Upen Patel, Marketing Manager, BASF Construction Chemicals (India) Pvt. Limited, Mumbai.

Background

Once concrete repairs and strengthening was considered as an activity of rejuvenating the old structures and making them capable of bearing loadings and environmental stresses in the future life. Today we are constructing more advanced and ever more-demanding structures with complex detailing. In case proper steps are not taken on quality control and proper supervision the concrete repairs and strengthening may start during the construction stage itself. Reduced emphasis on adequate quality assurance results in to construction errors and creates needs for repairs and strengthening during or after 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 it 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 a concrete structure.
  • Protection: Making the structure able to resist the likely deterioration due to the surroundings/environment.

Why does Concrete need Repairs?

There are many factors which lead to the need of repairs such as:
  • Corrosion of reinforcement due to carbonation, chlorides
  • Effects of Sulphates
  • Alkali silica reaction
  • Environmental pollution
  • Deicing salts
  • Acid rains
  • Oils
  • Freeze thaw cycles
  • Abrasion or erosion from wind or water borne agents
  • Plants or micro organisms
  • Overloading
  • Physical settlement
  • Impact
  • Earthquake
  • Fire
  • Chemical attack by aggressive chemicals, in industrial or marine environment 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 shrinkage & hardening stage shrinkage cracks
  • Inadequate or no curing
  • Insufficient concrete cover
  • Cast-in chlorides from contaminated water/aggregates/ /admixtures
  • Inadequate or excessive vibration during concreting
  • Shutterwork or reinforcement movement during placement of concrete
Generally, concrete structure requires repairs in the two cases– New construction and during the service life. Repairs in the new construction require a different approach then the repairs during service life and we shall deal one by one for better understanding. The repairs during service life have more steps and we will deal this 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 Deteriorate?

The reinforced concrete was designed with a basic understanding that it’s 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 actives 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 to water and forms weak carbonic/ sulfuric acid and enters the concrete through micro pores reducing the pH, resulting in to loss of passivation layer around the reinforcement. The reinforcement starts getting corroded resulting in to the rust. The rust times of the original volume of the metal creating bursting pressure in the concrete mass. The buildup of the pressure eventually cracks the concrete and makes the access for ingress of water and other water dissolved agents easy. 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 depth can be assessed by phenolphthalein liquid. The reaction is at its best at 50 – 75% relative humidity. The corrosive reaction is expressed as:

CO2+H2O+Ca(OH)2 >CaCO3+H2O

Chloride Attack: The main source of chlorides is the contaminated water or aggregates used during construction and the marine environment – direct contact with sea water or through wind borne chlorides in the splash zone. Chlorides ions first attack Fe2O3 - the passivating ferrous oxide layer on the steel reinforcement. Once reinforcement looses its passivation layer, it’s highly susceptible to electro-chemical corrosion further induced by chlorides ions. The water dissolved chloride ions forms electrochemical corrosion cell and establishes anodic and cathodic sites on the rebar.

The electro-chemical corrosion results in to pitting corrosion – reduction in the cross section of the rebar 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 structural member - making this as one of the most dangerous deterioration in the concrete structure. The corrosive reactions are expressed as:

Fe++ + 2Cl -> FeCl2

FeCl2 + H2O + OH ->Fe(OH)2 + H+ + 2Cl - 2Fe(OH)2 + ½O2 > Fe2O3 + 2H2O

(ANODE)

½O2 + H2O + 2e -> 2OH-

(CATHODE)

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 an important 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 25oF increase, 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 places 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 also reduces the resistance of chloride attack and hence not preferred in the marine situation.

Alkali - silica reaction (ASR): In the case of ASR the alkalireactive aggregates forms expansive gels in the concrete structure resulting in to cracking and spalling.

