Warm-mix asphalt technologies (WMA): an alternative to traditional hot-mix techniques
While the reputation and versatility of hot-mix asphalt has now been clearly established, there may nevertheless be a place for alternative solutions in the asphalt mix landscape.
“Warm” solutions – for which the finished product temperature is between 100°C and 130°C – have risen to the challenge of matching the performance of traditional asphalt while consuming less energy and emitting fewer greenhouse gas emissions.
After many years in which different technologies were developed, such as the addition of liquid or solid additives and the use of sequential coating techniques, foamed bitumen-based formulas have eventually become the preference of a large proportion of road builders on grounds of the easy implementation of this solution and the immediacy of its results.
Additive-based solutions allow for the lowering of the aggregate-coating temperature in different ways, e.g. by modifying the chemical properties of bitumen in order to reduce its viscosity, or by releasing small amounts of water at given temperatures, thus generating a foaming effect.
Solid additives are available in the form of powders or fibres. When they are packaged in bags, the additives may be poured directly into the mixer. When supplied in bulk or in big-bags, they are firstly stored in dedicated silos, metered by volume (via a rotary valve) or by weight (via a weighing screw) and then conveyed to the injection point by a screw or a pneumatic system.
Liquid additives are stored in dedicated tanks, which are sometimes equipped with a heating system if the additive requires heating so that it can be conveyed to the injection point. They are then pumped, metered via a volumetric or mass flow meter and then injected, either into the bitumen line, or directly into the mixer.
When the bitumen is brought into contact with small amounts of water before being added to the mixer, it changes state and becomes foam. The bitumen may then expand up to 20 times its initial volume, and one of the remarkable properties of this state allows it to coat the aggregates at significantly lower temperatures than normal.
Specifically, the foam kit consists of a water metering and (pressurised) injection system, a foam generator fitted to the bitumen line next to the mixer and an automated process management system, which may be a separate component or an integral part of the asphalt plant’s management software.
While foamed bitumen is now a perfectly controlled technique, it remains a cutting-edge technology, both in terms of the accuracy required for metering the reaction water and the importance of keeping the injection nozzles perfectly clean – which are two key points of the AQUABlack design.