1、Department of Cvili Engnieernig,Natoina lInsttiute of Technology Karnataka,Surathka,lMangaolre 575 025,IndiaABSTRACTPorous friction courses（PFCs）are mainly recommended as surface drainage layers on high-speed roadcorridors and runway pavements.Permeability and sound attenuation characteristics are c
2、onsidered to bet he indicesf orp erformancea ssessmento fP FCs.Oneo ft her easonsf or thel osso fp ermeabilityi nP FCs is densification under heavy traffic.But,resistance to ravelling too,is of main concern in the case of under-compacted PFCs.This paper summarises the details of laboratory investiga
3、tion on the characterization of PFC mixes corresponding to four different gradations and two binder contents,for three levels of the Marshallc ompaction.Thef indings oft he investigations uggestt hat thes electiono ft hec ompaction level for PFC mix design should be based on the design traffic level
4、,and the gradation selected.2009 Elsevier Ltd.All rights reserved.Keywords: Porous friction course Porous asphalt Marshall compaction Air voids Voids in coarse aggregate Permeability Moisture susceptibilityAbrasion loss1. IntroductionPorous friction courses（PFCs）are typical open-graded asphaltic mix
5、es,composed of relatively uniformly-graded aggregate and asphalt cement or modified binders,and are mainly used to serve as drainage layers,either at the pavement surface or within the pavement structure1.Pavements surfaced with open-graded asphaltic mixes were found to improve wet weather skid-resi
6、stance,minimize hydroplaning,reduce splash and spray,improve night visibility during wet weather conditions,and reduce traffic tyrenoise2,3.In countries like the United States of America,Japan,the United Kingdom,Malaysia,Australia,New Zealand,and South Africa,open-graded mixes are in use as surface
7、layers over high-speed and heavily trafficked highway pavements28.These are also recommended for surfacing runway pavements9-10.Many agencies around the world use different terminologies for open-graded mixes,and specifications that are slightly different.The various terminologies used include open-
8、graded asphalt（OGA）, porous asphalt（PA）,open-graded friction course（OGFC）,and porous friction course（PFC）. 1.1 BackgroundThe higha irv oidsc ontenti nP FCsc ontributet owardsp avement surface drainage and in attaining noise reduction.But,accelerated asphalt-film aging,may subsequently lead to loss o
9、f cohesion in the mastic,consequently poor adhesion between the mastic and aggregate results in ravelling.Experiences with the use of open-graded mixes in the USA indicated that ravelling was one of the major issues to be tackled11.Thus,structural durability of PFCs needst o bee nsured basedo n the
10、resistance to ravelling12.Some of the experiences with PFCs in Japan,the poor performance was related to the lack of compaction,or compaction at lower temper-atures than the specified6.Permeability and sound attenuation characteristics are the main measures of performance life of PFCs2.Studies perfo
11、rmed on test sections in Denmark,indicated that deteriorated mastic materials clogged the large-sized air voids in PFCs,which led to the formation of higher percentage of small-sized voids12.Clog-ging was found to be more pronounced on older pavement,especially along the wheel path,and also in the e
12、mergency lanes 13.Although,PFCs are composed of relatively uniformly-graded aggregate1,the use of thicker PFCs may undergo secondary densification during service,resulting in loss of permeability.Thus,the loss of permeability can be directly related to clogging of voids due to internal and external
13、materials,and densification under traffic2.An optimal design mix should ensure high air voids content and good resistance to ravelling.Procedure for mix design of PFCs recommended by various agencies include the evaluation of drain-down losses in loose hot mixes,air voids content,stone-on-stone cont
14、act condition,permeability,and the abrasion resistance using the Cantabro abrasion test method4,7,8,15.One of the major differences observed in the specifications by various agencies is that of the consideration of traffic volume in the optimum mix selection criteria.The minimum air voids content an
15、d the maximum abrasion loss specified by some of the agencies were dependent on the traffic volume4,7,8,whereas,this dependency was not considered by many agencies in the USA2,10,15,16.The level of Marshall compaction suggested by many agencies were found to be 50 blows per face,irrespective of the
16、traffic volume.In addition,in the recent past many researchers adopted the same level of the Marshall compaction for the design of PFC mixes1720.However,some of the researchers adopted or suggested the lower levels of the Marshall compaction,for characterizing the PFC mixes2124.1.2.Objective and sco
17、peThe main objective of this investigation was to characterize the PFC mixes for three levels of the Marshall compaction.The compaction levels investigated include 35,50,and 75 blows applied on each end of the specimen.The effect of each compaction level was studied on eight different PFC mixes,whic
18、h corresponded to four different aggregate gradations（G）,and two binder contents （BC）of 4.5 and 5.0%by mass of total mix,using the neat bitumen of 85100 penetration grade.The selection of gradation,binder type,and binder content were based on the previous studies performed on similar mixes25,26.Fig.
