The high maintenance charges and reconstruction costs which 4-inch and 5-inch pavements have required during the past seven years, particularly where the sub-soil is clay or adobe, lead to the inevitable conclusion that it is more economical to build thicker and more substantial pavements. We must expect enormously increased truck traffic. Wheel loads and speeds will increase unless legislation and larger license fees prevent. Congested truck traffic already requires an 8-inch minimum concrete thickness with substantial reinforcement wherever the subgrade demands it. For trunk roads we recommend pavements of an average thickness not less than 6 to 8 inches, dependent upon subgrade, drainage and traffic requirements. Wherever the sub-soil is treacherous and yielding, these concrete pavements require an amount of reinforcement proportional to the foundation difficulties to be overcome. The Highway Commission should have paid greater attention to the preparation of subgrades. Experience has shown that modern traffic will inevitably crack thin concrete pavements on subgrades of heavy clay or adobe unless every precaution is taken to provide drainage and to keep the moisture content as nearly constant as possible in the subgrade materials. It is recommended that clays and adobes be adulterated by rolling into them such materials as sand, gravel, and broken stone. Everything should be done that will tend to limit the tendency for change in adobe volume with change in moisture content. The adulteration and impregnation of clay and adobe sub-soils should always be extended to each side of the concrete slab to provide an equally secure foundation for side shoulders. These side foundations and shoulders should be not less than 3 feet in width wherever the clay or adobe is questionable. The type of reinforcement used by the California Highway Commission cannot prevent longitudinal cracks on adobe; it tends only to hold the broken slab together. While rock or gravel shoulders, oil bound and sealed, are recognized as unsatisfactory and uneconomical, if built for the purpose of widening a concrete road, it is our belief that such type of shoulder should be provided on all adobe and heavy clay sub-soil sections irrespective of width of pavement, primarily for the purpose of preventing storm water from working down and under the pavement to its detriment and possible destruction. In addition to keeping the moisture content more uniform, shoulder construction gives lateral support to the sides of pavement thus reducing the known tendency to break down along the edges. With wider concrete roads this type of shoulder would only be subjected to turnout traffic; in consequence, a longer span of life under proper maintenance may be expected. The cost of any measure is justified which tends to reduce or prevent longitudinal and corner cracks where pavements rest on flexible soils. Greater width in a pavement and steel reinforcement cannot alone be relied upon to prevent longitudinal and corner cracks. Shoulders must protect the edges and sides of the pavement. Greater width of permanent paved roadway is, however, of extreme importance. Fifteen-foot pavements without shoulders are unsatisfactory and dangerous. Modern trucks with an 8-foot body width cannot pass on a 15-foot roadway without inviting collision and accident. The width of all double track highways should never be less than 18 feet on tangents; they should be widened on curves and grades. Future traffic demands a minimum width of 20 feet for every important road. Near cities, or wherever the traffic is heavily congested, pavement widths should be not less than 24 feet. We cannot include technical details in this summary; therefore in the matter of the control of transverse cracks we merely state that properly designed expansion joints should be provided at stated intervals. The State should adopt a high standard of concrete mixture, developing higher ultimate strength than has heretofore obtained. Ten States use the same mixture as California and thirty-seven use a richer mix. Independent of all other considerations, it is illuminating to examine the probable life of 4-inch pavements completed in 1914-1915 and the ultimate cost at the end of the average life of the bonds issued to cover their initial cost. The first cost of these pavements 15 feet wide and 4 inches thick averages approximately $7,073.00 per mile (pavement and engineering costs only). Developing maintenance costs, including surfacing, until they reach a point where it is no longer sound financially to continue repairs, we find that economically these pavements have an average life of approximately 11 years. Adding interest to the original cost, plus maintenance, together with reconstruction costs and additional interest and maintenance to the average bond life noted (in this case 25 years) the ultimate costs are obtained. These ultimate (or 25-year) costs average approximately $67,800.00 per mile. The ultimate or 25-year costs of pavements laid in 1914-1915, if these had been constructed to a thickness of 7 