The asphalt road
construction industry is constantly exploring technological improvements that
will enhance the material’s performance, increase construction efficiency,
conserve resources, and improve environmental stewardship.
Hot Mix Asphalt (HMA), used
in construction of flexible asphalt pavements, has been traditionally produced
at a discharge temperature of 150°C - 160°C resulting in high energy (fuel)
costs and production of greenhouse gases. The growing concerns on environmental
issues such as global warming and carbon footprint has stimulated the
introduction of Warm Mix Asphalt (WMA) and its technologies in the asphalt road
construction industry, which serve the purpose of reducing greenhouse gas
emissions by reducing the mixing and compaction temperatures of asphalt mix.
Warm Mix Asphalt (WMA)
technologies produces asphalt mix at reduced temperatures, slightly above 100
°C, with performances and characteristics equivalent to or even sometimes
better than that of conventional HMA. Warm mix asphalt is usually produced at a
temperature ranging between 110 to 140 °C which is generally 10–50 °C lower
than the production temperature of hot mix asphalt. This temperature reduction
in WMA not only conserves energy but also reduces the emission of greenhouse
gases and the amount of fuel used during production.
WMA technologies reduce the
viscosity (the thickness) of the asphalt binder so that asphalt aggregates can
be coated at lower temperatures. The key is the addition of additives
(water-based, organic, chemical, or hybrids) to the asphalt mix. The additives
allow the asphalt binders and asphalt aggregates to be mixed at the lower
temperatures. Reducing the viscosity also makes the mixture easier to
manipulate and compact at the lower temperature.
WMA technology not only
allows for lower production temperatures, reducing energy consumption and
lowering greenhouse gas emissions, but minimizes fume and odour emissions
creating a cooler working environment for asphalt workers. It also improves
workability of mix, lower mixing and compaction temperatures, lowers aging,
facilitates cold weather paving and enhances the distance from which the mix
can be hauled to construction sites.
WARM
MIX ASPHALT TECHNOLOGIES
Several techniques exist
for the production of WMA. The three generally most accepted are those using
(i) organic additives; (ii) chemical additives; and, (iii) foaming techniques.
Organic
additives
Different organic additives
can be used to lower the viscosity of the binder (bitumen) at temperatures
above about 100°C. A commonly used
additive is a special paraffin wax produced by conversion of natural gas.
Organic additives typically give a temperature reduction of between 20–30 °C whilst
they also improve the deformation resistance of asphalt so modified.
Chemical
additivesÂ
Chemical additives do not
change the bitumen viscosity. As surfactants they work at the microscopic
interface of the aggregates and the bitumen. They regulate and reduce the
frictional forces at that interface at a range of temperatures, typically
between 140 and 85°C. It is therefore possible to mix the bitumen and
aggregates and to compact the mixture at a lower temperature. Chemical
additives may reduce the mix and compaction temperatures by about 20 – 40°C.
Foaming
techniques
A range of foaming
techniques is applied to reduce the viscosity of bitumen. Various means are
employed to introduce small amounts of water into the hot bitumen. The water
turns to steam, increases the volume of the bitumen and reduces its viscosity
for a short period. The expansion of the
bitumen allows the coating of the aggregates at lower temperatures and the
residual moisture supports the compaction of the asphalt on the construction
site. Production and paving temperatures can be reduced in parallel.
BENEFITS
OF WARM MIX ASPHALT
Reduction
in emissions: The prime objective of adopting warm mix
technology is to lower the production of harmful emissions and this is its
biggest advantage. About 30% decrease in emissions is observed by using
appropriate technology.
Energy
and cost savings: As the temperature requirement is low for
WMA, fuel consumption for heating is low. Hence, energy requirement is
considerably reduced. This in turn, directly results in cost cutting of fuel,
providing an economical product, as compared to HMA.
Elimination
of health hazards to workers: At high temperatures,
bitumen releases toxic fumes in traces that are harmful for the workers. They
can cause headache, rashes and cough. Even signs of cancer emerging due to
these fumes have been observed. A significant reduction in these fumes can be
achieved by using warm mix technology.
Enhanced
workability: WMA technologies are designed to lower the
viscosity of asphalt binders. This makes the asphalt mix easier to handle,
spread, and compact. WMA's enhanced workability translates into better handling
characteristics for asphalt workers, making the paving process safer and more
efficient.
Less
binder aging: Lower production temperatures can also
decrease the ageing of the bitumen during the production stage which can
additionally improve the thermal and fatigue cracking resistance of the
asphalt.
Potential
for Increased RAP in WMA: Reduced production temperatures minimize
binder aging which compensates for the aged RAP binder, thus allowing the use
of high proportions of RAP. Also, the improved workability enhances mixing and
compaction of RAP-containing mixtures at lower temperatures.
Longer
haul distance: As the difference between production and
ambient temperature is smaller for warm mixes compared to HMA, the rate of drop
in temperature is less for WMA with time, allowing a longer time for paving and
compaction.
Production
in cold weather: Additives used in WMA helps in compaction at
low temperatures also, allowing the paving to extend in cold weather also.
Further, the temperature difference between the mix and the environment is also
less, which helps in all season paving.
MACHINERY
REQUIRED FOR WARM MIX ASPHALT
The machinery used for the production, transportation, placement and compaction of WMA are the same as those needed for HMA, although minor modifications to the asphalt plant may be required depending on the WMA technology. Some technologies are added to the asphalt binder by the supplier and do not require additional equipment at the plant. Other technologies require more substantial modifications. Technologies involving water-based foaming techniques or mixture additives require additional equipment installed / integrated in the asphalt plant to measure and deliver the additive.
