Center for Asphalt Technology

H

ot-Mix Asphalt Materials, Mixture Design and Construction was first written in 1991, and the second edition was prepared in 1996. The third edition of the book was completed in 2009 and is now available from the National Asphalt Pavement Association (NAPA). It has been nearly 13 years since the second edition and many innovations have been developed and implemented in recent years. Prior to the development of the first edition of this book, there was very little comprehensive information available in one location on hot-mix asphalt (HMA). Only a few academic programs at universities throughout the country were available to address HMA technologies and pavement construction practices. In fact, engineers who worked with asphalt pavements received most of their experience on the job, resulting in a trial and error approach that detracted from the quality and cost effectiveness of HMA. The hot-mix asphalt industry has come to rely upon the Hot-Mix Asphalt Materials, Mixture Design and Construction textbook to inform and educate future leaders. The third edition remains dedicated to continuing this work and providing information on all aspects of producing and delivering quality hot-mix asphalt. In the third edition, the latest HMA technologies are incorporated and general background and basic fundamentals are updated. Chapter 1 provides the background and importance of this all inclusive text book on HMA pavements. Chapter 2 covers the production and classification of asphalt cement. This includes the basics of crude

refining, asphalt classifications and latest technologies in asphalt testing. Chapter 3 is about the production and characteristics of aggregates used in HMA. Basic aggregate mineralogy, crushing and characteristic of quality aggregates and gradation are provided in this chapter. Chapter 4 addresses mix design procedures including Marshall, Hveem and Superpave. The background for development of each method is provided and example mix designs are illustrated. Chapter 5 was completely rewritten to include the most recent technical developments in evaluating the mechanistic engineering properties of asphalt mixtures. Basic empirical tests such as Marshall, Hveem and wheel tracking tests are explained. This chapter also gives insight into tests that identify basic engineering properties of HMA. Resilient modulus, indirect tensile, dynamic modulus, flow number and complex modulus properties are discussed. In addition, the relationships between these tests and predicted field performance are provided. The chapter also provides introductory information on emerging concepts such as creep compliance and dissipated energy determinations. Chapter 6 provides information on the latest equipment and construction techniques for production and field operations required for quality HMA. This includes batch and drum mixers, calibration of plant operations, paver fundamentals, roller basics and density measurement equipment. Chapter 7 addresses issues and requirements of special mixtures and additives. This chapter incorporates new warm-mix technologies and includes stone mastic and recycled mix fundamentals. Chapter 8

gives information on performance measure and distresses associated with HMA. Present serviceability index, construction smoothness requirements and common visual distress present on HMA are discussed in this chapter. Chapter 9 addresses maintenance rehabilitation and reconstruction of HMA. This chapter discusses timing of maintenance procedures and typical application of procedures to preserve HMA pavement or correct deficiencies. The comprehensive coverage of the production, placement, and performance of HMA in the third edition of Hot Mix Asphalt Materials, Mixture Design and Construction is unparalleled. The asphalt paving industry is a key component of the transportation system. Over 94 percent of all paved roads have asphalt surfaces, with more than 500 million tons of HMA produced each year. The industry is very important to the financial well being of local, state, and national government agencies as areas with good transportation systems have been proven to be more successful in attracting new industry and creating jobs. The third edition of Hot-Mix Asphalt Materials, Mixture Design and Construction provides the new and practicing pavement engineer with the latest information needed to fulfill the promise of delivering high quality HMA pavement to the transportation community. HMAT This review was written by Monte Symons. Mr. Symons is retired from the Federal Highway Administration where he worked for over 30 years in various pavement and materials-related positions. He is currently the Director of the Airfield Asphalt Pavement Technology Program.

Hot Mix Asphalt Technology – MAY/JUNE 2010 • 13

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Delamination Delaminat

Eliminating in Asphalt Overlays

Louisiana Researchers Evaluate the Importance of Tack Coat By Skip Paul, P.E.

O

ne of the most avoidable causes of cracking and potholing in surface pavements is the type of delamination that occurs when the surface lift separates from the pavement structure below it due to insufficient bonding between the layers. Delamination is characterized by crescent-shaped cracks resulting from horizontal forces induced by traffic load. It is most commonly found in pavements subject to the stresses of stop and go traffic, or where vehicles turn frequently. This form of delamination can be prevented with a strong tack coat treatment prior to the placing of the surface course. The Louisiana Transportation Research Center is breaking new ground in this area with the ongoing study, Optimization of Tack Coat for HMA Placement (National Cooperative Highway Research Program project 9-40). In this work, Louisiana researchers are determining the optimum application methods, equipment type, calibration procedures, application rates, and asphalt binder materials for tack coats. They also will recommend revisions to relevant AASHTO methods and practices related to tack coats. Louay Mohammad, Ph.D., Louisiana State University Civil and Environmental Engineering professor and Manager

of the Engineering Materials Characterization Research Facility at the research center is currently spearheading the project as its principal investigator.

