2 Cracks in buildings are common occurrence Cracks in buildings are common occurrence. It develops when the stress in a component exceeds the strength.Stress is caused by either externally applied or internally generated force.External forces: Dead load, live load, wind load, seismic load, settlement of foundationInternal forces: Thermal moments, moisture changes, chemical action.
3 Classification:Cracks are broadly classified as : Structural Cracks or Non Structural cracksStructural cracks develop due to incorrect design, faulty construction or overloading.They endanger the safety of the building.Non structural cracks are mostly due to internally induced stresses and do not necessarily weaken the structure immediately. However in due course of time they allow the ingress of moisture that leads to corrosion of steel and thus weakening of structure.Vertical cracks in a long building or boundary wall, without expansion joint, is an example of non structural crack.
5 Non structural cracks do not endanger the safety of building, but they create an impression of faulty work or instability. They also spoil the external finish of the building due to penetration of moisture, thus increasing the cost of maintenance.Vertical cracks normally occur in walls due to horizontal movement and due to restraint provided by the foundation.Internal stresses could be compressive, tensile or shear. Normally all materials that crack are weak in tension and shear, such as masonry, concrete, plaster etc.
6 classification of cracks based on width. Thin cracks : Less than 1.00 mm wideMedium cracks : 1 to 2 mm wide.Wide cracks: More than 2 mm in width.Cracks may be of uniform width or varying width, straight, toothed, stepped, map pattern or random. They may be horizontal, vertical or diagonal.Cracks may be just skin deep or may extend into the body of the material.
10 Cracks need to be closely studied for correct diagnosis and treatment. Shrinkage cracks are wider & farther apart or thin & closely spaced. Thin cracks are generally less damaging even if in large number.Structures built today are more prone to cracks than before, due to thin section, fully stressed and high speed of construction. Thus measures for control of cracks have assumed much greater importance
11 PRINCIPAL CAUSES OF CRACKS: i Moisture changesii. Thermal variationiii. Elastic deformationiv. Creepv. Chemical reactionvi. Founsation movement and settlement of soilvii. Vegetation
12 Moisture Movement:Most building materials have pores in their structure. They expand on absorbing moisture and shrink on drying. These movements are reversible and cyclic in nature. The extent of movement depends upon the porosity of materials.Some irreversible movement also takes place, e.g. shrinkage of cement on initial drying.
13 From consideration of moisture movement materials are classified as under: Materials having very small moisture movement. e.g. marble, tiles etc.Materials having moderate moisture movement. e.g. bricks, concrete etc. In these materials some precaution is necessary.Materials having large moisture movement, e.g. timber, wood products, asbestos sheetsFor these materials special treatment such as protective coating of surface etc are required.
14 Irreversible shrinkage normally occurs in all Irreversible shrinkage normally occurs in all building materials that are cement based. This shrinkage is one of the main causes of cracking in structures.Initial shrinkage in cement concrete and mortar depends upon: cement & water content, maximum size-grading-quality of aggregates, duration-method-temperature of curing, presence of excessive fines in aggregates, humidity, type of cement, temperature of fresh concrete etc.
15 i. Curing is very important in minimizing initial. shrinkage i. Curing is very important in minimizing initial shrinkage. If proper curing is started immediately after initial set and continued for at least 10 days, drying shrinkage is much less.Presence of excessive silt, dust etc in sand and stone aggregates has considerable effect on shrinkage in concrete. It should not exceed 3%.iii Shrinkage is much less in coastal areas due to high humidity whereas it is very high in plains of UP.
16 PPC and low grade OPC have much less shrinkage. The ideal temperature range for concreting is 10oC to 30oC.vi. Sometimes cracks appear on freshly laid concrete even before it sets. This happens when rate of evaporation is higher than bleeding. In such circumstances, concrete should be covered with plastic sheet immediately after concreting and flood cured after initial set has occurred.
