Causes of Cracks in concrete structures:
The principal causes of occurrence of cracks in a building are as follows:
1. Permeability of concrete.
As deterioration process in concrete begins with penetration of various aggressive agents, low permeability is the key to its durability. Concrete permeability is controlled by factors like water-cement ratio, degree of hydration/curing, air voids due to deficient compaction, micro-cracks due to loading and cyclic exposure to thermal variations. The first three are allied to the concrete strength as well. The permeability of cement paste is a function of water-cement ratio given good quality materials, satisfactory proportioning and good construction practice; the permeability of the concrete is a direct function of the porosity and interconnection of pores of the cement paste.
2. Thermal movement:
Thermal movement is one of the most potent causes of cracking in buildings. All materials more or less expand on heating and contract on cooling. The thermal movement in a component depends on a number of factors such as temperature variations, dimensions, coefficient of thermal expansion and some other physical properties of materials. The coefficient of thermal expansion of brickwork in the vertical direction is fifty percent greater than that in the horizontal direction, because there is no restraint to movement in the vertical direction.
Thermal variations in the internal walls and intermediate floors are not much and thus do not cause cracking. It is mainly the external walls especially thin walls exposed to direct solar radiation and the roof which are subject to substantial thermal variation that are liable to cracking.
Thermal joints can be avoided by introducing expansion joints, control joints and slip joints. In structures having rigid frames or shell roofs where provision of movement joints is not structurally feasible, thermal stresses have to be taken into account in the structural design itself to enable the structure to withstand thermal stresses without developing any undesirable cracks.
Concrete when subjected to sustained loading exhibits a gradual and slow time dependant deformation known as creep. Creep increases with increase in water and cement content, water cement ratio and temperature. It decreases with increase in humidity of surrounding atmosphere and age of material at the time of loading. Use of admixtures and pozzolonas in concrete increases creep. Amount of creep in steel increases with rise in temperature.
4. Corrosion of Reinforcement
A properly designed and constructed concrete is initially water-tight and the reinforcement steel within it is well protected by a physical barrier of concrete cover which has low permeability and high density. Concrete also gives steel within it a chemical protection. Steel will not corrode as long as concrete around it is impervious and does not allow moisture or chlorides to penetrate within the cover area. Steel corrosion will also not occur as long as concrete surrounding it is alkaline in nature having a high pH value.
Concrete normally provides excellent protection to reinforcing steel. Notwithstanding this, there are large number of cases in which corrosion of reinforcement has caused damage to concrete structures within a few years from the time of construction. One of the most difficult problems in repairing a reinforced concrete element is to handle corrosion damage. Reinforcement corrosion caused by carbonation is arrested to a great extent through repairs executed in a sound manner. However, the treatment of chloride-induced corrosion is more difficult and more often the problem continues even after extensive repairs have been carried out. It invariably re-occurs in a short period of time. Repairing reinforcement corrosion involves a number of steps, namely, removal of carbonated concrete, cleaning of reinforcement application of protection coat, making good the reduced steel area, applying bond coat and cover replacement. Each step has to be executed with utmost care. When chlorides are present in concrete, it is extremely difficult to protect reinforcing steel from chloride attack particularly in cases where chlorides have entered through materials used in construction and residing in the hardened concrete.
This increase in volume causes high radial bursting stresses around reinforcing bars and result in local radial cracks. These splitting cracks results in the formation of longitudinal cracks parallel to the bar. Corrosion causes loss of mass, stiffness and bond and therefore concrete repair becomes inevitable as considerable loss of strength takes place
Reinforcement steel in concrete structures plays a very important role as concrete alone is not capable of resisting tensile forces to which it is often subjected. It is therefore important that a good physical and chemical bond must exist between reinforcement steel and concrete surrounding it. Due to inadequacy of structural design and /or construction, moisture and chemicals like chlorides penetrate concrete and attack steel. Steel oxidizes and rust is formed. This results in loss of bond between steel and concrete which ultimately weakens the structure.
The best control measure against corrosion is the use of concrete with low permeability. Increased concrete cover over the reinforcing bar is effective in delaying the corrosion process and also in resisting the splitting.
5. Moisture Movement:
Most of the building materials with pores in their structure in the form of intermolecular space expand on absorbing moisture and shrink on drying. These movements are cyclic in nature and are caused by increase or decrease in inter pore pressure with moisture changes.
Initial shrinkage occurs in all building materials that are cement/lime based such as concrete, mortar, masonry and plasters. Generally heavy aggregate concrete shows less shrinkage than light weight aggregate concrete.
Controlling shrinkage cracks.
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 undergone most of its initial shrinkage. In case of structural concrete shrinkage cracks are controlled by using temperature reinforcement. Plaster with coarse well graded sand or stone chip will suffer less from shrinkage cracks and is preferred for plastering for external face of walls.
Considering the building as a whole, an effective method of controlling shrinkage cracks is the provision of movement joints. The work done in cold weather will be less liable to shrinkage cracks than that in hot weather since movement due to thermal expansion of materials will be opposite to that of drying shrinkage.
6. Poor Construction practices.
The construction industry has in general fallen prey to non-technical persons most of whom have little or no knowledge of correct construction practices. There is a general lack of good construction practices either due to ignorance, carelessness, greed or negligence. Or worse still, a combination of all of these.
