FIGURE 6.-Producing six sizes of crushed stone with a modern portable crushing plant of 400-tons-per-hour capacity.

iron-bearing stone. A new process of upgrading gravel deposits containing nondurable stone has been developed recently and has been employed with some success in the Lake States. The process is called elastic fractionation and employs the principle that soft and hard stone have widely different elastic or bouncing properties which may be used in effecting the separation.

Under a new national highway research program adopted by the American Association of State Highway Officials jointly with 50 State highway departments, the "Development of Methods for Improving the Aggregates Used in Constructing Highway Pavements" will receive high priority attention.

Aggregate producing equipment

Considerable progress has been made in the methods and equipment used to produce aggregates. For example, in 1960 the bid price of macadam or crushed stone base placed on Federal-aid construction averaged $0.11 per square yard per inch of depth as compared with $0.135 in 1950, although there has been a substantial increase in the highway construction bid price index during that period. The ability to produce crushed stone at reduced prices has also had a favorable effect on the cost per ton of bituminous concrete pavements during this period.

This favorable situation has been brought about by improved equipment for nearly every phase of aggregate supplying. Improved pneumatic drilling equipment, improved and less expensive explosives, such as ammonium nitrate, vastly improved crushing, handling, hauling, and spreading equipment have all helped to introduce greater economy into this phase of highway construction. The favorable trend is expected to continue; and in the not too distant future, quarrying operation may be further improved by the introduction of electromagnetic radiation, ultrasonic methods, or perhaps nuclear explosives in isolated areas to reduce solid rock into processing sizes.

Base construction equipment

Natural granular materials are also widely used in base construction, both with and without stabilizing agents. Where stabilization is involved with cement, bitumens, or other materials, vastly improved equipment has been developed to process the material both at the roadsite and at central plants.

Base spreaders have been developed so that the aggregates can be spread with considerable accuracy and without segregation. Spreading equipment for applying both dry and liquid stabilizing agents is also highly developed. Single pass stabilizing mixers are available which can process the in-place material in a single pass while applying the stabilizing agent. One new model even provides the initial compaction. In order to improve control over material quantities and mixing, there is a trend toward the use of central plants for stabilized bases, particularly those employing calcium chloride and bituminous materials. Some of the new models are capable of processing more than 500 tons of materials per hour.

Since all surface lay-down machines operate either directly or indirectly on a prepared base of subgrade, it is essential that their profile be controlled to nearly the same degree of accuracy that is to be achieved in the surface course or courses.

Electronic controlled grading equipment.-We now have the electronically controlled fine grader attachments to a motor grader which provides considerable assistance toward the construction of smoother subgrades and soil-aggregate type base courses at faster rates and with less operating personnel. It, at the same time, reduces the amount of grading stakeout by at least 50 percent. The motor grader attachment also enables the crowning of road mix surfaces with greater accuracy. This device maintains the grader blade in a level position or to a desired crown slope without interference from the irregularities which are encountered by the grader wheels. With the operator's attention no longer required to maintain the vertical angle of the blade, he is able to concentrate most of his efforts on following stakes or a string or wire reference parallel to the finished grade. An automatic grade following system which operates similar to the motor grader device, has also been developed for a long wheelbase planer.

FIGURE 8.-Slope control panel for an electronically controlled motor grader blade.