There are different types of defects in concrete structures, and the reasons for the deterioration in concrete are either physical or chemical process. The following signs of deterioration are generally observed in concrete structure:
- Texture or colour of concrete
Signs of Defects/Deterioration in Concrete Structure:
Loading, faulty construction, differential settlement, impact on account of the accident, chemical reaction, weathering, excessive loss of prestressing, corrosion of steel, errors in design and detailing, earthquake, fire, etc
Shrinkage cracks, thermal cracks, cracks in fresh concrete etc
Measurement of crack width:
A simple small hand-held Tell-tell plus crack monitor may be used. Where greater accuracy of measurement is required, transducer or extensometer or strain gauges can be used.
Recording of crack:
During site visit Whenever cracks are noticed, the following should be recorded:
i) Location of the cracks
ii) Width, depth and length of the crack in detail
iii) Type and pattern of crack by visual inspection
iv) Whether active or dormant type
v) Behaviour under live load condition
Pattern of Cracks and Possible Causes:
|Structural Element||Location||Crack pattern/ Direction||Possible causes|
|Soffit of slab or girder||End of span||Longitudinal||(i) Bursting stress |
(ii) Lack of anchorage block reinforcement
(iii) Alkali-Silica reaction in concrete
|Soffit of slab or girder||Mid span||Longitudinal||(i) Alkali-Silica reaction in concrete|
(ii) Broken tendons
|Soffit of slab or girder||Mid span||Transverse||(i) Loss of prestress|
(ii) Excess of live load
|Web||End of span||Diagonal||(i) Sheer stressLoss of prestress|
|Web||End of span||Longitudinal||(i) Alkali-Silica reaction in concrete |
(ii) Duct Flotation
(iii) Broken tendons
|Web(Cantilever/ Continuous girders)||Over support||Vertical||(i) Loss of pre-stress|
|Top flange (I-girder or Box girder)||Mid span||Transverse||(i) Differential shrinkage|
|Top flange (I-girder or Box girder)||Over support||Longitudinal||(i) Alkali-Silica reaction in concrete|
(ii) Broken tendons
|Top flange (I-girder or Box girder)||Over support||Transverse||(i) Differential shrinkage|
(ii) Loss of prestress
(iii) Excess of live load
Type of Defects/Deterioration in Concrete Structure:
Spalling is the defects in Concrete of surface or sub-surface fatigue, which causes cracks to form in the running surface of the concrete. Spalling is the effect of surface or sub-surface fatigue; when a portion of concrete gets separated, showing a fracture parallel or inclined to surface, called spalling. Spall is flakes of a material that are broken off a larger solid body due to excessive rolling pressure.
Major causes of spalling:
- Corrosion of steel reinforcement
- The freezing of cracked or porous concrete
- Chemical attack
- Poor quality concrete
Separation of a thin surface layer of concrete from the existing concrete surface is called Delaminations. When concrete gets separated along a plane parallel to the surface of the concrete is called delamination.
Major causes of Delamination:
Due to corrosion of reinforcement, inadequate cover and fire delamination occurred.
Scaling known as local flaking or peeling of an existing surface of hardened concrete resulting in a continuous loss of mortar and aggregate over an area.
Major causes of Scaling:
It is observed where repeated freeze and thaw action on concrete occurred or when the concrete surface gets subjected to cycles of wetting and drying
Presence of rust/stain marks on the side and bottom surfaces of PSC girders, following the prestressing cable profile, indicate corrosion of prestressing cable is considered as a serious threat to the structural integrity of the member. Corrosion of steel is the most important deterioration of girders.
Major causes/influence of Rust streaks:
- pH value of old concrete,
- Inadequate cover of reinforcement
- Permeability of concrete
- Exposure to aggressive environment
- Incomplete grouting of ducts of tendons
- Besides, the HTS strands are more prone to corrosion by a phenomenon known as “STRESS CORROSION.”
Defects due to leaching happens gradually in structures in long-term contact with water resulting in the dilution of calcium from the concrete matrix, which is a major chemical component of concrete and is called leaching.
In case of deterioration due to leaching, the aggregates will be visible without the cementitious material
Erosion is the action of surface processes and it caused due to abrasive action deck top, or by high-velocity river water in piers may even lead to the formation of pocket or cavity
Loss of Camber Causes Defects in Concrete Structure:
Loss of camber may be caused by settlement, overloading, deterioration of concrete, due to stress corrosion. In the case of prestressed concrete girders, loss of camber may be due to loss of prestressing, which needs investigation.
Condition Survey for Defects/Deterioration in Concrete Structure:
Locations to be Specially Looked for defects in the concrete structure are as follows:
- The general condition of the structure and prestressed components
- overall condition of the concrete
- Cracks, delamination, leaching, Scaling or damage due to accident or any other causes
- Rusting or corrosion signs
- Condition of construction joints, the junction of girders and diaphragm, joints in segmental construction
- Condition of cable end sealing
- Damage due to abrasive action seepage
- Whether the seating of girder overbearing is uniform
- Around the bearing support and bottom of girder above bearing should be inspected for Spalling/crushing/cracking
- Condition of Expansion joints and wearing coat
- Bearing Condition which includes Rusting/corrosion, Unusual tilting movement of the piston, Rollers jumping off the guides and Condition of anchor bolts.