As each block gave 9 × 32 = 288 sq. ins., or 2 sq. ft., of wall surface, the cost of the wall per square foot was 19.5 cts. Assuming 40 per cent. hollow space, each block contained 1 cu. ft. of concrete, which cost 23 cts., or $6.21 per cu. yd., for materials and molding. Blocks in the wall cost $10.55 Per cu. yd. of concrete.
General Cost Data.—The following data are given by Prof. Spencer B. Newberry. The average weights of three sizes of hollow blocks are as follows:
| Size, ins. | P. C. Hollow Space. | Weight, lbs. |
| 8×9×32 | 33⅓ | 120 |
| 10×9×32 | 33⅓ | 150 |
| 12×9×32 | 33⅓ | 180 |
Costs of materials are assumed as follows:
| Item. | Per 100 lbs. |
| Cement, at $1.50 per bbl. | $0.40 |
| Hydrated lime, at $5 per ton | $0.25 |
| Sand, gravel or screenings, at 25 cts. per ton | $0.012 |
Mixed in batches of 750 lbs., sufficient for six 8-in. or four 12-in. blocks, the cost of materials per batch and per block will be for a 1-4 mixture as follows:
| Item. | Per Batch. | 8-in. Block. | 12-in. Block. |
| 150 lbs. cement | $0.60 | $0.10 | $0.15 |
| 600 lbs. sand | 0.072 | 0.012 | 0.018 |
| ——— | ——— | ——— | |
| Total | $0.672 | $0.112 | $0.168 |
In general a factory producing 600 8-in. blocks per day will require 25 men to operate it. At an average wage of $1.80 per day the following is considered as a fair estimate of cost:
| Item. | Per Day. | Per Block. |
| Materials for 600 blocks | $ 60 | $0.10 |
| 25 men, at $1.80 | 45 | 0.075 |
| Repairs | 10 | 0.017 |
| Office and miscellaneous | 20 | 0.034 |
| —— | ——— | |
| Total | $135 | $0.226 |
This gives for 8×9×32-in. blocks a cost of about $6.78 per cu. yd. of concrete for materials and molding or of 11.3 cts. per sq. ft. of face.