Concrete repair

Concrete spalling as a result of reinforcement corrosion on a concrete component. The exposed reinforcement is clearly visible.

Concrete repair is always necessary when components or structures made of concrete and reinforced concrete are impaired in their visual appearance or function due to their manufacture, use or exposure to media that damage concrete. Concrete repair describes technologies for restoring or extending the durability of components made of concrete and reinforced concrete. The peculiarities of concrete repair usually place higher demands on the specialist knowledge and the special material knowledge of the planners and executors than comparable activities in new buildings. The specialized concrete reconditioner carries out the value-preserving measures .

Basics

Concrete is a chemically unstable building material . Various internal and external influences can have a lasting effect on the durability of concrete. Through the typical use of concrete in conjunction with reinforcement of steel , further resulting durability of concrete influencing factors. Due to the variety of causes of damage and damage patterns on reinforced concrete structures, there are differentiated repair principles. For Germany, these are described in the directive on the protection and repair of concrete components (repair directive) issued by the German Committee for Reinforced Concrete ( DAfStb for short ). Since 2006, concrete repair has been regulated on a European level by the European standard EN 1504 (in Germany DIN EN 1504) products and systems for the protection and repair of concrete structures .

The basic principles can be summarized as follows:

1. Protection of the reinforcement surface from corrosion by

• Coating of the reinforcement
• electrochemical corrosion protection

2. Restoration of the concrete surface by

• Closure of cracks
• Reprofiling of imperfections

3. Protection of the concrete surface against the ingress of corrosive media

• Increasing the concrete cover of the reinforcement
• Application of surface protection systems

A prerequisite for a successful concrete repair is the knowledge of the specific cause of the damage and the derivation of the correct repair measures. A repair is successful when the actual condition of the construction comes as close as possible to the target condition. The target state of reinforced concrete structures is u. a. determined using the exposure classes according to DIN 1045. The need for repairs can be derived from the comparison of the specified target state of the construction and the actual state determined in the context of the state analysis. As part of the repair conception, damage- and component-related repair variants are derived from the conditions and exposures found. The most favorable repair steps in terms of economy and use are selected from these variants.

The building material concrete repair can only be carried out under the condition of a positive structural planning assessment of the construction or the reinforcement and repair measures to be carried out.

Damage analysis

Before repairing damage to a concrete component, the cause of the damage must be determined. The exact clarification is the prerequisite for a permanent restoration of the properties required for the purpose of the construction. Without clarifying and eliminating the cause of the damage, the same damage pattern will appear again sooner or later. Repair measures on concrete structures that are only geared towards architectural or functional requirements, i.e. only aiming at restoring the original condition without permanently eliminating the cause of the damage, are patchwork for time and mostly bad investments.

Reinforced concrete structures are composite components whose stability and durability can only be ensured by the interaction of steel and concrete in accordance with the rules of reinforced concrete construction. If damage occurs to such a structure, the question that has to be clarified is to what extent the bond behavior and thus the distribution of forces and stresses in the component on which the design is based still exists. Damage to reinforced concrete structures are often not caused by random defects in performance, but the first signs of the building existing constructive, concrete or construction material Technical Technical defects. Not only that damage will certainly recur if it is simply covered up with any measures, there is also the risk that minor damage, which is often only the first sign of existing construction defects, is covered up in such a way that more serious defects are not recognized and can later lead to considerable damage or even loss of the structural integrity of the component.

If execution documents are still available for the construction to be renovated (execution plans, reinforcement drawings, etc.), the documents can be used to check whether the actual load or stress corresponds to the assumptions made when the structure was created. If such documents are no longer available, which is often the case with older structures, the parameters required for the durability and stability of the structure must be determined on the basis of a sufficient number of random samples . These are primarily the strength of the concrete, the degree of carbonation , the position and condition of the reinforcing steel , exposure to chloride and cracks in the concrete. Only after a sufficient examination of the actual condition and clarification of the causes of the damage can one start creating a renovation concept.

The renovation concept describes a technically and economically feasible path from the actual state to the target state of the construction. Because of the often weakened stability, the requirements for the specialist knowledge and special material knowledge of the planning, performing and monitoring engineers and skilled workers are by no means lower, but are generally higher than for new buildings. For all measures that go beyond mere surface repairs, there is no statistical reason to differentiate between renovation and reinforcement. Regardless of whether a load-bearing cross-section is only restored or reinforced, the questions of force redistribution and the possible different deformation behavior must be clarified before deciding on a certain material to supplement the cross-section. This requires thorough knowledge of the behavior of building materials and components under the load, usage and environmental stresses that occur.