inches, with proper treatment of adobe subgrade, and using reinforcement where necessary over approximately half the mileage, would have been $53,050.00 per mile. This seems a conclusive demonstration of the type of pavement that would have provided the soundest investment of the people's money for the life of the bond. This analysis, therefore, confirms the recommendation as to thickness of pavement made in V. VII. Highway Bonds and Life of Pavements For the first and second bond issues the last bonds will be retired in 1962. Probably long before this time many of our present pavements will have become either worn out or obsolete and their general reconstruction will be necessary. Based on sound financial policy, pavements should live as long as the average life of bonds with which they are built. Some of our 4-inch pavements have broker down in 44 years; practically before the retirement of any of the bonds had begun. It is estimated that all of our roads will need replacement at the end of their economical life. The total length of life of our present roads is a matter of judgment and is speculative, but the economical life of the earlier pavements is definitely determined to average about eleven years. It is important to build more permanent roads with shorter life bonds. In this connection it is interesting to note that the State of New Jersey prohibits the use of a bond of a life over 20 years for the construction of any type of road. In that State the limit for sand and gravel road bonds is 5 years; for macadam, 10 years; for bituminous concrete, 15 years; for block or sheet asphalt, on concrete, and plain concrete 6 inches thick, 20 years. With our State bonds running many years beyond a 20-year life, it behooves us to consider the seriousness of a short life for our 4-inch pavements and to fundamentally change our policies in this regard. VIII. Present and Future Traffic-Traffic Laws and Their Violation The number of automobiles in California has increased from 98,399 in 1913 to 535,000 in 1920. In the same interval trucks have increased from 5,299 to 35,000. We may confidently expect a great increase in the growth of the number of trucks. Their size, weight and future speed is an uncertain question. During the past four months, 2,000 trucks have been weighed; 256 of these were found to be overloaded, in some instances as much as 100%. 500 trucks weighed on public scales gave loadings on each rear wheel ranging from 4,275 lbs. for 1-ton trucks to 11,280 lbs. for 5-ton trucks. Truck operators observe little relation between the manufacturer's rating and the load carried. As one expressed it: "They would haul any load they could start." The California State law provides a maximum weight per inch of tire in contact with the road of 800 lbs. By actual measurement on 500 trucks the average loads per inch width of tire on one rear wheel ranged from 831 lbs. on 1-ton trucks to 1,090 lbs. on 5-ton trucks. The maximum load per inch width of tire measured was 1,840 lbs. It is not so important to rigidly adhere to a rule limiting the maximum load of 800 lbs. per lineal inch of tire in contact with the pavement. It is far more important to limit the maximum load put upon any one wheel. The maximum load usually found on rear wheels of 31-ton and 4-ton trucks is approximately 10,000 lbs. The speeds of these trucks averaged 18.4 miles per hour for 1-ton capacity to 13 miles per hour for 6-ton trucks. While the State of California has built roads that have been demonstrated to be too frail and too narrow, it has also completely neglected to enforce the laws relative to overloading the pavement slab. It is apparent that if truck operators are permitted to use the highways without police supervision a few unreasonable truck drivers or operators will wreck any road system that may be built. Thinking people recognize that such a policy is improper, short-sighted and suicidal. There must be co-operation between truck dealers, truck operators and the State authorities in the enforcement of proper traffic laws. California should adopt a uniform Motor Vehicle Law. Such a law is necessary for interstate traffic. Manufacturers cannot be expected to design special trucks to meet different State laws. It is equally important that speed regulations should be enforced for trucks. The speeds at which these heavy loads travel on public highways produces impact. This road shock is especially destructive to the pavement. It seems a proper time has come to consider modifications to the present Motor Vehicle Tax as well as to the law governing weights of vehicles in view of the considerable additional mileage to be added to our existing paved highways and the steadily increasing maintenance and reconstruction costs. A more equitable adjustment of the tax will be secured by proportioning fees to be collected more directly to those vehicles which use the highways the most as well as to those vehicles which are most destructive. Examination of motor tax enactments in other states leads to the conclusion that the present California taxes are extremely