Dr Mansoor Ahmad Qazi
It is found that large numbers of houses in Kishtwar District are having minor cracks on their walls mostly on the exterior of the house. The visible cracks are not vulnerable. There are several reasons for these minor cracks on the walls of the houses. Some of the cracks are very old caused by the earthquake activities, as the whole area is in seismic zone IV of Seismic Zoning map of Jammu and Kashmir, which corresponds to highly seismically active area. The appearance of cracks in houses is very common. Its occurrence and development are due to the differential stresses set up in the component of the house. The differential stresses results due to external loads (dead, live, wind or seismic loads). It also results due to foundation movement and settlement of soil, thermal movements, moisture changes. The stress also develops due to chemical action, corrosion of reinforcements, the growth of vegetations etc. The differential stress affects the capacity of the structure to resist the load. Hence, it is necessary to understand the causes of cracking so as to prevent the cracks in house.
In general, the major causes of cracks in houses are as under:
1). Moisture: As we know, we use concrete, mortar, bricks, and timber, etc. for constructing a house. All the materials of construction expand on absorbing moisture and shrink on drying which set up the stress in components of house. Hence develops the cracks in walls due to change in moisture. The change in moisture condition of the construction materials is also known as moisture movement. Moisture movement depends on the porosity (intermolecular space) of the building materials.
2). Temperature: Material used in construction of houses such as concrete, mortar, burnt clay bricks, stone and timber, steel, etc. for the construction of house. They expand on heating and contract on cooling depending on their properties. Due to the expansion and contraction of materials, internal tensile stresses are set up in the component of the house. Hence it develops the wall cracks due to change in temperature. This effect is also known as thermal movement, which depends on the number of factors such as:
* Temperature variation,
* Dimensions of the house or structure,
* Coefficient of thermal expansion of construction materials,
* Color and surface characteristics in the sense of reflectivity,
* Thermal conductivity of building materials,
* Provision of an insulating or protective layer on terrace,
* Internal heat generated in concrete component,
* Other physical properties of the construction materials.
These types of cracks are observed on the exterior of the house.
3) Elastic Deformation: The materials of construction undergo deformation due to load under “Hook’s law”. Hence develops the cracks due to elastic deformation. The amount of deformation depends upon elastic modulus of the construction material. It also depends on the magnitude of loading and dimension of the components. Dead loads and live loads are the primary cause of elastic deformations in any structural components of a building.
4) Creep Movement: The construction materials result in deformation due to sustained load. The stresses are developed in the component of house due to gradual and slow time-dependent deformation and hence develops the cracks in walls. The tendency of the materials to deform permanently under stress is also known as creep movement.
Creep movement depends on many factors, such as:
4 Water cement ratio in mortar and concrete,
4Temperature and humidity,
4Use of admixtures in mortar or concrete,
4Age of concrete at the time of loading,
4Size and shape of the building components.
Creep movement increases with increase in water content, cement content and temperature. It also decreases with increase in humidity of the surrounding atmosphere and age of material at the time of loading.
5) Chemical Reaction: Due to chemical reactions, materials of construction results in the appreciable increase in the volume of materials. The internal stresses are set up in construction materials which may result in outward thrust and hence forms the wall cracks. The materials involved in chemical reaction also get weakened in strength. The soluble sulphates present in the soil, ground water or clay bricks react with tricalcium aluminates content of cement and hydraulic lime in the presence of moisture. It forms products which occupy much bigger volume than that of the original constituents. This expansive nature of materials results in weakening of masonry, concrete and plaster. Hence forms the wall cracks due to chemical reaction.
6) Corrosion of Reinforcement: Most of the cracks observed in recent RCC building are due to the corrosion of steel. Initially, they do not pose any threat to the safety of the structure. But corrosion is like cancer in the body, and it creates a vicious circle.
The main reasons of corrosion to reinforcement are:
* Poor quality reinforcement,
* Poor grade of concrete,
* Inadequate cover to reinforcement,
* Poor workmanship.
* Cracks allowing the moisture to enter.
Factors that affect the corrosion to reinforcement are:
* Presence of cracks in concrete,
* Permeability of concrete (Minimum cement in concrete 350 kg/m3 with water cement ratio should be 0.55)
* Carbonation in cement-based materials,
* Corrosion cells. (due to difference in moisture content, oxygen concentration, and presence of differential metal),
* Passage of electric current trough concrete,
* Alkali-aggregate reaction,
* Use of calcium chloride as accelerator,
* Ingress of sea water in concrete,
* The inadequacy of cover and impurities in curing water.
The iron oxide film is produced on the bar which increases bar’s volume, thus setting up internal stress in concrete. Hence develops the cracks in house due to corrosion.
In the course of time, it first causes thin cracks in line along the direction of reinforcement and later causes spalling of concrete. It also causes the dislodging of the cover of reinforcement from the body of the concrete. Hence it seriously damages the structure, if not attended in time. It can be dangerous and also very costly to repair (sometimes it is costlier than original cost) and be challenging.
7) Foundation Movement & Settlement: The movement of foundation and settlement of soil causes the shear cracks in the house. The cracks in walls occur due to unequal bearing pressure on the structure or due to bearing pressure on soil which is greater than SBC. The frequent drying and wetting of soil underneath the foundation cause the change in the volume of soil and hence due to the foundation movements, cracks appear in the house. Swelling and shrinking of expansive soil like black cotton soil also cause movement of foundation and ultimately cause cracks in house. They also occur due to the consideration of low safety factor in the design of foundation due to which internal stresses are set up which may cause the cracks in house. Sometimes local factors like types of soil (factor affecting design) are not considered, at the time of foundation design also results in cracks. The sudden forces of natural disasters (like flood, earthquake, or hurricane) also result in cracks, if the structure is not properly designed for shear forces.
