Concrete repair and corrosion control

Concrete repair and corrosion control

Corrosion of concrete happens due to various factors but it is necessary to repair the damage caused by such corrosion. In Part-1 of the two-part series,
Upen Patel, Business Director, BASF India
, dwells at length on the causes of deterioration and the remedy thereof
Once concrete repairs and strengthening was considered as an activity of rejuvenating the old structures and making them capable of loadings and environmental stresses in the future life. Today we are constructing more advanced and ever more-demanding structures with complex detailing and concrete repairs and strengthening starts during the construction stage itself. The complex and fast pace construction methods with reduced emphasis on adequate quality assurance results in to construction errors and creates needs for repairs and strengthening during construction. With the complex performance demands of the new structures and ever longer life expectancies makes concrete repairs, strengthening and protection procedures more and more demanding. This article is an attempt to present the fundamentals of concrete repairs and strengthening in a step-by-step process and focuses on the advantages and disadvantages of current practices and provides an insight in the futuristic but more simple to adopt techniques.
Basic Definitions
Repairs: To replace or correct deteriorated, damaged or faulty materials, components or elements of a concrete structure.
Strengthening: The process of restoring the capacity of weakened components or elements to their original design capacity or increasing the strength of components or elements of a concrete structure.
Protection: Making the structure capable to resist the likely deterioration due to the surrounding/ environment.
Why concrete needs repairs?
There are many factors which lead to the need of repairs such as:
• Corrosion of reinforcement due to carbonation, chlorides
• Sulphates
• Alkali silica reaction
• Environmental pollution
• Deicing salts
• Acid rains
• Marine environment
• Oils
• Freeze thaw cycles
• Abrasion or erosion from wind or water borne agents
• Plants or microorganisms
• Overloading
• Physical settlement
• Impact
• Earthquake
• Fire
• Chemical attack by aggressive chemicals, sewerage or even soft water
Also the deterioration gets aggravated due to errors/mistakes/poor workmanship during construction such as:
• Higher w/c ratio
• Honeycombs and compaction voids
• Bleeding and segregation
• Plastic shrinkages and hardening stage shrinkage cracks
• Inadequate or no curing
• In sufficient concrete cover
• Cast-in chlorides from contaminated water/aggregates
• Inadequate or excessive vibration during the concerting
• Shutterwork or reinforcement movement during placement of concrete
Generally concrete structure requires repairs in the two events- New construction and during the service life. Repairs in the new construction require different approach then the repairs during service life and we shall deal one by one to better understand. The repairs during service life have more steps and we shall deal with it first. The repairs during service life arise due to certain deterioration taken place and understanding of the same is very vital in the design of the repair solution.
Why concrete deteriorates?
The reinforced concrete was designed with a basic understanding that its a marriage of two carrying spouses - concrete and steel. Concrete protects steel from getting corroded and steel protects concrete from getting cracked due to bending. The marriage was designed to last forever but the environment facilitates entry of many agents who leads the marriage to divorce...Major agents and their activities are described as under:-
Carbonation: The high pH of concrete passivates steel reinforcement from getting corroded. The carbon dioxide / sulphur dioxide present in the atmosphere gets dissolved in the water and forms weak carbonic /sulphuric acid and enters the concrete reducing the pH, resulting in the loss of passivation layer around reinforcement. The reinforcement states getting corroded resulting in to the rust. The rust formed has 4- times the original volume of the metal creating bursting pressure in the concrete mass. The build up of the pressure eventually cracks the concrete and makes the access for ingress of corrosive water and other water dissolved agents easily. The quicker access aggravates the corrosion and structure starts deteriorating rapidly. Spalling of the concrete cover and formation of brown colored rust is a visual indication of the carbonation attack. The carbonation attack can be checked by phenolphthalein liquid. The reaction is at its best at 50-75 % relative humidity.
