Asphalt, mixed with lime, has been in use for making roads and pavements for years. Hydrated lime is known as an ‘anti-strip’ additive in asphalt for the making of roads and pavements but this magical mixture has presented a lot of other advantages.
In short, lime helps the binders and hot mix asphalt to resist rutting by providing stiffness to the mix. Lime provides toughness and hence, avoids fracture at low temperatures. Its ability to change oxidation chemistry in the binders facilitates it to reduce hardening with age. Moisture stability and durability is also improved by adding hydrated lime to the mixture. Apart from all these benefits, lime also provides the “filter effect”, which gives resistance to high-temperature rutting and adds fracture toughness at low temperatures.
Let us look at the benefits of hydrated lime and asphalt in detail. It acts as a multifunctional asphalt modifier. The properties of lime alone enable it to be tough and strong. When added with asphalt, and used in pavements, it reduces stripping, rutting, cracking, and aging, ultimately adding years to the life of the roads without added maintenance.
Due to the presence of moisture and certain climatic and other conditions like heat, heavy rainfall and traffic etcetera, the bond between the asphalt cement and the aggregate breaks down and causes stripping when the binder separates from the aggregate. Hydrated lime provides an excellent solution to this problem. It acts as an anti-stripping agent when added to the mixture. By reacting with asphalt and aggregates it strengthens the bond between the particles in the mixture.
Rutting is a very common problem in roads made from hot asphalt mix. It is caused when the elasticity is exceeded. When hydrated lime is added, a chemical reaction takes place which makes the mixture stiff at high temperatures. The best part about adding hydrated lime is that it becomes inert at lower temperatures. This property allows the mixtures to not become brittle at low temperatures.
Everything that reacts with oxygen, including us, ages. Oxidation and aging causes the roads to become fragile over time. The reaction of bitumen with lime makes the whole mixture inert with respect to the environment, lowering down the oxidation rate. This enables the roads to avoid fatigue and cracking. Cracks are often formed because of the micro cracks. The tiny particles of hydrated lime are able to intercept and deflect micro cracks. This gives the roads and pavements a longer life.
There are a lot of ways in which hydrated lime can be added to the hot mix asphalt. The most commonly used methods include injecting dry lime into the drum mixtures, adding dry lime on damp aggregate and the slurry method.
Recently, work is being done to recycle the old asphalt (only) pavements by using lime. Existing asphalt pavement is crushed using a milling machine, and hot lime slurry is added with asphalt mixture. The cold recycled mix is placed and compacted using conventional asphalt paving equipment, and produces a smooth based course for the new asphalt surface. The addition of lime results in superior cold recycled mixtures, with much greater strength and resistance to moisture damage.
Research and experience has proved that the use of lime in hot mix asphalt is beneficial for the roads and pavements from every aspect. All the aforementioned properties and advantages work together to produce the final product which has all the qualities that its previous counterpart lacked. Eventually less capital will be needed for the maintenance and conservation of the roads if this practice is continued.