Why tack coat? “A tack coat provides necessary bonding between pavement surface layers to make sure they act as a monolithic system to withstand the traffic and environmental loads,” explains Dr. Mohammad. “Strong tack coat bonding between pavement layers is critical to transfer radial tensile and shear stresses into the entire pavement structure.” Mohammad also stresses that an insufficient bond decreases the pavement bearing capacity and may cause slippage. In addition, insufficient bonding may cause tensile stresses to be concentrated at the bottom of the wearing course. Such concentrated stresses may accelerate fatigue cracking and lead to total pavement failure. To evaluate the quality of the bond strength of tack coat materials for this NCHRP research, a new test device named the Louisiana Tack Coat Quality Tester has been developed. Researchers evaluated three emulsions (CRS-1, SS-1h, and trackless) as well as PG 64-22 asphalt cement on an existing pavement surface with the Quality Tester. They found that each tack coat material developed its

16 • Hot Mix Asphalt Technology – MAY/JUNE 2010

One of the tell-tale signs of delamination crescent-shaped cracking. maximum tensile strength at a different temperature, and that each product’s softening point had a strong relationship to the temperature at which it exhibits its maximum tensile strength. They concluded that they can conduct the tack coat pull-off test in the field at the softening point. The research team also developed a direct shear device, known as the Louisiana Interlayer Shear Strength Tester to measure the interface shear strength of cylindrical specimens. The device is designed to fit into any universal testing machine. It has a nearly frictionless linear bearing to maintain vertical travel and can accommodate sensors that measure vertical and horizontal displacements. The Shear Strength Tester can also

This illustration depicts distress modes at the pavement interface under service conditions.

apply a constant normal load up to 689 kPa and it accommodates specimens with 100-mm or 150mm diameters. Researchers used the device to evaluate the interface shear strength of emulsified tack coats under a wide range of testing conditions commonly encountered in field applications. Three types of emulsified tack coats (CRS-1, SS-1h, and trackless) were considered at three application rates, 0.14/ m2, 0.28/m2, and 0.70 l/m2. In addition, a “no tack coat” condition was included in the analysis. The effects of construction conditions such as wet (rainfall) and dusty conditions were also evaluated. Laboratory direct shear tests were performed at 25ºC. To simulate these test conditions, cores were extracted from a full-scale test site at the Center’s Pavement Research

Facility. This test site was designed and constructed using conventional tack coat application and paving equipment over an existing asphalt pavement surface. A preliminary analysis of the results showed that the trackless tack coat produced the highest shear strength at the three application rates, while SS-1 and CRS-1 resulted in the medium and lowest strengths, respectively. The majority of the cases showed a statistically significant difference between clean and dusty conditions. However, no significant difference was found between dry and wet conditions.

Results on the roadways Dr. Mohammad explains, “It’s worth noting that the test method for the measurement of the interface bond strength using the

Researchers tested the interface shear strength of three different tack coats at different application rates. The trackless tack coat produced the highest shear strength at each application rate.

LISST device developed during this NCHRP project was successfully used in a recent forensic analysis of a distressed pavement.” Louisiana researchers also discovered that low interface shear strengths, less than 40 psi, measured between the wearing and binder course, confirmed the potential for future problems. In summary, to prevent future delamination failures, agencies should ensure the total structural design is sufficient and insist on good tack coat material that must be applied uniformly at sufficient rates to produce minimum shear strength at the interface of the top two layers.

References Mohammad, L.N., Bae, A., Elseifi, M., Button, J., and Scherocman, J. “Development of Pull-Off Test Device and Methodology to Evaluate the Bond Strength of Tack Coat Materials in the Field.” Journal of the Transportation Research Board, TRR No. 2126, 2009, pp. 1-11. Mohammad, L.N., Bae, A., Elseifi, M., Button, J., and Scherocman, J. “Interface Shear Strength Characteristics of Emulsified Tack Coats.” Journal of the Association of Asphalt Paving Technologists, Vol. 78, HMAT 2009, pp. 249-278. Harold “Skip” Paul is the Director of the Louisiana Transportation Research Center.