The analysis of the damage to a reinforced concrete structure is a prerequisite for the selection of suitable repair technologies and materials and thus the basis for a successful repair.

Investigation methods

The damage analysis consists of on-site investigations (on the building) and investigations in the laboratory (on building material samples obtained). The following examination methods are used.

Investigations on the structure

• The visual inventory serves to record the extent of damage, the existing exposures and the definition of the areas to be examined. Typical conditions of the building are documented in picture form.

• The non-destructive localization of the reinforcement is done with magnetic inductive measuring systems, which enable the determination of the position of individual reinforcing bars with high accuracy. The measuring systems available on the market can localize reinforcement down to a depth of approx. 10 cm.

• The non-destructive measurement of the concrete cover of the reinforcement is carried out with the same measuring system as the localization of the reinforcement. For the non-destructive determination of the concrete cover, it is necessary that the diameter of the reinforcing bars z. B. is known from planning documents.

• The non-destructive measurement of the compressive strength of the concrete takes place with the rebound hammer according to EN 12504-2. During the measurement, a bolt is thrown against the concrete surface with a defined force. The strength of the concrete structure can be inferred from the intensity of the rebound. In order to avoid influences from the unevenness of the concrete surface, each specified strength value stands for at least 10 individual tests. The non-destructive measurement of the compressive strength does not replace the testing of the concrete compressive strength according to EN 12390-3 on test bodies . The compressive strength, determined non-destructively, can be used to verify the uniformity of in-situ concrete and to represent areas or surfaces of poor quality or for damaged concrete in structures.

• The non-destructive evaluation of the corrosion status of the reinforcement can be carried out by measuring the potential field on the concrete surface. This requires the area to be recorded over a large area. The component-typical reference value of the potential field for the corrosion condition to be found is to be determined at an exploratory opening. The detection of corroded steels requires extensive experience and a critical assessment of the overall condition of the component.

• The non-destructive evaluation of the corrosion status of the reinforcement is based on the local exposure of the reinforcement at the point of the slightest concrete cover in the component. The assessment of the corrosion status is based on the visual appearance of the reinforcement surface.

• For the non-destructive measurement of the carbonation depth of the concrete , a fresh concrete fracture surface is sprayed with a suitable indicator - usually phenolphthalein solution . The progress of the concrete corrosion can be determined using the depth profile of the color change.

• The non-destructive measurement of the tensile strength of the concrete surface is carried out with the pull-off test. For this purpose, test stamps with a defined area are glued to the concrete. The stamps are then pulled off the surface with a - mostly electropneumatic - testing device with a defined loading speed. The measured value of the breaking force as well as the fracture pattern and the fracture depth allow statements about the quality of the concrete surface. Various minimum surface tensile strength values ​​are required for concrete repair systems.

• For the extended investigation of the concrete properties, it is necessary to take concrete samples . As a rule, this is done by removing drill cores using the wet drilling process. In order to weaken the component as little as possible, the reinforcement should be localized before drilling.

• The taking of rebar should rehearse just after release carried by the structural engineer. To determine the mechanical properties of reinforcing steel in the tensile test, it is necessary to take samples at least 35 cm long.

Research in the laboratory

• The observation of the concrete structure serves to visually determine the peculiarities of the concrete and the aggregate. These provide information on possible causes of damage and the degree of damage.

• The compressive strength of the concrete is determined by loading a concrete sample according to EN 12390-3 until it breaks. The compressive strength is calculated from the ratio of the loaded cross-sectional area and the breaking load .

• To determine the density of the concrete , the mass of a certain sample is related to the volume of the sample (EN 12390-7). The ratio corresponds to the bulk density .

• The chemical composition of the concrete is determined using a wide variety of analysis methods. The aim is to obtain information on the formulation of the concrete and its components. This information can be used to draw conclusions about the durability behavior. The selection of appropriate repair materials prevents any damaging reactions between the existing concrete and the addition. Frequently examined are:
  • the total sulphate content,
  • the total chloride content

• A wide variety of analysis methods (e.g. SEM ) are also used to determine components that are harmful to concrete . By determining the type and proportion of harmful substances, statements can be made about the cause of damage to the concrete structure.

