Concrete Repair: Principles from EN 1504 and Practical Considerations

Introduction
Many a times, even after taking all the due precautions and all quality control measures, Concrete structures deteriorate. One of the practical reasons is due to the fact, that at the time of planning or at the design table, it is very difficult to fully estimate the real stresses in practice that the concrete will have to carry. It is also not possible to suggest all the different types of coatings and membranes, also on account of economical considerations. The concrete then becomes vulnerable to ingress of water and other aggressive chemicals and conditions. Such problems can only be identified by being vigilant and by inspections conducted from time to time.

If neglected then the problems manifest into serious defects like spalling concrete, corrosion of reinforcement and in some cases, the degree of corrosion is so high that the repair becomes structural in nature. Such repairs are normally costly but are necessary for preserving the structures. The reports of maintenance and the records should be preserved to determine the suitability as well as the durability of the repair materials. In addition to having knowledge of structural behavior, the rehabilitation expert should also possess thorough knowledge of materials science not only in terms of advantages but also the disadvantages and limitations of the materials employed therein.

New generation materials and methods of restoring or repairing structures becomes an invaluable part of this idea. Understanding these materials is key to selecting the correct methodologies to address defects in the bridge structures. This paper covers selection ideologies, materials and methods for defect remediation in Concrete Structures.

Defects and damage occurring in concrete structures can be ascribed to varying causes, which frequently overlap. Table 1 taken from EN 1504, provides an excellent guideline summary of possible causes of concrete deterioration. Other factors, ofcourse include higher mechanical, chemical, thermal, and biological loads. Another area of concern is the durability of the repairs themselves. Generally, problems with the effectiveness of repairs have been as common as the repair materials themselves. Inspite of several researches in the field, the need for common understanding of performance requirements for repair materials and suitable methodologies has been felt all along. Figure 1 shows the factors affecting the durability of repairs.

Table 1:
Causes of Deterioration			Signs
Before & During Construction	Design errors	Incorrect joint spacing, restraints, incorrect load calculations excessively slender designs	Cracking
	Mix Design	Poor aggregate grading, too high or low cement content, incorrect water/cement ratio	Cracking, increase in permeability, surface sanding, increased carbonation rates (spalling as a result of corrosion)
	Poor Workmanship	Addition of water (incorrect w/c ratio Poor compaction Low cover to step Poor shuttering, Omitted or insufficient curing	pores, voids, cracks, blemishes exposed reinforcement, low strength, surface sanding, premature carbonation of Concrete
Environmental attacks	Excessive mechanical stresses	static or dynamic overloading I.e collision, explosion, abrasion etc	disintegration, cracking, wear
	Thermal stresses	temperature change freeze/thaw cycles, fire damages	cracking, reinforcement corrosion, surface sanding
	Chemical attack	aggressive gases (CO2, SO2) corrosive soils or water acids salts	disintegration, surface sanding, carbonation, reinforcement corrosion
	Biological effects	Plant, Micro-organisms Growth	Cracking, expansion effects, flaking

Internationally, over the past decades, the understanding of technical performance requirements of concrete repair and protection products has increased significantly. The new European Standard EN 1504 represents the culmination of over 15 years of consultation and inputs by professionals from all sectors of the concrete repair industry. EN 1504 was an attempt to bring in specifications based on performance properties of repair materials and also suggests the repair methodologies for a variety of situations. This article enumerates the steps for concrete rehabilitation in accordance with proven international standards.

