Concrete repair is the procedure of fixing a hardened concrete surface that over time has lost the ability to hold the binding concrete materials together because of environmental exposure or damage. Concrete repair is appropriate for cracks, physical impacts, surface scaling, or chipped-out surfaces.
Why Does Concrete Needs Repairs?
There are many factors that lead to the need for concrete repairs such as:
- Corrosion of reinforcement due to carbonation, chlorides
- Alkali silica reaction
- Environmental pollution
- Deicing salts
- Acid rains
- Marine environment
- Freeze-thaw cycles
- Abrasion or erosion from wind or water bore agents
- Plants or microorganisms
- Physical settlement
- The chemical attack by aggressive chemicals or even soft water
Also, the deterioration gets aggravated because of errors or mistakes or poor workmanship during construction such as:
- Higher w/c ratio
- Honeycombs and compaction voids
- Bleeding and segregation
- Plastic shrinkage and hardening stage shrinkage cracks
- Inadequate or no curing
- Insufficient concrete cover
- Cast-in chlorides from contaminated water/aggregates
- Inadequate or excessive vibration during the concreting
- Shutter work/reinforcement movement during placement of concrete
Generally, a concrete structure requires repairs in the two events – New construction and during the service life.
Repairs in the new construction require a different approach than 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 first. The repairs during service life arise due to certain deterioration that has taken place, and understanding the same is vital in the repair solution’s design.
Step-by-step Process to Successful Concrete Repairs
Following steps are essential for successful repairs
- Relating observations to causes
- Selecting methods and materials for repairs
- Preparation of drawings and specifications
- Selection of contractor
- Execution of the work
- Quality control
- Preserve records for future
Evaluate the current condition of the concrete structure. Structure analysis of the structure in its deteriorated condition, review of maintenance records, visual examination, destructive and non-destructive testing, and may lab analysis of concrete samples. Various popular tests utilized during the evaluation are summarized 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
- Chemical analysis of concrete at different depths
It’s most essential 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 should have compatibility with the existing structure. The compatibility may be defined as a balance 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 water Bourne slats- sulfates, chloride, SO2, CO2
2nd Compatibility: Chemical
- No negative chemical interaction with the substrate
- Absence of potentially dangerous substances such as Alkalies, chlorides
- No expansive ettringite formation with sulfates
3rd Compatibility: Electro-Chemical
- Higher resistance to corrosion current
- Must have conductivity & shouldn’t isolate substrate
- Effective passivation of rebars
4th Compatibility: Dimensional Stability
- Coefficient of Thermal Expansion: Different Coefficients of Thermal Expansion causes different movement, and hence shall be avoided.
- Modulus of Elasticity: Under compression materials of the different modules will cause stress at the interface and, hence, be avoided.
- Drying Shrinkage: Drying shrinkage of fresh mortar causes stresses at the interface; hence needs to be controlled to a minimum.
Repair Components & Right Materials
Treatment of Cracks
The most important criteria for selecting the right material for crack injection are based on the structural status of the crack. Is the crack arrive or dormant? Can be checked by monitoring the crack width. If the crack is live, stresses are still like to relieve & hence to avoid further cracking at any other location, it is important to inject and seal the live crack with flexible injection resin-like polyurethane based.
The dormant structural cracks can be sealed with epoxy or polyurethane resins meant for structural bonding. In case if the sealing is only meant for water tightness same can be accomplished by injecting with the re-swell capable acrylate injection resins. The surface cracks found within the concrete covers can be open, routed, and sealed used acrylic sealers as they are superficial but need effective sealing, leading to other forms of corrosion or deterioration. Many hairline cracks formed on the concrete surface can’t be opened & sealed and can be coated & sealed with high-elongation, flexible acrylic protective coatings instead.
Surface Preparation for Volume Replacements
Surface preparation is essential for an effective volume replacement job. The following components explain the surface preparation tasks.
- Remove all identified defective concrete.
- Saw cut perimeters – 15mm depth.
- Expose steel until no corrosion is evident.
- Expose the full perimeter of the steel & beyond by 25mm.
- Priming of the rebars: Prime instantly after cleaning. Apply a continuous coat of active zinc-rich epoxy primer or an appropriate corrosion protection system. Attention must be paid to the bottom of the bars.
- Priming of the concrete: Depending upon the need of the volume replacement materials, apply the right primer. In case of chloride contaminated area utilizes epoxy bonding agent.
Cosmetic Volume Replacements
While replacing the concrete volume within the cover is determined as cosmetic repairs. The aim is to replace the defective, deteriorated concrete cover with impervious polymer-modified mortar. The most cost-effective repair materials are ready to utilize re-profiling mortars or to utilize site-batched polymer modified repair mortars with 1:5:15 proportion of polymer: cement: sand. These mortars are not capable of achieving high compressive strengths but are able to provide effective corrosion barriers at economical costs. As the repair is within the cover zone, it doesn’t have important barring on the overall strength of the structure. The ready-to-use re-profiling 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 the concrete cover, it is very important that the member under repair is relieved from the imposed loads by supports. Surface preparation is very significant and any carelessness may cost the success of the structural repairs. There are several options available for volume replacement such as micro concrete, single-component patch repair mortars, two-component patch repair mortars, spray-applied micro-concrete, site-batched polymer-modified mortars, self-compacting concrete, shotcrete, pre-placed aggregate concrete. Larger volume concrete repairs can be conducted using self-compacting concrete, shotcrete or pre-placed aggregate concrete provided the interface between new and old concrete is taken care of properly.
Generally in our country, the 1st choice for volume replacement remains as form and pour micro-concrete while spray-applied micro-concrete is very popular internationally due to reduced needs for shuttering and ready availability of spray-applied micro-concrete. Patch repair mortars if need to be used must be ready to use, one-component type with fiber modification to avoid chances of cracking.
Products like two-component polymer-modified mortars and site-batched polymer-modified mortars known for their low compressive strengths and cannot be used to repair high-grade concrete. In most parts of our country, it’s not practical to produce site-batched polymer-modified mortars with greater than M20 grade compressive strength due to the 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 further, distress and deterioration.
While repairing the deteriorated structure it’s of utmost importance to prevent the rest of the sound structure from getting deteriorated by implementing proper corrosion control measures. There are various alternatives 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 & disadvantages of some of the most popular options:
Film forming coatings – Acrylics and resin based coatings
- Limited life expectancy
- Limited crack barding limits performance on the cracked substrate
- No passivation of corroding reinforcement
Migrating corrosion inhibitors – Surface applied method
- Easy to use
- Effectiveness depends on concrete porosity and chemical types
- Not effective against carbonation, sulfates, ASR
Cathodic protection – Impress current method
- Good life expectancy
- Very expensive
- Destructive, slow, and time consuming
- High application skills required
- Power break-down can disrupt the protection
- No protection against carbonation, sulfate, ASR attacks
Organo-functional silane based system
- Spray applied – Easy to use
- Organio functional silane-based – deep penetrative
- Corrosion inhibition effect- passivated rebar
- Well established and 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 drawing and specifications, selection of right contractor, execution of the work to the specifications, satisfactory quality control are similarly significant, repair project is more specialized and good site management & construction practices must be followed. Additionally, at the end of the job all the records relating to the diagnosis, material selection & execution of the work will be maintained & preserved for any reference needs in the future.