CONCRETE WORK
16. Plain concrete work is usually paid for by the cubic yard. The contractor furnishes all material, including the lumber, to make the forms; he also erects the forms and removes them after the concrete has been placed. There is no fixed practice regarding openings in walls. Usually, small openings under, say, 100 square feet in superficial area are considered as solid. All larger openings are deducted from the work when measuring for payment. In some localities, the actual volume of the concrete work is taken by the contractor as a basis of the cost. In either case, it is of prime importance that the architect and the contractor make some distinct agreement beforehand as to exactly how the concrete work is to be measured and paid for.
Reinforced-concrete work is also often measured by the cubic yard, although sometimes it is contracted for as a finished building. The steel reinforcement is sometimes included in the price. Often, patented steel reinforcement is bought separately and delivered to the contractor; at other times, the contractor buys the patented steel or else makes it and pays a royalty to the holder of the patent. Reinforced-concrete floors are sometimes measured by the cubic yard and sometimes by the square yard, according to agreement. Pavements are usually measured by the square foot or by the square yard.
In [Table II] are given the costs of stone concrete and gravel concrete. These figures do not include builder’s profit, cost of superintendence, or cost of forms. They are based on the following costs: Labor, 25 cents per hour; cement, $2 per barrel; sand, $1.50 per cubic yard; crushed stone, $1.65 per cubic yard; gravel, $1 per cubic yard.
To the values given in the table, the price of the wooden forms, both for material and erection, must be added. This of course varies considerably, according to whether the work is straight or has a number of corners and openings in it.
TABLE II
COST OF PLAIN STONE CONCRETE
| Mixture | Quantity | Cost | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Cement Parts | Sand Parts | Broken Stone Parts | Cement Barrels | Sand Cubic> Yards | Broken Stone Cubic Yards | Cement | Sand | Broken Stone | Labor | Total |
| 1 | 2 | 4 | 1.5 | .45 | .90 | $3.00 | $.68 | $1.49 | $.75 | $5.92 |
| 1 | 3 | 5 | 1.1 | .50 | .85 | 2.20 | .75 | 1.40 | .75 | 5.10 |
| 1 | 3 | 6 | 1.0 | .45 | .90 | 2.00 | .68 | 1.49 | .75 | 4.92 |
COST OF PLAIN GRAVEL CONCRETE
| Mixture | Quantity | Cost | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Cement Parts | Sand Parts | Gravel Parts | Cement Barrels | Sand Cubic> Yards | Gravel Cubic Yards | Cement | Sand | Gravel | Labor | Total |
| 1 | 2 | 4 | 1.3 | .4 | .80 | $2.60 | $.60 | $.80 | $.75 | $4.75 |
| 1 | 3 | 5 | 1.0 | .5 | .80 | 2.00 | .75 | .80 | .75 | 4.30 |
| 1 | 3 | 6 | .9 | .4 | .85 | 1.80 | .60 | .85 | .75 | 4.00 |
The price of forms, including both material and erection, may be said to vary from 50 cents for ordinary cellar work to $2 for heavy retaining walls per cubic yard of concrete placed.
DATA ON REINFORCED-CONCRETE
BUILDING
17. The cost of reinforced-concrete buildings varies with the market price of cement and the steel bars or metal used for reinforcing. At present, reinforced-concrete buildings of the factory type constructed of common, hard, stretcher-brick walls and reinforced-concrete floors, roof, and columns with foundations may be built for from $1.35 to $1.65 per square foot of floor area. Usually, the height of ceilings in factory buildings is about 14 feet from floor to floor, thus making the cost of this type of building approximately from 10 to 12 cents per cubic foot. For buildings of a better commercial type, with face-brick walls and terra-cotta trimmings, the cost per square foot of floor area will range from $1.65 to $1.90, making the cost per cubic foot from 12 to 14 cents.
Reinforced-concrete buildings, as a rule, exceed the cost of buildings of slow-burning construction of the same size by an amount about equal to the cost of the metallic reinforcement. In other words, a building of reinforced concrete costs from 10 to 25 per cent. more than the same building of slow-burning construction.
The price of reinforced concrete per cubic yard varies within wide limits, depending on the mass of concrete employed and the intricacies of the forms. In building construction, reinforced concrete, including the price of the forms, can usually be placed for from $12 to $18 per cubic yard, the cost of the steel reinforcement being added.
Where the building is of considerable height, the same forms used in the three lower stories may be used in constructing the balance of the building. In such a case, the cost of the forms will probably not exceed $6 for each cubic yard of concrete placed. If the building is only two or three stories in height, and the work is rushed by using new centering in each floor, the cost of the forms will range from $7 to $9 per cubic yard of concrete.
The cost of form work for floor construction will range from 10 to 12 cents per square foot. Column forms will cost in the neighborhood of 20 or 25 cents per running foot. The forms used for fine concrete-wall construction require considerable time and bracing, and generally must be executed with great care where finished work is required. Such forms will cost from 8 to 10 cents per square foot of form on both sides of the wall, but will greatly exceed this price if molded courses or paneled spandrels are to be formed and lettering is to be cut in the work.
