AASHTO SCCA-1997 Segregation Causes and Cures for Hot Mix Asphalt《热混沥青离析原因和养护》.pdf

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1、-, STD-AASHTO .- Published by the American Association of State Highway and Trans p o r t at i o n Off i ci a Is ? STD- AASHTO SRCH SCCA-ENGL 1997 063980Li 0045932 410 W i -?. egregation Prepared by Joint Task Force on Segregation of AASHTO Subcommittee on Construction; AASHTO Subcommittee on Materi

2、als; and National Asphalt Pavement Association Published by the American Association of State Highway and Transportation Officials STD-AASHTO SRCH SCCA-ENGL 1997 Ob39804 OOY5913 357 This publication is jointly published by the American Association of State Highway and Transportation Officials (AASHT

3、O) and the National Asphalt Pavement Association (NAPA). Many of the graphics included in this publication were courteously supplied by Astec Industries, Inc. It was prepared by the Joint Task Force on Segregation, established by the AASHTO Highway Subcommittee on Construction and the Highway Subcom

4、mittee on Materials, and NAPA. For AASHTO purposes, this is a committee report, published under the joint sponsorship of the Highway Subcommittee on Construction and the Highway Subcommittee on Materials. O 1997, jointly by the American Association of State Highway and Transportation Officials and t

5、he National Asphalt Pavement Association. All rights reserved. This publication may not be republished or copied (including mechanical reproductions) without written consent. To obtain this consent, contact either of the Associations at the addresses given below. American Association of State Highwa

6、y and Transportation Officials 444 North Capitol Street, N.W., Suite 249 Washington, D.C. 20001 National Asphalt Pavement Association NAPA Building - 5100 Forbes Boulevard Lanham, Maryland 20706-4413 AASHTO/FH WA/I ndustry Joint Task Force on Asphalt Segregation Kansas: Dean M. Testa, Chairman Alaba

7、ma, G. M. (Mike) Harper Indiana, Timothy D. Bertram New Jersey, Eileen Connolly New York, Richard P. Karis Oregon, Kenneth L. Stoneman FHWA, Joe Huerta Asphalt Institute, Bernie McCarthy Astec Industries, Znc., J. Don Brock National Asphalt Pavement Association, Dale Decker Redland Genstar, Inc., Ca

8、rl Blevins Vulcoin Materials Company, Ervin L. Dukatz, Jr. STD.AASHT0 SRCH SCCA-ENGL 1997 Ob3980q OOir5915 12T INTRODUCTION . 1 MEASURING SEGREGATION 3 MIX DESIGN . 4 STOCKPILING 7 ASPHALTPLANTS . 9 Cold Feed Bins 9 Hot Bins on a Batch Plant 9 DrumMixer . 10 Counter-flow Drum Mixer 13 Surge and Stor

9、age Bins . 14 TRUCK LOADING AND UNLOADING 18 PAVER . 20 MATERIAL TRANSFER VEHICLE . 23 DIAGNOSTIC SYSTEM 25 TESTING . 25 SUMMARY . 25 - STDmAASHTO SRCH SCCA-ENGL 1997 m Ob39804 004592b Obb m Hot Mix Asphalt (HMA) is a mixture of graded aggregate and asphalt cement designed to provide both strength a

10、nd durability. It is unique in that it is also flexible, making it adaptable over a wide range of climatic conditions. Considerable effort has been exerted over the last decade to improve the performance of Hot Mix Asphalt through better combinations of grading, the addition of polymers, fibers, and

11、 various other products. These products are intended to reduce rutting, add to durability, and lead to a longer life of the pavement. In order to ensure pavement performance, the HMA must be mixed, stored, transported, and placed uniformly and with the same gradation the lab designed. At each point

12、during the manufacturing process from stockpil- ing, aggregate blending, mixing, conveying, storing, truck loading, and placement at the paver, the mix has an opportunity to segregate, creating non-uniform mixes. A segregated spot may be “the birth of a pothole.” When segregation is present in a mix

13、ture, there is a concentration of coarse materials in some areas of the paved mat, while other areas contain a concentration of finer materials. Segregation creates non-uniform mixes that do not conform to the original job mix formula in gradation or asphalt content. The resulting pavement exhibits

14、poor structural and textural characteristics, provides poor performance and durability, and has a shorter life expectancy and higher maintenance costs. Problems associated with segregation are serious. Their elimination is essential to the production of high quality paving mixtures. Elimination of s

15、egregation is the responsibility of those who produce and place HMA, organizations who design the mix, owners who inspect the final product, and manufacturers who design and market paving equipment. There are basically five types of mix segregation that occur on the road. They are as follows: 1. Tru

