Concrete Repair and Protection

The European Union fully introduced all of the European Standards 1504 on 1st January 2009. These Standards define the assessment and diagnostic work required, the necessary products and systems including their performance, the alternative procedures and application methods, together with the quality control of the materials and the works on site. Principles 1 to 6 relate to defects in the concrete itself, Principles 7 to 11 relate to damage due to reinforcement corrosion.

Concrete is a composite material composed of water, fine and coarse granular material embedded in a hard paste, resulting from cement hydration, that fills the space among the aggregate particles and glues them together. Damages to concrete result from the presence of three simultaneous factors: interconnected porosity, presence of water or moisture and exposure to aggressive agents. A variety of processes contribute to concrete deterioration. They are classified as physical if resulting from a fire, water leakage, shrinkage, abrasion, erosion or freeze thaw cycle, chemical if resulting from carbonation, sulfate, chloride attack, bacterial or other biological action, efflorescence or leaching and alkali silica reaction, mechanical if caused by an overload, movement, or impact, and finally application damage if attributable to an inappropriate concrete mix design or placing.

Application damages include pinholes that develop as a result of plastic shrinkage due to an inappropriate water cement ratio of the mix design or lack of mold oil at the time of de-shuttering, honeycombs described as superficial damages taking the form of exposed aggregates up to a thickness of 20mm, that result from a lack of concrete vibration or from poor workability at the time of placing, and cracks that develop in the plastic phase of concrete such as plastic shrinkage cracks due to insufficient early curing and loss of water by fast evaporation.

It is customary before undertaking any repair, to carry out a field investigation and structural analysis. A visual inspection can help ascertain whether weathering factors, corrosion, segregation, cold joints or cracks caused the damage, with the latter divided into dry cracks and wet cracks. A more thorough physical inspection may identify defect in the form of delamination, hollow sound, or on large areas from chain drag sounding.

Upon a defect being identified a number of steps may be taken. First identify the damaged area by drawing a line around it. With a jack hammer the concrete in the damaged area is broken until reaching sound concrete. If the latter is reached before reaching the steel reinforcements than a mere surface patching may be sufficient. If on the other hand corroded steel reinforcements are reached before reaching sound concrete then break the concrete further until reaching sound concrete, before beginning repair works.

A structural engineer can take samples from the concrete for compressive strength testing. Low compressive strength indicates porosity and thus vulnerability of the concrete to chloride attacks and carbonation. A carbonation test may be carried out to find out the alkalinity of the concrete using a phenolphthalein solution. Carbonated surfaces exhibit a pH below 10 with no change in color while concrete with pH higher than 10 will exhibit a change in color to pink. A colorimetric test using a sodium nitrate solution may be carried out to ascertain the penetration depth of chlorides in the concrete.

It is always recommended that after completion of repair, or between two layers, and upon the repair mortar reaching its final setting, the repaired area be cured by traditional method or by the application of a curing compound. The repaired area should be protected with a polyethylene sheet against drying from wind or fast evaporation.

By defining key principles of concrete protection EN 1504 Part 9 has helped owners and professionals fully understand the problems and solutions throughout the different stages of the repair and protection process. The following is a brief overview of repair works and corresponding requirements of European standard EN 1504 for Concrete Repair and Protection Works, providing a general outline of principles, methods, criteria and product selection guide

I - Protection against ingress

The reduction or prevention of the ingress of adverse agents, such as water, other liquids, vapor, gas, chemicals and biological agents, including dissolved harmful substances, to achieve a water-repellent concrete surface can be handled using the following methods:

• 1.1 Hydrophobic Impregnations (EN 1504-2): A hydrophobic impregnation is defined as the treatment of concrete to produce a water-repellent surface. The pores and capillary network are not filled, but only lined with the hydrophobic material. This functions by reducing the surface tension of liquid water, preventing its passage through the pores, but still allowing water vapor diffusion each way.

