Cracking is a failure that occurs most often in structures. In order to treat the crack, one must know the causes of stresses that have resulted in the formation of cracks. An estimation of these stresses before the construction will help in bringing designs that will have the provision to anticipate for movements that are the reason for cracks. The occurrence of cracks may be in different ways but certain typical modes and characteristics exist for these. The cause of cracks can be clearly understood from the type and the magnitude of the cracking.
Causes of Cracks in Masonry Walls
The main reason behind the formation of cracks in the walls is the deformation. These deformations occurred may be at the macroscopic level or at the macroscopic level. Here different deformations as different causes of crack formation are explained below:
- Deflections in Slab or Roof Elements
- Differential changes due to temperature changes and stresses
- Roof Expansion and Construction
- Cracking due to Creep and Shrinkage
- Poor Detailing and Improper Construction
- Load Bearing Parameters
- Foundation Factors
Deflections in Slab or Roof Elements
Cause: Cracks can be developed in the wall slab interface, where the wall supports the slab. This is caused due to the rotation of the slab (supported by the wall) under the action of load over it. This rotation of the end of the slab will result in the upward left tensile force, which is the reason behind the crack formation. This type of crack is more visible in external walls than on the internal walls.
If the induced tensile stress at the outer surface is greater than the permissible stress of the wall, the crack formation will be there.
Remedy: These type of cracks can be accommodated only by the incorporation of the tensile reinforcement that is sufficient to handle the tensile stress. Another option is to separate the slab from the walls. A third was is to increase the stiffness of the slab so that the slab deflection can be reduced.But in practical, zero deflection is not possible.
Differential Changes due to Temperature Changes and Stresses
Cause: Differential expansion characteristics can be shown by two walls or a wall or column with different modulus of elasticity that will result in differential strains. These strains can be caused as a result of different stress level for the wall of same material. The external walls are subjected to temperature changes that are the main reason for temperature stresses and strain and hence crack is formed. These will bring cracks in the connecting walls.
Remedy: Choose materials that can resist such strains for internal and external walls. Also, the materials and design to bear the stress level during the construction of higher buildings.
Expansion of Roof and Its Construction
Cause: When compared to the foundation structure, the roof is subjected to high-temperature changes. The walls too are exposed to direct sunlight that will result in the small amount of deformation. The relative movement of the roofs and the walls are reflected as stresses in the wall section. When these stresses exceed the permissible value, long horizontal cracks are formed. This crack formation is a sure case to occur unless we provide provision for an independent movement.
Remedy: The main remedy under such situations is to provide a design that could facilitate such types of movements.
Creep and Shrinkage Cracking
Along the edge or at certain points of the masonry walls, restraints are provided. Stresses are developed in these areas when differential shrinkage has occurred between the masonry and the restraining media. In some situations, long time creep may be the source of stresses and the related movements.
Poor Details and Improper Construction
Poor construction includes:
- The wrong location of the control joints
- Window Alignment
- Placement of steel plates above opening
- Design of drain
- Location of Splices
Remedy: The usual procedure is to place the control joint first over the mortar. Only after this hardens, the raking is carried out for a depth of 20mm. This remaining mortar is the packing of the caulking.
- The issues of spalling can be reduced by taking care of run-off drain for the roof. This is caused due to the freeze and thaw action. The water falling over the wall will result in spalling.
- The windows opening will have a plane of weakness on either side of the opening. If the window opening is placed above another, proper alignment must be done for the sides of the opening.
- Spalling due to eccentric load can be prevented by placing steel plate above an opening.
- Use of Joint reinforcement will result in fine cracks instead of large single crack. Special care must be taken while splicing is done.
Load Bearing Factors
It is necessary that the designer decide the wall to be load bearing or non-load bearing ones. The load bearing walls will resist the crack formation by means of a clamping action. This increases the shear friction between the unit and the mortar joint.
Issues of cracking have emerged in masonry wall construction over a concrete foundation. The thermal expansion of the walls will work against the drying shrinkage of the concrete. This will create the extension of the masonry wall beyond the foundation.
During a lower temperature, the masonry wall will contract and the tensile strength of the same will not be sufficient to keep the masonry wall back. This will result in cracks near the corners.
Settlement of foundation is one of the main reason behind the formation of cracks. This is mainly experienced in structures supported in soil or silty clay soil. The detrimental settlement is caused by structures whose foundation are built based on a guess ( small construction).
The cracks due to the settlement are observed larger at the top and it will diminish as hair line cracks at the bottom. This is based on the relative direction of the settlement and the location with respect to the length of the wall. One must keep in mind that the differential settlement is the main issue, not the total settlement.