Hot Mix Asphalt Technology – MAY/JUNE 2010 • 17

The Evolution of Perpetual Pavements – An Overview By Dave Newcomb, P.E., Ph.D

N

APA, through the Asphalt Pavement Alliance (APA), has developed a new technical document – Perpetual Asphalt Pavements: A Synthesis, which can be found on the APA Web site at www. asphaltroads.org. The concept of Perpetual Pavements was introduced in 2000 by the Asphalt Pavement Alliance (APA). They defined a Perpetual Pavement as an asphalt pavement designed and built to last longer than 50 years without requiring major structural rehabilitation or reconstruction, and needing only periodic surface renewal in response to distresses confined to the top of the pavement. At that time, it was recognized that many well built, thick asphalt pavements that were categorized as either full-depth or deep strength pavements had been in service for decades with only minor periodic surface rehabilitation to remove defects and improve ride quality. The advantages of such pavements include: 1. Low life-cycle cost by avoiding deep pavement repairs or reconstruction, 2. Low user delay costs since minor surface rehabilitation of asphalt pavements only require short

work windows that can avoid peak traffic hours, and 3. Low environmental impact by reducing the amount of material resources over the pavement’s life and recycling any materials removed from the pavement surface. Pavement engineers have been producing long-lasting asphalt pavements since the 1960s. Research at institutions such as the University of Washington and the University of California has shown that well-constructed and well-designed flexible pavements can perform for extended periods of time. Many of these pavements in the past forty years were the products of full-depth or deep strength asphalt pavement designs, and both have design philosophies that have been shown to provide adequate strength over extended life cycles. It is significant that these pavements have endured an unprecedented amount of traffic growth. For instance, from 1970 to 1998, the FHWA estimates the average daily ton-miles of freight increased by 580 percent, and the average freight loading continues to increase 2.7 percent per year. As the demand on existing pavements in the U.S. increases with probably minimal funding for expansion and rehabilitation, efficient design

18 • Hot Mix Asphalt Technology – MAY/JUNE 2010

of new and rehabilitated sections through Perpetual Pavement design will become increasingly important. Congestion on the existing system requires pavements that can be maintained with minimal disruption of traffic. Full-depth pavements are constructed by placing asphalt pavement on modified or unmodified soil or subgrade material. Deep strength pavements consist of asphalt pavement layers on top of a thin granular base. Both of these design scenarios allow pavement engineers to employ a thinner total pavement section than if a thick granular base were used. By reducing the potential for fatigue cracking and by confining cracking to the upper removable/ replaceable layers, many of these pavements have far exceeded their design life of 20 years with minimal rehabilitation; therefore, they are considered to be superior pavements. Pavements which are either under-designed or poorly constructed exhibit structural distresses, such as fatigue cracking and rutting, before their design life is achieved. The successes seen in the full-depth and deep strength pavements are the result of designing and constructing pavements that

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Pavement Age (Years) PCCP

HMA

A study of Interstate Highways in Kansas by Professor Steve Cross showed that over a 40-year period asphalt pavements cost less than concrete pavements. Perpetual Pavements are economical. 1000 HMA (Lane Miles) PCCP (Lane Miles) 800

Lane Miles

resist these detriments to the pavement’s structure. In recent years, pavement engineers have begun to adopt a methodology of designing pavements to resist bottom-up fatigue cracking and deep structural rutting, the two most devastating pavement distresses, and through this change in thinking the idea of Perpetual Pavements or long-lasting pavements has evolved. The approach to the design of Perpetual Pavements requires a different strategy than that which has normally been applied to pavement design in the past. Empirical pavement design must rely on relationships between observations of pavement performance, a scale that represents traffic, some gross indicator of material quality such as a structural coefficient, and the thickness of the layers. For a given level of material quality, the thickness of the pavement increases with increasing traffic. A somewhat unified approach to designing Perpetual Pavements was adopted by a number of experts based on mechanisticempirical concepts originally proposed by Professor Carl Monismith in the design of the I-710 freeway in California. The premise to this approach was that pavement distresses with deep structural origins could be avoided if pavement responses such as stresses, strains, and deflections could be kept below thresholds where the distresses begin to occur. Thus, an asphalt pavement could be designed for an indefinite structural life by designing for the heaviest vehicles without being overly conservative. This contrasts to empirical methods that predated the Perpetual Pavement design approach. In those design procedures, greater volumes of heavy vehicles resulted in greater pavement thickness. This was due largely to the way these empirical

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31 - 40

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61 or More

Age (Ye a rs)

A study of Interstate Highways in Washington State by Professor Joe Mahoney showed that asphalt pavements are as old or older than concrete pavements. Asphalt pavements have a track record of long life. 1 Award 2 Awards 3 Awards 5 Awards 6 Awards 7 Awards

Since 2001, the Asphalt Pavement Alliance has given out more than 56 Perpetual Pavement Awards to agencies who have submitted long-lived pavement sections across the country. Hot Mix Asphalt Technology – MAY/JUNE 2010 • 19

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