Durable and Eco­nom­i­cal Tech­nol­ogy for Bi­tu­mi­nous Sur­fac­ing of Ru­ral Roads

NBM&CW - - CONTENTS - Prof. Prithvi Singh Kand­hal

In­tro­duc­tion

The Pradhan Mantri Grameen Sadak Yo­jna (PMGSY) has been one of the most suc­cess­ful flag­ship pro­grammes launched by for­mer Prime Min­is­ter Atal Bi­hari Va­j­payee in 2000 for con­struct­ing a wide net­work of ru­ral roads across In­dia. Dur­ing the last 17 years, a to­tal of 5,29,975 km of PMGSY roads have been con­structed con­nect­ing vil­lages of over 500 peo­ple, which is a com­mend­able task!

Two tech­nolo­gies (sur­face dress­ing and pre­mix car­pet) are spec­i­fied in In­dia’s ‘Ru­ral Roads Man­ual’ for bi­tu­mi­nous sur­fac­ing of ru­ral roads such as those un­der PMGSY. Ei­ther of these two tech­nolo­gies are per­mit­ted. Both are also stan­dard­ized by the In­dian Roads Congress (IRC).

Sur­face dress­ing (also called chip seal) con­sists of spray­ing a thick film of bi­tu­men over the com­pacted stone base layer (called wa­ter bound macadam) with the help of a bi­tu­men truck tank dis­trib­u­tor at a spec­i­fied rate. This is fol­lowed by spread­ing stone chips (ag­gre­gate) at a spec­i­fied rate by a mech­a­nized chip spreader (avail­able from many man­u­fac­tur­ers in Gu­jarat and cost­ing less than Rs. 2.5 lakhs). Stone chips are then com­pacted with a road roller so as to em­bed/fix them (about 70%) into the sprayed thick bi­tu­men layer. This com­pletes the sur­face dress­ing op­er­a­tion as il­lus­trated in Fig­ure 1.

The pre­mix car­pet (PMC) sim­ply con­sists of sin­gle size (about 12 mm) stone chips mixed with 3.5% bi­tu­men by weight at a hot mix plant. PMC is highly open graded be­cause the mix does not con­tain any fine ag­gre­gate (sand). This mix is laid in 20 mm thick­ness man­u­ally or with a paver. A sand seal coat is pro­vided at the top.

The ob­jec­tive of this pa­per is to dis­cuss these two tech­nolo­gies in de­tail and rec­om­mend the one which is bet­ter both in terms of dura­bil­ity and eco­nom­ics.

His­tory of Bi­tu­mi­nous Sur­fac­ing

When the au­thor was serv­ing as a high­way en­gi­neer in the Ra­jasthan PWD dur­ing the early 1960s, it was very com­mon to use bi­tu­mi­nous sur­face dress­ing (SD) or chip seal­ing on most types of roads. Sur­face dress­ing was very ef­fec­tive in wa­ter­proof­ing the wa­ter-bound macadam (WBM) roads be­cause of heavy bi­tu­men ap­pli­ca­tion rate fol­lowed by chip ap­pli­ca­tion. Sur­face dress­ing was sched­uled once in 3 or 4 years on all roads. Very few pot­holes dot­ted the roads at that time, and traf­fic vol­umes were gen­er­ally less. Road con­struc­tion was largely man­ual and hardly mech­a­nized. Bi­tu­men for sur­face dress­ing was ap­plied with per­fo­rated tin cans. Spread­ing the sur­face ag­gre­gate (chips) by hand was an art learnt through prac­tice, usu­ally by swirling the bas­ket con­tain­ing the ag­gre­gate.

As is of­ten usual with sur­face dress­ing, chips were dis­lo­cated and be­came loose if the treated road was opened too soon to traf­fic, or slow speeds were not main­tained just af­ter con­struc­tion. The fin­ished road sur­face was not black and, there­fore, not too ap­peal­ing to the pub­lic.

