LECTURE NOTES ON Rehabilitation & Retrofitting of structure Department of Civil Engineering
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.