Identification of Cracks

Introduction :

Cracks-The majority of cracks occur when the building or its components or the material of which the building is made upon it’s subjected to forces that are greater than those which it can withstand.

Cracks may occur in different components of a building due to-

• External applied force in a different component of the building is greater than that of its resisting capacity.
• Poor quality of material used Construction is not carried out by relevant drawing, prescribed job specifications, or work-man-like manner. etc.

Types of cracks: Cracks can be divided into two categories:

1. Structural cracks
2. Non-structural cracks

1. Structural cracks:

Structural cracks may arise due to

• Incorrect design
• overloading of the structural components
• Overloading of the soil in which the building is constructed.

Structural cracks endanger the stability of the buildings and may be difficult to be rectified. Extensive cracking of foundation walls, beams, columns or slabs, etc are examples of structural cracks.

2. Non-structural Cracks :

Non structural cracks are generally due to internal forces developed in the buildings on account of the changes in the size of building components due to

• Moisture variation
• Temperature variation
• Effect of gasses, liquid, and solids on the building.

The non-structural cracks can be repaired provided the reasons for the Cracks are identified and suitable remedial measures are taken to prevent the reoccurrence. Cracking in plaster, windows floors, etc are examples of non-structural cracking. The cracks can be broadly classified as vertical, or horizontal. diagonal, toothed cracks, irregular cracks, or random cracks.

Cracks due to foundation movement and settlement of soil: Shear cracks occur in buildings when there is a large differential settlement of foundation due to any of the following causes.

• Unequal bearing pressure under different parts of the structure
• Bearing pressure is more than the safe bearing strength of the soil.
• Low factor of safety in the design of foundation
• Local variation like supporting soil.

Preventions:

1. In case of shrinkable soil, adopt under reamed pile foundation.
2. The structural design of the foundation should be carried out in such a manner as to achieve uniform distribution of pressure on the ground to avoid differential settlement.
3. The foundation should be so proportioned that the safe bearings or soil is not exceeded.

Cracks due to Earthquake:

1. The crack may occur due to a sudden shift in the lower layer of the earth. The voids in the earth might have suddenly collapsed and been filled with soil from above.
2. Many geological events can trigger earth movement but in continuous movement. This results in a crack.

Preventions:

1. Construction of the foundation of the building on firm ground while doing construction.
2. Tie up the building worth connecting beams at foundation levne6 door level and roof level.

Question 1:

What is the mode of failure or damage of the beams in the figure above? Are the damages repairable? Elaborate

Answer 1(a)

Observation:

• In this picture we observe that, concrete get damage/ broken and reinforcement are visible at the bottom of beam.
• The concrete spalled beyond the reinforcement that is indicates the segregation.
• The top portion of beam is not exposed due to covered from the slab projection so it’s look safe and no major issue/ severe issue in the top of the beam.
• Crack developed in bottom and gradually increasing towards the top. The pressure on beam is more than its yielding point on the bottom support but the reinforcement is still active at the support top so there is no crack in top of beam.

Conclusion:

• At the time of earthquake, the stress on the beam is reversal, so this is a kind of flexural failure.
• During the earthquake, beam gets in tension zone and the structure feels in compression due to the stress reversal support at top.
• Bottom reinforcement is not sufficient to resist the tension in beam and the top compression resist by the slab due to concrete
• The bottom reinforcement of the beam lost their elastic limit, so that it is impossible to repair the bottom crack.

Answer 1(b)

Observation:

• In this picture we observed the cracks developed in column-beam corner
• Looks like the crack is much wider that have developed through the column-beam joint
• In the picture, the beam has separated from the column
• In picture shown, the concrete has not the crushed but only crack generated.

Conclusion:

• The reinforcement of the beam is not sufficient to anchor the vertical rebar of the column, that is insufficient development length of the rebar
• Due to inadequate reinforcement, beam not bonded to the column that’s why the horizontal member gets detached from the vertical member during the earthquake
• The lateral movement cannot be satisfied due to insufficient anchorage length of beam rebar so it keeps on separated from the column
• It is very tough to provide the development length in casted concrete between two members, so it is not repairable

Answer 1(c)

Observation:

• In this picture we observe that, cracks generated in column-beam junction
• The crack occurred in the entire width of the beam.
• Some cracks have developed in slab along the direction of beam.

Conclusion:

• This is a plastic hinge failure, the deformation in beam means no capability to resist the moment and its permit to free rotation
• The plastic hinge failure is happened at the joint only, so the damage occurs in the junction
• The main reason of damage are cyclic lateral loading and stress reversal at the junction
• As per the image, the slab has tried for handle the tension, but still it got cracked, so it clear that the tension is more on the slab and slab is unable to control damage
• To recover from the damage, we can strengthen the column-beam junction

Question 2:

Answer 2(a)

Observation:

• In this picture we observe that, the column has damage from the top, bottom of column is safe.
• The damage is all around the face of column
• No damage found in beam
• No connected walls are along with column, so this could be ground floor, probably parking area
• It is a most outer or external column of the building

Conclusion:

• From these observations its clear, this is a soft storey failure, technically plastic hinge formation at column
• Due to lateral movement in the building and absence of walls, columns got affected because of plastic hinge formed at the upper side of the ground floor
• Based on spall out the concrete from column and the severity of column is to be decide either repairable or non-repairable

Answer 2(b)

Observation:

• The breakage of concrete has diagonally developed in column, so this is not a crack it’s a completely crushed and concrete detached from column
• Lateral and longitudinal reinforcements are visible. Rebar has not damaged and not separated
• The clear vision of lateral reinforcement, i.e., shear links spacing is more than as per codel requirement which means that they have provided lesser shear reinforcement

Conclusion:

• By observing the picture, we can conclude this is a pure shear failure because the crack is in diagonal direction and the complete concrete had spalled and crushed
• The shear reinforcements were not sufficient so it had detached and spalled throughout the column. It had compelled concrete to breakdown into pieces.

Answer 2(c)

Observation:

• From the given picture, we can observe that the column is completely fail
• Damage happen at top of column and bottom of column is safe
• As per picture, there are two slabs connected with the column and the space between both slab is not the floor hight so this is probably for mechanical equipment
• The beam has temporary supported with the wooden piles after the column failure to avoid the collapse of the structure

Conclusion:

• From the observation we can conclude that the failure is short column failure.
• The column is attached with intermediate slab and the height of column is not similar then the other columns, so there is the heavy shear load on the column.
• The intermediate slab load is unintentional and not accounted in the design so this leads to the failure
• This failure is not repairable