The consistencies which are indicated by the terms damp, medium, and wet are defined as follows: (1) In damp consistency the per cent of water used gives the driest mixture which can be used in all five types of block machines; (2) medium consistency is halfway between the damp and the wet consistencies; (3) in wet consistency the per cent of water used gives the wettest mixture which can be used. The five cement-block machines which are in use were loaned to the laboratories under an arrangement made by a committee of the United Concrete Block Machine Manufacturers' Association, through the courtesies of the following companies: American Hydraulic Stone Company, Denver, Colo.; Miracle Pressed Stone Company, Minneapolis, Minn.; P. B. Miles Manufacturing Company, Jackson, Mich.; Dykema Company, Grand Rapids, Mich.; Century Machine Company, Jackson, Mich. These machines are shown in Pl. XX. PROGRAMME OF INVESTIGATIONS. The programme that has been adopted as a suggested outline covering all the investigations of mortar and concrete blocks is as follows: I. VARIABLES IN THE MANUFACTURE OF BLOCKS UNDER INVESTIGATION. A. Type of wall block—all plain face and standard ends: 1. With facing. a. One-piece wall block. (1) Hollow block. (a) Down face. x. Single air space. (b) Side face. x. Single air space. (2) Solid block. (b) Side face. b. Two-piece wall block. (1) With metallic bond. (2) Without metallic bond. 2. Without facing. a. One-piece wall block. (1) Hollow block. (a) Down face. x. Single air space. (b) Side face. A. Type of wall block—all plain face and standard ends-Continued. 2. Without facing-Continued. a. One-piece wall block-Continued. (2) Solid block. (a) Down face. b. Two-piece wall block. (1) With metallic bond. B. Materials used: 1. Cement, typical Portland. a. Single. (1) Sand. (2) Limestone. (3) Granite. (4) Gravel. (5) Cinder. b. Double, consisting of sand and— (1) Limestone. (2) Granite. I. VARIABLES IN THE MANUFACTURE OF BLOCKS UNDER INVESTIGATION-continued. C. Dimensions of specimen: 1. Outside. a. 8 by 8 by 16 inches. b. 9 by 12 by 24 inches. 2. Web-1 to 3 inches. 3. Air space-30 to 333 per cent. D. Consistency: 1. Damp. 2. Medium. 3. Wet. E. Proportions: 1. Mortar. a. 1:2. b. 1:4. c. 1:8 d. Balanced proportions for water proofing. 2. Concrete. a. 1:2:4. b. 1:2:5. c. 1:3:6. d. Balanced proportions for waterproofing. F. Process of manufacture: 1. Mixing. a. Hand. b. Machine. 2. Molding. a. Wet mixture-cast in molds in which test pieces remain until hard set. (1) Sand molds. (a) Poured without vibra- (b) Poured with vibration. (2) Metal molds. J. Use of waterproofing compounds: 1. Applied to surface. (a) Poured without vibra- 2. Added to material. Method, The mixing is performed in the same manner as described under "Beam section" (pp. 48-50). The mortar entering the blocks that have been made has been so dry that the forms could be removed as soon as the tamping was completed. Two men are employed in molding the blocks-one to shovel the material into the molds and one to tamp. The concrete is put in the molds in 3-inch layers and tamped with a hand or pneumatic tamper, care being taken to tamp all the blocks in the same manner and the same length of time. As soon as the tamping is finished, the sides of the forms are removed and the block is weighed and placed in the moist storage room. The weight of the blocks in any one batch is not permitted to vary more than 1 per cent. After the sides of the molds are removed the blocks are allowed to remain on the bottom for sixty hours. At the end of this time they are marked and piled in the order in which they are to be tested. Apparatus. All the mortar and concrete used in the tests are mixed in a one-half cubic yard Chicago cube mixer, which is mounted on skids and is motor driven. The discharging end of the mixer appears in the background at the left in Pl. XX, which shows the mixing and molding room.. Through the open doorway near the center of the picture can be seen the testing room. The water used is weighed in the barrel resting on the platform scales, the process being similar to that described under "Constituent-materials section" (p. 32). STORAGE. One of the moist rooms for the storage of blocks and cylinders is shown in Pl. XIX, B. There are two of these rooms, which are lined with moisture-proof paper and furnished with water and steam sprinklers near the ceiling. The specimens are sprinkled at regular intervals of eight hours. The five different types of blocks and corresponding cylinders may be seen in Pl. XIX, B. STRENGTH TESTS. Apparatus. The strength tests of the blocks and corresponding cylinders are made on a 200,000-pound 4-screw motor-driven testing machine, a portion of which will be seen through the doorway in Pl. XX. The blocks are first tested for transverse strength on a span of 20 inches, the load being applied at the center of the span. Pl. XXI, A, shows a building block in the testing machine ready for application of the load. An isometric drawing of each block is given to the operator and the position of the break is sketched upon the drawing, the distance of the break from one end of the block being measured and recorded on the drawing. The two pieces of the block resulting from the transverse test are placed in the testing machine one at a time and tested for compressive strength. A half block ready for testing is shown in place in the testing machine in Pl. XXI, B. The top and bottom of the block are bedded with thick sheets of asbestos, and the spherical bearing block (a) assures an even distribution of the load. Before these halves are tested, the top and bottom surfaces of each are traced, full size, on large sheets of paper for the purpose of getting the least area subjected to compressive stress. The load at the first crack and the breaking load are recorded. The ends of the cylinders are smoothed off with plaster of Paris and are tested as described under "Constituent-materials section" (pp. |