Chloride attack: The main source of chlorides is the contaminated water or aggregates during construction and marine environment - direct contact with sea water or through wind borne chlorides in the splash zone. Chlorides ions are the passivating ferrous oxide layer on the steel reinforcement. Once reinforcement loses its passivation layer, it is highly susceptible to electro-chemical corrosion further induced by chlorides ions. The water dissolved chlorides ions form electro-chemical corrosion cell and establishes anodic and cathodic sites on the re-bar.
The electro-chemical corrosion results in to pitting corrosion-reduction in the cross section of the re-bar at specific sites without noticeable deterioration of the concrete cover. The hidden reduction in the cross-section of the reinforcement can results in to sudden failure of the structure member-making this as one of the most dangerous deterioration in the concrete structure. There is no 'net use' of chloride ions during the corrosion process. Therefore, once enough chloride ions reach the steel to break the passivation layer only water, oxygen and a conductive medium is needed to maintain the corrosion reaction. Also note that since corrosion is a chemical reaction, temperature plays a role in the process. The higher the temperature the faster the corrosion reaction occurs. The general rule for the rate of chemical reactions is that for every 25 degree F increases, the reaction rate doubles.
Sulphate attack: The main source of sulphates is the ground water. The sulphates attack on concrete, by reacting with the C3A in the concrete. The reactive product is larger in the volume resulting in to the expansive cracking in the concrete mass. The spalling and cracking of concrete takes place without any deterioration of the reinforcement to start with. With the time other forms of corrosion such as carbonation, chlorides becomes aggravated due to quicker access to the reinforcement. The sulphate attack can be reduced by using sulphate resistant cement which has low C3A content; but this reduces the resistance of chloride attack and hence no more a preferred option in the marine situation.
Alkali-silica reaction (ASR): In the case of ASR the alkali-reactive aggregates forms expansive gels in the concrete structure resulting in to cracking and spalling.
Step-by-step process to successful repairs:-
Following steps are essential for successful repairs:-
• Evaluation
• Relating observations to causes
• Selecting methods and material for repairs
• Preparation of drawings and specifications
• Selection of contractor
• Execution of the work
• Quality control
• Preserve records for future
Evaluation
Evaluate the current condition of the concrete structure. Structural analysis of the structure in its deteriorated condition, review of records of any previous repair work accomplished, review of maintenance records, visual examination, destructive and noon-destructive testing and lab analysis of concrete samples. Some of the popular tests used during the evaluation are summarised as under:-
• Visual inspection and recording
• Hammer sounding / Rebound hammer test
• Phenolphthalein test for carbonation
• Silver-nitrate test for chloride attack
• Half-cell potential measurement
• Core-cutting
• Chemical analysis of concrete at different depths
Repair philosophy
It is most important to consider the full load envelope, which has been acting on the structure during the complete service life and in the future. The repair materials must have compatibility with the existing structure. The compatibility may be defined as a balance (equilibrium) of physical, chemical, electrochemical and dimensional properties between the repair material and the existing substrate in structural exposure conditions for a determined period of time.
1st Compatibility: Physical/Permeability
• Allow substrate to breath
• Prevent entry of water and waterborne salts - Sulphate, Chlorides, SO2, CO2 2nd Compatibility: Chemical
• No negative chemical interaction with the substrate
• Absence of potentially dangerous substances such as chlorides, alkalies
• No expansive ettringite formation of sulphate
3rd Compatibility: Electro-chemical
• Higher resistance to corrosion current
• Must have conductivity and should not isolate substrate
• Effective passivation of re-bars
4th Compatibility: Dimensional stability
Coefficient of Thermal Expansion: Different Coefficients of Thermal Expansion causes differential movement and hence shall be avoided.
Modules of Elasticity: Under compression materials of different module will cause stress at the interface and hence shall be avoided.
Drying shrinkage: Drying shrinkage of fresh mortar causes stresses at interface; hence needs to be controlled to minimum.
(Extract from the paper presented by the author at the Construction Chemicals International Conference 2012 held in Mumbai)
(Extract from the paper presented by the author at the Construction Chemicals International Conference 2012 held in Mumbai)

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