review of practices for improving ride quality and ... · subbase; substandard granular subbase...

1

Review of Practices for Improving Ride Quality and Periodical

Renewal of Bituminous Pavements in India

By

Prof. Prithvi Singh Kandhal* and Prof. A. Veeraragavan**

[Paper No. 662 published in the Journal of the Indian Roads Congress,

Volume 77-3, October – December 2016]

ABSTRACT

Improving ride quality and periodical renewal are the two major components of

rehabilitation of existing bituminous pavements, which are generally practiced across

India. The Ministry of Road Transport and Highways has provided guidelines through

circulars on how to improve ride quality and practices for periodical renewal of

bituminous pavements on national highway stretches. These practices are also

generally followed on other highways across India.

It has been observed that bituminous mixes/applications currently used in the so-

called Improving Ride Quality Programme (IRQP) and Periodical Renewal (PR) do

not generally produce durable bituminous pavements, which result in unsatisfactory

condition of roads especially during monsoons.

This paper gives a review of all practices currently used in IRPQ and PR across India.

Detailed guidelines have been given in this paper for selection of acceptable

bituminous mixes/applications based on durability and economics in case of IRQP

and PR. Primarily, recommendations have been made not to use open graded,

permeable bituminous mixes and use only dense graded bituminous mixes. Use of

surface dressing in lieu of premix carpet has also been recommended for low volume

traffic roads based on economics and durability.

INTRODUCTION

Two major components of rehabilitation of bituminous pavements generally practiced

across India are: improving ride quality and periodical renewal. (Unfortunately,

recycling has not made much headway in India as a means for rehabilitating

bituminous pavements.) Ride quality of bituminous pavements can deteriorate

prematurely due to several factors such as uneven consolidation of subgrade and/or

subbase; substandard granular subbase (GSB) and/or wet mix macadam (WMM); and

stripping, rutting and/or fatigue cracking of bituminous courses. Periodical renewal of

bituminous wearing courses is required because bitumen being exposed to sun and air

gets oxidized with time and becomes brittle resulting in raveling and subsequent

disintegration. Bituminous wearing courses can also develop top down cracking under

intense traffic loads.

* Associate Director Emeritus, National Center for Asphalt Technology (NCAT),

Auburn University, USA (currently Jaipur) < [email protected]>

** Professor of Civil Engineering, Indian Institute of Technology, Chennai

<[email protected]>

2

The Ministry of Road Transport and Highways has provided guidelines through

circulars on how to improve ride quality and practices for periodical renewal of

bituminous pavements on national highway stretches. These practices are also

generally followed on other highways across India. State highway agencies and other

agencies such as Border Roads Organization (BRO) have their own practices in

addition to those in the above guidelines.

It has been observed that bituminous mixes/applications used in the so-called

Improving Ride Quality Programme (IRQP) and Periodical Renewal (PR) do not

always produce durable bituminous pavements which result in unsatisfactory

condition of roads especially during monsoons.

CURRENT PRACTICES

As mentioned earlier in introduction, MORTH issued “Revised Guidelines for

Selection of National Highway Stretches for Improving Ride Quality Programme

(IRQP) and Periodic Renewals (PR)” in September 2002 (1). Although these

guidelines are meant for national highways, other organizations like state highway

agencies, Border Roads Organization, also use similar guidelines for other highways

across India. Some agencies also use other variations of bituminous

mixes/applications. So there is a significant proliferation of practices used in the

IRQP and PR of bituminous pavements across India.

Table 1 gives MORTH guidelines for IRQP in terms of various unbound and bound

pavement courses/applications depending on the thickness of existing pavement.

Table 1. Various MORTH Options in Improving Ride Quality Programme

(IRQP)

Existing pavement

thickness

Options

Less than 200 mm 225 mm WMM + 20 mm PMC with sand seal coat

Less than 200 mm 225 mm WMM + 20 mm MSS

Between 200 and 250

mm

150 mm WMM + 20 mm PMC with sand seal coat

Between 200 and 250

mm

150 mm WMM + 20 mm MSS

Between 250 and 300

mm

75 mm BUSG + 25 mm SDBC

Between 250 and 300

mm

75 mm BUSG + 25 mm MSS

More than 300 mm 50 mm BM + 25 mm SDBC

More than 300 mm 75 mm BM + 25 mm SDBC

Primarily, six courses/applications are included in Table 1: Wet Mix Macadam

(WMM), Bituminous Macadam (BM), Semi Dense Bituminous Concrete (SDBC),

Built Up Spray Grout (BUSG), Mixed Seal Surfacing (MSS), and Premix Carpet

(PMC). These courses/applications have been reviewed in the next section of this

paper.

3

Table 2 gives MORTH guidelines for PR in terms of bituminous courses/applications

depending on daily number of commercial vehicles. Primarily, the following

courses/applications are included in this table: Semi Dense Bituminous Concrete

(SDBC), Mixed Seal Surfacing (MSS), and Premix Carpet (PMC). These are also

reviewed in the next section.

Table 2 . MORTH Guidelines for Periodical Renewal (PR)

Number of Commercial Vehicles per

Day

Option

Less than 1500 20 mm PMC with sand seal coat

Less than 1500 20 mm MSS

More than 1500 25 mm SDBC

More than 1500 25 mm BC*

* Only if existing surface is also BC.

