Is Premix Carpet (PMC) really good for roads in India? The subject question may sound strange, unfortunately the answer is NO! It is a fact that Premix carpet (PMC) is not at all suitable for roads especially streets in towns and cities 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. 1 2 3
Option
Cost per sq m in rupees 210 205 70
Cost per km lane in rupees 7.88 lacs 7.69 lacs 2.62 lacs
20 mm PMC with sand seal coat 25 mm BC Grading 2 Surface dressing, single application with VG-10, nominal chip size 13.2 mm, mechanical means 4 Surface dressing, single 79 2.96 lacs application with VG-10, nominal chip size 13.2 mm, mechanical means with precoated chips 5 Surface dressing, double 140 5.24 lacs application with VG-10, chip size 13.2 mm, mechanical means 6 Surface dressing, double 149 5.58 lacs application with VG-10, chip size 13.2 mm, mechanical means with precoated chips 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
5 January 2016 “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.
