Density of Asphalt Concrete-How Much Is Needed?

Transcription

1 TRANSPORTATION RESEARCH RECORD Density of Asphalt Concrete-Ho Mch Is Needed? E. R. BROWN Density is one of the most important parameters in constrction of asphalt mixtres. A mixtre that is properly designed and compacted ill contain the optimm amont of air voids. Becase density of an asphalt mixtre varies throghot its life, the voids mst be lo enogh initially to prevent permeability of air and ater and high enogh after a fe years of traffic to prevent rtting cased by plastic flo. There are three primary methods of specifying density: (a) percent of control strip, (b) percent of laboratory density, and ( c) percent of theoretical maximm density. If sed correctly, all three methods can reslt in satisfactory compaction. The initial in-place air voids mst be belo approximately 8 percent and are determined by comparing blk density and theoretical maximm density (TMD). The final in-place air voids, hich mst be above approximately 3 percent, are estimated by comparing the blk density of laboratory-compacted samples and the TMD. The to methods that have been sed to measre blk density of asphalt mixtre are physical measrements of cores and se of the nclear gage. The nclear gage is fast and nondestrctive bt is not as accrate as the core method. The amont of voids in the asphalt mixtre is probably the factor that most affects performance throghot the life of an asphalt pavement. Voids are primarily controlled by asphalt content, compactive effort dring constrction, and additional compaction nder traffic. The density reqirements and the methods of measring density vary considerably from state to state. Some states constrct a control test strip, measre the density on the strip, and se that density as the target density for the project. Other states compact samples in the laboratory dring mix design and constrction and se that density as the target density. Finally, other states measre the theoretical maximm density (TMD) (ASTM D241) and se some percentage of that density as the target density. All of these techniqes have been sed sccessflly to bild ellperforming pavements, bt they have also been missed, reslting in poor performance. Which method shold be sed? Ho mch density shold be specified and obtained dring constrction to ensre good performance? A problem ith density is the method of measrement. The to primary methods that have been sed inclde measrement of blk density of cores taken from the in-place pavement, and se of a nclear gage to measre the in-place density. The nclear gage method is not considered as accrate as measring the density of cores. Many states se the nclear gage for developing rolling patterns bt specify that cores be taken and measred for acceptance or rejection of the in-place mix. Hoever, several states se the nclear gage for acceptance testing of the asphalt mixtre. The existing methods of specifying density of asphalt mixtres are compared in the folloing paragraphs. The rela- National Center for Asphalt Technology, Abrn University, 211 Ramsey Hall, Abrn, Ala tionship of each method to constrction and performance is discssed, along ith the varios methods of measring density dring constrction. DESIRED ASPHALT DENSITY The voids in an asphalt mixtre are directly related to density; ths, density mst be closely controlled to ensre that the voids stay ithin an acceptable range. Previos ork has shon that the initial in-place voids shold be no more than approximately 8 percent and shold not fall belo approximately 3 percent dring the life of the pavement. High voids lead to permeability of ater and air, reslting in ater damage, oxidation, raveling, and cracking. Lo voids lead to rtting and shoving of the asphalt mixtre. In a stdy for the state of Arkansas, Ford (1) shoed that asphalt mixtres shold be designed and constrcted so that the in-place air voids stay above 2.5 percent. He demonstrated that, as long as the voids are above that amont, the expected rt depth ill be no greater than 1 %2 in. (see Figre 1). Ford's ork as based on tests condcted on asphalt samples that ere obtained from in-place pavements. The rt depth reported as from actal measrements on these pavements. Bron and Cross (2), in a stdy of rtting of asphalt pavements, shoed that significant rtting as likely to occr once the in-place voids reached approximately 3 percent (see Figre 2). When a sitable aggregate as sed and the voids stayed above 3 percent, rtting as normally not a problem. Some of the projects evalated shoed significant rtting hen the in-place voids ere ell above 3 percent. It as speclated that rtting began after the voids had decreased to an nacceptable level. Once rtting began, the integrity of the mix as lost and the voids increased. For these mixes, it as generally fond that recompacting the mixtres in the laboratory ith standard compactive effort prodced lo voids, hich helped explain hy the rtting occrred. In a stdy of asphalt mixtres in Canada, Hber and Heiman (3) considered a nmber of cases of rtting. It as determined that one of the primary cases as lo voids (belo 3 percent) in the asphalt mixtres. Zbe ( 4) shoed that asphalt mixtres become permeable to ater at approximately 8 percent air voids (see Figre 3). As long as the voids ere belo 8 percent in the 1 projects stdied, permeability as not a problem. Hoever, the permeability increased qickly as the void level increased above 8 percent. In a stdy of segregated mixes, Bron et al. (5) shoed that the asphalt mixes ere impermeable to ater as long as the air void content as belo approximately 8 percent (see

