tracks from channelized traffic during high ambient temperature conditions.

Other new equipment developments.-Improvements have been made to several bituminous paving units used extensively on lower type bituminous surfaces. New distributor designs provide improved nozzles and circulating systems, making it possible to spray a uniform and evenly heated application at all nozzles. This is highly important for surface treatments and resealing activities where serviceability depends on good aggregate embedment in the bitumen.

Self-propelled aggregate spreaders have also been developed to work closely behind the bituminous distributor. These units are capable of placing a layer of cover stone over the newly applied asphalt which is uniform in both thickness and width while operating at high speeds. This assures proper embedment of the stone with minimum waste and gives neat appearance to the wearing course.

Settlement of repaired areas in bituminous pavements has always been a serious problem. The development of small vibratory rollers and plate type vibrators now make it possible to recompact both the base layers and the surface patch so that they will not settle to any substantial degree under traffic.

A number of heater-planer machines have been developed for smoothing up distorted high type pavements without discarding or covering up all of the existing paving material.

Addition of asbestos, rubber, or plastics to bitumen

The introduction of certain compounds to asphaltic materials and mixtures for the purpose of improving service performance to bituminous pavements is the subject of continuing research and experimentation. Additional compounds for this purpose are subjects of continuing research and development.

The addition of asbestos fiber in a finely divided form has proved to be a satisfactory ingredient for upgrading bituminous mixes. The asbestos material is actually a fraction of the mineral filler incorporated in the bituminous mix that tends to produce a more resilient pavement. These fibers have an electropositive surface which makes for retention of excessive binder, chemically and mechanically, within the mixture. The use of asbestos in connection with materials used in pavement construction had its beginning in 1917. The addition. of asbestos in hot bituminous mixtures has been expanded recently. Its use has reduced rolling time during placement operations and has made possible the opening of paved sections to traffic within minutes after rolling has been completed. This characteristic of the mixture may have considerable use in areas where urgent traffic requirements are a factor.

Rubber. The effect of the addition of rubber to bituminous concrete mixtures on experimental sections of highways has been under observation by the Bureau's Office of Engineering for a number of years. Based on the performance of 18 experimental projects,

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neither beneficial nor detrimental effects on the quality or durability of the pavements have been observed to date. Rubber compounds when added to standard cutback asphalts in surface treatment and seal coat applications, however, are reported by some States to achieve an improvement in the performance of the material. Based on the continued interest in rubber additives and admixtures, the Bureau developed a "tentative guide specification for rubberized asphalts" in 1960. It covers one grade of asphalt cement (85-100) and one rapid curing cutback (RC-3). Research and development of rubberized bituminous binders are being continued, principally through the rubber industry itself.

Epoxy blends.-As a result of industry's years of laboratory research and field developmental work, a new binder consisting of epoxy blends in asphaltic cements and liquid asphalts is now commercially available. These epoxy blends in asphalt have been developed for surface treatments, seal coatings, and plant-mix operations. The epoxy asphalt binder, when used in hot-mix bituminous concrete production, employs conventional methods and equipment in the mixing process. The behavior of the combined materials during mixing, hauling, spreading, and compaction is similar to the standard asphalt mixtures. During this process, and for some time after placement, the epoxy binder in a chemical-physical process converts into a thermosetting plastic, which is flexible and has high tensile strength. The material usage in highway work is limited because its cost is high. The prevailing prices of epoxy material are about 20 times the cost of an equivalent amount of asphalt. While these materials are promising, they are not fully developed.

Asphalt emulsions.-Industry is continuously researching and developing new asphaltic products, chiefly in the liquid asphalt category, to improve the workability and durability of the material and to widen the scope of its use. Frequently, these new products receive proprietary labels. Recent developments of asphalt in the emulsified form have made commercially available an emulsion in which the electrochemical charge of the asphalt can be controlled as desired. The development of an emulsifier, which affects a positively or negatively charged emulsion, improves its performance as a binder when used with certain aggregates. The prevailing electrochemical attraction of the aggregate determines the type of emulsifier to be used.

Color blends. Other uses of asphalt cements and liquid asphalts include the incorporation of color blends of the binder for such purposes as traffic lane delineation, edge markings, and similar demarcations. They are applied as a seal coating to the pavement. These uses have been developed for guiding the flow of traffic. Color-blend materials may also be placed on pavement areas requiring special delineation, such as interchange ramps, acceleration and deceleration lanes, and on shoulder areas. Colored polymeric binders without an asphalt component are under development in some areas. This material may be used as a substitute for asphalt color blends.

Other asphalt uses. As a structural material, asphalt concrete is being increasingly used for constructing curbs and gutters, waterways, ditches, berms, and similar structures. Special equipment has been designed and is available commercially for use in applying these materials for construction and maintenance operations. Research

and development of other mechanized techniques are continuing to advance the further use of bituminous mixtures, principally under industrial sponsorship and financing.

Although not a completely new application of bituminous materials, the extension of the use of an asphalt spray to retain mulch in place on roadside areas is becoming more widely used. The asphalt spray is also applied directly to a newly seeded slope to prevent erosion before stabilization of the area is effected by grass cover. The thin asphaltic film aids in germination of grass seeds and hastens the growth cycle of the ground cover.

Scope of use

PETROLEUM PRODUCTS

The substantial growth in highway construction volume since the middle 1950's and a gradual annual increase in outlays for highway maintenance have resulted in larger demands for petroleum products including fuel, oil, and lubricants.

It is estimated some 1.3 billion gallons will be consumed in highway construction and maintenance operations during the current year. Of this amount, about 800 million gallons will be used on highway construction.

Simplification of lubrication requirements

One of the most significant developments in the petroleum products field is the substantial reduction that has been made by manufacturers in the types and classes of oil and lubricants necessary to support equipment operations. This has introduced economies into both the warehousing and handling areas. One multipurpose grease now replaces three former automotive greases a chassis lubricant, a water pump grease, and a wheel-bearing lubricant. It also reduces the chances of misapplication. Triple purpose oil has also helped to reduce overall consumption. The widespread use of multigrade SAE 10W-30 motor oil is a good example.

Detergent oils

The use of additives in lubricating oils, such as metallic soap type chemicals to produce detergency by which the initial deposition of the products of fuel combustion and oil decomposition in new and clean engines is prevented, is an outstanding contribution to greater economy of equipment operations. Different additives produce a dispersion of soot and other impurities into a finely divided state, prevent oxidation and corrosion of bearings, and provide good lubrication for extreme pressure operations. Heavy duty detergent oil which was initially developed for diesel engines is now widely used in many gasoline engines as well.

The petroleum industry continues to explore the possibilities of making additional multipurpose lubricants available which will further benefit the highway contractor and the public highway department.

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