To over­come these per­ceived “prob­lems”, the pre­mix car­pet (PMC) was in­tro­duced with the IRC pub­lish­ing its spec­i­fi­ca­tion for the first time in 1962. As men­tioned ear­lier, road con­struc­tion was still man­ual. Sin­gle size chips (nom­i­nal size 12 mm) were ei­ther bro­ken by labour­ers by hand or ob­tained from stone crusher plants (if avail­able nearby). Hot bi­tu­men was ap­plied

as tack coat through per­fo­rated tin cans. The mix con­tain­ing al­most sin­gle size ag­gre­gate (11.2 mm to 13.2 mm) could eas­ily be coated with about 3-3.5% bi­tu­men ei­ther by hand on flat pans placed over wooden log fire; or small drums ro­tated by hand; or small portable mix­ing plants. Un­der such cir­cum­stances, graded ag­gre­gate could not be used.

The mix was taken in hand carts and spread over tack coated road sur­face us­ing hand rakes. Af­ter rolling, the road sur­face ap­peared “shin­ing” black, with no loose stones and im­pres­sive look­ing to the pub­lic, un­like sur­face dress­ing. It was re­al­ized that PMC was highly per­me­able to rain­wa­ter due to sin­gle size ag­gre­gate be­ing used in the mix. There­fore, the use of sand seal coat was war­ranted to seal the sur­face of the open graded mix. Sand was mixed with about 7 per­cent bi­tu­men, ap­plied on the open sur­face, and rolled. With the ad­vent of the PMC, sur­face dress­ing started to die across In­dia and is al­most non-ex­is­tent in many states such as Ra­jasthan. It is iron­i­cal that sur­face dress­ing is still be­ing used ex­ten­sively and suc­cess­fully on low to medium-traf­ficked roads in de­vel­oped coun­tries such as USA, Aus­tralia, New Zea­land, and South Africa. Some In­dian en­gi­neers ar­gue that sur­face dress­ing is suc­cess­ful in those coun­tries be­cause the con­struc­tion (bi­tu­men ap­pli­ca­tion and chip spread­ing) is mech­a­nized there. How­ever, the en­gi­neers can man­date bi­tu­men dis­trib­u­tors (al­ready avail­able for tack coat work) and mech­a­nized chip spread­ers (be­ing man­u­fac­tured in Gu­jarat). A typ­i­cal chip spreader is shown in Fig­ure 2.

The PMC has prob­a­bly served In­dia well for over 50 years es­pe­cially dur­ing the time mech­a­niza­tion was al­most not there. How­ever, due to sig­nif­i­cant in­crease in ve­hic­u­lar traf­fic, and PMC’s in­her­ent wa­ter­trap­ping char­ac­ter­is­tics, its ser­vice life has de­creased sig­nif­i­cantly in re­cent years. Time has now come to think out of the box and con­sider sur­face dress­ing in lieu of PMC for low to medium-traf­ficked roads, be­cause it is highly eco­nom­i­cal as well as highly ef­fec­tive in wa­ter-proof­ing the road pave­ment (as dis­cussed later).

Pre­mix Car­pet (PMC)

The un­de­sir­able wa­ter-trap­ping char­ac­ter­is­tic of the PMC, which causes pot­holes due to in­creased hy­draulic pres­sure un­der traf­fic, is dis­cussed be­low: To keep things in per­spec­tive, let’s com­pare PMC with open graded as­phalt fric­tion course (OGFC), which is used in de­vel­oped coun­tries pri­mar­ily for road safety. Al­though, OGFC is not used in In­dia, ex­pe­ri­ence with OGFC is ap­pli­ca­ble to PMC used in In­dia in cer­tain as­pects. Both are highly wa­ter per­me­able (por­ous) mixes and are placed 20mm thick. The OGFC is placed on dense bi­tu­mi­nous con­crete (sim­i­lar to BC Grad­ing 2) to pro­vide a skidresis­tant wear­ing sur­face dur­ing rain­fall or when the pave­ment is wet. The rain­wa­ter pen­e­trates the open sur­face of the OGFC; goes to its bot­tom; then flows within 20mm thick OGFC to­wards the shoul­ders; and then ex­its from the ex­posed edge of the OGFC onto the shoul­ders. Since there is no rain­wa­ter on the sur­face of OGFC, there is no hy­droplan­ing or skid­ding of mo­tor ve­hi­cles on its sur­face. OGFC is highly per­me­able to wa­ter since it has over 18% air voids. OGFC is durable de­spite high air voids be­cause it has about 6 per­cent poly­mer mod­i­fied bi­tu­men con­tent, which pro­vides a thick bi­tu­men film around the ag­gre­gate par­ti­cles.

