Concrete Construction Part 52

[Ill.u.s.tration: Fig. 262.--Center for Harlem Creek Sewer.]

The cost records of constructing the section of 29-ft. sewer so far built are not susceptible of complete a.n.a.lysis, but the following figures can be given. The prices of materials were as follows:

Cement, per barrel $1.80 Sand, per cubic yard 0.75 Broken stone, per cubic yard 1.00 Reinforcing bars, per pound 0.02 Vitrified brick, per 1,000 12.00

The wages paid different cla.s.ses of labor were:

Per hour.

Firemen $0.50 Laborers 0.175 Laborers 0.20 Laborers 0.25 Laborers 0.28 Laborers 0.3025 Bricklayers 0.66 2/3 Helpers $0.25 Carpenters 0.55 Engineers 0.50 Timekeepers 0.25 Watchmen 0.175 Hostlers 0.175 Teams 0.60

Taking up the several items of work in order, the excavation amounted to 21,400 cu. yds., of which 1,400 cu. yds. were rock excavation. The cost of excavation was as follows:

Total. Per cu. yd.

Earth, excavation $7,640 $0.38 Earth bracing 2,000 0.10 Rock excavation 1,400 1.00 Rock, dynamite, tools, etc. 560 0.40

The cost of crushing the excavated rock and returning it to the mixer was $1 per cu. yd.

The cost of the concrete work was as follows:

Per cu. yd.

1.30 bbl. cement at $1.80 $2.34 .044 cu. yd. sand at 75 cts. 0.33 1 cu. yd. broken stone at $1 1.00 ----- Total concrete materials $3.67

There were 1,600 cu. yds. of concrete placed at a cost of for:

Total. Per cu. yd.

Mixing and placing $1,180 $0.7375 Forms 2,000 1.25 Moving forms 400 0.25 ------ ------- Total for forms and labor $3,580 $2.2375

For reinforcing the concrete 86,600 lbs. of steel, or about 55 lbs. per cu. yd. were used. The cost of placing and bending this steel was as follows:

Total. Per lb.

Cost of placing $172 0.1986 ct.

Cost of bending 52 0.06 ct.

We can now summarize the cost of the concrete work proper of this sewer as follows:

Items. Per cu. yd.

Cement, sand and stone $3.67 55 lbs. steel at 2 cts. 1.10 Forms, labor and materials 1.25 Mixing and placing concrete labor 0.74 Placing steel at 0.1986 ct. per lb. 0.11 Bending steel at 0.06 ct. per lb. 0.03 Moving forms 0.25 ----- Total labor and materials $7.15

To get the total cost of the sewer proper we must add the cost of the vitrified brick invert paving. There were 71 cu. yds. of this paving and its cost was as follows:

Per cu. yd.

0.6 bbls. cement at $1.80 $1.08 0.25 cu. yd. sand at 75 cts. 0.19 450 bricks at $12 per M. 5.40 Labor laying, 71 cu. yds. at $180.33 2.54 ----- Total $9.21

None of the preceding figures includes the plant charges. The plant cost $12,000 and the cost of running it during the work described was $2,000.

In explanation it should be noted that the plant served for building some 1,340 lin. ft. of 27-ft. sewer as well as for the section described.

~SEWER AT MIDDLESBOROUGH, KY.~--In constructing an oval sewer 4 ft. high at Middlesborough, Ky., two steel forms in 10-ft. sections were used. As shown in Fig. 263, T-iron ribs were s.p.a.ced 5 ft. apart, fastened together at the top by longitudinal angle irons, and at the bottom by a sheet of steel 22 ins. wide, forming the bottom of the invert. The lagging for the sides consists of movable 5-ft. lengths of channel iron, secured by sliding bolts. After the bottom of the trench has been roughly shaped with concrete, a 10-ft. section of invert forms is lowered and suspended by the cross-beams, and the s.p.a.ce beneath packed with concrete; then a channel iron is slid into place and bolted, and concrete packed behind it, and so on until the invert is made. The next 10-ft. section is then built while the first is hardening. Upon the completion of the second section, the channel iron sides of the first section are removed, and then the rib framework is lifted out. Wood arch centers are then put in place and an inch of 1:2 plaster spread over the lagging before placing the concrete for the arch, which is 6 ins. thick.

[Ill.u.s.tration: Fig. 263.--Invert Form for Sewer Construction.]

The cost per 100 ft. of this sewer was as follows (prices being a.s.sumed for cement and labor):

Bottom concrete. Cost per 100 ft.

18.5 bbls. cement, at $1.50 $ 27.75 2.7 cu. yds. sand, at $1.00. 2.70 15 cu. yds. stone, at $1.00 15.00 17 days labor, at $1.50 25.50

Bottom concrete. Cost per 100 ft.

25.25 bbls. cement, at $1.50 37.85 7.5 cu. yds. sand, at $1.00 7.50 22 days labor, at $1.50 33.00

Sewer Arch.

26 bbls. cement, at $1.50 39.00 3.9 cu. yds. sand, at $1.00 3.90 13.6 cu. yds. stone, at $1.00 13.60 21 days labor, at $1.50 31.50 ------- Cost per 100 ft. $237.30

[Ill.u.s.tration: Fig. 264.--Sewer at Cleveland, Ohio.]

