Identification of Cracks

IDENTIFICATION OF 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.

1(a):

Flexural Failures

This failure mode occurs when the loads on the beam exceeds its flexural capacity. The shear strength of the beam should be greater than its flexural strength otherwise the shear failure would occur before flexural failure.

Flexural Tension Failure

Flexural tension failure initiates by yielding of steel reinforcement followed by crushing of concrete at compression side of the beam. It occurs when the beam is under-reinforced; the reinforcement ratio in the beam is low than balanced reinforced ratio as per ACI 318-14.

The signs of this type of failure are the development of cracks at the tension side of the beam which further extend to the compression side. These cracks are mostly vertical and located at the middle third of the beam. Great deflection is another sign of flexural tension failure.

In summary, the flexural tension failure happens gradually i.e., ductile failure and it is desired failure type in beam design process.

Observations:

1. The concrete is crushed and the reinforcement is fully exposed.

2. Concrete spalled behind the reinforcement, which may be a sign of cracking.

3. The upper part of the beam looks safe due to the advantage of plate flange.

4. The crack develops from the bottom up, which means that the yielding of the reinforcement occurs only at the bottom of the support.

Answer:

1. It’s a form of flexural failure due to earthquake stress reversal.

2. During stress reversal, the upper part of the support is in compression and the lower part is in tension.

3. The top compression is controlled by the slab plate advantage. The bottom tension stress was not satisfied by the reinforcement provided.

4. Since the reinforcement got yielded, it cannot be repaired.

1(b):

Beam-column joint plays an important role in the structural response of reinforced concrete structures, especially when these structures are subjected to cyclic or seismic loads. These joints usually experience high flexural and shear stresses from vertical and lateral loads. Shear stresses may arise from shear forces and/or from torsion straining actions in some cases such as edge beams, especially if loaded by heavy cladding. The stress distribution due to flexural and shear forces produce a diagonal crack pattern in the panel which leads to crush of the compressive strut, and consequently, to deterioration of strength and stiffness of the joint.

Observations:

1. Cracks occur along the edge of the beam-column.

2. Cracks appear to have developed in the structure as it appears much wider.

3. The beam looks separate from the column at the joints.

4. The concrete is not heavily crushed.

Answer:

1. It is a failure by pulling out the beam due to insufficient development of the length of the reinforcement in the joints.

2. During an earthquake, the horizontal movement of the structure aggravates this condition and separates the beam from the column.

3. Ensuring the anchorage length between two existing concretes is very difficult. So, it is not repairable.

1(c):

In framed structures, both steel and reinforced concrete, beam-column joints play a very crucial role in terms of seismic resistance. Under the effects of high lateral seismic loads, beam-column joints are subjected to high forces and moments and their behaviour have a significant influence on the response of the structure. Poor seismic performance of inadequately detailed joints can lead to the total or partial collapse of reinforced concrete frame structures. The use of low strength concrete, plain reinforcing bars, problematic anchorage details and inadequate transverse reinforcement in beam-column joints are the factors increasing the failure risk of the structures during severe earthquakes. In this paper, an experimental study on the cyclic behaviour of reinforced concrete exterior beam-column joints is presented.

Observations:

1. The cracks are exactly at the column-beam junction

2. Cracks in the beam developed along its entire width

3. The slab also developed multiple cracks along the width direction of the beam

Answer:

1. It’s a plastic hinge failure.

2. At first, the damage occurred only in the joint area.

3. Cyclic later loading and reversal of stress in the joints led to the damage.

4. Even the slab did its best to handle the stress but was unable to control the damage.

5. The slab is severely cracked, that is, it has undergone tension.

6. The joint can be strengthened.

2(a):

The required load-carrying capacity of columns damaged due to earthquake can be regained by retrofitting the inclined columns. During earthquakes, the top and bottom parts of a column are most vulnerable to shear forces. If the forces are large, the column may lose most of its strength and may get inclined under seismic and vertical loads.

The process explained in this article is ideal for retrofitting inclined columns. The retrofit process for columns damaged at the top involves estimating loads on damaged columns, jacking beams supported by the column, removing deteriorated concrete and deformed steel bars, installing reinforcements, and assembling formwork and pouring concrete. The concrete should be cured adequately after formwork removal.

Observations:

1. Concrete damaged at top of a column.

2. Concrete damaged all around the column face.

3. Beams are safe.

4. The location of the column looks at the edge of the column.

5. No walls are built along the column, which means it is probably on the ground floor.

Answer:

1. It's a soft storey failure, technically plastic hinge formation.

2. Due to the absence of infill Walls on the ground floor, the plastic hinge forms at the top of ground-storey columns during lateral movement.

3. Based on the development of concrete crushing, it can either be repairable or non-repairable.

2(b):

Torsional damage in columns is caused by the torsion moment effect. While diagonal cracks and spills occur in concrete on one side of the column, pressure ruptures and crushing occur diagonally on the other side. In places where there is crushing in the concrete with the effect of the earthquake, the impact of the load, the concrete is broken and poured.

Observations:

1. The developed concrete is completely broken diagonally.

2. Longitudinal reinforcements and braces are fine.

3. The shear-reinforced spacing looks wider, which probably means that less shear reinforcement is provided.

Answer:

1. It's a pure shear failure.

2. Inadequate share reinforcement forces the concrete to absorb all the shear stresses.

2(c):

Many factors affect the earthquake vulnerability of reinforced concrete (RC) structures, constituting a large part of the existing building stock. Short column in RC structures is one of the reasons for earthquake damage. Significant damages may occur due to brittle fractures in structural elements when the shear resistances are exceeded under the effect of high shear stress in short columns formed due to architectural and topographic reasons.

Observations:

1. The column completely failed at the top level.

2. Two slabs are attached to the column, probably one at mid-height of the column.

3. Wooden pikes can be proper support after the failure of the column and provide temporary support to prevent the entire structure from collapsing.

Answer:

1. It's a short column failure.

2. An additional slab or beam connected to a column in mid-height leads to this effect.

3. This leads to failure, especially if the connection is unintentional and not considered in the design.

4. The failure shown in the picture is not repairable at all.