Automated systems for today’s specialized asphalt mixes
In recent years the adverse impact of traffic on the country’s road network has increased significantly and will continue to do so. This trend is being caused not only by higher traffic volumes, but also by increased axle loadings and vehicle weights. These conditions have resulted in traditional road-surfacing materials being unable to meet current demands, resulting in high wear rates and costly reinstatement programmes.
Today’s road construction industry, however, is gradually being transformed by the more widespread use of a new range of specialized, performance-related asphalt surfacings. The introduction of certified procedures for thin surfacings and the restriction by the Highways Agency in the use of chipped, hot-rolled asphalt has resulted in a trend away from conventional mixes, with an increase in the use of alternative thin surfacings. A new generation of stone-mastic asphalt (SMA) ‘designer’ mixes has emerged as a result of advances in the technology associated with asphalt additives and modifiers, together with the parallel development of efficient, safe and accurate material-handling systems.
This range of advanced asphalt surfacings is the result of several other factors. For asphalt producers, who for many years have suffered in a relatively low-growth market, these new proprietary surfacings have been seen as an opportunity to increase their range of products to meet customer-driven specifications resulting from more stringent criteria laid down by the Highways Agency. They have also, of course, increased the value of their businesses.
SMA originated in Germany in the mid-1960s and proved extremely effective in combating wear and resisting damage from studded vehicle tyres. In view of its excellent characteristics and performance, a national standard was set up in 1984. Since then its popularity and use has spread throughout the countries of Europe, many of which have drawn up their own individual material quality standards.
SMA is a gap-graded, high stone content asphalt mix comprising a lattice of single-sized coarse aggregate in which the voids are filled with a bitumen/fine aggregate/filler mortar in a similar composition to a traditional UK-specification hot-rolled asphalt mix. The design of the mix after laying and compaction provides a voids content by volume of between 2–6%. This ensures that any mortar that migrates within the mix, as a result of traffic densification, will have space to fill.
The successful production of SMA has only been made possible with the use of stabilizing additives in the form of highly specialized cellulose fibres that prevent excessive drainage of the binder in the mix.
A relatively high bitumen content is used in the asphalt mortar to ensure good durability, and, to prevent the possibility of binder drainage from the hot mix, a stiffening agent (additive) in the form of cellulose fibres or a polymer is added to the binder. These fibres knit together, resulting in less drainage and higher elasticity.
SMA is a wearing coarse which, because of its inherent design structure, offers important functional, economic and technical advantages compared to conventional mixes. It provides excellent ride characteristics, combining high stability with high durability and, although applied in thin layers, gives an extended life span compared to conventional mixes.
During initial laying SMA has a lower requirement for aggregate due to its thinner construction and durability. It gives longer service life with lower maintenance and in wet weather offers better skid-resistance and a reduced risk of surface water — which can contribute to vehicles aquaplaning — because the water is held within the material rather than on the road surface. SMA provides tougher, quieter, more durable road surfaces, with less impact on the use of valuable environmental resources.
The cost per tonne can be slightly higher than that of a standard asphalt mix due to the higher binder content, high-quality aggregates and the need for a modified bitumen together with a drainage inhibitor. Tighter quality control of the asphalt production unit is essential to ensure the accuracy of batch weighing. With the recent launch of product approvals for thin-surfacing systems under the Highways Authority’s Product Approval Scheme (HAPAS), asphalt producers are now able to define and clarify these new performance specifications, thereby improving standards throughout the UK.
During the early 1990s asphalt producers in the UK started to develop these new surfacings, most of which required some form of additive as a modifier. This progress was accelerated with the advent of more widespread pelletization processes, which meant that certain materials could be handled more easily.
One of the major obstacles to the wider use of additives in the UK has been the effect of health and safety requirements at asphalt production sites, together with the lack of suitable automated handling systems capable of providing an accurate and safe means of transferring these additives directly into the asphalt mixer during the production cycle.