Scope of investigation

The scope of the examinations to be carried out is not specified. The minimum size, however, results from the building to be examined, the variety of its constructions and loads as well as the statistical reliability of the individual statements of an investigation.

Substrate preparation

Reinforcement exposed and cleaned with the concrete surface.

The substrate must be prepared accordingly in order to achieve sufficient adhesive strength. The repair of concrete is mainly done by applying replacement or protective layers. For this, the subsurface must be sufficiently stable. Damaged concrete rarely meets these requirements. Thus, before applying the repair materials, measures are required to ensure that the concrete surface is free from

• loose and crumbly parts and layers that easily peel off - it must not be powdered or sanded,
• Shells, detachments and cracks parallel to the surface,
• Gravel pockets and other hollow spaces,
• foreign substances (rubber abrasion, oil, vegetation, etc.).

The complete removal of carbonated concrete layers is not absolutely necessary, since carbonated areas of the old concrete are permanently realized by diffusion processes and can then again offer reliable corrosion protection for the reinforcement. According to the repair guideline of the DAfStb, in areas in which the mean carbonation depth has penetrated more than 15 mm behind the reinforcement layer, the concrete must be removed to the surface of the outer reinforcement layer.

In contrast to this, if there is chloride-induced corrosion of the reinforcement, the concrete must be completely removed to a depth at which the corrosion-inducing chloride content (0.5% based on the cement mass with slack reinforcement) is again below the level. Otherwise, the remaining chlorides would maintain the corrosion process even after repairs have been made underneath the newly applied concrete layer.

The concrete surface to be repaired must have certain minimum surface tensile strength. The surface of the exposed or exposed reinforcement must, depending on the type of repair, a the standard degree of purity Sa 2½ according to EN ISO comprise 12944-4 like state.

The following methods can be used to prepare the substrate:

• Chiselling is used to remove loose or cracked concrete and to expose corroded reinforcement. Electric or pneumatic prying tools are used. The work is very noisy and dusty. When mortising there is a risk that the mortising tools will damage the reinforcement steel or loosen the bond between reinforcement and concrete. The entry of vibrations into the structure can result in cracks.

• The wire brush can be used to prepare rudimentary concrete or reinforcement for the following repair steps in areas inaccessible for other cleaning processes. Other methods should be used where possible.

• The needle gun consists of a needle bundle that is pneumatically operated and thrown against the ground. In the process, loose or not firmly adhering components are separated off. With the needle gun, the requirements for regular substrate preparation can be met to some extent. The use of the needle gun is comparatively low in dust.

• By particles dry blasting , layers of concrete surface and rust layers can wear on the reinforcement surface. For this purpose, special blasting material (mostly slag) is thrown against the surface with air pressure (6 to 12 bar). Upon impact, the less solid components are released from the surface. The process is very noisy and dust-intensive.

• The shot peening is carried out in a closed circuit. In a special device, steel balls are thrown against the surface to be processed with a rapidly rotating centrifugal wheel. On impact, the less solid components of the surface loosen and are immediately sucked up again together with the balls. Balls and blasting material are separated within the device. The balls are used again immediately. The process is comparatively low in dust, but can only be used on flat, horizontal or slightly inclined surfaces.

High pressure water jets with the hand lance at 2000 bar.

• With high and maximum pressure water jets, the concrete surface is removed and the reinforcement is exposed with a strong water jet. A distinction is made between jets in the pressure range under high pressure, over 850 bar (maximum pressure) and over 2000 bar (ultra-maximum pressure). Economically, no concrete can be removed below 800 bar. In addition to the pressure, the intensity of the removal is influenced by the volume flow of the water and the shape of the nozzle. The usual diameters when using a hand lance are 0.8 mm to 1.3 mm. In the case of machine-operated nozzles, the diameter is 2 to over 4 mm, in the latter case approx. 1000 bar pressure is applied and approx. 450 l of water are pressed through the nozzle per minute. A drive power of approx. 1000 kW is necessary for the water pump. In addition to nozzles for targeted, selective removal, there are rotating nozzles for surface preparation. When using the water jet process, the formation of mist and the amount of water in the work area must be observed. The main advantage of the water jet process is that deep concrete removal is possible with comparatively little vibration of the component. The stress cracks caused by conventional methods such as caulking are largely avoided. Due to the low vibrations, almost no structure-borne noise is transmitted to the structure, but air-borne noise can be over 120 decibels with a free-radiating nozzle.

