SPECIALREPORT

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State of the Art:

Compaction of

Asphalt Pavements REFER TO: ,Acton nt

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NATIONAL RESEARCH COUNCIL

NATIONAL ACADEMY OF SCIENCES NATIONAL ACADEMY OF ENGINEERING

/

1972 HIGHWAY RESEARCH BOARD

OFFICERS

Alan M. Voorhees, Chairman William L. Garrison, First Vice Chairman Jay W. Brown, Second Vice Chairman W. N. Carey, Jr., Executive Director

EXECUTIVE COMMITTEE

A. E. Johnson, Executive Director, American Association of State Highway Officials ((,,x officio) F. C. Turner, Federal IlighwayAdministrator, U.S. Department of Transportation (cx officio) Carlos C. Villarreal, Urban Mass Transportation Administrator, U.S. Department of Transportation

(cx officio) Ernst Weber, Chairman, Division of Engineering, iVational Research Council ((!x officio) D. Grant Mickle, President, Highway Users Federation for Safety and Mobility (cx officio, Past

Chairman 1970) Charles E. Shumate, Executive Director, Colorado Department of Highways (cx officio, Past

Chairman 1971) Hendrik W. Bode, Gordon McKay Professor of Systems Engineering, Harvard University Jay W. Brown, Director of Road Operations, Florida Department of Transportation W. J. Burmeister, Executive Director, Wisconsin Asphalt Pavement Association Howard A. Coleman, Consultant, Missouri Portland Cement Company Douglas B. Fugate, Commissioner, Virginia Department of Highways William L. Garrison, Edward R. iVeidlein Professor of Environmental Engineering, University of

Pittsburgh Roger H. Gilman, Director of Planning and Development, Port of New York Authority George E. Holbrook, E. I. du Pont de iVemours and Company George Krambles, Superintendent of Research and Planning, Chicago TransitAuthority A. Scheffer Lang, Department of Civil Engineering, Massachusetts Institute of Technology John A. Legarra, Deputy State Highway Engineer, California Division of Highways William A. McConnell, Director, Product Test Operations Office, Product Development Group,

Ford Motor Company John J. McKetta, Department of Chemical Engineering, University of Texas John T. Middleton, Deputy Assistant Administrator, Office of Air Programs, Environmental

Protection Agency Elliott W. Montroll, Albert Einstein Professor of Physics, University of Rochester R. L. Peyton, Assistant State Highway Director, State Highway Commission of Kansas Milton Pikarsky, Commissioner of Public Works, Chicago David H. Stevens, Chairman, Maine State Highway Commission Alan M. Voorhees, President, Alan M. Voorhees and Associates, Inc. Robert N. Young, Executive Director, Regional Planning Council, Baltimore

( R SPECIALREPORT

131 it

State of the'Art:

Compaction

Asphalt Pavements

Subject Areas 31 Bituminous Materials and Mixes 33 Construction 40 Maintenance, General 41 Construction and Maintenance Equipment

HIGHWAY RESEARCH BOARD DIVISION OF ENGINEERING NATIONAL RESEARCH COUNCIL

NATIONAL ACADEMY OF SCIENCES—NATIONAL ACADEMY OF ENGINEERING Washington, D.C. 1972

NOTICE The study reported herein was undertaken under the aegis of the National Academy of Sciences -National Research Council with the approval of the Governing Board of the NRC. Such approval indicated that the Governing Board considered that the problem is of national significance, that solution of the problem required scientific or technical competence, and that the resources of NRC were particularly suitable to the conduct of the project. The institutional responsibilities of the NRC were then discharged in the following manner: The members of the study committee were selected for their individual scholarly competence and judgment, with due consid-eration for the balance and breadth of disciplines. Responsibility for all aspects of this report rests with the committee, except that opinions and conclusions attributed in the report to individuals are not necessarily those of the committee, the Highway Research Board, or the National Re-search Council.

Although the reports of Highway Research Board committees are not sub-mitted for approval to the Academy membership or to the Council of the Academy, each report is reviewed by a second group of appropriately qualified individuals according to procedures established and monitored by the Academy's Report Review Committee. Such reviews are intended to determine, inter alia, whether the major questions and relevant points of view have been addressed and whether the reported findings, conclusions, and recommendations arose from the available data and information. Dis-tribution of the report is approved, by the President of the Academy, only after satisfactory completion of this review process.

ISBN 0-309-02061-1 Library of Congress Catalog Card No. 72-86845 Price: $2.00 Available from Highway Research Board National Academy of Sciences 2101 Constitution Avenue, N. W. Washington, D.C. 20418

CONTENTS

FOREWORD ............................................ V

1 INTRODUCTION ......................................... 1

2 CONDITIONS OF MATERIALS ................................ 3

3 ENVIRONMENTAL CONDITIONS .............................. 8

4 THICKNESS OF PLACEMENT ................................ 12

5 EQUIPMENT ............................................ 13

6 ROLLING ........................................... 16

7 COMPLETED PAVEMENT .................................. 18

REFERENCES ........................................... 19

SELECTED ANNOTATED BIBLIOGRAPHY ....................... 20

APPENDIX A: STATE COMPACTION REQUIREMENTS ............... 21

APPENDIX B: STATE ROLLING EQUIPMENT REQUIREMENTS......... 23

SPONSORSHIP OF THIS SPECIAL REPORT ...................... 25

FOREWORD

This Special Report is a collection of information from published research in the field of asphalt compaction as well as from informal reports of methods and procedures handed down through the years. This information is intended to assist contractors, engineers, inspectors, and operators toward improvement of the performance and rid-ing qualities of asphalt highways.