2 28 TRANSPORTATION RESEARCH RECORD :r: 3 Log Y = (Log X) R 2 =.456, RMSE =.186 (f) (f) ~ 4 ::::L I Y = X R 2 =.36 :r:~ 2 _) ::: 1 -l( ~ 3 ~ f-- 2 ::: ::J 1 ::: AIR VOIDS, PERCENT FIGURE 1 Relationship beteen air voids and rt depth in Arkansas (1). 8-1-~~~~~~~..,..,.~.e--~~..a-..~r-><'l-.--,-, a 6 7 RECOMPACTED PERCENT FIGURE 2 Relationship beteen air voids and rt depth in rtting stdy by National Center for Asphalt Technology (2). Figre 4). The permeability increased rapidly as the void content increased above 8 percent. Santcci et al. (6) shoed that the retained penetration of asphalt cement is affected by the air voids in the pavement (see Figre 5). The loss in asphalt penetration as greatly increased for air voids significantly greater than 8 percent. They conclded that asphalt mixes mst be constrcted ith lo air voids (belo 8 percent) to prevent rapid oxidation that leads to cracking and raveling of the asphalt mixtre. From these stdies, it is apparent that asphall mixes mst be constrcted ith an initial air void content belo approximately 8 percent and that the final air void content after traffic shold remain above approximately 3 percent. The initial air void content is determined by comparing the inplace blk density and the TMD for the mix being evalated. The final in-place air voids are estimated on the basis of the mix design and field qality control testing. The voids obtained dring the mix design and laboratory compaction of samples dring constrction are sed to estimate the in-place voids after traffic. A Marshall hammer as selected to provide voids in laboratory-compacted samples eqal to the measred voids after traffic (7). SPECIFICATION OF ASPHALT DENSITY Percentage of Laboratory Density One method that has been sed to specify density is to reqire that the in-place material be compacted to some percentage of the laboratory density. The standard laboratory density is specified as 5 to 75 blos ith a Marshall hammer. In recent years most states have reqired 75 blos for high-volme roads. Some specifications reqire at least 95 percent of laboratory density, hereas others reqire at least 98 percent. Some specifications do not allo mixes to be compacted to a density greater than 1 percent of laboratory density. When mixes designed to have 4 percent voids are compacted to a density greater than 1 percent, prematre rtting is likely to occr. Several items are important for this method of specification to ork effectively. Samples of the mix prodced dring constrction have to be compacted in the laboratory to establish a reference density and to determine the air voids in the mix at that density. If the air voids are not satisfactory in the laboratory-compacted samples dring constrction, the mix mst be adjsted so that acceptable air voids are obtained. Most often, the adjstment simply involves a modification in the asphalt content. The density prodced dring the mix design shold not be sed as the reference density becase the laboratory properties ill be somehat different than test reslts on plant-prodced materials. Aggregates sometimes break don dring mix prodction, creating an increase in dst and ths altering the properties of the compacted asphalt mixtre. The density prodced ith a manal hammer has been shon to correlate ith density in the field after traffic (7). Hence, any other type of compaction (mechanical or otherise) mst be calibrated to prodce a density eqal to that obtained ith the hand hammer or, better yet, a density eqal

3 E. R. Bron 29 I- n::: (/) > I- :::::> (/).6 ""' I ;:: _J.4 m <{ 2 n:::.2 > I- :5 n::: FIELD PERMEABILITY ML/MIN FIGURE 3 Relationship beteen air voids and permeability in California stdy (4) VOIDS TOTAL MIX, PERCENT FIGURE 4 Relationship beteen air voids and permeability in Georgia stdy (5). to that obtained in the field after traffic. The procedres specified in ASTM D1559 and AASHTO T245 for the Marshall test reqire that the manal hammer be sed or that the method sed be calibrated ith the manal hammer. Density data from eight constrction projects are presented in Table 1. These data sho that the in-place density (8th percentile) after traffic is 2.2 lb/ft 3 higher than that obtained in the mix design. There are to likely reasons for this higher density after traffic. First, the mix probably changed somehat dring prodction to increase the laboratory density. Second, the laboratory compaction effort as probably insfficient and ths shold be increased to be more representative of traffic. As shon in the table, the density of the mixes recompacted ith the manal hammer compares closely to the in-place density. This finding emphasies both the need to compact samples in the laboratory dring constrction to verify voids in the mixtre and the need to se correct laboratory compactive effort. Sppose a mix is designed to provide 4 percent voids and is specified to be