• 1.2 Impregnations (EN 1504-2): An impregnation is defined as the treatment of concrete to reduce the surface porosity and to strengthen the surface. The pores and capillaries are then partly or totally blocked. This type of treatment usually results in a discontinuous thin film thickness on the surface blocking the pore system to aggressive agents by preventing the transport of liquid and gazes through the concrete surface.

• 1.3 Coating (EN 1504-2): It is one of the methods most commonly used. It is designed to provide an improved concrete surface, for increased resistance or performance against specific external influences. Fine surface cracks with a total movement of up to 0.3 mm can be safely repaired, then sealed and their movement accommodated by the use of elastic, crack bridging coatings, which are also waterproof and carbonation resistant. It is used when significant crack movements are expected or to accommodate thermal and dynamic movement in structures subject to wide temperature fluctuation, vibration, or which have been constructed with inadequate or insufficient jointing details.

• 1.4 Surface bandaging of cracks: It consists in locally applying over a crack a suitable material such as a textile-reinforced concrete, as a surface bandage, to prevent the ingress of aggressive media into the concrete. If lots of cracks are present, it might prove more economical to apply a continuous full-surface coating.

• 1.5 Filling of cracks (EN 1504-5): Cracks are filled to prevent the passage of aggressive agents and sealed. Non-moving cracks i.e. cracks that have been formed by initial shrinkage need only be fully exposed, repaired and filled with a suitable repair material in a way that substances cannot penetrate into or through the crack. Injection materials are classified as D for ductile and S for swelling

• 1.6 Transferring cracks into joints: Cracks to be treated to accommodate movement are widened so that a joint is formed to extend through the full depth of the repair and positioned to accommodate that movement. The cracks (joints) are then filled, sealed or covered with a suitably elastic or flexible material. Typical application includes single cracks or cracks with large movements. The decision to use a crack as a movement joint must be taken by a structural engineer.

• 1.7 Erecting external panels: Concrete surfaces are protected with external panels such as curtain wall or similar external facade cladding systems. The external panels protect the concrete surface from external weathering and aggressive materials attack or ingress. According to EN 1504-9 external panels are also used for Method 2.4 listed herein below to achieve moisture control. It is typically applied on concrete exposed to aggressive substances.

• 1.8 Applying membranes: Applying a preformed sheet or liquid applied membrane, over the concrete surface, which can be reinforced with fibers or meshes to enhance its mechanical strength, will fully protect the surface against the attack or ingress of deleterious materials. Typical membranes include polymer foils and bituminous sheets.

The selection of the most appropriate method is dependent on different parameters, including the type of deleterious material, the quality of the existing concrete and its surface, the objectives of the repair or protection works and the maintenance strategy.

II - Moisture control

It consists in adjusting and maintaining the moisture content in the concrete within a specified range of values.

• 2.1 Hydrophobic impregnations (EN 1504-2): A hydrophobic impregnation is defined as the treatment of concrete to produce a water-repellent surface. The pores and capillary network are not filled, but only lined with the hydrophobic material. It reduces the surface tension of liquid water, preventing its passage through pores, while still allowing water vapor diffusion. This method is used to treat concrete corroded by an alkali-silica reaction, sulfate attack or freeze-thaw.

• 2.2 Impregnations (EN 1504-2): An impregnation is defined as the treatment of concrete to reduce the surface porosity and to strengthen the surface. The pores and capillaries are then partly or totally filled. This type of treatment usually also results in a discontinuous thin on the surface. This serves to block the pore system to aggressive agents. Typical applications include horizontal surfaces such as floors.

• 2.3 Coatings (EN 1504-2): Surface coatings are defined as materials designed to provide an improved concrete surface, for increased resistance or performance against specific external influences. Fine surface cracks with a total movement of up to 0.3 mm can be safely repaired, then sealed and their movement accommodated by crack bridging coatings, which are also waterproof and carbonation resistant. This is to accommodate thermal and dynamic movement in structures subject to wide temperature fluctuation, vibration, or that have been constructed with inadequate or insufficient jointing details. The method is used to treat in early stages concrete corroded by alkali-silica reaction, sulfate attack and freeze-thaw.

