Concrete Construction: Methods and Costs by Gillette and Hill

METHODS AND COST OF CONSTRUCTING SIDEWALKS, PAVEMENTS AND CURB AND GUTTER. Next to pavement foundations the most extensive use of concrete in street work is for cement walks and concrete curb and gutter. Usually the mixing and placing of the concrete is hand work, practically the only exceptions being where pavement base, curbing and sidewalks are built all at once, using machine mixers. The same objections that have been raised to machine mixers in laying pavement foundation are raised against them for curb and walk construction, and owing to the much smaller yardage per lineal foot of street in walk and curb work these objections carry more force than they do in case of paving work. Another argument against the use of mixers is that both walk and curb and gutter work involve the use of forms and the application of mortar finish, the placing of which are really the limiting factors in the rate of progress permissible, and this rate is too slow to consume an output necessary to make a mixer plant economical as compared with hand mixing where so much transportation is involved. Concrete sidewalk and curb work are essentially hand mixing work; they, therefore, involve a careful study of the economies of hand mixing and wheelbarrow haulage which are fully discussed in Chapter II. CEMENT SIDEWALKS. Sidewalk construction consists in molding on a suitably prepared sub-base a concrete slab from 3½ to 7½ ins. thick, depending on practice, and finishing its top surface with a ½ to 1½-in. wearing surface of cement mortar. ~GENERAL METHOD OF CONSTRUCTION.~--The excavation and preparation of the sub-grade call for little notice beyond the warning that they should never be neglected. The authors have seen many thousands of feet of cement walk laid in the middle West in which the sub-base was placed directly on the natural sod, often covered with grass and weeds a foot high. Such practice is wholly vicious. The sod should always be removed and the surface soil excavated to a depth depending upon the climate and nature of the ground and the foundation bed well tamped. From 4 to 6 ins. depth of excavation will serve where the soil is reasonably hard and there are no heavy frosts; with opposite conditions a 12-in. excavation is none too deep. The thickness of the broken stone, gravel, cinder or sand sub-base should likewise be varied with the character of the soil, the conditions of natural drainage and the prevalence of frost. In well drained sandy soils 6 to 8 ins. of sub-base are sufficient, but in clayey soils with poor natural drainage the sub-base should be from 10 to 12 ins. thick at least; the local conditions will determine the thickness of sub-base necessary and in places it may be desirable to provide by artificial drainage against the accumulation of water under the concrete. Tile drains are better and cheaper than excessively deep foundations. The thorough tamping of the sub-base is essential to avoid settling and subsequent cracking of the concrete slab. This is a part of sidewalk work which is often neglected. Portland cement concrete, sand and broken stone or gravel mixtures in the proportions of 1-3-5 and 1-3-6 are used for base slabs. For walks up to 7 ft. wide the slab is made 3½ ins. thick for residence streets and 4½ to 5 ins. thick for business streets; for wider walks the thickness is increased to 7 ins. for 8-ft. width and 7½ ins. for 9 to 10-ft. width. Roughly the thickness of the walk in inches (base and top together) is made about equal to its width in feet. The concrete is deposited in a single layer and tamped thoroughly, either in separate blocks behind suitable forms or in a continuous slab which is while fresh cut through to make separate blocks. For walks up to 8 ft. wide the slab is divided by transverse joints spaced about the width of the walk apart, but for the wider walks the safety of this division depends upon the thickness of the base; an 8-ft. walk with a 5-in. base can safely be laid with joints 8 ft. apart, but if the slab is only 4 ins. thick it had better be laid in 4×4-ft. squares. The mode of procedure in base construction is as follows: The sub-base being laid, side forms held by stakes are placed as shown by Fig. 116, with the top edges of the boards exactly to the grade of the top surface of the finished walk. The concrete is then deposited between these side forms and tamped until it is brought up to the level marked by the templet A. If the plan is to deposit the base in sections transverse plates of 3/8 to ¼ in. steel are set across the walk between the side boards at proper intervals and the concrete tamped behind them; sometimes the concreting is done in alternate blocks. When the steel plate is withdrawn an open joint is left for expansion and contraction. Where the plan is to lay the base in one piece which is afterwards cut into blocks, the cutting is done with a spade or cleaver. [Illustration: Fig. 116.--Sketch Showing Method of Constructing Cement Walks.] [Illustration: Fig. 117.