The Process of Remediation

When a structure shows signs of distress or deterioration, the following steps should be taken in principle. EN 1504 recommends the steps for the entire procedure for repairs from assessment to maintenance. A guidance note (No. 4) from the Concrete Society UK recommends:

1. Assessment of damage to the structure
   1. Present Condition
   2. The design approach
   3. Exposure and environmental conditions the structure is exposed to
   4. Assessing the conditions and studying construction records
   5. Usage Conditions and history
   6. Intended future use
2. Choose Options of Repair Considering
   1. Intended Use, Design and Service Life
   2. Required Performance Characteristics to be specified by Structural Engg.
   3. Long-term performance of repair
   4. Opportunities for additional protection and monitoring
   5. Acceptable number and costs for future repair cycles
   6. Future Maintenance and access costs
   7. Properties of methods of preparing existing substrate
   8. Final Appearance of the repaired structure
3. Choose the appropriate repair Principles based on EN 1504: These principles address the methods for repair of both concrete and methods for mitigating corrosion of the reinforcement. These principles and the various methodologies available for repair of the concrete and protection of reinforcement corrosion are:

Concrete Defect Remediation
Principle	Description	Methodologies
Principle 1 [IP]	Protection Against Ingress of Substances	Impregnation / Hydrophobic Impregnation Surface coating with or without crack bridging properties Filling cracks Transferring cracks into joints Erecting external panels Applying membranes
Principle 2 [MC]	Moisture Control in the Structure	Hydrophobic impregnation Surface coating Overcladding Electrochemical Drying treatment
Principle 3 [CR]	Concrete Restoration	Applying mortar by hand Recasting with concrete Spraying concrete or mortar Replacing elements
Principle 4 [SS]	Structural Strengthening	Adding or replacing embedded or external reinforcing steel bars Installing anchored rebar in preformed or drilled holes in the concrete Bonded Plate reinforcement Adding mortar or concrete Injecting cracks, voids or interstices Filling cracks, voids or interstices Prestressing - (post tensioning)
Principle 5 [PR]	Physical Resistance	Overlay or Coating Impregnation
Principle 6 [RC]	Resistance to chemicals	Overlay or Coating Impregnation

Preventing Reinforcement Corrosion
Principle	Description	Methodologies
Principle 7 [RP]	Restoring or Preservation the Passivity of the Reinforcement	Increasing cover to reinforcement with additional cementitious mortar or concrete Replacing contaminated or carbonated concrete Electrochemical realkalisation of carbonated concrete Realkalisation of carbonated concrete by diffusion Electrochemical chloride extraction
Principle 8 [IR]	Increasing the electrical resistivity of the concrete.	Hydrophobic impregnation Surface coating Sheltering
Principle 9 [CC]	Cathodic control: Creating conditions by which cathodic areas of reinforcement are unable to drive an anodic reaction.	Limiting oxygen content at the cathode by: Saturation of surface Surface Coatings
Principle 10 [CP]	Cathodic protection of Reinforcement	Applying electrical potential (see EN 12696:2000, Cathodic protection of steel in concrete)
Principle 11 [CR]	Control of anodic area: Creating conditions where anodic reactions of steel are unable to participate in corrosion	Active Coating Barrier Coating Applying inhibitors to the concrete

1. Choose the appropriate methods of repair in accordance with the principles mentioned in Section 3 above
2. Choose appropriate materials for repair conforming with the principles and characteristics mentioned in Section 3 above
3. Specify ongoing requirements such as maintenance of records of repair, maintenance schedules and periodic evaluation of residual life of the structure

In these steps, the choice of procedure, base pretreatment and materials for concrete repair will depend on the existing degree of deterioration, the mechanism of deterioration, its causes and anticipated future stress. The reasons for evident defects or damage must be explored as thoroughly as possible. Following this, a thorough QA system needs to be established to ensure the durability of repairs.

The Concrete Repair System and Practical Considerations1. Surface Preparation
Concrete: The existing base has to be firm, stable and free from oils, impurities of all kind including form oil residues and any cement laitance. The loose and damaged concrete should be removed using light hammers and chisels. Care should be taken not to remove sound concrete to provide a good base to the repair material. Only mechanical means (rotary wire brushes, grinding, chipping, grit blasting, breakers) or hydraulic methods (use of water jets 10 to 250 MPa) should be used to remove concrete. Using hydraulic is the best means, as it reduces impact of damage to the substrate concrete.