The pre­mix car­pet (PMC) on the other hand, is sub­stan­tially more open graded and more por­ous than the OGFC be­cause the for­mer uses very coarse ag­gre­gate (nom­i­nal size of 11.2 to 13.2mm) and no fine ag­gre­gate. Its air void con­tent is es­ti­mated to be over 25%. Al­though, a sand seal coat is pro­vided on the sur­face of the PMC, it is not com­pletely ef­fec­tive in mak­ing the PMC wa­ter-proof at the sur­face. Even if there is a small patch where the PMC has lost its sand seal, the sur­face wa­ter on the road can pen­e­trate it at that spot, flow side­wards like in OGFC, and flood the en­tire PMC be­low the sand seal (Fig­ure 3). The hy­draulic pres­sure in­duced by traf­fic in the wa­ter trapped within the PMC be­low the seal coat is likely to cause strip­ping within the PMC and the un­der­ly­ing bi­tu­mi­nous course. If the un­der­ly­ing course is wa­ter bound macadam (WBM) or wa­ter mixed macadam (WMM), it would get sat­u­rated and lose its strength, es­pe­cially if it con­tains some plas­tic ma­te­rial.

The sur­face wa­ter per­me­abil­ity of an in­ser­vice PMC was de­ter­mined re­cently with a grease ring method. Al­though it is a sim­ple, crude, fall­ing head wa­ter per­me­abil­ity test, it does give some rel­a­tive per­me­abil­ity val­ues. A ring of about 225mm in di­am­e­ter and about 25mm high is made on the road sur­face to be tested us­ing heavy grease. Putty can also be used in lieu of heavy grease. The ring is filled with wa­ter upto a depth of 12.5mm and a timer is started. Time taken by the wa­ter to pen­e­trate and dis­ap­pear from the road sur­face is mea­sured in sec­onds as a mea­sure of rel­a­tive wa­ter per­me­abil­ity.

The first test (Fig. 4) was made on PMC with­out any seal coat. It was not even pos­si­ble to fill the ring with wa­ter be­cause it was pen­e­trat­ing the PMC as fast as it was poured. On fill­ing rapidly, wa­ter pen­e­trated fully in about 5 sec­onds. The sec­ond test (Fig. 5) was made on PMC with mod­er­ate amount of sand seal coat. The mea­sured field per­me­abil­ity was 105 sec­onds. The third test (Fig. 6) was made on PMC with ad­e­quate amount of sand seal coat. The mea­sured field per­me­abil­ity was 545 sec­onds, which is still con­sid­ered very high. It is not un­com­mon to see non­uni­form ap­pli­ca­tion of sand seal coat on PMC be­cause it is usu­ally spread man­u­ally. It is a mat­ter of great con­cern. Dur­ing a sim­i­lar test on BC wear­ing course, wa­ter re­mained at 12.5mm level for hours, there­fore, the field wa­ter per­me­abil­ity was al­most zero (Fig. 7).

More field per­me­abil­ity test data has been ob­tained by stu­dents at Rasta, Ban­ga­lore; IIT Guwahati; MNIT, Jaipur; and Kau­tilya Col­lege of En­gi­neer­ing, Jaipur. A steel ring (Fig­ure 8) rather than grease or putty ring was used to ex­pe­dite test­ing. Un­like BC, the test data has in­di­cated a very wide range of wa­ter per­me­abil­ity from very high (60 sec­onds) to al­most none. As men­tioned ear­lier, it ap­pears to be a func­tion of vari­abil­ity in sand seal coat ap­pli­ca­tion in terms of its qual­ity, quan­tity and mode of ap­pli­ca­tion. Prac­ti­cally, it is not pos­si­ble to ap­ply con­sis­tent and ad­e­quate amount of sand seal coat through­out a PMC project. Re­cent de­tailed in­ves­ti­ga­tions by IIT Guwahati have also shown PMC to be highly per­me­able to wa­ter.