REVIEW OF BITUMINOUS MIXES/APPLICATIONS FOR IRQP AND PR

This section gives a review of all bituminous mixes/applications currently used and

proposed to be used for IRQP and PR across India. The following have been

reviewed: Bituminous Macadam (BM), Semi Dense Bituminous Concrete (SDBC),

Dense Bituminous Macadam (DBM), Bituminous Concrete (BC), Premix Carpet

(PMC), Surface Dressing (SD), Mixed Seal Surfacing (MSS), and Built Up Spray

Grout (BUSG). The fundamental suitability of these mixes/applications in flexible

pavement has been examined (2,3).

Bituminous Macadam (BM)

Bituminous Macadam (BM) is an open graded, permeable, and recipe type mix

produced without any quality control on its volumetrics or strength (stability). The

primary problem with the BM mix is that being very open graded, it is highly

permeable and therefore will trap moisture or water. BM and SDBC were developed

several years ago, when conventional hot mix plants were not common. At that time,

hot mixing was done in small portable plants or concrete mixers in which much fine

aggregate could not be used due to limitations of the available heating and mixing

equipment. Now, good hot mix plants are normally available almost all across India.

Figures 1 and 2 show cross-sections where BM has been used as a base, binder or

profile corrective course (PCC) with no outlet for water thus creating a “bath tub”

situation within the pavement.

The fundamental question thus boils down to BM versus DBM. Should open graded

BM be deleted and densely graded DBM used instead in all cases? To answer that

question BM and DBM should be compared both from the engineering aspect

4

Fig. 1 Flexible pavement with BM as a base course or PCC

Fig. 2 Flexible pavement with BM as a base/binder course

(primary) and economical aspect (secondary). This has been done considering the

following factors (2):

Permeability: It has been acknowledged in many IRC and MORTH publications that

BM is a much more open mix compared to the densely graded DBM. The MORTH

Manual for Construction and Supervision of Bituminous Works (4) states on page 52,

“Because of the open-graded aggregate matrix, the voids content (in the BM) can be

as high as 20-25 percent.”

Figure 3 shows the open texture of compacted BM Grading 1. When these specimens

were placed under a water tap, the water readily passed through them indicating very

high permeability.

So undoubtedly the BM is a highly permeable mix compared to the dense graded

DBM. No permeable asphalt layer is desirable within the pavement structure (unless it

is specifically for drainage with proper outlets) whether it is a PCC, base course,

binder course or whatever. If this fundamental requirement is disregarded, the

potential for premature pavement distress

is increased. A permeable layer always attracts and traps water, moisture or moisture

5

Fig. 3 Open surface texture of compacted BM Grading 1

vapour. Water can come from the top, from the sides, or from the unbound courses

underneath (5,6). This leads to stripping of bitumen in the permeable layer as well as

in the adjacent layers overlying or underlying it due to traffic action thereby resulting

in premature failure of the road (Fig. 4).

Fig. 4 Premature failure of a road in Eastern India

due to presence of water in the BM

Structural Strength: Many highway agencies across the world give structural value

to a BM type mix (used for drainage with proper outlets) of 50% of dense graded

DBM type mix. IRC Publications 37 and 81 on flexible pavement design state that 7

mm of DBM is equal to 10 mm of BM. In either case, the DBM is far superior to the

BM in terms of structural strength and fatigue life.

Use as a PCC: It has been surmised that BM is a good material for profile corrective

course (PCC) because it resists reflection cracking. It does not appear that this

conclusion is based on any research. No other country is using a permeable, water-

trapping type mix for PCC. Only dense graded mixes such as DBM or BC are used

for transverse or longitudinal profile correction in other countries (7) in courses called

scratch courses, levelling courses or wedge courses, which are same as India’s PCC.

6

Cost Considerations: The use of BM is quite often made on the premise that BM is

cheaper than DBM and, therefore, it is suitable for use in developing country like

India. That is simply not correct as discussed below.

Comparative cost analysis of BM and DBM has been done based on the 2013 Basic

Schedule of Rates (BSR) of BM and DBM obtained from the Rajasthan PWD Circle

in Jaipur. The cost of BM Grading 2 in place is Rs 5,354 per cu m and the cost of

DBM Grading 2 in place is Rs. 6,851 per cu m. Considering that 100 mm thick BM is

equal to 70 mm thick DBM as per IRC guidelines, the actual cost of DBM in place

comes out to be Rs. 4,796 per cu m. That is a saving of Rs. 558 per cu m or about 10

percent, when DBM is used in lieu of BM. That is a lot of savings on a road project.

The preceding cost analyses have clearly established that DBM is much cheaper than

the BM on equivalency cost basis. This fact should not be ignored by pavement

designers.

Traffic Conditions: According to some highway engineers, BM is intended for low-

traffic roads only, although it is being used indiscriminately on national highways and

state highways. However, the fact remains that any layer, which traps water, should

not be used whether it is a low-volume or high-volume road. The concept of perpetual

pavement or long lasting pavements is relevant even for less traffic roads.

Even the Central Road Research Institute (CRRI) has stated in 2008 the following in

their investigational report (8) on premature failure of NH-91 in Uttar Pradesh: “For

ensuring long term pavement performance, focus now must shift to the use of dense

graded bituminous mixes (such as DBM and BC) rather than the open graded and

semi dense bituminous mixes (such as BM and SDBC)…….Bituminous Macadam is

a highly permeable mix which is prone to rutting and water induced damage.

