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1、Characterization of porous friction course mixes for differentMarshall compaction effortsDepartment 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 ro
2、adcorridors and runway pavements.Permeability and sound attenuation characteristics are considered 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 t
3、he case of under-compacted PFCs.This paper summarises the details of laboratory investigation 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 electi
4、ono ft hec ompaction level for PFC mix design should be based on the design traffic level,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 susceptibilityAbras
5、ion loss1. IntroductionPorous friction courses(PFCs)are typical open-graded asphaltic mixes,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.Paveme
6、nts surfaced with open-graded asphaltic mixes were found to improve wet weather skid-resistance,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
7、,Malaysia,Australia,New Zealand,and South Africa,open-graded mixes are in use as surface 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,an
8、d specifications that are slightly different.The various terminologies used include open-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 att
9、aining noise reduction.But,accelerated asphalt-film aging,may subsequently lead to loss of 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 is
10、sues to be tackled11.Thus,structural durability of PFCs needst o bee nsured basedo n the 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 att
11、enuation characteristics are the main measures of performance life of PFCs2.Studies performed 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
12、to be more pronounced on older pavement,especially along the wheel path,and also in the emergency 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 o
13、f permeability can be directly related to clogging of voids due to internal and external 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
14、 evaluation of drain-down losses in loose hot mixes,air voids content,stone-on-stone contact 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
15、 of traffic volume in the optimum mix selection criteria.The minimum air voids content and 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 comp
16、action suggested by many agencies were found to be 50 blows per face,irrespective of the 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 lev
17、els of the Marshall compaction,for characterizing the PFC mixes2124.1.2.Objective and scopeThe 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 s
18、pecimen.The effect of each compaction level was studied on eight different PFC mixes,which 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,a
19、nd binder content were based on the previous studies performed on similar mixes25,26.Fig.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 pr
20、eparation and test planStraight-run bitumen and crushed stone aggregates are the major constituents 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 obt
21、ained from local stone-crushing plants were used in this study.Table 2 shows some of the 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,con
22、stituting 2%by mass of total aggregates weighing 1000 g.The procedure adopted for preparation 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 grav
23、ity (Gmb),air voids content(Va),stone-on-stone contact condition,per-meability(K),moisture 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.Thr
24、ee observations were made on each experimental mix.3. Characterization of PFC mixes3.1. Bulk 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 mi
25、x was determined using the geometric measurements of diameter,height,and the mass of the 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
26、 compaction effort will lead to densification of the mix,resulting in higher densities.Further,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 gradati
27、on and compaction level.The mixes with gradations G1 and G4 exhibited higher and lower Gmb 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 af
28、fected the Gmb values significantly.3.2.Air voidsThe air voids(Va)content in a compacted 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
29、a decrease in Va.Fig.3 shows the individual plot of Va for each mix.It also indicates the 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 v
30、alues were found to be in the range of 1022%,and 1020%,respectively.As per the standard requirements 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
31、.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 requirement 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 co
32、rresponding to the gradation G4,i.e.,M19M24 were found to have mean Va of more than15%.The 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
33、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-stone contact condition was verified