compacted to at least 95 percent of laboratory density. This specification ill reslt in p to 9 percent voids immediately after compaction and shold reslt in approximately 4 percent voids after several years of traffic. The initial voids (9 percent) may be a little high ith this specification; hoever, the final voids ( 4 percent) shold be acceptable. The high initial voids may reslt in increased oxidation, casing more cracking and raveling if not sbjected to significant traffic to provide frther compaction. If this mix is sbjected to a high volme of traffic, a small rt (5 percent of layer thickness, or V1 in. for a 2-in. layer) ill reslt after additional channelied compaction nder traffic increases the density from 95 to 1 percent of laboratory density. If a mix is designed to have 4 percent air voids and is compacted to a density greater than 1 percent, immediate failre cased by rtting is likely. If the laboratory compactive effort is satisfactory, it is not practical for the mix to be compacted to a density greater than 1 percent. Hence, any project that continally approaches or exceeds 1 percent of laboratory density is likely the reslt of lo laboratory density, not excessive compaction in the field. This method of specifying compaction ill reslt in good performance of properly designed mixes if (a) laboratory samples are compacted dring constrction to establish reference density, (b) correct laboratory compaction techniqes are sed, and (c) a minimm compaction reqirement is set to ensre that in-place air voids after compaction do not exceed approximately 8 percent. Percentage of Theoretical Maximm Density A second method often sed to specify compaction reqires that the asphalt mixtre be compacted to some minimm percentage of the TMD. This procedre is a direct method of specifying maximm in-place air voids and an indirect method of controlling compaction. It involves taking a sample of the asphalt mixtre dring constrction and condcting tests to measre the TMD (ASTM D241). The blk density of the asphalt mixtre is measred after compaction and compared to the TMD, providing a direct measrement of in-place voids.

4 3 f- n:::: L... ~ n:::: ~ n:::: f AIR VOID CONTENT, PERCENT FIGURE 5 Relationship beteen retained penetration and air voids (6). For instance, a mixtre compacted to 93 percent of the TMD ill have 7 percent air voids. This type of compaction specification reqires that the TMD, hich is the reference density, be measred rotinely dring constrction. The TMD measred dring mix design shold not be sed as a reference for the mix being prodced at an asphalt plant. As stated before, the materials change hen 15 TRANSPORTATION RESEARCH R ECORD 1282 heated and mixed at an asphalt plant, hence the TMD mst be measred on these plant-prodced materials. Some states do not compact samples of asphalt mixtre in the laboratory dring constrction. Many bitminos engineers do not believe that laboratory compaction of samples is necessary becase the relative density is no the TMD and the time normally spent on compacting and testing laboratory samples can be sed to condct other tests. To control the constrction process adeqately, samples mst be taken dring constrction and compacted in the laboratory. The voids in the laboratory-compacted samples mst be measred and evalated to determine the final expected in-place voids. 1t is not orthhile to compact an asphalt mixtre to 7 or 8 percent air voids initially if the voids are going to be redced to 1 or 2 percent after one smmer of traffic. The only ay to estimate the final in-place voids (one of the most critical properties of an asphalt mixtre) is to compact samples in the laboratory sing the specified techniqe (manal or eqivalent) and to measre the voids. If the voids are not acceptable, the mix (sally asphalt content) mst be modified to prodce acceptable voids. This type of density specification is often missed. On many projects, so mch emphasis has been placed on the initial inplace voids after compaction that the asphalt content has been arbitrarily increased to redce the initial in-place voids to an acceptable range. This increase in asphalt content is often done hen paving in cold eather or at other times hen compaction is difficlt. The increase in asphalt content ill loer the air voids in laboratory-compacted mixes to an ndesirable level and ill likely reslt in rtting hen sbjected to a significant amont of traffic. If voids are high dring constrction, more compactive effort, improved roller patterns, or a modified mix design shold be sed to increase density. An increase in asphalt redces the TMD and typically increases the actal density, hich can significantly decrease the voids in the mix after being exposed to traffic. This method of specifying density does encorage higher asphalt content and higher filler content; hoever, it can be correctly sed if properly monitored. Laboratory compaction TABLE 1 COMPARISON OF JOB MIX FORMULA DENSITY, IN-PLACE DENSITY, AND RECOMPACTED DENSITY In-Place Density Recompacted Density JKF Density (8 percentile) (75-blo Hand Hammer) Project (pcf) (pcf) (pcf) l Average