Early attempts to add materials into the mix centred on pre-weighed bags which were thrown manually into the paddle mixer by operatives working in a hostile and unsafe environment. Other asphalt producers used bag-splitters and conveying devices, which also gave rise to inaccurate weighing, together with the physical drawback of manhandling bags of 500kg and 1,000kg up to the mixer level, via hoists. As well as being unsafe and unpopular with operators, these methods did not ensure the addition of an accurate and consistent percentage of additive to every batch.
Since 1995 Allchem International Ltd, the London-based chemical suppliers, have been responsible for the introduction of specialized branded stabilizing additives into the UK market. With increasing demand for such products, Allchem recognized that the introduction of a cost-effective material-handling system would maximize the product benefit offered to customers. Consequently, in collaboration with two leading specialist suppliers of pelletized fibre additives and handling systems, Allchem developed — under the Flomac name — their own fully automatic systems for the metering, pre-blending and conveying of pelletized additives (cellulose fibres, pigments, binders etc) used in the production of thin surface dressings, SMAs and porous asphalts.
These systems were developed from field trails on the use of additives by asphalt producers, which, combined with Allchem’s own pelletized additive technology, produced an effective means of safely adding materials into a paddle mixer. This has resulted in more widespread use by local authorities of thin surfacings and coloured asphalts throughout the road network for cycleways, bus lanes, road junctions etc.
Seeing the benefits of such mixes, many asphalt producers have embraced this new technology and now offer specific, Highways Agency-approved, SMA-based designer mixes under their own individual brand names. Information sheets produced by the suppliers give mix characteristics, technical data and application suitability. The trend towards these customer-driven mixes, to suit a constantly changing and expanding market, has led to an acceleration of research and development by the suppliers in a drive to push ahead with more performance-based products. In terms of economics, these specialized products, which have found several niche markets, are becoming more popular not only because of the value-added price per tonne, but also because of the high-performance characteristics associated with their use. Interestingly, a high percentage of asphalt plants now sold have additive-handling systems included in their specification.
In basic terms, these materials can be added to a specific asphalt mix to change its characteristics and, for a number of reasons, determined by the additive used, greatly improve the end product.
Additives can be in the form of loose fibres, powder pigments or polymer-modified bitumen (PMB). Because of the associated health and safety implications of handling loose fibres and powder pigments, the industry was forced to adopt the safer, free-flowing pelletized alternatives.
Advances in polymer technology have resulted in the development of performance products that offer the advantage of allowing modification on site, rather than the customer buying-in a branded modified bitumen and then having the separation problems associated with the storing of these substances.
The two types of polymer in general use are SBS (styrene butadiene styrene), which increases cracking resistance and is generally more tolerant at low temperatures, and EVA (ethyl vinyl acetate), which increases surface resistance to rutting and ravelling and has more elasticity to cope with higher temperatures. Significantly, modern, highly efficient material-handling systems that are capable of accurate metering have given site managers the option to modify mixes on site and to decide which particular polymer best suits the customer’s application.
Granules are easy to deliver in bulk with no manual handling needed. They are free-flowing and their consistent pellet size and product density allow for easy conveying and accurate weighing by volume. The bitumen held in the pellets gives rise to little or no dust emission during production runs.
In recent years the use of rubber crumb in mixes has become more widespread, particularly for surfacings used in the growing leisure market. When laid as a surface treatment on sports areas it provides the shock-absorbing element that is vital for cushioning athletes’ feet against impact. On children’s play areas, a rubber-crumb treatment provides a safer playing surface, thereby reducing accident levels. Standard addition is between 15% and 20% of the total mix.
There are several types of delivery system available to asphalt producers, each designed specifically for differing customer needs in terms of price and specification. Flomac have become brand leaders in such equipment and offer proven systems based on an accurate batch-weighing and material-transfer unit with associated storage silos.
