Now, in road construction in India, flexible pavement is favored over cement concrete roads due to the fact that it may be reinforced and enhanced in stages as traffic increases. Additionally, the surfaces of these roads may be milled and recycled for rehabilitation, which is a significant benefit. Flexible pavements are also less costly in terms of initial investment and upkeep.
The ultimate characteristic of bitumen is its rigidity, but the original method of evaluating bitumen was by eating it in the mouth; hence, the test temperature at that time was 37°C (which is the temperature of human body). In 1903, the American Society for Testing and Materials (ASTM) authorized the grading of bitumen by means of a 25°C penetration test. The lower penetration value suggested bitumen that was firmer, whereas the greater penetration value indicated bitumen that was softer. The penetration-based grading system lasted until 1970 and is still in use in many nations, including India (for road construction in India), to this day. In 1970, a viscosity-based grading system was adopted in the United States to solve several construction issues, such as the tender mix caused by 60/70 penetration grade bitumen (because to its low viscosity at 135°C) and performance-related issues, such as rutting at high pavement temperatures.
Based on penetration values, the British Standard (BS: 12591) defines nine classes of bitumen. In BS: 12591, viscosity at 60°C and 135°C are optional qualities, in addition to wax content, Frass Breaking point, and Aging Properties. Currently, ASTM:D-946 defines “Penetration-Based Specification” for bitumen, but ASTM:D-3381 specifies “Viscosity-Based Grading System.” Similarly, the AASHTO standard provides two grading systems, creating a hybrid grading system. Some U.S. states adopted the hybrid grading system in 1970, prior to moving to the viscosity-based system. This hybrid system in the United States reportedly failed for the following two reasons:
- Bitumen providers were unable to concurrently meet both criteria for road construction.
- Performance could not represent superior quality.
The climate determines the performance grade bitumen used for Super Pave. PG-64-16 bitumen, for instance, is acceptable for a site where the average maximum pavement temperature for 7 days is as high as 64°C and the lowest pavement temperature is as low as -16°C. Therefore, PG-64-16 bitumen will be required for a project site in India with a maximum 7-day pavement temperature of 63°C in summer and a minimum 7-day pavement temperature of -1°C in winter. CRRI experiments have shown that Performance Bitumen and Viscosity Bitumen grades are relatively similar.
Indian Situation and Related Issues
In road construction in India, bitumen is graded based on a penetration test done at 25 °C, and penetration bitumen 60/70 is often utilized. Developed more than a century ago, the empirical penetration test consists of allowing a conventional penetration needle loaded with 100 grams to enter bitumen (kept at 25°C) for five seconds. The penetration is measured in millimeters and represents the bitumen’s relative hardness. Bitumen becomes softer as penetration increases. A penetration of 0.6 to 0.7 millimeters suggests bitumen with a 60/70 penetration grade.
Summertime rutting is the most prevalent issue with the performance of bituminous concrete roads (50 mm or thicker) around the globe, including India. In the 60 to 70°C temperature range (average road surface temperature on a hot summer day), the bitumen becomes soft and begins to push and shove beneath heavy truck tires, causing rutting and corrugations in the roadway’s wheel tracks.
Bitumen produced from various petroleum crude sources and/or refining procedures may have the same penetration grade at 25°C but dramatically varying hardness between 60 and 70°C. Those that are very soft (low viscosity) are more susceptible to rutting/corrugations than those that are not as soft (high viscosity). Therefore, it is apparent that the consistency (viscosity) of the paving bitumen at high temperature (such as 60°C) must always be measured in order to identify whether or not it would induce rutting. Due to this issue, a requirement to test and grade bitumen viscosity at 60°C (rather than 25°C) was adopted in the United States around 30 years ago. At 60°C, the viscosity of bitumen is measured in poises using a simple viscometer. The asphalt cement (bitumen) viscosity grades AC-30 Grade, AC-20 Grade, and AC-10 Grade were developed. In the 1970s, these were introduced in North America according to climate, with greater viscosity classes chosen for hotter climates.
In India, the current bitumen specification has a 60/70 penetration grade, whereas until recently it was bitumen 80/100 and equivalent to an AC-10 viscosity grade. The use of the softer AC-10 grade is suitable for colder climates but is unacceptable for India because it is extremely soft and will increase the likelihood of rutting caused by heavy trucks. To reduce the rutting potential of our pavements, it is necessary to use at least an AC-30 grade in India, and for southern India, an AC-40 grade.