Concrete Construction: Methods and Costs

by Halbert Powers Gillette; Charles Shattuck Hill

The sewer was circular, having an inner diameter of 42 ins., the thickness of the invert and the arch alike was 8 ins. Figure 266 is a cross-section. The concrete was 1 of Portland cement to 6 of gravel. There were 11 concrete blocks in the ring of the arch, each block being 24 ins. long, 8 ins. thick, 8 ins. wide on the outside of the arch and 5¾ ins. wide on the inside of the arch. A block weighed 90 lbs. which was too heavy for rapid laying; blocks 18 ins. long would have been better. Some 8,500 blocks were made. Molds were of 2-in. lumber, lined with tin, for after a little use it was found the concrete would stick to the wood when the mold was removed. The four sides of the mold formed the extrados, the intrados, and the two ends of the block; the other two sides being left open. When put together the mold was laid upon a 1-in. board, 12×30 ins., reinforced by cleats across the bottom. The sides of the molds were held together with screws or wedge clamps. When the blocks had set, the sides of the molds were removed, and the blocks were left on the 12×30-in. boards for 3 days, then piled up, being watered several times each day for a week.

A gang of 14 men made the blocks; 2 screening gravel through 1-in. mesh screen; 4 mixing concrete; 4 molders; 3 shifting and watering blocks, and 1 foreman. With a little practice each molder could turn out 175 blocks a day; and since each block measured ¾ cu. ft., the output of the 14 men was 19½ cu. yds. a day. Mr. Gifford informs us that the wages were $1.50 a day for all the men, except the foreman. The daily wages of the 14 men were $22, so that the labor of making the blocks was $1.10 per cu. yd.

Fig. 266.—Sewer with Monolithic Invert and Block Arch.

Each batch of concrete, containing ½ bbl. of Portland cement costing $1.35 per bbl., made 18 blocks. (1 bbl. per cu. yd.) Since the gravel cost nothing, except the labor of screening it, the total cost of each block was 11 to 12 cts., which includes 0.85 cent for use of molds and mold boards, which were an entire loss. At 12 cts. per block the cost was $4.32 per cu. yd.

The contract price was $3 per lin. ft. of this sewer, as against a bid of $3.40 per ft. for a brick sewer.

When the trenching had reached to the level of the top of the invert, two rows of stakes were driven in the bottom, stakes being 6 ft. apart in each row, and rows being a distance apart ¼-in. greater than the outer diameter of the sewer. These stakes were driven to such a grade that the top of a 2×4-in. cap or "runner" set edgewise on top of them was at the proper grade of the top of the invert. The excavation for the invert was then begun, and finished to the proper curve by the aid of a templet drawn along the 2×4-in. runners. In gravel it was impossible to hold the true curve of the invert bottom. Concrete was then placed for the invert. To hold up the sides of the invert concrete, a board served as a support for the insides, but regular forms were more satisfactory in every respect except that they were in the way of the workmen. A form was tried, its length being 6 ft. It was built like the center for an arch, except that the sheeting was omitted on the lower part of the invert. It was suspended from the cross-pieces resting on the "runners." After the concrete had been rounded in place, the form was removed and the invert trued up. This form worked well in soil that could be excavated a number of feet ahead, so that the forms could be drawn ahead instead of having to be lifted out; but in soil, where the concreting must immediately follow the excavation for the invert, the form is in the way. The invert was trued up by drawing along the runners a semi-circular templet having a radius of 21½ ins. Then a ½-in. coat of 1-2 mortar was roughly troweled on the green concrete. Another templet having a 21-in. radius was then drawn along the runners to finish the invert.

When the plaster had hardened, two courses of concrete blocks were laid on each shoulder of the invert, using a 1-2-¼ mortar, the ¼ part being lime paste. The lime made the mortar more plastic and easier to trowel. Then the form for the arch was placed, and as each 8-ft. section of the arch was built, a grout of 1-1 mortar was poured over the top to fill the joints. Earth was thrown on each shoulder and tamped, and the center moved ahead.

Common laborers were used for all the invert work, except the plastering which was done by masons who were paid 30 cts. per hr. Masons were also used to lay the concrete blocks in the arch. Mr. Gifford states that two masons would lay at the rate of 100 lin. ft. of arch per day, if enough invert were prepared in advance. As there were 11 blocks in the ring of the arch, this rate would be equivalent to 7½ cu. yds. of arch laid per mason per day.