• The scarfing is used in some cases for the removal of contaminated concrete surfaces. It cannot be used on exposed reinforcement because there is a risk that the steel, which has suddenly been heated to flame temperature, will expand and the bond between steel and concrete will loosen in the steels that are still in the concrete.

• Sanding the subsurface . A newly approved process for removing undesirable substances from concrete surfaces. An angle grinder with a diamond grinding wheel is usually used for this. Disadvantage of this method: The substrate has a smooth surface, which can be problematic when applying further layers (grip).

Crack repair

Cracks in the concrete allow the access of corrosive media to the concrete structure and the reinforcement. Due to the dissolution of the bond, cracks can impair the load-bearing capacity of a component made of reinforced concrete. The following principles are used to repair cracks:

• Closing the cracks as protection against the penetration of pollutants into the component
• Sealing the cracks as protection against penetration of the component
• Extensible connection of the crack flanks with elastic materials for permanent limited mobility closure of the crack
• Force-fit connection of the crack flanks to produce a tensile and pressure-resistant bond in the component

The type of materials and technologies used depends on the properties of the crack:

• Type of crack (close to the surface, continuous)
• Crack course (vertical, diagonal, reticulate ...)
• Crack width
• Crack movements (short-term, daily, long-term)
• Crack condition (water-bearing, moist, dry, soiling ...)

The following procedures are used to repair cracks:

• Impregnation - only near-surface crack filling without pressure
• Injection - crack filling under pressure

Due to the process, the following materials are used:

• Epoxy resin EP - for impregnation, injection for frictional connection
• Polyurethane resin PUR injection for flexible bonding
• Zementleim ZL, cement suspension ZS - impregnation, injection for frictional connection

The devices and special resins available today allow cracks with a crack width of down to 0.1 mm to be grouted. The injection of cement paste or suspension requires larger crack widths (cement paste 0.8 mm, cement suspension 0.2 mm).

Extensible connection

Two-component polyurethane resins are injected to seal cracked components and when the joint flanks move; they still have a certain elasticity after curing and retain their sealing function thanks to their good adhesion to the crack flanks - even if the component is slightly deformed. When repairing water-bearing cracks, fast-foaming polyurethanes (SPUR) are used at the water entry point in the rear third or behind the crack. These SPUR only have a temporary sealing function, which enables the subsequent use of the regular materials.

Frictional connection

If tensile and pressure-resistant connections of the crack flanks are necessary for static reasons, cement emulsions, cement suspensions or epoxy resin are injected. As the crack flanks are always jagged as a result of the coarse aggregates contained in the concrete, the ability of the concrete to absorb forces that do not run at right angles, but also diagonally or even parallel to the crack banks is restored when the concrete is largely filled.

Corrosion protection of the reinforcement

If, despite all the coatings on the concrete surface, there is a risk that corrosive media will continue to have access to the reinforcement, corrosion protection measures must be taken. These are based on the direct, dense coating of the reinforcement surface or the electrochemical prevention of corrosion of the reinforcement surface.

• In the coating of the reinforcement reaktionserhärtende systems (PC epoxy resin) may be used, while the minimum film thickness is 300 microns. Plastic -modified cement-bound systems (PCC) are designed with a minimum layer thickness of 1000 µm. The corrosion protection must be carried out in at least two work steps. For better control, the materials can be pigmented differently for both operations. Reaction-hardening systems can be sanded with fire-dried quartz sand to improve the bond (physically) to the concrete replacement mortar. When coating the reinforcement, it is imperative to ensure that the reinforcement surface is completely covered, otherwise very small anodes (coating defects) stand against large cathodes, which increases and accelerates the rate of corrosion. Transition areas between reinforcement and concrete must be coated so that they overlap by a few millimeters, but in principle the corrosion protection must only be applied to the steel.

