| Course Name: Basic Civil Engineering – Asphalt | |
| Asphalt is that black stuff that has potholes in it and ruins your car, right? Or, that stuff that folks in hard hats are always playing with, blocking traffic and making lives miserable – a good excuse when you are late to work. The typical person only thinks about asphalt when it affects them detrimentally. But as a Construction professional who works with asphalt, our job is to make sure they never see us. How can we do that? This one hour online course outlines the problems that can arise when working with asphalt and the current solutions, helping you remain as invisible as possible. This course includes a multiple choice quiz at the end. |
At the conclusion of this course, you will:
- Be more familiar with the state of the asphalt industry and its history
- Have reviewed the basics of asphaltic concrete and stone matrix asphalt
- Be knowledgeable in asphaltic concrete maintenance
- Have looked closely at several case histories investigating asphalt failures
Introduction
A quick look at the data available on asphaltic concrete shows problems. The product is not perfect, even though ninety-six percent of all paved roads and streets in the U.S. – almost two million miles – are surfaced with asphalt and approximately 500 million tons of hot mix asphalt is placed every year.
Some of the problems with asphalt are poor longitudinal joint construction, rutting, deformation, cracking, abrasion, creep and slippage, and excessive permeability.
This course will touch on many of the problems, and the solutions presently entertained.
State of the Industry
Ninety-six percent of all paved roads and streets in the U.S. – almost two million miles – are surfaced with asphalt. This has given rise to the following kind of “tongue in cheek” attitude:
Pave the Earth
Pave the Earth started as a newsgroup, in which the group’s credo was spelled out and various corollaries were endlessly debated. It has now spilled over onto a set of web pages, which give the background and philosophy behind the goal of covering the entire planet with a layer of asphalt.
All of this is followed with a religious-like fervor. Indeed, most references call it Holy Asphalt, and it is considered sacrilege to suggest substituting concrete. (Concrete, you see, requires seams to allow for expansion and contraction. Seams are not good because they slow down the cars.)
The gist of the philosophy is as follows:
The world would be a much better place if it were covered by asphalt, so you could drive all over the globe at high speeds.
The HMA (Hot Mix Asphalt) Industry
The HMA (Hot Mix Asphalt) industry directly employs approximately 300,000 people, and indirectly accounts for an additional 600,000 jobs. On average, the industry produces and places approximately 500 million tons of hot mix asphalt annually valued at some $20 billion. When combined with state and federal employees associated with the construction and maintenance of asphalt surfaced roads, the HMA industry has a significant impact on the economic vitality of the nation. This also means that there are a lot of people looking over shoulders making sure that the industry is doing the best job possible.
Superpave
One of the major things that has come of the industry is what is known as “Superpave”. Many misconceptions abound about Superpave, and one of the reasons is that the program has been adopted before the program has been completed. Another is the expense of converting over to the Superpave Gyratory Compactor (SGC). Many smaller agencies are staying with the tried and true until they have to change, but I believe that will happen as the Superpave program is completed.
The Superpave Mixture Design and Analysis System was developed in the early 1990’s under the Strategic Highway Research Program (SHRP). Originally, the Superpave design method for Hot-Mix Asphalt (HMA) mixtures consisted of three proposed phases:
- materials selection,
- aggregate blending, and
- volumetric analysis on specimens compacted using the Superpave Gyratory Compactor (SGC).
The fourth step, which will provide a method to analyze the mixture properties and to determine performance potential, is not yet available for adoption. Testing for this step is expected to be completed in August of 2003:
Project 9-19, FY 1998
Superpave Support and Performance Models Management
| Res. Agency: | University of Maryland |
| Principal Invest: | Matthew W. Witczak |
| Effective Date: | May 10, 1999 |
| Completion Date: | August 9, 2003 |
| Funds: | $2,501,323 |
| NCHRP Staff: | Edward T. Harrigan |
Background
This project will continue work started in July 1995 under sponsorship of the Federal Highway Administration (FHWA). The original FHWA project was divided into two phases. Phase I, completed in September 1996, evaluated the Superpave performance models developed through the Strategic Highway Research Program.