Concrete removal should not reduce the structural integrity of the structure. The removal technique shall not damage either the reinforcement bars or concrete substrate. The depth of contamination should be established when determining the depth of concrete removal. Remove concrete along the reinforcement bars until non-corroded steel is reached. Edges around a patch repair shall be cut at an angle between 90 ° and 135 ° to avoid edges that may break.

Reinforcement: Sand blasting or wire brushing is an effective water-free method to clean reinforcement. In this case, suitable wire-brushes are used to remove the loose material, dust and laitance. Use of rust removers and water is not recommended to protect the reinforcement from further corrosion, as it may reduce the pH of the surrounding substrate and promote further corrosion. The reinforcement shall be prepared till bare metal is seen. Additional reinforcement if needed should be incorporated at this time by anchoring new steel, lapping or welding.

Protection of Exposed Reinforcement

All exposed reinforcement cleaned to bare metal should be protected immediately after preparation by using suitable corrosion inhibiting active or barrier coatings. These are proprietary materials to be used to provide corrosion resistance to the cleaned reinforcement, prior to application of the polymer modified mortar / concrete system. The materials that can be used to provide corrosion protection include:

Active coatings for reinfor- cement are coatings, which contain Portland cement or electrochemically active pigments, that may function as inhibitors or may provide localized cathodic protection. Portland cement is considered to be an active pigment due to its high alkalinity. Typical product that can be used is a one-component polymer modified mineral-based corrosion protection coat, which can be used for most repair applications.

Barrier coatings are coatings, which isolate the reinforcement from pore water in the surrounding cementitious matrix. Typical product that can be used is a two component Epoxy Resin-based Zinc Rich Primer and Coating Material for use in repairs subject to aggressive environmental and chemical attacks.

Treatment of Cracks

The sealing of cracks in concrete is a special measure in itself, which becomes absolutely essential in the event of the following widths of cracks: More than 0.3 mm in dry areas/rooms or more than 0.2 mm on buildings in the open or more than 0.1 mm where the structure has a high corrosion risk. Low-viscosity epoxy-resins can be used to treat fine cracks (0.1 to 1 mm). For bigger cracks (0.4 to 2 mm) mineral injection grouts are also available. This injection must be carried out before application of the final treatment.


Bonding Coats

These are also proprietary materials used for bonding of fresh concrete to hardened concrete using adhesive bonded joints where it forms a part of the structure and is required to act compositely. The materials that can be used to provide bonding include:

1. A one component polymer modified mineral-based corrosion protection and bonding coat, which can be used for most repair applications
2. A two component Epoxy Resin-based solvent-free, universal bonding agent and coating for use in repairs, subject to aggressive environmental and chemical attacks
3. Polymer modified Cement

Here too, use of manufactured materials described in 4a and 4b are recommended.

Polymer Concrete / Mortar (PCC) Repair

Small areas and patches less than 100 mm thick are usually repaired with hand / trowel or spray applied polymer repair mortars. Some of these products are proprietary products. On the other hand, these mortars can also be site batched using polymer additives. In most cases, a pre-bagged manufactured ready-to-use polymer modified mortar is preferred. These manufactured products can also be available in special grades allowing thicknesses more than 40 mm to be applied in a single operation. PCC Types include:

a. Site Batched Polymer Modified Mortar: This is a mixture of OPC, well graded, clean, washed, quartz sand, Styrene Acrylic or SBR-based polymer additive and water as required for consistency. The SBR / Acrylic polymer for use in the polymer modified cementitious mortar (PMC) shall be obtained from a reputed manufacturer and should impart following properties to the mortar:

Pot Life: Approx. 45 to 60 minutes
Compressive strength at 28 days: ≥ 45 MPa
Flexural strength at 28 days: ≥ 4 MPa
Air Content: ≤ 3%
Adhesive Bond Strength to Concrete: ≥ 2 MPa
Capillary Absorption: ≤ 0.5 kg/m2.h0.5

