Organization Sponsored Jointly by

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1 W. Maupin, Jr. G. Scientist Research opinions, findings, and conclusions expressed (The are those of author and not necessarily report Highway & Transportation Research Virginia Organization Sponsored Jointly by Cooperative Council Virginia n Cooperation with U. S. Department of Transportation I Highway Administration Federal REPORT FINAL SILOS SEGREGATION OF ASPHALT MIXES CAUSED BY SURGE by this in of those sponsoring agencies.) (A and of Highways & Transportation Department University of Virginia) Charlottesville, Virginia March 1982 VHTRC 82-R47

2 BITUMINOUS RESEARCH ADVISORY COMMITTEE A. D. BARNHART, Chairman, District Materials Engineer, VDHgT R. L. ALWOOD, District Materials Engineer, VDH&T J. D. BARKLEY II, Resident Engineer, VDHgT P. F. CECCHINI, District Engineer, VDHgT J. L. CORLEY, District Engineer, VDHgT W. R. DAVIDSON, AssistaNt Maintenance Engineer, VDHgT C. E. ECHOLS, Asst. Prof. of Civil Engineering, U. Va. R. V. FIELDING, Materials Engineer, VDHgT R. J. GIBSON, Resident Engineer, VDHgT W. L. HAYDEN, Asst. Materials Engineer, VDHgT C. S. HUGHES iii, Highway Research Senior Scientist, VHgTRC A. B. JOHNSON, Assistant Construction Engineer, VDHgT J. T. LOVE, Materials Engineer, Materials Division, VDHgT H. E. MARSHALL, District Engineer, FHWA T. W. NEAL, JR., Chemistry Lab. Supvr., Materials Div., VDHgT R. D. WALKER, Chairman, Dept. of Civil Engineering, VPI g SU ii

3 Although problem is encountered primarily on jobs mixes. surge silos, it has been related to or factors such using report discusses changes in equipment and production This that reportedly have all eviated problem, including procedures devised to eliminate segregation in surge bin. specifications discussed is a field project in which an attempt was made Also prevent segregation by changes to equipment and to and paving processes. production ABSTRACT of asphalt mixes continues to be a problem Segregation. Virginia, particularly with base mixes and coarse surface in as mix design and handling or paving techniques. iii

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5 SEGREGATION 0F ASPHALT MIXES CAUSED BY SURGE SILOS W. Maupin, Jr. G. Scientist Research past 15 to 20 years, surge and storage silos Over been used to store hot plant mix so as to achieve increased have in production and decreased production costs. flexibility surge silo enables a plant to supply more than A type of one at any one time, and economy is realized by shortened mix loading times, increased plant production, and increased truck 1980 segregation attributed to use of surge In occurred on two paving projects in Virginia. Subsequently, silos was suggested at a meeti.ng of Research Council's it research advisory committee that bituminous limited investi- a of segregation problem caused by surge silos be gation The purpose of this investigation was (i) to undertaken. extent of problem statewide by determine questionnaire a and (2) to examine literature and make personal contacts survey, seek possible solutions. It was anticipated that to could be tried in field if segregation solutions en- was questionnaire was distributed to state's eight A materials engineers and seven replies district received. were purpose was (i) to determine extent of segregation The and wher similar types of segregation were problem common all districts, and (2) to solicit information on any means to had proven successful in alleviating or eliminating that 20 surge silos have been Approximately are being or by paving contractors who supply asphalt mixes to used Department of Highways and Transportation. These Virginia are distinguished from storage silos on basis of how long FINAL REPORT by PURPOSE AND SCOPE rate of paving.(l,2) countered in Q UE S T I 0 NNAI RE S URV EY problem.