It is quite ev­i­dent from the pre­ced­ing field ex­per­i­ments that gen­er­ally the PMC with sand seal coat would eas­ily take in and trap wa­ter dur­ing rains in many cases. Once the PMC is sat­u­rated with wa­ter, the hy­draulic pres­sure re­sult­ing from traf­fic above can

loosen up the sand seal in other ar­eas of the PMC. This phe­nom­e­non has been ob­served on Jaipur streets (Fig­ure 9). As al­ready men­tioned, the hy­draulic pres­sure also causes strip­ping in the PMC as well as in the un­der­ly­ing bi­tu­mi­nous cour­ses, if any. That is why; PMC de­te­ri­o­rates rather rapidly dur­ing mon­soons, es­pe­cially in towns and cities where streets usu­ally get flooded. The av­er­age life of PMC in Jaipur is about 1-2 years. Its bi­tu­men con­tent is about 3.5 per­cent.

Ob­vi­ously, there are cases where PMC with good, uni­form sand seal coat and/ or very dry cli­mate has en­dured well. How­ever, fun­da­men­tally, the ques­tion is why to place a highly por­ous bi­tu­mi­nous mix like PMC in the first place and then try to seal it. There is no avail­able data as to what depth, if any, the es­ti­mated 6mm thick sand seal coat re­ally pen­e­trates the 20mm thick PMC when rolled.

PMC is used only in In­dia; it is not used in de­vel­oped coun­tries. There are nu­mer­ous other ques­tions re­lated to PMC which need to be an­swered: to­tal air voids in PMC; ab­so­lute vol­ume of sand seal coat; un­filled voids in PMC; depth of sand seal pen­e­tra­tion in PMC; its per­for­mance and dura­bil­ity, etc. It is sur­pris­ing to note as to why no such re­search was con­ducted in In­dia dur­ing the last 60 years to an­swer these le­git­i­mate ques­tions. On the other hand, hun­dreds of re­search pa­pers have been pub­lished across the world on sur­face dress­ing in terms of its ra­tio­nal de­sign, con­struc­tion and per­for­mance. There is hardly any re­search in In­dia where PMC is used, es­pe­cially on its struc­ture, vol­u­met­ric, per­for­mance and dura­bil­ity. It ap­pears there is only a “con­ven­tional wis­dom” that PMC does work and is “good” for In­dia and, there­fore, there is no need for any re­search on it.

No pub­lished data on the av­er­age life of PMC in In­dia could be found ei­ther. Some PMGSY en­gi­neers re­vealed its av­er­age ser­vice life to be 2 years with­out sig­nif­i­cant distress such as rav­el­ling and pot­holes. Above all, the cost of PMC is about three times the cost of sur­face dress­ing. This is not ac­cept­able.

If the PMC is a panacea for low to medium traf­ficked roads in In­dia, why is this tech­nol­ogy not used in de­vel­oped coun­tries? How­ever, that would re­quire sound en­gi­neer­ing jus­ti­fi­ca­tion, which is al­most non-ex­is­tent and hard to come by in case of PMC.

Time has come now to ban PMC al­to­gether be­cause its con­tin­ued use can­not be jus­ti­fied tech­ni­cally as well as eco­nom­i­cally, be it city streets, low vol­ume roads (such as PMGSY), or medium to high vol­ume roads.

Sur­face Dress­ing

The al­ter­na­tive for PMC in In­dia is sur­face dress­ing, which, as de­scribed ear­lier, has the fol­low­ing ad­van­tages:

• It is very cheap at ₹2.62 lakhs per km com­pared to PMC’s ₹7.88 lakhs per km.

• Due to thick sprayed film of bi­tu­men, it is highly ef­fec­tive in wa­ter­proof­ing the road. Wa­ter is the big­gest en­emy of bi­tu­mi­nous road and should be kept away from the road sur­face.