This Special Report was prepared by the Highway Research Board Committee on Flexible Pavement Construction at the request of other elements of the Board. Be-cause of the wide differences in procedures used throughout the country, the committee felt that every possible source of information should be sought in preparing a com-prehensive collection of information.

Early in this effort a subcommittee consisting of Wes Beaty (chairman), Verdi Adam, and Charles Foster was selected as a steering group to organize a conference on com-paction. This conference was held during the 48th Annual Meeting of the Highway Re-search Board. It was moderated by Richard Stander, and informal papers were pre-sented by W. R. Lovering, Charles Foster, Richard G. Ahlvin, and W. B. Gibboney, with J. Scherocman.

A printout on the subject of compaction was obtained from the Board's Highway Re-search Information Service, and papers from the conference and tapes of comments were organized. Frank Drake prepared a rough draft of a statement and submitted it to the committee members for evaluation and comment. A subcommittee consisting of Charles F. Parker (chairman), Lansing Tuttle, David W. Rand, Charles W. Beagle, and David G. Tunnicliff was named and charged with organizing and preparing this state-of-the-art report. The committee is grateful to John Lyons for diligently replacing David Tunnicliff, who resigned because of interference with other duties.

V

INTRODUCTION

THE SCOPE of this report is confined to hot-mix asphalt pavements. Hot-mix asphalt paving consists of combinations of aggregates that are uniformly mixed and coated with asphalt cement. Both the aggregates and the asphalt must be heated prior to mixing, hence the term hot-mix.

Compaction is the act of applying mechanical effort by means of rollers or other equipment to increase the unit weight or density of a hot bituminous mixture. The unit weight sought by the rolling action is usually determined in the laboratory as part of the mixture design. The results obtained in the field are compared and expressed as a percentage of the density of the laboratory-compacted sample.

Theoretical maximum density, inferred from computation, is defined as the com-paction of aggregate and asphalt to the ultimate density, i.e., a voidless mass having 100 percent compaction. Any material rolled or otherwise compacted in the field is measured and defined as being compacted to a certain percent of the theoretical maxi-mum density. In general, 95 percent laboratory compaction is equivalent to 92 percent theoretical maximum density.

Most current specifications require asphalt mixtures to be compacted to a specified density. Appendix A contains a comparison of density requirements and the basis of measurement for the various states. Some agencies specify that the mixtures must be compacted to a specified laboratory- controlledand established density, whereas others use a theoretical density based on ultimate compaction. Figure 1 shows the various stages of compaction and the resistance to compaction. Figures 2 through 5 show how the structural characteristics of completed pavements are greatly affected by the den-sity of the mixture used. It is also shown that the density obtained is closely related to the temperature of the mixture at time of compaction.

Figure 1. Relation between various stages of compaction

and resistance to compaction (after McLeod, 5).

/ INTER- COMCTION

BREAKDOWN FINAL ' MEDIATE BY

ROLLING ROLLING

k~

ROLLING - TRAFFIC

% LABORATORY COMPACTED DENSITY - 100

Although there are no specific requirements pertaining to permeability, it is common knowledge that bituminous mixtures with a low void content harden to a lesser degree than do mixtures with a high void content. A mixture with a high void content, such as 8 percent, allows a breathing action to take place. When this occurs, the continuous exposure of the asphalt cement to air and water causes the asphalt to oxidize and harden. When asphalt is oxidized and hardened, it becomes brittle and subject to cracking and general disintegration.

There are many other contributing factors to the oxidizing process, such as low asphalt content, high voids in the mineral aggregate, and soft, absorptive aggregate. It is generally accepted that well-compacted mixtures aid in the development of tensile and compressive strength, resistance to shear and raveling, stability, resistance to the entry of moisture and air, and protection for base course or subgrade.

2 CONDITIONS OF MATERIALS

ALTHOUGH it is always desirable to use highest quality materials, in many cases it is necessary to use local materials because of economics. This often requires that spec-ifications be altered to fit local conditions. This is sound practice, but it requires solid engineering judgment. Often, case histories supply valuable information when such changes in specifications are necessary. It is extremely important that specifi-cations for any project be practical and that they not cause undue work or delay.

AGGREGATE S

Good quality aggregate is an important factor. But regardless of the quality, many problems are encountered if the aggregate lacks uniformity. Unfortunately, laboratory tests do not often detect the extent of uniformity. An experienced engineer can deter-mine the degree of uniformity by visually inspecting stockpiles of materials to be used.

In general, the use of fractured material results in high stability values. Often, it is desirable to use various percentages of both fractured and unfractured material to control stability and workability.

MINERAL FILLER

Both the type and the quantity of mineral filler (passing the No. 200 sieve) have an effect on the compaction and quality of the mixture. The importance of the use of lime as a filler is often neglected. Lime is a well-recognized waterproofing agent and in many cases improves the adhesion of the asphalt to the aggregate. Filler often makes possible the use of a softer grade of asphalt (6). On the other hand, an excess of filler may increase the viscosity of the asphalt and result in mixtures that are difficult to place and often require an increase in the placing temperature. As a general rule, the percentage of filler passing the No. 200 sieve should never exceed the percentage of asphalt used in the mixture.

