Alkali–silica reaction

The alkali -silica reaction ( ASR short ) or even alkali or alkali reaction bustle, colloquially known as concrete cancer, is the name for the chemical reaction between alkalis of the cement in the concrete and aggregates with alkali- silica. The term alkali-aggregate reaction (AAR) summarizes similar processes, of which the most important is the ASR. Is arising out of slaked lime Ca ( OH) 2 and SiO2 crystal by crystallization and a wollastonite and other calcium silicates, such as Ca (OH) 2 • SiO 2.

Cause and consequences

In the literature there are different considerations for reaction. The starting point is the alkalinity of pure cement, which is determined by the calcium hydroxide. It falls at pH values ​​of about 12.6. Silica in the form of quartz is significantly dissolved until a pH of 13. Admixtures of sodium or potassium, to increase the alkalinity beyond this value. The onset of pozzolanic reaction is usually intended as it reduces the unwanted calcium hydroxide. Since it runs over the years, and damage sometimes appear after a few years, it is probably partly responsible for the damage of concrete due to ASR. Depending on the reaction conditions and the resulting theory CSH ( calcium silicate hydrate eng. ) can establish a diffusion barrier, which favors the access of alkali atoms to silicon-rich phases. There is then formed a swellable alkali - silica gel or a swellable CSH gel breaks due to the increase in volume of concrete from inside.

Alkali -sensitive rocks

Are considered to be sensitive to alkali rocks containing amorphous or finely crystalline silicates, such as opal sandstone and porous Flint. In particular, in northern Germany, found in large quantities Opalsandsteine ​​and Grauwackevorkommen in the Lausitz region can contain harmful amounts of alkali-soluble silica. By use of cements with low effective alkali content ( cement letters: NA) and by limiting the cement content in the concrete, with the use of concrete aggregates with alkali sensitive ingredients, the alkali reactions can usually be avoided. Further details can be found in the alkali - Guideline of the German Committee for Reinforced Concrete.

Affected Structures

Affected are only concrete parts that come in contact with water, especially concrete roadways and railway sleepers. Concrete in buildings, which is kept permanently dry, is not affected by the AKR problem, according to previous knowledge.

Due to damage from alkali reaction, among other things, built 1965/66 Lachswehr bridge in Lübeck had to be demolished two years later.

The mid-1970s began with the Deutsche Reichsbahn in track construction Baltic gravel admix for prestressed concrete sleepers, causing the concrete very quickly crystallized and initially received unusual strength. Under constant load, the crystallization continued until the destruction of the concrete. This affected several thousand kilometers of track, mainly due to the heavily used main lines had an enormous impact on the operation of trains. It took decades ( until about 1992), to the German Reichsbahn could replace all affected kilometers superstructure.

A similar response was observed in the concrete of the built- thresholds of the high-speed line Berlin- Hamburg 2007. The renovation took place in 2009.

In May 2009, the Federal Ministry of Transport announced that about 320 km concrete road of the German motorway network are affected. Of these, for example in Hesse alone affected 79 km of the busy A5, in Saxony and Saxony -Anhalt is the A14. In addition, strongly affected is also the A9 (Munich - Berlin ), which was renewed until 2006 mostly and is in need of repair again. An employee of the Institute for Building Materials Research in Duisburg pointed out that träten by " concrete cancer " damages due to a kind of incubation period is usually only five to ten years after completion of the highways in appearance. Already in 1992 the geologist and mineralogist Gerhard Hempel from Weimar pointed out, however, that the risk of ASR damage could be reduced by selecting the right aggregates.

References and Notes