• Min­i­mizes ox­i­da­tion of bi­tu­men be­cause it is present as a thick film and stone chips pro­vide pro­tec­tion from sun rays

• En­vi­ron­ment-friendly be­cause chips need not be heated

• Con­trac­tors can­not use lower bi­tu­men ap­pli­ca­tion rate be­cause af­ter con­struc­tion the stone chips would be lost with the move­ment of traf­fic

• In­dian Roads Congress has an ex­cel­lent stan­dard for de­sign­ing and en­sur­ing good qual­ity sur­face dress­ing

• Sur­face dress­ing is used suc­cess­fully in both de­vel­op­ing and de­vel­oped coun­tries (such as USA, Europe, and Aus­tralia) for black­top­ping low and medium traf­ficked roads. No other coun­try be­sides In­dia uses pre­mix car­pet

• Hun­dreds of re­search pa­pers have been pub­lished on sur­face dress­ing across the world to fine tune this tech­nol­ogy. (The au­thor has pub­lished two pa­pers in in­ter­na­tional jour­nals).

For low to medium-traf­ficked roads where PMC is used right now, it is rec­om­mended to use sin­gle or dou­ble sur­face treat­ment. If ‘black” road sur­face is de­sir­able for sur­face dress­ing to “im­press” mo­tor­ing pub­lic as

well as min­i­mize chip loss, pre­coated chips can be used. It should be noted all these al­ter­na­tives are much cheaper than the PMC (as shown in Ta­ble 1). Note that the cost of sin­gle coat sur­face dress­ing is only 1/3 of the cost of PMC. The cost of the same with pre­coated chips in­creases to only 40% of the cost of PMC. It is not un­der­stood as to why it can­not be used on low vol­ume roads such as PMGSY, which would save In­dia thou­sands of crores of ru­pees ev­ery year. Even if dou­ble sur­face dress­ing with pre­coated chips is used, its cost is only three-fourth the cost of PMC. Ob­vi­ously, the high­way agen­cies have to man­date the use of mech­a­nized bi­tu­men dis­trib­u­tor and chip spreader, which are al­ready avail­able in In­dia, to en­sure the func­tional suc­cess of sur­face dress­ing.

Bi­tu­mi­nous Con­crete in lieu of Pre­mix Car­pet

For medium to heavy-traf­ficked roads and city roads, use BC Grad­ing 2 in lieu of the PMC. Al­though it is per­mis­si­ble to lay BC Grad­ing 2 in 25-40 mm depth ac­cord­ing to IRC:111- 2009, it is prefer­able to use 40mm depth to en­sure ad­e­quate com­paction dur­ing con­struc­tion (thin lifts cool rapidly). Al­though the ini­tial cost of 40mm BC Grad­ing 2 is about 50% more than the cost of 20mm PMC, BC Grad­ing 2 is ac­tu­ally 24.1% cheaper than the PMC based on life­cy­cle cost anal­y­sis (LCCA) given in the An­nex­ure. This is a very con­ser­va­tive anal­y­sis in that the re­main­ing ser­vice life, sal­vage value, main­te­nance ex­penses, and user op­er­at­ing costs were not con­sid­ered, which all favour BC. There­fore, sav­ings will be much more than 24.1 per­cent. More im­por­tantly, BC Grad­ing 2 pro­vides sig­nif­i­cant struc­tural strength to the road pave­ment for fu­ture traf­fic growth, whereas PMC has al­most zero struc­tural strength. If there is some re­luc­tance to use sur­face dress­ing in lieu of the PMC on ru­ral roads, 40mm BC Grad­ing 2 can be placed di­rectly on wa­ter bound macadam (WBM) or wa­ter mixed macadam (WMM), as prac­ticed in de­vel­oped coun­tries.

Enor­mous Cost Sav­ings

Cost sav­ings of over ₹22,000 crores could have re­sulted if sur­face dress­ing was used in lieu of PMC since the in­cep­tion of the Pradhan Mantri Grameen Sadak Yo­jna (PMGSY) in 2000. In ad­di­tion, if the changeover is made now, cost sav­ings of over ₹4,400 crores can be re­al­ized ev­ery year in resur­fac­ing al­ready con­structed ru­ral roads. This big sav­ing can be used in build­ing and up­grad­ing more lengths of ru­ral roads in In­dia.