8) Growth of Vegetation: Due to fast growing trees near the structure, it causes cracks due to growing (spreading) of roots under the foundation. The plants begin to grow in the fissures of walls, and if these plants are not removed well in time, it develops the wall cracks in house. Cracks also occur due to the upward thrust on a portion of the building, when old trees are cut off, and the soil that had been dehydrated earlier by roots swells up on getting moisture from rain. Many factors affect the house i.e. change in moisture, change in the temperature and behavior of the building materials. The chemical composition of the construction materials, design criteria and workmanship of construction also affect the cracks in house. Hence it is necessary to evaluate the cause of cracks based on above factors before attempting any repairing of cracks in house. Any attempt to repair the cracks without understanding the causes will make cracks reappear from time to time.
9) Earthquakes: Permanent ground deformations can tear a structure apart. Some foundation types are better able to resist these permanent ground deformations than others. For example, the use of pile foundations, with the piles extending beneath the anticipated zone of soil liquefaction, can be effective in mitigating the hazard’s effects. The use of heavily reinforced mats can also be effective in resisting moderate ground deformation due to fault rupture or lateral spreading. Most earthquake-induced building damage, however, is a result of ground shaking. When the ground shakes at a building site, the building’s foundations vibrate in a manner that’s similar to the surrounding ground. Brittle elements tend to break and lose strength. (Examples of brittle elements include unreinforced masonry walls that crack when overstressed in shear, and unconfined concrete elements that crush under compressive overloads.) Ductile elements are able to deform beyond their elastic strength limit and continue to carry load. (Examples of ductile elements include tension braces and adequately braced beams in moment frames.
Seismic Strengthening (Retrofitting): It will involve actions for upgrading the seismic resistance of an existing building so that it becomes safer under the occurrence of probable future earthquakes. The seismic behavior of existing buildings is affected by their original structural inadequacies, material degradation due to aging and alterations carried out during use over time. The complete replacement of such buildings in a given area is just not possible due to a number of social, cultural and financial problems. Therefore, seismic strengthening of existing undamaged or damaged buildings is a basic requirement. Seismic strengthening including structural restoration and cosmetic repairs may sometimes cost upto 20 to 25 per cent of the cost of rebuilding although usually it may not exceed 12 to 15 per cent. Hence justification of strengthening work must be fully considered from cost point of view. The main items of seismic strengthening of masonry building could be some or all of the following actions: i) Modification of roofs, ii) Substitution or strengthening of floors, iii) Modification in the building plan, iv) Strengthening of walls including provision of horizontal and vertical bands or belts, introduction of ‘through’ or ‘header’ stones in thick stone walls, and injection grouting etc., v) Adding diagonal bracings in Dhajji diwari panels, vi) Strengthening of foundations is rarely found necessary (but it is usually very difficult and expensive).
10) Blasting effect: Effects of ground vibrations on nearby structures and people resulting from blasting operations have become a major environmental problem and concern to engineers, contractors as well as to the general public. Understanding of propagation characteristics of stress waves produced by blasting and structural response to ground vibration are essential in planning and design of safe blasting operations. The rapid detonation of explosives confined in blast holes produces stresses and strains in the rock which fracture the rock in the immediate area of the blast. Research data are reviewed which indicate that the peak particle velocity of the ground is the best criterion for evaluating vibration in terms of its potential to cause damage. Particle velocity is the speed at which the individual earth particles are moving (or vibrating) as the earth wave passes a particular location. Blasting complaints are consuming an increasing amount of time and resources. This worsening situation has spawned new methods for relaying the public’s concerns. The direct measurement of crack response to both long term environmental and blast vibration effects with the same sensor exemplifies one new approach, which can be helpful in the process of educating quarry neighbors about the large impact of the environmental changes. This comparison of long-term and vibratory crack response demonstrates that the silent response of cracks to environmental changes is larger than that produced by blasting, which is felt and heard. The damage and destruction in stone masonry and brick masonry walls is because of the violation of the most basic rules of masonry construction; such as absence of ‘through’ stones and ‘long corner’ stones in stone walls.
* Use of mud mortar in stone or brick masonry makes them very weak against severe or even moderate earth shaking.
* In case of stone and brick masonry in cement mortar, inadequate curing has been observed as one of the causes of failure.
* In all types of masonry construction, the absence of earthquake resisting features has also contributed significantly to the failures. Provision of such features will improve the earthquake resistance of even mud mortar masonry.
In responding to blasting complaints, whether they are claims of annoyance or structure damage, it is always the best practice to:
* Respond immediately and do not delay direct contact with the complainant;
* Make personal contact;
* Be knowledgeable of the blast creating concern and be prepared to explain any unusual circumstances that may have contributed to higher than usual offsite noise or vibrations;
* Provide assurance that blasting is not causing any damages; and
* Be willing to respond to any future concern.
It is concluded that we cannot hold responsible a single factor for such cracks on the walls of the houses. It is the responsibility of the Administration to make a good public-relations program and aware the peoples of the area to mitigate such problems in future by consulting Architectures, Geologists, Civil Engineers before starting the construction of a house.
(The author is Manager (Geology), CVPPPL, Kwar HE Project, Kishtwar).