34、 based on the percent voids in coarse aggregate of the compacted mixture(VCAm)and the percent voids in coarse aggregate of the coarse aggregate alone(VCAd)in dry-rod-ded test,conducted according to the ASTM C29/C29 M31.The stone-on-stone contact conditions were confirmed,when the ratios of VCAm/VCAd
35、 were below one.The VCAd and VCAm values were computed using the following equations:VCAd=(Gcaws)(Gcaw)VCAm=100(GmbPcA/GcA)where GCA=bulk specific gravity of the coarse aggregate; s =bulk density of the coarse aggregate fraction in the dry-rodded condition; w=density of water;and PCA=percent coarse
36、aggregate in the total mixture.Generally,PFC mixes characterized with higher Gmb,higher PCA,or a combination of these,result in lower VCAm,ensuring better stone-on-stone contact conditions.The ratio of VCAm/VCAd for each replicate mix is shown in Fig.4.It can be noticed that each replicate of the mi
37、xes M7M24(corresponding to gradations G2,G3,and G4),satisfied the stone-on-stone contact condition,even under lowest compaction level of 35 blows.Thus,the mixes corresponding to the gradation,between the bands of G2 and G4,are expected to offer best performance.The mixes corresponding to gradation G
38、1 failed to satisfy the requirement.This might be due to the presence of more than 20%of aggregates passing 4.75 mm sieve17. The mixes investigated in the present study,based on the 95%confidence interval for the mean of the ratio of VCAm/VCAd,it can be observed that the mixes M4(G1-50-5.0),M5(G1-75
39、-4.5),and M6(G1-75-5.0)may probably satisfy the requirement for stone-on-stone contact condition.3.3. PermeabilityThe coefficients of permeability of PFC specimens were determined using the falling-head method29.The details on the test procedure adopted can be found elsewhere25,26.Permeability(K)val
40、ue of each replicate,the mean K value of each mix tested,and the 95%confidence interval for the mean K,of the experiment are shown in Fig.5.The ASTM D 706415suggests a minimum K of 100 m/day.The mean K values of mixes corresponding to gradation G4,compacted with 50 blows or lesser, and mixes corresp
41、onding to gradation G3,compacted with 35 blows,were found to satisfy this requirement.The mixes corresponding to gradation G1 exhibited poor permeability,with no individual value more than 20 m/day.The mixes compacted witha compaction efforts of 75 blows,had K values of lesser than 50 m/day,except i
42、n the case of mixes with gradation G4.For mixes compacted with 50 blows,the mean K values were found to be in the range of 8158 m/day.For mixes compacted with 35 blows,the mean K values were higher by a factor in the range of 1.32.2.In the case of mixes compacted with an effort of 75 blows,the mean
43、K values were reduced by a factor of 0.30.7.3.5.Analysis of variance(ANOVA)for responsesThe influences of main treatment factors such as the compaction effort(MC),gradation(G),binder content(BC),and the interactions between the main treatment factors(MC*BC,BC*G,MC*G,and MC*BC*G),were statistically a
44、nalysed for the response properties Va,VCAm,and K.The tests for analysis of variance(ANOVA) were carried out using a three-factor model32as shown in Eq.(3).Statistical analyses were carried out with the aid of the statistical software,MINITAB (Release 15,trial version).yijkl=+i+j+k+（）ij+（）ik+（）jk+（）
45、ijk+ijklwhere yijkl is the response of an experiment of lth replicate; is the mean of response; is the variation due to the Marshall compaction (MC)levels; is the variation due to binder content(BC); is the variation due to gradation(G); is the error term;i,j,and k are number of levels in MC,BC,and
46、G,respectively;and l indicatesthe replicate of the experiment.The ANOVA tests for each of the responses were carried out in two groups,Group A comprises the responses for mixes M1M24(corresponding to all four gradations),and Group B comprising the responses for mixes M7M24(corresponding to the grada
47、tions G2,G3,and G4).Here,the null-hypothesis(H0)is that the mean value of the responses due the effect of a particular source of variation is same.A confidence level of 95%was selected to test the H0.If,the F-static(F)of the response was found to be lesser or equal to F-critical(F0),then H0 was acce
48、pted.Otherwise,it was rejected.The results of the ANOVA tests are presented in Table 4.The common observations made based on the results of the ANOVA tests are given below:(i)The responses corresponding to volumetric properties(Vaand VCAm),and the permeability(K)were significantly influenced by the
49、compaction levels and the gradations(MC andG),and the interactions between the compaction levels andthe binder contents(MC*BC).(ii)The bitumen content was found to have a significant effect only on the air voids content(Va)and the permeability(K).(iii)The R2 values of the responses indicate that the model expressed as in Eq.(3)fits the response-data better.T
