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A SIMPLE AD EFFECTIVE METHOD OF REPAIRIG POTHOLES I IDIA

By

Prof. Prithvi Singh Kandhal*

[This paper was published as Paper o. 544 in the Journal of the Indian Roads Congress, Volume 69-3,

October-December 2008. Author’s response to comments made on this paper are also given at the end.]

ABSTRACT

Development of potholes on roads and streets of India after the onset of monsoons is a common phenomenon. Quite often, potholes are repaired with antiquated techniques such as placing soil or bare aggregate in the pothole because no hot mix asphalt is available during monsoons. An economical, generic, readymade stockpile cold patching mix has been proposed, which is manufactured in a batch type hot mix plant using local aggregates. This generic mix can be placed without preparing the pothole such as drying, squaring the edges, cleaning, and tack coating. This patching mix developed by the author was adjudged as the best performer in the readymade bituminous patching mix category in a nationwide field evaluation research project conducted under the US Strategic Highway Research Program (SHRP). Since that study was conducted in regions with different climatic conditions (including hot and wet), it is believed this patching mix will be equally successful in India. Recent field trials of this mix in Rajasthan during monsoons have been highly successful. The mix can be stockpiled and remains workable for at least 6 months and, therefore, it can be used throughout the year including the rainy season. A detailed specification ready to be used by highway agencies in India is given in the paper.

1. ITRODUCTIO

Development of potholes on Indian roads and streets after the onset of monsoons is a common phenomenon. Every year there is a public outcry and newspapers are full of pictures showing potholed road pavements. Hot mix asphalt plants are usually shut down during monsoons and no hot bituminous mix is available for filling potholes. Therefore, many potholes are either not repaired or repaired with antiquated techniques. It is quite common for the government agencies to repair potholes as follows:

• Fill the pothole with soil, which is usually washed away in the first rain.

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* Associate Director Emeritus, National Center for Asphalt Technology (NCAT), Auburn University, Alabama, USA (currently, Karanpura House, 50 Raj Bhawan Road, Civil Lines, Jaipur 302 006) E-mail: pkandhal@eng.auburn.edu

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• Fill the pothole with bare stones (Photos 1 and 2). This not only provides a rough riding surface especially for motorcycles but the stone particles start to

Photo 1. Pothole filled with soil and aggregate

Photo 2. Pothole filled with bare aggregate ravel out of the pothole during subsequent rains. If the stones survive in the potholes, these are not usually removed and pothole properly patched when the road or street is overlaid with hot mix. This results in reappearance of the same pothole during next year’s monsoon.

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• Very deep potholes are filled with cement bags full of sand, which is undesirable.

• Sometimes a penetration macadam type approach is used. A tack coat is applied to the bottom of the pothole. A layer of large stone is spread in the pothole. Bituminous binder is applied to the stone layer and then another layer of stone is spread and compacted. This penetration macadam is extremely porous to water and gets saturated with water when it rains. Even when the road is resurfaced with hot mix, the pothole redevelops at the same place year after year. Photo 3 shows an example of the repeated failure of this technique in India.

Photo 3. Reappearance of pothole repaired with the penetration macadam technique The following are the major disadvantages of potholes when they are not repaired at all or repaired with the preceding improper techniques:

• Inconvenience to motoring public

• User delays which cost money in terms of their time

• Extra fuel consumed when the vehicles have to stop or go very slow

• Excessive wear and tear of the vehicles, which require frequent repairs

• Increased air pollution caused by vehicle emission due to slow or stopped traffic

• Accidents resulting from the tendency of the vehicles to go around the potholes

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• Increased real cost of repairing potholes in terms of labour because the government agencies have to keep on repairing the same potholes several times during the monsoon

• Significant environmental impact due to most of the reasons mentioned above such as increased fuel usage, increased air pollution, increased consumption of energy in repairing the potholes with stopgap techniques.

There is a need to implement a simple and effective method of repairing potholes not only during monsoons but also throughout the year. This can be achieved by using the latest cold mix asphalt technology for producing and stockpiling readymade bituminous pothole patching mix. A conventional batch type hot mix asphalt plant is used to produce a large stockpile of patching mix prior to the onset of monsoons. A properly designed and produced stockpile mix can maintain its workability (ease of handling), ease of placement, and compaction at ambient temperatures for at least 6 months. In other words, this cold mix can be used not only during monsoons but afterwards as well to repair isolated potholes and utility cuts. A hot mix is not economical and suitable for repairing isolated potholes and utility cuts because the hot mix keeps on cooling in the truck and therefore cannot be compacted adequately to provide a durable patch. Various manufacturers have promoted many proprietary pothole-patching products. These mixes are usually tailor made with one stone type under strictly controlled conditions and supplied in drums or bags. The cost of such materials is very high, which makes their use prohibitive from the economic standpoint. Therefore, there is a need to specify a generic patching mix, which can be produced by all hot mix asphalt plants in India using local aggregates. Such a generic patching mix was developed by the author1 while serving as Chief Asphalt Engineer of the Pennsylvania Department of Transportation (DOT) in the US. Field trials of that patching mix were highly successful in Pennsylvania throughout the year including hot, rainy summers. Therefore, this mix is considered suitable for Indian climatic conditions as well. Recent field trials of this mix in Rajasthan during monsoons have been highly successful as reported later.

2. CHALLEGES OF MIX DESIG It is difficult to design stockpile patching mixtures because the properties required in stockpiling and handling and after the material is placed in the pothole are contradictory. Some of these contradictory requirements are as follows:

1. Aggregate gradation--For good mixture workability, an open gradation is desired. After the mix is placed, however, a denser gradation is needed to improve durability. 2. Aggregate shape--To obtain good workability, angular aggregate shape should be avoided. However, once the mix is in place, a high degree of angularity is desirable for better stability. 3. Binder viscosity--Lower binder viscosity is desired for storageability and workability, but after placement higher viscosity is desirable as soon as possible for better cohesion of the mixture.

