Deterioration of Concrete - Types and Causes
Different Types of Deterioration observed are:
1. Spalling
2. Cracking
3. Debonding of the Joints
4. Erosion
5. Corrosion of Concrete Through Chemical Attack
6. Sagging of Beams and Floors, bowing or inclination
7. Excessive Efflorescence, staining or discoloration
8. Damages Due to Vibration
9. Faults in Design, Material, and Workmanship
Spalling of Concrete
The spalling in concrete is caused due to
1. Corrosion of steel reinforcement or other embedded metal
2. The freezing of the porous or the cracked concrete
3. The chemical attack on concrete
4. Concrete that is of poor quality
5. Overloading or insufficient reinforcement
6. The thermal shock creating fire or firefighting activities
7. Mechanical damage due to accidents
8. The bearing of the concrete member or another with joint that is insufficient or that is choked
9. The bearing edge forming too close to the edge or at the end of a concrete member
Cracking
The safety or the bearing capacity of the structure is not endangered by the cracks formed in the concrete. All the possible effects of cracks are considered in the context of the cause, location, statical system and environment and the structure utilization. The cracks caused may be the effect of fault or the both. The reinforced concrete is not wholly monolithic.
The hypothesis that is followed during the design and the structural analysis of reinforced concrete structure is that the concrete will crack in the tension zone before the reinforcement bars will take the tension. Hence the cracks formed in this part of the member will have no direct effect on the immediate load bearing capacity of the member, even if the cracks are of considerable width.
The cracks formed in the cover will result in the penetration of the corrosion accelerating agents to reach steel. This will affect and break the corrosion protection of the reinforcement.
Note: Special Consideration must be given to sealing by injection the cracks that are wider than those allowed by the area’s building code standards.
It has to be considered that the cracks will influence the stiffness and the dynamic response of the structure, wherever this criterion is not considered in the design. The possibilities of unforeseen cracks in the prestressed concrete structures will result in fatigue failure when subjected to repeated loading.
The cracks in the compression zone of a load bearing member indicate the lack of shear resistance, this will result in the ultimate failure of the structure.
Cracks can be classified into different types. Principle method of classification is based on the primary cause of cracks.
Types of Cracks
1. Cracks in Fresh Concrete
2. Thermal Cracks
3. Shrinkage Cracks
4. Durability Cracks
5. Cracks formed From Loading
6. Other Cracks
Before choosing a specific repair method, the cause of crack identified have to be determined. It is essential to find whether the cracks are active or dormant. The periodic observations by utilizing tell-tales helps to check the crack behaviour at intervals.
The functional requirements and the cause of cracking is the factors on which the acceptable width of a crack is dependent. The National codes -design codes worldwide limit the crack width. For theoretically calculated values they are admissible limits. These will differ from the observed values practically.
Debonding Of the Joints
This problem of deterioration is found where there is a large change in the cross-sectional area or where dissimilar materials are bonded to concrete. The examples include:
1. At the epoxy resin expansion, joint nosings
2. Areas where the mastic asphalt waterproofing material is used
3. In areas where a thin topping layer like granolithic flooring is laid
4. Areas where sealants are used
Erosion
This is mechanical action due to the usage, weather and the water which leads to abrasive wear formed due to sliding, scraping or percussion. The erosion that is caused by the jet of water with a very high velocity can have the same mechanism as in the cavitation.
Corrosion of Concrete Through Chemical Attack
The cement paste which is highly alkaline in nature (due to hydration reaction and products) is subjected to chemical attack. This can occur only through the dissolution into the pore water of the cement paste. Hence, the resistance of the concrete to chemical attack will depend on the:
Ø Permeability of the concrete
Ø Type and size of the pore system
Ø Type of the cement
This will involve the following types:
1. The attack in the form of dissolution: Here, the compounds that are easily soluble can be washed out of the concrete or from the cement paste by the continuous access of the water.
2. The attack in the form of expansion: Compounds that are heavy soluble remain in the cement paste pores and crystallizes. If these compounds more space, they will create additional expansive stresses that will result in the bursting of the structure of cement paste.
3. Chemical or the electrochemical attack: These are caused by the acids, organic and the inorganic salts, alkali-carbonate reactions, soft water, aggressive gases or due to the electrochemical attack.
Sagging of Beams and Floors, Bowing or Inclination of Vertical Members
Settlement, overloading, deterioration or the failure of the concrete, corrosion or detachment of the tendons or the reinforcement or the conversion of the high alumina cement may cause the problems of bowing in walls or columns, opening or the closing of the joints or the sagging of the floors or the beams.
Excessive Efflorescence, Staining or Discolouration
The main reasons for these types of deterioration are the:
1. Undergone Chemical Attack
2. Presence of salts in the concrete constituents
3. Porosity in the concrete
4. Have Passage of water
5. Conversion of high-alumina cement
6. The concrete cover is small
7. The concrete permeability
8. Incomplete grouting of the ducts of tendons
Vibration Caused Damages
The vibration will bring seldom problems for the concrete. But checks is very necessary where heavy industrial processes is carried out.
Faults Found in Design, Material, and Workmanship
Faults found in design, the materials used and workmanship increases the risks of excessive deterioration. A proper and thorough investigation of the statistical calculation and drawings shows that whether a structural component has reserved or whether it requires any strengthening or repair.
The Faults can be Caused due to the Failure of Design, which will Include:
Ø Reduction of the dimension of concrete members (below the permitted minimum value
Ø Inadequate concrete cover
Ø Error in the calculation and human incompetence
Ø Inadequate assessment of the construction stages and temporary scaffoldings and the formwork used
Ø Incorrect design consideration for the tendon curvature
Ø Complex interaction of the numerous parameters for the impact and the impulse loading
Ø Difficulty in the prediction of structural response to these loads
The Faults in Materials include:
Ø Poor Quality Cement, aggregate, admixture, water
Ø Incorrect type of cement, aggregate, admixture, water
The faults in Workmanship include:
Ø Materials Storage
Ø Installation of formwork and scaffolding
Ø Batching
Ø Mixing
Ø Compaction
Ø Placing
Ø Curing