~INTERCEPTING SEWERS, CLEVELAND, O.~--An intercepting sewer some 3 miles long, of the form and construction shown in Fig. 264, was built at Cleveland, Ohio, in 1904. The construction consists of a plain concrete invert lined with two courses of shale bricks, and having two rows of anchor bars set in the side walls so that the bars of one row are staggered with respect to those of the other row. The anchor bars are 2-in. steel, and are s.p.a.ced 30 ins. apart in each row. To the anchor bars are bolted arch reinforcing bars arranged as shown, and these arch bars have bolted to them eight lines of 1-in. longitudinal bars. A natural cement concrete is used for the invert and side walls. The arch is Portland cement concrete of normally a 1-3-7, 1-in. screened stone mixture, but where the voids in the broken stone exceeded 40 per cent., it is a 1-3-6 mixture. The invert bricks are laid in Portland cement mortar and the arch has a mortar lining and is waterproofed with 1-in.

of mortar on top.

_Forms._--Separate forms were used for the invert and for the arch ring.

Regarding these, the engineer, Mr. Walter C. Parmley, remarks:

One of the first forms used in the sewer was like a piece of segmental arch centering inverted, and with the lagging nailed fast to the ribs.

The trouble with this form is that it is difficult to tamp concrete under the bottom portion of the form, and hence a very rough surface is produced. Much better results were obtained by omitting the lagging boards on the bottom and at the sides till a point was reached where the inclination of the concrete surface was about 45. The concrete for the bottom could then be worked down between the ribs, thorough tamping done, and a good surface obtained. The ribs serve as a guide, so that the workman produces the proper shape. From this point up to the vertical, good results can be secured with the ribs attached to the lagging. Some contractors found it more convenient to use ribs that were connected with each other by a skeleton framework only, and then to slip the lagging in, one piece at a time. For some of the sewers, in which the brick lining was not carried quite up to the spring line, a separate side form of skeleton ribs and loose lagging was set upon brace legs bearing on the bottom of the invert. This form carried the concrete from about 2 ft. below to about 2 ft. above the springing line. The arch ribs then became segmental and rested upon the middle braces. This method has the advantage of using ribs that are lighter and more easily handled than those that are semi-circular. For arch centering, it is necessary and convenient to use independent ribs and loose lagging, for the centers can then be carried forward piece-meal, the falsework upholding the green arch and re-erected at the advance end of the work. In these matters each contractor prefers to use his own ingenuity, and so long as the work is properly built, the engineer can well give him considerable lat.i.tude as to use of methods. One thing, however, the engineer must insist upon--that all centering and falsework be as nearly rigid as possible. Even a slight settlement of the centers at the crown under the load of concrete and back-fill will cause the arch to kick out at the quarters, and if the green concrete arch is not cracked at the crown, it will be crushed on the inside, about half way between the crown and springing line. A reinforced arch is no more immune to this danger than is a plain concrete arch. However, with a few days of hardening, although the damage may be serious, the danger of actual collapse is less. A point to be guarded against, especially in reinforced construction, is any foolish act on the part of contractor or workman, due to his overconfidence in the strength of the structure because it contains embedded steel.

The mode of procedure in constructing the arch ring was to erect the centers with lagging complete. The lagging was then covered with building paper waterproofed with paraffine. The arch reinforcing bars were then bolted to the anchor bars and the longitudinals connected up.

The lining of Portland cement mortar was first laid on the lagging.

Before this mortar had set, concrete was rammed in between it and the sheeting to a height of 18 ins. above the springing line, and then the remainder of the concrete placed without outside forms. The top of the arch ring was finally finished with a 1-in. mortar coat. In regard to the concrete, Mr. Parmley remarks:

"Concrete will flush up to the forms and produce a better surface, and the voids in the stone will be much better filled if it is so wet as to require but little tamping; moreover there is less danger of obtaining a weak, porous wall should a workman neglect thorough tamping, than there is where only a moist mixture is used. It is also to the contractor's interest to use wet concrete for much less labor is required in mixing and placing it. Small broken stone or gravel is preferable in concrete for sewers. The walls being comparatively thin, unless there be a considerable excess of mortar, if coa.r.s.e stones are used, the concrete will be honeycombed with voids. The stones should be well graded in size from large to fine, but the largest fragments should not exceed 1 ins.

in greatest dimension."

_Cost._--A number of records of cost of constructing short sections of the sewer described are given by Mr. Parmley, as follows:

Labor placing anchor bars. Per day.

1 man, at $3.50 $3.50 1 man, at $1.75 1.75 4 hours carrying steel at 20 cts. 0.80 ----- $6.05

The anchor bars were placed for 40 lin. ft. of sewer, or about 1,504 lbs. of metal at a cost of 0.4 ct. per lb.

The concreting gang for the sides consisted of:

5 men wheeling and mixing at $1.75 $8.75 1 man tamping 1.75 2/3 time man lowering brick and concrete at $2.25 1.50 1 man carrying concrete 1.75 ------ $13.75

This gang built the side wall for 40 ft. of sewer daily, or 13 cu. yds.

Cost of labor per cu. yd. was, therefore, $1.06. The concrete was tamped behind the brick lining as the latter was built up by the mason.

Cost of single ring brick lining at sides:

2 masons at 70 cents per hour $1.20 1 man mixing mortar 2.25 1/3 time man lowering at $2.25 0.75 3 men wheeling sand, filling buckets and dumping 5.25 ------ Total labor for 40 lin. ft. of sewer $19.45 Quant.i.ty of brick masonry laid, cu. yd. 6.38 Labor per cu. yd. 3.05

An account was kept of labor performed on 85 lin. ft. of arch work, or 14 1-6 ft. daily. The force was as follows: