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Premix Carpet is Killing Our Roads in Towns and Cities

The subject title may sound sensational; unfortunately it is a fact. Premix carpet (PC)

is the oldest hot mix in India. It is a good, economical, bituminous wearing course

mix to be placed directly on water bound macadam (WBM) of low-volume rural

roads. Since such roads are not constructed by mechanical means, the quality of

WBM may not be consistent. The 20 mm thick PC being an open graded mix is

flexible enough to adapt to any uneven consolidation of WBM.

The premix carpet is also provided with a bituminous sand seal coat to minimize

direct penetration of rainwater into it. This system works reasonably well on rural

roads where the rainwater drains away from the PC to earthen shoulders. However,

the PC is not suitable at all for roads and streets in towns and cities where surface

drainage 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 PC) is the only solution.

To keep things in perspective, let’s compare PC with open graded 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 PC 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 more than 5% polymer modified bitumen

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

The premix carpet (PC) 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 PC, it is

not completely effective in making the PC waterproof at the surface. Even if there is a

small patch where the PC 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 PC

below the sand seal (Figure 1). This is analogous to porous 20 mm OGFC (PC 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). I have 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.

Once the PC is saturated with water, the hydraulic pressure resulting from traffic

above can loosen up the sand seal in other areas of the PC. I have observed this

phenomenon on Jaipur streets (Figure 4). The hydraulic pressure also causes stripping

in the PC as well as in the underlying bituminous course. That is why PC deteriorates

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Figure 1. Surface water entering the premix carpet (PC) through an unsealed area

saturating it under the seal as well, causing stripping within PC 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 PC and adjacent BC courses.

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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.

Figure 4. Failure of premix carpet (PC) during the first monsoon within Jaipur city

rather rapidly during monsoons when streets are flooded in towns and cities. The

average life of PC in Jaipur is about 1-2 years. Its bitumen content is about 3.3

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percent. [It took me about half hour to calculate the bitumen content of PC because

the Indian specification only indirectly specifies it. The specification states the

quantity of aggregate in cu m to be used in 100 sq m of road and the quantity of VG-

30 bitumen to be used in kg per cu m of aggregate. The specification encourages PC

to be made in a hot mix plant. Therefore, it would be helpful to the contractors as well

as the engineers to specify bitumen content by weight of mix. While suggesting

revisions to MORTH specifications I have suggested this change.]

It does not make any sense as to why we place a highly porous bituminous mix like

PC and then try to seal it. Why not place a dense BC Grading 2 to begin with, which

has an average life of 7-8 years, and will definitely fare much better than PC when

subjected to repeated short-term flooding due to inadequate drainage system in our

towns and cities. Although 25 mm BC Grading 2 is about 50% more expensive than

the 20 mm thick PC, it is much more economical from life/cycle considerations. 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. In addition, 25 mm of BC Grading 2 contributes to the structural strength

of the road whereas 20 mm of PC has “zero” structural strength. Therefore, imposing

a ban on the use of PC within towns and cities and substituting BC Grading 2 for

resurfacing is a common sense decision.

Faced with deteriorated road situation in Jaipur during current monsoons (2010), the

Jaipur Development Authority (JDA) has accepted my common sense

recommendation to use only BC Grading 2 for hot patch repairs as well as resurfacing

of city roads. Both PC (average life of 1-2 years) and Semi Dense Bituminous

Concrete (average life of 3-4 years) are being banned. I sincerely hope similar

measures would be taken by other towns and cities in India so that peoples’ tax

money is not wasted with these “disaster” mixes and the public is also not

inconvenienced from potholed roads every monsoon.

The least you can do is to forward this common sense information to the mayor and

engineers of your town or city.

Prof. Prithvi Singh Kandhal

Jaipur, Rajasthan

6 September 2010

E-mail: pkandhal@gmail.com

Homepage: www.eng.auburn.edu/users/kandhps

“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