Lime has been used in hot mix asphalt (HMA) to reduce moisture sensitivity and stripping since 1910 in the United States. While hydrated lime has long been an acknowledged anti-strip additive for asphalt pavements, recent studies confirm that lime imparts other important benefits:
- It stiffens the binder and HMA to resist rutting;
- It improves toughness and resistance to fracture growth at low temperatures;
- Lime changes oxidation chemistry in the binder to reduce age hardening; and
- Lime alters clay fines to improve moisture stability and durability.
Lime is also useful to upgrade marginal aggregates. In addition to the chemical effects that lime imparts to reduce stripping potential and the aging impact resulting from oxidative hardening, the “filler effect” of lime improves resistance to high-temperature rutting and adds fracture toughness at low temperatures.
A Multi-Functional Asphalt Modifier
As numerous state highway authorities have found, modifications made to HMA with hydrated lime will add years to the pavement’s life. These modifications can reduce stripping, rutting, cracking, and aging. Hydrated lime substantially improves each of these properties when used alone, and also works well in conjunction with polymer additives, helping to create pavement systems that will perform to the highest expectations for many years.
A Superior Anti-Stripping Agent
Stripping occurs when the bond between the asphalt cement and the aggregate breaks down due to the presence of moisture, and the binder separates from the aggregate. Certain types of aggregates are particularly susceptible to stripping. Climatic characteristics such as heat, heavy rains, freeze/thaw cycles, and traffic play a major role in stripping. Hydrated lime is the most effective anti-stripping agent available and is widely specified by states with serious stripping problems.
When lime is added to hot mix, it reacts with aggregates, strengthening the bond between the bitumen and the stone. At the same time that it treats the aggregate, lime also reacts with the asphalt itself. Lime reacts with highly polar molecules that can otherwise react in the mix to form water-soluble soaps that promote stripping. When those molecules react with lime, they form insoluble salts that no longer attract water. In addition, dispersion of the tiny hydrated lime particles throughout the mix makes it stiffer and tougher, reducing the likelihood the bond between the asphalt cement and the aggregate will be broken mechanically, even if water is not present.
Improves Stiffness and Reduces Rutting
Rutting is permanent deformation of the asphalt, caused when elasticity is exceeded. The ability of hydrated lime to make an asphalt mix stiffer, tougher, and resistant to rutting is a reflection of its superior performance as an active mineral filler. 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.
Stiffening that results from addition of hydrated lime can increase the performance grade (PG) rating of asphalt cement. Depending upon the amount of lime used (generally 10 to 20% by weight of asphalt), the PG rating may increase by one full grade. In other words, a PG 64-22 can be increased to a PG 70-22. Adding lime will not, however, cause the mix to become more brittle at lower temperatures. At low temperatures hydrated lime becomes less chemically active and behaves like any other inert filler.
Reduces Oxidation and Aging
Oxidation and aging occur over time to generate a brittle pavement. Polar molecules react with the environment, breaking apart and contributing to pavement failure. Adding hydrated lime to asphalt cements reduces the rate at which the asphalt oxidizes and ages. This is a result of the chemical reactions that occur between the calcium hydroxide and the highly polar molecules in the bitumen. Hydrated lime combines with the polar molecules at the time it is added to the asphalt and thus, the polar molecules do not react with the environment. Consequently, the asphalt cement remains flexible and protected from brittle cracking for years longer than it would without the contribution of lime.
Cracking can result from causes other than aging, such as fatigue and low temperatures. Hydrated lime reduces asphalt cracking from these causes as well. Although, in general, stiffer asphalt mixes crack more, addition of lime improves fatigue characteristics and reduces cracking. Cracking often occurs due to the formation of micro-cracks. These micro-cracks are intercepted and deflected by tiny particles of hydrated lime. Lime reduces cracking more than inactive fillers because of 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 micro-cracks, preventing them from growing together into large cracks that can cause pavement failure.
The numerous benefits resulting from addition of hydrated lime to HMA work together to produce a superior, high performance product. Though the benefits above have been described individually, all of them work synergistically, contributing in multiple ways to improvement of the final product. Synergistic benefits also accrue when lime is used in conjunction with polymer modifiers. Research shows that in some situations, lime and polymers used together can produce improvements greater than when each is used alone.
Adding Hydrated Lime to Hot Mix Asphalt
Hydrated lime can be added to hot mix asphalts in a variety of ways. As a general rule, the application rate is one percent by weight of the mix. However, in cases where severe stripping is anticipated the application amount may increase. The most commonly used methods of addition are described below:
Dry Injection into Drum Mixers
This method was pioneered by the State of Georgia in the mid-1980s, when the state decided to require addition of lime to all of its HMA. One percent hydrated lime by weight of the mix is used and is added to the drum at the same time as the mineral filler. Georgia required modifications to the drum mixer to minimize the loss of lime when it is added. The hydrated lime comes into direct contact with the aggregate, thereby improving the bond between the bitumen and the stone, while the balance enters the bitumen. That portion of the lime can react with the polar molecules that contribute to both stripping and oxidation, while simultaneously stiffening and toughening the mix. The dry method is the simplest of the commonly used application methods. Since using lime, Georgia has significantly reduced its severe stripping problems as well as most of its rutting problems.
There are also other drum methods, such as ASTEC’s double barrel mixer, for example. In this system, fine materials can be added efficiently because they enter the mix in a turbulence-free zone.
Dry Lime on Damp Aggregate Method
This method is the one most commonly used throughout the country. It involves metering the lime onto a cold feed belt carrying aggregate that has been wet to approximately 2-3% over its saturated-surface-dry (SSD) condition. The lime-treated aggregate is then run through a pug mill to insure thorough mixing before it is fed into the plant. Lime is applied to damp aggregate in order to insure more complete coverage of the stone than is achieved using the dry method. Lime that does not adhere to the stone is dispersed throughout the mix where it will contribute to the other improvements that have been described. The “dry on damp” method of adding hydrated lime to hot mix is also relatively simple, but driving off the additional water required by the process uses additional fuel and may slow down plant production to some degree. In a variation on this method, the aggregate and lime are marinated in a stockpile before use to provide additional time for the lime to react with the surface of the stone and further improve anti-stripping performance.
This method utilizes a slurry mixture of lime and water that is applied at a metered rate to the aggregate, insuring superior coverage of the stone surfaces. The aggregate can then either be fed directly into the plant or marinated in stockpile for some period of time, allowing the lime to react with the aggregate. Because the lime is bound to the stone, it is also the method that results in the least dispersion of the lime throughout the rest of the mix.
Use of Lime to Recycle Asphalt Pavement
A new and growing use for lime is in cold, in-place recycling for the rehabilitation of distressed asphalt pavements. Existing asphalt pavement is pulverized using a milling machine, and a hot lime slurry is added with asphalt emulsion. 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 early strength and resistance to moisture damage.