--"Jointer" for Cement Sidewalk Work.] Portland cement mortar mixed 1 to 1½ to 1 to 2 is used for the wearing surface, and is laid from ½ in. to 1½ ins. thick, depending upon the width of the walk and the thickness of the base. As a rule the mortar is mixed rather stiff; it is placed with trowels in one coat usually, but sometimes in two coats, and less often by tamping. The mortar coat is brought up flush with the top edges of the side forms by means of the templet B, and the top finished by floating and troweling. The wearing coat is next divided into sections corresponding with the sections into which the base is divided, by cutting through it with a trowel guided by a straight edge and then rounding the edges of the cut with a special tool called a jointer and shown by Fig. 117. An edger, Fig. 118, is then run around the outside edges of the block to round them. The laying of the mortar surface must always follow closely the laying of the base so that the two will set together. [Illustration: Fig. 118.--"Edger" for Cement Sidewalk Work.] ~BONDING OF WEARING SURFACE AND BASE.~--Trouble in securing a perfect bond between the wearing surface and the base usually comes from one or more of the following causes: (1) Applying the surface after the base concrete has set. While several means are available for bonding fresh to old concrete as described in Chapter XXIV, the better practice is not to resort to them except in case of necessity but to follow so close with the surfacing that the base will not have had time to take initial set. (2) Poor mixing and tamping of this base concrete. (3) Use of clayey gravel or an accumulation of dirt on the surface. In tamping clayey gravel the water flushes the clay to the surface and prevents the best bond. (4) Poor troweling, that is failure to press and work the mortar coat into the base concrete. Some contractors advocate tamping the mortar coat to obviate this danger. Conversely, to make the surface coat adhere firmly to the base it must be placed before the base concrete has set; the base concrete must be thoroughly cleaned or kept clean from surface dirt; the surface coat must be tamped or troweled forcibly into the base concrete so as to press out all air and the film of water which collects on top of the concrete base. ~PROTECTION OF WORK FROM SUN AND FROST.~--Sun and frost cause scaling and hair cracks. For work in freezing weather the water, sand and gravel should be heated or salt used to retard freezing until the walk can be finished; it may then be protected from further action of the frost by covering it first with paper and then with a mattress of sawdust, shavings or sand and covering the whole with a tarpaulin. Methods of heating concrete materials and rules for compounding salt solutions are given in Chapter VII. The danger from sun arises from the too rapid drying out of the surface coating; the task then is to hold the moisture in the work until the mixture has completely hardened. Portable frames composed of tarpaulin stretched over 2×4-in. strips may be laid over the finished walk to protect it from the direct rays of the sun; these frames can be readily removed to permit sprinkling. Practice varies in the matter of sprinkling, but it is the safe practice in hot weather to sprinkle frequently for several days. Moisture is absolutely necessary to the perfect hardening of cement work and a surplus is always better than a scarcity. In California the common practise is to cover the cement walk, as soon as it has hardened, with earth which is left on for several days. ~CAUSE AND PREVENTION OF CRACKS.~--Cracks in cement walks are of two kinds, fractures caused by any one of several construction faults and which reach through the surface coating or through both surface and base, and hair cracks which are simply skin fractures. Large cracks are the result of constructive faults and one of the most common of these is poor foundation construction; other causes are poor mixing and tamping of the base, too large blocks for thickness of the work, failure to cut joints through work. Hair cracks are the result of flushing the neat cement to the surface by excessive troweling or the use of too wet a mixture. The prevention of cracks obviously lies in seeing that the construction faults cited do not exist. If expansion joints are not provided, a long stretch of cement walk will expand on a hot day and bulge up at some point of weakness breaking the walk. ~COST OF CEMENT WALKS.~--The cost of cement walks is commonly estimated in cents per square foot, including the necessary excavation and the cinder or gravel foundation. The excavation usually costs about 13 cts. per cu. yd., and if the earth is loaded into wagons the loading costs another 10 cts. per cu. yd., wages being 15 cts. per hr. The cost of carting depends upon the length of haul, and may be estimated from data given in Chapter III. If the total cost of excavation is 27 cts. per cu. yd., and if the excavation is 12 ins. deep, we have a cost of 1 ct. per sq. ft. for excavation alone. Usually the excavation is not so deep, and often the earth from the excavation can be sold for filling lots. In estimating the quantity of cement required for walks, it is well to remember that 100 sq. ft. of walk 1 in. thick require practically 0.3 cu. yd. concrete. If the concrete base is 3 ins. thick, we have 0.3 × 3, or 0.9 cu. yd. per 100 sq. ft. of walk. And by using the tables in