The sand to be used in the mortar shall be graded quartz sand and the sand content shall be in accordance with the desired consistency. A dry mortar of quartz sand and cement (OPC) shall be prepared as per the proportions recommended by the manufacturer. The quantity of polymer shall be measured by weight only. Add the measured polymer and part of the water in a mixing vessel; start mixing with a mechanical mixer. Add the powder mortar to the liquid in the mixing vessel, while continually mixing using the mechanical mixer. Mix until a homogenous lump-free mortar is achieved. Add water while mixing to get required consistency. Never add the liquid to the mortar as it hampers the dispersion of the polymer.

b. Prebagged / Manufactured Polymer Modified Mortar: The premixed / manufactured PCC for Structural Repairs to be used should have the following properties:

Air Content: ≤ 3%
Compressive Strength: ≥ 45 MPa
Bond Strength to Concrete: ≥ 2 MPa
Chloride Ion Content: < 0.05%
Capillary Absorption: ≤ 0.5 kg/m2.h0.5
Adhesive Bond Strength to Concrete: ≥ 2 MPa

These mortars are most suitable for repairs when the section to be replaced ranges from a depth of 5 mm to 50 mm. In case of depths, more than 100 mm need to be placed, additional layers maybe needed. Follow manufacturer’s recommendations for mixing, placement, compaction and curing of the polymer modified repair mortar. Apply the mortar by hand or by spray, ensuring no voids are left in the fill area. Care should be taken to fill in the complete area, even behind the exposed reinforcement. In case of large structural elements, a ready to use non-shrink micro concrete can be used.

The forms, if any, can be removed in 48 hours. Cure the exposed surfaces for 7 days or more, preferably using acrylic-based curing compounds (so as not to hinder bond in subsequent applications).

Fine Filling

To achieve a visually uniform surface and to provide additional preventive protection, the repaired concrete surface should be fine-filled. This is done with a fine polymer modified, concrete cosmetic or fine sand and a mixing liquid composed of water and or the polymer component. Depending on the structure involved, it can be used in overhead work or as a fine-filler under elastic, crack-bridging systems.

Concrete Surface Protection, Carbonation inhibitor, Coloured finishes

On completion of the work described above, the entire concrete surface must be provided with a final seal or final coating. This is mandatory to maintain status quo of corrosion. Such surface treatments perform several duties at one time. Firstly, all the concrete is protected from further stress due to aggressive pollutants in the air and from progressive carbonation.

These coating systems must have to a high CO2 resistance if they are to be effective in protecting against carbonation and, on the other hand, they must not have a negative effect on the buildings water vapor diffusion rate. These materials should also be water resistant, crack bridging, UV-resistant and breathable. In case of exposed surface, it may also be imperative to use hydrophobic impregnations for protecting the structures from water ingress.


Conclusion

Concrete Repair is the creation or recreation of existing cover to the damaged concrete. Durable concrete repair is an art as much as it is a science. We in India are in the stage of developing codes for repair. In the meanwhile, we can always resort to established codes of practice available internationally. EN 1504 (Parts 1 to 10) is an excellent guideline on systems for assessment, specification, methods and material selection for concrete repair. The principles and methods and a brief outline of the concrete repair system has been enumerated in this article. These methods can be followed for durable repair of RCC structures. The system from assessment to completion of repair and further maintenance should be rigorously followed for durability. Protective coatings should also be used as they help improve the durability of repairs as well as that of the structures. It should be remembered, protect the concrete and the steel will take care of itself.

References:

1. EN 1504 (Parts 1 to 10)
2. Repair Guidance: Note No. 4 – Scope of EN 1504, Concrete Society UK
3. IRC SP 40
4. IRC SP 80
5. Concrete Repair Guide ACI 546R-04
6. Technical Report No. 38: Patch Repair of Reinforced Concrete by Concrete Society UK