To Prevent Cracks Due to Moisture Movement.
- Select materials having small moisture movement eg bricks, lime stones, marble etc.,
- Plan for less richer cement content, larger size of aggregates and less water content.
- Porus aggregates (from sand stone, clinker etc) prone for high shrinkage.
- Plan for offsets in walls for length of more than 600mm.
- Use of of composite cement-lime mortar of 1:1:6 mix or weaker for plastering work.
- Plan for proper expansion/control/slip joints.
- For brick work 2weeks time in summer and 3 weeks’ time in winter should be allowed before using from the date of removal from kilns.
- Delay plastering work till masonry dried after proper curing.
- Proper curing immediately on initial setting brings down drying shrinkage.
To prevent Cracks Due to Thermal Movement
- Dark coloured and rough textured materials on exteriors have lower reflectitivity and react more for thermal expansions
- Plan for a layer of adequate thickness of good reflective surface over concrete roof slabs to minimize these cracks
- Slip joint should be introduced between slab and its supporting wall or the some length from the supporting wall or the slab should bear only on part width of the wall
- Mortar for parapet masonry should be 1cement: 1 lime: 6 sand
- Construction of masonry over the slab should be deferred as much as possible (at least one month) so that concrete undergoes some drying shrinkage prior to the construction of parapet.
- Good bond should be ensured between parapet masonry and concrete slab
- The bearing portion of the wall is rendered smooth with plaster, allowed to set and partly dry, and then given a thick coat of whitewash before casting the slab so that there is a minimum bond between the slab and the support. To ensure more efficient functioning of this joint, in place of whitewashing 2 or 3 layers of tarred paper are placed over the plastered surface to allow for easy sliding between RCC slab and the supporting masonry
- To avoid cracks near door frames provide groove.
To Prevent Cracks Due to Elastic Deformations
- When large spans cannot be avoided, deflection of slabs or beams could be reduced by increasing depth of slabs and beams so as to increase their stiffness.
- Adoption of bearing arrangement and provision of a groove in plaster at the junction of wall and ceiling will be of some help in mitigating the cracks.
- Allow adequate time lag between work of wall masonry and fixing of tiles.
To prevent Cracks due to Movements Due to Creep
- Do not provide brickwork over a flexural RCC member (beam or slab) before removal of centering and allow a time interval of at least 2 weeks between removal of centering and construction of partition or panel wall over it.
- When brick masonry is to be laid abutting an RCC column, defer brickwork as much as possible.
- When RCC and brickwork occur in combination and are to be plastered over, allow sufficient time (at least one month) to RCC and brickwork to undergo initial shrinkage and creep before taking up plaster work.
- A panel walls in RCC framed structures: (i) as far as possible, all framework should be completed before taking up masonry work of cladding and partitions which should be started from top storey downward. (ii) Provide horizontal movement joint between the top of brick panel and soffit of beams.
- Partitions supported on floor slab or beam: (i) Provide upward camber in floor slab/beam so as to counteract deflection. (ii) Defer construction of partitions and plaster work as much as possible (iii) Provide horizontal expansion joints between the top of masonry and soffit of beam/slab, filling the gaps with some mastic compound.
To prevent Cracks due to Movement due to Chemical Reaction
- For structural concrete in foundation, if sulphate content in soil exceeds 0.2 per cent or in groundwater exceed 300 ppm, use very dense concrete and either increase richness of mix to 1:1.5:3 or use sulphate resisting Portland cement/super-sulphated cement or adopt a combination of the two methods depending upon the sulphate content of the soil.
- Cracking caused in concrete due to carbonation can be avoided or minimized by ensuing use of Exposed concrete items in thin sections, such as sunshades, fins and louvers of buildings, are with concrete of richer mix (say 1:1.5:3).
To Prevent cracks due to Foundation Movement and Settlement of Soil
- Plan for under-reamed piles in foundation for construction on shrinkable soils
- Plan for plinth protection around the building
- Slip / expansion joints to ensure that new construction is not bonded with the old construction and the two parts (Old and new) are separated right from bottom to the top. When plastering the new work a deep groove should be formed separating the new work from the old.
- For filling deep – say exceeding 1.0m, Soil used for filling should be free from organic matter, brick-bats and debris filling should be done in layers not exceeding 25 cm in thickness and each layer should be watered and well rammed.
- If filling is more than 1 meter in depth, process of flooding and compaction should be carried out after every meter of fill.
To Prevent Cracks Due to Cracking Due to Vegetation
- Do not let trees grow too close to buildings, compound-walls, garden walls, etc, taking extra care if soil under the foundation happens to be shrinkable soil/ clay. If any saplings of trees start growing in fissures of walls, etc. remove them at the earliest opportunity.
- If some large trees exist close to a building and these are not causing any problem, as far as possible, do not disturb these trees if soil under the foundation happens to be shrinkable clay.
- If, from any site intended for new construction, vegetation including trees is removed and the soil is shrinkable clay, do not commence construction activity on that soil until it has undergone expansion after absorbing moisture and has stabilized.