Prior to commencing repair of concrete structures, it is always advisable to investigate the possible causes of the degradation. Nothing will be gained by carrying out a repair in haste.
Causes of deterioration may be divided into recurring and non-recurring:
- If the recurrence of deterioration is acceptably low, then it is normally acceptable to restore the structure as nearly as possible to its original state.
- If there is an unacceptable risk of recurrence, the structure should be repaired and the fundamental cause of deterioration should be eliminated or controlled to acceptable limits.
Once the cause of deterioration is known, a decision on the extent of repair is required. This includes the parameters of durability, strength, function and appearance of the structure after the repair process is completed.
After the decision is made, the choice of repair material and repair technique are investigated.
Causes of the concrete deterioration of a concrete structure:
- Structural Defects: Design or construction errors resulting in stress cracking or foundation movement.
- Corrosion of Steel Reinforcement: Concrete deficiencies in cover, compaction, presence of chloride, rusting resulting in spalling (i.e. concrete cancer) or cracking of overlying concrete.
- Chemical Attack: Etching or softening of the surface may be caused by acid attack or concrete may crack or spall due to sulphate attack.
- Fire Damage: A fire can cause spalling and cracking.
- Substances in Concrete Aggregates: Alkali aggregate or sulphate reaction causing concrete expansion with cracking and spalling. This occurs slowly over many years.
- Thermal Contraction or Expansion: Concrete contracts and expands with variations of temperature and when the contraction or expansion is restrained cracking or spalling may occur.
- Shrinkage During Curing: As concrete sets, shrinkage occurs and if this is restrained cracking may occur soon after construction.
- Structural Creep: Inadequate design may fail to allow for long term creep due to the shortening of columns or deflection of suspended floor slabs and beams causing cracking or damage.
- Plastic Shrinkage: Evaporation of the water content of concrete during the curing process may cause fine plastic cracks if not fully protected by curing compounds.
- Plastic Settlement: New concrete can hang on formwork or reinforcement causing cracks in columns, beams or walls.
- Construction Faults: Lack of cover over steel reinforcement, honeycombing in concrete, voids in concrete, blow-holes in concrete surfaces.
Identification of the causes of the deterioration of the concrete structure, extent of damage and recommended remediation methods:
- Visual inspection
- Review of construction documentation such as specifications, drawings, geotechnical reports, quality control on concrete batching and concrete mix designs.
Depending on the extent of deterioration testing may include any combination of the following:
- Visual examination
- Laboratory testing of samples
- Non-destructive testing
- Chloride content
- Sulphate content
- Concrete strength
- Moisture content
- Compression tests
- Cover over steel
- Reinforcement deterioration
- Nature of cracking
- Aggregate quality
- Concrete mix design
- Rate of deterioration
- Rebound Schmidt Hammer tests
- Ultrasonic measurements
- Pull-off tests
- Break-off tests
- Core tests
Generally experienced concrete repair technicians will identify the possible causes and the recommended solutions after a visual examination of the degraded areas.
This depends on the extent of damage and the feasibility of repair.
- Extent of corrosion of reinforcement
- Extent of loss of strength of reinforcement
- Extent of loss of the bond between the reinforcement and the concrete
- Extent of deficiency in concrete cover over reinforcement
- Extent of deflection due to cracking in the tensioned areas
- Extent of honeycombing in concrete
- Extent of porosity of concrete
- Extent of damage and loss of strength due to sulphate attack.
- Extent and width of cracking
This will determine the nature and type of repairs required or whether repairs are feasible.
If repairs are not feasible then complete replacement of the damaged structural element, such as, beams, columns, slabs, stairs etc may be unavoidable.
THE MORAL TO THE STORY IS THAT IMMEDIATE ACTION MUST BE TAKEN WHEN THE PROPERTY OWNER BECOMES AWARE OF THE EARLY SYMPTOMS OF CONCRETE DEGRADATION OR DETERIORATION IN ORDER TO AVOID A MASSIVE ESCALATION IN THE COST OF REMEDIATION OR , IN THE EXTREME, THE NEED FOR COMPLETE REINSTATEMENT OF THE STRUCTURAL ELEMENTS.