ASPHALT CEMENT

The grade of asphalt affects the compaction effort. A mixture of good design will usually present favorable rolling conditions. A consideration often neglected is that different mixtures, particularly with different grades of asphalt cement, require dif-ferent temperatures for the most efficient compaction. For example, mixtures that have a high percentage of fines, especially those passing the No. 200 sieve, require higher compaction temperatures than do coarser mixtures such as binder mixtures. A low-penetration or high-viscosity asphalt requires a higher placing temperature than does a high-penetration or low-viscosity asphalt.

n

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cm 400

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200

Figure 2. Effect of temperatures at compaction (Marshall method) on percent voids of bituminous concrete wearing course (after Parker, 1).

330 325 300 273 250 225 200 175 130 123 100

COMPACTIQN TEMPERATURES IN DEGREES FAHRENHEIT

Figure 3. Effect of temperatures at compaction on Marshall stability of bituminous concrete wearing course (after Parker, 1).

S

120

100

80

350 325 300 275 250 225 200 175 150 125 100

COMPACTION TEMPERATURES IN DEGREES FAHRENHEIT

Figure 4. Effects of temperatures at compaction (Marshall method) on Hveem stabilometer values of bituminous concrete binder course (after Parker, i)•

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I2 0

I 4 U.

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40

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350 323 300 275 250 225 200 175 150 125

COMPACTION TEMPERATURES IN DEGREES FAHRENHEIT

Figure 5. Effect of temperatures at compaction (Marshall method) on Hveem cohesiometer values of bituminous concrete binder course (after Parker, i)•

in

N

350 325 300 275 230 225 200 175 150 125

COMPACTION TEMPERATURES IN DEGREES FAHRENHEIT

5

0

MIX DESIGN

The design of asphalt paving mixtures is largely a matter of selecting and propor-tioning raw materials to obtain a desired product. An important feature often over-looked is uniformity. It is extremely important to use only those materials of high uniformity and minimum segregation. For example, if a stockpile contains material of only one size, there would be no segregation because there would be no variation. Likewise, the use of closely graded materials results in minimum segregation, which often requires additional stockpiles and cold feed bins. However, in general, the greater the number of materials, the better is the control of the mixture.

Asphalt content is primarily a function of durability; however, the percentage of asphalt used in a mixture will affect compaction characteristics. The practices of various states are given elsewhere (7).

MOISTURE

Moisture is known to affect asphalt mixtures, particularly their stability and dura-bility. There are two forms of moisture content: When hard aggregate is used, any moisture in the mixture is in the mix itself; when soft or absorptive aggregate is used, the moisture can be essentially in the aggregate particles. This is often more notice-able with coarse or maximum-size aggregate such as in base courses, particularly when absorptive or porous-type aggregates such as slag and vesicular basalt are used. The drying process is often not of sufficient duration to remove the absorbed moisture. When this occurs, additional moisture is sometimes removed after the mixture has been placed if its temperature is high enough. During compaction, such mixtures often appear to have an excess of asphalt. Mixtures with excess moisture content are difficult to compact because they are unstable.

MIX TEMPERATURE

Temperature of the mixture is probably the most important single factor .in the com-paction of an asphalt pavement. The effect of compaction temperature was studied ex-tensively by Parker, and Figures 2 through 5 are from his report (1). In the figures it is assumed that 100 percent compaction was obtained at 275 F. Figure 2 shows that there is little, if any, increase in density when compaction occurs at temperatures higher than 275 F. Density decreases rapidly, however, when compaction occurs at lower temperatures. For example, at 150 F, there is an increase in voids of more than 400 percent.

Temperature also affects other characteristics. For example, Figure 3 shows how stability (using Marshall test procedures) decreases as the temperature at compaction decreases. As Figure 3 shows, there is some increase in stability when compaction occurs at temperatures higher than 275 F; however, there is a marked decrease in stability when compaction occurs at lower temperatures, especially below 200 F. Fig-ures 4 and 5 show that similar stability variations occur when Hveem stabilometer and cohesiometer values are used (4).

Figure 6. Effect of asphalt viscosity on compaction (after McLeod, J.

15: LOW VISCOSITY ASPHALT 2.41

150 PENE1RATON AT 77°F 941 - - VISCOSITY AT 275-F.S.F. I20SC9 240

U. D 149 - ______________ ..239 0 ___________

238 148 - 237

U) OD 147 12,36

______ Ow 235

146 "14GH VISCOSiTY ASPHALT - - 2.34 U) z

14° PENETRATION AT 77°F 92

275°F.S.F. 230 SEGS 2.33

DING COMPACTOR - 2.32 - IrVEEMKNEAAT

5OO P.S.I. I 15.0 TAMP 2.31 14 50 190 230 270 310 355

COMPACTION TEMPERATURE OF

Results obtained elsewhere (2) show that, when compaction occurs at high tempera-tures, deep lift retards the rate of heat transfer; i.e., by using the thick lift, the rate of cooling is very slow, which allows additional time to compact the mixture at high temperatures.

The temperature of the aggregate controls the temperature of the mixture; a mixing temperature that is correlated with the viscosity of the asphalt is normally specified. There are other variables, such as percentage of filler, other gradation factors, am-bient temperature, subgrade temperature, and wind velocity, that must be considered, too. Figure 6 shows the variation in compaction due to high- and low-viscosity asphaits. For example, the figure shows that, to obtain a density of 148 lb/cu ft with a low-viscosity asphalt would require a compaction temperature of 240 F, whereas to obtain the same density when using a high-viscosity asphalt would require a compaction tem-perature of approximately 310 F. Other factors that affect the mix temperature are discussed in the following section on environmental conditions.

