Premix Carpet (PMC) is Killing Our Roads in Towns and Cities

The subject title may sound sensational, unfortunately it is a fact. Premix carpet

(PMC) is not at all suitable for roads/streets in towns and cities in India for the

following reasons. First, PMC has undesirable water-trapping characteristic which

causes potholes due to stripping of bitumen from the aggregate resulting from

hydraulic pressure under traffic. Second, surface drainage in towns and cities in India

is usually inadequate. If it rains even for half hour, flooding of roads and streets is a

common sight in India. Under such situation use of dense graded Bituminous

Concrete (BC) Grading 2 (and not the PMC) is the only solution as discussed later.

Even SDBC, MSS and BUSG are not desirable because they too are water-trapping.

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. 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

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 water on roads

in cities and towns can penetrate it at that spot, flow sideward like in OGFC, and

flood the entire PMC below the sand seal (Figure 1). 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.

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 2). This writer

has observed this phenomenon while conducting forensic investigation in Australia

(Figure 3). 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 1. 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. 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.

Figure 3. 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. 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 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.

It is not uncommon to see non-uniform application of sand seal coat on PMC because

Fig. 4. Field permeability of PMC without any sand seal coat

Fig. 5. Field permeability of PMC with moderate sand seal coat

Fig. 6. Field permeability of PMC with adequate sand seal coat

it is usually spread manually (Fig. 7). 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. 8).

Fig. 7. PMC surface with non-uniform application of sand seal coat

Fig. 8. Field permeability of BC Grading 2

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. 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 writer has observed this phenomenon on

Jaipur streets (Figure 9). As already mentioned, the hydraulic pressure also causes

stripping in the PMC as well as in the underlying bituminous course. 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 9. Failure of premix carpet (PMC) during the first monsoon within

Jaipur city

Fundamentally it does not make any sense as to why we place a highly porous

bituminous mix like PMC in the first place and then try to seal it. We do not have any

idea 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 amazing 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 some engineers

just have a “gut” feeling that PMC does work and is “good” for India and therefore

there is no need for any research on it.

This writer could not find any published data on average life of PMC in India 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 cannot be exported to developed countries in this global world. However,

that would require fundamental, sound engineering justification which is almost

nonexistent 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 in rural area (including those under the

jurisdictions of towns and cities), 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, use precoated chips. 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. [It is not

understood as to why surface dressing cannot be used on low volume roads such as

PMGSY either; that would save India thousands of crores of rupees every year. Just

imagine how many thousands of additional kilometers of PMGSY roads can be built

with the savings.] Even if double surface dressing with precoated chips is used its cost

is three-fourth (3/4) of the cost of PMC.

Now is the time to take this matter seriously especially when the excuse that surface

dressing is not completely mechanized is no longer valid. Bitumen distributors are

already used for tack coating. Mechanized chip spreaders are available in Gujarat. The

service life of surface dressing is not considered less than the service life of the PMC.

So what’s the excuse now when the whole world is using surface dressing with

success and the Indian Roads Congress (IRC) has a very good standard specification

for surface dressing and most states have this item included in their Basic Schedule of

Rates?

Table 1. Comparison of Costs for PMC and Recommended Alternates

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

4 Surface dressing, single

application with VG-10, nominal

chip size 13.2 mm, mechanical

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

6 Surface dressing, double

application with VG-10, chip size

13.2 mm, mechanical means with

precoated chips

149 5.58 lacs

7 40 mm BC Grading 2 320 12.00 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. Rates based on 2013 Rajasthan

PWD Basic Schedule of Rates. Lane width = 3.75 m

Besides significantly lower construction cost, surface dressing offers the following

functional advantages compared to PMC:

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 (with buildings on the side) 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 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 1).

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). This was 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.

Even based on initial costs, if a city has to do carpeting of 150 km of roads every year,

it can resurface only 100 km with BC Grading 2 in the first year and the remaining 50

km can be done in the second year. What’s the problem when the PMC is going to fail

in 1-2 years and BC would last for 7-8 years? It is no rocket science. 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.

Faced with deteriorated road situation in Jaipur during 2010 monsoon, the Jaipur

Development Authority (JDA) accepted this writer’s common sense recommendation

to use only BC Grading 2 for hot patch repairs as well as resurfacing of all city roads.

Both PMC (average life of 1-2 years) and Semi Dense Bituminous Concrete (average

life of 3-4 years) were banned. Results have been outstanding in that roads resurfaced

with BC Grading 2 about 3 to 4 years ago have not developed potholes as was the

case in earlier years. It is hoped similar measures would be taken by other towns and

cities in India so that peoples’ tax money is not wasted with these “disaster” PMC

type mixes and the public is also not inconvenienced from potholed roads every

monsoon. Let’s ban the premix carpet which is really killing our roads in towns and

cities!

Prof. Prithvi Singh Kandhal

Jaipur

29 June 2014

“American roads are not good because America is rich, but America is rich because

American roads are good.” - John F. Kennedy

“Never doubt that a small group of citizens can change the world. Indeed, it is the

only thing that ever has.” -Margaret Mead

ABOUT THE WRITER

Prof. Prithvi Kandhal is currently Associate Director Emeritus of the National Center

for Asphalt Technology (NCAT) at Auburn University, Alabama, USA. Prior to

joining NCAT in 1988, Prof. Kandhal served as Chief Asphalt Engineer of the

Pennsylvania Department of Transportation for 17 years.

Prof. Kandhal has served as Chairman of the US Transportation Research Board

(TRB) Committee on Bituminous Mixtures. He also served as Chairman of ASTM

Committee DO4 on Road and Paving Materials, which is responsible for over 200

standards used worldwide. He is also past President of the Association of Asphalt

Paving Technologists (AAPT), which has members in all continents of the world.

Prof. Kandhal has published over 120 papers in the area of asphalt paving technology.

He also co-authored the first-ever textbook on hot mix asphalt technology, which is

being used in over 25 universities in the US.

Prof. Kandhal has been a practicing highway engineer in India for 20 years and in the

US for 30 years. In recent years, he has drafted many standards for the Indian Roads

Congress (IRC) including specifications for dense graded bituminous mixes, stone

matrix asphalt (SMA) and readymade pothole patching mix, which have been

adopted. He was also instrumental single-handedly in introducing viscosity grading of

bitumen (VG Grades) in India in lieu of penetration grading in 2005.

In August 2011, Prof. Kandhal was inducted on the “Wall of Honor” established at

the largest asphalt road research center in the world. In April 2012, he received the

Honorary Membership which is considered equivalent to “Lifetime Achievement

Award in Asphalt Road Technology” from the International Association of Asphalt

Paving Technologists during their annual banquet held in Austin, Texas, USA.