Benefits of using lime in asphalt
Hydrated lime is a beneficial addition to asphalt as it:
- Results in improved durability - up to 25% longer life
- Increases moisture resistance
- Increases ageing resistance in some instances
- Has been shown to enhance stiffness and mechanical properties
- Is abundantly available in the UK
The BLA recently presented a paper on the use of hydrated lime in asphalt, which has been published in the International Journal of Pavement Engineering and Asphalt Technology (available here).
Introduction to the use of lime in asphalt
Hydrated lime (calcium hydroxide – Ca(OH)2) is used as a modifier that improves performance in multiple ways to help create high performance asphalt pavements. Hydrated lime has been recognised for many years as the premier asphalt modifier to correct stripping problems. As hydrated lime use has grown worldwide, particularly in the USA, other benefits have been identified in the laboratory and in the field.
The BLA have produced a leaflet summarising these benefits which can be downloaded here.
Marshall asphalt mixture designs (available here) that include hydrated lime have been specified for airfield runways and taxiways over decades, to deliver durable surfacing that has fewer costly maintenance interventions and delivers a lower risk of aggregate plucking or stripping that would be a potential catastrophic risk to aircraft jet engines from loose aggregates.
A survey of USA State Highway Authorities (available here) using hydrated lime in asphalt concluded that the addition of hydrated lime resulted in a median 25% increase in the average life expectancy of Interstate Highways, and 33% increase for State and US Highways. The median increased average life expectancy for these roads was from 10 years to 13.5 years, and from 10 years to 14.5 years respectively.
Similar increases have recently been reported by the highway authority responsible for strategic roads in the North of France, where hydrated lime additions are now mandated.
The BLA have produced a paper summarising the evidence on service life extension (available here).
A highway review conducted in the USA Transport Research Board (available here) noted that additions of hydrated lime are beneficial to preventing moisture-related damage in asphalt mixtures. Many other evaluations have confirmed the same including recent research results from the University of Nottingham (available here) specifically evaluating asphalt mixtures using UK aggregates.
Moisture that penetrates asphalt surface courses may result in a weakening of the bond between the aggregate and the bitumen in the mixture, and ultimately lead to stripping. Left uncorrected, in small areas, this results in potholes, and over larger areas, cracks in the road surface develop, which allow water ingress to the lower pavement layers, thus potentially compromising the highway structure.
As a result of its ability to promote moisture resistance, hydrated lime is widely added across the USA as an 'anti-strip' agent. Hydrated lime is also gaining favour as an addition for asphalt mixtures across Europe – noted by the European Lime Association (available here). Although the European highways experience is not as extensive as USA practice, hydrated lime is increasingly specified is currently mandatory in some French and Dutch road systems.
Ageing of bitumen in asphalt results in mixtures becoming more brittle, which in turn lowers the ability of the surface course to distribute traffic loading effectively, and may ultimately lead to cracking.
Evidence suggests that the resistance to ageing provided by hydrated lime results from chemical acid/base reactions between the hydrated lime and the bitumen (available here), which result in the development of fewer carboxylic acids than usually associated with ageing. However, it is also noted that there may be other chemical mechanisms at work.
The BLA have produced a paper summarising the evidence on reduction of oxidation (available here).
Stiffening and mechanical properties
Hydrated lime stiffens the asphalt with the extent of the stiffening understood to be dependent on bitumen source and on the temperature. This means that mixture designs which use appropriate binder quantities remain important.
The modulus and strength of asphalt mixtures may be increased with the addition of hydrated lime.
As a result of increased stiffness, strength and modulus, rutting resistance and fatigue resistance may be improved. Field trials in Germany (available here) over 11 years indicate improved rutting resistance where hydrated lime is added to asphalt mixtures.
Additions of hydrated lime
Hydrated lime is most commonly added as a 1% -2% by mass of aggregate filler replacement. Hydrated lime can be added to the asphalt mix by various methods, e.g. directly into the drum; as a mixed filler; as dry powder to the damp aggregate; or as a lime slurry.
The Safe Handling of Lime document (available here) gives some general guidelines on the handling requirements for lime products.
However, please refer to the supplier's Safety Data Sheets for the complete safety information referring to an individual product being considered.
Information on asphalt is available from:
The BLA have produced several technical and summary papers on the use of hydrated lime in asphalt:
A conference paper titled: Developing the case for extending service life of asphalt surfacings using hydrated lime – available here.
A leaflet summarising the benefits of hydrated lime additions to asphalt - available here.
A technical paper summarising the service life of asphalts containing hydrated lime – available here.
A technical paper summarising the oxidation resistance of asphalts containing hydrated lime – available here.
The 2017 British Lime Association Conference – Lime in Road Solutions brought together over 75 delegates from across the highways supply chain to discuss the use of hydrated lime in asphalt and for soil stabilisation. You can read a summary of the conference here, and access the presentations from the day here.
The European Lime Association have undertaken a critical literature reviews of the use of hydrate lime in asphalt is available here.
There are several relevant papers available from the US National Lime Association are available here.