|
Lime has been used in hot mix asphalt (HMA) to reduce moisture sensitivity and stripping since 1910 in the United States. But while hydrated lime has long been an acknowledged anti-strip additive for asphalt pavements, it also imparts other important benefits:
Lime is also useful to upgrade marginal aggregates. In addition to the chemical effects that reduce stripping potential and the aging impact resulting from oxidative hardening, the filler effect improves resistance to high-temperature rutting and adds fracture toughness at low temperatures. Hydrated Lime Is A Multi-Functional Asphalt Modifier As numerous state highway authorities have found, modifications made to hot mix asphalt with hydrated lime will add years to its 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. Life cycle cost analysis demonstrates that lime is cost-effective. In 2001, nearly 400,000 metric tons of lime were used in asphalt in the U.S. For a more detailed fact sheet on lime’s role as a multifunctional asphalt modifier, see http://www.lime.org/HydratedLime.pdf. For a more lengthy report and literature review, see http://www.lime.org/ABenefit.pdf. Hydrated Lime Is 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, and environmental 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, the 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. Hydrated Lime 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 it more resistant to rutting and fatigue cracking. The stiffening that results from the addition of hydrated lime can increase the PG rating of an asphalt cement. Depending upon the amount 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. The addition of the lime will not, however, cause the mix to become more brittle at lower temperatures. At low temperatures the hydrated lime becomes less chemically active and behaves like any other inert filler. Hydrated Lime 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 that it is added to the asphalt and thus, they 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, the addition of lime improves fatigue characteristics and reduces cracking. Cracking often occurs due to the formation of microcracks. These microcracks 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 microcracks, preventing them from growing together into large cracks that can cause pavement failure. Hydrated Lime: Synergistic Benefits The broad array of benefits that result from the addition of hydrated lime to hot mix asphalt work together to produce a superior, high performance product. Though the benefits have been described individually, all of them work synergistically, contributing in multiple ways to the improvement of the final product. Synergistic benefits also accrue when lime is used in conjunction with polymer modifiers. Research has shown that in some situations lime and polymers used together can produce improvements greater than each of them 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, though 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 the addition of lime to all of its hot mix asphalt. 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 has required modifications to the drum mixer to minimize the loss of lime when it is added. The hydrated lime comes in contact with the aggregate itself, directly 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 to implement of the commonly used application methods. (Since using lime, Georgia has significantly reduced its severe stripping problems as well as the majority 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. Slurry Method: 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 for Recycling of 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 along 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. |
USE OF LIME FOR SOIL STABILIZATION AND MODIFICATION Lime can be used to treat soils in order to improve their workability and load-bearing characteristics in a number of situations. Quicklime is frequently used to dry wet soils at construction sites and elsewhere, reducing downtime and providing an improved working surface. An even more significant use of lime is in the modification and stabilization of soil beneath road and similar construction projects. Use of lime can substantially increase the stability, impermeability, and load-bearing capacity of the subgrade. Both quicklime and hydrated lime may be used for this purpose. Application of lime to subgrades can provide significantly improved engineering properties. There are essentially two forms of improvement: modification and stabilization. LIME AND SOIL MODIFICATION: Lime is an excellent choice for short-term modification of soil properties. Lime can modify almost all fine-grained soils, but the most dramatic improvement occurs in clay soils of moderate to high plasticity. Modification occurs because calcium cations supplied by the hydrated lime replace the cations normally present on the surface of the clay mineral, promoted by the high pH environment of the lime-water system. Thus, the clay surface mineralogy is altered, producing the following benefits:
LIME AND SOIL STABILIZATION: Soil stabilization occurs when lime is added to a reactive soil to generate long-term strength gain through a pozzolanic reaction. This reaction produces stable calcium silicate hydrates and calcium aluminate hydrates as the calcium from the lime reacts with the aluminates and silicates solubilized from the clay. The full-term pozzolanic reaction can continue for a very long period of time, even decades -- as long as enough lime is present and the pH remains high (above 10). As a result, lime treatment can produce high and long-lasting strength gains. The key to pozzolanic reactivity and stabilization is a reactive soil, a good mix design protocol, and reliable construction practices. Benefits of soil stabilization include:
These performance benefits translate into short- and long-term economic benefits.
In addition to stabilization of new materials, lime is an excellent choice for the reclamation of roadbases. As more and more governmental entities are choosing to reclaim existing roadbases rather than replace them, this use of lime will become even more important. Lime stabilization is not difficult to carry out. After proper mix design and testing is performed, in-place mixing is usually used to add the appropriate amount of lime to soil, mixed to an appropriate depth. Pulverization and mixing is used to thoroughly combine the lime and soil. For heavy clays, preliminary mixing may be followed by 24 to 48 hours (or more) of moist curing, followed by final mixing. For maximum development of strength and durability, proper compaction is necessary. Proper curing is also important. If sulfates are present at levels greater than 0.3 percent, special procedures are required.
|
USE OF LIME IN BUILDING CONSTRUCTION Masonry Mortars, Stuccos, and Plasters—Lime has been used as a primary ingredient in masonry mortars for centuries, and this important use continues to the present day in both historic and contemporary applications. Mortars madewith lime and cement exhibit superior workability balanced with appropriate compressive strength, as well as low water permeability and superior bond strength. Lime is a major constituent in exterior and interior stuccos and plasters, enhancing the strength, durability, and workability of these finishes. All of these lime applications are supported by ASTM specifications and standards. Papers and articles on a variety of building lime applications are available at www.buildinglime.org.
Limewash– Limewash is a versatile, accommodating, and robust surface covering that is compatible with a variety of building surfaces. It is maintainable, beautiful, stable, and long lasting. A copy of a paper on limewash presented at the 2005 International Building Lime Symposium is available here. Site Preparation – Lime can be used to dry up wet sites. Lime can also react with clays in the soil to provide a more stabile base for building construction. For more information on these uses click here. Autoclaved Aerated Concrete (AAC) —Lime is also employed in the manufacture of innovative lightweight cellular concrete products, such as autoclaved aerated concrete (also called “aircrete”), which can be formed into block as well as large masonry units or insulation slabs. The 2005 International Building Lime Symposium included a paper on AAC. Other Concrete Products—Hydrated lime can be added to the concrete mix in making block and other concrete products, in order to produce a denser, more water-resistant product. By adding greater plasticity to the mix, lime also produces concrete products with more precise edges and corners, improves reflectivity, and reduces loss through breakage. Calcium Silicate Brick—Calcium silicate (sand-lime) brick is employed in standard masonry construction in the same manner as common clay brick. Sand is mixed with high calcium lime (quick or hydrated) in a wet state, and then molded into bricks and autoclaved. The lime reacts with silica to form complex hydro(di)calcium silicates that bind the brick and provide high dimensional stability. Lime is also used to make hollow sand-lime building block, tile, slabs, and pipe. Insulation
Materials—Some insulating materials, molded as units, contain lime
and diatomaceous earth or lime and silica. In these products, lime serves
as a binding agent, reacting chemically with the available silica present
in the mix to form calcium silicates. The lime-silica reaction is also
employed in making microporite insulation.
|
 |