In Phase II, begun in November 1997, the contractor was tasked with development and validation of an advanced material characterization model and the associated calibration and testing procedures for hot mix asphalt used in highway pavements. It also includes a discrete but important task to identify a simple performance test to complement the Superpave volumetric mix design method.
Most highway agencies in the United States have now adopted the volumetric mixture design method. However, as indicated, there is not yet a strength test to compliment the Superpave volumetric mixture design method, this is not expected until after the testing (my guess is 2004 or 2005). The traditional Marshall and Hveem mixture design methods had associated strength tests. Even though the Marshall and Hveem stability tests were empirical, they did provide some measure of the mix quality. There is much work going on to develop a strength test. Considering that approximately 2 million tons of HMA is placed in the U.S. during a typical construction day, contractors and state agencies must have some means as soon as practical to better evaluate performance potential of HMA. These test methods do not have to be perfect, but they should be available for assuring good mix performance. Research from WesTrack, NCHRP 9-7 (Field Procedures and Equipment to Implement SHRP Asphalt Specifications), and other experimental construction projects have shown that the Superpave volumetric mixture design method alone is not sufficient to ensure reliable mixture performance over a wide range of materials, traffic and climatic conditions.
The final series of tests involve simulative tests which primarily include wheel tracking tests. The Asphalt Pavement Analyzer (APA), Hamburg Wheel-Tracking Device (HWTD), and French Rutting Tester (FRT) appear to provide reasonable results and do have some data correlating with performance. They do seem to simulate what happens in the field. Mechanistic tests are being studied by others (NCHRP 9-19) and may be available for adoption in the near future (2005). It is also interesting to point out that most tests that have been evaluated for their ability to predict performance have actually been compared to one of these wheel-tracking devices since they do simulate rutting in the laboratory.
Asphaltic Concrete
Asphaltic Concrete is made up of many parts. Air, aggregate, binder, and admixtures to name a few. In Florida, rubber is mandated to be included.
Objectives of Asphalt Paving Mix Design
The design of asphalt paving mixes, as with other engineering material designs, is largely a matter of selecting and proportioning materials to obtain the desired properties in the finished construction product. The overall objective for the design of asphalt paving mixes is to determine (within the limits of the project specifications) a cost-effective blend and gradation of aggregates and asphalt that yields a mix having:
- Sufficient asphalt to ensure a durable pavement.
- Sufficient mix stability to satisfy the demands of traffic without distortion or displacement.
- Sufficient voids in the total compacted mix to allow for a slight amount of additional compaction under traffic loading and a slight amount of asphalt expansion due to temperature increases without flushing, bleeding, and loss of stability.
- Maximum void content to limit the permeability of harmful air and moisture into the mixture.
- Sufficient workability to permit efficient placement of the mix without segregation and without sacrificing stability and performance.
- For surface mixes, proper aggregate texture and hardness to provide sufficient skid resistance in unfavorable weather conditions.
Asphalt has its problems, as evidenced by the body of work done on it. Below is a short list of some of the documents available. They say coffee is the most investigated food we consume, I think asphaltic concrete must be the most investigated building material we use!
