Figure 11-42.—Pneumatic-tired roller.lifts, a smooth drum is all the compaction equipmentrequired. The smooth drum compacts lifts of- 4 to 8inches and seals the surface to allow the excess water todrain. Unlike the sheepsfoot drum, smooth drum rollersconcentrate the full width of the drum. The totaldynamic force is slightly less, because more of the drumis in contact with the ground.PNEUMATIC-TIRED ROLLERSThe pneumatic-tired rollers are widely used forcompaction of subgrades, bases, bituminous mixes, andmany types of material. They have rubber tires insteadof steel tires or drums and generally feature two tandemaxles, with three or four tires on the front axle and fouror five tires on the rear, as shown in figure 11-42. Theyare aligned so the rear tires cover the spaces left betweenthe tracks of the front tires. The tires are mounted in pairsthat can oscillate, or singly with spring action, so tirescan move down into soft spots that would be bridged bya steel drum. The rubber tires add to their downwardpressure a kneading effect, as material is pressedtoward spaces between the tires.Pneumatic-tired rollers can be ballasted to adjust theweight. Depending on size and type, the weight mayvary from 10 to 35 tons. However, more important thangross weight is the weight per wheel for the materialbeing compacted.Figure 11-43.—Action of a pneumatic-tired roller.CAUTIONPneumatic rollers ballast with water are topheavy and are very unstable when operating onuneven terrain.11-22
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