There are many ways in which concrete can become damaged, including mechanical, chemical, biological, electrochemical, and even indirectly, such as by corrosion of the reinforcement.
Several methods are available for protecting and repairing concrete, standing out the principles included in the EN 1504-9 standard:
- Protection against ingress – Principle 1 of EN 1504-9
- Moisture control – Principle 2 of EN 1504-9
- Concrete restoration – Principle 3 of EN 1504-9
- Structural strengthening – Principle 4 of EN 1504-9
- Increasing physical resistance – Principle 5 of EN 1504-9
- Resistance to chemicals – Principle 6 of EN 1504-9
Protection against ingress
This is principle 1 for repairing and protecting concrete according to the EN 1504-9 standard. Protection against ingress prevents liquid water and aggressive substances from entering the concrete through pores or cracks caused, for example, by degradation processes. As a result, pores and cracks are closed and the surface is strengthened.
Different methods are available to fulfil this principle, as follows:
Impregnation
Applying impregnations can prevent liquid substances and aggressive media from infiltrating concrete surfaces. This method is commonly used to prevent moisture and water vapour from penetrating concrete. Two different types of products can be used. Hydrophobic compounds are used to hydrophobize the concrete surface pores. Silicones, such as silanes and siloxanes, are the most common. Silanes or siloxanes react chemically with the silicate structure of the concrete, creating an effective barrier against water and the penetration of aggressive agents while remaining permeable to water vapour. Moreover, other products use compounds that can be absorbed into the concrete’s pores, filling or partially filling the gaps. Two sub-types of products are used: those that react with the components of the cementitious sealant, such as crystallizers and silicate-type materials like sodium silicate, and materials that react and cure independently, such as low-viscosity epoxies, methacrylates, and polyesters.
Surface treatments – coatings
A surface protection system provides an additional barrier against the ingress of undesirable aggressive media and liquid water onto the steel reinforcement of a rehabilitated concrete structure. The penetration of water and carbon dioxide are the main aggressive media, but other aggressive media can also penetrate concrete depending on its specific exposure. Furthermore, surface protection systems improve the overall appearance of the concrete structure to be repaired by unifying its surface in colour.Surface bandaging of cracks
Bandaging is used to cover stabilized cracks. This technology is only used when a crack is of purely aesthetic importance and does not compromise the structure’s structural integrity. The ingress of aggressive media into the structure will be prevented by this method. A strip of banding fabric is glued over the crack so that the crack runs through the middle of the strip. It is necessary to use a suitable adhesive for adhering. Once the fabric has adhered and dried, a surface finish, coating, or paint should be applied. This method is one of the less common procedures used for concrete repair.
Filling of cracks – injection
This method consists of the injection of specific materials (e.g. PU resins, EP resins, thin mortars) into, behind or around a structure or element to form a barrier against liquids or vapours. Most grouting materials will solidify after curing. The purpose of grouted barriers is to prevent penetrations through an element or structure. In typical applications, cracks and joints are sealed against penetration of water (to ensure that they are watertight). This method is one of the most common procedures used for concrete repair.
Joint sealing
The purpose of joint sealing is to create a barrier to liquids by placing material in or overlapping rehabilitated joints. There are many applications for joint sealing, including sealing water cracks, rolling joints, floor joints, and tank joints.
Erecting external panels
The installation of external elements can provide protection against aggressive media and liquid water intrusion. External elements prevent aggressive liquids from penetrating concrete surfaces. This procedure involves the modification of structural design or adjustment of operating conditions, and therefore, generally, these procedures are not considered repairs but strengthening or rehabilitation.
Moisture control
This is principle 2 for repairing and protecting concrete according to the EN 1504-9 standard. Concrete should be protected by this method only when its moisture content is below the critical value for the material or if the procedure used allows harmless moisture to escape. The capacity to renew these protection systems becomes a key criterion in their selection since they may not last as long as the concrete structure.
Impregnation
This treatment partially reduces the penetration of liquid media into the concrete and its main effect is to harden the surface and prevent the ingress of liquid substances and aggressive media.
