BOND IN BRICK ARCHES. The only matter requiring special mention in connection with brick arches is the bond to be employed. When the thickness of the arch exceeds a brick and a half, the bond from the soffit outward requires attention. There are three principal methods employed in bonding brick arches. (1) The arch may be built in concentric rings; i.e., all the brick may be laid as stretchers, with only the tenacity of the mortar to unite the several rings (see Fig. 223). This form of construction is frequently called rowlock bond. (2) Part of the brick may be laid as stretchers and part as headers, as in ordinary walls, by thickening the outer ends of the joints—either by using more mortar or by driving in thin pieces of slate—so that there shall be the same number of bricks in each ring (see Fig. 224). This form of construction is known as header and stretcher bond, or is described as being laid with continuous radial joints. (3) Block in course bond is formed by dividing the arch into sections similar in shape to the voussoirs of stone arches, and laying the brick in each section with any desired bond, but making the radial joints between the sections continuous from intrados to extrados. With this form of construction, it is customary to lay one section in rowlock bond and the other with radial joints continuous from intrados to extrados, the latter section being much narrower than the former (see Fig. 225).
1. The objection to laying the arch in concentric rings is that, since the rings act nearly or quite independently of each other, the proportion of the load carried by each can not be determined. A ring may be called upon to support considerably more than its proper share of the load. This is by far the most common form of bonding in brick arches, and that this difficulty does not more often manifest itself is doubtless due to the very low unit working pressure employed.
The mean pressure on brick masonry arches ordinarily varies from 20 to 40 pounds per square inch, under which condition a single ring might carry the entire pressure (see 4 622-27). The objection' to this form of bond can be partially removed by using the very best cement mortar between the rings.
The advantages of the ring bond, particularly for tunnel and sewer arches, are: a. It gives 4-inch toothings for connecting with the succeeding section, while the others give only 2-inch toothings along much of the outline. b. It requires less cement, is more rapidly laid, and is less liable to be poorly executed. c. It possesses certain advantages in facilities for drainage, when laid in the presence of water.
2. The objection to laying the arch with continuous radial joints is that the outer ends of the joints, being thicker than the inner, will yield more than the latter as the centers are removed, and hence concentrate the pressure on the intrados. This objection is not serious when this bond is employed in a narrow section between two larger sections laid in rowlock courses (see Fig. 225).
3. When the brickwork is to be subject to a heavy pressure, some form of the block in course bond should be employed. For economy of labor, the "blocks" of headers should be placed at such a distance apart that between each pair of them there shall be one more course of stretchers in the outer than in the inner ring; but a moment's consideration will show that this would make each section about half as long as the radius of the arch,—which, of course, is too long to be of any material benefit. Hence, this method neces sitates the use of thin bricks at the ends of the rings.