Fig. 121.—Box Form for Concrete Curb.
Fig. 122.—Continuous Form for Concrete Curb.
FORM CONSTRUCTION.—The form construction for curb and gutter work is determined by the general plan of construction followed,—whether monolithic or two-piece construction. In monolithic construction two types of forms are employed, sectional or box forms and continuous forms. A good example of box form is shown by Fig. 121. This form was designed for a curb 14 ins. high at the back, 6 ins. high in front and 24 ins. from face of curb to outer edge of gutter, constructed in sections 7 ft. long. The form, it will be observed, is a complete box, in which alternate sections of curb are molded and after having set are filled between using the same form but dispensing with the end boards which are replaced by the completed sections of curb. A fairly representative example of continuous form is shown by Fig. 122; in this construction a continuous line of plank is set to form the back of the curb and another line to form the face of the gutter slab, both lines being held in place by stakes. When the gutter slab concrete has been placed and surfaced the form for the front of the curb is set as shown and the upper portion of the curb wall concreted behind it. The method in detail of constructing curb and gutter, with this type of form, at Ottawa, Ont., is described in a succeeding section. Here the joints were formed by inserting a partition of ⅜-in. boiler plate every 12 ft., which was withdrawn just previous to finishing up the surface; the sections between partitions were concreted continuously. Another method is to make the partitions of plank, concrete every other section, then remove the partition plank and concrete the remaining spaces against the previously finished work. A different method of supporting the plank forming the face of the curb wall, is to clamp it to the back form (Fig. 123), spacers being inserted to keep the two their proper distances apart. The forms shown by Figs. 121 to 123 are for monolithic curb and gutter. In two-piece construction where the curb wall is constructed on the finished gutter slab practically the same method of construction is employed as is illustrated by Fig. 122 except that no attempt is made to concrete the curb wall before the slab concrete has begun to set. The more common and the preferable method of two-piece construction is illustrated by Fig. 124; the curb proper is built first using the simple box form shown at the right hand, then the water table is built using the completed curb as the form for the back and a board held by stakes as a form for the front. This board is set with its top edge exactly to the grade of the finished water table so as to serve as a guide for one end of the template, the other end of which rides on the top of the finished curb wall. Forms for curves at street intersections are best constructed by driving stakes to the exact arc of the curve and bending a ⅜-in. steel plate around them or bending and nailing ⅞×1¼-in. strips. Soaking the wood strips thoroughly will make them bend easily. The cost of form work in constructing curb and gutter is chiefly labor cost in erecting and taking down the forms.
Fig. 123.—Continuous Form for Concrete Curb.
Fig. 124.—Form for Two-Piece Curb Construction.
CONCRETE MIXTURES AND CONCRETING.—The curb body is usually made of a 1-3-5 or 6 concrete and the curb finish of a 1-2 mortar. Portland cement is employed almost exclusively. The concrete mixture commonly used is of such consistency that thorough ramming is necessary to flush the cement to the surface. The cubical contents of combined curb and gutter of the forms illustrated will run from 3 to 5 cu. yds. per 100 ft., and about one-eighth of this will be facing mortar 1 in. thick; thus a curb running 5 cu. yds. per 100 ft. will contain per 100 ft. about 0.83 cu. yd. of mortar and 4.17 cu. yds. of concrete. The usual method of concreting is to erect the forms for the back of the curb wall and the front of the gutter slab and concrete to the height of the water table clear across; then shape the exposed top of the water table to section and place the mortar finish, and then erect the face form for the gutter wall, bring the concrete backing and vertical face finish up together and, finally, finish the top. The finish coat is placed by troweling on the horizontal surfaces; on the vertical face of the curb wall it may be placed in any one of several ways. Frequently the mortar coat is simply plastered against the face board and filled behind with concrete. Another method is to lay a 1-in. board against the inside of the form, concrete behind it, then withdraw the board, fill the space with mortar and tamp concrete and mortar to a thorough bond. The special face forms shown in Chapter VIII may be used in place of the board. The securing of a good bond between the backing concrete and the mortar facing is governed by the same conditions that govern sidewalk work.
COST OF CURB AND GUTTER.—The cost of concrete curb and gutter is commonly estimated in cents per lineal foot. The cost of excavating, loading and carting will run about the same per cubic yard as for sidewalks. Excavating the trench and preparing the sub-grade usually runs from ½ ct. to 2 cts. per foot of curb, but sometimes it amounts to 3 cts. Placing the sub-base will cost for placing and tamping 1 ct. per ft., to which is to be added the cost of materials; a 6-in. sub-base 30 ins. wide contains 4.7 cu. yds., tamped measure, of materials per 100 ft. The amount of materials per foot depends upon the cross-section of the curb; it equals in cubic yards the area of cross-section in square feet divided by 27, and of this volume about one-eighth will be 1-2 mortar and seven-eighths 1-3-6 concrete. The tables in Chapter II give the amounts of materials per cubic yard of these mixtures; the product of these quantities and the cost of the materials on the ground gives the cost. The labor cost of mixing and placing, including the form work, will run from 10 to 14 cts. per foot. In round figures curb and gutter of the section shown in the accompanying illustrations may be estimated to cost in the neighborhood of 40 cts. per lineal foot. The following sections give records of cost of individual jobs of curb and gutter construction.