Bituminous Macadam, though is widely used at present, but needs to be gradually

replaced with DBM in the coming years, because it is not cost effective in the long

run and does not perform better during the design life of a pavement subjected to

heavy traffic. Similarly, the use of Semi-Dense Bituminous Concrete is also needed to

be discouraged as it suffers from "pessimism" voids, which have potential to trap

water resulting into damage due to moisture. It should be substituted by Bituminous

Concrete as it is a better performing mix and is also cost effective in the long run.”

It is evident from the preceding discussion that BM should not be used at all in

flexible pavement.

Semi Dense Bituminous Concrete (SDBC)

There is no engineering logic in using a “semi-dense” mix when only dense,

continuously graded mixes are technically desirable. In most developed countries (7)

either dense mixes (HMA) are provided or the open graded asphalt friction course

(OGFC) is provided as wearing course. Semi-dense mixes which are neither dense

graded nor open graded, contain the so-called “pessimum” voids when constructed.

Terrel and Shute (9) advanced the concept of “pessimum” voids for stripping.

Pessimum represents opposite of optimum. The objective is to stay out of the

“pessimum” void range. A “semi-dense” mix, which has a potential for having

“pessimum” voids in it, is likely to trap moisture/water and cause stripping. As

7

mentioned earlier, this also has been acknowledged by the CRRI while investigating

NH-91 in Uttar Pradesh (8).

Unfortunately, the use of SDBC has been advertently promoted to some extent

because only SDBC Grading 2 has been specified by MORTH for a thin layer of 25

mm. However, the fact remains that thin 25 mm mat cools rapidly after lay down and

it is not possible to compact it to the desired level. This results in high permeability

and reduced life. It should also be noted that BC is only 10 percent more expensive

than the SDBC as is evident from the following prices obtained from the 2013

Schedule of Rates of Rajasthan PWD, Jaipur Circle:

Semi-Dense Bituminous Concrete (SDBC) Grading 2 Rs. 7,758 per cu m

Bituminous Concrete (BC) Grading 2 Rs. 8,553 per cu m

Unfortunately across India the extremely undesirable combination of BM and SDBC

continues to be used even in heavy rain areas like northeast India. It is simply

unacceptable. Rainwater permeates through the semi dense SDBC (or its cracks) and

is stored in the underlying BM “bath tub”. The water or moisture vapor from the BM

can cause stripping in the BM as well as in the overlying SDBC, quite often also

causing debonding (scaling) of the SDBC from the BM. This scaling results in

numerous “shallow potholes” on the road as shown in Figure 5. These shallow

potholes are sometimes repaired with premix carpet (PMC) mix which can perpetuate

the problem.

Figure 5. Shallow potholes (scaling) on SDBC on highway in South India

Although MORTH has rightly deleted SDBC in its revised 2013 Specification (10), it

is being used at the present time by some states.

Dense Bituminous Macadam (DBM)

At the present time the dense bituminous macadam (DBM) is specified for use as a

base course and/or binder course. Two gradations of the DBM are specified in Section

505 of 2013 MORTH specifications: Grading 1 has a NMAS (nominal maximum

aggregate size) of 37.5 mm and Grading 2 has a NMAS of 25 mm.

Table 3 gives the existing MORTH composition of DBM Gradings 1 and 2. The

specified percentage of fine aggregate is the same in both gradings (28-42 percent),

8

the main difference is just some large size aggregate particles (25-45 mm size) are

contained in Grading 1. Use of large stone mix (NMAS of 37.5 mm or larger) has

several disadvantages such as segregation (Figure 6) and high permeability (2). These

disadvantages outweigh the “marginal” gain in stability, if any, over a 25 mm NMAS

mix. Since Grading 1 is highly permeable, it has to be sealed or overlaid before rainy

season otherwise water will penetrate it and damage the underlying WMM course.

Experienced Indian highway engineers advise this but the solution is to simply ban

the problematic DBM Grading 1 altogether and use only the DBM Grading 2. On

many national highways in India deteriorated DBM Grading 1 in the lower lift of the

total DBM, which was disintegrated due to stripping, could not be retrieved intact by

coring. One case is shown in Figure 7.

Figure 6. Segregation of DBM Grading 1 (37.5 mm NMAS mix) resulting in

honeycombing

Figure 7. Deteriorated DBM Grading 1 used in lower DBM lift could not

retrieved intact while coring

TABLE 3. EXISTING MORTH GRADATIONS FOR DENSE BITUMEN

MACADAM (DBM) (Ref. 10)

9

Grading 1 2

Nominal Aggregate Size 40 mm 25 mm

Lift Thickness 80-100 mm 50-75 mm

Sieve, mm Percent Passing

45 100

37.5 95-100 100

26.5 63-93 90-100

19 - 71-95

13.2 55-75 56-80

9.5 - -

4.75 38-54 38-54

2.36 28-42 28-42

1.18 - -

0.6 - -

0.3 7-21 7-21

0.15 - -

0.075 2-8 2-8

Bitumen Content, % Min. 4.0 Min. 4.5

Based on the preceding discussion, problematic DBM Grading 1 should not be used in

flexible pavement.

Bituminous Concrete (BC)

Two gradings of the Bituminous Concrete (BC) have been specified in Section 507 of

the MORTH Specifications (2013). According to MORTH, BC can be used for

wearing and profile corrective courses. Grading 1 has a NMAS of 19 mm and

Grading 2 has a NMAS of 13 mm.