Surface treatments – coatings
This technology provides the surface of the rehabilitated concrete structure with an additional barrier against intrusion by liquids and aggressive media, preventing ingress to steel reinforcements.
Erecting external panels
Shielding and cladding elements provide protection against aggressive media and liquid water ingress. By installing external elements, aggressive liquids cannot penetrate the protected concrete surface. As mentioned above, the procedure is more or less a modification of the structural design or a modification of the operating conditions. Therefore, in a general sense, these procedures are not considered to be repair procedures but strengthening or rehabilitation interventions.
Electrochemical protection
Electrochemical protection includes the use of active electro-osmosis, which works on the principle of two electrodes. The positive electrode is inserted into the wall, and the negative electrode into the ground. The electrodes are usually made of potted metals. When an electric current passes through the electrode the water (2H2O) is decomposed into 2H2 + O2. This method is one of the less common methods used for moisture control.
Concrete restoration
This is principle 3 for repairing and protecting concrete according to the EN 1504-9 standard. Principle 3 of the EN 1504-9 standard, concrete restoration, mortar, or concrete reprofiling, aims to protect and repair concrete structures by restoring their original shape and thickness of the cover layer over the reinforcement, as well as strengthening the structure.
Reprofiling can be accomplished through various techniques, including mortar or concrete masonry application, spraying, or concreting. The repair material to be used may be cementitious, polymer-cement, or polymer. It is essential to choose a material that can provide alkaline passivation to the reinforcement.
Restoration of a concrete structure involves restoring its original form and function or replacing damaged parts.
Hand-applied mortar
Hand application is a commonly used method for non-flowing materials on vertical surfaces and ceilings. The mixed material is applied to the prepared surface either manually with a flat trowel or using a troweling machine.
During the application process, it is crucial to ensure perfect bonding with the substrate, particularly by pressing the material into the porous structure of the exposed concrete to restore the original concrete construction.
To improve the cohesion of the subsequent layers, each layer needs to be roughened. This method is suitable for superficial repairs to areas without steel reinforcement. If steel reinforcement is present, achieving compaction and perfect wrapping becomes difficult due to poor cohesion between layers and voids around the embedded steel reinforcement.
Nonetheless, it is still one of the most used repair procedures for reinforced concrete structures and elements.
Concreting
Using this technology, large-volume repairs and rehabilitation of concrete elements can be carried out and their original shape and function can be regained when deep damage occurs. The procedure is also widely used to modify the layout and shape of the structure being repaired. Various technological methods can be used for concreting, such as ramming, full-depth rehabilitation, pumping into formwork or separate concreting with grouting.
Ramming
Ramming involves applying mortar or concrete into voids with zero or near-zero settlement during consistency testing. The consistency of the mortar must be such that it can be formed without
excessive water shedding. To achieve cohesion, mortar must be compacted to increase density and ensure close contact with the concrete.
The compaction process can be used on ceilings, vertical surfaces, and flat surfaces. To prevent surface polishing, compaction must be performed with a hardwood mandrel. During the first week following the intervention, it needs to be treated by constant wetting. Each mortar packing rehabilitation must be carried out in layers. The most advantageous use of this method is for small cavities such as rod holes, small areas, surface defects or rib bottoms.
Full-depth rehabilitation
Sometimes full-depth rehabilitation can be better than surface rehabilitation. Suppose, for example, that the concrete has extensive surface damage, then rebuilding the entire disturbed section might be a more economical and durable option. However, care should be taken to minimise shrinkage in the reconstructed section. New concrete shrinks, causing stresses in the reconstructed section and at the interface between old and new materials. When the tensile stresses are not compensated, undesirable cracking can occur. Therefore, low-shrinkage concrete mixes should be used.
Pumping into formworks
In remedial pumping into formworks, formwork is erected, and remediation material is pumped into the cavity formed by the formwork and existing concrete. A variety of repair materials can be used for this technology, with pumpability being a necessary requirement. Different types of pumps can be used based on aggregate size. Prior to erecting the formwork, all surfaces that may cause air pockets during pumping must be cut off or vents must be installed.
