Alkali-silica reaction

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Surface of a concrete pillar marked by the alkali-silica reaction

The alkali-silica reaction ( AKR for short ) or just alkali reaction or alkali drifting , colloquially also concrete crustaceans , is the chemical reaction between the alkalis of the cement paste in the concrete and the aggregate with alkali-soluble silica . The term alkali aggregate reaction ( AAR ) summarizes similar processes, of which the ASR is the most important. There arise from slaked lime (Ca (OH) 2 ) and quartz (SiO 2 ) by crystal formation u. a. Wollastonite and other calcium silicates , e.g. B. Ca (OH) 2  • SiO 2 .

The reaction can cause severe damage to concrete structures such as bridges and highway pavements. It occurs when the concrete is exposed to moisture and made with gravel that contains too much soluble silica. In Germany, the need for renovation for the concrete cancer-infested airport runways alone was estimated at 1.2 billion euros in 2016.

Cause and consequences

There are different considerations regarding the course of the reaction in the literature. The starting point is the alkalinity of pure cement , which is determined by the calcium hydroxide. Ca (OH) 2 precipitates at pH values above 12.6. Silicon dioxide in the form of quartz, on the other hand, is only noticeably dissolved from a pH value of 13. Additions of sodium or potassium increase the alkalinity above this value. The pozzolanic reaction that sets in is usually wanted because it breaks down the unwanted calcium hydroxide:

AKR catalysis by NaOH or KOH

The empirical formula above does not correctly reflect the actual AKR reaction, which is catalyzed by alkali hydroxides ( NaOH / KOH ) at very high pH values. In detail, the following two reactions take place, which when taken together result in the sum formula above:

2 Na (OH) + H 4 SiO 4 → Na 2 H 2 SiO 4 + 2 H 2 O
Na 2 H 2 SiO 4 + Ca (OH) 2 → CaH 2 SiO 4 + 2 NaOH

Without NaOH or KOH and their high pH values ​​(~ 13.5), the amorphous silicate would not be dissolved and the reaction would not proceed. In addition, the water-soluble sodium or potassium silicate is very hygroscopic and swells in the process. When the rock pores are filled and the very viscous gel cannot escape, the hydraulic pressure in the concrete increases and leads to breakage.

Since it expires over years and damage sometimes only occurs years later, it is probably jointly responsible for the damage to concrete by the ASR.

Depending on the reaction conditions and the resulting theory CSH (may English calcium silicate hydrate ) to establish a diffusion barrier, which favors the entry of alkali metal atoms to silicon-rich phases. A swellable alkali-silica gel or also a swellable CSH gel is then formed there, which breaks up the concrete from the inside by increasing its volume.

Alkali-sensitive rocks

Are considered to be sensitive to alkali rocks , the amorphous or fine crystalline silicates contain such. B. Opal sandstone and porous flint. In particular, the opal sandstones occurring in large quantities in northern Germany and the greywacke deposits in Lusatia can contain harmful amounts of alkali-soluble silica. By using cements with a low alkali content (marked with "(na)" after the standard designation) and by limiting the cement content in the concrete, the alkali reaction can usually be avoided when using concrete aggregates with alkali-sensitive components. Further information can be found in the alkali guideline of the German Committee for Reinforced Concrete .

Affected structures

Only concrete parts that come into contact with water are affected, especially concrete roadways and railway sleepers. As far as we know, concrete in buildings that is kept permanently dry is not affected by the ASR problem.

Due to damage from the alkali reaction, the salmon weir bridge in Lübeck , built in 1965/66, had to be demolished two years later.

In the mid-1970s, East German pebbles began to be added to the manufacture of prestressed concrete sleepers, whereby the concrete crystallized very quickly and initially acquired unusual strength. Under constant stress, the crystallization continued for years until the concrete was completely destroyed. Several thousand kilometers of railway lines were affected, which had an enormous impact on train operations, especially on the heavily used main lines. It took until the end of the 1990s for all the affected lines to be rehabilitated.

A similar reaction was found in 2007 in the concrete of the sleepers used on the Berlin – Hamburg railway line . The renovation took place in 2009.

In May 2009, the Federal Ministry of Transport reported that around 320 kilometers of concrete lanes on the German motorway network were affected. Z. E.g. in Hesse alone 79 kilometers of the heavily frequented federal motorway 5 , in Saxony and Saxony-Anhalt the federal motorway 14 is affected. The Federal Motorway 9 (Munich - Berlin), which was largely renewed by 2006 and is again in need of renovation, is also badly affected . An employee of the Institute for Building Materials Research in Duisburg pointed out that damage caused by "concrete cancer" usually only appears five to ten years after the motorway was completed due to a kind of incubation period . As early as 1992, however, the geologist and mineralogist Gerhard Hempel from Weimar pointed out that the risk of ASR damage could be reduced by choosing the right aggregate.

Individual evidence

  1. ↑ Motorways as graves of millions: concrete cancer eats up tax money , n-tv
  2. ^ Literature review in Christian Öttl: The damaging alkali reaction of broken Upper Rhine aggregates in concrete. (2004) (PDF; 7.3 MB)
  3. ↑ Technical rule alkali guideline: 2013-10 DAfStb guideline - Preventive measures against harmful alkali reactions in concrete (alkali guideline) ( beuth.de ).
  4. ↑ Dietmar Seher u. Tobias Bolsmann: "Concrete cancer" is eating away at highways. Westdeutsche Allgemeine Zeitung , accessed on May 14, 2010 .
  5. ↑ Nicole Preuss: Concrete cancer continues to eat its way through Saxony's highways. Sächsische Zeitung, accessed on April 7, 2010 .
  6. ^ Steffen Winter: Blooming motorways. Spiegel Online , accessed May 13, 2010 .
  7. ↑ Winfried Borchert: Researcher: Concrete cancer on the A14 was avoidable. Mitteldeutsche Zeitung, accessed on May 13, 2010 .

literature

  • Aggregate behavior in concrete - alkali-silica reaction. - Geoscientific. Mitt. Von Thüringen 2000, supplement 9, pp. 153-181 .

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