17 Dry bricks are water hungry and should not be used without wetting. Vii Cyclic changes are very prominent in porous bricks.Dry bricks are water hungry and should not be used without wetting.Brick should fully absorb water before it is laid in mortar.Also fresh bricks from kilns should not be used before 3-4 weeks to avoid cracks in bricks.Viii Next slide shows how a small return wall cracks due to expansion.
19 Measures to control shrinkage cracks: In structural concrete shrinkage cracks are controlled by using temperature reinforcement. Even in plain concrete a minimum 0.12% steel is recommended by IS: Codes. It is more effective if smaller dia bars- closely spaced are usedCracks in masonry can be minimized by using rich mortar and delaying plaster work till masonry has dried after curing.
20 In plaster over masonry the cracks could be controlled by providing good bonding with the wall. Deep raking of brick joints provide good key between the wall and the plaster.Cement plaster richer than 1:6 is harmful for external wall exposed to high temperature variations, since it will create a stronger membrane than the surface of brick.Coarse well graded sand should be used for plaster. Fine sand will result in crazy cracks.
21 Strong bond between concrete and plaster prevents shrinkage cracks, if rendering is done as early as possible after removal of shuttering. Key to plaster is provided by hacking and applying cement slurry just before renderingShrinkage cracks affect the appearance and finish and not the structural stability.Cracks in walls generally get localized at weak sections, such as doors and window openings or staircase walls. In external walls shrinkage cracks generally run downwards from window sill to the lintel of the lower storey.
22 Construction joint should be avoided in concrete Construction joint should be avoided in concrete. However if necessary it should be located at one-fourth of span away from preceding beam.Shrinkage in woodwork should be concealed thro’ proper treatment of joints. Beadings often serve the desired purpose.Paint both sides of Asbestos sheets with hydrophobic colourless chemical.Non seasoned timber should not be used. Allow sufficient period for seasoning and reduction in moisture.
24 Protect the surface and edges of plywood/ vaneer/ blockboard etc with hydrophobic colourless chemical.If rich finish is provided to external surface, such as grit plaster, it should be done in panels of not more than 0.50 sq.m each. by providing grooves of 8-10mm in both directions.No concreting should be done at temperatures beyond 10oC to 30oC range.
25 Under burnt bricks absorb more water, thus permitting greater moisture movement and shrinkage cracks. Such bricks should be sorted out and removed from the stack.If external finish is provided with sandstones, it should be coated with hydrophobic chemicals that form a water repellant film.Acrylic or PU based chemicals are preferred. The film will prevent the absorption of water.
26 Thermal Movement:Thermal movement is one of the most potent causes of cracking and call for serious consideration.It depends upon: temperature variation, dimensions, co-efficient of expansion, colour, surface characteristic, thermal conductivity, insulation, internally generated heat etc.
27 Internal walls do not suffer thermal movement appreciably Internal walls do not suffer thermal movement appreciably. However external walls and roof slab, undergo severe expansion and thus liable for cracking.When concreting is done in summer at high temperature, contraction due to drop in temperature in winter is high leading to thermal cracks.
28 When roof in a load bearing structure undergoes expansion, horizontal cracks occur in cross walls.
30 To prevent such cracks: Slab should be provided thermal insulation.Span of the slab should not be large.Slip joint should be provided.Slab should either project beyond the wall or rest only partially on the supporting wall.Plaster on inside wall and ceiling should be made discontinuous by providing 10mm wide grove on the ceiling.
34 Formation of horizontal cracks at the support of brick parapet wall over a cantilevered slab occurs frequently.
35 There are no simple solutions to avoid such cracks There are no simple solutions to avoid such cracks. However the following are recommended:Construct RCC parapet instead of brick.In balconies, the supporting beam itself may be inverted. Beam may be kept 150 wide to give sleek look.Defer the construction of brick parapet as much as possible to allow shrinkage of supporting slab.Apply bonding chemical and hack the concrete surface before laying the bricks.Use rich mortar in brick masonry for parapet.Provide 10mm wide groove in plaster at junction of brick wall and slab.
36 General measures for prevention of thermal cracks include provision of expansion joints, slip joints, control joints etc.