The roof slabs were raised from the casting beds by means of the locomotive crane and hooks, as shown by Fig. 245, and loaded onto cars; eight slabs made a carload. The cars were run to the work, where the gin poles hoisted the slabs one at a time to cars running on a track built on timbers laid on top of the roof girders. A small derrick on rafters picked the slabs from the hand car and set them in place. A gang of 15 men erected from 18 to 20 slabs per 10-hour day. With average wages at 21 cts. per hour the cost of erection was (15 × $2.10) ÷ 19 = $1.66 per slab, or $1.79 per cu. yd. The total cost of slabs in place was thus:
| Item. | Per slab. | Per cu. yd. |
| Molding | $6.73 | $7.27 |
| Erecting | 1.66 | 1.79 |
| —— | —— | |
| Total | $8.39 | $9.06 |
In studying these cost figures their limitations must be kept in mind. Because of the character of the available data quantities had in several cases to be estimated from the working drawings. The cost of lumber for and of framing column and girder forms is not included, but this is partly balanced at least by the assumption that each form was erected complete for each column and girder, which was not the case, as has been stated. Cost of plant is not included nor is cost of shoring the columns until girders and struts were placed, nor are several minor miscellaneous items.
HOLLOW BLOCK WALL CONSTRUCTION.—Three general processes of molding hollow wall blocks of concrete are employed: (1) A dry mixture is heavily tamped into a mold and the block is immediately released and set aside for curing; (2) a liquid is poured into molds, where the block remains until hard: (3) a medium wet mixture is compressed into a mold by hydraulic presses or other means of securing great pressure. The molds used may be simple wooden boxes with removable sides or mechanical molds of comparative complexity. Generally mechanical molds, or concrete block machines as they are commonly called, will be used. There are a score or more kinds of block machines all differing in construction and mode of operation. None of them will be described here, but those interested may consult "Concrete Block Manufacture" by H. H. Rice or "Manufacture of Concrete Blocks and Their Use in Building Construction" by H. H. Rice, Wm. M. Torrance and others.
Factory Buildings, Grand Rapids, Mich.—The buildings ranged from one to four stories high and altogether occupied some 74,000 sq. ft. of ground. The owners installed a block making plant fully equipped with curing racks, two Ideal machines, two National concrete mixers, 5 h.p. gasoline engine, platens, tools and a Chase industrial railway.
The walls were constructed of 24-in. square pilasters of blocks arranged as shown by Fig. 246, connected by curtain wall belt courses of single blocks. The blocks were 8×8×16 ins., and after molding the faces were bush hammered and the edges tooled. The pilasters, consisting of four blocks laid around an 8×8-in. hollow space, were solidified by pouring the 8×8-in. space and all but the three outside block cavities with wet concrete. The interior of the building was of regulation mill construction, and as the pilasters reached the heights for beam supports cast iron plates with downward flanges were set in the concrete. These plates had a cast pin projecting upward to fasten the beam end.
Fig. 246.—Concrete Block Pilaster for a Factory Building.
The materials used for the block were Sandusky Portland cement and ¾-in. bank gravel well balanced from fine to coarse. The blocks were molded with 1-3 mortar faces, the mortar being waterproofed by a mixture of Medusa waterproofing compound. All concrete was machine mixed. The men operating the block machines were paid 1 ct. for each block molded, so that their pay depended upon the energy with which they worked. The men handling materials and engaged in handling and curing the blocks were paid $1.75 per day. The gravel was shoveled from the railway cars onto the screens and from the screen piles to the mixers. The gang was organized as follows:
| Item. | Per day. |
| 8 men handling materials, at $1.75 | $14.00 |
| 5 men operating molds, at 1 ct. per block | 15.00 |
| 1 man mixing facing mortar, at $1.75 | 1.75 |
| 2 men loading blocks onto trucks, at $1.75 | 3.50 |
| 2 men unloading blocks from trucks, at $1.75 | 3.50 |
| 3 men sprinkling blocks, at $1.75 | 5.25 |
| ——— | |
| Total, 21 men molding and curing blocks | $43.00 |