Construction of hollow and solid concrete block masonry - Code of practice
2005 Edition

According to IS 2572 (Clause 6.9.2 and Table 1), recommended mortar mixes vary depending on the type of work: For normal masonry without reinforcement, use a cement:lime:sand ratio of 1:1:9-10; for masonry with reinforcement, use 1:7-8 cement to sand. In severe conditions such as foundations or heavily loaded structures, the ratios adjust to 1:1:6-7 (cement:lime:sand) and 1:4-5 (cement:sand) respectively. Partition walls (100 mm thick) require 1:1:7-8 and 1:5-6 ratios respectively. Hollow blocks typically demand a weaker mortar mix to prevent cracking, with superstructure hollow block masonry often using a composite mortar of 1 cement:1 lime:9-10 sand. For foundations, hollows should be filled with concrete (1 cement:3 sand:6 gravel). Sand proportions may be modified based on grading to maintain durability and minimize cracking.

IS 2572 specifies that vertical joints should have mortar applied on the vertical edges of both front and rear shells, either on the placed unit or on the one being set next, ensuring thorough compaction. For two-cell blocks, any slight indentations on vertical sides must be filled with mortar to enhance lateral strength. Horizontal joints require a uniform mortar layer approximately 1 cm thick covering the entire top surface, including shells and webs, providing full bedding. For walls subjected to light loads, face shell bedding (mortar on just the front and rear shells) is acceptable to reduce water permeability. Mortar should not be spread prematurely to avoid stiffening, and joints should be raked to about 1 cm depth after laying each course to promote good plaster adhesion. This method ensures effective load transfer and long-lasting masonry.

IS 2572 recommends applying a 12 mm thick cement plaster with a 1:3 cement-to-sand mix in two coats over the exterior surfaces of basement walls starting just beneath ground level, extending downward and over the footing's edge. In wet subsoil conditions, this plaster should be coated with asphalt for enhanced waterproofing. Annual maintenance before monsoon involves inspecting for cracks, sealing them with cement grout, and applying two coats of cement paint. Additionally, in damp soils, richer mortar mixes should be employed for foundation and basement masonry, accompanied by a damp-proof course such as a 25 mm thick 1:2 cement mortar layer or an approved bituminous membrane to prevent moisture ingress.

IS 2572 stipulates that intersecting load-bearing concrete block walls must be firmly bonded or tied to ensure monolithic structural behaviour. When constructed simultaneously, a true masonry bond is formed by embedding half the blocks of each wall into the other at intersections, ensuring mechanical interlock. If built separately, vertical pockets spaced up to 200 mm apart are left in the first wall, into which the blocks of the second wall are anchored. Additional reinforcement can be provided by rigid ties, such as round steel bars embedded in mortar joints or wrapped around vertical reinforcements in grouted cores, or flexible ties in the form of steel tie bars acting as metal anchors. These methods ensure adequate load transfer and structural stability at wall intersections.

To control cracks due to shrinkage and expansion, IS 2572 advises using blocks with moisture content not exceeding 40% of maximum water absorption in environments with relative humidity of 60% or more, and 25% if humidity is below 60%. Control joints should be provided at intervals of 8 to 10 meters in free-standing walls and 15 to 18 meters in walls connected by cross walls, as well as at junctions between load-bearing and non-load-bearing walls and at column connections. For thermal expansion, expansion joints must be included in walls longer than 30 meters. Mortar joints should use lean mixes containing hydrated lime, marble powder, and gypsum to enhance plasticity and facilitate autogenous healing, thus reducing the occurrence of cracks.