Drainage 6. On clay and adobe soils drainage is essential. On sidehill work, drain ditches excavated eighteen inches in depth should be filled either with broken stone or tile and the water carried away from the base of the pavement. A uniform moisture content cannot be maintained in a sub-grade when pools of water occasionally stand along the sides of the pavement. In sandy or loam soils the grade may be held down to within a few inches above prevailing ground levels. On heavier soils, however, it should be raised to at least twelve inches above the general ground levels. Experiments 7. The prevention of shrinkage and expansion in clay or adobe soils under a rigid pavement is beset with such uncertainty and is so essential to the permanence of any type of pavement that it is of great importance that both laboratory and field experiments should be made by the State to determine the most effective manner of accomplishing this. The method by which clay soils absorb and evaporate moisture should be studied, together with the volume of shrinkage or expansion and the forces that are exerted thereby. A continuing series of levels should be taken on pavements that are now laid on adobe sub-soils in order to determine the amount of movement that occurs. As the road construction program of California on the part of the State and Counties involves a past and prospective outlay of over one hundred million dollars, and as so many of the problems involved, both with reference to the subgrade and slab, are yet not completely solved, it is important that extensive laboratory and field investigations should be undertaken under the direction of the Highway Commission or by some adequately equipped organization, such as might be obtained in the Engineering Department of the State University. Road problems are encountered in California which are unique. It is not adequate for the investigations to be carried out by the Federal Good Roads Bureau in the East. Such studies should be conducted through a term of years. They particularly should include methods by which the moisture contents under the pavements could be kept uniform. While investigations of this nature have been started in the laboratories of the State University for this report, time has not been sufficient to carry them to the desired finality. Inspection 8. No specification for a sub-base will be satisfactory unless it is rigidly enforced in the field. To some contractors much of the detailed procedure which involves expense to them is deemed unnecessary and unduly exacting, and there may result a continued pressure to avoid the performance of some of the requirements. It is, therefore, necessary that the work on subgrade should be inspected with vigor and exactness, because the whole future life of the pavement rests on the adequacy of the subgrade. Expenditures 9. The average cost per mile of State roads in Southern California to date has been $18,097.00. It has been found that the average annual cost per mile of maintaining, widening and resurfacing these roads amounts to $615.00. When to this is added the average interest costs and bond retiring fund, the total mean annual expense per mile for these roads to the people of the state is $1,353.00. Experience has shown that all types of roads, including certain experimental sections. that have been built with unusual care and stability, have failed under heavy traffic when placed on poor subgrades, and that even the lighter types of pavements, when placed on good foundations, have given quite satisfactory service. It follows, therefore, that heavy expenditures are justified in the preparation of the subgrade; and in fact that no type of pavement should be built on heavy, undrained soil until it is put in satisfactory condition, irrespective of cost. (Signed) ROBT. MORTON, Chief Engineer, San Diego Highway Commission. LAWRENCE MOYE, County Surveyor, Tulare County. J. B. LIPPINCOTT, Consulting Engineer. REPORT OF THE COMMITTEE ON THICKNESS, WIDTH AND REINFORCEMENT OF CONCRETE SLAB MR. CHARLES PETIT, County Engineer, Ventura County, Chairman MR. OWEN O'NEILL, County Engineer, Santa Barbara County (a) The Committee on "Thickness, Width and Reinforcement of Concrete Slab" has made a field inspection of concrete pavements in Monterey, San Luis Obispo, Santa Barbara, Ventura, Los Angeles, Orange and San Diego Counties in Southern California. Some members of the committee also made an inspection of selected sections of concrete paving in Alameda, Contra Costa, Solano, Sacramento, San Joaquin and Stanislaus Counties in Northern California. Highways in Oregon and Washington were also visited. The inspection in the Southern California counties covered both State and county highways and included plain and reinforced pavements. The cross-section and manner of reinforcing some sections of pavement examined are contained in Appendix "A" to this report, page 149. From this field inspection and from a study of the available literature on the subject of reinforcement, thickness and width of slab, the committee makes the following observations. and recommendations: (b) The most serious defect of the concrete pavements examined, but not the only defect, is the ocurrence of cracks in the concrete. The reason that cracking is considered a most serious defect is discussed later in the report. Cracks of the most common occurence may be classified as follows: 1. Transverse cracks roughly at right angles to the center line of the pavement. 2. Random cracks, sometimes called "Surface Cracks" or "Hair Cracks." 3. Cracks surrounding circular or irregular spots where the pavement slab has settled under load. 4. Longitudinal cracks, roughly parallel to the center line of the pavement. 5. Diagonal cracks or corner cracks occurring where the transverse cracks or expansion joints intersect the edge of the pavement slab. (c) All cracks in concrete pavement as classified in Section b of this report, are caused by tensile stresses in the concrete. Theoretically, cracks could be caused by direct shear of compression under excessive load, but to get this kind of failure it would be necessary to have an unyielding subgrade. Otherwise the slab would yield sufficiently to produce tensile stresses which would be the ruling factor in the formation of the crack. An exception to this should be noted where breaking by direct compression occurs at the edge of the concrete slab, or adjacent to an unprotected expansion joint. This kind of failure manifests itself in the breaking of small fragments of the slab, which disintegrate into the original aggregate by raveling, rather than in the formation of pronounced cracks. Tensile stresses in the concrete slab are set up by (a) temperature, (b) drying of the concrete, (c) beam and slab action under the dead and live load, (d) heaving of the subgrade under the pavement. (d) Transverse cracks in narrow concrete pavements are caused by a combination of temperature and setting stresses. The formation of these cracks can be eliminated by placing sufficient expansion joints in the pavement when it is laid. While the committee does not consider a transverse crack to be a serious defect in the pavement, yet we would advocate the use of expansion joints, especially for slabs thicker than four inches. We do not favor especially designed or armored joints, a plain joint with asphaltic filler being sufficient. We consider the constructed expansion joint preferable for the following reasons: (1) It presents a better appearance by the regular intervals and straight lines. (2) The intermediate cracks between the regular joints of about thirty-foot intervals are merely hair cracks. |