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ENVIRONMENTAL CONDITIONS

CERTAIN CONDITIONS that are not immediately alterable, such as foundation solidar-ity and temperature and ambient temperature, will always be present at the paving site. When these conditions will interfere with proper placement of the mix, work can be halted until the conditions are corrected. On the other hand, however, adjustments in spreading can also be made to alleviate the danger.

FOUNDATION SOLIDARITY

Foundation condition has a direct relation to compaction of the overlaid asphalt pavement. Foundation density is important; however, stability is equally important. It is possible to have foundation density and yet lack stability. For example, it would be practically impossible to straighten a bent nail using a rubber block as an anvil, whereas, with an anvil made of steel, the process would be simple. By analogy, placing thin overlays on unstable foundations is futile; however, the use of thick layers, such as 4 in., can reduce compaction problems on unstable foundations.

AMBIENT TEMPERATURES

When a mixture is placed on a foundation, heat is transferred from the hot mixture to the colder foundation. Heat is transferred by convection and conduction from the mixture to the air and subgrade at different rates and amounts that depend on numerous conditions. The effect on cooling the mixture and reducing rolling time is well known.

The effect of the ambient temperature is shown in Figures 7 and 8. The figures show that as the ambient temperature decreases the time allowable for compaction decreases rapidly.

Figure 9 shows the effect of various base temperatures on the cooling rate of a 2-in. mat. The figure indicates that the laydown temperature of the mat should be increased as the temperature of the base decreases to obtain the same time interval for com-paction.

Studies have shown that little is accomplished during compaction after the tempera-ture of the mix drops below 175 F, and this value is generally accepted by the industry. Figure 10 shows the initial laydown temperature required for various base temperatures and mat thickness if the mixture is to cool to 175 F in 15 minutes.

Figure 11 shows the temperature-time relationship under actual field conditions. The three curves in the figure show the relationship under the conditions outlined at the top of the figure. The temperature readings were obtained by using an infrared elec-tronic thermometer that provided an instantaneous temperature reading of the surface of the mixture. Before discussing the data presented, the following additional condi-tions should be stated:

1-i- I '/4 WEARING COURSE THERMOMETER PLACED AT MID DEPTH

I pFL,AVER I 1AMBIENT AIR TEMPERATURE 40- 50°F

10 20 30 40 50 60 70 80 90 TIME IN MINUTES

350

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D 250

Q.

U) i.- ISO

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50

00

The ideal temperature for initial compaction of hot mix is 275 F; All compaction should be completed before the temperature of - the mat drops

below 175 F; and Studies indicate that the typical time that elapses between spreading and rolling

is about 16 minutes, and experience also indicates that it is difficult to compact the mat within 8 minutes of placing.

With these criteria in mind, note that the conditions in Figure 11 can be met when a 3-in, mat is placed under adverse conditions. Notice also that this is about the mini-mum thickness that can be safely handled. The roller operator will have to be alert to the situation.

The i',4-1%-in. curve indicates that the three conditions can be met but initial compaction time will be critical. Any mat thinner than this probably will not meet the established criteria because it will cool too quickly. The 7/8-in. curve shows that it is practically impossible to properly spread and compact a very thin layer under adverse conditions.

Another factor related to the ambient temperature is wind velocity. Figure 12 shows the effect of wind velocity on time required for a 2-in, mat placed on a 50-deg base to cool to 175 F. For example, it can be seen that with a laydown temperature of 280 F and a 20-knot wind, the mixture will cool to 175 F in 13 minutes. At the same laydown temperature and a 10-knot wind, the time required to cool to 175 F is 15'/4 minutes.

Figure 7. Cooling rate of hot-mix asphalt'concrete Figure 8. Cooling rate of hot-mix asphalt- behind spreader when ambient temperature is 86 F. concrete behind spreader when ambient

temperature is 40 to 50 F.

100 0

- DENSE GRADED ASPHALT CONCRETE- - COMPACTED ThICKNESS 1.5"

THERMOMETER PLACED ATMID DEPTH OFLAYER

AMBIENT AIR TEMPERATURE 86°F

30 60 90 120 ISO 180 ELAPSED TIME IN MINUTES SINCE SPREADING

400

U- 350

i 300

U. 250

Uj 200

ISO

10

Figure 9. Effect of base temperature on cooling rate (after Foster, 9).

EMENNEEN

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IL 0

280

UJ 260

Ui

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MINUTES To COOL TO 175 OF

Figure 10. Laydown temperature required under various

conditions (after Foster, 9).

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LAYDOWN TEMPERATURE OF

20

100

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Figure 11. Cooling rate of bituminous pavement.

350

300

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- Figure 12. Effect of wind velocity on 2-in, mat placed on 50 F base.

300 I 1 I I 1 I I I...'I

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280

260

Ui I-

g 240

-j

220 4 6 8 tO 2 14 16 18

MINUTES TO COOL TO 175 OF

11

250

200

C

150

I 00

50

00 10 20 30 40 50 60 70 80 90

TIME IN MINUTES

THICKNESS OF PLACEMENT

AT ONE TIME it was believed that the thinner the lift the easier it was to compact. This has now been disproved, largely by the work of Charles Beagle (2). In general, the determination of placement thickness has been related to the maximum-size aggre-gate in the mixture. Although this is still a factor, it now applies more to thin over-lays and surface mixtures. For example, Figure 10 shows the cooling rate for various mat thicknessess. Thus, with a base temperature of 60 F and a mat thickness of 3 in., the laydown temperature would be approximately 230 F. However, with the same base temperature of 60 F and a mat thickness of 1'/2 in., the laydown temperature required would be approximately 315 F. Obviously, greater time is allowed for compaction with greater mat thickness due to the decrease in heat transfer with increased mat thickness.

