Optimising Bitumen Performance
Background
Petroleum based bitumen is a product of consistently high quality. Its production involves a heavy investment in high-technology equipment and a commitment to quality management at all stages of crude oil selection, manufacture, blending, storage and distribution. By retaining and checking samples of consignments of bitumen, the composition and quality of the bitumen at the time it was delivered to the contractor can be confirmed.The actual performance of the bitumen when it is doing its job is just one part of the total performance of an asphalt mixture or sprayed seal. Asphalt performance depends on a number of factors, including the design of the mixture, the way it was mixed, the quality of mineral aggregate used and how it was laid and compacted. Sprayed seal performance depends on similar factors and can be significantly affected by weather conditions at the time of spraying and the cleanliness of the aggregate. As bitumen is an integral part of both asphalt mixtures and sprayed seals, advice on how to avoid and correct errors would need to cover many factors and types of applications. This subject is outside the scope of the information provided on this website.
However, it is possible to put forward the following essential guidelines to optimising the performance of bitumen, mainly in hot mix asphalt, on the basis of what is known about bitumen itself. Some of the points are mentioned elsewhere but they have been repeated in this section for convenient reference.
General Advice
Minimise the time during which hot bitumen is exposed to air. Bitumen hardens and loses some of its adhesive property when it is exposed to a combination of air and excessive heat. Careless mixing of bitumen and aggregate is an example of a practice that exposes bitumen to the air and heat for longer than the minimum time required to coat the aggregate. It is therefore likely to reduce the bitumen’s performance as an adhesive.Keep bitumen dry and away from contamination by water at all times. When hot bitumen comes into contact with water it will foam. In extreme cases it will violently boil over from storage vessels due to the presence of small quantities of water.
Keep bitumen within the range of temperatures recommended for the type and grade. Bitumen solidifies as it cools and becomes unworkable and the viscosity may increase due to oxidation if it is overheated. Keep asphalt at the lowest temperature that will permit it to be thoroughly mixed, transported and compacted to the specified density. The maximum bitumen storage temperature relating to the grade should not be exceeded during the mixing process.
Temperature control is a major factor in the quality control of asphalt mixes.
Avoid contaminating the bitumen, particularly with petroleum
based solvents that reduce the flashpoint and the viscosity,
and increase the penetration. Just 0.1% diesel in an asphalt
mixture can lower the flash point and viscosity, and increase
the penetration significantly.
Stability is an essential quality in the performance of asphalt. It depends on friction and cohesion within the mixture. Too much bitumen will act as a lubricant for the aggregate particles, reducing friction and therefore the stability of the mix. The cohesion, or internal binding force in the mixture, increases as the proportion of bitumen in the mixture is increased, until it reaches the optimum. If the amount of bitumen increases further, stability decreases. It is therefore essential to ensure that the optimum proportion of bitumen is used in any mixture.
To quote the Asphalt Institute of America directly: “Finished mix should be frequently observed. No testing method or device is as quick and convenient as the human eye.”
The following are useful examples of what can be seen by simple observation. They can be treated as early warning signs that further checks should be made:
Stability is an essential quality in the performance of asphalt. It depends on friction and cohesion within the mixture. Too much bitumen will act as a lubricant for the aggregate particles, reducing friction and therefore the stability of the mix. The cohesion, or internal binding force in the mixture, increases as the proportion of bitumen in the mixture is increased, until it reaches the optimum. If the amount of bitumen increases further, stability decreases. It is therefore essential to ensure that the optimum proportion of bitumen is used in any mixture.
To quote the Asphalt Institute of America directly: “Finished mix should be frequently observed. No testing method or device is as quick and convenient as the human eye.”
The following are useful examples of what can be seen by simple observation. They can be treated as early warning signs that further checks should be made:
- blue smoke coming from the mixture can indicate overheating
- steam coming from the mixture indicates the presence of too much moisture
- if the distribution of bitumen on the aggregate is not uniform, the temperature in the mixing operation may be too low
- if the mixture looks stiff and dull rather than black and shiny there may be too little bitumen in the mixture, or overheating has excessively oxidised the binder
- if the mixture lies slumped on the floor of the truck the mixture may contain too much bitumen
