LANJUTAN MAKLUMAT PERKHIDMATAN FORENSIK JKR MALAYSIA

ACKNOWLEDGEMENT – PAPER CONTRIBUTOR

Dr Lim CCIr. Dr. Lim Char Ching, Director of Engineering of Road Branch has been with JKR Malaysia for more than 30 years and is The Subject Matter Expert for Forensic Works.

Ir. Dr. Lim was involved in numerous investigations and structural remedial works. He has published and presented more than 35 technical papers in International and National Journals and Conferences and written more than 380 reports on structural investigation and assessment and conducted more than 150 lectures in related fields.Decorative Divider 01

WHAT CONDITION NEEDED OUR SERVICE

Identification of the cause and level of distress/damage is a very important step for successful repair and to achieve the designed service life. Several new techniques such as new radar mapping, infrared thermography and impact echo to detect flaws in concrete structures has been used to identify invisible defects in concrete. Various steps in distress diagnosis include determination of concrete compressive strength from nondestructive tests, signature/vibration analysis, load tests, tests for state of corrosion etc. Distress diagnosis is an important step for implementing a proper repair scheme to achieve the designed life of the structure.Bookmark Divider 02

SYMPTOMS OF DISTRESS

The most common symptoms indicating structure distress is crack.  A crack is defined as a fracture in concrete which extends partly or completely through the member.  Cracks in concrete occur because of tensile stresses introduced in the concrete exceeding the tensile capacity of the concrete.

Table 1-2 below shows the relating symptoms to causes of Distress and Deterioration of concrete.Distress symtoms

TYPES OF CRACKS

There are two (2) types of cracks in concrete structures:

1. Non – Structural Cracks in Concrete:

Non-structural cracks are caused by changes in the moisture content and thermal movement. These cracks normally do not endanger the safety but may look unsightly, create an impression of faulty work or give a feeling of instability.

Shrinkage on slab
Shrinkage crack on slab
Shrinkage on wall
Shrinkage crack on concrete wall
Shrinkage on Slab roof
Typical random pattern shrinkage cracks at roof slab
Shrinkage on plaster
Shrinkage crack at plaster along interface between beam and brick
Shrinkage on beam
Shrinkage cracks could also be in the form of equally spaced transverse cracks especially in beams or columns

Non – Structural cracks occur mostly due to internally induced stresses in building materials. These cracks normally do not endanger the safety but may look unsightly, create an impression of faulty work or give a feeling of instability.

They often occur where an opening created a variance in the reinforcement in the concrete, such as around the windows, doors, and pipes. Non-structural cracks are the result of lateral pressure from the soil outside the foundation, and will typically leak when the soil is saturated. Types of non-structural cracks can include cracks that appear along form seams, shrinkage cracks, and hairline cracks.

2.  Structural Cracks in Concrete:

Structural cracks are those which result from incorrect design, faulty construction or overloading and these may endanger the safety of the structure.  The description on each types of cracks are given below:

Crack structural shear
Typical structural crack due to shear on wall
Crack structural bending.png
Crack due to bending stress on beam
Crack structural bending2
Crack due to bending stress on beams

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CAUSES OF CRACKS

a. Cracks – corrosion induced

These cracks occur when the underlying reinforcement had corroded and its expansive products exerted pressure on the concrete causing it to crack.  The cracks normally follow the line of reinforcement. Corrosion of reinforcement is normally caused by:

  1. Carbonation of concrete whereby carbon dioxide in the air enter the concrete and react in the presence of moisture making the concrete less alkaline and susceptible to corrosion, or
  2. Chloride ingress from marine environment and causing the reinforcement to corrode when its concentration exceeded the threshold level.

Delamination is the loss of coating adhesion to a surface or between coating layers and tends to destroy a coating’s strength and durability, as well as the material’s aesthetic appearance.

Concrete delamination is found most frequently in bridge decks and is caused by the corrosion of reinforcing steel or freezing and thawing.

b. Cracks – load induced

These are structural cracks which are caused either by:

  1. deficiency in design
  2. increased in loading than the intended design, or
  3. construction fault

c. Cracks – Settlement

These cracks occur when there is movement of the foundation either through subsidence or heaving.  This normally resulted in cracking at the apron and perimeter drain.  Sometimes the movement of the foundation is significant enough to result in cracking of the wall.  The crack is normally predominantly diagonal and follows the vertical and horizontal mortar joints in brickwork.  Some examples on cracking due to differential settlement are shown below:

Cracks Load induced settlement
Cracks at pedestrian footpath due to soil settlement
Cracks Load induced settlement2
Severe cracks at the pedestrian footpath due to soil settlement
Cracks Load induced differential
Cracks at wall due to differential

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Crack Monitoring

Monitoring the changes in crack width across a crack helps determine the cause of cracking and what remedial work should be specified. Two important reasons cracks need to be monitored when they’re detected is to indicate serious defects that threaten a structure’s stability and cracks that go ignored can devalue a property regardless of the threat they pose

Having the ability to assess and continuously monitor cracks when they appear is crucial.  Therefore, having a rigid methodology as well as suitable tools that are capable of accurate measurements.

Crack Detection microscope
Crack detection microscope
Crack Detection gauge
Avongard crack gauge
Crack Detection marking
Marking for installing DEMEC gauge
Crack Monitoring result
Example of crack monitoring result: Typical crack monitoring result indicating active crack

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⇒  Coming soon; second forensic show case project. 

 

 

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