Concrete degradation

Damage to concrete structures occur due to the stress from the environment and on errors in the processing of building material. Concrete is produced in different quality and used for various tasks. The versatility offered by this construction material, and the fact that it is usually produced in its final form at the construction site, often lead to execution or planning errors.

Concrete is - even if he is often referred to as - no " universal building material". There are loads for which it is less suitable, whether because of its chemical composition, except that you squeezed it into molds and claimed by forces for which it is problematic due to its material-induced brittleness.

It has long been of the opinion that concrete structures virtually throughout their useful time require no maintenance. The experience of the last decades has shown that concrete structures are appropriate to entertain and that minor damage, if they are not immediately restored and the damage causes eliminated relatively quickly grow into larger damages that are to be removed only with great effort.

Today there are a number of special procedures and a variety of stresses, materials adapted for concrete repair. But before you go to the elimination of an occurred damage, the cause of damage needs to be clarified. The detection and termination of damage causes requires a thorough knowledge of the behavior of building materials and components under the last- occurring, use- and environmental stresses.

Typical causes of damage

Spalling and delamination

Concrete has a low tensile strength at very high compressive strength. The component of a recorded tensile stresses must therefore usually inserted by steel bars ( rebar ) are taken (reinforced concrete ). Steel is a corrosion- prone building materials, rusts very quickly when it is exposed to unprotected atmospheric oxygen and humidity. Concrete is highly alkaline and has the important property of forming by its alkalinity, a passivation layer on the steel, and so to protect it from rust. By reaction with CO2 from the air (see carbonation (concrete ) ) of the concrete loses its alkalinity but with time and then is no longer able to protect the embedded steel bars from corrosion. Therefore, the standards prescribe a minimum thickness of concrete cover depending on use and environmental conditions or exposure class. Due to the insufficient concrete cover, protection against corrosion is no longer guaranteed. The material forming corrosion product (rust) has the more times the volume of the original steel, thus the protective concrete cover is blown off by the forming pressure. This damage is more likely to enter, the thinner, more porous and less alkaline, the concrete cover of the steel.

Destruction by chemical attack

Many substances are inclined, as soon as they come with certain other molecules / atoms in contact, to enter into new chemical compounds. Thereby, the original material properties are more or less varied. This also applies to the material as concrete. His inclination to take such new chemical compounds and thus the danger that concrete components are attacked by chemical agents depends not only on the chemical composition and concentration of forces acting on the concrete materials also highly dependent on the tightness of the concrete. Thus, whether the substances act only on the surface, or if they penetrate into the component and can act from the inside. Aim is to support the penetration of chemically aggressive liquids or gases through the air pores and cracks of the concrete. A distinction is made between the dissolving attack and the driving attack.

Santander solvent attack

Concrete consists mainly of cement verkittetem by natural rock. Little resistance to chemical attack is particularly the cement man-made, while existing in its present chemical composition for many millions of years nature rocks are much less susceptible to chemical attack. Cement stone is as basic product very little resistant to acids. Since the components of the various cements are almost all soluble in acid can be expected in principle, no resistance to organic and inorganic acids of the hardened cement, the cement stone. The complicated lime -alumina compounds of the cement are transformed by acid attack in water-soluble compounds, which can then be removed by water and atmospheric conditions. This cohesion between aggregate and cement paste is first loosened and destroyed with progressive attack. As long as the concrete skin is still undisturbed, the attack can always start only from the surface. The larger the attack surface, with progressive opening and fracturing of the concrete outer skin but, the faster proceeds the destruction.

Impulsive aggression

Impulsive aggression is when the forces acting on the concrete materials upon reaction with the cement stone, in some cases, with the additives ( alkali and bustle ), new products with a much larger volume form. The larger space requirement then leads out to blow up the concrete from the inside. A typical example of this is also the sulfate activity. Interaction sulfate-containing gases or solutions to the concrete, then it comes by reaction between the sulfates and the tricalcium aluminate of the cement ( C3A ) in the formation of ettringite. This makes the volume of raw materials increased eightfold, the concrete is blown up from the inside out. This damage often occurs in sewers of concrete. Here is formed under the conditions prevailing above all in low-lying channel systems conditions (low flow rate, relatively high temperature and lack of ventilation ) by bacterial decomposition of sulfur-containing organic substances contained in waste water, such as proteins, which smells like rotten eggs hydrogen sulfide gas. This gas can be oxidized by other bacteria or by atmospheric oxygen to form sulfates and these may cause the sulfate activity.

Destruction by fire

Concrete is a non-flammable and highly resistant to fire load of building materials. Nevertheless, also occur in the typical normal fire temperatures up to 1000 ° C for damage, the effects of fire duration and type of construction are dependent.

