Articles & FactsSafer Asphalt RoadsE-Z LIME PlusOne of the Keys to a Safer AsphaltWhat industry insiders call "hot mix" cover most highways. Basically, it is a combination of heavy crude oil; tar-like hydrocarbon products mixed with various sizes of rock, called aggregate. Hot mix has a small amount of void space, about 5 to 7 percent, amid the rock and tar and is essentially impermeable. Rain falls on it and, although roads are slightly banked toward the outside to allow runoff, can puddle in low spots or grooves formed by the tires of cars over time. The result, when rain conditions and road geometry combine in the worst possible way, can be hydroplaning and high-speed crashes. Porous friction course, on the other hand, uses larger and homogeneous aggregate, typically chunks from 3/8 to 1/2 inch in size. Although it doesn't look like it to the naked eye, the mixture of that rock and the heavy hydrocarbons has 18 percent to 22 percent void space in it after it dries. And, again, though it doesn't look like it, those channels are interconnected. To glue the rocks more strongly often an additive is used to make the tar-like glue stronger and thicker. E-Z lime plus is a product that does both and at less cost. So rain almost instantly soaks into the pavement, then runs downhill through that network of channels and exits harmlessly on the side of the road. Aside from combating hydroplaning, the pavement also significantly decreases the spray that flies up from puddles on the road and thus improves visibility. It is also much quieter in both dry and wet weather. When the pavement changes from gray to black the decrease in road noise is unmistakable. So, why isn't the highway department putting it everywhere? While porous friction course asphalt is about 10 percent more expensive per ton than hot mix, it actually can cost less per lane-mile to apply. With the much greater porosity, the asphalt is less dense than hot mix, so a ton takes up more space. And while the department typically lays down as much as 2 1/2 inches of hot mix, the porous asphalt layer is usually about 1 1/2 inches. Porous friction course asphalt, however, is not as structurally strong as hot mix, so it is typically laid over an existing, stronger base layer of either concrete or hot mix. And, because the 21st century version of it has special polymer, cellulose and lime additives to make it more durable than the pavements rejected 20 years ago, many contractors at this point lack the equipment to mix it. EZ-lime plus and it's simple addition unit installed at the hot plant solves this problem. E-Z lime includes the polymers and additives in the new technology of pelletization. These pellets store easily, add easily and work better than regular asphalt when applied in a roadway system. And, oddly, the surface works best on high-volume, high-speed roads where the suction created by cars passing overhead provides a sort of vacuuming effect for the pores in the asphalt. That requires stronger glue for the rocks but preserves the ability of the pavement to absorb water. Multi-functional Performance of Hydrated Lime in Asphalt PavementsEZ-Lime™ and EZ-Lime Plus™Quality and Environmental Improvements Hydrated lime has been studied in laboratories and used in the field for over 40 years. During that time it has earned the reputation as the premier asphalt additive to mitigate moisture sensitivity/adhesion failure. In addition, lime contributes other benefits to asphalt pavements ranging from improved rutting and fatigue resistance, to retarding the rate at which pavements oxidize and age. An excellent survey of hydrated lime’s benefits entitled, “The Benefits of Hydrated Lime in Hot Mix Asphalt” was written by Drs. Dallas Little and Jon Epps for the National Lime Association (NLA). The paper can be downloaded from the NLA’s website at www.lime.org. Following is a summary of some of hydrated lime’s benefits. Also, the paper on EZ-lime and EZ-lime Plus pellets for use in lieu of the powder and added directly to the asphalt oil (bitumen) is available upon request at www.internationalconstructionchemicals.com Moisture sensitivity – Hydrated lime mitigates moisture sensitivity in two major ways. First, when applied to the surface of aggregates (particularly siliceous aggregates) it improves compatibility between the binder and aggregate (Kennedy and Ping, 1991). Lime also reacts with acid components of the asphalt binder to create insoluble calcium salts that are hydrophobic. The elimination of the acid components in the binder promotes the formation of strong nitrogen bonds between the asphalt and the aggregate (Petersen, 2005). Fatigue resistance and fracture toughness – As described previously, hydrated lime reacts with acid components of the asphalt binder by adsorbing them onto the surface of the calcium. The newly formed compounds have a greater effective volume than the traditional fillers that comprise the asphalt mastic. The calcium-based particles intercept micro-cracks and deflect