Construction of Stone Masonry - Code of Practice, Part 1: Rubble Stone Masonry
1992 Edition

Per the guidelines outlined in Clause 6 and Table 2 of IS 1597 Part 1, dense stones such as granite and gneisses are recommended for masonry submerged in water or in contact with soil. For masonry exposed to smoke or chemical fumes, granite and quartzite are suitable. Fine quality masonry often uses sandstone, while ornamental or carved work prefers softer stones like marble and sandstone. Stones should be sized to be manageable by hand, with length not exceeding three times the height, breadth on the base limited to three-quarters of the wall thickness but at least 150 mm, and height up to 300 mm.

The mortar composition depends on local experience, stone variety, and environmental conditions (Clause 7.4). Mortars should be workable, adhere well, set quickly, and develop sufficient compressive strength. Typical mortar types include cement with fine aggregate, a blend of cement, lime, and fine aggregate, or lime with fine aggregate such as limestone or pozzolana sand. The fine aggregates must meet IS 2116 specifications, and detailed mix proportions are provided in IS 2250. Mortar selection also considers stone type and exposure conditions.

For walls up to 600 mm thick, through bond stones must be used. These should overlap by at least 150 mm and be arranged in a line extending from the face to the back. In the case of porous stones like limestone and sandstone, bond stones should penetrate approximately two-thirds into the wall to prevent moisture penetration. Bond stones or sets must be spaced at intervals of 1.5 to 1.8 meters horizontally in every course. Quoin stones are laid alternately as headers and stretchers, placed squarely on their beds which are roughly chisel dressed to a minimum depth of 100 mm, ensuring close-fitting joints and breaking vertical joint lines to enhance wall integrity.

Timber frames require holdfasts of sufficient size and strength securely embedded within the stone masonry, typically placed in chases filled with cement concrete. Holdfasts made of iron must be coated with bitumen to prevent corrosion. Timber surfaces in contact with stone should be treated with preservatives to protect against insect and termite damage. Frames should be fixed concurrently with masonry to avoid gaps. For steel frames and beams, timber should not be embedded directly to prevent dry rot. Timber ends rest on corbels or brackets, or if built-in, they must be treated and spaced for ventilation. Steel ends embedded in masonry must be coated with bitumen or provided space for repainting and encased in M-15 grade concrete or stone templates to ensure durability.

The code emphasizes external rendering as a crucial measure to enhance the weather resistance of solid rubble walls, especially those at least 400 mm thick. According to Clause 7.2 and Table 3, solid rubble walls are recommended only in sheltered conditions, while externally rendered walls are suitable for sheltered and moderate exposures but not for severe exposure without rendering. Rain penetration depends on the water absorption properties of the stone and mortar as well as the presence of cracks, which act as seepage channels. Prolonged wet weather can saturate masonry, leading to internal decay and disfiguration due to moisture carrying sulphuric fumes. Proper installation of damp-proof courses as per IS 2212 and maintenance of crack-free masonry are essential to prevent moisture ingress.