As discussed earlier, DBM Grading 2 was selected as base course. Now, there is a

need to select a binder course and two wearing (surface) course. BC Grading 1 with a

NMAS of 19 mm is suitable for a binder course because by definition it binds the base

course (NMAS of 25 mm) and the wearing course (NMAS of 13 mm) with an

intermediate (transition) NMAS of 19 mm. BC Grading 2 with a NMAS of 13 mm is

suitable for a wearing course. Therefore, BC Grading 1 should be renamed as a binder

course and used as such in the pavement design in lieu of the upper lift of DBM. This

conforms to general practice in developed countries.

There is a need to add a new BC gradation with a NMAS of 9.5 mm, which can be

used for light to medium traffic, and in urban areas to provide smooth and highly

impermeable and durable bituminous road surface. BC Grading 3 is also suitable for

thin asphalt lifts and should be preferred over BC Grading 2. This BC gradation with

a NMAS of 9.5 mm is being used successfully across the US even on interstate

(national) highways. Such a gradation was proposed in the IRC paper by Kandhal,

Sinha and Veeraragavan (2). All three BC gradations are shown in Table 4.

TABLE 4. AGGREGATE GRADING FOR BITUMINOUS CONCRETE (BC)

GRADINGS 1, 2 AND 3

10

SPECIFICATION BC GRADING NUMBER*

Grading 1

2 3

Nominal maximum

aggregate size

19 mm 13.2 mm 9.5 mm

Layer thickness 50 mm 25/40 mm 25/40 mm

IS Sieve size (mm) Percent passing by weight

26.5 100

19 90-100 100

13.2 59-79 90-100 100

9.5 52-72 70-88 90-100

4.75 35-55 53-71 55-75

2.36 28-44 42-58 40-55

1.18 20-34 34-48 29-44

0.6 15-27 26-38 21-33

0.3 10-20 18-28 14-25

0.15 5-13 12-20 7-15

0.075 2-8 4-10 4-7

Bitumen content (min.) 5.2% 5.4% 5.7%

Note: BC Grading 1 should be used as binder course; BC Gradings 2 and 3

should be used for wearing courses. Proposed BC Grading 3 should be

preferred over BC Grading 2 for thin asphalt lifts and city streets.

Premix Carpet (PMC)

Before the premix carpet (PMC) is discussed, a little history is in order. When the first

author was serving as highway engineer in the Rajasthan PWD during 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 water-proofing the 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. Traffic volumes were generally less during that time period. 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

aggregate.

As is 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.

Too 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

11

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, no loose stone and

impressive to public unlike surface dressing. It was realized that the 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% 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. This is ironical that surface dressing is

still being used extensively and successfully on low to medium-trafficked roads in

developed countries such as US, 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 require/mandate bitumen distributors (already available

for tack coat work) and mechanized chip spreaders (being manufactured in Gujarat)

(11).

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 water-trapping characteristics its service life has decreased

significantly in recent years. Time has come now 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 discussed later) as well as highly effective in water-proofing

the road pavement.

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 20 mm thick. The OGFC is placed on dense bituminous concrete (similar

to BC Grading 2) to provide a skid resistant 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 20 mm thick OGFC towards the shoulders; and then

exits from the exposed edge of the OGFC onto 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 (12). The

OGFC is durable despite high air voids because it has about 6% polymer modified

bitumen content, which provides thick bitumen film around the aggregate particles.

The premix carpet (PMC) on the other hand is substantially more open graded and

more porous (permeable to water) than the OGFC because the former uses very

coarse aggregate (nominal size of 11.2 to 13.2 mm). Its air void content is estimated

to be over 25 percent. Although a sand seal coat is provided on the surface of the

12

PMC, it is not completely effective in making the PMC waterproof 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 side wards like in OGFC, and flood the

entire PMC below the sand seal (Figure 8). 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

WMM or WBM, it would get saturated and lose its strength especially if it contains

some plastic material.

Figure 8. 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.

Intrusion of water from the unsealed areas of PMC is analogous to porous 20 mm

OGFC (PMC in our case) overlaid by dense BC which has cracks. Surface water can

penetrate the OGFC through cracks and flood the entire OGFC (Figure 9). The first

author has observed this phenomenon while conducting forensic investigation in

Australia (Figure 10). It was hard to believe the sight of water oozing out of the

OGFC although it had not rained for weeks. That is why; OGFC is always milled off

before placing a dense bituminous surfacing.

Figure 9. Premix carpet (or OGFC) sandwiched between two BC courses can be

saturated with surface water entering through the cracks in the top BC course,

causing stripping in the PMC and adjacent BC courses.

13

Figure 10. Free water oozing out of the OGFC sandwiched between two BC

courses. It was observed when a section of the road was cut by cold milling.

The surface water permeability of an in-service PMC was determined recently with a

grease ring method. Although it is simple, crude, falling head water permeability test,

it does give some relative permeability values. A ring about 225 mm in diameter and

about 25 mm 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 up to a depth of

12.5 mm and timer is started. Time taken by the water to penetrate and disappear from

the road surface is measured in seconds as measure of relative water permeability.

The first test (Fig. 11) 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. 12) was made on PMC with moderate amount of sand seal coat. The

measured field permeability was 105 seconds. The third test (Fig. 13) was made on

PMC with adequate amount of sand seal coat. The measured field permeability was

545 seconds. It is not uncommon to see non-uniform application of sand seal coat on

PMC because it is usually spread manually (Fig. 14). It is a matter of great concern.

During a similar test on BC wearing course, water remained at 12.5 mm level for

hours and therefore the field water permeability was almost zero (Fig. 15).

