This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
When bags of cement are sunk to the bottom of cylinders and rammed down to form foundations where loose cement would be washed away, they virtually become in time solid blocks separated by the bags only. Bags of concrete used likewise for similar constructions may be readily packed together and well jointed by divers so as to make solid work. In both these instances the junctions between the indurated blocks may be appropriately termed from analogy bag joints.
Much that has been said under the Bricklayer with regard to this joint is applicable here also. In order to withstand perpendicular pressure the joint should be left full and square for the whole depth, and whenever ironwork is built in the strictest attention has to be paid to verticality of axis and horizontality of bearing surface. In ashlar a tooled or plain draft is worked round the edge of the bed out of winding (except in the case of skew arches, &c), the rest of the bed between the drafts being only roughly, though neatly, pointed or picked to afford a good key, care being taken that nothing protrudes above the drafts. Usually the bed is boasted, and often accurately worked so that no part lies more than 1/8 in. below the edge of a banker rule applied to opposite drafts. The block should be set firmly without pinning or levelling upon the cementing medium laid over the upper bed similarly wrought of the course below. Good work is executed with lias lime and Portland cement grouting. Foundation stones and base stones consisting of flag stones or landings running under piers require level beds and surfaces, and fair joints. Bed stones, base stones, and templates have been noticed under Bricklayer. Post stones for heavy iron columns should be thick, and have a base of about six feet super. The top beds of all stones forming a bearing surface for setting or fixing the longitudinal or vertical iron or timber elements of a structure must be dressed fair, and the bottom ones left square and full. Drums, or frusta, or tambours of columns unless bedded accurately, will flush or fracture at the arris. A sheet of lead, cut rather less than the bed so as to leave about an inch between it and the outer edges of the stone, prevents flushing or the splintering off of spalls. When Portland cement is used the joints should be raked clean out all round about an inch in depth before it sets. One mode of applying cement grout is as follows : - Four small pieces of sheet lead about 3 in. square being disposed between two adjacent frusta so as to keep the beds asunder and equally bear the weight, the joint is pointed with mortar, leaving a couple of small holes. A half-inch hole is then drilled from a point about 3 in. above the joint so as to enter the bed 2 in. from the face of the stone. The grout is then to be poured in till the joint is full. The surplus water will escape through the holes left in the pointing, and when the cement is set the joints are to be cleaned out an inch from the face, and stopped and pointed as desired. Where granite bearing blocks are carried by iron cylinders filled with concrete or brickwork, such filling should be allowed to project an inch or so above the iron casing, so that the blocks may be wholly bedded thereon without touching the iron. As a general rule, the more important the work the more necessary is it that the top and bottom beds of stone should be carefully dressed to an even surface, so that the layers of fine mortar introduced between them may be of uniform thickness. Other particulars relating to bed joint will be found in the Smiths' Section, as well as under asphalte, bond, cramped, dowelled, lead, etc, in this. Birdsmouth, - A rebate sunk on the edge of a stone, as well as the joint resulting therefrom, is sometimes so called.