Flomac Premium or Multi systems
The Flomac system has been designed to handle one or more materials. Each is stored in a bulk silo and conveyed within an enclosed centre-less auger to a batch weigh hopper. From here the material is transferred by a positive-pressure pneumatic conveying unit to the mixer. With this arrangement, material is weighed gravimetrically in controlled conditions using high-accuracy load-cells. This allows multiple ingredients to be batched (ie pigment pellets, binders and other additives), thus maintaining precise batch quantities and lower production costs. The system, once linked to the main asphalt plant controls, is fully automatic, thereby preserving plant mixing-cycle times.
The system offers constant batch production as well as the option to handle multiple additives via one system, as opposed to having individual systems for each additive. This saves on costs and space, and the equipment is easier to link into an existing plant control system.
The following is a general guide to the types of additives and modifiers available:
- cellulose fibres —highly fibrillated cellulose-fibre pellets, designed to stabilize the asphalt mix and help prevent binder drainage
- pigments (iron oxide red, green, yellow, red, blue, white)—100% dust-free premium-grade iron oxide pellets and other pigment pellets disperse faster in the asphalt mix and give a stronger colour than powdered pigments
- polymer modifiers (EVA,SBS, latex) —EVA in pellet form and SBS and latex in free-flowing crumb form for direct addition into the ??asphalt mixer box, offering better control over polymer content in the bitumen mix
- clear binder —synthetic asphalt binder in pellet form giving colouring flexibility and brighter coloured asphalt surfacings
- rubber crumb — high-quality recycled car and truck tyres for maintaining binder elasticity and reducing deformation, noise pollution and moisture damage. Used in sports tracks and play areas
- adhesion modifier — wetting and adhesion agent in flake form, giving improved bitumen–aggregate bonding, greater viscosity and better temperature performance
- uintaite (natural asphalt) — cost-effective alternative to using polymers for basecourse or wearing- course applications. Natural asphalt stiffens the binder and gives greater resistance ??to deformation.
Recent examples of Flomac systems in use in the UK:
At Tarmac’s Bankfield Quarry near Clitheroe, Lancashire, a Flomac Premium Multi system was installed to handle the introduction of both iron oxide pigment and cellulose-fibre pellets into the plant’s paddle mixer. The system stores, meters and weighs the materials needed for individual mix specifications and is positioned adjacent to the asphalt plant. A fully galvanized steel bulk silo, which can be filled automatically via a connection to a road tanker, stores the fibre pellets. Alongside this, red oxide pellets are stored in a 3m3 capacity silo which is gravity fed from above, from 500kg and 1,000kg bags, using a fork-lift.
When the controls activate the system, the material destined for the mix travels by screw conveyor to a batch weigh hopper located within the Flomac unit. After weighing the material is transferred by a positive-pressure pneumatic conveyor directly into the mixer box. The system is linked into the main plant controls to ensure accurate batching within the plant mixing cycle.
The handling system was chosen by Tarmac because, with more stringent health and safety legislation being imposed, the equipment provides a safer and easier means of adding material to the mix, giving consistent batches with accurate weighing cycles.
In the Midlands, at Aggregate Industries’ Croft Quarry in Leicestershire, another Flomac system allows the use of specialized pigments. With this Multi system there are two separate storage silos; one holds fibre pellets and the other contains a red pigment additive. When the system is called upon to deliver one of these materials into the mix, it travels via a centre-less auger to a Compac batch-weigh unit where the material is accurately weighed by a gravimetric system with load-cells and then discharged by rotary valve into a blowing line and transported through ducting at the side of the plant up into the mixer box via a volumetric blower system.
Existing plant systems
Most new asphalt plants commissioned within the UK market have been specified with some form of additive-handling system but, as the market for designer asphalts grows, operators with existing plants have also been made aware of the benefits of providing a wider range of specifications and are adding these handling systems to their plants. All Flomac systems have a design configuration which is easily adaptable for retrofit installation, while the use of standard components means systems can be customized to suit individual plant layouts.
As technology advances in the use of surface coatings, a combination of both legislation and economics is likely to push forward the more widespread use of additives and modifiers in bituminous mixes. This in turn will accelerate developments in efficient material-handling systems for the next generation of asphalt plants, be it in hot- or cold-mix technology.