Step-by-step process to successful repairs:

Essential steps for successful repairs
1. Evaluation
2. Relating observations to causes
3. Selecting methods and materials for repairs
4. Preparation of drawings and specifications
5. Selection of contractor
6. Execution of the work
7. Quality control
8. 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 nondestructive testing, and any lab analysis of concrete samples. Some of the popular tests used during the evaluation are summarized as under:
  • Visual inspection & recording
  • Hammer sounding/Rebound hammer test
  • Phenolphthalein test for carbonation depth
  • 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 completed 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 & water Bourne salts – Sulphates,Chlorides, SO2, CO2 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 with sulphates

  • 3rd Compatibility: Electro- Chemical:

  • Higher resistance to corrosion current
  • Must have conductivity and should not isolate substrate
  • Effective passivation of rebars

  • 4th Compatibility: Dimensional stability:

  • Coefficient of Thermal Expansion : Different Coefficients of Thermal Expansion causes differential movement, and hence shall be avoided
  • Modulus of Elasticity: Under compression materials of different moduli will causes 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.
  • Repair Components & Right Materials

    Treatment of cracks

    The most important criteria for selecting the right material for crack injection is based on the structural status of the crack. Is the crack alive or dormant? Can be checked by monitoring the crack width. If the crack is live, stresses are still like to relieve and hence to avoid further cracking at any other location, it is important to inject and seal the live crack with flexible injection resin such as polyurethane based. The dormant structural cracks can be sealed with epoxy/polyurethane resins meant for structural bonding. In case if the sealing is only meant for watertightness same can be achieved by injecting with the re-swellable acrylate injection resins. The surface cracks found within the concrete covers can be open, routed and sealed used acrylic sealers as they are superficial nature but needs effective sealing as can leads to other form of corrosion/ deterioration. Many hairline cracks formed on the surface of concrete cannot be opened and sealed and can be coated and sealed with high-elongation, flexible acrylic protective coatings instead.

    Surface Preparation for Volume Replacements

    The surface preparation is a prerequisite for an effective volume replacement job. Following components explains the surface preparation tasks:
    • Remove all identified defective concrete.
    • Saw cut perimeters - 15mm depth.
    • Expose steel until no corrosion is evident.
    • Expose the full circumference of the steel and beyond by 25mm.
    • Priming of the rebars: Prime immediately after cleaning. Apply a continuous coat of active zinc rich epoxy primer or appropriate corrosion protection system. Attention must be paid to the underside of the bars.
    Priming of the concrete: Depending upon the need of the volume replacement materials, apply right primer. In case of chloride contaminated area use epoxy bonding agent.

    Cosmetic Volume Replacements

    While replacing the concrete volume within the cover is defined as cosmetic repairs. The aim is to replace defective, deteriorated concrete cover with impervious polymer-modified mortar. The most cost effective repair materials are ready to use Re-Profiling mortars or to use site-batched polymer modified repair mortars with 1:5:15 proportion of polymer: cement: sand. These mortars are not capable of achieving the high compressive strengths but are able to provide effective corrosion barrier at economical costs. As the repair is within the cover zone, it does not have significant barring on the overall strength of the structure. The ready to use reprofiling mortars are shrinkage compensated and have good thixotropy enabling up to 50mm thickness built-ups in single operations; while polymer modified site-batched mortars may need multi-layer applications on thick applications.

    Structural Volume Replacements

    While replacing the volume of the concrete beyond concrete cover, it is very important that the member under repair is relived from the imposed loads by supports during repair period. Surface preparation is very important and any negligence may cost the success of the structural repairs. There are many options available for volume replacement such as – microconcrete, single component patch repair mortars, two-component patch repair mortars, spray applied micro-concrete, site-batched polymer modified mortars, selfcompacting concrete, shotcrete, pre-placed aggregate concrete. Large volume concrete repairs can be conducted using selfcompacting concrete, shotcrete or pre-placed aggregate concrete provided the interface between new and old concrete is taken care properly. Generally, in India the 1st choice for volume replacement remains as form and pour microconcrete while spray applied micro-concrete is very popular internationally due to reduced needs for shuttering and ready availability of spray applied microconcretes. Patch repair mortars if need to be used must be ready to use, one component type with fibre modification to avoid chances of cracking. Product like twocomponent polymer modified mortars and site-batched polymer modified mortars known for their low compressive strengths and can not be used to repair high grade concrete. In most parts of India, it’s not practical to produce sitebatched polymer modified mortars with > M20 grade compressive strengths due to limitation of local fine aggregate gradation! If such low strength mortars are used to replace concrete, it leads to higher stress concentration on the rest of the structure resulting in to further distress and deterioration.

