Raising environmental concerns, fuel costs and the steadily declining natural resources affect not only the cost of asphalt mixes per se, but is forcing the industry to go green. Today, every professional in the field understands that going green is not just a fashion statement or eyewash, but it is posing a very crucial question about survival. It is impossible to be profitable without being green.
There are numerous methods that the road building industry has adapted to reduce their energy consumption and to be more carbon friendly. The urgency is so great that at each level starting form equipment design to production and laying techniques, focus has to be consistently placed on reducing the carbon emission.
This technical note describes the various techniques available and presently being implemented world over for production of Low energy asphalts or Carbon friendly mixes. Production techniques for warm and half warm mix asphalt either use chemical or mechanical solutions. The modifications made to existing plants in order to produce low energy asphalt by chemical methods (additives) are minor, whereas more significant modifications must be made to mechanical methods (foam bitumen or sequential coating). This paper covers the possible solutions and provides A summary on its advantages.
BeginningsIt is highly unlikely that we can put an exact date on the emergence of the concept. Since the mid 20th century, and the industrial development of hot mix asphalt (HMA), engineers have had a cheap energy source at their disposal with the environment only a minor concern. Good Practice stipulated that the asphalt had to be around 150°C. Since bitumen is hydrophobic, coating is that much easier when the aggregates are dry. The European production standard nevertheless states that the maximum residual water content of HMA is 0.5%. In the 1970s, when TSMs (dryer drum mixers at parallel flow) developed, excellent results at 130°C were nevertheless noted, with a residual water content of 1%, especially during compaction.
Coating attempts with foam bitumen subsequently developed in the 1990s in the UK, South Africa, the Netherlands and France. The concept of warm and half warm mix asphalt was first mentioned in the late 1990s, particularly by JENKINS , and alternative techniques to foam bitumen flourished in all countries, notably Scandinavia, with sequential coating techniques using several types of bitumen.
A typology of the mixes was officially described by Dr. Jacques Bonvallet from FAYAT Group in 2000 , with reference to the heating and drying energy used during the process. The 100°C border is widely discussed and there are two schools of thought: one that recommends staying shy of the 100°C mark for the sake of making vaporisation energy savings (1/3 of the total energy), and the other which recommends vaporising water while remaining within the 110 to 130°C range, which also saves around 30%.
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