| Report No. | Title | Authors | Price |
| 02-04 | Development of Mix Design Criteria for 4.75 Mixes (26 pages) | L.A. Cooley, Jr. R.S. James, & M.S. Buchanan | $6.00 |
| 02-03 | Evaluation of Eight Logitudinal Joint Construction techniques for Asphalt Pavements in Pennsylvania (26 pages) | P.S. Kandhal, T.L. Ramirez & P.M. Ingram | $6.00 |
| 02-02 | Coarse Vs. Fine-Graded uperpave Mixtures: Comparative Evaluation of Resistance to Rutting (22 pages) | P.S. Kandhal & L.A. Cooley Jr. | $6.00 |
| 02-01 | Case Studies of the Tender Zone in Coarse-Graded Superpave Mixes (21 pages) | M.S. Buchanan & L.A. Cooley, Jr. | $5.00 |
| 01-05A | Performance Testing for Hot-Mix Asphalt (Executive Summary) (16 pages) | E.R. Brown, P.S. Kandhal & J. Zhang | $4.00 |
| 01-05 | Performance Testing for Hot-Mix Asphalt (79 pages) | E.R. Brown, P.S. Kandhal & J. Zhang | $9.00 |
| 01-04 | Effects of Re-heating and Compaction Temperature on Hot Mix Asphalt Volumetrics (28 pages) | M.H. Hunter & E.R. Brown | $6.00 |
| 01-03 | Development of Critical Field Permeability & Pavement Density Values for Coarse-Graded Superpave Pavements (24 pages) | L.A. Cooley, Jr., E.R. Brown & S. Maghsoodloo | $6.00 |
| 01-02 | As-Built Properties of Experimental Sections on the 2000 NCAT Pavement Test Track (94 pages) | R. Buzz Powell | $10.00 |
| 01-01 | Premature Failure of Asphalt Overlays From Stripping: Case Histories (44 pages) | P.S. Kandhal & I.J. Rickards | $7.00 |
| 00-06 | Use of Normal Propyl Bromide Solvents For Extraction and Recovery of Asphalt Cements (24 pages) | M. Stroup-Gradiner & J.W. Nelson | $6.00 |
| 00-05 | Evaluation of OGFC Mixtures Containing Cellulose Fibers (24 pages) | L.A. Cooley, Jr., E.R. Brown & D.E. Watson | $6.00 |
| 00-04 | Loaded Wheel Testers in the United States: State of the Practice (26 pages) | L.A. Cooley, Jr., P.S. Kandhal, M.S. Buchanan, F.Fee & A. Epps | $6.00 |
| 00-03 | Evaluation of the Effect of Flat & Elongated Particles on the Performance of Hot Mix Asphalt Mixtures (31 pages) | M.S. Buchanan | $6.00 |
| 00-02 | Hot Mix Asphalt Tender Zone (14 pages) | E.R Brown, B. Lord, D. Decker & D. Newcomb | $4.00 |
| 00-01 | Design, Construction and Performance of New-generation Open-graded Friction Courses (38 pages) | R.B. Mallick, P.S. Kandhal, L.A. Cooley Jr. & D.E. Watson | $6.00 |
| 99-7 | Development of a New Test Method for Measuring Bulk Specific Gravity of Fine Aggregates (30 pages) | P.S. Kandhal, R.B. Mallick & M. Hunter | $6.00 |
| 99-6 | Limited Round Robin Asphalt Content Test Using Troxler Furnace (10 pages) | R.B. Mallick & E.R. Brown | $4.00 |
| 99-5 | Automated Aggregate Grading Analysis: Development & Use (43 pages) | M.S. Buchanan & J.E. Haddock | $7.00 |
| 99-4 | Evaluation of Asphalt Pavement Analyzer for HMA Mix Design (34 pages) | P.S. Kandhal & R.B. Mallick | $6.00 |
| 99-3 | Design of New-Generation Open-Graded Friction Courses (59 pages) | P.S. Kandhal & R.B. Mallick | $7.00 |
| 99-2 | No-Tack Inlay on Milled Surface: Project Report (10 pages) | L.A. Cooley Jr. | $4.00 |
| 99-1 | Permeability of Superpave Mixtures: Evaluation of Field Permeameters (63 pages) | L.A. Cooley Jr. | $9.00 |
| 98-7 | Open Graded Asphalt Friction Course: State of the Practice (31 pages) | P.S. Kandhal & R.B. Mallick | $6.00 |
| 98-6 | Asphalt Mix for Intersections (27 pages) | P.S. Kandhal, R.B. Mallick & E.R. Brown | $6.00 |
| 98-5 | Evaluation of Superpave Gyratory Compactor (24 pages) | R.B. Mallick, S. Buchanan, E.R. Brown & M. Hunter | $6.00 |
| 98-4 | Aggregate Toughness/Abrasion Resistance & Durability/Soundness Tests Related to Asphalt Concrete Performance in Pavements (25 pages) | Y. Wu, F. Parker & P.S. Kandhal | $6.00 |
Materials – Aggregate
Increasing demands to provide clean, specification materials has made washing a common step in aggregate processing. Even in crushing operations where the material often would be considered clean enough (containing minimal deleterious particles), washing may be needed to affect the final product gradation or may be a requirement of the material specification. Washing has taken varied paths over the years. Some producers pass material under a spray bar, acting at best as a means of dust control, and call it washed. Others spend millions of dollars on sophisticated material-processing studies and equipment to wash every last particle of rock and sand.