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4. Binder content--Greater residual bitumen content in the mixture is needed to obtain thicker films on the aggregate for stickiness and durability, but there is a potential binder drainage problem in the stockpile just after stockpiling while the mix is hot.

Use of highly absorptive aggregates can also pose problems. High moisture content in such aggregates often causes stripping and/or binder drainage problems in the stockpile. Selective absorption of the lighter fractions of the bituminous binder by such aggregates leaves a bituminous film that has undesirable characteristics and is significantly different from the original bituminous binder used.

In case of cold patching mixtures, it is not possible to use conventional methods of mix design generally used for hot bituminous mixes such as the Marshall method. Not only are the specimen preparation and testing difficult, but also the desired design criteria for the stockpile patching mixtures are unknown.

3. EW COCEPTS

In the past, the use of larger-sized aggregate (12.5-19.0 mm) in the stockpile mixture was promoted in the US to obtain higher stability. Such a mixture can be successful if the patching technique is ideal (for example, making edges vertical, cleaning, applying tack coat, and compacting adequately). However, ideal patching techniques are not always used and mixtures that contain large aggregate start to ravel under traffic, which results in premature failure of the patch. Another concept is to disregard the stability and make the mixture finer and more pliable so that it will be more tolerant of abuse during placement and perform under traffic. This finer mix, if placed less than 75 mm (3 in) deep in one lift in a confined area, should be stable. For deeper and/or larger holes, the mixture has to be compacted in layers.

The cohesive and adhesive qualities of a mix depend mainly on the composition of the

mortar (bituminous binder plus fines). If there are excessive fines or dust (material passing a 0.075-mm or No. 200 sieve) in the mixture, the mortar will be lean, less tacky, and friable. It is no coincidence that most of the expensive commercial patching products are made from clean stone. Several extraction tests run on such products have revealed that the fines (the minus 200 fraction) are usually less than 1 percent. In the absence of excessive fines, mixtures are very tacky; therefore, tack coating of the pothole will not be required. Many of the Pennsylvania DOT's earlier conventional stockpile patching mixtures had not performed satisfactorily because of excessive fines. Such mixes are dull and friable and lack cohesive and adhesive qualities.

4. RECOMMEDED IMPROVED FORMULATIO

4.1 In view of the challenges of mix design and new concepts, the characteristics discussed below appeared desirable to the author1 for development of a satisfactory and economical stockpile patching mixture.

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4.1.1 Finer and Predominantly One-sized Gradation

A gradation consisting of 100 percent passing the 9.5 mm or 4.75 mm sieve has the following advantages: 1. The mix is pliable and workable. 2. Due to increased surface area, more bituminous binder can be incorporated into the mix to improve its durability. 3. The mix remains pliable for a prolonged period of time and continues to densify easily under traffic and will continue to adapt to the changing geometry of the pothole. This characteristic enhances its chances of survival. Normally, a finer dense gradation will not have good workability. However, if it is

made of predominantly one-sized aggregate (100 percent passing the 9.5 mm or 4.75 mm sieve and mostly retained on the 1.18 mm sieve), the following advantages result: (a) the workability of the mixture is increased significantly, and (b) the mixture can cure effectively.

4.1.2 Clean Aggregate

As discussed earlier, it is very important to keep the dust content (minus 200 fraction) in the mixture as low as possible to impart tackiness. This would significantly improve the adhesive and cohesive properties of the mixture. 4.1.3 Angular Aggregate Shape

Angular aggregate shape is desirable for better stability. Since a finer and predominantly one-sized gradation is used, the effect of aggregate angularity on the workability of the mix is minimal. Angular crushed stone aggregate is an ideal material.

4.1.4 Use of Least Absorptive Aggregate

Highly absorbent aggregates should be avoided. The aggregate water absorption should preferably be limited to approximately 1 percent.

4.1.5 Adequate Binder Content It has been determined that at least 4.5 percent residual bituminous binder (total binder minus diluent) is required in a stockpile patching mixture made from an aggregate whose water absorption is 1.0 percent or less. If the aggregate absorbs water in excess of 1.0 percent, the residual binder content should be increased by a similar amount. For example, an aggregate that absorbs 1.5 percent water should have 5.0 percent minimum residual bituminous binder. The factor limiting the maximum amount of the bituminous binder is drainage in the stockpile just after manufacture. The drainage can be minimized or eliminated by using lower mix temperatures and limiting the stockpile height to 1.2 m (4 ft) during the first 48 hours.

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4.1.6 Proper Type and Amount of Antistripping Agent

The antistripping agent is a very important part of the formulation of the stockpile mixture. A mixture should retain its coating in the stockpile under adverse weather conditions, during handling, and in the pothole after placement. A stockpile patching mixture, which is more pervious than a densely graded hot mix, has to withstand by far the most severe weather and traffic effects. It has to survive in conditions that led to the creation of the pothole in the first place (such as poor base, inadequate drainage, and deteriorated adjacent pavement). Rain or melting snow provides water. The pneumatic tires of vehicles provide high pressures. This combination can emulsify the bituminous binder or displace it from the aggregate. If sufficient stripping occurs as a result of this action, the traffic will dislodge the aggregate particles. There are many commercially available antistripping agents in the market for use with the medium-curing (MC) cutback bitumens. Experience has shown that there is no single additive that will work with all aggregate types. Therefore, it is essential that the type of antistripping agent and its amount be selected after testing with the aggregate that is actually being used in the mix. Pennsylvania DOT requires its bituminous suppliers to conduct the wet coating test, static immersion test, and stripping test with the job aggregate.

4.2 Specifications

The salient features of Pennsylvania's stockpile patching material specifications are discussed below. A detailed specification ready to be used by highway agencies in India is given in the appendix.