The building or structure during construction is in its formative period like a child in mother’s womb. It is very important that the child’s mother is well nourished and maintains good health during the pregnancy, so that her child is healthily formed. Similarly for a healthy building it is absolutely necessary for the construction agency and the owner to ensure good quality materials selection and good construction practices. All the way to building completion every step must be properly supervised and controlled without cutting corners.
Some of the main causes for poor construction practices and inadequate quality of buildings are given below:
Improper selection of materials.
Selection of poor quality cheap materials.
Inadequate and improper proportioning of mix constituents of concrete, mortar etc.
Inadequate control on various steps of concrete production such as batching, mixing, transporting, placing, finishing and curing
Inadequate quality control and supervision causing large voids (honey combs) and cracks resulting in leakages and ultimately causing faster deterioration of concrete.
Improper construction joints between subsequent concrete pours or between concrete framework and masonry.
Addition of excess water in concrete and mortar mixes.
Poor quality of plumbing and sanitation materials and practices.
7. Poor structural design and specifications
Very often, the building loses its durability on the blue print itself or at the time of preparation of specifications for concrete materials, concrete and various other related parameters.
It is of crucial that the designer and specifier must first consider the environmental conditions existing around the building site. It is also equally important to do geotechnical (soil) investigations to determine the type of foundations, the type of concrete materials to be used in concrete and the grade of concrete depending on chemicals present in ground water and subsoil.
It is critical for the structural designer and architect to know whether the agency proposed to carry out the construction has the requisite skills and experience to execute their designs. Often complicated designs with dense reinforcement steel in slender sections result in poor quality construction. In addition, inadequate skills and poor experience of the contractor, ultimately causes deterioration of the building.
Closely spaced of reinforcement steel bars due to inadequate detailing and slender concrete shapes causes segregation. If concrete is placed carelessly into the formwork mould, concrete hits the reinforcement steel and segregates causing fine materials to stick to the steel, obstructing its placement and is lost from the concrete mix while the coarse material falls below causing large porosity (honeycombs).
Slender structural members like canopies (chajjas), fins and parapets often become the first target of aggressive environment because of dense reinforcement, poor detailing, less cover of concrete to the reinforcement steel. Added to all this, low grade of concrete and poor construction practices can make the things worse. It is necessary for the structural consultant to provide adequate reinforcement steel to prevent structural members from developing large cracks when loaded.
To sum up, the following precautions are required to be taken by the Architects, Structural Consultants and Specifiers:
Proper specification for concrete materials and concrete.
Proper specifications to take care of environmental as well as sub – soil conditions.
Constructable and adequate structural design.
Proper quality and thickness of concrete cover around the reinforcement steel.
Planning proper reinforcement layout and detailing the same in slender structures to facilitate proper placing of concrete without segregation.
Selection of proper agency to construct their designs.
Architects and Engineers are parents of the buildings they plan and design and therefore their contribution to the health and life of the building is quite significant. Once the plans are drawn the structural designs and specifications are prepared, it is then the turn of the agency to construct the building and bring the blue print to reality. Special care must be taken in the design and detailing of structures and the structure should be inspected continuously during all phases of construction to supplement the careful design and detailing.
8. Poor Maintenance
A structure needs to be maintained after a lapse of certain period from its construction completion. Some structures may need a very early look into their deterioration problems, while others can sustain themselves very well for many years depending on the quality of design and construction.
Regular external painting of the building to some extent helps in protecting the building against moisture and other chemical attacks. Water-proofing and protective coating on reinforcement steel or concrete are all second line of defence and the success of their protection will greatly depend on the quality of concrete.
Leakages should be attended to at the earliest possible before corrosion of steel inside concrete starts and spalling of concrete takes place. Spalled concrete will lose its strength and stiffness, besides; it will increase the rate of corrosion as rusted steel bars are now fully exposed to aggressive environment. It is not only essential to repair the deteriorated concrete but it is equally important to prevent the moisture and aggressive chemicals to enter concrete and prevent further deterioration.
9. Movement due to Chemical reactions.
The concrete may crack as a result of expansive reactions between aggregate containing active silica and alkalines derived from cement hydrations. The alkali silica reaction results in the formation of swelling gel, which tends to draw water form other portions of concrete. This causes local expansion results in cracks in the structure.
To control Cracks due to alkali-silica reactions, low alkali cement, pozzolona and proper aggregates should be used.
10. Indiscriminate addition and alterations.
There have been some building collapses in our country due to indiscriminate additions and alterations done by interior decorators at the instance of their clients.
Generally, the first target of modifications is the balcony. Due to the requirement to occupy more floor area, balconies are generally enclosed and modified for different usages.
Balconies and canopies are generally cantilever RCC slabs. Due to additional loading they deflect and develop cracks. As the steel reinforcement in these slabs have less concrete cover and the balcony and canopy slab is exposed to more aggressive external environment, corrosion of steel reinforcement takes place and repairs become necessary.
The loft tanks are generally installed in toilets or kitchens, which are humid areas of the buildings. The structure in addition to being overloaded is also more prone to corrosion of reinforcement steel in these areas and therefore deteriorates and if not repaired, part of the building can even collapse.