In these two extremes, probably neither producer is still in business: one for failing tightening specifications, and the other for not being cost effective.
To accomplish both functions, there are some basic requirements and a few dos and don’ts in washing aggregate, particularly sand materials. There are also ways to use more fines in sand products.
Start in the Deposit
Do enough analysis of the rock deposit to know what is there and where. This helps determine a mining plan based on the desired products. There is no need to mine if it is not economically feasible to process enough of the material into product.
Too often, saleable material is brought all the way to the processing plant and then lost to a less profitable product pile; or worse, as is most often the case with fines, wasted to the settling pond. And in the settling pond, the material adds an additional cost when it has to be mucked out and hauled away to a low-profit, non-spec product pile or thrown back in a hole.
Sizing Wash Equipment
Knowing the consistency of the deposit also plays a major part in sizing wash equipment for the optimum utilization of the material. In deposits high in fines (minus 100 and 200 mesh), the configuration as well as the size of the equipment in a processing plant, will have to be considered to most effectively process the material.
Correct water requirements are key to proper wet sand processing. Three factors are necessary to properly size washing equipment and to determine the water required to process sand:
- feed material gradation;
- amount of feed material (tph); and
- product gradation(s).
Washing equipment should not be sized based only on feed material tonnage, especially when sizing equipment for a high-fines feed material and high-fines products.
The amount (gpm) of water slurry fed to a classifying tank or dewatering screw determines the velocity of the liquid portion of the slurry as it moves through the tank. This velocity is important. Assuming that all the sand particles have the same density, each particle in the slurry feed settles at a rate particular to its size.
Proper determination of the dilution of the sand feed, and in particular the silts (minus 200 mesh in most cases), is the first step in sizing fine-material washing equipment. If the slurry velocity through the tank or screw is too fast, the time required for the finest sand particles (minus 100 and 200 mesh) to settle may be insufficient. As a result, some good sand particles that could be put in the product stockpiles may be kept in suspension and carried over the tank or screw weirs and lost to the settling pond.
Too little dilution and the slurry will be too thick to allow the sand particles to separate and settle. In this instance, the weir overflow will contain sand fractions of almost all sizes and silting in often occurs. This is when the corners and sides of the classifying tank or screw fill in with sand, reducing the tank volume, increasing the slurry velocity and causing the sand feed material to go directly over the weir.
Build up and silting in occurs as the specific gravity of the overflow slurry rises above 1.065. Maximum efficiency occurs when the slurry’s specific gravity is between 1.025 and 1.030. Additional water will carry away more fines unless the tank area is oversized.
Sizing hydraulic sand classifying equipment can be a complicated job that must be handled correctly to realize success in producing a specification product.
Sizing a Hydraulic Sand Classifying Tank
A major error in sizing a classifying tank is working solely with the tons-per-hour (tph) of feed. This can be misleading because you may not be able to produce the desired specification product at the desired rate, unless you first address the amount of water required in the tank for proper classification.
In hydraulic classification, water does the work. If the tank is the wrong size for the amount of water required, then not only will production rates fall, but production of a specification product may be next to impossible. With insufficient water to produce the correct dilution rate, the sand and water slurry may be too thick, causing hindered settling. Under these conditions, the feed material may never get classified or separated, making it difficult to reblend to the proper specifications.
When sizing a classifier, it is important to work closely with the application engineer of the equipment manufacturer or another trusted expert. At minimum, the operator needs to supply the following information:
- Feed rate in tph to the classifying tank.
- Feed material gradation or sieve analysis.
- Bulk density of the material.
- Amount of water in gallons per minute (gpm) entering the tank with the feed.
- Exact product specifications to be produced.
Other information that helps the application engineer fine tune the process includes:
- The type of material to be processed (granite, limestone, crushed or natural, dirty or clean).
- Material feed system (conveyor, wet screen, dredge, etc.).
- Desired plant layout (portable, skid or stationary).