Fig. 11. Field permeability of PMC without any sand seal coat

14

Fig. 12. Field permeability of PMC with moderate sand seal coat

Fig. 13. Field permeability of PMC with adequate sand seal coat

Fig. 14. PMC surface with non-uniform application of sand seal coat

15

Fig. 15. Field permeability of BC Grading 2

Fig. 16. Steel ring used for field permeability test

More field permeability test data have been obtained by students at Rasta, Bangalore;

IIT Guwahati; MNIT, Jaipur; and Kautilya College of Engineering, Jaipur. A steel

ring (Figure 16) rather than grease or putty ring was used to expedite testing. Unlike

BC, test data has indicated a very wide range of water permeability from very high

permeability (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, its quantity

and its mode of application. Practically, it is not possible to apply consistent and

adequate amount of sand seal coat throughout a PMC project.

Recent investigations by IIT Guwahati have also shown PMC to be highly permeable

to water (13).

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

16

can loosen up the sand seal in other areas of the PMC. This phenomenon has been

observed on Jaipur streets (Figure 17). As already mentioned, the hydraulic pressure

also causes stripping in the PMC as well as in the underlying bituminous courses.

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.

Figure 17. Failure of premix carpet (PMC) during the first monsoon within

Jaipur city

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 6 mm thick sand seal coat

really penetrates the 20 mm thick PMC when rolled.

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; etc. etc. It is surprising to note as to why no such

research was conducted in India for the last 60 years to answer these legitimate

questions. On the other hand, hundreds of research papers have been published across

the world in case of surface dressing in terms of its rational design, construction and

performance. How come hardly any research has been conducted in India where PMC

is used, especially on its structure, volumetrics, performance and durability? It

appears there is conventional wisdom only that PMC does work and is “good” for

India and therefore there is no need for any research on it.

No published data on average life of PMC in India could be found in the literature

either. Some PMGSY engineers revealed its average life to be 2 years without

significant distress such as ravelling and potholes. This is not acceptable.

If the PMC is a panacea for low to medium trafficked roads in India, why this

technology is not used in developed countries? However, that would require

17

fundamental, sound engineering justification which is almost non-existent and hard to

come by in case of the PMC.

Therefore, time has come now to ban the PMC altogether because its continued use

cannot be justified technically as well as economically anywhere; be it city streets,

low volume roads (such as PMGSY), or medium to high volume roads.

So what are the alternatives for PMC in India? The discussion follows.

For low to medium-trafficked roads where PMC is used right now, 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 5. Note that the cost of single coat surface dressing is only 1/3 of the cost of

PMC. It is not understood as to why it cannot be used on low volume roads such as

PMGSY; that 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 (3/4)

of the cost of PMC.

Table 5. Comparison of Costs for PMC and Recommended Alternates (base year

2013)

No. Option Cost per sq m in

rupees

Cost per km lane

in rupees

1 20 mm PMC with sand seal coat 210 7.88 lacs

2 25 mm BC Grading 2 205 7.69 lacs

3 Surface dressing, single

application with VG-10, nominal

chip size 13.2 mm, mechanical

means

70 2.62 lacs




means with precoated chips

79 2.96 lacs

5 Surface dressing, double

application with VG-10, chip size

13.2 mm, mechanical means

140 5.24 lacs



13.2 mm, mechanical means with

precoated chips

149 5.58 lacs


Notes: All options except surface dressing include one tack coat. Precoated chips

coated with 1% VG-10 costs Rupees 1,107 per cu m. In case of double surface

dressing, only top application used precoated chips. Lane width = 3.75 m

Pandey and his associates have made the following observation recently, “While a

wearing course of surface dressing has been known for its durability all over the

world on low volume roads, the wearing course of premix carpet and seal coat is very

common in India though frequent pothole repair and patching well within five years

18

of construction are not uncommon…The thick film of bitumen in the surface dressing

oxidises slowly and retains flexibility for a longer period.” (14).

Section 7.3.3 on Type of Bituminous Surfacing of IRC:SP:72-2007, “Design of

Flexible Pavements for Low Volume Rural Roads” (15) states the following. “For the

low volume rural roads, when a bituminous surfacing needs to be provided, two

alternatives viz, Surface Dressing and 20 mm Premix Carpet are generally available.

The recently revised and vastly improved IRC Specifications for Surface Dressing

(16) adopt the concept of Average Least Dimension (ALD) of stone chips and take

into account, the factors of traffic, climate and type of chipping. A standardised chart

is used for the determination of design binder content and chipping application rate.

The adoption of the revised IRC specifications makes Surface Dressing both suitable

and economical for low traffic volume conditions, as borne out by its popularity in

several countries abroad.”

The Indian Roads Congress has a very good standard specification for surface

dressing; it should replace the PMC as soon as possible.

Besides significantly lower construction cost, surface dressing offers the following

functional advantages compared to PMC (11):

1. Excellent sealing of road surface, which does not allow ingress of rainwater

into the lower layers thus resulting in a durable pavement

2. Minimizes oxidation of bitumen because it exists in thick film and stone chips

provide protection from sun rays

3. Higher resistance to skidding which reduces accident hazards

4. Retards reflection cracking because of flexible behaviour

5. Environmental friendly because chips need not be heated

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.

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 (17), it is preferable to use 40 mm depth to ensure adequate

compaction during construction (thin lifts cool rapidly). It is ironical that the cost of

25 mm BC Grading 2 is lower than the cost of PMC (Table 5).