The remediation materials will be mixed and pumped into defined cavities from below and above. Large areas may be divided into partitions to make them more manageable. As the cavity fills, the
pump applies pressure to the formwork, compacts the material and leads to the consolidation of the repair material, which adheres tightly to the existing concrete. Even though this method offers
several advantages compared to other techniques, such as shotcrete, hand placement, and separate concreting, it is very labour-intensive and costly, making it a less commonly used technique.
Trowelling and application of cover layers
Using trowels and applying cover layers to repair concrete structures can provide a secondary solution to various concrete problems. They can be used to improve drainage, workability, or load-bearing capacity, or as protection for the underlying concrete from aggressive environments. Additionally, the capping layer often addresses surface damage issues. Various materials can be used for screeds and capping layers, ranging from very thin to very thick. Care must be taken when carrying out the work to avoid insufficient compaction, spreading or inadequate cohesion with the substrate. Polymers or polymer-modified mortars are used for thin screeds (up to 3 mm).
Spraying of concrete or mortar – torquing
As the name implies, spraying technology involves spraying mortar or concrete onto a concrete substrate. There are several ways to spray concrete or mortar, depending on when water is added.
For dry spraying, water is added at the nozzle, while in wet spraying, water is added before all the ingredients and water are pre-mixed and fed into the nozzle. Large, heavily damaged structures are often repaired by spraying, whether dry or wet. Prefabricated, industrially produced mixtures should be preferred to achieve uniform quality and properties of the sprayed layer.ç
Replacing elements
Whenever an element is badly damaged, it may be better to remove it completely and replace it with a new one. There should always be consideration of whether removing the element will not affect the structural stability of the whole construction.
Structural strengthening
This is principle 4 for repairing and protecting concrete according to the EN 1504-9 standard. When applying the principle of structural strengthening, it must be considered that any rehabilitation of the concrete must not induce stresses that will negatively impact the original structure.
Adding or replacing embedded or external reinforcing bars
New rebars can be added to replace corroded or inadequately sized rebars. A careful connection between the new reinforcement and the original reinforcement is imperative in order to ensure the structure’s load-bearing capacity is enhanced. To prevent the reinforcement from being pulled out by tensile forces, the minimum length of reinforcement in the concrete should be maintained at the anchorage length.
Adding reinforcement anchored in pre-formed or drilled holes
A concrete structure can be strengthened and upgraded by glueing reinforcement into grooves and holes prepared in advance. Reinforcement is glued into the ductile part of the concrete section. This method has the disadvantage of being labour-intensive compared to using external reinforcement.
Bonding plate reinforcement
Bonded plates, such as carbon-fibre reinforced polymer (CFRP) or steel plates, can be used to increase the load-bearing capacity of existing structures. The pads are bonded to the pre-prepared surface. Verification of sufficient tensile strength of the surface layers of the concrete cross-section is required in advance.
Filling of cracks, voids or interstices
Crack filling follows the same principle as grouting. The resulting voids and gaps in nests and cavities can be filled with a high-viscosity mortar.
Prestressing – (post-tensioning)
A prestressing reinforcement is placed through or around a concrete structure element and then tensioned to strengthen the element or transfer forces. It must be considered that this method may induce a change in the structural system.
Increasing physical resistance
This is principle 5 for repairing and protecting concrete according to the EN 1504-9 standard. In the case of structural damage caused by inadequate resistance to mechanical stresses on the structure’s surface (low abrasion resistance, low toughness, etc.), it is appropriate to (after clearly identifying the causes) increase the mechanical resistance of the surface’s structure. For this purpose, impregnations and coatings can be used.
Resistance to chemicals
This is principle 6 for repairing and protecting concrete according to the EN 1504-9 standard. When structural damage is caused by the insufficient resistance of the old concrete to specific chemicals (chemical solutions, bacteria-induced chemicals, etc.), it is necessary (after clear identification of the chemicals) to increase the structure’s chemical resistance. In order to accomplish this, impregnations, coatings, and concrete restoration techniques can be used, whose procedures are described in the complete guide, link below.