Warm mix asphalt is said to offer many benefits including fewer emissions and energy savings, but further research is needed to "validate its expected performance and added value" claims one organisation. Patrick Smith reports
Late last year, the United States Federal Highways Administration (FHWA) noted in a report that European countries "are using technologies that allow a reduction in the temperatures at which asphalt mixes are produced and placed."The warm mix asphalt (WMA) technologies, says the FHWA, offer an immediate benefit by reducing the energy consumption to heat traditional hot mix asphalt (HMA) to temperatures in excess of 150°C at the production plant. Such temperatures are needed to allow the asphalt binder to become viscous enough to completely coat the aggregate in the HMA, have good workability during laying and compaction, and durability during traffic exposure.
Mix production for WMA can typically be done at temperatures of 10-37°C below the standard HMA temperatures of 149-176°C.
However, notes the FHWA, the additional benefits with the decreased production temperature include reduced emissions from burning fuels, fumes, and odours generated at the plant and the paving site.
"Warm asphalt mixes will also allow longer haul distances and a longer construction season if the mixes are produced at more normal operating temperatures. There is another potential added advantage in that oxidative hardening of the asphalt will be minimised with the lower operating temperatures and this may result in changes in pavement performance such as reduced thermal cracking, fatigue cracking, and preventing the mix to be tender when placed," says the FHWA, which looked at a number of WMA technologies, some using special additives, and all of which appear to allow the production of WMA by reducing the viscosity of the asphalt binder at a given temperature.
"This reduced viscosity allows the aggregate to be fully coated at a lower temperature than what is traditionally required in HMA production. However, some of these technologies require significant equipment modifications," says the FHWA.
Various projects using WMA have been carried out, and a Warm Mix Asphalt Technical Working Group (WMA TWG) initiated by the National Asphalt Pavement Association) and FHWA has been set up to evaluate and validate WMA technologies and to implement proactive WMA policies, practices, and procedures that contribute to a high quality, cost effective transportation infrastructure.
However, the FHWA adds: "This technology could have a significant impact on transportation construction projects in and around non-attainment areas such as large metropolitan areas that have air quality restrictions. The reduction in fuel usage to produce the mix would also have a significant impact on the cost of transportation construction projects.
"The benefits of these technologies to the United States in terms of energy savings and air quality improvements are promising but these technologies need further investigation and research in order to validate their expected performance and added value. It is important to note that producing HMA at lower temperatures is the desired product to achieve these benefits, not the particular technology that is used to produce the WMA mix."
The
The compact plant can work in restricted site applications in built-up areas while its low emissions allow it to meet tight air quality regulations required for many urban locations.
The plant manufactures roadbuilding materials, including HMA and its environmentally-friendly credentials are further boosted because it can use the innovative
The easily transported plant is quick to set up, has a comparatively small footprint and cuts transport costs and fuel consumption by reducing the need to transport materials to a site from a conventional fixed plant.
Its drum features three zones, for loading material, drying and heating and mixing. A feeder system controls the flow of aggregates, while the angle of the drum can be varied to delay material output. The binder temperature is limited to 130°C, to avoid altering the binder characteristics, with the binder temperature being maintained by a heat transfer fluid. The temperature is some 40°C lower than that required for conventional asphalt supply methods, thus significantly reducing energy consumption/tonne produced. The low temperature also means that emission levels/tonne are far lower from the Beluga equipment (both from the heating system and the material) than for a conventional plant. Emissions are virtually eliminated by a telescopic exhaust stack that features 19m² of flitration in all.
The plant produces material in 1tonne batches and has an output of 7tonnes/hour (for standard aggregates and a moisture content of 3% to 4%), so it is not suited to large scale paving operations, but Fayat believes the Beluga provides a highly efficient, low emission and low environmental impact solution that offers lower running costs than conventional plants in urban road repair operations.