12

EQU PM ENT

THIS REPORT considers only mechanically placed mixtures. Hand-placed mixtures and areas inaccessible to conventional rollers and placing equipment bear special con-sideration and are not covered here.

Appendix B contains a comparison of the equipment type and size 'specified by the various state highway departments.

PAVE RS

There are several types and makes of payers. Each has certain unique features and characteristics, and each is basically sound. All payers embrace the concept of initial compaction.

STEEL-TIRED ROLLERS•

The use of steel-tired rollers has been extensively covered elsewhere (1, 8). The importance of using large-diameter rolls and reducing drawbar pull has been empha-sized for many years.

Figure 13 shows what is perhaps one of the most important factors in the compac-tion of bituminous pavements. The most desirable roller is obviously the one requiring the least drawbar pull. (Drawbar pull is defined as a measure of the horizontal force required to move a roller drum in a horizontal direction.) A decrease in roll diameter causes an increase in displacement, or pushing, and may result in decompaction of the mixture. Thus, a larger diameter is preferable to the small diameter. Figure 13 shows that the only effective method of decreasing the drawbar pull is to increase the diameter of the roll. In the words of F. N. Hveem (10), "If more of our compaction rollers had 6-ft wheels, many of our asphalt compaction troubles would disappear."

The normal width of a 60-in, diameter roll is 54 in., whereas for 72-in, diameter rolls the normal width is 60 in. Figure 14 (1) shows the effect of both the 54-in, and 60-in, width rolls when using various laps during the compaction process. The figure indicates that about 15 percent of rolling time would be saved by using the larger rolls. However, according to the manufacturers, rollers with large diameter drums and greater widths are no longer in demand. They are mentioned here to point out that com-paction efforts used to be concentrated on the elimination of lateral movement (shoving) in the mix. This principle was good then and is still important. Similarly, research by Parker (8) and others points out that successful compaction is the direct result of compressing the mixture downward in a vertical direction while it is hot.

PNEUMATIC-TIRED ROLLERS

There are two general types of pneumatic-tired rollers: small-diameter tires and large-diameter tires. They range in weight from 5 to 35 tons. Because of its greater

13

400

1200

1000

400

600

400 -

200

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ID ID ID

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0N ID ID ID P

0 0

ID q ID ID

0.27 COEF. 0.30 COEF. 0,48 COEF.

BITUMINOUS CONCRETE SANDY — CLAY OILEDCLAY UNTREATED_MACADAM

LAP OF ROLLER PATTERN

I!

IFliru 0 ID so

I I—

U- 0 0

14

Figure 13. Comparison of drawbar pull for various conditions (after Figure 14. Comparison of roller Parker, a,). coverage (square yards per hour).

Figure 15. Comparison of roller action.

STEEL FIXED ADJUSTABLE TIRE TIRE TIRE

ROLLER PRESSURE PRESSURE

I __

15

width, the large-diameter tire can compress better without shoving the mix laterally. The strong point of this roller is its kneading characteristics during the compaction process.

With the pneumatic-type roller, the contact area may be adjusted by tire pressure (Fig. 15). In addition, adjustments can be made in ballast. For these reasons, there is more flexibility in adjustments of pneumatic-tired rollers than in the others. In actual field practice, many of these benefits are overlooked.

There are many advantages and disadvantages to the use of both the pneumatic-tired and the steel-tired roller. Many believe that there are too many variables in-volved with the use of pneumatic-tired rollers and therefore the steel-tired roller is a more fixed and practical piece of equipment. It has the further advantage of being able to roll at high temperatures with no pickup problems.

Figure 15 shows the three types of rolling. Note the steel-tired roller and its ac-tion of decompression ahead of the roll. As previously explained, an increase in diameter of the rolls results in a decrease in drawbar pull, which minimizes the de-compaction effect. The same decompaction occurs with pneumatic tired rollers, but they have the advantage that the tire pressure may be varied. A lower tire pressure has the same effect as increasing the diameter of the rolls and minimizing the decom-paction effect.

VIBRATORY ROLLERS

Vibratory rollers are becoming popular as a tool for compaction. However, at this time insufficient knowledge is available to draw intelligent conclusions. Vibratory equipment, with the capabilities that appear to be forthcoming, can find its place along with other types. When sufficient knowledge is gained, this information will be pub-lished.

ROLLING

MOST MIXTURES compact quite readily if they are spread and rolled at temperatures that ensure proper asphalt viscosity. The asphalt mixture is placed by the paver in the form of a mat. The aggregate particles have been oriented into position by the paver, vibrators, or tampers. The mixture is consolidated by the paver to about 80 percent of its eventual density. In the final analysis, the art of compaction consists of rolling the freshly spread mixture in its ideal compaction temperature with the least amount of shoving of the mixture, the least amount of cracking in the mixture, and finally the least amount of pickup on the roller drums or tires.