The most frequently used form of anti-corrosion coating are plastic-modified cement-based systems (PCC). They are not considered a closed reinforcement coating (principle C). PCC corrosion protection has cement as the main binder and therefore cannot form a closed system. The corrosion protection is based on a catalytic effect due to the high cement content. The cleaned reinforcement steel forms a closed passive layer on the surface very quickly and intensively (high cement content = high alkalinity), which actively protects it from corrosion. Possibly The same material can also act as a bonding agent on the concrete excavated surface, but it must be applied in a further work step after the corrosion protection has completely hardened and immediately before the coarse mortar (fresh on fresh) is applied.

• With cathodic corrosion protection, the reinforcement is protected by polarization induced by external currents, often with inert anodes. For this purpose, a particularly resistant mesh electrode is inserted into a coating system (usually spray mortar). When a voltage is applied, the reinforcement becomes the cathodic and the mesh electrode the anodic part of the corrosion cell. With this form, there is no more material loss on the reinforcement. The durability of the anode should correspond to the remaining useful life of the component.

A new solution is the use of glass fiber reinforcement, which is very well suited for use in aggressive environments due to its corrosion resistance and resistance to acids and bases.

Concrete replacement

Defects and breakouts in the concrete surface are re-profiled with concrete replacement systems after the substrate has been prepared accordingly, if no additional increase in the concrete cover is required. Depending on the expected exposure or subsequent coatings, concrete replacement is carried out with plastic-modified cement-bound mortars or - depending on the underlying regulations - with pure cement-bound mortars. Epoxy resin mortars (PC) are i. d. Usually only used in special cases (chemical exposure, speed, etc.). These are only spatially applicable to a limited extent, especially for traffic areas (area ≤ 1 m²) and are not taken into account here.

The replacement with purely mineral systems occurs due to the better compression of the material in the spraying process (spray mortar, sprayed concrete). Plastic-modified systems can be processed by hand. To improve the adhesive bond between the existing concrete and the concrete replacement, it is necessary to apply an adhesive bridge by hand before reprofiling with plastic-modified systems.

A mean surface tensile strength of 1.5 N / mm² is required for a successful concrete replacement . The smallest individual value of a test series may be 1.0 N / mm².

The selection of the repair mortar and concrete is made taking into account the stress classes

• Resilience class M 1 - only if there are requirements to restore the component geometry
• Resilience class M 2 - with extended requirements for the carbonation resistance and the application in the case of dynamic loads
• Resilience class M 3 - when taken into account within the framework of the load-bearing capacity or usability verification

Adhesive bridge

The application of an adhesive bridge serves to improve the bond between the repair concrete and the substrate and must therefore always be carried out 'fresh on fresh'. Two variants are used: on the one hand, purely cement-bound adhesive bridges (grain size 0/2 mm) and, on the other hand, cement-bound and polymer-modified systems. In addition to improving the bond, adhesive bridges serve as a moisture barrier between the two layers on sufficiently pre-wetted concrete (starting at 24 hours). The effectiveness of cement-bound adhesive bridges increases with penetration into the concrete substrate and must therefore be applied to the 'matt-damp' repair area.

It is usually applied with a coarse brush or a paintbrush. Some products meet the requirements for mineral corrosion protection of the reinforcement. With these products, the bonding bridge and the corrosion protection can be applied with the same material, but in separate operations.

Unfortunately, no machine spraying methods for applying the bonding agent are known and developed so far.

Repair mortar

Only plastic-modified cement-bound mortars are used as repair mortars ( repair mortars and patch mortars are also obsolete ) due to the special requirements - in particular for the bond with the substrate also in the edge area of ​​breakouts and the tightness of the concrete cover . The type and scope of the organic remuneration of the mortar depends on the required properties. The following use cases are distinguished in the set of rules:

• PCC I - for horizontal and slightly inclined surfaces
• PCC II - for any installation position, even upside down

Plastic-modified mortar can also be applied over a large area using the dry spray method as SPCC. With this type of application, no bonding bridge is generally required on the existing surface. SPCC must not be used for horizontal surfaces and for load-bearing components. It is mostly used to increase the concrete cover over a large area. Due to its tightness and a workable minimum thickness of 10 mm, it is well suited for this.

Spray mortar

Spray mortar is the application of cement-bound mortar by spraying. In contrast to shotcrete, sprayed mortar has a smaller aggregate grain size . Due to the largest grain of 4 mm, thinner layers and the restoration of the concrete surface can be implemented in closely reinforced components. Spray mortars according to DIN 1045 are mostly processed using the dry spray method . Plastic-modified spray mortars have a relatively high resistance to carbonation and are processed using either the wet spray method or the dry spray method .

