Blown Joint

A joint may be blown either through the swelling of cement, or in work exposed to the action of waves from hydrostatic pressure on the air confined in the joint.

Bolted Joint, - Massive iron castings intended to act as footing pieces to the ends of arched principals, or as base plates to the ribs of cast iron arches, are held down on masonry foundations by means of holding down or anchor bolts from 2 in. to 3 in. diameter, which are built into the work with heads downwards at points a few courses below the bed joints of the castings, and pass upwards through drilled holes or holes cut in the joints. Similarly, masonry in breakwaters, lighthouses, etc, exposed to the fury of wave action, however well joggled and dowelled, may be reinforced in strength by iron bars or straps connected with rods or bolts passing through drilled holes in the blocks of several courses, and well screwed up with proper nuts. Where concrete is used, rod bolts 3 in. or 4 in. diameter may be conveniently secured to underlying courses of masonry, and left projecting upwards with twisted ends to be irremovably set and fixed in the concrete that is afterwards deposited to consolidate round and about them. Works on a smaller scale require blocks of stone to be mortised for lewis bolts which are run with lead, or else the stones are bored right through to receive larger bolts. In either case the screwed ends of the bolts pass through the bolt holes in the ironwork, and after the bearing surfaces are truly levelled and adjusted as elsewhere explained, the ironwork is finally bedded upon the masonry, and the two are tightly united with nuts and washers. Doorcases likewise are secured to stone door jambs by J- in. bolts and nuts run with lead. Cast iron chairs supporting the feet of iron rafters are bolted down to stone templates, etc, with lewis or dovetail bolts leaded thereto; and amongst numberless other applications of the screw bolt to mason's work may be noticed the securing of guard piles to the face of quay walls, which is well effected by its aid.

Bond Or Bonding Joint

The size of the blocks in ashlar masonry makes the contrivance called bond, described in the Bricklayers' Section, of great value from its contributing to the formation of walls of no mean strength without the adventitious aid of cement, provided only that the stones are set with beds and joints accurately dressed both square and smooth. Bond, through its instrumentality, by enlisting as it were to its service the vertical pressure due to the operation of gravity, causes the weight of each block to fall upon two or more adjacent stones of the course below, by which means they become more or less united; and the same process occurring also with the resultant pressure on each block, not only is the wall rendered firm, but by a precisely similar action the pressure produced by inequality of load is widely distributed before it is transmitted to the ground, so that in fact the strength of ashlar necessarily depends upon the excellence of its bond. The various kinds of masonry, however, - whether coursed or uncoursed ashlar, block-in-course walling, squared rubble coursed or uncoursed, random rubble, or any other description compounded of one or more of these - have but the one common bond of the broken joint. In the present day neither the preparation of the stones nor the way they are dressed with the pick, point, or tool, nor the mode of arranging them in the courses, gives a nomenclature to the bonding as such, but merely to the general disposition of the stones. There are one or two exceptions, however, which are unimportant, inasmuch as the terms are but little used. Cross bond signifies that the cross and back joints break joint at the adjacent courses, as, for instance, when there are two and three stones alternately in the width of each course. Dog's tooth bond is an appellation given to a simpler kind of cross bond by which binders or headers cross alternately from opposite sides and overlap in the centre of the wall for a space equal to 1/3 its thickness. Notwithstanding what has been said in adulation of bond, it is but fair to observe that no matter how good it is, and however well cemented, it is yet insufficient without other devices, such as dovetailing, joggling, cramping, and bolting both blocks and courses to obtain that approach to monolithic strength which constitutes one of the goals of modern engineering. This is particularly the case when the load or impact does not act vertically. Moreover, good bond, though of the highest importance and even thus assisted, is of little avail without careful bedding, and this again cannot be accomplished without accurately cut beds and joints, fine mortar, and perfectly solid work free from all dry packing, interstices, vacuities, and splits, especially under the through stones, which pass right through from back to front, or other headers sometimes called inbonds or binders, which only lap with their tails to form the cross bond. Unless these rest on consolidated beds, they act with leverage and set up internal dislocating forces. With all descriptions of masonry it is comparatively easy to break the vertical joints in the facing or backing, but the transverse bond being out of sight, and the hearting or filling of thick walls consisting often of rubble, inducements are thus offered to arrange and bed the stones in the interior of walls with an absence of that painstaking and systematic care so frequently observable at the more conspicuous parts. Hence, careful and firm and intelligent supervision is very necessary. The chief feature of good bond in all varieties of masonry for ordinary building purposes, over and above what has been already animadverted upon, is a proper proportion of equal-spaced, equal-sized headers, showing along each course an aggregate length equal to from 1/6 to of the whole. Where voids occur, the headers should show a combined area holding a somewhat similar proportion to the total area of the facade. This should be supplemented by a fair overlapping of all the stones, and a careful selection of the best and largest blocks for the corners and chief openings. In the case of masonry built to resist percussive action, horizontal bonding courses extending throughout from face to back, vertical cross walls and counterforts are expedients more or less often resorted to, but in all cases the hearting and the contents of the pockets, if intended to add to the strength of the structure, must be either of the best hydraulic concrete carefully deposited and rammed, or else of well-packed and thoroughly grouted rubble. Analogous to the use of hoop-iron bond in brickwork, chain bond is resorted to in masonry. This consists of one or more rows of wrought-iron bars 4 in. by in., or thereabouts, let into grooves in the bed joints and completely embedded in Portland cement. The bars are connected by cross pieces when the walls are thick, the whole being well secured by bolts, or otherwise, at the joints and angles. Chain bond is further noticed under Collar Joint.