The pre­ced­ing cost sav­ings have been es­ti­mated based on the fol­low­ing as­sump­tions:

• PMGSY roads con­structed so far: 5,29,975 km

• Ra­tio of pre­mix car­pet and sur­face dress­ing: 80:20

• Cost of pre­mix car­pet and sur­face dress­ing per km: ₹7.88 lakhs and ₹2.62 lakhs, re­spec­tively (based on 2012 Ra­jasthan PWD Ba­sic Sched­ule of Rates)

• 5-year cy­cle of re­newal of sur­face for ex­ist­ing roads.

Im­ple­men­ta­tion of Sur­face Dress­ing

In­di­vid­u­als can­not be blamed for this col­lec­tive “con­ven­tional wis­dom” of us­ing pre­mix car­pet for over 50 years de­spite the fact that it is highly ex­pen­sive and is an un­de­sir­able tech­nol­ogy. How­ever, some en­gi­neers who “like” pre­mix car­pet would like to main­tain the sta­tus quo and would make one or more of the fol­low­ing mis­lead­ing state­ments (au­thor’s re­sponse is in paren­the­sis):

1. Pre­mix car­pet is “good” for In­dia. (It is just an un­founded vague state­ment; if it is good why have we not been able to con­vince and ex­port this tech­nol­ogy to rest of the world?)

2. I have seen it per­form well. (Not all jobs fail, but fun­da­men­tally it is wa­ter-trap­ping

and there­fore pre­ma­ture pothol­ing is highly likely).

3. It pro­vides struc­tural strength to the road. (This is ab­so­lutely wrong. Both sur­face dress­ing and pre­mix car­pet do not con­trib­ute to struc­tural strength of the road).

4. It pro­vides a black and shiny sur­face and is laid some­times with a paver, which pleases the pub­lic. (This seems to in­di­cate pub­lic per­cep­tion is more im­por­tant than proper en­gi­neer­ing and the wel­fare of our coun­try in terms of costs and per­for­mance).

5. Let’s un­der­take some re­search projects to com­pare the per­for­mance of pre­mix car­pet ver­sus sur­face dress­ing. (This is just a de­lay­ing tac­tic. Sat­is­fac­tory per­for­mance of sur­face dress­ing has been proven and re­ported world­wide. More­over, In­dia’s Ru­ral Roads Man­ual al­lows ei­ther one so why not use sur­face dress­ing which costs only one-third of pre­mix car­pet. The Man­ual also states that sur­face dress­ing is an age-old tech­nique that re­ally seals the road well).

6. Let’s set up some demon­stra­tion project to “rein­tro­duce” sur­face dress­ing. (Again, it is just a de­lay­ing tac­tic. Why do it when we have an ex­cel­lent and easy to fol­low IRC stan­dard for sur­face dress­ing for our road en­gi­neers? Most states have sur­face dress­ing as item in their Ba­sic Sched­ule of Rates. If no­tices invit­ing ten­ders are is­sued, In­dian en­gi­neers/ con­trac­tors are smart to ex­e­cute the con­tracts with­out any prob­lem).

Sum­mary

In­dia’s Ru­ral Roads Man­ual per­mits two tech­nolo­gies: sur­face dress­ing or pre­mix car­pet for bi­tu­mi­nous sur­fac­ing of ru­ral roads. Both have been stan­dard­ized by the In­dian Roads Congress. How­ever, pre­mix car­pet is highly ex­pen­sive (three times the cost of sur­face dress­ing); gen­er­ally, soaks up wa­ter read­ily which re­sults in early pot­holes; and not used any­where in the world. Sur­face dress­ing is cheap (only onethird the cost of pre­mix car­pet); makes the road highly water­proof thus less po­ten­tial for pot­holes; and is used suc­cess­fully through­out the world. There­fore, there is no doubt that sur­face dress­ing has sig­nif­i­cant ad­van­tages over pre­mix car­pet in terms of life (per­for­mance) and cost. Be­sides new con­struc­tion, sav­ings of over ₹4,400 crore can be re­al­ized per year for resur­fac­ing ex­ist­ing ru­ral roads if sur­face dress­ing is used in lieu of pre­mix car­pet.

