654. The use of concrete for curbs and gutters is rapidly increasing. Curbing is sometimes molded and afterward put in place like stone curbing, but the greatest advantages in the use of concrete for this purpose are only attained by molding in place the curb and gutter as one structure.
The Parkhurst combined curb and gutter is a patented form that has proved very satisfactory. This form has a projection of about one inch at the back and another along the bottom just below the curb, this feature being patented.
A combined curb and gutter may consist of a curb four to six inches wide at the top, and five to seven inches at the bottom, and have a face of six to seven inches above the gutter. The upper face corner of the curb and the angle between curb and gutter should be rounded with a radius of one and one-half to two inches. The-gutter is sixteen to twenty inches wide, and from six to nine inches thick, with top surface conforming to the grade of the street.
655. The sub-base should consist of a layer of broken stone six inches thick, or six to twelve inches of cinders thoroughly rammed. The preparation of the foundation should be similar to that required for a pavement, care being taken that the sub-base be thoroughly drained, tile being used if necessary. forms to receive the concrete are held in place by stakes, the molds being carefully set to grade. The sub-base may now be covered by a layer of four to six inches of Portland concrete of only moderate richness, as one to three to six, and the concrete to form the curb and gutter placed upon it before it has set, or a six inch layer to form the gutter may be placed directly on the sub-base.
656. Concrete to form the curb and gutter should be of good quality, not more than two and one-half parts sand to one part Portland cement being used for the mortar, and sufficient mortar used to entirely fill the voids in the stone. The broken stone for this concrete should be rather fine, with few, if any, pieces larger than one inch in greatest dimension. The exposed faces receive a top-dressing, or wearing surface, of onehalf inch to one inch of granolithic containing not more than one and one-half parts of trap or granite, pea size, to one part Portland cement. This coating is applied as soon as possible after the concrete is placed, as in sidewalk work. The surface is troweled or floated, but a smooth, glossy finish is avoided.
The curb and gutter may well be laid in alternate blocks about six feet long, but a somewhat neater appearance is secured by making the work continuous, and cutting it entirely through at intervals of six feet to provide for slight movement. As the molds may be used repeatedly, they should be substantially made. Special forms are of course required at corners, catch basins, etc. As in other concrete construction, the work should be protected from injury and kept moist for at least a week.
657. On business streets it is desirable to build the sidewalk close to the curb, with only a joint between, the grade of the walk conforming to the curb and sloping up toward the building line one-quarter inch to the foot. On residence streets the walk should be separated from the curb by a park strip, the walk being high enough to give drainage toward the curb.
Steel facing is sometimes used for curbs subjected to exceptional wear, as in front of shipping warehouses and freight sheds. Where these are applied, they should cover the top and the upper part of the face of the curb and must be well anchored, by bolts or special webs, to a substantial mass of concrete, otherwise they will work loose and defeat the object for which they are used.
At Champaign, lll.,1 a curb was built seven inches high and five inches thick, the gutter, six inches thick, extending nineteen inches into the roadway from the face of the curb. The foundation consisted of six inches of gravel or cinders well rammed. The concrete was composed of one part Portland cement to five parts fine gravel, and the finishing coat, one inch thick, was of one part Portland cement to one part clean, sharp, coarse sand. The Cost per foot was thirty-nine cents, including all excavation.
A similar curb at Urbana, 111., was 4 1/2 inches thick at the top, 5 inches at the base and 7 1/4 inches high; the gutter being 5 inches thick and extending 18 inches into the roadway. The foundation was composed of eight inches of cinders or gravel. The concrete was of one part Portland cement to five parts clean gravel, and the finishing coat was one inch thick, composed of one part Portland cement to two parts sharp sand. The price per linear foot, including the excavation, removal of old curbing, and refilling, was forty-six cents.
1 W. H. Tarrant, Engineer, Proc. 111. Soc. Engr. and Surveyors, 1899.
At South Bend, cement curb alone, 6 inches wide at top, 7 inches at bottom and 16 inches depth, with the upper half composed entirely of one to two Portland cement mortar, has been constructed for eighteen cents per linear foot.