Damage to concrete structures
Damage to concrete structures occurs due to environmental stresses and errors in the processing of the building material. Concrete is produced in different qualities and used for a wide variety of tasks. The wide range of possible uses that this building material offers, as well as the fact that it is usually only manufactured in its final form on the construction site , often leads to execution or planning errors.
Concrete is - even if it is often called that - not a "universal building material". There are stresses for which it is less suitable, be it because of its chemical composition, be it that it is forced into shapes and stressed by forces for which it is problematic due to its material- related brittleness .
For a long time it was believed that concrete structures require practically no maintenance during their entire service life. Experience over the last few decades has shown that concrete structures must also be properly maintained and that minor damage, if it is not repaired immediately and the causes of the damage are eliminated, relatively quickly grows into larger damage that can only be removed with great effort.
Today there is a whole range of special processes and materials for concrete repair that are adapted to the most varied of stresses . However, before starting to repair the damage that has occurred, the cause of the damage must be clarified. Recognizing and eliminating the causes of damage requires thorough knowledge of the behavior of building materials and components under the loads, usage and environmental stresses that occur.
Typical causes of damage
Chipping and peeling
With a very high compressive strength, concrete has only a low tensile strength . The tensile stresses to be absorbed by a component therefore usually have to be absorbed by inserted steel bars (reinforcing steel) ( reinforced concrete ). Steel is a building material that is susceptible to corrosion and rusts very quickly if it is exposed to atmospheric oxygen and moisture without protection. Concrete is highly alkaline and, due to its alkalinity, has the important property of forming a passivation layer on the steel and thus protecting it from rust . However, through reaction with the CO 2 in the air (see carbonation (concrete) ), the concrete loses its alkalinity over time and is then no longer able to protect the embedded steel rods from corrosion. The standards therefore stipulate a minimum thickness for the concrete cover depending on the load and environmental conditions or exposure class . The insufficient concrete cover means that protection against corrosion is no longer guaranteed. The corrosion product (rust) that forms has a volume that is several times the volume of the original steel, so the protective concrete cover is blown off by the pressure that forms. This damage is more likely to occur the thinner, more porous and less alkaline the concrete cover of the steel.
Destruction by chemical attack
Many substances tend to form new chemical compounds as soon as they come into contact with certain other molecules / atoms. This changes the original material properties more or less. This also applies to concrete as a building material. Its tendency to enter into such new chemical compounds and thus the risk of concrete components being attacked by chemical substances depends not only on the chemical composition and concentration of the substances acting on the concrete, but also very much on the tightness of the concrete, i.e. on whether the substances are used only act on the surface or whether they can also penetrate the component and act from the inside. The penetration of chemically aggressive liquids or gases through the air pores and cracks in the concrete is particularly encouraged. A distinction is made between the releasing attack and the driving attack.
Dissolving attack
Concrete essentially consists of natural stone cemented with cement . As a basic product, cement stone is particularly poorly resistant to acids. The lime-alumina compounds of the cement stone are transformed into water-soluble compounds by the acid attack, which can then be removed by water and atmospheric agents. This initially loosens the cohesion between aggregate and cement stone and destroys it as the attack progresses. As long as the concrete skin is still undisturbed, the attack can only ever begin from the surface. However, the larger the area of attack becomes as the concrete outer skin continues to open and fracture, the faster the destruction proceeds.
In the long term, rainwater and other very soft water with a hardness of less than 3 ° dH can leach concrete . Water low in calcium and magnesium and running over the concrete dissolves calcium hydroxide and washes it out. A hydrolytic decomposition of the hydrate phases can then take place. The denser the concrete, the less the effect.
When animal and vegetable oils and fats come into contact with concrete, they are split in an alkaline environment. The organic fatty acids released during this saponification form a lime soap with the calcium hydroxide , which results in a local decrease in strength.
Mineral oil products (which do not contain glyceric acid esters ) are not saponified. However, larger quantities can cause a loss of strength of the concrete by up to 25% due to their lubricating effect.
Driving attack
A driving attack occurs when the substances acting on the concrete form new products with a significantly larger volume when they react with the cement stone, and in some cases with the aggregates ( alkali drift ). The greater space requirement then leads to the concrete blasting from the inside. A typical example of this is also sulphate driving . If sulphate-containing gases or solutions act on the concrete, a reaction between the sulphates and the tricalcium aluminate of the cement paste (C3A) leads to the formation of ettringite . The volume of the raw materials increases eightfold, the concrete is blasted from the inside out. This damage is common in concrete sewers . Under the conditions prevailing in deep-seated sewer systems (low flow velocity, relatively high temperature and lack of ventilation), the hydrogen sulfide gas, which smells like rotten eggs, is formed here by bacterial decomposition of the organic sulfur-containing substances (such as proteins) contained in the wastewater . This gas can be oxidized to sulfates by other bacteria or by atmospheric oxygen and these can cause sulfate drifting.
Is known gypsum or Ettringittreiben also from the Monument restoration. If concrete is used, for example, to stabilize foundations that were originally walled up with mortar containing gypsum, sulphate ions can migrate into the concrete in a moist environment and cause harmful crystal growth.
Destruction by fire
Concrete is a non-flammable building material that is very resistant to fire exposure. Nevertheless, even at temperatures of up to 1000 ° C, which are typical for normal fires, damage occurs, the effects of which depend on the duration of the fire and the type of construction.
concrete
The drop in concrete strength is minimal up to approx. 200 ° C. The strength drops faster at higher temperatures and at 500 ° C it can be as low as 50% of the normal compressive and splitting tensile strength. Due to the poor thermal conductivity , with normal fire exposure, temperatures relevant for stability only occur in the top centimeters, while the core of the concrete structure is usually less affected. This usually results in flaking as a result of the development of steam due to the residual moisture in the concrete.
