cover any doubts that may exist. In addition, maintenance data obtained embraced pavements of richer concrete mixture than California specifications and the additional cost due to a 1:2:31⁄2 mixture is considered here. Adulteration of adobe subsoil is assumed as an application of six inches (loose) of gravel and a partial incorporation of the same into the top layer of the subsoil before rolling and compacting. Steel reinforcement is based on square steel bars, 3-inch, spaced 12 inches on centers transversely and two 1-inch longitudinal rods each placed six inches from outer edges of pavement. One-half of all pavement is considered treated as above. Table XII following, is the development of cost of an average mile of pavement seven inches thick, with the special treatment noted, to a life of 25 years, and we have a final cost of $53,050.00 per mile. Against this is the final cost of the four-inch pavement, of $67,800.00 per mile. It appears from this examination of California highway pavements that on main trunk lines subjected to heavy traffic, particularly where subsoil conditions are questionable, thin concrete slabs are entirely unsound for pavement design. It is recognized that the increase in fast-moving heavily loaded traffic has been enormous since these pavements were built, and it is estimated that this traffic will continue to increase in successive years, but that loading will be limited. Past experience should provide a tangible basis for future guidance, and if this experience in California is analyzed as we have attempted to do, it becomes apparent that the adoption of a five-inch slab at the present day may prove only a repetition of the experience with the four-inch slab of the past, and we have gained nothing. That practically every community in California has been educated to the value of hard paved roads is shown in the voting of county bond issues for highways to supplement the great State system. The people are willing to pay for good roads, and road officials and engineers in charge of these important problems must recognize that road design calls for comprehensive and intensive study, foresight and courage, if the road funds of the county, State or country are to be economically expended. The important principle that we advance here is an old one; first cost is not always a wise guide in the design of engineering structures, and miles of highway can well be set aside in our future work for quality in construction. Of vital concern in this same connection is the record of pavement thickness adopted by other States and Table XIII following is a record obtained by correspondence with thirtyfive Highway Departments to this end. It will be noted that many states are calling for pavements eight and nine inches thick and that of all of the states listed, none use as light a pavement as the recently-adopted 5-inch minimum of the California Highway Commission. From every standpoint of comparison that the subject may be approached, the fallacy of the four-inch pavements is shown. It is our belief that the five-inch pavement will be the subject of a like comparison and is in no way suited to modern traffic conditions on our main heavily traveled highways. Memorandum Total mileage of highways represented in Tables X and XI following = 483.9. As numerous sections in this total are 18 feet wide and a few 20 and 24, this mileage has been expanded to the equivalent mileage of 15-foot pavement 504., and this latter figure used in determining the cost per mile $67,860,00. |