4.2.1 Production

The stockpile patching mix must be produced using a conventional batch type hot mix plant, which is operated at relatively low temperatures.

4.2.2 Bituminous Materials

Since it is a cold mix, medium curing cutback bitumen conforming to IS: 217 Specification for Cutback Bitumen should be used as a binder. MC-800 grade cutback bitumen should be used in production when the patching mix is intended to be used during most of the year. MC-250 grade cutback bitumen should be used in production when the patching mix is intended to be used during colder winter months or in high altitude places. This is because MC-250 has relatively low viscosity at 60 C compared to MC-800 and therefore will provide better workability at cold ambient temperatures. It is recommended that the highway agencies try both MC grades and select the one, which gives better performance in terms of stockpile workability and stability of the patch under local conditions. Recently MC-800 was used by the author successfully in Rajasthan during monsoons. Since MC cutback bitumen binders use kerosene as a diluent, it can be argued to discourage their use considering loss of energy and environmental concerns. However, not repairing the potholes (especially during monsoon season when hot mix plants are shut

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down) or repairing the potholes with inappropriate materials which do not stay in place would waste far more energy in terms of fuel and create far more air pollution when the vehicles have to crawl or stop on potholed roads. Unfortunately, no pure emulsified bitumens are available, which are as effective as medium curing cutbacks. It is of no use using conventional emulsified bitumen, which will not perform as intended. This case is similar to the use of MC-30 and MC-70 medium curing cutback bitumen grades, which are very effective materials for use as a prime coat in road construction. Contrary to what some suppliers might say, there are no emulsified bitumens, which are as effective. That is why; many states in the US make exception and do allow medium curing cutback bitumens when they are used for priming coat or producing stockpile patching mix. Emulsified cutback bitumens (inverted emulsion with oil as a continuous phase) have been tried for prime coat and stockpile patching mix, but they are not as effective as conventional cutback bitumens. Moreover, emulsified cutback bitumens also contain substantial amounts of diluent such as kerosene. More developmental work is needed in producing effective emulsified cutback bitumens. MC cutback bitumens should be treated with antistripping agents to meet the requirements of the wet coating test, the static immersion test, and the stripping test performed with the actual job aggregate. These tests are described in the annexure to the specifications given in the appendix. The use of antistripping agents should be mandatory because the patching mix should not be susceptible to moisture or water damage (stripping) so as to survive in a hostile environment of a pothole. Under no circumstances, MC cutback bitumen should be allowed to be made by contractors or department by blending kerosene with paving bitumen. It should only be obtained from approved manufactures, which are certified to produce MC cutback bitumen conforming to all requirements of IS: 217 Specification for Cutback Bitumen.

4.2.3 Composition of Mixture

The patching mix should conform to the gradation given in Table 1.

Table 1. Gradation of Stockpile Patching Mix

Sieve size, mm Percent passing

9.5 100

4.75 40 – 100

2.36 10 – 40

1.18 0 - 10

0.075 0 – 2

It should be noted that the material passing 0.075 mm sieve is restricted to 2 % maximum (based on washed gradation). This is very essential for the quality and durability of the patching mix.

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As far as possible, aggregate with 1.0 % or less water absorption should be used. If the water absorption exceeds 1.0 %, the amount of residual bituminous material should be increased as shown in Table 2.

Table 2. Minimum Residual Bitumen Content by Weight of Mix

Aggregate water absorption,

%

Minimum residual bitumen content,

%

1.0 or less 4.5

1.1 to 1.5 5.0

1.6 to 2.0 5.5

Example to explain the residual bitumen content: If the total amount of MC cutback bitumen in the mix is 6.0 % and the cutback has 80 % bitumen (residual) and 20 % diluent (such as kerosene), the residual bitumen content in the mix is 6.0 times 80 %, which is equal to 4.8 %. The contractor must obtain the actual percentage of residual bitumen in the MC cutback from the approved supplier.

4.2.4 Preparation of Mixture

As mentioned earlier, the stockpile patching mix should be produced in a conventional batch hot mix plant. The mix should be such that it can be stocked, handled, placed, and finished without stripping of the bitumen from the aggregate. To help prevent stripping and avoid heat buildup in a stockpile (which may burn the entire stockpile), the mixed material should not be stockpiled no higher than 1.5 m for the first 48 hours. The mineral aggregate should be clean and surface dry before mixing. The temperature of aggregate and bituminous material should comply with those shown in Table 3.

Table 3. Temperature Ranges for Producing Stockpile Patching Mix

Bituminous material Aggregate temperature, C Bitumen temperature, C

MC-250 5 – 65 55 -80

MC-800 5 – 65 75 - 95

Since the range of aggregate temperature is rather low and the maximum aggregate temperature is restricted to 65 C, it may not be possible to dry the aggregate within this temperature range. Therefore, the aggregate is quite often processed in a dryer at high temperatures and allowed to cool before the bituminous material is added. Drying the aggregate at high temperatures will also help in reducing the fines (material passing 0.075 mm sieve), which will go into the baghouse. The resulting mix then would have fines less than 2 % as required in the stringent gradation specifications.

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High aggregate temperature while mixing with the cutback bitumen will not only cause excessive loss of diluent from the cutback but will also pose a safety hazard in the plant pug mill. Proper and adequate venting of the pug mill is necessary. 4.2.5 Acceptance of Completed Mixture

The following two tests should be performed by the contractor (in presence of the Department representative) on the mixture, freshly prepared or taken from a stockpile at any time during its storage life (usually 6 months).

