Hot Mix Asphalt
Hot Mix Asphalt (HMA) is a composite material comprised of two major ingredients; aggregate and binder. The aggregate is usually obtained from quarry operations (or through recycling) and the binder is a petroleum product, sometimes occurring naturally but usually the by-product of refining crude oil. The function of the binder is to basically coat the aggregate, creating a stable mixture of aggregate and asphalt that can resist numerous stresses induced by highway traffic and the environment.
Asphalt pavements are a crucial part of the UK’s strategy for building a high performance transportation network for the future. Asphalt construction is fast and relatively simple; it is economical, safe, quiet and the most sustainable solution to the future aspirations of the UK road network. Hydrated lime can be used as a modifier that improves performance in multiple ways to create high performance asphalt pavements.
The benefits of lime
A growing use for hydrated lime, particularly in the United States, is as an additive to the aggregates that can be applied either in a dry or slurry state. Hydrated lime tends to change the surface chemistry or molecular polarity of the aggregate surface, resulting in a stronger adhesion at the interface between the aggregate and asphalt binder. This is a particularly important factor for HMA’s which are constantly subjected to changing environmental conditions and traffic wheel loads. The environment plays an important role in conditioning the pavement due to the presence of moisture, the fluctuations in temperature, and the ageing of HMA mixtures. Combining this with the stresses from repeated traffic loads, a physical separation between the asphalt binder and aggregate may begin to occur. As the binder is displaced, moisture moves in to capture the aggregates surface, a process which is known as ‘water stripping’ or ‘water sensitivity’.
The performance of an HMA mixture is primarily measured in terms of its resistance to rutting, fatigue, low temperature cracking, and ravelling. The resistance of HMA to these distresses can to some degree be evaluated using performance tests and the measurement of its susceptibility to moisture and temperature.
In order to compensate for the problem of moisture damage, many manufacturers use anti-stripping agents, which may include lime. Experience in the US has shown us that lime is currently the most suitable additive for the widest range of aggregates and asphalts. Hydrated lime addition levels of 1.0 to 1.5% by weight of the aggregate are usually sufficient to reduce water stripping.
In addition, hydrated lime added as a mineral filler, has been shown to increase viscosity of the binder, as well as increasing the stiffness, tensile strength, compressive strength and resistance to rutting, all of which increase the durability of the mix. Rutting is permanent deformation of the asphalt, caused when elasticity of the material is exceeded. Unlike most mineral fillers, lime is chemically active rather than inert. It reacts with the bitumen, removing undesirable components at the same time that its tiny particles disperse throughout the mix, making the pavement more resistant to rutting and fatigue cracking.
Hydrated lime also has the ability to reduce cracking that can result from causes other than ageing, such as fatigue at low temperatures. Cracking often occurs due to the formation of microcracks. These microcracks are intercepted and deflected by tiny particles of hydrated lime. Lime tends to reduce cracking more than inactive fillers due to the reaction between the lime and the polar molecules in the asphalt cement, which increases the effective volume of the lime particles by surrounding them with large organic chains. Consequently, the lime particles are better able to intercept and deflect microcracks, preventing them from growing together into large cracks that can ultimately end in pavement failure.
Overall, the broad array of benefits that result from the addition of hydrated lime to HMA work together to produce a superior high performance product. Although the benefits here have been described individually, they all work synergistically, contributing in multiple ways to the improvement of the final product. Synergistic benefits also occur when lime is used in conjunction with polymer modifiers, and recent research has shown that in certain circumstances lime and polymers when used together can in fact produce improvements greater then each of them used alone.
Adding Hydrated Lime to Hot Mix Asphalt
Hydrated lime can be added to HMA in a number of ways. This can be done as part of a mixed filler aggregate or through a separate system. Adding hydrated lime to HMA is a simple process, on which BLA members can advise. A general rule of thumb for the application rate tends to be one percent by weight of the mix, though in cases where severe stripping is anticipated the application may increase.
Both powdered hydrated lime and milk of lime meeting the requirements of Types CL 70, 80 or 90 are most suitable, along with Type S dolomitic limes.
The future of Lime in Asphalt:
Hydrated lime has been recognised for many years as the premier asphalt modifier to correct water stripping problems. As its use has grown worldwide (particularly in the US) many other benefits have been identified, both in the laboratory as well as numerous field projects. The need to produce high performance asphalt pavements increases the importance of lime as a multi-functional asphalt modifier. Transport professionals and the public demand high performance asphalt pavements and hydrated lime provides an important tool to help meet those demands.