them, preventing the formation of macro-cracks. Fatigue tests routinely demonstrate that asphalt mixtures containing hydrated lime can accommodate considerably more fatigue without failing than can mixtures made with common fillers (Little and Petersen, 2005). Rutting resistance – Hydrated lime often has more than 50% of its bulk smaller than 5 mm. Those small particles disperse throughout the asphalt mastic stiffening the mix through the “filler effect”. These properties are improved by adding the lime directly to the bitumen during manufacture. That stiffening effect which has been shown to be substantially greater at high temperatures than equivalently sized traditional fillers does not adversely affect the low temperature properties of the mix. At low temperatures hydrated lime’s activity is reduced and it behaves as traditional filler (Lesueur et al, 1998). Current data shows EZ-lime Plus pellets added to bitumen increases the Superpave PG-grade by one to two grade increases allowing for higher traffic loading. Oxidation and aging – The reactions between hydrated lime and the acid components (generally carboxylic acids and 2-quinoline groups) of asphalt binders reduce the viscosity producing components of the asphalt. As a consequence the asphalt binders retain greater ductility for a longer period of time than they otherwise would. That greater ductility represents a reduced rate of oxidation in the pavement along with a reduction of the brittleness that contributes to cracking (Petersen et al, 1987). Life cycle improvement – The synergistic benefits that the addition of hydrated lime contributes to asphalt pavements combine to extend the life of the pavements. A study completed for the Nevada Department of Transportation compared the field performance of equivalent pavements with and without lime modification using both laboratory tests and analysis of the state’s pavement management records. The study concluded that lime’s contributions extended the customary eight-year life of the pavements by an average of three years. The savings attributed to the additional pavement life, along with savings resulting from reduced maintenance totaled 38% compared to a 12% higher first cost for the asphalt mix (Sebaaly et al, 2001). Another study modeled life cycle costs of lime treated pavements versus non treated pavements based upon actual data provided by the Federal Highway Administration (FHWA), ten states, and ten contractors. The analysis identified life cycle cost savings ranging from “$2 to $4 per square yard; or, $14,000 to $30,000/lane mile [3% to 40% of project life cycle costs]” (Hicks and Scholz, 2001). The cost savings were based solely on an analysis of moisture sensitivity and did not consider the numerous other benefits of hydrated lime which would further increase the life cycle benefits. Numerous United States Departments of Transportation have successfully used hydrated lime in asphalt for many years, including
The reports available show an introduction with the following methods:
All of these methods have proven successful in producing asphalt mixes with enhanced performance. Records indicate that mixing with the bitumen prior to adding the aggregate accomplishes many of the benefits with addition of 30% less hydrated lime. Test data shows a more effective utilization of the hydrated lime with the use of EZ-lime pellets than current wasteful and environmentally dangerous slurries, marinating (rain water run-off dangers) and blowing dust at the manufacturing facilities. This dust issue is a real safety concern with many contractors in the USA. Regardless of the method used, hydrated lime effectively reacts with both the aggregate and the binder in asphalt mixes. The list of resources at the end of this report provides additional information on the use of hydrated lime in asphalt and contact information for technical experts within lime companies. In the final analysis there is no doubt that hydrated lime is not only the best moisture sensitivity additive available, but also an additive that contributes many other benefits to asphalt pavements. Those benefits work together synergistically to extend pavement life and improve performance. Even the best liquid antistripping additives only affect the adhesion between bitumen and aggregate. Studies that compare hydrated lime and liquid antistripping additives over multiple freeze/thaw cycles conclude that ultimately lime lasts longer and performs better (Kennedy and Ping, 1991). Considering the many other benefits that hydrated lime contributes to asphalt mixtures it is undoubtedly the best (and most economical) choice for high performance pavements. ReferencesEric Berger, PE. – Lhoist/Chemical Lime Co. Ft. Worth, Texas Association of Asphalt Paving Technologists, www.asphalttechnology.org The Lime Association will provide copies of the following papers on request.
Hydrated Lime by Eric BergerClick here to read the article in Adobe .pdf file format
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