    Special Applications

    Application such as under-water, chemical exposures, pavement repairs, abrasion/impact damage, and heritage restorations requires additional considerations while repairs and now many specialized products are available for such applications some of them are summarized as below:

    Underwater Repairs

    • Micro-concrete: Emaco S46UW – Anti-washout grout
    • Patch repair mortar: Emaco S90 – Thixotropic patching mortar
    • Crack Injection resins: Concresive 1316 – waterinsensitive resin
    • Re-profiling/sealing mortar: Subcote S – Resin based

    Chemical Resistance Situations

    • Epoxy concrete: Concresive 2929
    • Patch repair mortar: Concresive ERL – Epoxy liner
    • Re-profiling/sealing mortar: Concresive 2200 – Resin based

    Pavement repairs

    • Patch repair mortar: Emaco R650 – Rapid hardening, cementitious
    • Re-profiling mortar: Concresive 1418T – Rapid hardening, resinous
    More details on individual products can be available from local BASF offices.

    Corrosion Control

    While repairing the deteriorated structure it’s of utmost importance to prevent the rest of sound structure from getting deteriorated by implementing proper corrosion control measures. There are various options available; the right selection needs to be based on the need of the situation and the practicality of the options. Following is a basic introduction with the advantages and disadvantages of some of the most popular options:

    A. Film forming coatings – Acrylics & resin based coatings:

    • Limited life expectancy ( 5 ~ 10 years)
    • Limited crack bridging limits performance on cracked substrate
    • No passivation of corroding Reinforcement

    B. Migrating corrosion inhibitors - surface applied method

    • Easy to use
    • Effectiveness depends on chemical types & concrete porosity
    • Not effective against Carbonation, sulphates, ASR.

    C. Cathodic Protection – Impress Current method

    • Good life expectancy (> 20 years)
    • Very expensive
    • Destructive, Slow and time consuming
    • High application skills required
    • Power break-down can disrupt the protection
    • No protection against Carbonation, sulphate, ASR attacks

    D. Cathodic Protection – Impress Current method

    • Low to medium life expectancy ( 5 - 10 years)
    • Destructive method, requires replacements in future
    • Lump-sum applications, Design guidelines not followed
    • Not practical in heavy reinforced members
    • Corrosion continues > instead of steel embedded zinc corrodes
    • Not effective against Carbonation, sulphates, ASR.

    E. Protectosil CIT –Organo-functional silane based system

    • Spray applied - Easy to use
    • Organo functional Silane based - deep penetrative
    • Monomer structure – chemically reacts, no degradation in UV
    • Corrosion inhibition effect – repassivates rebar
    • Effective in all 4 types of corrosion as it hydrophobize concrete
    • Well established & tested worldwide
    • Non destructive method
    • Measurable reduction of corrosion current Effective till treated concrete thickness lasts!

    Other Important Aspects of Repairs

    While right diagnosis and selection of materials is the core of the repair project; other aspects such as Preparation of drawings and specifications, Selection of right contractor, Execution of the work to the specifications, adequate Quality control are equally important. Repair project is more specialized and good site management and construction practices have to be followed. Also at the end of the job all the records relating to the diagnosis, material selection and execution of the work will be maintained and preserved for any reference needs in future.

    Conclusion

    With the right focus on the causes, evaluation and selection of correct repair materials; the resultant repairs would be long lasting and we would be able to avoid expensive repetitions of repairs in the future.

    NBMCW October 2007