29-01-2013, 01:08 PM
A PROJECT REPORT ON CRACKS IN BUILDINGS; CAUSES AND PREVENTION
CAUSES AND PREVENTION.docx (Size: 2.4 MB / Downloads: 611)
ABSTRACT
Cracking in structures is of common occurrence and engineers are often required to look into their causes and to carry out suitable repairs and remedial measures. For repairs and remedies to be effective, it is essential that the engineer should have proper understanding of various causes of cracking. For investigating the causes it is necessary to observe carefully location, shape, size, depth, behaviour and other characteristics of the cracks, and to collect information about specifications of the job, time of construction and past history of the structure. It will also be necessary for the engineer to know as to when the cracks first came to notice and whether the cracks are structural or non-structural.
Structural cracks are those which are due to incorrect design, faulty construction or overloading and these may endanger the safety of a building. Non-structural cracks which are due to moisture changes, thermal variations, elastic deformation, creep, chemical reaction, foundation movement and settlement of soil, vegetation, etc. 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, sometimes these cracks may, because of penetration of moisture through cracks or weathering action, result in corrosion of reinforcement and thus may render the structure unsafe.
INTRODUCTION
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 crack-prone 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 from rain can easily reach the inside and spoil the finish of a modern building which has thin walls. Thus measures for control of cracks in buildings have assumed much greater importance on account of the present trends in construction. Cracks in buildings are of common occurrence. A building component develops cracks whenever stress in the component exceeds its strength. Stress 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 variations, moisture changes, chemical action, etc.
MOISTURE CHANGES
As a general rule, most of the building materials having pores in their mortar, burnt clay bricks, some stones, timber, etc. Expand on absorbing moisture and shrink on drying. These movements are reversible, that is Cyclic in nature and is caused by increase or decrease in the inter-pore pressure with moisture changes, extent of movement depending on molecular structure and porosity of a material.
Reversible Movement
From consideration of moisture movement of reversible nature, materials could be broadly classified as under:
a) Materials having very small moisture movement, as for example, burnt clay bricks, igneous rocks, limestone, marble, gypsum plaster, metals, etc. The use of these materials does not call for many precautions.
b) Materials having small to moderate moisture movement, as for example, concrete, sand-lime bricks, sandstones, cement and lime mortars, etc. In the use of these materials some precautions in design and construction are necessary.
Based on research, range of reversible moisture movement of some of the commonly used building materials is given in Table 1.
CRACKS IN FRESHLY LAID CEMENT CONCRETE
In freshly laid cement concrete pavements and slabs, sometimes cracks occur before concrete has set due to plastic shrinkage. This happens if concrete surface loses water faster than bleeding action brings it to top of concrete at the surface results in shrinkage and as concrete in plastic state cannot resist any tension; short cracks develop in the material. These cracks may be 5 to 10 cm in depth and their width could be as much as 3 mm. Once formed these cracks stay and may, apart from being unsightly affect serviceability of the job. In order, to prevent plastic shrinkage of concrete, it is necessary to take steps so as to slow down the rate of evaporation from the surface of freshly laid concrete. Immediately after placing of concrete, solid particles of the ingredients of concrete begin to settle down by gravity action and water rises to the surface. This process — known as bleeding—produces a layer of water at the surface and continues till concrete has set. As long as rate of evaporation is lower than the rate of bleeding, there is a continuous layer of water at the surface, as evidenced by the appearance of ‘water sheen' on the surface and shrinkage does not occur.
CRACKS IN BRICK WORK DUE TO EXPANTION
When clay bricks (or other clay products) are fired, because of high temperature (900°C to 1000°C), not only intermolecular water but also water that forms a part of the molecular structure of clay, is driven out. After burning, as the temperature of bricks falls down, the moisture-hungry bricks start absorbing moisture from the environment and undergo gradual expansion, bulk of this expansion being irreversible.
Extent of irreversible expansion depends on the nature of soil, that is, its chemical and mineralogical composition and the maximum temperature of burning. When bricks are fired at very high temperature, as in the case of engineering bricks, because of fusion of soil particles, there is discontinuity in the pores and as a result, water absorption and moisture movements are less.
MEASURES FOR CONTROLLING CRACKS DUE TO SHRINKAGE
(I) to avoid cracks in brickwork on account of initial expansion, a minimum period varying from 1 week to 2 weeks is recommended by authorities for storage of bricks after these are removed from Kilns.
(ii) Shrinkage cracks in masonry could be minimized by avoiding use of rich cement mortar in masonry and by delaying plaster work till masonry has dried after proper curing and has undergone most of its initial shrinkage.
(iii) Use of precast tiles in case of terrazzo flooring is an example of this measure. In case of in-situ/terrazzo flooring, cracks are controlled by laying the floor in small alternate panels or by introducing strips of glass, aluminium or some plastic material at close intervals in a grid pattern, so as to render the shrinkage cracks imperceptibly small.
(iv) In case of structural concrete, shrinkage cracks are controlled by use of reinforcement, commonly termed as 'temperature reinforcement'. This reinforcement is intended to control shrinkage as well as temperature effect in concrete and is more effective if bars are small in diameter and are thus closely spaced, so that, only thin cracks which are less perceptible, occur.
THERMAL VARIATIONS
It is a well known phenomenon of science that all materials, more or less, expand on heating and contract on cooling. Magnitude of movement, however, varies for different materials depending on their molecular structure and other properties. When there is some restraint to
Movement of a component of a structure, internal stresses are set up in the component, resulting in cracks due to tensile or shear stresses. Extent of thermal movement in a component depends on a number of factors, such as temperature variation, dimensions, co-efficient of expansion and some other physical properties of the materials.
• Data contained in this table is from 'Principles of modern buildings'. Vol. I1 excepting item (iii), which is from the 'Performance of high" rise masonry structures 18 and item (vi) which is from ' Thermal movements and expansion joints in buildings. Coefficients of Thermal expansion of some of the common building materials are given in Table 2.
ELASTIC DEFORMATION
Structural components of a building such as walls, columns, beams and slabs, generally consisting of materials like masonry, concrete, steel, etc, undergo elastic deformation due to load in accordance with Hook's law, the amount of deformation depending upon elastic modulus of the material, magnitude of loading and dimensions of the components. This deformation, under circumstances such as those mentioned below, causes cracking in some portions:
a) When walls are unevenly loaded with wide variations in stress in different parts, excessive shear strain is developed which causes cracking in walls.
b) When a beam or slab of large span undergoes excessive deflection and there is not much vertical load above the supports, ends of beam/slab curl up causing cracks in supporting masonry.
MOVEMENT DUE TO CREEP
Some building items, such as concrete, brickwork and timber, when subjected to sustained loads not only undergo instantaneous elastic deformation, but also exhibit a gradual and slow time-dependent deformation known as creep or plastic strain. The latter is made up of delayed elastic strain which recovers when load is removed, and viscous strain which appears as permanent set and remains after removal of load. This phenomenon known as creep is explained in Fig. 1.6.
BACKGROUND
The afflicted structure forms part of the Gram Shabha Hall at Lohian. The building was designed for 500 persons. The building is one story and it consists of Main Hall with area 20x 30m, kitchen, store, and rooms for persons. Figure 2.1 shows the layout of the building. The building was under construction and the work was reached the finishing stage. Attention was drawn towards the main beams in the Hall where structural distress in the form of flexural and shear cracking had been observed. Cracking was first noticed in August and by September; it had progressed to the extent that the client requested immediate action by the contractor. Figure 2.2: shows the cracks in the beams.