Reports were made periodically of the performance and behavior of the test sections. The report of performance after 2 years indicated that the experimental material was less flexible than the standard asphalt binder and edge cracking developed rapidly. Raveling, and loss of fine aggregates, soon occurred which produced a rough-textured surface. The experimental binder did not satisfactorily bond with itself or other paving mixtures, causing an opening of the centerline and transverse construction joints.

As the coal-tar binder was laid, wherever hand placement and compaction was required on the surface and approaches the work was difficult. In regard to the roughness of pavement, the surface composed of coal-tar binder material deteriorated more rapidly than the standard asphalt binder mixes. Another factor was the excessive fumes given off by the coal-tar mix, creating a difficult working condition for the construction personnel.

Some of the above deficiencies were observed in the standard asphalt binder, but they have not developed to such a severe degree in the test sections. It may be possible that better results might be obtained if this new material were treated as a tar rather than as a substitute for asphaltic cement.

Aggregates and mineral fillers

The mineral aggregate comprises the bulk component of a bituminous concrete and is equally important in the performance and durability to the binding agent-be it asphalt or tar. The physical and chemical characteristics of the aggregate and the gradation of the articles within a mix influence greatly the stability and suitability of the product when subjected to traffic and weather.

Aggregate gradation chart by Bureau. A new aggregate gradation chart, which will be especially useful in developing more realistic specifications and in evaluating individual gradation for bituminous concrete mixtures, has been recently developed by the Bureau of Public Roads. The new chart can be used to convert a nonconforming State specification gradation to the simplified practices gradation. Effects of mineral fillers.-It has been demonstrated by research that mineral aggregate material, passing a 200 sieve, has a significant influence on properties and performance of bituminous mixtures. By contrast, the effects of fillers cannot be precisely predicted from easily measured characteristics of the filler itself. Studies were made to ascertain effects of mineral fillers on bituminous mixtures and to assess their suitability for pavement performance. Variations in the character, grading of aggregates, and the amount and characteristics of asphalt cements used also affect test findings when evaluating specific materials for use as fillers.

The strength and stability of bituminous mixtures are affected by the shape and texture of the fine mineral aggregates incorporated in the bituminous concrete mix. Research is being undertaken to determine what influence the shape and texture of the fine material will have on the behavior of the mix in service. It has been found that mixtures containing mineral aggregates having a high percentage of angular and rough-textured characteristics generally developed a greater strength than the same mix consisting of round, smoothtextured aggregates.

Spectroscopy tests.-A new procedure to assess the uniformity and mineralogical nature of mineral fillers for bituminous mixes is the use

of infrared light spectroscopy. It is being adopted as a relatively new practice. Extension and further development of this technique and equipment adaptation are almost assured by virtue of the highway needs in this area.

For economic reasons and because of the growing scarcity of high quality aggregates in some areas, it is essential to make as much use as possible of aggregates that are locally available. This often necessitates tailoring specification requirements to fit the physical characteristics of such local aggregates to whatever extent may be compatible with producing high quality construction at economical prices.

Bituminous stabilization and protection.-Soils and aggregate materials are sometimes treated with liquid asphalts and emulsions to provide sufficient adhesion and cohesion between the particles to allow for compaction and stability. This has permitted the use of natural materials which have been previously unusable, reducing construction costs and conserving the supply of higher quality materials for other highway purposes.

Linked with the addition of bituminous materials in soils and aggregates for improved stability is the desired effect of the reduction of moisture absorption by these materials when placed in base course and subgrade layers. This process has also promoted the use of many aggregates not usable in high-type pavement construction. The placing of an asphalt membrane on the bottom, sides, and top of base courses, subgrades, and embankments, or to encompass all three structures in one application is being performed to prevent the infiltration and absorption of moistures in these structures.

New equipment and procedures for bituminous paving

There are a number of significant changes in both the appearance and performance of a modern bituminous paving train.

The paving contractor has his choice of using a batch or a continuous plant in practically all States; but in either case, it will be equipped with automatic electric or hydraulic controls which assure that the plant will go through the various operations following a predetermined material makeup and processing schedule with a minimum of human supervision.

In a modern plant, the aggregate dryers have been increased in diameter. This is the result of an extensive research study on the part of industry to speed up and provide more efficient drying. The dryer may be equipped with an automatic burner unit by which the intensity of the flame is automatically adjusted to the moisture conditions of the cold aggregate.