Stone Matrix Asphalt
Stone Matrix Asphalt (SMA) has been proven to resist permanent deformation in Europe and has shown promise in the United States as a stable and durable surface mixture. SMA mixtures were developed in Europe and have been used successfully for the past twenty years to provide resistance to rutting under heavy loads and wear from studded tires. The SMA also shows potential for improved long term performance and durability. The success in Europe has encouraged the U.S. to adopt the use of SMA mixtures particularly on high volume roads such as Interstates and urban intersections. However, this new methodology has to be evaluated using U.S. materials and construction methods to insure satisfactory performance in the U.S. .
Placing Asphaltic Concrete
Field density
Aggregate gradation
% Asphaltic Extraction
Stability
The author has had considerable experience with contractors not “living up to” their claims; the field densities were not any where near the lab densities advertised, the gradation was not as advertised, the asphalt extraction was well below that stipulated and the stability was off.
As has been demonstrated, the density in a one inch layer of asphalt can only be obtained within the first 5 minutes of being laid. The rolling patterns are crucial, and if not started immediately, the appropriate density cannot be reached. Your inspectors need to be aware of this and deductions can be made for asphalt that fails to be compacted correctly. Some of the discrepancies can be found as follows:
Effects of Re-heating and Compaction Temperature on Hot Mix Asphalt Volumetrics
by Michael H. Huner and E.R. Brown
ABSTRACTThe need for accurate, consistent volumetric measurements of hot mix asphalt (HMA) has become increasingly important in the past few years. This change has come about because more and more states are utilizing volumetrics to design the HMA mixtures and then to evaluate them during construction. Since volumetrics are now widely used for quality assurance, it has become a major concern for both the state and the contractor to measure these properties with accuracy and reliability. Minor changes in volumetric properties may be the difference in whether a contractor receives full pay or reduced pay for produced mixtures.
It is believed that differences in how mixtures are handled and tested have played a role in discrepancies between government agency and contractor test results. The objective of this study was to evaluate the effects of re-heating and compaction temperatures on the volumetric properties of HMA mixtures. These effects were studied with two experiments. In the first experiment, mix was compacted after 0, 3 and 20 hours storage. In the second experiment, mix was compacted at three different temperatures; standard target compaction temperature for the grade of asphalt cement in the mixture, target -14º and target +14º. These two conditions generally vary from laboratory to laboratory and are believed to cause changes in mixture properties. Fine and coarse graded mixtures comprised of granite and sandstone aggregate with PG64 and PG76 binder were compacted with the Superpave Gyratory Compactor (SGC) and their volumetric properties measured.
Hot Mix Asphalt Volumetrics
There are three volumetric properties most commonly measured to evaluate the physical characteristics of HMA: voids in total mix (VTM), voids in mineral aggregate (VMA), and voids filled with asphalt cement (VFA). These mixture properties are defined as follows:
Voids in Total Mix (VTM) – The total volume of the small pockets of air between the coated aggregate particles throughout a compacted paving mixture, expressed as a percent of the volume of the compacted paving mixture.
Voids in the Mineral Aggregate (VMA) – The volume of intergranular void space between the aggregate particles of a compacted paving mixture that includes the air voids and asphalt cement not absorbed into the aggregates.
Voids Filled with Asphalt Cement (VFA) – The volume of the VMA, expressed as a percentage, that is filled with asphalt cement.
These properties are measured during mix design and production of HMA.
CONCLUSIONSAfter significant tests were run, there was no significant effect on the volumetrics of samples found. Thus the results of the labs has found to be correctly obtained and no quarter can be given.
Asphaltic Concrete Maintenance
Recommended Procedures to Evaluate and Optimize Performance
Predicting performance of HMA is very difficult due to the complexity of HMA, the complexity of the underlying unbound layers and varying environmental conditions. Presently, there are no specific methods being used nationally to design and control HMA to control rutting, fatigue cracking, low-temperature cracking, and friction properties. There are moisture susceptibility tests that are being used nationally but these tests are not very effective. Some additional guidance is needed to minimize the occurrence of these distresses. There are several studies underway, that should be completed in the near future, to develop additional tests to predict performance. When these improved tests are developed then the guidance provided in this course may be superseded regarding the additional guidance be provided. However, until better tests and methods of analysis are available, the guidance discussed below is available to help provide some indication of performance. Specific guidance is only provided for permanent deformation. The author believes that this guidance is the best available at this time
Permanent Deformation
Permanent deformation is probably the most important performance property to be controlled during mix design and QC/QA. Permanent deformation problems usually show up early in the mix life and typically result in the need for major repair whereas other distresses take much longer to develop. Several tests were considered for measuring rutting potential. Tests that appear ready for immediate adoption include the following three wheel tracking tests: Asphalt Pavement Analyzer (APA), Hamburg Wheel-Tracking Device (HWTD), and French Rutting Tester (FRT). Several factors were used to select these tests: availability of equipment, cost, test time, applicability for QC/QA, performance data, criteria, and ease of use.