Although the initial cost of 40 mm BC Grading 2 is about 50% more than the cost of

20 mm PMC, BC Grading 2 is actually 24.1% cheaper than the PMC based on life

cycle cost analysis (LCCA) given in Annexure. This is a very conservative analysis in

that the remaining service life, salvage value, maintenance expenses, and user

operating costs were not even 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 to offer (18).

Mixed Seal Surfacing (MSS)

19

The Indian Roads Congress adopted the Mixed Seal Surfacing (MSS) specification

IRC:SP:78-2008 (19) probably as an alternate to the PMC; both are applied in 20 mm

thickness. Since hot mix asphalt plants are now widely available across India, it was

considered practical and easy to adopt a hot mix which would encompass or

incorporate both the PMC and the sand seal in one mix rather than two different

applications. This would also reduce cost of construction. As shown in Table 6, two

gradations are specified for MSS: one is “closed” gradation (Type A with NMAS of

9.5 mm) and the other is “open” gradation (Type B with NMAS of 9.5 mm or 12.5

mm). Since the “closed” graded mix has NMAS of 9.5 mm it can be placed in 20 mm

thick course similar to PMC. Both are recipe type mixes with no mix design

requirements such as Marshall required for BC.

TABLE 6. AGGREGATE GRADINGS FOR MIX SEAL SURFACING

IS Sieve size, mm Type A Type B

% Passing by weight % Passing by weight

13.2 --- 100

11.2 100 88-100

5.6 52-88 31-52

2.8 14-38 5-25

0.090 0-5 0-5

Both MSS mixes are not really dense graded mixes similar to BC Grading 2. Recent

research conducted by IIT Guwahati (13) has demonstrated MSS is highly permeable

to water compared with BC mixes. The question is: why not adopt well designed

dense graded BC Grading 3 with NMAS of 9.5 mm proposed earlier under BC to

achieve the same purpose? Gradations of BC Grading 2 and proposed BC Grading 3

are included in Table 4 for comparison. That would ensure an almost impermeable

and durable mix, also designed with the Marshall Method. Grading 3 is even used as a

wearing course on US interstate (national) highways with satisfactory performance.

Therefore, MSS should be replaced with a more densely graded, and more durable BC

Grading 3.

Built-up Spray Grout (BUSG)

Built-up Spray Grout (BUSG) has been recommended as a base course for flexible

pavements. It is not a bituminous mix; rather a two-layer composite construction of

compacted, almost singe sized crushed aggregates with application of hot bitumen

after each layer. Single sized key aggregate is then applied at the top. Obviously, this

type of bituminous construction is highly permeable because the sprayed bitumen

does not fill the voids in the coarse aggregate adequately. Therefore, this type of

construction is considered water trapping. This has been proven from the fact that

potholes repaired with BUSG technique quite often reappear in a year or two.

Therefore, BUSG should not be used at all in India. Although BUSG has been rightly

deleted from recently revised 2013 MORTH Specification (10), it is still used by

some highway agencies.

20

RECOMMENDATIONS FOR IMPROVING RIDE QUALITY PROGRAMME

(IRQP)

Improving Ride Quality Programme (IRQP) is practiced across India for the implied

purpose. As mentioned earlier, MORTH issued revised guidelines for IRQP for

national highway stretches in September 2002 (1). The guidelines are also used by the

state highway agencies for roads other than national highways with all kinds of

variations as noted from NITs published in newspapers. Table 7 gives various options

for IRQP along with costs based on 2013 Rajasthan PWD Basic Schedule of Rates

(BSR). The first seven options are listed in the MORTH circular of September 2002.

Options 8 and 9 have been used by some states. Option 10 has been proposed in this

paper in lieu of Options 1 through 9 as discussed later.

Based on the preceding detailed technical discussion, BM, SDBC, PMC, MSS and

BUSG should be deleted from the Indian Specifications (both MORTH and IRC) and

therefore should be excluded from Table 7. Use of WMM (see options 1, 2, 3, and 4

in Table 7) is not advised if IRQP is being conducted on an existing road consisting of

bituminous course because that would mean abandoning its structural contribution (in

terms of tensile strength) to the rehabilitated pavement system.

Table 7. Comparison of Costs for Various Options in Improving Ride Quality

Programme (IRQP)

No. Option Cost per sq m

in rupees

Cost per km-lane

in rupees

1 225 mm WMM + 20 mm PMC with sand

seal coat

541.34 20.30 lacs

2 225 mm WMM + 20 mm MSS 511.34 19.18 lacs

3 150 mm WMM + 20 mm PMC with sand

seal coat

446.22 16.73 lacs

4 150 mm WMM + 20 mm MSS 416.22 15.61 lacs

5 75 mm BUSG + 25 mm SDBC 461.00 17.29 lacs

6 50 mm BM + 25 mm SDBC 521.50 19.56 lacs

7 75 mm BM + 25 mm SDBC 684.25 25.66 lacs

8 50 mm BM + 20 mm MSS 518.50 19.44 lacs

9 50 mm BM + 20 mm PMC with sand seal

coat

561.50 21.06 lacs

10 60 mm BC Grading 2 in two applications:

20 mm (average) Scratch or leveling

course + 40 mm wearing course

525.20 19.70 lacs

Notes: All options except BUSG include two tack coats. Lane width = 3.75 m

Therefore, Option 10 consisting of 60 mm BC Grading 2 in two applications: 20 mm

(average) scratch or leveling course applied with a paver to fill depressions followed

by 40 mm uniform thick wearing course in recommended. In developed countries,

21

dense graded wearing course mix is used for leveling for practical purposes and also

for ease in feathering of mat thickness. It also provides a dense wearing course. Both

leveling and wearing courses provide highest structural strength to the pavement

compared to the first nine options.