• 2.4 Erecting external panels: As long as the concrete surface is not exposed, no water can penetrate and the reinforcement cannot corrode. The method is used to treat in early stages concrete corroded by alkali-silica reaction, sulfate attack and freeze-thaw. Panel requirements are not specified in EN 1504.

• 2.5 Electrochemical treatment: By applying an electric potential in the structure, moisture can be moved towards the negatively charged cathode area.

III - Concrete restoration

The selection of the appropriate method of replacing and restoring concrete depends on a number of parameters including the extent of damage. Method 3.1, hand applied mortar, is more economical for limited damage while Method 3.2, recasting with concrete and mortar would be normally chosen in areas heavily congested with reinforcements.

• 3.1 Hand applied mortar (EN 1504-3): It is particularly suitable and economical in case of limited damage. It replaces defective concrete with new mortar or concrete by hand without strengthening the structure. It is often recommended after conducting patch repairs to fully coat the surface to minimize carbonation and differences in appearance.

• 3.2 Recasting with concrete or mortar (EN 1504-3): It is suitable for all types of concrete surfaces but in particular in the presence of heavily congested rebars. In general, the same methodology shall apply as if new concrete was poured.

• 3.3 Spraying concrete or mortar (EN 1504-3): This is the preferred method for vertical surfaces such as slabs and decks, or where the area of repair is far from the mortar preparation place. With sprayed concrete a better quality may be expected. When considering sprayed concrete or mortar as a repair method, it is important to ascertain whether the sprayed material is classified as Class R4 or Class R5, and determine the rebound rate, seeking to minimize it, and high build to achieve non-sag layer thicknesses.

• 3.4 Replacing elements: The economics of replacing whole or part of the structure with precast elements is a determining factor, which should be considered before selecting this method.

IV - Structural strengthening

This principle suggests methods for increasing or restoring the structural load bearing capacity of an element of a concrete structure such as a parking deck and bridge. A certified structural engineer should perform the design and engineering and determine the size, configuration and location of reinforcements. The selection of the appropriate method depends on many factors, including the type of structure, maintenance requirements, cost benefit analysis, and environment and the points for anchorages into the concrete in accordance with EN 1504 Part 6 and the relevant European Technical Approval Guidelines.

• 4.1 Adding or replacing embedded or external reinforcing bars: Product requirements are same as for new construction. It is used for structures with insufficient load-bearing capacities, which suffer from an advanced state of corrosion.

• 4.2 Adding reinforcement anchored in pre-formed or drilled holes (EN 1504-6): European and national approvals should be consulted to ascertain the procedure materials and requirements regarding hole preparation.

• 4.3 Bonding plate reinforcement (EN 1504-4): A high strength sheet is glued to the concrete surface. It is adapted to structures with insufficient load bearing capacities.

• 4.4 Adding mortar or concrete (EN 1504-3 and EN 1504-4): The method consists in placing, usually on large areas using spray equipment, new concrete on top of old instead of replacing old concrete.

Surface cleanliness of grooves and anchor holes cut in the concrete should be prepared in accordance with EN 1504 Part 10 Sections 7.2.2 and 7.2.3.

Repair mortars are classified in accordance to EN 1504-3 as Class 3 or Class 4. Epoxy based patching mortars a shear strength ≥ 6 N/mm² must be achieved

• 4.5 Injecting cracks, voids or interstices (EN 1504-5): This method is used for cracks, voids, or interstices in areas with high requirements to load bearing capacity. Special attention should be given to moisture condition in the crack, and to crack movements to restore to initial defect-free condition.

• 4.6 Filling cracks, voids or interstices (EN 1504-5): Cracks should be cleaned and prepared in accordance with EN 1504 Part 10 Section 7.2.2 guidelines. Inert cracks, voids or interstices that are wide enough should be filled by gravity, pouring or using an epoxy mortar. The injection and sealing of cracks with low viscosity epoxy resin can help restore the structure of a building to its initial condition albeit without the structure gaining in strength. Injection material are classified by specifying the transmitting force / Load transfer.