1. Water Resistance Test (See Annexure I, Test C)

2. Workability Test (See Annexure I, Test D) The water resistance test will indicate whether the patching mix has a potential for stripping in the pothole in presence of water. If the mix fails this test, it means a proper type and/or amount of antis tripping agent has not been used in the mix. If the mix fails in workability it could be due to improper bitumen type, low bitumen content, excessive fines or improper gradation. There is a general tendency in certain parts of India to consistently use bitumen content significantly lower than the design bitumen content. This will simply not work in case of cold stockpile mix, which will not last long in a pothole. Even one-half percent lower bitumen content will make the mix unfit for use. Also, the washed gradation, especially the percentage of material passing the 0.075 mm sieve (dust) should not exceed 2 percent in any case to ensure its quality. Therefore, the user agency should invariably get the mix composition of the produced mix tested by an approved independent testing laboratory before acceptance.

5. PERFORMACE HISTORY

5.1 Performance in the US

This bituminous stockpile mix was developed by the author in the late 1970s in Pennsylvania1. This generic patching mix was placed side by side in many potholes with other types of patching materials including some expensive, proprietary products to compare its performance in the field (Photos 4 and 5). After its performance was found very satisfactory, it was adopted as a standard cold patching mix in Section 485 of the Pennsylvania DOT specification. That is why it is also called Section 485 material and has been used up to this date.

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Photo 4. Different cold patching mixes being tried in the field

Photo 5. Same pothole filled with two types of cold patching mix

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A nationwide evaluation 2,3 of various bituminous stockpile patching mixture was conducted in the US during 1991 and 1992 under the US Strategic Highway Research Program (SHRP) Project H-106. Many states with different climate were involved in the field evaluation. The H-106 field experiment was part of the most expensive field experiment of its kind ever undertaken. Data were collected on the performance and cost effectiveness of various cold mix materials and procedures for repairing potholes in bituminous pavements. At the conclusion of this 4-year field study, this generic Section 485 material was adjudged first in ranking among all readymade cold patching materials including many proprietary products. It had a survival rate of 78 % after about 4 years, which is considered very high. It is believed this readymade bituminous stockpile patching mix should be equally successful in India when used throughout the year including the rainy season. During the rainy season, there is no need to dry the pothole; just the water in the pothole needs to be swept off before placing this mix (Photo 6). One of the main advantages of cold stockpile patching mix is that crews can take needed amount of the mix in truck (Photo 7) to the job site and return unused portion to the stockpile. The mix is placed in the pothole with a shovel (Photo 8) without any preparation and compacted with truck tyres (Photo 9) if a roller is not readily available. In India, rammers with long handles or equal can be used effectively. Photo 10 shows a view of some completed patches. For better performance it is recommended to place and compact this mix in 75-mm layers in deep potholes. It is quite normal for the patching mix stockpile to form a hard crust after some time because the diluent is lost near the surface. However, this hard crust (usually about 150 mm thick) protects the mix within it from losing diluent. The hard crust can be broken easily to access the soft material inside. The hard crust also helps in shedding rainwater on a conical stockpile. If possible, cold mix stockpiles should be kept under a shed (Photo 11).

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Photo 6. Water in the pothole needs to be swept off before placing cold patching mix

Photo 7. Truck carrying cold patching mix

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Photo 8. Cold patching mix being placed in the pothole with a shovel

Photo 9. Cold patching mix being rolled with truck tyre

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Photo 10. Completed cold mix patches

Photo 11. Optional shed for cold patching mi x stockpile

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5.2 Performance in India

This generic cold patching mix meeting the specification given in the Appendix was produced on a limited scale in Rajasthan during the 2008 monsoons. Potholes on NH 11 (Jaipur-Agra Road) and some main streets of Jaipur were patched with this mix. The gradation of the produced mix was: 100 % passing 9.5 mm; 73 % passing 4.75 mm; 12 % passing 2.36 mm; 3 % passing 1.18 mm; and 1 % passing 0.075 mm sieves. MC-800 cutback bitumen was obtained from an approved supplier. Since the quartzite aggregate used in the mix is susceptible to stripping, it was necessary to use 2 % liquid antistripping agent by weight of MC-800 to meet the requirements of wet coating test, static immersion test, and stripping test (see annexure to Appendix). The residual bitumen content in the patching mix was 4.5 percent. Many potholes were completely wetted with water to simulate rainy season before the patching mix was placed in them. All patches are intact and their performance has been excellent under traffic and after subjected to rains. The edges of the patches have almost blended with the existing road surface with no signs of raveling. Photo 12 shows the repair of a pothole on NH 11 (Jaipur-Agra Road). Photo 12 (a) shows the mix being placed in a pothole; Photo 12 (b) shows patching mix being compacted with a hand rammer; Photo 12 (c) shows completed patch; Photo 12 (d) shows the same patch after subjected to traffic on NH 11 and after rains; and Photo 12 (e) shows the close up of the same patch in service. Photo 13 shows the repair of a pothole on an important road in Jaipur. Photo 13 (a) shows the pothole after wetting with water; Photo 13 (b) shows the mix being compacted with a hand rammer in this wetted pothole; Photo 13 (c) shows the completed patch; Photo 13 (d) shows the same patch after subjected to traffic and rains; and Photo 13 (e) shows the close up of the same patch, which has completely blended with the existing road surface. Research is continuing to develop test methods for laboratory evaluation of the performance of cold patching mixes5. Once the use of such mixes becomes prevalent in India, such test methods can be developed suited for Indian conditions.

