Like any other composite pavement, it is called upon to perform two functions; to safely sustain the weight of loads passing over it, and to resist wear and abrasion of its surface. A material and form of construction that meets the first requirement may not meet the second. Experience has proved that ordinary 1:3:6 concrete makes an entirely satisfactory foundation for any pavement, but it lacks the hardness and strength to successfully resist the surface abrasion of travel. To secure this quality a richer and harder concrete is called for, but it is unnecessary that the foundation should be equally hard. To construct the pavement in two courses as here specified would seem to be the logical way, especially as it decreases the total cost, and should make a more durable pavement.
[61]. Some engineers advocate a greater total thickness of the pavement than is here specified (6 inches). Considering that this pavement should never be used on heavy traveled streets, a total thickness of concrete of six inches will have ample strength to carry the loads to which it will be subjected. If so, it is a useless waste of money to increase the thickness of the concrete.
[62]. The use of limestone for the top course (unless it is of very superior quality) is not advisable or economical unless the cost of trap rock is so high as to be prohibitive, which, considering its superior durability under the wear of travel, will not often be the case.
[63]. It is advisable to remove the screenings from this surface mixture for two reasons: first, to secure greater uniformity of composition. If the screenings are allowed to remain in the aggregate, there is danger of segregation into patches of different sized aggregate and different ratios of materials, which it is very important to avoid, and second, the small fragments of stone are more likely to be crushed under the concentrated weight of wheels than the larger masses, and to thus start disintegration. Lack of uniformity in the composition and homogeneity in this surface-course concrete is especially to be guarded against, otherwise the surface of the pavement will wear unevenly and depressions and ruts are likely to result.
[64]. The ideal composition of this surface-course concrete is one where the stone forms the largest possible part of the mass consistent with sufficient mortar to fill the voids and thoroughly bind the fragments of stone together.
[65]. The importance of securing high quality and great uniformity in the surface course cannot be urged too strongly.
[66]. This requirement must be strictly enforced. Otherwise there will be danger that the two courses may not properly adhere to each other. It is the writer’s experience that if this rule is observed there will be no danger of the two courses separating.
[67]. The purpose of this rolling is mainly to evenly compress the mass and thus secure its uniform density. It also produces a truer surface than can usually be secured by ramming alone.
[68]. Among engineers there is quite a wide difference of opinion as to the proper spacing of expansion joints, and, in fact, as to the necessity or advisability of providing them at all. It has been suggested that it might be better to omit them entirely, allowing the pavement to form its own expansion joints by cracking along lines where natural forces dictate. Such cracks by their irregularity give a bad appearance to the surface, but observation seems to indicate that the edges of these natural joints wear as well as those made by expansion joints. Further observation and experience is needed in the matter. In most concrete it is known that some contraction takes place during the setting of the cement, regardless of temperature changes, and cracking is probably due as much to this permanent contraction as to that caused by low temperature. The coefficient of expansion of concrete by heat is variable but so small that expansion joints ⅛ inch wide every fifty feet along the street should provide for temperature changes.
[69]. If the expansion joints are not thus filled with bituminous cement they will become filled and packed with incompressible stone, sand, etc., that will not permit expansion.