Terex WMA system
"Since its introduction in mid-2008, dozens of Terex WMA systems have been installed in the field, successfully reducing mix temperatures by 10-320C," says the company.
The system design consists of two main components: an expansion chamber and a programmable logic control (PLC) skid package. The single mixing point expansion chamber is based on proven foamed asphalt technology pioneered by Terex in 1998. Once the hot AC and water combine in the expansion chamber, the foamed asphalt is immediately injected into the drum via an injection pipe that has a variable number of spray nozzles to evenly and thoroughly coat the heated aggregate.
For field installations, the warm mix PLC unit comes complete in a water skid package, and it interacts with plant controls to meter a predetermined percentage of water with liquid AC. The skid also includes an 832.8litre water tank and a variable frequency 3.73kW electric motor that drives an accurate water pump.
Terex says its WMA system can be installed on any unitised counterflow drum mix plant, and for new plant purchases the control unit is integrated into the plant controls package. Also contained in the skid package, a water meter verifies the correct percentage of water is injected with the hot AC inside the expansion chamber.
"By incorporating the meter, the Terex system goes a step beyond most other foamed asphalt units, which leave the operator to assume the correct percentage of water is added," says Joseph Musil, product manager at Terex Roadbuilding.
The Terex warm mix system works by injecting a variable amount of water with hot oil inside the exclusive expansion chamber. The system injects and mixes up to 4% of water by mass weight of the liquid AC. Higher percentages of water result in more asphalt foaming action.
"In typical applications, producers inject 2% water," says Musil.
The Terex system can produce up to 544tonnes of asphalt at a 4% injection rate, and producers can quickly switch between running warm and traditional hot mix designs with the new system.
"Mixing at lower temperatures helps to reduce odour and particulate, carbon dioxide and nitrous oxide emissions at the plant, while at the same time virtually eliminate 'blue smoke' at the truck and paver. WMA helps to reduce the plant's fuel consumption. Additionally, lower temperatures inside the drum potentially result in a longer drum shell life," says Terex.
Green Pac System
The company says that by offering the Green Pac System it expands the benefits of its exclusive Double Barrel Green warm mix system to any asphalt plant owner or operator.
Astec says that the Double Barrel Green system saves energy and eliminates smoke and emissions without compromising mix quality. It uses water to produce a foamed warm mix asphalt that is odourless, smokeless and longer lasting.
"The Double Barrel Green System does not require the addition of expensive commercial additives. Instead, the injection of water along with the liquid asphalt cement causes the liquid asphalt to foam and expand in volume. The foaming action helps the liquid asphalt coat the aggregate at a temperature that normally is in the range of 230-270°F versus traditional temperatures of 300-340°F.
"With the Green Pac System, an operator can decrease fuel consumption 14% by decreasing the mix temperature just 50°. Other benefits include the elimination of smoke and smell, an increase in production, and the ability to run higher percentages of recycled asphalt pavement (RAP)."
Patented RED dryer
KVM unveils green credentialsDanish company KVM International has launched its newly-developed, patented equipment for the temperature measurement of aggregates in drying drums.
The company, which produces a variety of equipment, including asphalt plants and solutions for the concrete industry, is eager to expand its markets internationally and promote its new energy-saving 'green production' system.
Erik Clausen, director, asphalt, said the system consists of four temperature probes fitted in the drying drum which, via a wireless transmitter, send the signals to a receiver. This is placed in the control cabin where the signals are transmitted to four displays showing the temperature for each probe.
"By combining several process parameters with these temperatures, it is possible to regulate the burner at an optimum so the energy consumption for the drying process is minimised, and the waste of materials by the start/ stop of production is almost eliminated," explained Clausen.
The company has just sold its first asphalt plant into the Netherlands, a semi-mobile Freja capable of producing up to 240tonnes/hr (other KVM models include the Viking, 160tonnes/hr, and the Odin, 320tonnes/hr), and it is keen to expand its markets.Italian manufacturer
The counter flow-type dryer drum has been developed with a new technology, which includes a special combustor drum at high temperature and a new inside blading system.