When processing plastic-modified cement-bound mortar in the spraying process, one speaks of SPCC. The processing of this mortar places higher demands on personnel (nozzle driver's license) and technology.

Concrete repair with shotcrete

The concrete spraying process is predominantly used for larger areas, for thicker layers and when stability is at risk . An earth-moist concrete mix is ​​sprayed onto the surfaces to be rehabilitated, which have been cleaned and roughened beforehand by sand or water jets. Due to the high impact energy, the fresh concrete is pressed into the concrete pores, which were previously opened by radiation. After hardening, this results in the good bond between new and old concrete, which is typical for this process. An additional bonding agent is therefore not required. The tensile and shear strength in the connection joint largely correspond to the values ​​that can be expected for concrete parts made in one cast.

The aftertreatment of the shotcrete layer applied to a concrete component for repair is of particular importance. Here, the water required for cement hydration is withdrawn from the young concrete not only, as in new buildings, by the surrounding atmosphere , but also by the usually dry old concrete. The old concrete should therefore be damp and the relatively thin shotcrete layers must be provided with enough moisture in the first few days after application to prevent them from shrinking too quickly at a point in time when the concrete and, above all, the connection joint are still not strong.

If it is necessary to install additional reinforcement in the new shotcrete shell to be installed, it is laid in the required locations and in the required cross-sections in accordance with the rules of reinforced concrete construction before spraying. The introduction of force into the added reinforcement generally takes place via the bond between the reinforcement and the shotcrete. The connection to the existing reinforcement is predominantly made by overlapping joints , by means of reinforcing steels set in drill holes and cast, and in special cases also by welding.

Shotcrete

Shotcrete with a maximum grain size of 4 mm or more is used to restore deeper, larger defects in the concrete. In the context of concrete repairs, sprayed concrete is mostly processed using the dry spraying method.

When replacing concrete by spraying, no bonding bridge is required on the existing concrete surface. Corrosion protection of the reinforcement would be destroyed if the shotcrete hit. For this reason, sprayed mortar and concrete are only used in areas in which either the reinforcement is sufficiently covered for the subsequent stress or the concrete surface is coated with a surface protection system.

The surfaces of sprayed mortar and sprayed concrete are usually peeled off as spray-rough. Rubbing out both materials can lead to unevenness and structural disturbances on the surface. For the final surface leveling, the application of a plastic-modified spatula is better suited.

The replacement of missing concrete parts to produce the required cross-section is done with shotcrete or with plastic-modified mortars , depending on the size and depth of the new areas to be applied . The choice of a material suitable for the respective load or use of the component is a decisive prerequisite for the durability of the measure. The main thing to consider is the difference in deformation and fire behavior between new and old concrete.

Pure cement mortars, which are used to fill seals or to compensate for imperfections in concrete, are fine-grained and rich in cement for processing reasons. As a result, they have a strong tendency to shrink, which creates the risk of cavities or cracks forming, especially at the seal edges. This is why plastic-modified cement mortars are usually used for such smaller patches. These are hydraulically setting mortars to which plastic dispersions are added to change the properties of fresh and hardened concrete. By adding plastics dispersed in the water, these so-called patch mortars are made more elastic, that is, made less susceptible to cracking, and at the same time their water retention capacity is improved, that is, the need for post-treatment is reduced. The dispersion is either added in liquid form shortly before processing as a special component or, in the case of dry ready-mixed mortars, is dissolved by mixing water into the mixture. If the water is removed from the plastic dispersion through evaporation and hydration of the cement paste after installation , the plastic particles stick together and thus act as an additional binding agent. So there are two different hardening processes that take place side by side. The cement hardens by absorbing water (hydration), i.e. by a chemical process, while the plastic particles harden by drying out, i.e. physically. After hardening, the individual plastic particles (large molecules) are, figuratively speaking, embedded as ball bearings between the individual cement particles and thus act to compensate for deformation of the building material. In order to achieve sufficient adhesive strength, an inlet primer (adhesive bridge) made of a thin plastic coating must first be applied before the mortar is installed.

Resin injections for crack injection

The repair of deeper cracks in concrete components is carried out by injecting liquid multi-component resins under high pressure. The devices and special resins available today allow the grouting of cracks down to 0.1 mm.