5.3 Cost Analysis

The readymade stockpile patching material is expected to cost significantly more than a typical hot mix. However, economical analyses have shown4 that premium mixes need only extend the average service life by a modest amount to provide a lower annualized cost than conventional mixes. For example, a $ 120/ton mix lasting 2 years provides substantial savings compared to a $ 30/ton mix lasting 1 year. Typically, the hot bituminous concrete (BC) mix with 5 % viscosity graded VG-30 (equivalent to old 60/70 penetration grade) costs about Rs. 2,700 per ton at the hot mix plant. The cost of aggregate, mixing, etc. in a hot mix plant are expected to be about the

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Photo 12 (a). Mix being placed in a pothole on NH 11 (Jaipur-Agra Road)

Photo 12 (b). Mix being compacted with a hand rammer

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Photo 12 (c). Completed patch on NH 11

Photo 12 (d). Same patch (circled) after subjected to traffic and rains

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Photo 12 (e). Close up of the same patch under traffic on NH 11

Photo 13 (a). Pothole on Jaipur Road after wetting to simulate rain

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Photo 13 (b). Mix being compacted in the wet pothole

Photo 13 (c). Completed patch of the wet pothole

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Photo 13 (d). Same patch after subjected to traffic and rains (black patch behind the car)

Photo 13 (e). Close up of the black patch after traffic and rains (centre of picture)

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same for BC and cold patching mix, although the later may incur less heating costs. The increase cost of patching mix is primarily due to the higher cost of MC-800 cutback bitumen (about Rs. 55,000 per ton) compared to the cost of VG-30 paving bitumen (about Rs. 32,000 per ton). Assuming the cold mix contains 4.5 % residual bitumen (about 5.6 % MC-800 cutback), its estimated cost should be about Rs. 4,175 per ton compared to BC mix, which costs about Rs. 2,700 per ton. That is an increase of about 55 percent. This increase is not significant considering that patching mix is durable, can be used during monsoons, and can be used at isolated locations (including utility cuts) throughout the year. Based on life cycle costs, the use of this cold mix is significantly economical than repeated, labour intensive filling of potholes with soil and/or aggregate during the rainy season. Also, consider that the potholes can remain unfilled during the rainy season (which usually is the case) because no hot mix is available. The cost to the motoring public in such situations in terms of user delays, excessive wastage of petrol/diesel due to slowing or stoppage by far exceed multiple times the difference between the cost of BC and the cold mix. Moreover, the cost of this generic patching mix (which can be produced by all hot mix plants using local materials) is less than one-third of some proprietary patching products supplied in bags, which can cost as much as Rs.13,000 per ton. [Addendum by the Author as of 25 July, 2010: Comparative cost analyses of hot mix versus

cold mix patching were made again considering the present Schedule of Rates of the

Rajasthan P.W.D. It was determined that the production cost of cold patching mix was

21.8% more than that of the hot mix FOB hot mix plant. This is due to higher cost of MC-

800 compared to VG-30, and higher amount of MC-800 (6%) required than VG-30 (5%).

However, the cost of in-place cold mix patching was only 5% more than that of in-place hot

mix patching, because the latter requires extra labour and tack coat material. Considering

that (a) hot mix cannot be used during rainy season during which period potholes grow

bigger and deeper requiring more hot mix eventually,(b) wastage of hot mix due to cooling,

(c) user delay costs, (d) wear and tear of motor vehicles, and (e) excessive wastage of

diesel/petrol by motor vehicles,etc., the cost of timely in-place cold patching is likely to be

less than half that of the in-place hot mix patching.]

5.4 Unwarranted Shutdown of Hot Mix Plants during Monsoons

For long stretches of repairs, which cannot be called potholes, it is recommended to use hot mix because the cold patching mix is not expected to perform well. It is quite common that the hot mix plants in the vicinity of metropolitan areas (for example, Jaipur) are completely shut down during the monsoons. Therefore, potholes and long stretches of raveled roads and streets (Photo 14 shows an example) remain unattended during the rainy season causing a lot of discomfort to the motoring public. In many regions like Rajasthan it does not rain every day during the monsoon season. There are always some days without any rainfall during which hot mix can be produced for road repairs/resurfacing. There are many countries in the world (including some states in the US) where rainfall is spread out throughout the year and can occur on any day. If a similar policy of complete shutdown of plants is followed there, no hot mix will be available there during the whole year. Two arguments are offered to completely shut down hot mix asphalt plants in India. First, the

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price of hot mix will be high and second, the hot mix will not stick to damp existing surface. Both are largely fallacies based on experience in other countries. Due to competition, there is no significant increase in the price of hot mix. Generally, road surfaces dry out on the next day following rain. If not, an emulsion tack coat can be applied to existing slightly damp road surfaces before placing the hot mix. In case of sudden rains, most specifications in the US require plants to shut down immediately but allow the hot mix in transit trucks to be placed on the roadway during the break in rain. The author has not seen any significant road failures resulting from this practice; otherwise it would not be permitted at all in the specifications. Therefore, the practice of shutting down all asphalt mix plants during the monsoons needs to be discontinued.

Photo 14. Long stretch of a raveled bituminous street pavement

6. COCLUSIOS AD RECOMMEDATIOS

• Development of potholes on Indian roads and streets after the onset of monsoons is a common phenomenon. Unfortunately, there is no standard readymade cold patching material available in India, which can be used during the rainy season when hot mix plants are usually shut down.

• It is difficult to design stockpile patching mixtures because the properties required in

stockpiling and handling and after the material is placed in the pothole are contradictory.

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Some of these contradictory requirements pertain to aggregate gradation, aggregate shape, and binder viscosity.

• New concepts have been postulated to meet the challenges of designing a stockpile cold mix.

• A generic cold patching mix was developed by the author in view of challenges of mix design and new concepts. The desirable characteristics of such a mix are: fine and predominantly one-sized aggregate, clean aggregate, angular aggregate, use of least absorptive aggregate, adequate binder content, and proper type and amount of antistripping agent.

• This cold patching mix is manufactured in a batch type hot mix plant using local aggregates. This mix can be placed without preparing the pothole such as drying, squaring the edges, cleaning, and tack coating.

