Building Maintenance And Repair
(CRACKS IN BUILDING) Date 18, Sept., 2012 By Anuja sohani (BE civil)
INTRODUCTION Cracks result in building when applied forces are greater than those which the building or its part can withstand. LOOPHOLES OF CRACKS ¬ BUILDING BECOME UNSAFE ¬ IT SPOIL THE APPERANCE OF BUILDING ¬ DAMPNESS IN THE BUILDING ¬ REDUCe MARKET VALUE ¬ INCREASE IN MAINTAINENCE COST CAUSES OF CRACKS Non Structural cracks ¬ Moisture changes ¬ Thermal movement ¬ Elastic deformation ¬ Chemical reaction ¬ Foundation movement and settlement of soil ¬ Vegetation Structural cracks ¬ due to incorrect design ¬ faulty construction ¬ Overloading Moisture Changes Ρ Building materials expand on absorbing moisture and shrink on drying. These are generally reversible. Shrinkage in concrete or mortar depends on a number of factors Ρ Cement concrete Ρ Water Content Ρ Aggregates Ρ Curing Ρ Excessive fines Ρ Temperature: Thermal Movement Ρ All materials expand on heat and contract on cool. It is one of the most potent causes of cracking in buildings and needs attention. ELASTIC DEFORMATION Ρ When two materials having wide different elastic properties are built together under the effect of load, different shear stresses in these materials create cracks at the junction CHEMICAL REACTION Ρ Chemical reactions in building materials increase their volume and internal stress causes cracks. The components of structure also weaken due to chemical reactions. Ρ Some common instances of chemical reactions are following.
¬ Sulphate attack on cement products ¬ Carbonation in cement based materials ¬ Corrosion of reinforcement in concrete ¬ Alkali aggregate reaction FOUNDATION MOVEMENT AND SETTLEMENT OF SOIL Ρ Unequal bearing pressure under different parts of the structure Ρ Bearing pressure being in excess of safe bearing strength of the soil Ρ Low factor of safety in the design of foundations Ρ Local variation in the nature of supporting soil VEGETATION Ρ The roots of trees located in the vicinity of a wall can create cracks in walls due to growth of roots under foundation. EARTH QUAKE Ρ Crack may occur due to sudden shift in lower layer of the earth. The voids in the earth might have suddenly collapsed and be filled with soil from the above. Many geological events can trigger earth movements but is continuous movement. This results in cracks. PREVENTIONS ¬ 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 more prone to CRACKS. ¬ 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. MOISTURE CHANGE ¬ Do not use excessive cement in the mortar mix. ¬ Use minimum quantity of water according to water cement ratio. ¬ Proper curing should be started as soon as initial setting has taken place and be continued for at least seven to ten days. THERMAL CRACKS Ρ To prevent thermal cracks expansion joints, control joints and joints in case of change of shape and direction of wing in a structure are to be provided ELASTIC DEFORMATION Ρ Create slip joints under the support of RCC slab on walls. Masonry work on RCC slabs and beams should not be started before drying RCC slab and beam. Ρ Provide horizontal movement joints between the top of brick panel and RCC beam/slab CHEMICAL REACTION Ρ Use dense and good quality concrete i.e. richer mix of cement concrete 1:1.5:3 to prevent cracks. Ρ Avoid bricks containing too much soluble sulphates (more than 5 %) and use rich mortar in such cases. Ρ Use expansion and control joint at closure intervals FOUNDATION MOVEMENT AND SETTLEMENT OF SOIL Ρ Prevention of moisture penetration in the surrounding areas has to be ensured by providing a waterproof blanket around the plinth. Ρ The masonry wall below ground level should also be separated from the adjoining soil by replacing the existing soil with coarse grain material. Ρ The design of foundation must be based on sound engineering principles and good practice. Vegetation Ρ Do not let trees grow too close to the buildings, compound walls etc. Remove any saplings of trees as soon as
possible if they start growing in or near of walls etc. Earth Quake Ρ Construct the foundation of buildings on firm ground while doing construction. Tie up the building with connecting beams at foundation level, door level and roof level. REPAIR OF CRACKS Ρ Cracks must be repaired immediately when observed otherwise the life of structure will reduce causing structural failure. Ρ They not only weaken the building but spoil the beauty of walls also. Repair cracks giving hollow sound Ρ Remove plaster applying proper cut Ρ Prepare the surface thoroughly Ρ Apply fresh cement mortar and finish the surface Repair Cracks on Joint of RCC Slab and Brickwork Ρ Remove plaster and old material Ρ Apply cut and prepare the surface Ρ Apply cement mortar Ρ Make groove Ρ Do curing and finishing CONCLUSION ¬ Cracks must be repaired immediately when observed otherwise the life of structure will reduce causing structural failure. ¬ The cracks can be prevented at the time of construction ¬ They not only weaken the building but spoil the beauty of walls also. ¬ This write up tells you about repair of cracks in plaster, RCC member, Brick walls, Floor and Fine Cracks.