TYPES OF ROLLING

In general, compaction of an asphalt pavement is a three-phase activity: breakdown rolling, intermediate rolling, and finish rolling. It is sometimes difficult to separate the breakdown rolling from the intermediate rolling. Normally, the first coverage of the roller would constitute breakdown rolling. Often there is confusion over the number of passes and coverages. Coverage constitutes the number of passes required to com-pletely cover the area. This will vary with the vidft of the rolls and lap of rolls for each pass (Fig. 14). Aother advantage of the use of large-diametei thatawgX passes are re uired fo 1eie.coerage. C2

o er pattern should be established and a plan agreed on b tween the engineer and the contractor to ensure uniformity of compaction. The entire pavement should be sub-jected to the same number of passes and coverages by the compaction equipment. This does not rule out the possibility of exceptions where more or less coverage may be required. Rollers should be kept in motion on the hot pavement; when required to stop, they should be on the completed and cool pavement or entirely off the pavement. The rolls should be kept clean and coated with an approved solution or water to pre-vent pickup.

Normally, the following order is used when a single lane is being paved: out-side edge, breakdown, intermediate rolling, and finish rolling. When a series of lanes or a lane abutting a previously placed lane is being paved, the order used is as follows: longitudinal joint, outside edge, breakdown rolling, intermediate rolling, and finish rolling.

Breakdown rolling is normally accomplished with steel-tired rollers. A high-stability mix made of crushed material and having a low asphalt content will support a light load that may never compact the mixture. This type of mix will require an ex-tremely heavy load to compress it. However, it must compress without shoving or displacement. The so-called tender mixtures will require a much lighter roller to compress without displacement; other factors include the use of large diameter rolls

16

\

17

The second or intermediate rolling should follow the breakdown rolling as closely as possible, while the mixture is still plastic and at a temperature that will result in maximum density. Regardless of type of roller used, the rolling pattern should be developed in the same manner as the breakdown rolling. This pattern should be con-tinued until the desired density is achieved.

Finish rolling is done essentially to improve the surface and to remove any roller marks. This phase should be done while the material has not dropped in temperature below 175 F. Often it is difficult to draw the line between intermediate and finish rolling.

Much could be written about mix design relative to ability to compact the mixture. However, it is not the purpose of this report to discuss mix components and composi-tion. It is assumed that the mixture has been placed, and we are concerned here with the compaction of that mixture.

Determination of the number of rollers required is a difficult problem, because of the many variables involved. All of the variables discussed in this report must be considered. Appendix B gives the practices of the various states on roller re-quirements.

7 COMPLETED PAVEMENT

IF ALL has gone well, the individual aggregates and asphalt cement that started in the material storage yard have been combined and transformed into a pavement that has the following characteristics: smooth surface, adequate density, uniform density, and uniform surface texture.

Density plays an important role in the life of a pavement, not only by reducing oxidation and impermeability but also by influencing the smoothness and uniformity of the surface. Traffic will assist in increasing the density, but its greatest effect will be in obtaining a uniform surface texture and appearance.

McLeod summarized the situation aptly when he wrote (11):

At the present time pavement compaction takes place in two stages. The first stage consists of compaction by mechanical equipment during construction while the paving mixture is hot. We are usually satisfied if the pavement is compacted to from 95 to 97 percent of laboratory-compacted density during this first stage. The second stage of compaction occurs under traffic, and, because it occurs slowly at ambient temperature, it usually requires from 2 to 4 years for traffic to complete the compaction of a pavement to 100 percent of laboratory-compacted density. In the meantime, the resistance to compaction and hardening of the asphalt may become so high that traffic is unable to compact the pavement to 100 percent laboratory density:

This all points to the importance of compaction and underscores the fact that every effort should be made to obtain maximum density during construction.

18

REFERENCES

Parker, C. F. Steel-Tired Rollers. HRB Bull. 246, 1960, pp. 1-40. Beagle, C. W. Compaction of Deep Lift Bituminous Stabilized Base. Proc. AAPT, Vol. 35, 1966. Foster, C. R. The Current State of the Art of Compaction Procedures. Paper presented at 48th Annual Meeting of Highway Research Board, Jan. 1969. The Asphalt Handbook. The Asphalt Institute, Manual Series No. 4, July 1962, 398 pp. McLeod, N. W. Influence of Viscosity of Asphalt-Cements on Compaction of Paving Mixtures in the Field. Highway Research Record 158, 1967, pp. 76-115. Richardson, C. The Modern Asphalt Pavement. The Scientific Press, 1908. Bituminous Aggregate Base Course: Survey of State Practices. HRB Spec. Rept. 117, 1971, 67 pp. Parker, C. F. Paving the Maine Turnpike. Proc. AAPT, Vol. 25, 1956. Foster, C. R. A Study of Cessation Requirements for Constructing Hot Mix Asphalt Pavements. National Asphalt Paving Assn., 1970. Hveem, F. N. Pick Your Rollers for Top Blacktop Profit. Day Construction Equipment and Materials, Oct. 1963. McLeod, N. W. Discussion of paper. Proc. AAPT, Vol. 40, 1971, p. .294.

19

SELECTEDANNOTATED BIBLIOGRAPHY

Tilison, G. W. Street Pavements and Paving Materials. John Wiley and Sons, 1901.

An early treatise on the subject, surprisingly modern in scope and cognition.

Marker, V., et al. Proc. AAPT, Vol. 36, 1967.

Discusses significance of pavement permeability as related to pavement durability.

Gibboney, W. B. The Current State of Development of Compaction Equipment and Its Use. Paper presented at 48th Annual Meeting of Highway Research Board, Jan. 1968.

States that the purpose of compaction is to bring about surface stability that will not be further affected by traffic.