• This patching mix was adjudged as the best performer in the readymade mix category in a nationwide field evaluation research project conducted under the Strategic Highway Research Program (SHRP) in the US. It had a 4-year survival rate of 78 percent. Since that study was conducted in regions with different climatic conditions (including hot and wet), it is believed this patching mix will be equally successful in India. Recent field trials of this mix in Rajasthan by the author have shown excellent performance when it was used during the monsoons.

• The mix can be stockpiled and remains workable for at least 6 months and, therefore, it can be used throughout the year including the rainy season.

• Economical analyses of such mixes show they are cost effective despite their higher cost compared to hot mix.

• A detailed specification ready to be used by highway agencies in India is given in the paper.

• A case has been made not to shut down hot mix plants completely during the monsoon season in India so that major repairs/resurfacing can be undertaken on non-rainy days like other countries in the world.

7. ACKOWLEDGEMETS

The cold patching mix used in recent field trials in Rajasthan was prepared by R. V. Murthy of IJM (India) and applied by their crew on NH 11 (Jaipur – Agra Road). The application of this cold patching mix on roads in Jaipur was made with the assistance of Dr. A. N. Arora; faculty (Pawan Kalla) and engineering students of the Malviya National Institute of Technology (MNIT), Jaipur, who will also continue to monitor and document the long range performance of these patches.

8. REFERECES

1. Kandhal, P. S. and D. B. Mellott. Rational Approach to Design of Bituminous Stockpile Patching Mixtures. TRB, Transportation Research Record 821, 1981.

2. Wilson, T. P. Strategic Highway Research Program Pothole Repair Materials and

Procedures. TRB, Transportation Research Record 1392, 1993.

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3. Berlin, M. and E. Hunt. Asphalt Concrete Patching Material Evaluation. Oregon

DOT Report No. OR-RD-01-19, June 2001. 4. Thomas, H. R. and D. A. Anderson. Evaluation of Experimental Cold- Stockpiled

Patching Materials for Repairs in Cold and Wet Weather. TRB, Transportation Research Record 1268, 1990.

5. Estakhri, C. J. and J. Button. Test Methods for Evaluation of Cold-Applied

Bituminous Patching Mixtures, TRB, Transportation Research Record 1590, 1997.

APPEDIX

Proposed Specification for Readymade Stockpile Bituminous Patching Mix

1. Scope

The material shall consist of plant mixed readymade stockpile patching bituminous mixture composed of mineral aggregate coated with bituminous material. The material shall be capable of being stocked for at least six months without stripping and shall be workable at all times. This material is intended for patching holes up to 75 mm (3 inches) deep. 2. Materials

2.1 Bitumen

Medium Curing Cutback Bitumen MC-800 or MC-250 conforming to Indian Standards Specification IS: 217 Specification for Cutback Bitumen shall be used in preparing the stockpile mix and shall be supplied by a certified manufacturer of this product. Unless otherwise specified, MC-800 grade shall be used. MC-250 grade may be specified if the stockpile is intended to be used during winter or at high altitude locations. For proper mixing, the bitumen shall be heated as specified in Section 3. MC Cutback Bitumen shall be treated with a proper type and amount of an antistripping agent by the approved bitumen supplier so that when combined with the proposed job aggregate the resulting mix shall pass the Wet Coating Test, Static Immersion Test and Stripping Test as given in Annexure I. The contractor shall furnish the sample of the job aggregate each year to the bitumen supplier for these coating and stripping tests and obtain a certificate that the bitumen material has been treated to suit the job aggregate. This yearly certificate must be on file and shall be available at the hot mix plant when required by the Engineer. The contractor shall also forward a copy to the Engineer. Under no circumstances, the contractor or the department shall be permitted to manufacture the MC cutback by blending paving bitumen and kerosene.

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2.2 Coarse Aggregate

2.2.1 The coarse aggregate shall consist of crushed rock, crushed gravel or other hard

material retained on 2.36 mm sieve. It shall be clean, hard, durable and cubical shape, free from dust and soft organic and other deleterious substances. The aggregate shall satisfy the physical requirements specified in Table 1.

2.2.2. Where crushed gravel is proposed for use as aggregate, not less than 90% by

weight of the crushed material retained on 4.75mm sieve shall have at least two fractured faces resulting from crushing operation.

TABLE 1. Physical Properties of Coarse Aggregate

Property Test Requirement

Test method

Cleanliness Grain size analysis Max. 2% passing 0.075 micron

IS 2386 Part I

Particle shape Flakiness & Elongation Index (combined)

Max. 40% IS 2386 Part I

Los Angeles Abrasion Value

Max. 40% IS 2386 Part IV Strength *

Aggregate Impact Value Max. 30% IS 2386 Part IV

Soundness (Sodium or Magnesium), 5 cycles

Sodium Sulphate Max. 12% IS 2386 Part V

Durability

Magnesium Sulphate Max. 18% IS 2386 Part V

Water absorption Water absorption Max. 2% IS 2386 Part III

* The coarse aggregate may satisfy either of the two strength tests.

2.3 Fine Aggregate

Fine aggregate shall consist of crushed mineral material passing 2.36 mm sieve and retained on 75-micron sieve. It shall be clean, hard, durable, and free from dust and soft organic and other deleterious substances. No natural sand shall be permitted.

2.4. Composition of Mixtures

When tested in accordance with IS:2386 Part 1 (wet sieving method), the combined aggregate grading shall fall within the limits shown in Table 2. As far as possible an aggregate with water absorption of 1.0 or less shall be used. The amount of residual bitumen binder (total cutback bitumen minus diluent such as kerosene) in the mix shall be

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as shown in Table 3. The stockpile mix shall be rejected if it does not meet the grading (especially the 0.075 mm sieve) and the minimum residual bitumen content. The produced mix shall be tested by an independent approved testing laboratory before its acceptance by the Engineer. Table 2. Gradation of Stockpile Patching Mix

Sieve size, mm Percent passing

9.5 100

4.75 40 – 100

2.36 10 – 40

1.18 0 - 10

0.075 0 – 2

Table 3. Minimum Residual Bitumen Content by Weight of Mix

Aggregate water absorption,

%

Minimum residual bitumen content,

%

Less than 1.0 4.5

1.1 to 1.5 5.0

1.6 to 2.0 5.5

Based on the characteristics of the aggregate and the performance of the mix, the Engineer can specify amount of residual bitumen higher than that shown in Table 3.

