Cracking in Masonry Wall and Brick Boundary Walls
Brittle Cracking in Masonry Wall is one of the most common failure modes in brick wall structures. Stone and brick masonry walls are widely used for load bearing and non-load bearing applications. Brick masonry is most commonly used as load bearing structure for non-RC framed structures in residential buildings. Other major applications include boundary walls, masonry columns and partition walls.
Due to its negligible flexural strength and lack of ductility, masonry walls are highly susceptible to cracking, if not properly designed. The foundation or supporting system plays the most critical role ensuring the service life of masonry structures. Cracks in masonry walls can compromise the integrity of structure and safety of users since the failure can sudden without enough warning.
This article discusses, the cracks and failures in unreinforced masonry brick walls used as boundary walls and partition. Brick walls are primarily designed for in-plane loads (gravity loads), although it can resist certain magnitude of out-of-plane loads. The capacity of masonry walls depends on bond strength, section modulus in both vertical and lateral directions.
Masonry wall failure patterns:
Depending upon the type of supports provided and the aspect ratio of the wall panel, masonry walls can exhibit three types of failure patterns, which are:
- Horizontal Failure
- Vertical failure
- Diagonal failure
Horizontal failure occurs due to inadequate bending strength in vertical direction under the action of vertical loads. Similarly, vertical failure occurs when the applied horizontal load exceeds the horizontal bending capacity of the wall. The load acting could be a collision load, earth pressure or water pressure.
Diagonal cracks occur primarily due to relative rotation of masonry units or blocks and without any external load action. Diagonal failure is the most common form of failure in boundary walls. Hence, the primary prevention method to avoid the diagonal cracking is to avoid relative rotation or deflection.
Causes of Diagonal Cracking in Masonry Brick Wall
The nature of diagonal cracking in masonry usually along the mortar joints or along other weak bricks in the panel. And the magnitude of cracking depends on the magnitude of deflection or movement it has undergone. Masonry crack can be complete or partial narrow opening by breaking or splitting. As mentioned above, relative movement of masonry brick units can occur due to various reasons. Few of those are disused below.
Excessive deflection of support beams or slabs
Beams supporting masonry units are usually designed for tight serviceability requirements and deflection limited below L/600. This is to avoid cracking of walls supported on it. Since these are non-structural walls, one of the common ways of repairing is to replaster the walls if further deformation of beams is not expected.
Foundation subsidence causes Cracking in Masonry Wall:
In case of boundary walls, the wall panels are usually founded on stone masonry. If the bearing capacity of soil is inadequate to support the wall self-weight, it leads to foundation subsidence. This leads to bending of wall panels and cracking. This can also occur if the bearing capacity of soil is reduced due to saturation or liquefaction.
Underpinning is one of the solutions for the cracks caused by subsidence. Lifting and re-leveling methods are also used. Ground beams are also widely adopted where bearing capacity of soil cannot be relied upon.
Soil heaving is the reverse of subsidence where the founding ground experiences an upward movement causing the wall foundation to heave. Foundation heaving can lift a whole wall structure, in reality the heaving is non uniform and causes differential uplift along the length of wall. Heaving of soil can occur due to seasonal variation of moisture content in the soil or expansion of soil. This is quite common in walls founded on expansive clay.
To avoid these kind of failure, soil stabilization or treatment is often necessary. Ground beams can also resist heaving actions of soil.
Differential settlement or rise of foundation causes relative rotation of brick units and leads to cracking. Differential settlement can occur due to localized soil failure or loss of bearing capacity. Whereas, differential rise if foundation could be due to roots of trees growing into or underneath the foundation.
How to avoid Cracks in Masonry and Design Considerations
Maintaining Service deflection limits
One of the main design considerations to avoid diagonal cracks in boundary wall and partition wall is to limit the service deflection limit. Span/600 is considered appropriate in most of the applications.
Vertical Expansion Joints in Bricks
Brick vertical expansion joints are introduced at intersection of walls or at intermediate lengths; this can decrease the possibility of cracks due to differential settlement. Vertical joints are usually placed at corners, offsets, setbacks, openings, wall intersections, changes in direction, certain heights, and long walls.
Usage of CMU walls:
Blocks of ordinary concrete, cinder concrete, or hollow tile are usually called Concrete Masonry Units (CMU). They usually are much larger compared to the ordinary bricks. Moreover, CMU’s have lesser water absorption compared to bricks and so we can avoid cracking due to expansion.
CMU wall can also be reinforced by filling the voids in the blocks with concrete/rebars. Particularly at corners, wall-ends, and openings, while other are left empty. Usage of steel reinforcement normally gives a CMU wall with greater lateral strength.
Construction of Ground beams:
The design of ground beams is to span between pad foundations or piles. The need of the ground beam is to connect the piles and to provide a podium for superstructure erection. These ground beams resist the settlement and also increases the bearing capacity of indirectly because of the pressure generated. Ground beams can also withstand the ground movements primarily clay heave in the foundation.
When wall is subjected to out-of-plane or lateral loads like wind loads and earthquake loads, the strength of unreinforced masonry walls can be increased by providing ties. This will enhance the stiffness of the walls.