• 4.7 Prestressing - post-tensioning: This method is used to reinforce structures. Structural engineers can specify pre-stressed composite reinforcements using high strength lightweight carbon fiber reinforced plates.

V - Increasing resistance to physical or mechanical attack

Concrete structures can be damaged by any, or a combination, of any of the following factors:

• Increased mechanical load
• Wear and tear from abrasion
• Hydraulic abrasion from water and water borne solids
• Surface breakdown from the effects of freeze – thaw cycles

Methods to achieve the goal outlined herein are as follows:

• 5.1 Coating (EN 1504-2): The application of a reactive coating, with the right characteristics, improves concrete physical and mechanical properties, which may result in higher resistance to abrasion and mechanical attack. Typical applications include floor surfaces exposed to abrasion or impact.

• 5.2 Impregnations (EN 1504-2): By filling pores partly or totally, impregnations, which take the form of a discontinuous very thin film strengthen surfaces and reduce their porosity. Typical applications include floor surfaces exposed to abrasion or impact. Criteria that apply are as follows:

  • Abrasion (Taber-Test): 30% improvement in comparison to non impregnated sample
  • Penetration depth: >5 mm
  • Capillary absorption: w < (0.1 Kg/m2 x √h)
  • Impact resistance: Class I to Class III
    
    

• 5.3 Adding mortar or concrete: Methods and suitable systems to achieve Principle 3 Concrete restoration objectives are as defined herein. The products have to fulfill the requirements of EN 1504-3, Class R4 or R3. The civil engineer may seek to fulfill additional requirement on specific structures such as resistance to hydraulic abrasion.

VI - Resistance to chemicals

This principle deals with increasing resistance of the concrete surface to deteriorations from chemical attack. As the chemical resistance of a concrete structure is dependent on a variety of factors, as a prerequisite to any repair work, the type and concentration of the chemicals, temperatures and likely duration of exposure and other field conditions must be ascertained. Protective coatings include the following: products based on acrylic, epoxy, polyurethane, silicate, polymer modified and epoxy-cements.

• 6.1 Coatings (EN 1504-2): Reactive coating with the right characteristics can improve concrete chemical resistance.

• 6.2 Impregnations (EN 1504-2): By filling pores partly or totally, impregnations, which take the form of a discontinuous very thin film strengthen surfaces and reduce their porosity. The performance criterion is the resistance to chemical attack after 30 days exposure. Typical applications include surfaces exposed to severe chemical attack.

• 6.3 Adding mortar or concrete (EN 1504-3): Specific requirements such as combinations of cement and epoxies, watertight or other special cements, if any, have to be specified by the engineer as no specific criteria are required by the standard.

VII - Preserving or restoring passivity

Steel in concrete is usually in a passive non-corroding state. A layer of oxides, which strongly adheres to the underlying steel, protects it from reacting with oxygen and water to form rust. The reinforcing steel of a concrete structure will corrode upon a combination of these three different factors occurring:

1. Presence of oxygen
2. Loss of passivity and
3. Sufficient moisture in the surrounding concrete.

For corrosion to occur the three conditions must be fulfilled. The principle aims at creating chemical conditions in which the surface of the reinforcement is maintained in or returned to a passive condition. When concrete carbonates, a more neutral environment replaces the alkaline environment that would normally protect steel from corrosion. Similarly, and somewhat faster, in the presence of seawater or de-icing salts, chlorides induce corrosion and rust as the passive oxide layer on steel bars is disrupted. To recommend the appropriate repair method to be adopted, it is important to identify the extent of damage, ascertain site conditions, and whether loss of passivity resulted from carbonation or chloride attack. An important factor in ensuring the durability of concrete is its impermeability, which can be best achieved by good compaction and curing of a low water cement ratio and high cement content concrete.

• 7.1 Increasing cover with additional mortar or concrete (EN 104-3): This is a preventive method intended to protect steel rebars from carbonation or chlorides by adding a layer of concrete or mortar. It should not be applied if steel is no longer passive.