16、ck End Segregation: Figure 1 shows a roadway with segregated spots, often referred to as wings, occurring longitudinally and on each side of the lane being paved. These wings constitute spots of coarse aggregate separated from the uniform mixture. They are typically much more open graded than design

17、ed and, if severe enough, will deteriorate in a short period of time, leading to a pothole in the road. Less severe truck end segregation may not appear until the road has been under traffic for several months. Truck end segregation is usually caused by improper truck loading, silo segregation, impr

18、oper truck unloading, or running the hopper empty between loads. Figure 1. Truck End Segregation. - STD-AASHTO SRCH SCCA-ENGL 3997 Ob39804 0045937 TT2 2 Segregation - Causes and Cures 2. Centerline Segregation: The second most common type of segregation is centerline segregation (shown in Figure 2).

19、 This normally occurs in the center of the lane and is primarily attributable to coarse aggregate as it is discharged from the conveyor system into the auger area. It then rolls underneath the auger chain drive or gearbox and concentrates in the center of the mat being laid. 3. Joint/Edge Segregatio

20、n: Figure 3 shows that the next most prevalent type of segregation, which occurs at the outside or the outer edges of the pavement being placed. This commonly occurs from the augers not being run at sufficient speeds on the paver, allowing the coarse aggregate to roll to the outside of the mat. 4. T

21、ruck End Segregationlone Side: This type of segregation is shown in Figure 4. It is a special case of truck end segregation, but is most com- monly caused by improper loading of the batcher on the hot storage bin. 5. Random Segregation: Random type segregation is shown in Figure 5. The cause of this

22、 type of segregation is the most difficult to find. Generally, this type of segregation occurs when improper mixing is taking place either in the batch plant or drum mixer, but can also occur in other places in the process. Figure 2. Centerline Segregation. Figure 3. JointlEdge Segregation. Figure 4

23、. Truck End Segregation/One Side. Figure 5. Random Segregation. STD.AASHTQ SRCH SCCA-ENGL 1997 Ob39804 0045918 939 Segregation - Causes and Cures 3 Historically, segregation has been measured on a subjective basis with the inspectors making a determination based on visual information only. This beco

24、mes very difficult since, in many cases, particularly on surface mix, segregation may not appear for several months after the pavement has been opened to traffic. Visual observation can only identify surface segregation. Thus, segregation below the surface may be undetected. The agencies that have b

25、een most successful in minimizing segregation have been extremely demanding of performance and have not allowed any visible type of segregation to occur. This places a very tough burden on both the inspector and the contractor placing the mix and can lead to disputes. In an effort to develop a non-d

26、estructive method of measurement, one state has written a specification using a nuclear gauge to measure density longitudinally along the pave- ment, looking for variations in density. A varia- tion of more than 5 lb/ft3 (80 kg/m3) (highest to lowest) or 2.5 lb/ft3 (40 kg/m3) (drop from the average

27、to lowest) has been considered to be unsatisfactory. This method has been found to be successful in detecting truck end segregation. manufacturer to try to determine if a more accurate measurement might be the use of the asphalt content, utilizing the longitudinal variation technique noted. At this

28、writing, a fully validated, non-destructive segregation measurement test has not been developed. Segregation frequently occurs to some degree. The aggregate particle sizes in Hot Mix Asphalt vary substantially in diameter. The large particles are as much as 10,000 to 12,000 times the diameter of the

29、 smallest particles. The largest particle can often weigh as much as 1,000,000 times as the smallest particle. To believe that total uniformity can always be achieved would be naive. However, variations of as much as 10% on the #4 (4.75 mm) sieve should be viewed as excessive. The following sections

30、 give a road map to determine where the segregation is occurring and how to prevent it. A diagnostic chart included with this document shows the five different types of segregation. This information will assist in determining the type of segregation occurring and provide help in solving the problem.

31、 The National Center for Asphalt Technology NCAT) is presently working with an equipment 4 Segregation - Causes and Cures Proper mix design is important in the effort to eliminate segregation. Mixes are categorized as dense-graded (uniform aggregate particle size distribution) or gap-graded (a size

32、range in the aggregate grading which contains little or no material). Mixes that are dense-graded may be forgiving by compensating for mistakes in the plant operation or laydown operations without affecting the mix performance significantly. In order to avoid segregation, gap-graded mixes may requir

33、e greater attention to detail during placing and handling than dense-graded mixes. If the mix is gap-graded to a sufficient degree and has a low asphalt content, it may be very difficult to produce without segregation, regardless of the techniques used. Extra care and/or the use of mix additives may