One recommended approach is to use the APA with cylinders compacted in the Superpave Gyratory Compactor. Samples compacted for volumetric testing could be tested thus minimizing number of samples required. This will allow QC/QA tests to be quickly conducted without requiring additional compacted specimens.
Fatigue Cracking
There has been much research done on the effects of HMA properties on fatigue. Certainly the HMA properties have an effect on fatigue but the most important factor to help control fatigue is to ensure that the pavement is structurally sound. Since the classical bottom-up fatigue is controlled primarily by the pavement structure, there is no way that a mix test can be used alone to accurately predict fatigue. However, steps can be taken to minimize fatigue problems. Some of these steps include: use as much asphalt in the mix as allowable without rutting problems, select the proper grade of asphalt, do not overheat the asphalt during construction, keep the filler to asphalt ratio lower, compact the mix to a relatively low void level, etc. This is general guidance but this is the approach that is generally used to ensure good fatigue resistance. A more definitive way to control fatigue is needed but is not presently available.
Thermal Cracking
Thermal cracking is a problem in colder climates and guidance is needed to minimize this problem. At the present time the best guidance to minimize thermal cracking is to select the proper low temperature grade of the PG asphalt binder for the project location. Other steps during construction can be helpful. For example, do not overheat the asphalt. This will result in stiffening of the binder and will therefore encourage thermal cracking. It is also important to compact the HMA to a relatively low air void level to minimize any future oxidation.
Moisture Susceptibility
Moisture susceptibility is typically a problem that can cause the asphalt binder to strip from the aggregate leading to raveling and disintegration of the mixture. AASHTO T-283 has been used for several years to help control stripping. This test does not appear to be a very accurate indicator of stripping but it does help to minimize the problem. The Hamburg test has also been shown to identify mixes that tend to strip. There are things during the construction process that can help to minimize stripping potential. Of course liquid and lime anti strip agents can be used. Other items include good compaction and complete drying of aggregate.
Premature Failure of Asphalt Overlays from Stripping Case Histories
By Kandhal and Rickards
ABSTRACT
Separation and removal of asphalt binder from aggregate surface due primarily to the action of moisture and/or moisture vapor is generally termed “stripping.” In the identification of the cause of stripping, practitioners have historically tended to focus their attention on the sensitivity of the aggregate and asphalt system in the presence of moisture. The authors classify this stripping as a physio-chemical incompatibility of the asphalt system, and the classical moisture sensitivity tests are relevant.
Under saturated conditions, all asphalt mixes may fail as a consequence of cyclical hydraulic stress physically scouring the asphalt binder from the aggregate. The authors classify this stripping as a mechanical failure of the asphalt pavement system, and the classical moisture sensitivity tests are irrelevant. While under saturated conditions, a less moisture sensitive asphalt system may survive longer, it is probable that failure is deferred and not avoided.
Stripping became a major problem in the United States in the late 1970s. Premature failures of asphalt overlays within two years of construction are not uncommon.
The term “stripping” is applied to hot mix asphalt (HMA) mixtures that generally exhibit separation and removal of asphalt binder film from aggregate surfaces due primarily to the action of moisture and/or moisture vapor. Although stripping of HMA has been mentioned sporadically in literature since the early twentieth century, it became a major problem in the U.S. in the late 1970s. Several HMA related developments took place in the 1970s, which may or may not have contributed to the onset of stripping problems in the U.S. It may be interesting to list some of these developments as follows:
• The 1972 Clean Air Act required baghouses in HMA plants to collect fines which are partially or fully added back to the mix. Prior to 1972, these very fine dust particles were released into the atmosphere and were not incorporated in the mix.