19、1 shows the details of the four aggregate gradations investigated.Table 1 provides details of the coding method adopted for designating the mixes for various compaction levels and mix compositions.2. Specimen preparation and test planStraight-run bitumen and crushed stone aggregates are the major co
20、nstituents of PFC mixes.The straight-run paving grade bitumen used in the present investigation was supplied by Mangalore Refinery and Petrochemicals Limited（MRPL）,Manga-lore.Crushed granite stone aggregates obtained from local stone-crushing plants were used in this study.Table 2 shows some of the
21、physical properties of bitumen and aggregates tested in accordance with the requirements of the ASTM D 94627and ASTM D706415,respectively.Ordinary Portland Cement（OPC）was used as a part of the mineral filler,constituting 2%by mass of total aggregates weighing 1000 g.The procedure adopted for prepara
22、tion of the PFC specimens was quite the same as that adopted for dense graded asphalt,as suggested in Asphalt Institute Manual Series-228.The properties of compacted mixes investigated include bulk specific gravity （Gmb）,air voids content（Va）,stone-on-stone contact condition,per-meability（K）,moistur
23、e susceptibility,and unaged abrasion loss.These properties were evaluated in accordance with the guidelines of the ASTM D 706415.Table 3 provides details on the tests performed for various experimental mixes.Three observations were made on each experimental mix.3. Characterization of PFC mixes3.1. B
24、ulk specific gravity of compacted mixThe tests for bulk specific gravity of compacted mixes（Gmb） were performed on 24 mixes as described in Table 3,with three replicates for each mix.The Gmb of each compacted mix was determined using the geometric measurements of diameter,height,and the mass of the
25、specimen in air,in accordance with ASTM D 706415.The individual and mean Gmb values were found to be in the range of 1.9352.249,and 1.9842.226,respectively,as shown in Fig.2.It is evident that an increase in the compaction effort will lead to densification of the mix,resulting in higher densities.Fu
26、rther,it is evident from the 95%confidence intervals that there is no significant difference between the mean Gmb values among the mixes with binder contents of 4.5%and 5.0%when compared,for a particular gradation and compaction level.The mixes with gradations G1 and G4 exhibited higher and lower Gm
27、b values,respectively,especially since G4 comprised of coarser aggregates.The mean Gmb values for the mixes with gradations G2 and G3 are approximately the same,as the minor differences in gradations have not affected the Gmb values significantly.3.2.Air voidsThe air voids（Va）content in a compacted
28、mix is related to the Gmb and the theoretical maximum density（Gmm）of the uncompacted mix,determined in accordance with the ASTM D 204130.The increase in the compaction effort results in higher Gmb, resulting in a decrease in Va.Fig.3 shows the individual plot of Va for each mix.It also indicates the
29、 mean Va of the individual mixes tested,and the 95%confidence interval for the mean.The line connecting the mean Va shows the trend in variations of the Va with the compaction effort.The individual and mean Va values were found to be in the range of 1022%,and 1020%,respectively.As per the standard r
30、equirements of ASTM D 706415,PFC mixes should have a minimum Va of 18%.The mean Va corresponding to the mixes M19（G4-35-4.5）,M20（G4-35-5.0）,and M21（G4-50-4.5）satisfied this requirement,while,the mixes M7（G2-35-4.5）,M8（G2-35-5.0）,M9 （G2-50-4.5）,M13（G3-35-4.5）,and M15（G3-50-4.5）seem to satisfy this re
31、quirement at a confidence level of 95%.However,in the mixes tested for 75 blows,the individual,mean,and upper limits of 95%confidence interval of mean Va failed to satisfy the minimum Va requirement.All mixes corresponding to the gradation G4,i.e.,M19M24 were found to have mean Va of more than15%.Th
32、e reduction in the mean Va due to change in the compaction level from 50 blows to 75 blows was found to be in the range of 0.63.2%.Similarly,change in compaction level from 50 to 35blows,resulted in an increase in mean Va in the range of 0.22.9%. 3.2. Verification of stone-on-stone contact conditionThe presence of stone-on-stone contact condition in the coarse aggregate skeleton is considered to be must,for the best performance of PFC15.In the compacted PFC mix,stone-on