3. Preparation of Mixture

The stockpile patching mix shall be produced in a conventional batch type hot mix plant. The mix should be such that it can be stocked, handled, placed, and finished without stripping of the bitumen from the aggregate. To help prevent stripping and avoid heat buildup in a stockpile (which may burn the entire stockpile), the mixed material should not be stockpiled no higher than 1.5 m for the first 48 hours. The stockpile then can be raised in height and made conical in shape. The mineral aggregate should be clean and surface dry before mixing. The temperature of aggregate and bituminous material should comply with those shown in Table 4.

Table 4. Temperature Ranges for Producing Stockpile Patching Mix

Bituminous material Aggregate temperature, C Bitumen temperature, C

MC-250 5 – 65 55 -80

MC-800 5 – 65 75 - 95

Since the range of aggregate temperature is rather low and the maximum aggregate temperature is restricted to 65 C, it may not be possible to dry the aggregate within this temperature range. Therefore, the aggregate can be processed in a dryer at high

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temperatures and allowed to cool before the bituminous binder is added. Pre-drying the aggregate at high temperatures will also help in reducing the fines (material passing 0.075 mm sieve), which will go into the baghouse. The resulting mix then would have fines less than 2 % as required in the stringent gradation specifications. 4. Acceptance of Mixture

The composition of the produced mix (gradation and bitumen content) shall be tested by an independent, approved testing laboratory before acceptance by the Engineer. The following two tests shall be performed by the contractor (in presence of a Department representative) on the mixture, freshly prepared or taken from a stockpile at any time during its storage life (usually 6 months).

• Water Resistance Test (See Annexure I, Test C)

• Workability Test (See Annexure I, Test D) The water resistance test would indicate whether the patching mix has a potential for stripping in the pothole in presence of water. If the mix fails this test, it means a proper type and/or amount of an antistripping agent has not been used in the bituminous binder. If the mix fails in workability it could be due to improper bitumen type, low bitumen content, excessive fines or improper gradation. Stocked patching material may be rejected, at any time during the six month period if, in the opinion of the Engineer, the patching material has stripped (more than 10% uncoated particles) or otherwise become unfit for use. When the patching material has been delivered directly to a Department stockpile before Department approval, it will be the contractor's responsibility to remove any unacceptable material within two weeks of notification. Refusal by the contractor to remove unacceptable material from the Department stocking area will be sufficient grounds to suspend the contractor from the Department's bidding list for patching material, until such time as the problem is resolved to the satisfaction of the Department.

5. Measurement for Payment

The tonnage of the readymade stockpile patching mix shall be measured and determined from the actual plant batch reports as recorded by a representative of the Engineer assigned to the work.

6. Rate

The readymade bituminous stockpile patching material shall be paid for at the contract unit price per ton, FOB the hot mix plant, at the work site, or at other destination as specified in the contract. A reasonable amount of earnest money will be kept by the Department for 6

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months or on depletion of the stockpile, whichever is earlier, to ensure the product is satisfactory for use and meets all tests specified earlier during the life of the product.

AEXURE I

A. Wet Coating Test

Heat the unwashed job aggregate, cutback bitumen and distilled water to 40 C in a suitable oven. Weigh 100 g of dry aggregate into a suitable mixing container (such as seamless tin can, 16 oz capacity). Add 3 ml of distilled water. Mix thoroughly with a spatula until the aggregate particles are uniformly wetted. Add cutback bitumen equivalent to 5.0 +/ 0.2 g of bitumen residue. Mix rigorously with the spatula until all aggregate is coated, but not more than 5 minutes. Transfer the contents into a 400 ml beaker containing 150 ml of distilled water (22 – 32 C). Let stand for 15 minutes and visually determine the percent of retained coating, which should be at least 98 percent.

B. Static-Immersion Test

The coated aggregate as prepared in the preceding wet coating test shall remain immersed in the beaker of distilled water (22 – 32 C) for 24 hours. At the end of this period, visually determine the percent of retained coating while the sample remains immersed in water, which should be at least 95 percent.

C. Water Resistance Test

Fifty grams of patching mix, whether freshly prepared or taken from the stockpile, shall be heated at 120 C in a laboratory oven for 1 hour, cooled to 95 C in laboratory air, and then placed in 400 ml of boiling water in a 600-ml glass beaker and stirred with a glass rod at the rate of 1 revolution per second for 3 minutes. The water shall be decanted and the mix shall be spread on an absorbent paper for visual observation of the coating. The aggregate shall be at least 90 % coated with a bituminous film.

D. Workability Test

Approximately 2.5 kg of the patching mix shall be cooled to –7 C in a freezer. After cooling, the mixture shall be capable of being broken up readily with a spatula that has a blade length of approximately 200 mm. This test shall be performed when the mix is produced and thereafter anytime during storage. If the mix is not workable at –7 C, it shall be rejected and the composition of the mix shall be properly modified (for example, by increasing the bitumen content and/or gradation changes). This test is also applicable in areas with hot climate because it amplifies the workability characteristics of the mix by using a lower test temperature.