Cost of Option 10 is very comparable to the other 9 options listed in Table 7 even

without considering life-cycle costs. More importantly, Option 10 is also considered

most durable and would therefore revolutionize the conditions of roads across India.

There are hardly any logical technical and economic reasons to use Options 1 through

9 in lieu of Option 10. If so warranted for the ride quality and/or pavement design, the

thickness of BC Grading 2 scratch or leveling course can be increased from 20 mm to

30 mm in Option 10.

MORTH Circular also recommends use of CRMB or PMB in bituminous mixes used

for IRQP. Whereas PMB with elastomers is acceptable and should be used in courses

within 100-150 mm from the road surface; CRMB should not be used because of its

inadequate specifications and lack of quality control in the field (20,21).

RECOMMENDATIONS FOR PERIDICAL RENEWAL (PR)

Periodical Renewal (PR) is also practiced across India for the implied purpose.

MORTH also issued revised guidelines for PR for national highway stretches in

September 2002 (1). Again, the guidelines are also used by the state highway agencies

for roads other than national highways with all kinds of odd variations as noted from

NITs published in newspapers. Options 1, 2, 3, and 4 in Table 8 are listed in the

MORTH guidelines circular along with costs based on 2013 Rajasthan PWD Basic

Schedule of Rates (BSR).

Based on the preceding detailed technical discussion, problematic SDBC, PMC, and

MSS should be deleted from the Indian Specifications (MORTH and IRC) and

therefore should not be used for PR as well.

As mentioned earlier, single or double surface dressing with or without precoated

chips (Options 5, 6, 7, and 8 in Table 8) should be used for low to medium trafficked

roads. Surface dressing would really waterproof the pavement structure compared to

PMC and MSS. It is unbelievable to note that the cost of PMC is about three times the

cost of singe surface dressing. Even double surface dressing with precoated chips is

much cheaper than the PMC. It should be noted that surface dressing is used world

wide with high degree of success. PMC is used only in India despite its high costs and

unacceptable durability.

For medium to heavy trafficked roads, BC Grading 2 listed as Option 9 should be

used. Although the cost of 25 mm BC Grading 2 (NMAS of 12.5 mm) is comparable

to problematic SDBC, PMC and MSS, it would be better to use BC Grading 3

(NMAS of 9.5 mm) to facilitate thin lift paving. However, thin lifts do cool rapidly

and it is difficult to obtain adequate compaction. Therefore, use of 40 mm BC

Grading 2 should also be considered in terms of constructability, structural strength,

longevity, and lower life cycle costs. Budget constraints are unfairly used often to

justify use of undesirable bituminous mixes/applications.

22

MORTH Circular also recommends use of CRMB or PMB in bituminous mixes used

for PR. Whereas PMB with elastomers is acceptable and should be used; CRMB

should not be used as explained earlier.

Table 8. Comparison of Various Options for Bituminous Periodical Renewal

(PR) with Costs

No. Option Cost per sq m in

rupees

Cost per km lane in

rupees

1 20 mm PMC with sand seal coat 210 7.88 lacs

2 20 mm MSS 180 6.75 lacs

3 25 mm SDBC 183 6.86 lacs





means

70 2.62 lacs




means with precoated chips

79 2.96 lacs



13.2 mm, mechanical means

140 5.24 lacs



13.2 mm, mechanical means with

precoated chips

149 5.58 lacs


Notes: All options except surface dressing include one tack coat. Precoated chips

coated with 1% VG-10 costs Rupees 1,107 per cu m. In case of double surface

dressing, only top application used precoated chips. Lane width = 3.75 m

MAIN CONCLUSIONS AND RECOMMENDATIONS

Of some ten types of bituminous paving mixes used in India, seven are open graded

(water-trapping) mixes. Examples: Bituminous Macadam (BM) Gradings 1 and 2;

Semi Dense Bituminous Concrete (SDBC) Gradings 1 and 2; Dense Bituminous

Macadam (DBM) Grading 1; Premix Carpet (PMC); and Mixed Seal Surfacing

(MSS). The Built-Up Spray Grout (BUSG) is no different. Their use generally results

in poor performing roads in India. The remaining three are dense graded (and

therefore desirable) mixes. These are: Dense Bituminous Macadam (DBM) Grading

2; Bituminous Concrete (BC) Grading 1; and Bituminous Concrete (BC) Grading 2.

Therefore, these seven bituminous mixes should not be used in Improving Ride

Quality Programme (IRQP), and in Periodical Renewal (PR) programme. Detailed

guidelines have been given in this paper for selection of acceptable bituminous

23

mixes/applications based on durability and economics in case of IRQP and PR. They

are given briefly as follows:

Improving Ride Quality Programme (IRQP). Do not use WMM if the existing

road consists of bituminous course(s). Use 60 mm BC Grading 2 in two applications:

20 mm (average) scratch or leveling course (with a paver) followed by a 40 mm

wearing course. The thickness of the scratch or leveling course can be increased from

20 mm to 30 mm if so required. Of all the options given in MORTH guidelines

circular this is not only economical but would also result in a highly strong, durable,

and smooth road pavement.