6 stored. Usually, a mix is stored in mix-is surge silo for a than 12 hours, whereas it is left in storage silos, for less longer periods. (3) districts have used or presently use surge silos; All most of usage is concentrated in Culpeper, however,, Staunton, and Suffolk districts. Five of Richmond, seven responding to questionnaire have experienced districts all cases where problems were noted, segregated areas were In visually at regular intervals in pavement, and detected areas appeared to coincide with beginning or end of se truckload of mix. In most cases segregation was not each by routine gradation analysis performed on detected sampled from truck at hot mix plant. If mix mix and intermediate mixes were reported to be more Base to segregate than were fine surface mixes. prone Virginia, various methods have been used in attempting In correct segregation, most common being" to loading, trucks with 3 dumps (front, rear, and i. of truck) rar than with middle single dump; a maintaining plant mix at a level above cone 3. silo; of he reported attempts at correction, segregation was eliminated In only two of nine cases. Although improvements in noted in were of nine.cases, segregation was still considered detri- six to quality of pavement. In only one case was mental of silo disallowed when modifications proved-to be use No one seemed to consider segregation to be ineffective. in asphalt mix supplied from surge silos, and segregation two have had little experience with silos. remaining segregated in truck, sampling procedure was not was as to detect it. such 2. modifying hot mix plant and silo; 4. dumping paver wings a minimum number of times; and 5. changing source of materials and mix design. related to a specific brand of s'lo.

7 literature search was made and highway departments, A organizations, and equipment manufacturers were national for information on causes of segregation in contacted and solutions to problem. mixes.of information gared in literature Much was similar to that obtained in questionnaire survey search from a conveyor or stationary chute into a silo. (4) discharged however, segregation that originates in cold Sometimes, bin, drum-mixer, and hot bins is erroneously attributed feed use of a silo. to Virginia officials contacted indicated that y West encountered problems with- silo only when it was operated had low levels or when trucks were loaded through an empt. y silo. at Carolina reported having detected some problems wroth North mixes, and y attributed se to use of a silo on coarse basis of inspections of plant mix before it was introduced so urce of segregation can be manner in Anor mix is transferred from silo to truck. If which **J. E. Grady, Jr. personal correspondence Nobember LITERATURE SEARCH AND PERSONAL CONTACTS Causes of Segregation was cited by persons contacted during study. Brock and noted that segregation can be introduced if mix is has into silos and after it was dumped into truck.** is dumped in center of truck body, coarse mix tends to roll to edges. material segregation can occur during paving Finally, Here, problems can be encountered with mixes that operation. prone to segregate, and in case of a mix that has are partially segregated, problem may be exacerbated. become *G. W. Steele" personal correspondence October 1981.

8 some modifications to silos that can be undertaken to lists segregation Improved methods of charging silo eliminate a batcher or rotating chute. A batcher collects mix at use of silo and drops it when full. It is important that top be dropped straight into center of batcher and material batcher be allowed to fill completely each time before that A rotating chute deposits mix vertically as it rotates dumping. silo. It is important that it be designed to deposit around mix proper distance from edge of silo and also it be checked to ensure that a hole does not form as that in Figure i.(4) Brock reports that "All bins being indicated today are equipped with eir a batcher or manufactured has noted that it is possible to add baffles in Foster silo to prevent segregation, and that placing a cone inside silo near discharge gates reportedly helps remix (Figure 2). (5) material noting ir porblems with segregation, West Upon officials revised ir specifications. They now Virginia that "Permission may be granted for bin usage that specify results in mixtures that have gradation and consistently also specify that "affixed to each bin and visible to They operator shall be an indicating device, which will loading activated when material in bin reaches top of be sloped portion of bin." overcome its problem with coarse mixes, North Carolina To not require use of specific equipment to prevent does discharge from storage bin " Producers have used upon of methods cited in responses to previously some effects of storage on properties of binder, The of temperature of mix, and segregation were maintenance and a sampling and testing plan was devised. Before determined silo is approved samples are taken at specified interval.s a to Problem S61utions In technical bulletin previously cited, Brock (4) rotating chute." properties of no less quality than specified mixtures temperature directly from plant's mixing operation..." discharged rar it attempts to minimize problem by segregation, that " mixture meet job mix formula requirements requiring discussed questionnaire. 1973, New Jersey conducted a study to develop a In for-evaluating surge and storage bins for acceptance. (6) procedure when silo is being filled and at similar intervals when

