The cost, of formwork is given in summary as follows:
| Lagging per M. ft. (used twice): | |
| Lumber at $23 | $11.50 |
| Erection | 15.00 |
| ——— | |
| Total cost erected | $26.50 |
| Studding and rough boards used in place of lagging per M. ft. (used twice): | |
| Lumber at $25 | $12.50 |
| Erection | 10.00 |
| ——— | |
| Total cost erected | $22.50 |
| Wales per M. ft. (used six times): | |
| Lumber at $36 | $ 6.00 |
| Erection | 10.00 |
| ——— | |
| Total cost erected | $16.00 |
The total cost of the main arch span centers to the District of Columbia was $54,000 or $59 per lineal foot of arch span, or $37.33 per M. ft. B. M. The cost of center erection and demolition was as follows:
| Erection below springing line per M. ft. | $15 |
| Erection above springing line per M. ft. | 25 |
| Demolition | 5 |
The salvage on the centers amounted to $11 per M. ft. B. M.
The spandrel arch centers were each used twice and cost per M. ft. B. M. for
| Lumber at $25 per M. ft. | $12.50 |
| Erecting at $25 per M. ft. | 25.00 |
| Moving at $5 per M. ft. | 5.00 |
| Total per M ft. | 42.50 |
Molding Concrete Blocks.—The bridge is trimmed throughout with molded concrete blocks, comprising belt courses, quoin stones, chain stones, ring stones, brackets and dentils. The blocks were made of a 1-2-4½ concrete faced with a 1-3 mixture of Dragon Portland cement and bluestone screenings from ⅜-in. size to dust. They were cast in wooden molds with collapsible sides held together by iron rods. Each mold was provided with six bottoms so that the molded block could be left standing on the bottom to harden while the side pieces were being used for molding another block. The molding was done on a perfectly level and tight floor on mud sills, the perfect level of the molding platform having been found to be an important factor in securing a uniform casting. The blocks were molded with the principal showing face down and the secondary showing faces vertical. The facing mortar was placed first and then the concrete backing. Care was taken to tamp the concrete so as to force the concrete stone into but not through the facing. Mr. Douglas remarks that the back of the block should always be at the top in molding since the laitance or slime always flushes to the surface making a weak skin which will develop hair cracks. In this work the backs of the blocks were mortised by embedding wooden cubes in the wet concrete and removing them when the concrete had set. These mortises bonded the blocks with the mass concrete backing. The blocks were left to harden for at least 30 days and preferably for 60 days and were then bush hammered on the showing faces, some of the work being done by hand and some with pneumatic tools.