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Owing to the introduction of smooth-riding "plant-mix" and "road-mix" shortly after the development of the new type of sealing, the motorist began to be dissatisfied with any surface showing slight irregularities. While the exposed chip sealing in itself did not become wavy or irregular, the underlying macadam was not in all cases constructed to a very high standard of smoothness and' also the roller-compacted macadam continued to deform under traffic. This meant that the seal coat did not retain its smooth-riding qualities. It is gratifying to report that, as a result of further research and experimental work, a smoothriding stabilized gravel surface which does not readily deform can now be constructed, and therefore the exposed chip sealing which takes up the contour of the underlying macadam can be laid to provide a surface which retains its smooth-riding qualities. The stabilized gravel crust is somewhat dusty and is not suitable for the direct application of ordinary thin bituminous courses. This led to the development and use of thin or low viscosity tar primers, which kill the dust, penetrate and bind the surface, and form a bituminous skin to which a light asphaltic treatment will strongly adhere. The stabilized top-course has not only provided much smoother roads with inferior local materials but also the development of this type of work has been of considerable benefit to local industry! Prior to the need for tar primers, the gasworks, especially those with vertical retorts, found it difficult to dispose of any appreciable quantity of their tar, and large quantities were simply burnt under the furnaces. Sales had fallen to a very low figure, but, so rapidly has the advantage of tarpriming work been recognized, that during last season the gasworks of New Zealand could not meet the demand for tar primers. Much research has been conducted throughout the world into the construction of stable road foundations or subgrades, and the laying of durable metal crusts. Tests as developed in the United States of America have been used to some extent by the Board, but it is believed that for metal-crust, work the tests developed in this country are more satisfactory. The stabilized top-course as now adopted by the Board consists of a mineral aggregate densely graded from about 1 in. or § in. in size down to and including some cohesive clay. This is well mixed and evenly spread out by long wheel-base power-graders. While grading and planing continue, the aggregate is sprinkled from water-carts to the plastic stage, and then compacted by pneumatic-tired vehicles and ordinary traffic. A very dense and smooth-riding surface results. While it had been recognized to some extent that aggregates would not compact to a rigid mass unless a certain percentage of fine mineral material was incorporated it was not evident as to what quantity was required, or how fine the material should be, and there was no suitable method of determining the binding properties of the fines. By considerable laboratory research and road experimental work these problems have been sufficiently solved to allow the Board to lay down guiding principles and to adopt certain standards, subject to possible modification as continued experience in the various districts and under different climatic conditions throughout the Dominion may indicate. A short description of the method of ascertaining the binding or cementing properties of the fine clayey portion of the aggregate may be of interest. The total sand-clay material passing the 50-mesh sieve (U.S. series) is moistened to a plastic state, packed into standard cement/sand moulds, and allowed to dry out to constant weight. The briquettes so formed are than tested for tension strength m an ordinary tension machine, or by other improvised means. It has been found that sand-clay binders with a tension strength of 10 lb. per square inch provide sufficient binding in a metal crust. Tests of over 150 lb. per square inch have been obtained in some instances. In addition to the tension, the shrinkage of the binder fraction is determined by measuring the difference in length of the wet and dry briquette. The lineal shrinkage should not exceed 5 per cent of the wet length. The Board has continued its policy of improving old and out-of-shape bituminous surfaces. Many of the existing sealed and paved highways which have become far too rough and irregular for the safety and comfort of modern traffic are gradually being brought up to standard by the application of thin " evener " or " smoothing " courses. For the most part these treatments have so far consisted of a mixture of densely graded aggregate and liquid asphaltic material mixed either on the road or in a central plant, and usually known as road-mix "or " plant-mix." These mixtures are spread out and planed over the road with long wheel-base power-graders and road-planing machines, while ordinary traffic effects compaction. The surface provided is exceptionally smooth-riding, and besides making for increased safety it reduces to a minimum the damaging effect of impact on the road structure. This densely graded, low bitumen content type of smoothing course depends for stability very largely upon the grading of the aggregate, and therefore does not have the flexibility of the bituminous carpets with heavier films of soft asphalts. As a consequence of this condition " plant-mix " or " road-mix," whether as initial treatment or as smoothing course, should not be laid on any base which is not uniformly firm and rigid. Bituminous surfaces which have starred and cracked, unless they can first be repaired and sealed up to provide a uniformly rigid crust, are unsuitable for the application of plant-mix" or " road-mix." Bough, but sound, bituminous concrete (hot mix) and bituminous macadam (penetration) can be greatly improved by a " plant-mix" or " road-mix " smoothing course. In the case of old, cracked, and starred thin-sealing courses in which the bituminous binder has become hard and brittle due to old age, instead of applying a smoothing course, it has generally been more satisfactory to scarify and remove the old sealing from out-of-shape and rough sections, reshape the gravel with power-graders, and apply an entirely new seal coat,

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