For many years the author's practice has been to stress the concrete in compression only six hundred (600) pounds per square inch; but lately the Joint Committee of the Technical Societies has reported in favor of adopting six hundred and fifty (650) pounds. When a good aggregate is procurable, the author has no objection to this increase of eight per cent; but otherwise he prefers to adhere to his old custom, especially as by so doing he adds only two per cent to the cost of the concrete in those cases where the section can be reduced by using the higher intensity. Very often, though, no such reduction is practicable, and the saving on the entire job reduces to only one per cent.

The actual reduction in the amount of concrete in a beam due to this difference of intensity of working stress is about six per cent, but this is partially offset by an increase in the amount of steel required. The combination of the intensities of 600 for concrete and 16,000 for steel requires a percentage of 0.68 for reinforcing steel, while with 650 and 16,000 that figure is increased to 0.77. Moreover, construction work is simplified by using more concrete and less steel, also better concrete is secured; hence the author does not think that there is much real advantage in adopting the higher intensity. The general complaint from contractors on his work has been "too much steel"; and adopting the higher intensity for concrete would make matters worse. Moreover, the use of heavier concrete sections tends to keep down the shear intensity, which is always desirable.

Frequently there is a choice between a heavy concrete section with no shear reinforcement, and a lighter section with such reinforcement. In superstructure it is generally preferable to adopt the latter and in substructure the former.

Pavings

The subject of the economics of pavings is treated fully in Chapter XXI. The economy lies in the cost of the pavement itself, excepting that brick is heavier than the other paving materials; and, consequently, it adds to the cost of a concrete structure, the amount being from two to three per cent.

It is poor economy to neglect the expansion joints in block pavements; because expansion is sometimes quite destructive, not only causing waves in the pavement but sometimes actually pushing off the curbs.

Handrails

Handrails for reinforced-concrete bridges should always be made of concrete, in order to harmonize with the rest of the structure. A steel railing on a concrete bridge is ruinous to the general appearance, and, therefore, should not be tolerated. The first costs of the two types of railing are about alike; but the concrete one, being heavier, requires more