5 E. R. Bron tests mst be condcted dring constrction to ensre that the voids are maintained ithin an acceptable range. The TMD mst be measred on the actal material being placed to ensre an accrate measrement. Additional asphalt mst never be added for the sole prpose of redcing in-place voids. If the in-place voids are too high and the mixtre has been properly designed, more compactive effort mst be exerted to decrease them. In particlar, more asphalt shold not be added to decrease voids hen paving in cold eather. Again, more compactive effort mst be applied to the asphalt mix. Percentage of Control Strip Density A third method sed to specify density is to compare the blk density of the in-place asphalt mixtre to that of a previosly constrcted control strip. The control strip is constrcted sing standard compaction techniqes. Most specifications reqire that it be compacted to some minimm percentage of the standard laboratory density or the TMD. If the specifications do not reqire a minimm density, the inspector mst closely evalate the contractor's compaction eqipment and rolling procedres to ensre that reasonable compactive effort is being applied to the asphalt mix. Any significant changes in the mix dring constrction shold reqire that a ne test strip be constrcted and evalated. This method of density control is probably the least desirable of the three methods discssed. Althogh it does allo the compactibility of a mixtre to be evalated, it is difficlt for an inspector to kno hen a contractor has applied a reasonable compactive effort to the control strip. Many items affect density, and a change in any of them may alter the reslts obtained from a control strip. Some of the items that affect density inclde gradation (especially for content of particles passing No. 2 sieve), asphalt content, moistre content, mix temperatre, air temperatre, layer thickness, roller eight, roller pattern, and roller speed. As stated earlier, a minimm density is normally reqired for the control strip. This reqirement ensres that the contractor does apply some minimal effort dring compaction. The specification reqires a minimm density in the control strip and then a minimm percentage of the control strip density in the remaining ork. This specification cold be made simpler by reqiring the compacted mix to meet some percentage of the laboratory density or TMD. For example, assme a specification reqires that a control strip have a density of at least 94 percent of the TMD and that all asphalt mix placed after the control strip have a density at least 98 percent of that of the control strip. This specification cold be simplified by reqiring that the mixtre be compacted in place to a minimm density of 92 percent of the TMD. These to examples of specifying density reslt in similar compaction reqirements. The control strip method of specifying density can achieve satisfactory reslts. Hoever, the specifications shold reqire that the initial in-place voids in the asphalt mixtre do not exceed approximately 8 percent and that the final in-place voids do not decrease belo approximately 3 percent. This reqires that samples be compacted in the laboratory dring constrction to estimate the final in-place voids and that the initial in-place air voids be measred dring the constrction process. As long as sfficient testing is performed to ensre that the initial and final in-place voids are acceptable, then this procedre can be sed satisfactorily to specify compaction reqirements. MEASUREMENT OF ASPHALT DENSITY Core Method The core method is the referee procedre for density measrement and is the standard to hich other methods are compared. It reqires a significant amont of time becase the pavement has to cool before cores can be taken and the cores mst be air-dried to obtain dry eight. In most cases, the density reslts are obtained the day after constrction. After ctting the core from the pavement, the material otside the layer in hich density is being measred mst be removed. In some cases, paper or other material is placed on the existing srface before overlaying to redce the bond beteen layers. The core can then be separated easily so that the density of the asphalt layer being placed can be measred. The location mst be careflly marked so that the core can be taken over the paper. There are some problems in sing paper to break the bond beteen to layers. Becase there is a lack of bond in this location, this method may reslt in loer density over the paper. This approach also identifies the location at hich cores ill be taken and, hence, may reslt in some additional rolling in these locations by the contractor. This method of taking cores is not reliable and is not idely sed today. The method most often sed to obtain core samples is to randomly locate samples, ct the core fll depth, and sa or otherise separate the layers being tested from the remaining material. This method is sally the most accrate ay to evalate the overall density of the pavement and is the least disrptive to the paving operation. A problem that sometimes occrs in measring the blk density of a core is failre to allo the core sfficient time to dry before obtaining the dry eight. The core shold be alloed to air-dry before the density is measred. Drying in an oven at an elevated temperatre may reslt in distortion of the core and, hence, reslt in an