Cracks in Buildings:
Cracks in buildings are of common occurrence. A building component develops cracks whenever stresses in the component exceed its strength. Stresses in a building component could be caused by externally applied forces, such as dead, live, wind or seismic loads or foundation settlement or it could be induced internally due to thermal movements, moisture changes, chemical action etc.
Types of cracks:
Cracks could be broadly classified as structural and non-structural cracks. Structural Cracks are those, which are due to incorrect design, faulty construction or overloading and these may endanger the safety of a building. Extensive cracking of the RCC Beam is an instance of structural cracking. Non-structural cracks are mostly due to internally induced stresses in building materials and these generally do not directly result in structural weakening. In course of time, however, some non-structural cracks may, because of penetration of moisture through cracks or weathering action, result in corrosion of reinforcement and thus may render the structure unsafe. Vertical cracks in a long compound wall due to shrinkage or thermal movement is an instance of non-structural cracking. Non-structural cracks normally do not endanger the safety of a building, but may look unsightly or may create the impression of faulty work or may give a feeling of instability.
Internally induced stresses in building components lead to dimensional changes and whenever there is a restraint to movement as is generally the case, cracking occurs. Due to dimensional changes caused by moisture or heat, building components tend to move away from stiff portions of the building, which act as fixed points. In case of symmetrical structures, the centre of the structure acts as the fixed point and movement takes place away from the centre. A building as a whole can easily move in the vertical direction, but due to the presence of sub structure and foundation there is a restraining action in horizontal movement of the superstructure (building above ground), thus vertical cracks occur in the walls more frequently. Volume changes due to chemical action within a component result in either expansion or contraction and as a result cracks occur in components.
Internal stresses in building components could be compressive, tensile or shear. Most of the building materials that are subject to cracking, namely masonry, concrete, mortar etc are weak in tension and shear and thus forces of even small magnitude are able to cause cracking. It is possible to distinguish between tensile and shear cracks by closely examining their physical characteristics.
Figs Shown below bring out the differences between tensile and shear cracks
Cracks may vary appreciably in width from very thin hair cracks barely visible to naked eye (about 0.01 mm in width) to gaping cracks 5 mm or more in width. A commonly known classification of cracks, based on their width is:
a) Thin - less than 1mm in width
b) Medium - 1 to 2 mm in width
c) Wide - more than 2 mm in width
Cracks may be of uniform width or may be narrow at one end. Cracks could be straight, toothed, stepped, map pattern or random and may be vertical, horizontal or diagonal. Cracks may be only at the surface or may extend to more than one layer of materials. Occurrence of closely spaced fine cracks at the surface of a material is sometimes called crazing. Cracks from different causes have varying characteristics and it is by careful observation of these characteristics that one can correctly diagnose the cause or causes of cracking and adopt appropriate remedial measures
Depending on certain properties of building materials, shrinkage cracks may be wider but further apart, or may be thin but more closely spaced. As a general rule, thin cracks, even though closely spaced and greater in number, are less damaging to the structure and are not so objectionable from aesthetic and other considerations as a fewer number of wide cracks.
Modern structures are comparatively tall and slender, have thin walls are designed for higher stresses and are built at a fast pace. These structures are therefore, more prone to cracks as compared with old structures, which used to be low, had thick walls, were lightly stressed and were built at a slow pace. Moreover moisture can easily reach the inside of the modern buildings due to the usage of thin walls. Thus measures for control of cracks in buildings assume much greater importance than ever before.