All walls must be expected to consolidate and settle down when weight comes upon them, but so long as they settle equally no injury is done; inequality of settlement, however slight, is dangerous, and produces unsightly cracks in the masonry.
A want of uniformity in construction leads to such results, and other evils are involved, among which is instability when exposed to the action of fire. With regard to this, Captain Shaw, the Chief of the London Fire Brigade, says, "The walls of a most pretentious and imposing building, of sufficient thickness, and apparently constructed of sound stones, are found to crack at an early stage of a fire, and perhaps to fall down altogether, and then it is discovered that they have been only a deception, having been constructed externally of stone and internally of brick."1
It will be as well to notice two or three forms of composite walls, in order that their structure and defects may be described.
In all compound walls the backing should have joints as nearly as possible equal in number and thickness to those in the face, so that the back and front may settle down under pressure to the same extent; if not, the joints should be in cement or quick-setting mortar, in order that they may become consolidated before any pressure comes upon them.
It is a common practice, especially in using single Flemish bond, to build the face work with better bricks, and with thinner joints, than the backing. This leads to unsound work, and should not be allowed.
1 Fire Surveys, by Captain Shaw, C.B.
In such cases, on account of the joints of the backing being thicker than those of the face work, the courses will not be of the same depth in front and back. For example, it may require eight or nine courses of the face to gain the same height as six or seven in the backing (see Fig. 1), and it is only when they happen to come to a level, as at aa (once in every eight courses or so), that headers can be introduced. Even the few that can thus be used are liable to be broken off by inequality of settlement, caused by the difference in the thickness of the joints. This may be partly remedied by using thinner bricks in the backing, so as to have the same number of joints in face and back; but even then the difference in thickness of the joints in facing and backing tends to cause unequal settlement, unless the work is built in very quick-setting mortar which will harden before any weight comes upon it.
A further result of this practice is that, in order to economise the more expensive face bricks, dishonest bricklayers will cut nearly all the headers in half, and use "false headers" throughout the work, so that there is a detached slice, 4½ inches thick, on the face, having no bond whatever with the remainder of the wall.
Such constructions are liable in an aggravated degree to the evils pointed out as existing in walls built with different qualities of bricks. The coarser and more numerous joints in the brick backing are sure to consolidate to a greater extent than the few and fine joints of the ashlar, and thus tend to cause a separation of the face and backing; or, if this is prevented by bond stones, the facing will probably bulge outwards.
In building such work the ashlar stones should be of heights equal to an exact number of courses of the brickwork, in order that they may bond in with it; the stones should be properly square throughout, with the back joints vertical, so as to leave no vacuities between the facing and the brickwork, for these could not be properly filled in without the expense of cutting bricks to fit the irregularities.
Rubble Ashlar consists of an ashlar stone face with rubble backing (see Fig. 3), and is subject, even to a still greater extent than brick ashlar, to the evils caused by unequal settlement.
To avoid these evils, the stones and joints of the rubble backing should, as before mentioned, be made as nearly as possible of the same thickness as those in the ashlar facing, or, if the joints are necessarily thicker, there should be fewer of them, so that the total quantity of mortar in the backing and face may be about the same. This can seldom be economically arranged in practice, but it should be remembered that the more numerous and coarser the rubble joints, the worse the construction becomes.
The ashlar should be bonded in with "through-stones" or "headers," as previously described; their vertical joints should be carefully dressed for some distance in from the face, and their beds should be level throughout; the back joint and sides of the tails of the stones, may, however, be left rough, the latter may even taper in plan with advantage, and they should extend into the wall for unequal distances, so as to make a good bond with the rubble, the headers from which should reach well in between the bond stones of the ashlar. Through stones may be omitted altogether, headers being inserted at intervals on each side extending about 2/3 across the thickness of the wall.
Care must be taken that the stones in the ashlar facing have a depth of bed at least equal to the height of the stone. In common work the facing often consists merely of slabs of stone having not more than from 4 to 6 inches bed, with a thin scale of rubble on the opposite side, the interval being filled in with small rubbish, or by a large quantity of mortar, which has been known to bulge the wall by its hydrostatic pressure.
The ashlar facing is in all respects, except those above mentioned, built as described in the section on ashlar, Part I., and the backing may be of random rubble done in courses from 10 to 14 inches high, according to the depth of the stones in the facing.
Fig. 3 is the section of a wall 3 feet thick, with an ashlar facing composed of good substantial stone.