error in density measrement. Measring density of a core that is not completely dry ill reslt in an erroneosly high density vale. Brati and Eloghbi (8) shoed that the variability of density test reslts as less hen measred ith cores than hen measred ith a nclear device (described in the folloing section). They examined three nclear gages on to constrction projects and fond that there as a statistically significant difference in the average density hen measred ith cores and nclear gages. Nclear Gages Nclear gages have been sed for a nmber of years to measre the blk density of asphalt mixtres. This techniqe has the advantage of being rapid and nondestrctive. Most density measrements on asphalt mix have been done in the backscatter mode. fo this method, the gage is set on top of the pavement and a reading is taken that represents the average density for the top several inches of material. For 31

6 32 instance, the average density may be representative of the top 6 in. of material, bt the layer being evalated may only be 2 in. thick. Part of the error is removed by calibrating the nclear gage to provide the same density as that provided by cores. Hoever, errors still exist becase of variations in thickness and density in the nderlying layers. In recent years, a nclear gage has been developed to measre the density of thin lifts. This gage shold provide greater accracy in density measrements hen compared to the previos gage, bt sfficient tests to sho overall accracy have not been developed. The best se of nclear gages is in development of rolling patterns and qick determination of approximate density. Becase of the possibility of error, nclear gages shold never be sed alone for acceptance testing. Some cores shold be taken rotinely to verify the accracy of the gage and to ensre lhal an acceptable density is obtained. Many projects have heen constrcted in hich the nclear gage as the only method sed to measre density. This practice is not recommended becase, even if the gage is calibrated daily, problems can develop that reslt in inaccrate readings. CONCLUSION AND SUMMARY The amont of vmds 1s the most important property of an asphalt mixtre. Becase voids vary throghot the life of the pavement, the initial and final voids mst be controlled. Initial in-place voids are determined by comparing the blk density to the TMD. They shold not exceed approximately 8 percent. Final voids are controlled by compacting samples (sing a manal hammer or similar method) in the laboratory dring the constrction process. The voids in these samples ill be representative of the final in-place voids if correct compactive effort is sed. The final in-place voids shold not be belo approximately 3 percent. Typically, the mix design is performed to provide 4 percent voids in the mix. As long as a specification is ritten to ensre that maximm voids do not exceed 8 percent and minimm voids do not fall belo 3 percent, then density can be specified as a percentage of laboratory density, control strip density, or TMD. All three methods of specifying density ill provide acceptable reslts TRANSPORTATION RESEARCH RECORD 1282 if properly sed, bt the TMD method has been grossly missed. The method of measring density mst be controlled becase voids are directly related to density. The nclear gage is qick and nondestrctive bt is not as accrate as cores. Some cores shold alays be taken dring the constrction process to verify that acceptable initial in-place density is obtained. ACKNOWLEDGMENT Information for this stdy as obtained from ongoing research, conversations ith state bitminos engineers, and a revie of recent literatre on compaction. REFERENCES I.. -. ord. l'avcmcnt l)en it cation Related to Asphalt Mix harncteri tic. In 1i'f111sportMio11 Re earcl1 Record 117., TRB, National Re carch onci l, Washington, D. ' E. R. Bron ind. ros. A tdy of ln Plcc Rtting or phnlt Pavements. Presented at JC), 9 Anmml Meeting of the As.soci ati n of phalt Paving Tcchnologi t~. Na. h illc, Tenn A. Hber and. H. Heiman. Effect of Asphalt oncrctc Parameters on Rtting Performance: A Ficlcl Investigntion. Proc., Associmio11 of Asp/i(l/t Paving Tech11ologis1.r. 11 hville, Tenn., Vol. 56, 1987, pp. 3 - l. 4. E. Zbe. Compaction SLUdics of Asphaltic Concrete Pave1m:t 11~ Related to the Water P rmeability Test. Presented at 41st Annal Meeting of the Highay Research Board.Wahington. D.., I E. R. Bron. R. ollins, and J. IC Hron ficld. lnve rigati n of egrcgation of Asphalt Mixtllrc. in the Hile of Gcorgi. In Trt111 poriatio11 Rese"rch Record 1217, TR.B. National Research Concil, Washington. D L. E. Santcci, D. D. AUen, and R. L. Coats. The Effects of Moistre and ompacti n n the Qality of Asphalt Pavements. Proc.. Association f Aspllr Pm illg Tecl111/ogis1., Nashville, Tenn., Vol. 54, 1985, pp T. D. White. Marshall Procedres for Design a.nd Qality omrol of A phalt Mixtres. Pre.. A~ soci{l/ion of Asphalt Paving Tecli - 11/ogists, a lvlllc. Tenn., Vol r>p J. L. Bnni. Jr. and G. B. - loghbi. Correlation of cle<ir Dcnity Reslts.ith ore Densities. In Tra11sµor1io11 lk:11rc/1 Re - ord 1126, TRB, ational Re earch oncil. Wa hington. D , pp Pblicaiion of this paper sponsored by Committee on Flexible Pave ment Constrction and Rehabilitation.