Code of practice for construction of stone masonry, Part 2: Ashlar masonry
1992 Edition

According to Clause 5.1 of IS 1597 Part 2, the materials for ashlar masonry should conform to those outlined in Clause 5 of IS 1597 Part 1 (1992). Table 2 in Part 1 details commonly used stones and their suitable applications. Commonly recommended stones for ashlar masonry include granite, sandstone, limestone, and basalt, chosen based on their strength, durability, and exposure conditions. Granite and basalt are preferred for heavy load-bearing and exposed applications, while sandstone and limestone are suitable for moderate loads and decorative uses. It is essential to select stones with a uniform texture, free from cracks and defects to ensure durability.

IS 1597 Part 2 details specific dressing and finishing techniques for ashlar masonry types. For plain ashlar, stones must be finely dressed with flat, smooth faces and sharp, square edges, maintaining a surface deviation not exceeding 3 mm from a straight edge. The edges should have a fine chisel draft 2 to 5 cm wide, with rough tooling inside the drafts. Rough or rock-faced ashlar features similar edge chisel dressing but with a rough tooled interior face. Hammer-dressed ashlar involves stones squared and laid with fine joints but with hammer-textured faces, typically larger than plain ashlar stones. Quoins must be squared with straight, vertical corners free from chipping. These dressing standards ensure tight joints, uniform appearance, and aesthetic quality.

Mortar types for ashlar masonry follow IS 1597 Part 1, Clause 7.4, commonly involving cement mortar with cement-to-sand ratios of 1:3 or 1:4 for strength and durability. Lime mortar may also be used for heritage or aesthetic purposes but is less typical. Joint thickness in ashlar masonry is specified to be thin and consistent, usually not exceeding 10 mm, to promote tight fitting and effective bonding. Proper compaction and curing of mortar joints are critical to the durability and structural performance of the masonry.

Bond stones in ashlar masonry should be arranged according to wall thickness and stone absorbency. For walls up to 60 cm thick, through bond stones extending fully from the outer to inner face are recommended. For walls exceeding 60 cm thickness, sets of two or more overlapping bond stones with a minimum overlap of 15 cm should be provided in a line from face to back. In the case of highly absorbent stones such as porous limestone or sandstone, bond stones must penetrate about two-thirds into the wall thickness, with overlapping sets used regardless of thickness. Bond stones or sets should be spaced at intervals of 1.5 to 1.8 meters clear in each course. These provisions ensure structural integrity and minimize moisture penetration.

Structural stability for ashlar masonry as per IS 1597 Part 2 involves selecting quality, durable stones compliant with IS 1597 Part 1, Clause 5, and choosing stones based on strength and durability from Table 2 in Part 1. Design must ensure proper bonding with uniform, thin joints, maintain minimum thickness and height-to-thickness ratios to avoid buckling, and provide appropriate mortar quality and joint thickness (typically 10-15 mm). Load paths should be well-defined to prevent eccentric loads, and lateral stability should be enhanced through buttresses or pilasters when required. Minimum wall thickness is typically at least one-tenth of the wall height. These considerations collectively contribute to the masonry’s strength and durability.