transfer the stress between the bars by bond and shear. WIRE MESH, or fabric, is usually spliced by overlapping a distance of 4 in. SPLICES SHOULD BE AVOIDED

AT SECTIONS OF MAXIMUM STRESS.

SPLICES OF VERTICAL REINFORCEMENT in columns and struts should provide sufficient lap to transfer the stress by bond. Splices made with PIPE SLEEVES, or THREADED CONNECTIONS, are usually less desirable than lap-splices. Where the sizes of superimposed columns are reduced at the floor-levels, it is necessary to bend the rods of the lower tier of columns a short distance from the upper extremity, so that they coincide for purpose of splicing with those above. The Joint Committee, 1924, requires that when offset, the slope of the inclined portion from the axis of the member shall not be more than 1 in in 6 in. For

large offsets, more than 3 or 4 in on a side, DOWELS should be used instead of attempting to bend the main reinforcement.

5. Protective Covering of Reinforcement. The INSULATION required around the reinforcement for different classes of work is a very important element of the design. In assembling and placing the reinforcement particular attention should be paid at RECESSES, REVEALS, ORNAMENTAL MARKINGS, etc., to insure sufficient insulation at these points to provide against corrosion. Appropriate means should also be employed to prevent the rusting of BONDS provided for future additions. A brush coat of cement-grout should be considered, for this purpose, as of only temporary value. The insulation of slabsteel is obtained by the chairs used in the positioning of the reinforcement, or by placing precast blocks of cement-mortar beneath the bars. The practice

of supporting the steel of the floor-construction upon pieces of STONE, or GRAVEL, of uncertain thickness should never be permitted. In the case of wall-reinforcement some adequate, fool-proof method should be used to obtain the required thickness of protective concrete.

In placing COLUMN-STEEL it is particularly important to insure proper protection, and for this purpose a satisfactory practice is to employ cement-mortar dough-nuts, which are slipped over the vertical rods and are cast of such thickness as to give the designed distance between the reinforcement and the face of the forms. These are ordinarily made 6 in in diameter with a 2-in hole in the center and 2 in thick; two doughnuts are placed on each of the corner verticals of each column. Fig. 13 illustrates another method of insuring this very important matter of the insulation of column steel. The u-shaped blocks, cast as shown, are hung over the column hoops. The other cement-mortar blocks, shown in the illustration, are for use as SPREADERS between wall forms.

6. Checking Reinforcement. The reinforcement for each section of a building should be in place a sufficient period in advance of the depositing of the concrete to permit a thorough inspection of all elements of the work. Inspection should proceed systematically, checking the SIZE, NUMBER, and LOCATION of the bars, bay by bay, with particular attention to the HEIGHT OF THE STEEL at the various critical sections of the floor-construction.

CHAPTER XX

CONCRETE

1. THE SCOPE OF THE PROBLEM

1. General Considerations. The erection of a concrete building is a manufacturing process carried on in the field, and the skill required for efficient construction is fully equal to that demanded for the design itself. In the drafting-room the COMPRESSIVE STRENGTH of the concrete is assumed as a basis of computation, and the field-methods must be such as to produce CONCRETE

OF THE DESIGNED STRENGTH FOR THE LEAST COST.

To make the best choice from available materials, to GRADE anⱭ PROPORTION them, as well as to MIX, CONVEY, DEPOSIT, and properly CURE structural concrete, are difficult matters, merely from the viewpoint of obtaining satisfactory physical results. When to these requirements is added the necessity of economy, the problem becomes still more complex. In the first place it is necessary to make a thorough survey of the available aggregates. These should be sampled and tested, both separately and when combined in varying proportions with the cement to be used. The STANDARD TESTS at present applied are mentioned in proper sequence under II. Concrete Materials, and are printed in full in the Appendix.

The materials having been chosen, a certain combination of which furnishes the most economical concrete that may be expected entirely to fulfill the designrequirements, TECHNICAL SUPERVISION must continue throughout the progress of the job in order carefully to control the quality of the concrete, which should be checked by frequent compression-tests of samples taken at the job. Besides the technical supervision necessary to insure a high quality of work, efficient construction requires the selection of a suitable cONCRETE-PLANT, the economical use of materials and the coordination of all the human and mechanical elements entering upon the work.

2. CONCRETE MATERIALS

Concrete is a mixture of cement, aggregates, and water; when combined in proper proportions these materials form a plastic mass which hardens through the hydration of the cement. If the chemical action is to take place at a normal rate, and the concrete acquire at various ages a compressive strength approaching the maximum obtainable from the materials and proportions employed, it is necessary that it be SUFFICIENTLY WARM during the early stages of hardening. An adequate degree of MOISTURE must also be present in the surrounding air,