Reduction in mixing time for bituminous concrete. Although no major design changes have been made at the pugmill, the wet mixing may now be timed for 20 or 25 seconds instead of 45 seconds or more that generally prevailed 10 or 12 years ago and which still applies in some States.

A practical method has been developed for determining when a bituminous concrete mix is mixed. A recent study by the Virginia Council of Investigation and Research, at 17 batch plant setups, revealed that it is entirely feasible and desirable to determine wet mixing time by the Ross count method for individual plants and mix design. The Ross count method involves the evaluation of aggregate particle coating by visual observation.

Virginia mixing time study.-The Virginia study showed that a 98to 100-percent coating of aggregate particles, on the road, can be achieved in an average of about 20 seconds of mixing at the pugmill. Also, that a substantial amount of coating occurs in the discharge, hauling, and placing operations.

An experiment is being started in Ohio to determine the possible advantages in the new impact method of mixing bituminous concrete related to improved durability, stability, and economy for both construction and maintenance. Under the impact method, hot liquid bitumen is pumped into the mixing chamber at high pressure through special nozzles which atomize the bitumen to effect a more thorough coating of aggregates. Under present conventional methods, the hot liquid bitumen flows through a spray bar under low pressure or is introduced into the mixing chamber by force of gravity. It is claimed that the impact method requires a shorter mixing cycle.

Effect of overmixing on production and costs.-Studies conducted by the Oregon State Highway Department show that every second of unnecessary mixing time increases the cost of mixing by 2 percent. A reduction of only 10 seconds of wet mixing can increase the potential seasonal output of a mixing plant by 23 miles of two-lane bituminous. concrete pavement. The trend toward reduced mixing time for hot bituminous mixes not only results in a substantial increase in production, but also reduces the oxidizing tendencies from excessive mixing at high temperatures.

New paving equipment

On the large bituminous paving jobs, 15-ton capacity hauling trucks are becoming more evident instead of the 8- or 9-ton conventional haulers.

The bituminous paver-finishers have gone through substantial design changes and several new models have been placed on the market in recent years. Higher working speeds have been provided, and hopper capacities have been increased. Speeds are available up to 100 feet per minute, but a 50-feet-per-minute maximum is more likely to prevail because of mixing plant and hauling capacity. Last year, a single paver handled the total output of three mixing plants in Iowa (over 500 tons per hour) while operating on base construction at average speeds of about 54 feet per minute. This laydown rate is about three times as fast as former rates using older equipment designs.

The higher vehicle speeds prevalent on expressways built to interstate standards have created a demand for smoother riding pavements. There is also a need for a practical method to smooth out distorted pavements in resurfacing operations.

Electronic grade and slope controls for bituminous finishers. A number of manually operated devices for controlling the paver have been developed by contractors and paving engineers which have helped to improve laydown smoothness. The equipment manufacturing industry has sponsored the development of several different electronic devices on pavers for automatically controlling transverse slope and longitudinal profile from a single grade-line reference such as a steel piano wire mounted on adjustable steel pins and installed parallel to finished grade. With field testing complete, several paver manufacturers are making this device available for the current construction season. Some improvement in grade reference systems is needed before the full potentials of this device can be realized. The automatic grade and slope following device not only assures smoother pavements, but in addition it permits the construction of the various bituminous layers in closer conformance to the design thickness and conserves the higer priced materials.

Sealing joints with infrared heat. One of the problems associated with hot-mix lay down is the bonding of longitudinal joints of abutting pavement courses. A new infrared heating device, fueled with propane, which is attached to conventional type bituminous finishers now makes it possible to obtain smooth, well-bonded joints with a single laydown machine. The design of this unit is based on the property of dark objects or materials of absorbing infrared energy which is converted to heat.

Compaction with pneumatic-tired rollers. There are some new procedures in compacting hot bituminous mixes. The self-propelled pneumatic roller operating with tire contact pressures of about 80 pounds per square inch is now generally used at the intermediate stage following the breakdown rolling. While no substantial increase in average density is achieved on most mixes, the kneading action. appears to provide an aggregate particle placement similar to what would be produced by the tire loads and inflations employed by the trucking industry. A more uniform stability is built into the pavement during construction which will minimize rutting in the wheel