Periodical Renewal. Single or double surface dressing with or without precoated

chips should be used for low to medium trafficked roads. Surface dressing would

really waterproof the pavement structure compared to PMC and MSS. The cost of

PMC is about three times the cost of single surface dressing. Even double surface

dressing with precoated chips is much cheaper than the PMC. It should be noted that

surface dressing is used world wide with high success. PMC is used only in India

despite lack of research on it, its high costs, its generally high permeability, and its

unacceptable durability.

For medium to heavy trafficked roads and city streets, BC Grading 2 should be used.

Proposed BC Grading 3 (NMAS of 9.5 mm) can be considered for thin lift paving.

However, thin lifts do cool rapidly and it is difficult to obtain adequate compaction.

Therefore, use of 40 mm BC Grading 2 should also be considered in terms of

constructability, structural strength, longevity, and lower life cycle costs.

The preceding recommendations have the potential to obtain more durable flexible

pavements in rural and urban India regardless of traffic intensity.

ACKNOWLEDGMENTS

Valuable comments/suggestions given by Dr. Mittar Dhir, former Director of Central

Road Research Institute (CRRI), R.S. Shukla, former Head, Flexible Pavement

Division of CRRI; Prof. A.N. Arora, Kautilya College of Engineering, Jaipur; Kirori

Mal Modi, Paving Contractor; and Sanjay Garg, MORTH are acknowledged.

REFERENCES

1. MORTH. Revised Guidelines for Selection of National Highway Stretches for

Improving Ride Quality Programme (IRQP) and Periodic Renewals (PR). Circular

No. RW/NH-33044/10/2000-S&R dated 26 September 2002.

2. Kandhal, P.S., V.K. Sinha and A. Veeraragavan. A Critical Review of Bituminous

Mixes Used in India. Journal of the Indian Roads Congress, Volume 69-2, July-

September 2008.

24

3. Kandhal, P.S., A. Veeraragavan, and R.K. Jain. Guidelines for Long Lasting

Bituminous Pavements. Journal of the Indian Roads Congress, Volume 71-3, 2010.

4. Ministry of Road Transport & Highways. Manual for Construction and Supervision

of Flexible Pavement Works. Indian Roads Congress, New Delhi, November 2001.

5. Kandhal, P.S. Moisture Susceptibility of HMA Mixes: Identification of Problem

and Recommended Solutions. National Asphalt Pavement Association, Quality

Improvement Publication (QIP) No. 119, December 1992.

6. Kandhal, P.S., C.W. Lubold, and F.L. Roberts. Water Damage to Asphalt Overlays:

Case Histories. Proceedings, Association of Asphalt Paving Technologists, Vol. 58,

l989.

7. Roberts, F.L., P.S. Kandhal, E.R. Brown, D.Y. Lee, and T.W. Kennedy. Hot Mix

Asphalt Materials, Mixture Design and Construction. NCAT Textbook, NAPA

Education Foundation, Lanham, Maryland, Second Edition, 1996.

8. Central Road Research Institute. Investigation of NH-91 in Uttar Pradesh. Indian

Roads Congress, Report on Road Research in India, 2008.

9. Terrel, R. L. and J. W. Shute. Summary Report on Water Sensitivity. SHRP Report

SHRP-A/IR-89-003, November 1989.

10. Ministry of Road Transport & Highways. Specifications for Road and Bridge

Works, Fifth Revision, 2013, Indian Roads Congress, New Delhi.

11. Government of Gujarat. Roads and Buildings Department. Surface Dressing: An

Effective But Inexpensive Maintenance Technique. Accessed at:

http://www.scribd.com/doc/94645292/12-Final-Surface-Dressing-Rawal-Gujarat on

23 June 2014.

12. Kandhal, P.S. Design, Construction, and Maintenance of Open-Graded Asphalt

Friction Courses. National Asphalt Pavement Association Information Series 115,

May 2002.

13. Choudhary, Rajan, S.K. Singh, A. Kumar, and S.S. Porwal. Permeability

Characteristics of Bituminous Premix Carpet and Mixed Seal Surfacing. Journal of

the Indian Roads Congress. Volume 77-2, July-September 2016.

14. Saboo, N., M.A. Reddy and B.B. Pandey. Durable Wearing Course for

Bituminous Pavements. Indian Roads Congress, Indian Highways, May 2014.

15. Indian Roads Congress. Guidelines for the Design of Flexible Pavements for Low

Volume Rural Roads. IRC:SP:72-2007

16. Indian Roads Congress. Standard Specifications and Code of Practice for Design

and Construction of Surface Dressing. IRC: 110 -2005.

http://www.scribd.com/doc/94645292/12-Final-Surface-Dressing-Rawal-Gujarat

25

17. Indian Roads Congress. Specifications for Dense Graded Bituminous Mixes.

IRC:111-2009.

18. Kandhal, P.S. Bituminous Road Construction in India. Textbook cum Reference

Book. Prentice Hall of India. New Delhi, July 2016.

19. Indian Roads Congress. Specifications for Mixed Seal Surfacing (MSS).

IRC:SP:78-2008.

20. Kandhal, P. S. Quality Control Requirements for Using Crumb Rubber Modified

Bitumen (CRMB). Journal of the Indian Roads Congress, Volume 67-1, April-June

2006.

21. Kandhal, P.S. and M.P. Dhir. Use of Modified Binders in India: Current

Imperatives. Journal of the Indian Roads Congress, Volume 72-3, October-

December 2011.

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 lacs

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

• 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 6 years 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 lacs

NPV of BC Grading 2 = 12.00 lacs

26

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.

************************************

review of practices for improving ride quality and ... · subbase; substandard granular subbase...

Documents