Concrete

The decrease of concrete strength is minimal until about 200 ° C. At higher temperatures, the strength falls off faster, and can be dropped at 500 ° C already up to 50 % of normal pressure and splitting tensile strength. Because of the poor thermal conductivity occur during normal fire load related to the stability of temperatures but only in the top few centimeters, while the core of the concrete structure is usually less affected. It usually occurs as a result of steam flaking development by the residual moisture in concrete.

Rebars

Reinforcing steel is much more temperature sensitive than concrete. Even at relatively low fire temperatures, the steel begins to stretch. This is the faster, the smaller is the concrete cover. By stretching the steel it comes to flaking of the concrete cover ( due to the better thermal conductivity of the steel heats up in areas where the concrete is even cooler. This leads to expansion differences between steel and concrete that lead to spalling of the concrete cover. ), whereby the steel is then directly exposed to the action of fire. At about 500 ° C, the steel reaches its yield point, there are high-quality, cold-formed steels in general more sensitive to fire temperatures. When prestressing steel the critical limit is only slightly above 350 ° C. Decreases in a reinforced concrete component, the yield point of the steel to be absorbed by it, the voltage, the resistance of the member is low. It will initially deform and fail in another load.

Even if it does not come through the fire to a component failure, the resistance of the member is massively weakened due to overstretching of the steel and the composite loss and needs to be upgraded. This can, for example, done by underpinning or the subsequent bonding of reinforcement of flat steel or carbon blades.

Damage caused by chloride exposure

Although chlorides attack the concrete directly, they can - if sufficient moisture is present - lead to pitting corrosion of the reinforcing steel in the concrete. Damage caused by chloride exposure can occur due to fire or road salt.

Chloride exposure to fire

Combustion of PVC plastics it comes, especially in industrial fires, the chloride load of reinforced concrete structures. During the thermal decomposition of PVC, hydrogen chloride is released and condensed in connection with the combustion of moisture in the form of hydrochloric acid to the colder, generally further from the fire source away concrete surfaces. The respective penetration depth is mainly dependent on the tightness of the concrete next to the amount of liberated hydrogen chloride.

Chloride contamination by road salt

When ice or snow formation, the busy and committed concrete surfaces with Frosttaumitteln sprinkled usually with de-icing salts. The use for the next salt ( NaCl), containing a large proportion chloride. When defrosting a sodium chloride solution forms. Reach the chlorides to the reinforcement, so there is always the risk of pitting corrosion, in particular for the sensitive prestressing steel. Particularly at risk are bridges and parking decks. The combined operations are not being conducted at the surface, where they are easy to recognize, but in the interior of the component to the reinforcement through selective destruction. You can therefore be the point at which they are detected, have already led to severe impairment of stability.

Cracks

In the inhomogeneous components Material concrete already exist on the production of fine cracks, resulting for example from the contraction of the cement at discharge of excess water. These fine, resulting in the first hours of hardening cracks are barely visible, and no shortage or even loss. But thermal or mechanical stresses in the component may be implemented at these microcracks and they increase to macro cracks. Since it takes a certain amount of elongation in the composite building material of reinforced concrete, to the built-in for receiving tensile reinforcement is able to absorb these stresses alone, cracks in load- induced deformation is to be avoided not completely. In the static analysis of reinforced concrete structures is assumed that the so-called state II, the concrete is cracked in the tension zone. It therefore jokingly speaks at reinforced concrete by a " cracked construction ".

To ensure adequate serviceability and durability of a structure, but it is necessary to limit the selection of appropriate concrete and steel cross-sectional areas and through proper distribution of reinforcing the cracks to a harmless level, so that under normal atmospheric exposure, there is no risk of corrosion of the reinforcement. So cracks usually represent not constitute a technical fault, if its width is less than 0.3 mm. But at larger crack widths to form gates for water and oxygen, possibly also for invading aggressive substances, and endanger the corrosion protection of reinforcing steels.

In the assessment of a crack is to distinguish between pure surface cracking and separating cracks. The former do not represent a threat to the structural component, but often jeopardize the guaranteed only through an intact concrete cover corrosion protection of reinforcement. The divisive fault lines running through a larger part of the building through it, however, no longer provide the necessary stability for the transmission of forces.

For components that have a sealing function except a constructive yet, such as swimming pools, water tanks, or buildings in groundwater ( white tanks ), it is not always necessary that a crack through the entire wall thickness passes through. For the occurrence of leaks, it may in thin plates under unfavorable circumstances sufficient that a crack extends to the reinforcement, since the liquid along a path of the steel bars usually present imperfections in compact concrete structure seeks out and somewhere - often a long way from the intrusion away - can emerge again.

Standardize

• DIN 1045 - Concrete, reinforced and prestressed concrete