18. In estimating the cost of reinforced-concrete slabs, the cost of the centering, the concrete, and the steel reinforcements must be considered. The cost of centering for slab work varies from 5 to 7 cents per square foot, the latter figure probably being more nearly correct for usual conditions. The concrete for plain reinforced-concrete slab construction on steel beams can be placed for about $5.60 per cubic yard, or 1¾ cents for a square foot 1 inch in thickness. At present market values, the steel for reinforcing slabs can usually be considered as costing about 3 cents a pound, the pound price increasing as the rods decrease in size.
[Table III] gives the approximate cost, in cents per square foot, of constructing different thicknesses of reinforced-concrete slabs on steel-beam construction with the different sizes of reinforcing bars usually employed. The prices include cost of centering, concrete, and steel reinforcement.
19. [Table III] cannot be used for estimating the cost of a reinforced-concrete floor with reinforced-concrete beams and girders. In the construction of such a floor, the centering is much more costly than where steel beams are used for the support of the floor slab. On an average, the centering for the reinforced-concrete floor systems, including the studding and shoring, will cost from 20 to 22 cents per square foot, the sides of the beam and girder forms being included in the square-foot estimate. This cost is materially reduced where the centering is used over and over again for the construction of upper floors, so that where the building is six or eight stories in height, the average cost of the centering will not exceed 10 or 12 cents per square foot, including shoring, as just mentioned. Owing to the difficulty encountered in placing the concrete in the beams, the tamping required with slice bars, etc., and the expense of placing the reinforcement, the concrete for such construction will cost about $7.50 or $8 per cubic yard. The steel for the entire construction will usually average from 3 to 4 cents a pound, depending on whether plain rods or deformed bars are used, or whether the system is made up of loose rods or fabricated frames. In estimating the cost of such a floor system, the centering should be figured by a carefully itemized estimate, or roughly from the preceding figures. The amount of concrete in both the slab beams and columns should be estimated, and the total number of cubic yards required for the entire work should be determined; then the unit price for providing and placing the concrete should be carefully considered with reference to local conditions of labor and cost of material.
TABLE III
COST IN CENTS PER SQUARE FOOT OF
REINFORCED-CONCRETE FLOOR SLABS
SUPPORTED ON STEEL BEAMS
CEMENT CELLAR FLOORS
20. In determining the cost of cement cellar floors, the concrete proper and the top coat should be considered separately. The concrete proper is usually reckoned by the cubic yard. About 1 hour more is required to lay a cubic yard of floor than is necessary for plain concrete work. Therefore, in estimating, 25 cents per cubic yard, or the wages of a laborer for 1 hour, must be added to the figures given in [Table II]. A 1-3-6 mixture is generally used. For stone concrete, then, the cost of the concrete proper for a cement cellar floor would be $5.17 per cubic yard, exclusive of the cost of supervision and the builder’s profit.
An analysis of the cost per square yard of top coat 1 inch thick for a cement cellar floor is as follows:
| Cost Cents | ||
| ¼ | hour, plasterer’s time, at 45 cents per hour | 11¼ |
| ¼ | hour, laborer’s time, at 25 cents per hour | 6¼ |
| ¹/₁₅ | barrel of cement, at $2 per barrel | 13⅓ |
| ⅛ | barrel of white sand, at 75 cents per barrel | 9⅜ |
| Approximate cost per square yard | 40 |
To the foregoing should be added the cost of supervision and builder’s profit.
CONCRETE BUILDING BLOCKS
21. Concrete building blocks usually present about 2 square feet of surface in the wall and are generally 8 inches thick, thus making a volume of 1⅓ cubic feet. Since one-third of this volume as a rule is air space, the actual volume of concrete is a little less than 1 cubic foot. The materials in a block of this kind, if used in a 1-4 mixture, will cost about 18 cents. If the block is 12 inches thick instead of 8 inches, the materials will cost about half again as much, or 27 cents. The cost of labor to manufacture these blocks depends on whether they are made in great quantities, and the wages paid the workmen, etc., and varies from 6 to 10½ cents for each block. To lay a block in the wall costs from 5 to 10 cents, 7 cents being about the average price. This price includes the mortar used in laying. For teaming and haulage, an allowance of 5 cents per block is usually sufficient. The cost per block, 8 inches thick, set in the wall is then as follows:
| Maximum Cost Cents | Minimum Cost Cents | ||
| Materials | 18 | 18 | |
| Labor | 10½ | 6 | |
| Placing | 10 | 5 | |
| Haulage | 5 | none | |
| Total | 43½ | 29 |
These results divided by 2 will give the cost of the wall per square foot, since each block is supposed to have a surface of 2 square feet. If the wall is 12 inches thick instead of 8 inches, one-half as much again should be added to the price.