The innovative combustor included in the dryer is made of a special steel support system with refractory bricks inside for high temperature resistance, arranged to protect the materials that are to be dried (virgin aggregates and RAP) from direct flame irradiation; to aid the elimination of possibly harmful bituminous steams and dust through post-combustion, and to optimise combustion and decrease fuel consumption.
"Thanks to the special inside blading, there is no contact between the flame and the RAP, thus removing any thermic shock to the bitumen and allowing the release of its humidity inside," said Bernardi Impianti.
The RAP, extracted from any available feeder through the collecting belt is, together with the virgin aggregates, put into the dryer.
"Here, thanks to the softening of the bitumen and to the release of humidity, we obtain a mixture of virgin aggregates covered with bitumen but with no problems of obstructions or adherence to the plant's other machinery, such as elevators and screening unit."
The RED dryer can also operate as a traditional dryer to treat virgin aggregates without any additional RAP. This also offers savings on fuel, says Bernardi Impianti.
'Flexible all-rounder'
The Swiss-based internationalThe model range, described as the "flexible all-rounder," can produce 120-320tonnes/hr and mixer variations are 2tonnes, 3tonnes and 4tonnes, while hot aggregate silos are 30tonnes, 56tonnes, 80tonnes and 120tonnes, as single and double lane models. Mixed material storage silos are 60tonnes, 100tonnes and 200tonnes, and are modular and expandable.
The plant incorporates flexibility, extendability and a significantly larger selection of variants.
"The plant has a modular construction design based on standardised container dimensions and its transportation is therefore cost-effective. A greater variety of hot aggregate silos and modular housing make this plant a real all-rounder: processing of recycling material (RA), granulate admixture and the production of low temperature asphalt are just a few examples," says Ammann.
Ammann America's CEO Robert Feiman, said this company is now focusing on continuous asphalt plants.
"We are developing a new range and the first one will be delivered at the end of August/early September. They will be versatile because they allow for a high amount of recycling, warm mix technology and colour options."
Using standard elements such as burners and baghouses, they will feature an extended twin-shaft mixer developed specially for continuous asphalt, and will offer the potential for the use of fillers, fibres and polymers.
Custom engineered
Stansteel, which manufactures asphalt plants and offers equipment product solutions, highlights its Accu-Shear module for making warm mix in asphalt plants.Increased output of asphalt plant mixer
The 8.4m x2.5m x3.2m unit weighs 15tonnes and uses the new Ammann Amix 2_4 mixer (also on show), which has maximum capacity of 4.3tonnes and output of 350tonnes/hr. It is powered by two 45kW motors.
The aggregates weighing system is 5.3m³ and bitumen filling is by gravity and spray bar.
The SIM Ammann SpeedyBatch 280 is one of four SpeedyBatch models, which also includes the 150, 180 and 210.The assembly, says Stansteel, is custom engineered to offer the advantages of accurately injecting water, other liquid additives or a combination of these materials.
"Liquid Asphalt and water do not mix naturally: in fact they will separate if possible. The principal that the Accu-Shear operates under is the shearing process of forcing the two (or more) liquids to mix together. The colloidal pump and the mixing action that takes place with the application of power are similar to an emulsion process," says Stansteel.
"By mechanically blending in lieu of simply injecting, the producer avoids the inherent nature of laminar fluid flow. By positively blending the additives with the liquid asphalt the foaming action is dramatically increased."
To maintain the integrity of warm mix on batch plants, Stansteel engineers have devised the Accu-Shear B which also utilises energy to blend the liquids. The Accu-Shear B is custom engineered for each facility and is configured based on the tonne/hr rate and the cycle time.
"Stansteel engineers, field technicians and former asphalt plant producers and owners joined forces to develop the Accu-Shear Warm Mix System.
"The jury is still out on determining if warm mix is an everyday product."
