There is unfortunately a considerable variation in the methods of measuring brickwork, the variation depending on local trade customs. Brickwork is often paid for by the perch. The volume of a perch was originally taken from a similar volume of stone masonry, the unit being a section of the wall one rod (16 1/2 feet) long and one foot high. Since the usual custom made such a wall 18 inches thick, the volume 24f cubic feet came to be considered as one perch of masonry; then this number was modified to the round number 25 cubic feet, for convenience of computation. The construction of walls one foot thick and with the same face unit of measurement, gave rise to a unit volume of 16 1/2 cubic feet, which was also called a perch. Such units have undoubtedly arisen from the fact that it requires more work per cubic yard to build a thin wall than a thick wall, and the brick mason desires a unit of measurement more nearly in accordance with the labor involved.

Brick is generally paid for by the cubic yard or by the thousand, and the bidder must make his own allowance, if necessary, for any extra work due to thin walls. The number of brick per cubic yard depends on the thickness of the joints and on the size of the bricks. A very slight variation in the thickness of the joint will change very materially the number of brick per cubic yard, and also the amount of mortar. The exact values (according to the size of the brick and the thickness of the mortar joint) are as given below; but the values are not closely to be depended on, because of these variations:

Quantities of Brick and Mortar

 Kind of Brick Size (Inches) Thickness OF Joints No. OF Brick per Cubic Yard MORTAR Per Cubic Yard of Masonry Per 1,000 Brick Common brick 8 1/4 X 4 X 2 1/4 1/2 in. 430 .34 cu yd. .80 cu. yd. ,, ,, 8 1/4 X 4 X 2 1/4 1/4 in. 516 .21 ,, ,, .40 ,, ,, Pressed ,, 8 3/8 X 4 1/8 X 2 1/4 1/8 in. 544 .11 ,, ,, .21 ,, ,,

It is very common and convenient to estimate that 1,000 brick will make two cubic yards of masonry. The number of brick per cubic yard given above is the equivalent of 16, 19, and 20 brick per cubic foot. Bricklayers (backed up by their unions) sometimes demand pay per 1,000 brick laid, but compute the number on the basis of 7 1/2 bricks per superficial foot of a wall 4 inches thick, 15 bricks for a "9-inch wall," and 22 1/2 bricks for a "13-inch wall." The number actually used in a 13-inch wall varies from 17 to 20.

## 154. Cost Of Brickwork

A laborer should handle 2,000 brick per hour in loading them from a car to a wagon. If they are not unloaded by dumping, it will require as much time again to unload them. A mason should lay from 1,200 to 1,500 brick per 9-hour day on ordinary wall work. For large, massive foundation work with thick walls, the number should rise to 3,000 per day. On the other hand, the number may drop to 200 or 300 on the best grade of pressed-brick work. About one helper is required for each mason. Masons' wages vary from 40 to 60 cents per hour; helpers' wages are about one-half as much.

## 155. Impermeability

As previously stated, brick is very porous; ordinary cement mortar is not water-tight; and therefore, when it is desirable to make brick masonry impervious to water, some special method must be adopted as described in Part I, under the head of "Waterproofing."

## 156. Efflorescence

This name is applied to the white deposit which frequently forms on brickwork and concrete, and has already been described in Part I. The Sylvester wash has frequently been used as a preventive, and with fairly good results. Diluted acid has been used successfully to remove the efflorescence. These methods have already been described in Part I.

## 157. Brick Piers

A brick pier, as a general rule, is the only form of brickwork that is subjected to its full resistance. Sections of walls under bearing plates also receive a comparatively large load; but only a few courses receive the full load, and therefore a greater unit-stress may be allowed than for piers.

Kidder gives the following formulae for the safe strength of brick piers exceeding 6 diameters in height:

 Piers laid with rich lime mortar, (1) Safe load in pounds per square inch = 110-5 H/D. (1) Piers laid with 1 to 2 Natural cement mortar. Safe load in pounds per square inch = = 140 - 5 1/2 H/D (2) Piers laid with 1 to 3 Portland cement mortar. Safe load in pounds per square inch = 200 - 6 H/D. (3)

In the above formulae, H is the height of the column in feet, and D is the diameter of the column in feet.

For example, a column 16 feet in height and 1 3/4 feet square, laid with rich lime mortar, may be subjected to a load of 65 pounds per square inch, or 9,360 pounds per square foot; for a 1 to 2 natural cement mortar, 90 pounds per square inch, or 12,960 pounds per square foot; and for a 1 to 3 Portland cement mortar, 146 pounds per square inch, or 20,914 pounds per square foot.

The building laws of some cities require a bonding stone spaced every 3 to 4 feet, when brick piers are used This stone is 5 to 8 inches thick, and is the full size of the pier. Engineers and architects are divided in their opinion as to the results obtained by using the bonding stone.

## Concrete

158. Concrete is extensively used for constructing the many different types of foundations, retaining walls, dams, culverts, etc. The ingredients of which concrete is made, the proportioning and the methods of mixing these materials, etc., have been discussed in Part I. Methods of mixing and handling concrete by machinery will be discussed in Part IV. Various details of the use of concrete in the construction of foundations, etc., will be discussed during the treatment of the several kinds of work.