• 7.2 Replacing contaminated or carbonated concrete (EN 1504-3): To repair damaged areas, defective concrete should be removed, and steel reinforcements protected with a new alkaline concrete. This traditional method is adapted for all types of concrete structures.

• 7.3 Electrochemical re-alkalization of carbonated concrete: It is a method that restores a high pH value to carbonated concrete using a temporary impressed electrical current and a highly alkaline electrolyte

• 7.4 Re-alkalization of carbonated concrete by diffusion: It is a method for increasing the alkalinity of cement by diffusion of alkaline solutions such as sodium carbonate, potassium hydroxide or a mix of these two and sodium hydroxide, without the use of an electrical current. The results obtained through direct contact can prove effective in reestablishing the high pH of concrete, albeit with a slight decrease in compressive strength and mortar adherence to re-alkalized substrates.

• 7.5 Electrochemical chloride extraction: It is an electrical process that extracts and removes chloride ions from chloride-contaminated reinforced concrete structures.

VIII - Increasing the electrical resistivity of concrete

The higher the resistivity, the lower is the amount of free moisture available in the pores and thus the lower the corrosion.

• 8.1 Hydrophobic impregnations (EN1504-2): It is an impregnation with a hydrophobic agent to yield a water-repelling surface that prolongs the service life of the structure. It is accomplished by protecting the reinforcement bars from chlorides or by changing the moisture content inside the concrete. By treating the concrete with a water-repellent agent, the properties of the surface layer become hydrophobic. Water is stopped from entering, while water vapor can still pass through, thus reducing chloride ingress and stopping rain from penetrating the surface.

• 8.2 Impregnations (EN 1504-2): By filling pores partly or totally, impregnations, which take the form of a discontinuous very thin film strengthen surfaces and reduce their porosity.

• 8.3 Coatings (EN 1504-2): Surface coatings are based on synthetic polymers and other products whose primary function is to protect the surface from the environment and thus lower the amount of free moisture.

IX - Cathodic control

It consists in creating conditions in which potentially cathodic areas of reinforcement are unable to drive an anodic reaction.

• 9.1 Limiting oxygen content (at the cathode): The method seeks to reduce corrosion by ensuring oxygen does not reach the reinforcement surface. This is achieved by water saturation of the concrete in touch with the reinforcement or by an appropriate surface coating.

X - Cathodic protection

It is particularly appropriate to deal with chloride contamination, which it can control regardless of levels. It is standardized in EN ISO 12696.

• 10.1 Applying an electrical potential: It is achieved by connecting the steel to be protected to a more easily corroded “sacrificial metal” to act as the anode. The ability to evaluate the level of performance achieved by cathodic protection is an advantage over alternative methods.

XI - Control of anodic areas

The method aims at creating conditions in which potentially anodic areas of reinforcement are unable to take part in the corrosion reaction.

• 11.1 Active coating of the reinforcement (EN 1504-7): As a prelude to coating, uncover the rebar, creating enough space around it, and remove any rust and loose particles. It is particularly suitable for temporary protection of exposed rebars, and when there is little space for concrete repair. Coatings such as zinc-based materials that promote passivation can be used. As an alternative, coatings with anodic inhibitors (EN 1504-7) can be selected.

• 11.2 Barrier coating of the reinforcement (EN 1504-7): As a prelude to coating, uncover the rebar, creating enough space around it, and remove any rust and loose particles. Coatings such as epoxy-based resin can be used as a barrier coating, which isolate the reinforcement from harmful agents. This method can be considered when a concrete protection is difficult to achieve. However, unless it can be applied in sufficient thickness and quality to protect the reinforcement, there is a risk of corrosion underneath the coating particularly as the shielded reinforcement will no longer benefit from the alkalinity of the concrete surrounding it.

• 11.3 Applying corrosion inhibitors in or to the concrete: It consists in the surface application of a corrosion inhibitor designed to penetrate the concrete cover to the rebar. It is thus not the preferred solution for low permeability and high cover thickness concrete. Alternatively, corrosion inhibitors may be mixed with repair mortars to increase the electrical polarization of the concrete.