Kiefer; R. W. Effect of Compaction on the Properties of Bituminous Concrete. ASTM Spec. Tech; Publ. 294, 1960.

Documents influence of temperature on compaction process.

Bahri, G. R., and Rader, L. F. Effects of Asphalt Viscosity on Physical Properties of Asphaltic Concrete. Highway Research Record 67, 1965, pp. 59-83.

Further substantiates work of Parker (1) and Kiefer (above).

Lovering, W. R. The Need for Compaction of Asphalt Concrete. The Asphalt Institute, Sacramento. Contains pertinent statements on the necessity and purposes of compaction.

Abson, G. Method and Apparatus for the Recovery of Asphalt. Proc. ASTM, Vol. 33, Part 2, 1933.

Presents a method for recovering and testing asphalt from pavements. This method opened door to understanding of changes that occur in asphalt in pavements.

Epps, J. A., Gallaway, R. M., Harper, W. J., and Scott, W. W., Jr. Long-Term Compaction of Asphalt Concrete Pavements. Texas Transportation Institute, Texas A&M Univ., Res. Rept. 90-2F.

Summarizes a large amount of data.

20

Appendix A STATE COMPACTION REQUIREMENTS

THE FOLLOWING TABLE gives a comparison of compaction requirements according to the various state specifications.

Compaction Numerical Based on

Speci- Density or Percent Lab. or Design fication Procedure Compaction Theoretical Method

State Date Requirement Required Value If Lab.

Alabama 1964 Procedure Arizona 1969 Density 92

Arkansas 1959 Density 92

California 1971 Procedure Colorado 1971 Density 93 Connecticut 1969 Density 95 Delaware 1970 Density 95 District of

Columbia Density 94 Florida 1966 Procedure 95 Georgia 1966 Density 98 Hawaii 1969 Density-

procedure 95 Idaho 1967 Procedure Illinois 1971 Density 93

Indiana 1971 Procedure Iowa 1964 Density 96 Kansas 1966 Density 96

Kentucky 1965 Procedure Louisiana 1966 Density 97 Maine 1968 Density 93

Maryland 1968 Procedure and density 94

Massachusetts 1967 Density 95

Theoretical D-2041 max.

Theoretical max.

Lab. Hveem Lab. Marshall Lab. Marshall

Lab.' Marshall Lab. Lab.

Theoretical max.

Lab. Hveem Lab. AASHO

T -167

Lab. Marshall Theoretical D-2041

max.

Lab. Marshall Lab. Massachu-

setts 21

QA

State

Speci- fication Date

Density or Procedure Requirement

Numerical Percent Compaction Required

Compaction Based on Lab. or Theoretical Value

Design Method If Lab.

Michigan 1970 Procedure Minnesota 1968 Density 97 Lab. Marshall Mississippi 1967 Density 97 Lab. 75 blows

Marshall Missouri 1968 Density 94-97 Theoretical

max. Montana 1966 Procedure Nebraska 1965 Density 90 Theoretical Nebraska

max. New Hampshire 1969 Density 95 Lab. Marshall New Mexico 1970 Density 96 Lab. North Carolina 1965 Density 95 Lab. Marshall North Dakota 1971 Density 95 Lab. Marshall Ohio 1971 Procedure Oklahoma 95 Lab. Oregon 1970 Density 92 Lab. Oregon Pennsylvania 1970 Density 95 Lab. Marshall Rhode Island 1965 Density 95 Lab. South Carolina 1964 Procedure South Dakota 1969 Density 95 Lab. Marshall Tennessee 1968 Density 95 Lab. Marshall Texas 1962 Density Mm. 94, max. Lab. C-14, THD

99, optim. 97 Bull. Utah 1970 Density 96 Lab. Marshall,

Utah Dept. Vermont 1964 Density 92 Lab. Vermont

Dept. Virginia 1970 Density 92 Theoretical

max. Washington 1963 Procedure Wisconsin 1969 Density 95 Lab. Marshall Wyoming 1967 Interim

density 95 Lab.

Appendix B STATE ROLLING EQUIPMENT REQUIREMENTS

THE FOLLOWING TABLE gives the rolling equipment types and sizes specified by the various states.

State

Speci-fication Date Knockdown Interim Final

Alabama 1964 10 T steel 7 T tandem Arizona 1969 2-pneu. 35 psi P-60 psi 8 T tandem Arkansas 1959 8 ton steel 10 T steel California' 1971 12 ton, 3 wheel Colorado 1971 Number, weight, and type sufficient to obtain density Connecticut 1969 3 wheel or tandem Pneu. 60-90 psi 10 T tandem

10 ton Delaware 1970 Number, weight, and type sufficient to obtain density Florida 1966 Equipment specified on individual project basis Georgia 1966 3 wheel or 2 axle tandem P050-80-psi Tandem

osc. 250-350/mm. Hawaii 1969 Tandem or 3 axle 3 pass., P-90 psi 8 T tandem Idaho 1967 2 axle tandem 3 pass., pneu. 3 axle tandem Illinois 1971 Not spec. Pneu. 25 ton 3 axle tandem Indiana' 1971 3 wheel Pneu. 60-90 psi 15 T tandem Iowa 1964 3 wheel Pneu. 45-55 psi 3 axle tandem Kansas 1966 Steel Pneu. Size not spec. Kentucky 1965 3 wheel or 10 Pneu. 2 or 3 axle

tandem tandem Louisiana 1966 10 T or 3 wheel Hi Int. Pneu. Tandem Maine 1968 Number, weight, and type sufficient to obtain density Maryland 1968 8 T steel Pneu. 8 T steel Massachusetts 1967 240 lb/in, steel Pneu. 285 lb/in, steel Michigan 1970 8-10 T Pneu. 85 psi 8 T Minnesota 1968 Steel Pneu. Tandem steel Mississippi 1967 10 T 3 wheel 12 T pneu. 80 psi 8-10 tandem