9 CHUTE ROTATING HOLE) (WITH Figure i. Rotating distribution chute. Figure 2. Silo with cone.

10 is being emptied, and gradations are required to agree. it statistical analysis is performed on individual sieves and A results must be within certain tolerances. If results satisfactory, silo is approved unconditionally. f are indicate no segregation until last several loads, results conditional approval might be issued under which use of a equipment manufacturers --Barber Greene* and Two Industries, Inc. (4) provided information on causes ASTEC mix segregation, including those originating in silos. of of ir suggestions for eliminating segregation were Many minimize or prevent segregation when transferring To mix from silo to truck, consecutive dumps should made at front, rear, and middle as shown in Figure 3. be addition to controlling segregation, this practice promotes In Figure 3. Truck loading sequence. last several loads is not allowed. same as those given by respondents to survey questionnaire. of material as it is dumped and goes through remixing paver *James A. Scherocman" personal correspondence September

11 reduce or eliminate segregation during paving To Brock suggests use of following practices- operations, Do not empty paver hopper between truck i. of mix. loads Do not raise paver wings after every truck load 2. mix is emptied. of Raise truck body sufficiently to keep paver 3. full of mix. hopper control segregation at any stage, precautions should To taken in sampling and testing. The acceptability of a be processed through a surge silo can be determined visually mix by performing an extraction and gradation analysis. The or analysis cannot, of course, indicate segregation that gradation in paving process after sample has been taken or occurs sample has been taken so as not to include segregated if In Virginia re have been several instances where material. was visible in pavement but none was revealed segregation gradation analyses on samples taken from truck. by of contractors who placed a mix with excessive One in Virginia in 1980 was required to modify his segregation and procedures to obtain an acceptable mix before equipment could be used in Upon advise from silo surge silo if changes eliminated or reduced segregation determine an acceptable level. to 200-ton (181 Mg) capacity silo was filled with The tons (132 Mg) of 1-2 mix from drum-mixer before any Increase opening of rear hopper gates on paver. Reduce speed of conveyor and auger. 5. Reduce paver speed. (4) 6. FIELD PROJECT contractor made changes to cold feed, manufacturer, conveyor, rotating chute at top of silo, drum-mixer, procedure, paver adjustment and operations, and truck-loading mix design. A trial section of pavement was placed to mix was removed. Some segregation developed in center of

12 paver h0pper; however, when hopper was kept full. was not noticeable. Before change was made, segregation also tried, and with it appearance of mix in was appeared to be improved. After all adjustments were pavement segregation did not appear objectionable; refore, made, system was tentatively approved for use on state work. pawing started approximately two months later, When was observed for possible segregation. Some segregation operation observed in center of paver hopper and this carried was to center of pavement lane. The truck body was through to keep hopper full and segregation in middle raised improved, but mix n tended to segregate on sides was segregation was significantly improved over that The previous year and it was not felt to be detrimental; noted paving was allowed to continue. It was not possible refore, conclude which changes contributed most to improvements, to y were all made at same time. Field engineers because that segregation tended to reappear after several reported is recommended that findings of this study be It to selected field projects. Because segregation is applied mix through a surge silo. The mix should be sampled surface various stages of production and paving process to at source and magnitude of segregation. When determine of segregation has been identified, appropriate source that were identified in present study should corrections attempted. be segregation was evident in center of pavement slight A second paver with better conveyor and auger controls lane. of hopper. possibly because of a lack of attention and adherence to.weeks, original changes. or cases of segregation believed to be caused by No silos were reported to author. surge RECOMMENDATION FOR IMPLEMENTATION a problem only with coarse, mixes, paving projects usually be selected that involve production of coarse should

13 Hard Look at Economics vs. Technology and Investment," A Contractor and Builder, May 4, Michiga.n Truck Loadout, "Fast Worthwhile," Rainy Day Insurance Make Hot Mix Roads and Streets, October Storage with Silicone and Inert Gas," Roads and Streets, Extended July J. Don, Brock, T-101, "SEGREGATION- Causes and Cures," Technical ASTEC Industries, Incorporated, Bulletin EFERE CES Rheinfrank, Lamson, Jr. "Hot-Mix Surge Binning or Storing" l Brock, J Don, and Ronald B. Cox, "Safe Hot-Storage Time " National Asphalt "Hot-Mix Foster, Charles R., Storage, Pavement Association, October Edgar J. "Storage of Hot Bituminous Hellriegel, New Jersey Department of Transportation, Concrete October Mixes,"

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