UNIT-1 INTRODUCTION Cracks in the building are of common occurrence in a building It is due to exceeding stress in a building components Causes of the cracks are mainly by increase in live load and dead load, seismic load etc., Classification of cracks Cracks can be classified into two categories viz., Structural cracks Non-structural cracks Structural cracks It arises due to incorrect designs, overloading of structural components Expenses cracking of foundation walls, beams and columns or slab etc., PHOTO OF STRUCTURAL CRACKS Non structural cracks They are due to internal forces developed in materials due to moisture variations, temperature variation, crazing, effects of gases ,liquids etc., They can be broadly classified into vertical, horizontal, diagonal, smoothened cracks PHOTO OF NON STRUCTURAL CRACKS DIRECTION OF THE CRACKS Vertical Horizontal Diagonal Straight Toothed
Variable and irregular WIDTH OF CRACKS It can be measured through instrument and tell-tale signs. The changes in the length of the cracks should be noted. Cracks measuring devices CAUSES OF CRACKS Major causes of cracks Movements of the ground Over loading Effect of gases, liquids and solids Effect of changes of temperature General causes such as vibrations Movements of grounds Due to mining subsidence, land slips, earthquakes, moisture changes due to shrinkable soils. Overloading Overloading of the building Overloading of the building parts results in cracks Overloading forced may be due to External ( excessive wind/snow loads) Internal ( from heavy machinery etc.,) Effects of gases, liquids and solids Gases Only gases like Co2 (carbon dioxide) is likely to produce cracks. It causes Carbonation of porous cement products Leads into an overall shrinkage crazing cracks
Liquids Water is the most commonly used liquid when not taken care it can be hazardous Construction water i.e., that in the utilization of water during the construction process Effects of water Physical(i.e. due to change in water content) Chemical ( directly or indirectly affecting other materials) General vibrations Vibrations can cause cracks in buildings only when their amplitude of vibrations are high. Apart from vibrations caused due to earthquakes, the vibrations caused due to heavy machinery, traffic, sonic booms are also responsible for the occurrence of cracks in buildings. THERMAL MOVEMENT All materials expand on heat and contract on cool. Thermal movement in components of structure creates cracks due to tensile of shear stresses One of the most potent causes of cracking in buildings and need attention GENERAL PRECAUTION TO AVOIDING CRACKS Before laying up foundation, the type of foundation to be used should be decided based on the safe bearing capacity of soil. Providing R.C deep beam or an involved T-beam with adequate reinforcements to withstand the stress due to differential ground movements. This method is expensive Construction operations such as cutting for roads drainages etc., close to the structures should be avoided this will results in reduction of soil moisture with consequent shrinkage of soil beneath the foundation of the structure. In buildings close to the water courses are noticed in many places PLACING OF CONCRETE Concrete should not be placed in heavy rains unless suitable shelter is provided. To avoid segregation, concrete should not be dropped from a height of more than 1m. Working on freshly laid concrete should be avoided
While placing the concrete in R.C.C members the alignment of formwork should not be disturbed. Concrete should be laid continuously to avoid irregular and unsightly lines. Internal surface of the forms either steel or wood should have even surfaces and should be oiled so that the concrete may not stick to it MATERIAL QUALITY Aggregate should be hard, sound, durable, non-absorbent and capable of of developing good bond with mortar. Water shall be clean and free from alkaline and acid materials and suitable for drinking purposes. TEST TO BE CARRIED OUT Slump test to be carried out for the control of addition of water and workability. Consistency of concrete should also be tested. A slump of 7.5 to 10cm may be allowed for building work LAYING TECHNIQUE AND CURING METHOD Concrete should be laid in layers and should be compacted while laying with wooden tamping rods or with mechanical vibrators until a dense concrete is obtained After two hours of laying concrete, when the concrete has begun to harden, it shall be kept damp by covering with wet gunny bags or wet sand for 24 hours Evaluation of cracks To determine the effects of cracks in the building. First the cracks location and extent should be noted down for the adopting suitable methods of repair and the future problems due to that cracks. Crack widths should be measured to the accuracy of 0.001 in (0.025mm) using a crack comparator. Movements should be recorded with movement sensors. Based on the reports from the location and width the suitable methods is adopted Crack as narrow as 0.002 in can be bonded by the injection of epoxy. Epoxy injection can alone be used to restore the flexural stiffness. For water retaining structure cracks it can be repaired by the autogenous healing.