An­nex­ure Life cy­cle cost anal­y­sis (LCCA) of Pre­mix Car­pet (PMC) and Bi­tu­mi­nous Con­crete (BC) Grad­ing 2

Anal­y­sis pe­riod = 6 years

As­sump­tions:

• Av­er­age life of 20 mm PMC with sand seal coat = 3 years (real 2 years)

• Av­er­age life of 40 mm BC Grad­ing 2 = 6 years (real 7-8 years)

[This means 20 mm PMC will be re­quired for re­ha­bil­i­ta­tion of the pave­ment af­ter 3 years.]

• Cost of 20 mm PMC per km lane = 7.88 lakhs

• Cost of 40 mm BC Grad­ing 2 per km lane = 12.00 lakhs

• Real dis­count rate = 4%

• Net present value (NPV) = Ini­tial cost + Re­hab cost (1/(1+r)n)

• Sal­vage value con­sid­ered equal af­ter the 6 years anal­y­sis pe­riod (al­though BC would have a sig­nif­i­cantly higher struc­tural strength whereas PMC strength is al­most zero)

• Re­main­ing ser­vice life af­ter anal­y­sis pe­riod of 6 years con­sid­ered equal (al­though BC would have more ser­vice life be­cause it is dense graded)

• No main­te­nance ac­tiv­ity con­sid­ered dur­ing the 6-year pe­riod (al­though PMC is likely to re­quire some ac­tiv­ity)

• User op­er­at­ing costs con­sid­ered equal (al­though BC would pro­vide a smoother ride and less op­er­at­ing costs). Deter­min­is­tic Ap­proach was used for LCCA, which is easy and is used tra­di­tion­ally. The Net Present Value (NPV) was cal­cu­lated for PMC and BC for the 6-year pe­riod as fol­lows:

NPV of PMC = 14.89 lakhs

NPV of BC Grad­ing 2 = 12.00 lakhs

This means, PMC is 24.1% more ex­pen­sive than BC Grad­ing 2. If the re­main­ing ser­vice life, sal­vage value, main­te­nance costs, and user op­er­at­ing costs are con­sid­ered (which all are in favour of BC), PMC would be much more ex­pen­sive than 24.1 per­cent, which was cal­cu­lated with very con­ser­va­tive as­sump­tions, listed above.

About the au­thor

Prof. Prithvi Singh Kand­hal is As­so­ciate Di­rec­tor Emer­i­tus of the Na­tional Cen­ter for As­phalt Tech­nol­ogy (NCAT) based at Auburn Univer­sity, Alabama, U.S.A. NCAT is the largest as­phalt (bi­tu­men) road tech­nol­ogy cen­ter in the world. Prof. Kand­hal has been a prac­tic­ing high­way en­gi­neer in In­dia for over 20 years and in the US for 30 years. He has drafted many stan­dards for the In­dian Roads Congress in­clud­ing spec­i­fi­ca­tions for dense graded bi­tu­mi­nous mixes, stone ma­trix as­phalt and ready­made pot­hole patch­ing mix.

Fig­ure 1: Schematic of sur­face dress­ing

Fig­ure 3: Sur­face wa­ter en­ter­ing the pre­mix car­pet (PMC) through an un­sealed area sat­u­rat­ing it un­der the seal as well, caus­ing strip­ping within PMC and the un­der­ly­ing bi­tu­mi­nous course when sub­jected to traf­fic loads

Fig­ure 2: Typ­i­cal chip spreader man­u­fac­tured in Gu­jarat

Fig­ure 4: Field per­me­abil­ity of PMC with­out any sand seal coat

Fig­ure 5: Field per­me­abil­ity of PMC with mod­er­ate sand seal coat

Fig­ure 6: Field per­me­abil­ity of PMC with ad­e­quate sand seal coat

Fig­ure 7: Field per­me­abil­ity of BC Grad­ing 2

Fig­ure 9: Fail­ure of pre­mix car­pet (PMC) dur­ing the first mon­soon within Jaipur city

Fig­ure 8: Steel ring used for field per­me­abil­ity test

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