Reinforcing steel
Reinforcing steel is much more sensitive to temperature than concrete. The steel begins to stretch even at relatively low fire temperatures. The smaller the concrete cover, the faster this happens. The expansion of the steel causes the concrete cover to flake off (because of the better thermal conductivity of the steel, it also heats up in areas where the concrete is even cooler. This leads to expansion differences between steel and concrete, which lead to the concrete cover flaking off. ), whereby the steel is then directly exposed to the effects of fire. From around 200 ° C, the stiffness and strength parameters of steel drop considerably. At around 500 ° C, the yield point has generally dropped to the level of the existing stress in the reinforcement bar, while high-quality and cold-formed steels are generally more sensitive to fire temperatures. For prestressing steel , the critical limit is just over 350 ° C. If the yield point of the steel in a reinforced concrete component falls below the stress it has to absorb, then the component's load-bearing capacity is exhausted. It will initially deform strongly and fail with further loading or further temperature rise.
Even if the component does not fail as a result of the fire, the load-bearing capacity of the component is massively weakened due to the overstretching of the steel and the loss of bond and must be strengthened. This can e.g. B. done by underpinning or the subsequent gluing of reinforcement made of flat steel or carbon fiber lamellas.
Damage from exposure to chloride
Although chlorides do not attack the concrete directly, they can - if there is sufficient moisture - lead to pitting corrosion of the reinforcing steel in the concrete. Damage from the effects of chloride can occur due to fire or de-icing salt.
Chloride pollution from fire
When PVC plastics are burned , reinforced concrete components are exposed to chloride, especially in the event of industrial fires. During the thermal decomposition of PVC, hydrogen chloride is released and condenses in connection with the combustion moisture in the form of hydrochloric acid on colder concrete surfaces, usually further away from the source of the fire. In addition to the amount of hydrogen chloride released, the respective penetration depth depends primarily on the tightness of the concrete.
Chloride pollution from de-icing salt
When ice or snow forms, the concrete surfaces driven on and walked on are sprinkled with frost-defrosting agents , usually with de-icing salts . The salt (NaCl) used contains a large proportion of chloride. A sodium chloride solution forms when thawed. If the chlorides get on the reinforcement, there is always the risk of pitting corrosion , especially for the sensitive prestressing steel . Bridge structures and parking decks are particularly at risk . The damage processes do not take place on the surface, where they can be easily recognized, but on the inside of the component on the reinforcement through selective destruction. Therefore, by the time they are recognized, they can already have seriously impaired stability .
Cracks
In the inhomogeneous component building material concrete, fine cracks already exist from the point of manufacture, which for example result from the contraction of the cement paste when excess water is released. These fine cracks that occur in the first few hours of hardening are barely noticeable and no defects or even damage. However, thermal or mechanical stresses in the component can start at these microcracks and enlarge them to macrocracks. As reinforced concrete requires a certain amount of elongation before the reinforcement installed to absorb tensile stresses is able to absorb these stresses on its own, cracks from load-related deformation cannot be completely avoided. The static calculation of reinforced concrete components assumes that in the so-called state II the concrete has cracked in the tension zone. Therefore, reinforced concrete is also jokingly referred to as a “cracked construction”.
In order to ensure adequate serviceability and durability of a construction, it is necessary to limit the cracks to a harmless level by choosing appropriate concrete and steel cross-sectional areas and by correctly distributing the reinforcement, so that there is no risk of corrosion for the reinforcement under normal atmospheric loads. As a rule, cracks do not represent a technical defect as long as their width remains below 0.3 mm. With larger crack widths, however, they form entrance gates for water and oxygen, possibly also for aggressive substances that penetrate, and endanger the corrosion protection of the reinforcing steel .
When assessing a crack, a distinction must be made between pure surface cracks and separating cracks. The former do not pose a structural hazard to the component, but often endanger the corrosion protection of the reinforcement, which is only guaranteed by an intact concrete cover. The separating cracks that go through a larger part of the structure, on the other hand, no longer guarantee the transmission of forces required for stability.
For components that have a sealing function in addition to a structural function, such as swimming pools , drinking water tanks , or structures in the groundwater ( white tubs ), it is not always necessary for a crack to go through the entire wall thickness. In the case of thin components, under unfavorable circumstances, it may be sufficient for leaks to occur that a crack extends to the reinforcement, as the liquid seeks its way along the imperfections that are usually present around the steel rods in the compact concrete structure and somewhere - often a long way from the concrete Penetration point removed - can escape again.
Norms
- DIN 1045 parts 2 and 3 - Structures made of concrete, reinforced concrete and prestressed concrete
- EN 1992 - Design and construction of reinforced and prestressed concrete structures
- EN 13670 - Execution of concrete structures
literature
- G. Ruffert: Lexicon of concrete repairs . Fraunhofer-IRB Verlag, Stuttgart 1999, ISBN 3-8167-4710-8 .
- J. Stark, B. Wicht: Durability of concrete . Birkhäuser Verlag, Basel 2001, ISBN 3-7643-6513-7 , p. 188 ff .
- RP Gieler, A. Dimmig-Osburg: Plastics for building protection and concrete repair . Birkhäuser Verlag, Berlin 2006, ISBN 3-7643-6345-2 .
Web links
- Further information on efflorescence on concrete surfaces (PDF file; 0.2 MB)
- Further information on driving attack
Individual evidence
- ↑ a b c Roland Benedix: Construction chemistry: Introduction to chemistry for civil engineers and architects , Springer Verlag
- ↑ Compendium Zement Beton - 5 Structural properties of cement , p. 161ff; In: VDZ-online.de