Some precautions necessary in the molding and handling of large concrete blocks were discovered in this work and merit mention. In designing blocks for molding it is necessary to avoid thin flanges or the flanges will crack and break off; blocks molded with a 2¼ in. flange projecting 1¾ ins. gave such trouble from cracking on this work that a flange 5 ins. thick was substituted. Provide for the method of handling the block so that dog or lewis holes will not come in the showing faces. Dog holes can be made with a pick when the concrete is three or four weeks old. When it is not practicable to use dogs, two-pin lewises can be used. The lewis holes should be cast in the block and should be of larger size than for granite; they should not be located too near the mortar faces. In turning blocks it is necessary to provide some sort of cushion for them to turn on or broken arrises will result. When the work will permit, it is desirable to round the arrises to about a ⅜-in. radius.
The following general figures of the cost of block work are available. Foreman cutters were paid $5 per day; foreman concrete workers $3 per day; stonecutters $4 per day; concrete laborers $1.70 per day, and common laborers $1.50 to $1.70 per day. Plain and ornamental blocks cost about the same, the large size of the ornamental blocks bringing down the cost. The following is given as the average cost of block work per cubic yard:
| Cement | $ 1.95 |
| Sand | 0.35 |
| Stone | 1.14 |
| Forms, lumber and making | 0.80 |
| Mixing and placing concrete | 1.50 |
| Dressing | 4.73 |
| Handling and setting | 2.00 |
| Superintendence, plant, incidentals at 25 per cent. | 3.12 |
| Condemnation at 5 per cent. | 0.78 |
| ——— | |
| Total cost blocks in place | $16.37 |
It will be seen that the largest single item in the above summary of costs is the item of dressing. This was done, as stated above, partly by hand and partly by pneumatic tools. Hand tooling cost about twice as much as machine tooling, but its appearance was generally better. The average cost of tooling the several forms of blocks is shown by Table XIX. For 42,190 sq. ft. the average cost was 26 cts. per sq. ft. or $2.34 per sq. yd., or $4.73 per cu. yd. of block work. This tooling was done by stone cutters, and was unusually high in cost.
Mass Concrete Work.—All parts of the bridge except the molded block trim were built of concrete deposited in place. Briefly, the molded blocks were set first and then backed up with the mass concrete deposited in forms and on centers. The only features of this work that call for particular description are those in connection with the main arch ring and the spandrel arch construction.
The main arch rings were concreted in transverse sections; Fig. 158 shows the size and order of construction of these sections. Back forms were necessary up to an angle of 45° from the spring line after which the concrete was made somewhat drier and back forms were not used. After Sections 1, 2, 3 and 4 had been concreted they were allowed to set and then the struts and back forms were taken out and the intervening sections were concreted. The large Sections 6 and 7 were concreted in five sections each, in order to permit the taking out of the timber struts supporting the sections above. The concrete in all sections was placed in horizontal layers as a rule and it is the judgment of the engineers in charge of this work that this is the preferable method.
Table XIX.—Showing Cost of Tooling Concrete Ornamental Blocks for Connecticut Avenue Bridge.
| Description. 1: 2: 4½ Concrete Backing. 1: 3 (Mortar Face). | Per Cubic Foot. | Per Superficial Foot of Showing Face. | ||||||
| Total Number Stones Cut. | Number cubic feet in each. | Total cubic feet cut. | Cost per cubic foot. | Superficial feet in each. | Total superficial feet. | Cost per superficial foot. | Number Super. ft. to one cubic foot. | |
| Brackets under Lamps and Rail Posts (Cap and Base) | 344 | 16.0 | 5,500 | $0.27 | 10.5 | 3,630 | $0.41 | 0.66 |
| Moulding under coping | 770 | 5.9 | 4,560 | 0.30 | 3.8 | 2,930 | 0.47 | 0.64 |
| Dentils between Moulding | 520 | 5.5 | 2,860 | 0.20 | 8.0 | 4,160 | 0.14 | 1.45 |
| Coping | 494 | 61.2 | 30,220 | 0.12 | 35.4 | 17,490 | 0.21 | 0.58 |
| Pedestal (3 courses) | 162 | 27.2 | 4,400 | 0.15 | 14.1 | 2,290 | 0.29 | 0.52 |
| Rail Posts (Top and Base) | 296 | 7.1 | 2,100 | 0.50 | 17.3 | 5,100 | 0.21 | 2.43 |
| Lamp Posts and Parapets over Piers (Top and Base) | 248 | 22.9 | 5,690 | 0.17 | 26.5 | 6,580 | 0.15 | 1.16 |
| Average of above—Totals | 2,834 | 19.5 | 55,330 | $0.17 | 14.8 | 43,190 | $0.26 | 0.77 |