• Many crude oil sources changed in 1973 due to the Arab Oil Embargo. Although not proven, some people believe that the quality of some asphalt binders changed.
• Drum mixers came into use in HMA plants, which dried the aggregate and mixed it with asphalt binder in the same drum.
• Vibratory rollers became common and the use of pneumatic tired rollers for intermediate compaction was mostly phased out. Some asphalt paving technologists believe the neumatic tired rollers are helpful in sealing the fresh HMA mat (thus making it almost impermeable at the surface) due to kneading action.
• The use of open-graded friction course (OGFC) or plant mixed seal coats became common in some states. The Federal Highway Administration encouraged the use of OGFC to improve the skid resistance of HMA wearing courses.
• The use of siliceous aggregates which are relatively more prone to stripping, increased to obtain increased skid resistance in HMA pavements.
• PCC pavements on interstates built in the 1950s increasingly required asphalt overlays in the 1970s. The subsurface drainage of PCC pavements was generally inadequate. Overlaying the 4-lane PCC pavements along with paving the shoulders and median created a very wide asphalt surface trapping the moisture and/or moisture vapor (1).
• Asphalt contents in HMA mixtures generally decreased (reducing binder film thickness) to obtain increased rut resistance.
• Last but a very important factor, truck traffic (and tire pressures) had increased substantially on interstate and primary highways by 1970s and continues to increase.
Although the stripping problem is prevalent in most of the United States, it is puzzling to note that it has not been identified as a major problem in the northeastern United States where a wide variety of aggregates (including siliceous aggregates) and asphalt binder sources are used.
Numerous papers have been published during the last 20 years on the possible causes of stripping, methods for predicting stripping potential of HMA mixtures, and use of antistripping agents to minimize or prevent stripping. However, very few papers are available in the literature, which have evaluated this phenomenon considering the pavement permeability and drainage in the total highway pavement system or the interaction between different HMA courses including open-graded friction courses. This paper presents four field case histories where premature failure of HMA pavement occurred due to stripping. The four projects in Pennsylvania, Oklahoma (two), and New South Wales, Australia were investigated by the first author. The second author was co-investigator for the Australian project. Unlike wet coring which is commonly used, HMA pavement layers were sampled with a jack hammer without adding any water. The actual moisture profiles obtained in HMA pavements, which are generally not found in the literature, have been reported. Thus, the stripping phenomenon in a specific HMA course has not been evaluated in isolation but in the context of the total pavement system. The discussion of four case histories follows. In all cases, critical asphalt pavement layers were substantially saturated and this is believed to have preceded the resulting stripping.
PENNSYLVANIA TURNPIKE (CUMBERLAND COUNTY)
Pennsylvania Turnpike Mile Post (MP) 209.5 to 218.0 received an asphalt overlay consisting of 37 mm thick ID-2 wearing course (it is a dense-graded 9.5 mm nominal size mix) in 1994. The percentage of material passing 4.75, 2.36, and 0.075 mm was 71, 45, and 4.5 percent, respectively, with a design asphalt content of 6.3 percent. The overlay consisting of crushed gravel aggregate HMA mixture was placed during the period of April-November 1994 after milling the existing road surface to an average depth of 40 mm. This project started to exhibit premature pavement distress in 1996 primarily on the westbound (W.B.) slow lane from MP 215.5 to 218.0. The section from MP 209.5 to 215.5 did not develop any significant pavement distress. The project was inspected in July 1996 to investigate the probable cause of the distress. The following observations were recorded during the inspection.
Typical telltale signs of moisture-induced stripping: fines brought up to the surface by water (mud stains), flushing of the surface, and potholing, were clearly visible on the W.B. slow lane from MP 215.5 to 218.0 (Figure 1). Potholes had developed in both wheel tracks of the W.B. slow lane between MP 215.5 and 218.0. There were more potholes in the inside wheel track compared to the outside wheel track (Figure 2). Rutting of the pavement had also started to develop in many areas
General Observations and Recommendations:
1. Water and/or water vapor was getting into the pavement structural system from underneath primarily through the longitudinal and transverse joints, cracks in the PCC pavement and the disintegrated concrete itself at some places. With pavement almost saturated, the pore water pressure developed by differential thermal expansion and cyclic stresses (compression/decompression) from the traffic ruptures the asphalt-aggregate bond causing stripping. Extensive stripping was observed in the old limestone binder course. It is highly likely that this course was already stripped when the new gravel wearing course was placed in 1994.
