The usual method of finishing the surface dressing is to roll the chippings with a 3-wheel roller weighing between 3 and 8 tons. Some authorities use steam rollers of ro or 12 tons and some do not use any roller at all. The cost of rolling is small; Grigson? gives it as 0.06d. per sq. yd. for fully mechanized work. The decision on what type of roller, if any, to use depends therefore upon its effect on the life and quality of the dressing. A road experiment was made in 1948 to compare methods of compaction. Ten surface dressings, each of at least 200 yd. in length, were laid; limestone chippings were used for fivc of the dressings and crushed quartzite gravel for the other five. Sections with each type of chipping wcre compacted by the following methods : (i) 3-ton diesel roller ; (ii) 6a-ton diesel roller ; (iii) 12-ton steam-roller ; and (iv) traffic only (by stopping each vehicle and asking the driver not to exceed 10 m.p.h.)
In addition, onc section with limestone aggregate was compacted by a towed pneumatic-tyred roller; this proved so difficult to manoeuvre on a narrow road that it was not used for the corresponding section with gravel chippings, which was compacted by traffic controlled only by large notice boards asking drivers not to exceed 10 m.p.h. After six months all the dressings were in good condition except where there was an obvious deficiency of binder.
It is too early to draw final conclusions, but the results suggest that (a) the use of the different steel-tyred rollers had little effect on the appearance of the dressing, except that the number of chippings crushed was greater with the heavier rollers;
(b) no detrimental effect resulted when no roller was used, providing the traffic was controlled to a speed of 10 m.p.h. or less for 15 minutes after application of the dressing; (c) in the experimental work the use of a large notice asking drivers to reduce speed to 10 m.p.h. had sufficient effect to prcvent any damage to the surface dressing. It was felt, however, that, in the long run, a more positive means of restricting spced would be necessary.
Recommendations
(i) Heavy rollers which crush the chippings should not be used, but lighter rollers of 6 tons or less help to finish the dressing before the trafic can damage it.
(ii) Except on the most lightly trafficked roads, it is inadvisable to dispense with a roller unless it has been found that traffic can be slowed down to a speed that will bed down the chippings rather than throw them off the road.