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To prevent or minimize the occurrences of non-structural cracks it is necessary to understand the basic causes and mechanism of cracking and ceratin properties of building materials which may lead to dimensional changes of the structural components. The principal mechanism causing non-structural cracks in the building are:
- Moisture change
- Elastic deformation
- Chemical reaction
- Growth of vegetation
- Thermal movement
- Creep
- Foundation movement and settlement of soil
- Growth of vegetation
Moisture Change
Most of the buildings material like concrete, mortar, burnt clay brick, timber , plywood etc. are porous in their structure in the form or inter-molecular shrinks on drying. These movements are reversible i.e. cyclic in nature and are caused by increase or decrease in the inter-pore pressure with moisture change. Extent of movement depends upon molecular structure and porosity of a material.
Apart from reversible movement certain materials undergo some irreversible movement due to initial moisture changes after their manufacture or construction. The incidences of irreversible movement in materials are shrinkage / plastic shrinkage and expansion of burnt clay bricks and other clay products on removal from kilns i.e. initial expansion.
Thermal Movement
All materials more or less expand on heating and contracts on cooling. When this movement is restraint, internal stresses are set – up in the component, and may cause cracks due to tensile or shear stress. Thermal movement is one of the most potent causes of crackling in buildings and calls for careful consideration. The extent of thermal movement depends upon.
- Ambient temperature variation
- Co-efficient of thermal expansion:- Expansion of cement mortar and concrete is almost twice of the bricks and brick work. Movement in brickwork in vertical direction is 50% more than in horizontal direction.
- Dimensions of components: - The cracks due to thermal movement is caused either due to external heat i.e. due to variation in ambient temperature, or due to internally generated heat i.e. due to heat of hydration in mass concrete during construction.
Cracks in the building component due to thermal movement opens and closes alternatively with charges in the ambient temperature. The concreting done in summer is more liable for cracking due to drop in temperature in winter since thermal contraction and drying shrinkage act in unison. Whereas the concrete job done in the winter is less liable to cracking through it may require wider expansion joints.
Generally thermal variation in the internal walls and intermediate floors are not much and thus do not cause cracking. It is mainly the external walls exposed to direct solar radiation, and the roof, which are subjected to substantial thermal variation, are more liable to cracking.
Elastic Deformation
Structural components of a building undergo elastic deformation due to dead and the super imposed live loads. The amount of deformation depends upon elastic modulus, meningitides of loading and the dimension of the component. This elastic deformation under certain circumstances causes crackling in the building
- When walls are unevenly loaded with wide variations in stress in different parts, excessive shear is developed which causes crackling in walls.
- When a beam or slab of large span undergoes excessive deflection and there is slab curl up causing cracks in supporting masonry.
- When two materials, having widely different elastic properties, are built side by side, under the effect of load, shear stresses are set up at the interface of the two materials, resulting in cracks at the junction. Such a situation is commonly encountered in the construction of RCC framed structure and brick masonry panel (external) and partition ( internal ) walls.
Creep
In concrete, the extent of creep depends on
- Water and cement content
- Water cement ratio
- Temperature and humidity
- Use of admixture and pozzalans
- Age or strength of concrete at the time of loading
- Size and shape of the component
Creep increases with water and cement content, water cement ratio and temperature; it decreases with increase in humidity of surroundings and the age or strength of the material at the time of loading.
Use of admixtures and pozzalons in concrete increases the creep. Creep also increases with the increase in surface to volume ratio of component. In brickwork the creep depends upon stress/ strength ratio therefore the creep in brick work with weak mortar is generally higher. For example, for same quality of brick, creep of brick in 1:1:6 mortar is 2 to 3 times that of bricks work in 1:1:3 mortar. Generally creep in brickwork is approximately 20 to 25% that of concrete. In brick work it ceases after 4 months while in concrete it may continue up to a year or so, and most of creep takes place in 1 st month thereafter it pace slows down.
The major effect of creep in concrete is the substantial increase in the deformation of structural members , which may be to the extent of 2-3 times the initial elastic deformation. This deformation sometimes causes cracks in brick masonry of frame and load bearing structures. When the deformation due to elastic strain and creep occurs in conjunction with shortening of an RCC member due to shrinkage, cracking is more severe and damaging.
Chemical reaction
Certain chemical reactions in building materials result in appreciable increase in volume of materials, due to which internal stresses are set up which may results in outward thrust and formation of cracks. The material involved in chemical reaction are.
- Sulphate attack
- Carbonisation in cement based material
- Corrosion of reinforcement in concrete and brick work
- Alkali-aggregate reaction
Foundation movement and settlement of soil
Shear cracks in buildings occurs when there is a large differential settlement of foundation due to one of the following causes.
- Unequal bearing pressure under different parts of the structure.
- Bearing pressure being in excess of safe bearing strengths of the soil.
- Low factor of safety in the design of foundations.
- Local variations in the nature of the supporting soil, which remained undetected and could not be taken care of the foundation design at the time of construction.
- Foundation resting in active zone on expensive soil.
Growth of Vegetation
Roots of a tree generally spread horizontally on all sides to the extent of height of the tree above the ground and when the trees are located in vicinity of walls , they can cause cracks in walls due to expensive action of roots growing under the foundations.
Sometimes plants take root and begin to grow in fissures of walls, because of seeds contained in bird droppings. If these plants are not removed well in time, these may in course of time develop and causes severe cracking of wall.
Growing roots of trees cause de-hydration of soil which may shrink and cause foundation settlement, or in areas where old trees had been cut off to make way for building construction roots had de-hydrated the soil on receiving moisture from some sources, such as rain etc. The soil swells up and causes an upward thrust on a portion of the building resulting in cracks in the building.
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