Depending on the pressing task, a distinction is essentially made between two methods in which different products are used: Injections for sealing and non-positive injections.

Waterproofing injections

To seal cracked components, two-component polyurethane resins are pressed, which after hardening still have a certain elasticity and, thanks to their good adhesion to the crack flanks, retain their sealing function even if the component is slightly deformed. If there is a strong rush of water, polyurethane foams , so-called water stoppers, are used, in which the injected resin reacts on contact with water to form closed pores with a strong increase in volume. However, since the large pores of this foamed resin are destroyed again or become water-permeable with the constant influx of water, only a preliminary seal against the influx of water is possible before the final seal can be made with stable resin types.

Non-positive injections

If the stability of a structure is endangered by cracks, force-fit injections may be helpful. The purpose of this measure is to largely fill the space between the crack flanks with a high-strength resin, usually epoxy resin, in order to enable the concrete to transfer tensile forces again. Since the crack flanks are always jagged as a result of the coarse aggregates contained in the concrete, the ability of the concrete to absorb forces that do not run at right angles, but also diagonally or even parallel to the crack banks is restored if the material is largely backfilled.

Reinforcement

Increase or restore the load-bearing capacity of the structure

• Cross-sectional enlargement
• Additional reinforcement
  • Steel bars and mats
  • Steel lamellas
  • Carbon fiber fins
  • Fiberglass reinforcement

surface protection

The goals of surface protection include:

• Leveling the concrete surface - spatula
• Surface protection systems

The different use cases are divided into different classes depending on national law. In Germany (or in the EU) there are a total of 12 different surface protection systems, which are briefly described below.

• OS-1: hydrophobization of the substrate. This is only a subsequent protection against the ingress of water. Similar to the silicification in masonry, hydrophobic materials suck themselves into the concrete and are apparently no longer recognizable.
• OS-2
• OS-3: Surface protection without crack-bridging effect. These are all possible water-emulsifying substances that are used for the color design of concrete components.

monitoring

The two sets of rules governing concrete repairs, the Repair Guideline and the Additional Technical Contractual Conditions and Guidelines for Civil Engineering (ZTV-ING) prescribe two forms of monitoring: In the case of self- monitoring, the company carrying out the work monitors itself; in the case of external monitoring, a recognized monitoring body also monitors . In principle, self-monitoring must always be carried out. It includes the implementation of the necessary tests and their logging.

In the case of measures in accordance with the repair guidelines of the German Committee for Reinforced Concrete (DAfStb), the competent planner must determine, among other things, whether the planned measure is necessary for maintaining stability. Protective and concrete repair measures according to the ZTV-ING are always to be regarded as relevant to stability due to the specifications of the Federal Building Ministry (BMVBS).

The Federal Quality Association for the repair of concrete structures e. V. is approved as an external monitoring body both by the German Institute for Building Technology (DIBt) for measures according to the repair guideline and by the BMVBS for measures according to the ZTV-ING.

See also

• Damage to concrete structures
• Windsor probe
• SonReb method

Norms

• Repair guideline of the DAfStb from 2001 including the three corrections
• DIN 1045 structures made of concrete, reinforced concrete and prestressed concrete
• DIN 18349 VOB procurement and contract regulations for construction works - Part C: General technical contract conditions for construction works (ATV); Concrete maintenance work
• EN 1504 Products and systems for the protection and repair of concrete structures

literature

• German Committee for Reinforced Concrete (DAfStb): Directive on the protection and repair of concrete components (repair directive) . Beuth-Verlag, Berlin 2001.
• Federal Ministry of Transport, Building and Housing: Additional technical contract conditions and guidelines for civil engineering (ZTV-ING) . Verkehrsblatt-Verlag, Dortmund 2010.
• Günther Ruffert : Lexicon of concrete repairs . Fraunhofer-IRB Verlag, Stuttgart 1999, ISBN 3-8167-4710-8 .
• RP Gieler, A. Dimmig-Osburg: Plastics for building protection and concrete repair . Birkhauser Verlag, Berlin 2006, ISBN 978-3-7643-6345-1 .

Web links

• http://www.betonerhaltung.com - website of the Federal Quality Association for the repair of concrete structures e. V.
• http://www.dafstb.de - Website of the DAfStb, German Committee for Reinforced Concrete e. V. (set of rules)