8-10 steel tandem

Missouri ' 1968 8-12 T 2 wheel Pneu. 80+ psi 10 T 2-3 wheel tandem or 3 tandem wheel roller

23

24

State

Speci-fication Date Knockdown Interim Final

Montana 1966 Steel Pneu. Steel Nebraska 1965 10 T 3 wheel 14 T pneu. 80 psi 15 T 3 axle

roller or 10 T tandem

Nevada 1968 10 T tandem or 3 Pneu. 9 wheel 8 T 2 axle wheel roller 1,000-2, 000 lb

per wheel New Hampshire 1969 Steel wheel Pneu. 55 to 90 3 axle tandem New Jersey 1961 10 T 3 wheel or 8 ton 3 axle or 3

300 lb/lin. in. axle tandem New Mexico 1970 The number, weight, and type of roller to obtain compaction New York 1962 10-12 T 3 wheel 11 T pneu. with 3 axle tandem

8-10 T tandem 80 psi North Carolina 1965 10-12 T 3 wheel Pneu. Tandem North Dakota 1971 Pneu. 40-90 psi Pneu. 40-90 psi Tandem

8 ton 2 axle steel Ohio 1971 10 T 3 wheel Pneu. 85 psi 12-20 T 3 axle

tendem Oregon 1970 Type, number, and weight to achieve compaction Pennsylvania 1970 10 T 3 wheel or Pneu. 60-95 psi Tandem

10 T tandem Rhode Island 1965 At least 2 rollers, weight, and type sufficient to obtain com-

paction South Carolina 1964 8-12 T 2 axle Pneu. 40-90 psi 12-18 T 3 axle

tandem tandem South Dakota 1969 Number, weight, and type sufficient to obtain required density Tennessee 1968 Steel wheel Pneu. 85 psi Tandem Texas 1962 8 T 2 axle 10 ton Pneu. 10 T 3 axle

3 wheel tandem Utah 1970 Number, weight, and type sufficient to obtain required density

8-942 Vermont 1964 Steel roller Pneu. 3 axle steel Virginia 1970 Steel wheel, static or urban and/or pneumatic sufficient to

compact mix Washington 1963 8 T steel wheel Pneu. 40-80 psi Tandem steel West Virginia 1968 3 wheel steel, 10-12 T type, weight and number to achieve

compaction Wisconsin 1969 8-12 T 3 wheel Pneu. 30 psi 12-15 T 3 axle

or tandem tandem Wyoming 1967 Number, weight, and type sufficient to obtain density

SPONSORSHIP OF THIS SPECIAL REPORT

GROUP 2—DESIGN AND CONSTRUCTION OF TRANSPORTATION FACILITIES John L. Beaton, California Division of Highways, chairman

CONSTRUCTION SECTION Robert D. Schmidt, Illinois Department of Transportation, chairman

Committee on Flexible Pavement Construction Frank M. Drake, The Asphalt Institute, chairman

Verdi Adam, Charles W. Beagle, R. W. Beaty, Bruce W. Butt, W. L. Echstenkamper, Jon A. Epps, D. S. Fletcher, Charles R. Foster, J. Frank Jorgensen, R. V. LeClerc, John J. Lyons, Duncan A. McCrae, E. C. Meredith, Charles F. Parker, Edward T. Perry, John T. Reinhard, James M. Rice, Orrin Riley, Roy M. Rucker, Vernon L. Schrimper, R. R. Stander, David G. Tunnicliff, Lansing Tuttle, P. J. White

William G. Gunderman, Highway Research Board staff

25

THE National Academy of Sciences is a private, honorary organization of more than 800 scientists and engineers elected on the basis of outstanding contributions to knowledge. Established by a congressional act of incorporation signed by Abraham Lincoln on March 3, 1863, and supported by private and public funds, the Academy works to further science and its use for the general welfare by bringing together the most qualified indi-viduals to deal with scientific and technological problems of broad significance.

Under the terms of its congressional charter, the Academy is also called upon to act as an official—yet independent—adviser to the federal government in any matter of science and technology. This provision accounts for the close ties that have always existed be-tween the Academy and the government, although the Academy is not a governmental agency and its activities are not limited to those on behalf of the government.

The National Academy of Engineering was established on December 5, 1964. On that date the Council of the National Academy of Sciences, under the authority of its act of incorporation, adopted articles of organization bringing the National Academy of Engi-neering into being, independent and autonomous in its organization and the election of its members, and closely coordinated with the National Academy of Sciences in its ad-visory activities. The two Academies join in the furtherance of science and engineering and share the responsibility of advising the federal government, upon request, on any sub-ject of science or technology.

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The Highway Research Board is an agency of the Division of Engineering. The Board was established November 11, 1920, under the auspices of the National Research Council as a cooperative organization of the highway technologists of America. The pur-pose of the Board is to advance knowledge of the nature and performance of transporta-tion systems through the stimulation of research and dissemination of information de-rived therefrom. It is supported in this effort by the state highway departments, the U.S. Department of Transportation, and many other organizations interested in the develop-ment of transportation.

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