moderate and not in accord with more equitable tendencies. IX. Comparison of California Highways With Those of Other States It is a fact, established by correspondence with 35 States, that California design is far behind modern specifications in use elsewhere for width, thickness and richness of concrete mixture. This fact in itself is ample cause for reflection. Until October, 1920, after these investigations began, California was building a standard pavement 4 inches thick. The present standard is 5 inches, reinforced. Of the thirty-five States for which we have records, no other State has a standard pavement as thin as 5 inches. Many other States started their highway program five and more years ago, with thin and narrow slabs and lean mixes of concrete, but they have developed a more substantial standard construction as they saw the growth and development of traffic. Their pavements are now wider, thicker and richer than our own. It is true that California has changed its concrete mix from 1:22:5 to 1:2:4, and has very lately increased slab thickness to 5 inches and introduced reinforcement, but in making these changes California has been slower to act and has not gone so far as other states in the building of heavier and more durable pavements. X. Research and Experimental Investigations The California Highway Commission has not sufficiently and adequately conducted research and experimentation. In work of the magnitude of our highway program, the Commission should set aside a budget of sufficient funds for the continuous study of special problems. Some of these are: 1. Relative capacity to distribute pressure on sub-soils by a-rigid pavements of concrete. c-macadam. 2. Tests of heavy clays and adobes. 3. Investigation of sub-soil drainage, a-in clays and adobes. b-in compressible soils in the presence of 4. Level records on pavements from selected sites. 5. The effects of alkali. 6. Tests on full sized pavements to determine more fully the relative advantages of reinforcement vs. greater thickness of slab and varying richness of mix of concrete; a study of different types of reinforcement. 7. Studies of the absorptive power of adobe and clay to take water out of the bottom of a freshly poured concrete slab. 8. A study of the mechanical adulteration of clay and adobe subgrades; an inquiry into possible chemical treatment. 9. Traffic studies; the effect of impact; of roughness, of surface of pavement; design of truck, character of tires, speed, etc. These and other tests suggested by modern practice should be programmed. They would be expensive; would take time and would require a specially trained staff of assistants, but the magnitude of the work justifies such expenditures and would teach the Commission and its engineers how to avoid otherwise costly mistakes. XI. Public Opinion The public has not been sufficiently informed that adequate permanent roads would cost more than the estimates and bond issues have indicated. To achieve mileage of highway, roads have been built that are dangerously narrow and entirely too frail. In the three bond issues a total of 5,560 miles of State roads are proposed; only 1,402 miles have been paved, or 25%. Practically no roads specified in the third bond issue have been built. Public opinion in California demands more permanent types of construction rather than mileage of highways. County roads where the taxation is more directly placed on the beneficiaries are in some instances better roads than those built by the Highway Commission. We predict that the California public will not continue to support frail construction on long life bonds. XII. Conclusion This report has called attention to numerous failures of concrete roads, but it is not intended to convey the impression that the undersigned are opposed to the construction of concrete highways. The contrary is true. We believe that the most satisfactory type of pavement to stand heavy traffic conditions, particularly on insecure foundations, is an adequately and properly built concrete slab. The California Highway Commission, while perhaps not legally and technically responsible for some of the conditions described in this report, is however, the State authority having general supervision and jurisdiction over our expensive highway program. At least it is the advisory authority to which all branches of the State Government turn. The Commission and its engineering staff must take direct responsibility for the present highway situation. Judging from past experience, all future work should be subject at intervals to careful inspection and review by competent representatives of the people. Detailed reports have been submitted to the Automobile Club of Southern California and the California State Automobile Association by their respective engineers and the above is a general review and summary made by the undersigned. J. B. LIPPINCOTT, Consulting Engineer, Los Angeles. H. J. BRUNNIER, Consulting Structural Engineer, San Francisco. CHARLES DERLETH, Jr., Civil and Consulting Engineer, WALTER C. HOWE, Consulting Highway Engineer, |