The moisture or moisture vapor in this old limestone binder course initiated the stripping at the bottom of the new gravel wearing course. If stripping takes place in any layer, the asphalt binder has separated from the aggregate surface allowing the fines to migrate upwards and appear as a white or gray spot. The stripped asphalt binder also starts to migrate upwards causing the flushing of the pavement surface. A pothole then develops in the flushed area which has almost bare aggregates underneath. All typical symptoms of stripping: white or gray spots, flushing, and potholes, were present on this project in the distressed area. Briefly, stripping of the pavement layers underlying the new gravel wearing course had already taken place due to inadequate subsurface drainage conditions. These conditions in turn initiated stripping in the new gravel wearing course (placed in 1994) at the bottom and the stripping was progressing upwards.
2. Although the segment of this project between MP 215.5 and 209.5 is not showing any significant distress at this time, stripping has already taken place in the pavement layers underlying the new gravel wearing course. Therefore, this segment is also expected to develop problems, similar to the distressed segment between MP 215.5 and 218.0, in the near future.
The delay in the development of distress at the surface cannot be explained. It could be due to different construction/subsurface drainage conditions which could not be established. The following recommendations based on the experience of the author were made to rectify the subsurface drainage problem and to reconstruct the asphalt overlays:
1. Mill off all asphalt overlays (about 200 mm) down to PCC pavement. Rubblize the PCC pavement. Place a 100-mm thick layer of asphalt treated permeable material (ATPM) drainage course right over the rubblized PCC pavement. The ATPM should be connected on both sides to the longitudinal edge drains. The ATPM primarily consists of AASHTO No. 57 or 67 aggregate (no fine aggregate) coated with 1-1/2 to 2-1/2 percent asphalt binder. It has been used successfully on I-90 near Erie in similar applications. The structural coefficient of ATPM is believed to be about 0.30. The ATPM should be overlaid with HMA consisting of a binder course and a wearing course of adequate thicknesses to meet the structural design requirements. (Since this investigation, the Pennsylvania Turnpike Commission has undertaken reconstruction of some segments of the Turnpike. The reconstruction involves removal of all HMA courses and the PCC pavement and providing an ATPM at the bottom of new HMA courses.)
2. Consideration should be given to the use of 1-1/2% of hydrated lime (by weight of aggregate) as an antistripping agent in all HMA mixes which are used on the Turnpike in situations similar to this project. Whereas the use of hydrated lime can not be a substitute for proper subsurface and/or surface drainage system, it can increase the resistance of the HMA mix to stripping.
AASHTO T283 (modified Lottman test) with a freeze and thaw cycle should be used to determine the resistance of the HMA mixes to moisture- induced damage.
Friction Properties
Friction is one of the most important properties of an HMA mixture. There are good methods to measure the in-place friction, but there are also poor methods to evaluate mixes in the lab for friction. Several state DOTs have methods that they use but these have not been adopted nationally. More work is needed to evaluate these local procedures for national adoption.
There are several things that can be done in design and construction to improve friction. The primary concern is friction during wet weather. Use of a mix such as open-graded friction course (OGFC) has been shown to be effective in increasing friction in wet weather. Other methods that can be used are to use aggregate that does not tend to polish, use mixes that are not over asphalted, use crushed aggregates etc. Coarse textured mixes such as SMA have been shown to provide good friction in wet weather. At the present time, past experience with local materials is the best information available for providing good friction.
Conclusions
Permanent Deformation
Permanent deformation is probably the most important performance property to be controlled during mix design
Fatigue Cracking
The most important factor to help control fatigue is to ensure that the pavement is structurally sound.
Thermal Cracking
Select the proper low temperature grade of the PG asphalt binder for the project location and do not overheat the asphalt.
Moisture Susceptibility
Good compaction and complete drying of aggregate.
Friction Properties
At the present time past experience with local materials is the best information available for providing good friction.