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Response to Comments on Paper o. 544, “A Simple and Effective

Method of Repairing Potholes in India” by Author: Prof. Prithvi Singh

Kandhal

[The following response to comments made on the paper were published in the Journal of the Indian

Roads Congress, Volume 69-4, April – June, 2009. ]

Shri Bhattacharya has asked if there is any effect if the pothole is cut in a regular shape with vertical edge. The proposed cold mix is capable of being placed without any preparation such as cutting to a regular shape and making a vertical edge. It has a 4-year survival rate of 78% as mentioned in the paper, which is quite satisfactory considering that the pothole does not require any preparation. No comparative performance studies have been conducted on the use of this mix in potholes with and without vertical edges. Shri Bhattacharya should try this mix first without any preparation. Concerning deep potholes, it is mentioned in Section 5.1 of the paper that mix should be placed in about 75 mm thick layers, each layer to be compacted with a hand rammer. In response to Shri Biswas, if the mix is properly placed and compacted it can be and has been used on high-speed highways based on experience in the US and NH 11 between Jaipur and Agra. It should be ensured that the compacted mix is about 10 mm above the existing road surface to allow for some additional compaction by traffic. Shri Ghosh has asked whether the mix prepared with cutback bitumen will lose its binding quality when stored for six months. If the mix is stored in sealed bags, there is no loss of kerosene type solvent used in the medium curing cutback MC-800. In case the mix is stockpiled under an open shed, the stockpile forms a thin, hard crust due to loss of solvent. As mentioned in Section 5.1 of the paper, this hard crust prevents any further loss of solvent in the cutback bitumen and the mix remains pliable for six months. This hard crust is easily broken and used with the soft material. If possible, the mix should be kept under an open shed to avoid direct sun. Shri Jain is referred to Section 5.3 of the paper, which gives a detailed, comparative cost analysis of the cold pothole repair mix. Repairing potholes does require money, but it has to be done to maintain the serviceability of our roads and streets for the convenience of public. The author agrees with Shri Mahajan that it was unfortunate no discussion was held on this paper at the IRC session because the session started late. However, the author strongly disagrees with his statement that “the theory presented may not be applicable in all cases and in all weather conditions”. It is not a “theory” but a real mix which has been used successfully in the US for several years under different climatic conditions (hot/cold, dry/wet) more severe than those prevailing in India. Moreover, this mix has now been used successfully during monsoons on NH 11 (Jaipur-Agra Section) and on Jaipur city streets.

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All this is well documented in the paper. One can safely call for tenders for supply of this mix today in India based on the specifications appended to the paper. The author agrees with Shri Rao that our roads should be free of potholes. But why at least once in three years, why not all the time? If a highway agency has a stock of this cold mix, potholes can be repaired as and when they develop even during monsoons without waiting for hot mix. In response to Shri Sahay, it is stated that this pothole mix is also suitable for deep potholes. However, as mentioned earlier, the mix should be placed and compacted in layers not exceeding about 75 mm in thickness. For very deep potholes such as 0.3 to 0.5 m in depth mentioned by Shri Sahay, it is recommended to fill the potholes with WMM first and compact it and then fill the top 150 mm with this mix in two layers. Shri Vakharia has stated that cold patching material is not popular for patch repairs in India because it is costlier than a typical hot mix. As per detailed cost analyses given in Section 5.3 of the paper, this cold mix is about 55% more expensive than a conventional BC hot mix FOB plant. However, it is still very cost effective considering that the mix is stockpileable and can be used throughout the year including monsoons. Unlike hot mix, there is no wastage because the unused mix can be brought back and redeposited on the stockpile. It can also be used to repair a very few (say 2 or3) potholes for which sending a truckload of hot mix is neither justified nor economical. Right now, our engineers usually wait until a large number of potholes are developed to justify a truckload of hot mix. Even then, by the time half truck load is used, the hot mix in the truck usually cools down and the resulting hot mix patch repair is not durable. Also, unlike hot mix, placement of this cold mix does not require pothole preparation such as squaring it, cleaning it, and applying tack coat. All these operations cost money. Considering all these factors there is no question this cold mix is highly cost effective compared with hot mix. As mentioned in the paper, proprietary cold patching mix being sold in bags costs about Rs. 13,000 per ton, which is more than 3 times the cost of this generic patching mix which can be made with local materials. [See the addendum to the paper by the author in Section 5.3, in which the most recent cost

analyses have been given as of July 2010. The cost of cold mix was determined to be 21.8%

higher than that of the hot mix FOB hot mix plant. The cost of in-place cold patching was

determined to be only 5% more than that of the in-place hot mix patching. If other factors

such as deterioration of potholes during monsoons, wastage of mix due to cooling, costs of

user delays, extra fuel wasted, etc. are considered, the cost of cold in-place patching is

estimated to be less than half of the cost of in-place hot mix patching.]

Shri Vakharia should please note that the medium curing cutback contains about 80% bitumen and 20% kerosene type solvent, there is no water in it. In closing, the author would like to add the following. Since the paper was published, there is growing interest in this generic, cheap cold patching mix by some contractors who have to maintain BOT and PMGSY roads during the concession or warranty period. As some isolated potholes develop on these roads, a bag or two of this mix can be carried even in a

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car and potholes repaired promptly and effectively. These contractors cannot afford to procure small amounts of hot mix because of logistics, cost and/or unavailability. If there are any questions/problems associated with the production and usage of this mix the author can be contacted for voluntary assistance via e-mail kandhps@auburn.edu It is a mix with proven performance only if it is produced strictly in conformance with the specification appended to the paper. There are plenty of excuses for the people who do not want to try and/or use this mix. Some will say it has to be evaluated by CRRI or it has to be specified by the IRC. They do not see any problem in continuing to use soil, bare aggregate, etc. to fill the potholes or doing nothing causing misery to public. What is needed now is the will and determination of our young engineers to use it on our rural roads and urban streets so that the highway community does not get a bad name every year due to roads and streets dotted with potholes and also the motoring public is served. As a start, they can use a small concrete mixer to make about ten 50-kg bags of this mix (it would cost less than Rs. 1500) following the recommended specification and try the mix in some potholes even when the potholes are wet.

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