34、 minimize segregation problems with gap-graded mixes. Probably the single most important mix design criterion for segregation susceptibility is asphalt content. Low asphalt content mixes tend to segregate much more than mixes containing high asphalt content, regardless of the gradation. Figure 6. Ma

35、ximum Density Line. 1 o1 90 80 70 Gap-graded mixes can be successfully used. However, the mixes may require fibers or poly- mers, enabling the use of a higher asphalt content that makes the film thicker. In many mixes, a slight increase in asphalt content (often as little as 0.2 percent) will reduce

36、 segregation significantly. Increased film thickness dampens particle-to-par- ticle contact and reduces the tendency for mix to separate at transfer points throughout the process. The maximum density line can be used as a guide to understanding aggregate gradation. The maximum density line represent

37、s a gradation in which the aggregate particles fit together in their densest possible arrangement. To plot the maximum density line, use the FHWA 0.45 Power Gradation Chart as shown in Figure 6. Draw a straight line from the maximum aggregate size through the origin. The maximum aggregate size is de

38、fined as one sieve size larger than the nominal maximum size; the nominal maximum size is defined as one sieve size larger than the first sieve to retain more than 10 percent of the aggregate. c 40 O p! 30 20 10 O Horizontal scale represents sieve sizes raised io the .45 power STD-AASHTO SRCH SCCA-E

39、NGL 1997 Ob39804 00459210 597 SS Experience dictates that mixes with gradations that fall directly on the maximum density line should not be produced. Often there is insuffi- cient room in the mixture for the liquid asphalt, and a plastic type mix results. Another problem arises when the mix design

40、is near the maximum density line. Gradation variations in stockpile materials cause the curve to vary back and forth across the maximum density line, thereby gap-grading the mix. It is suggested that the mix designer select approximately two to four percentage points above the maximum density curve

41、if a fine textured mix is desired. A gradation blend two to four points below the curve should be selected if a coarse textured mix is desired. See Figure 7). These bowed up and bowed down Figure 7. Selecting Mixines. Figure 8. “S“ Curves Tend to Segregate. 1 O0 90 Segregation - Causes and Cures 5 c

42、urves usually result in a good, forgiving mix. Detailed discussion of mix design is beyond the scope of this publication. Rarely does a mix that lies on the maximum density line contain sufficient voids in the mineral aggregate (VMA), especially if the design has a relatively high percentage of minu

43、s No. 200 (0.075 mm) material. A grading selected on a line approximately parallel to the maximum density line will produce a uniformly graded mix that will have little tendency to segregate. However, the maximum density line should be used only as a guideline for uniform grading. Other criteria suc

44、h as VMA, air voids, stability, and other specifications must also be met. 80 70 I- 5 40 u er 30 CL 20 10 O Horizontal scale represents sieve sizes raised io the .45 power 100 90 80 70 20 10 O Horizontal scale represents sieve sizes raised to the .45 power STD.AASHTO SRCH SCCA-ENGL 1777 m ob37804 OO

45、LWL 423 m 6 Segregation - Causes and Cures Some mixes have gradations that make an “S” across the maximum density line, as shown in Figure 8. These mixes tend to have segregation problems. The slightly bowed curve shown in Figure 9 has resulted in good performance. But the potential benefit that a d

46、esigner tries to achieve by gap-grading requires special care in handling to avoid segregation problems. Figure 9. Slightly Bowed Curves May Do We1 I. Figure I O. Same Mix 4 versus 7 Points. lot 90 80 When plotting a mix gradation, plot as many sieve sizes as possible. Figure 10 illustrates how plot

47、- ting only a few points can result in a misleading graph. When only 4 sieve sizes are plotted as shown in Figure 10 by the thin solid line, the curve may indicate a good mix. But when 7 sieve sizes are plotted, shown in Figure 10 by the dashed line, it is easy to see that the mix is actually gap-gr

48、aded. Thus, a complete range of sizes should be included in the sieve analysis. 70 Ca cn 3 50 + 40 u 30 60 a 20 O 200 so 40 20 10 Horizontal scale represents sieve sizes raised to the .45 power 100 90 80 70 + u 40 E 30 20 10 O STD.AASHTQ SRCH SCCA-ENGL I1997 Ob39804 O045922 3bT = Segregation - Cause

49、s and Cures 7 Proper stockpiling techniques are needed to ensure that the material will be uniform when fed to the hot mix plant. High stockpiles are very sensitive to segregation for single aggregate blends. Figure 11 shows a typical example of a single aggregate stockpile. In this example, segregation has occurred because a conveying system was improperly used to form the stockpile. Large particles have rolled to the outside of the pile thereby segregating the material. Fine material coming off the belt tends to fall toward the rear of the stockpile, furthe