Alkali Silica Reaction ASR

The reaction mechanism between alkah and reactive sihceous aggregate is complex. It requires the presence of hydroxyl, alkali metal and calcium ions and water. SimpHfying, the hydroxyl ions provoke the destruction of atomic bonds of the siliceous compounds, the alkah ions react with Si(OH)4 complexes to form a fluid (Na,K)-Si-OH gel, which then exchanges Na and K for Ca, upon which the gel solidifies. The solidified gel absorbs water and swells. 

This sweUing can induce tensile stresses within the concrete and lead to the appearance of cracking (with a pattern that depends on the geometry of the stracture, the layout of the reinforcement and the level of tensile stress). For example, in floors or in foundations, elements that are usually only tightly reinforced and do not have significant tensile stress, the typical pattern of cracking is the so-called map cracking with randomly distributed cracks, from which whitish gel may leak. Another typical phenomenon is pop-outs, that is, the expulsion of small portions of concrete. 

Development of ASR may be very slow and its effects may show even after long periods (up to several decades) [17]. The principal factors influencing this reaction are analysed below. 

The extent of reaction depends on the amount of reactive silica present in the aggregate mix. As a rule there is a range of concentrations of reactive components over which expansion occurs, with a maximum that is called pessimum. As the quantity of reactive silica increases beyond the value of the pessimum, expansion decreases gradually until it becomes negligible. For opal, for example, the reactive range is 2-10% by volume, with the pessimum at about 5%, while for other, less reactive, types of silica it may lie at contents of about 30%. For mixes of aggregates from various sources, there may be no pessimum at all. 

Environment. ASR can occur only in moist environments it has in fact been observed that in environments with a relative humidity below 80-90%, alkahs and reactive aggregate can co-exist without causing any damage. The temperature influences the reaction, by favouring it as the temperature increases. 

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It is important to repeat that we are only discussing corrosion repairs. Other problems, structural defects alkali-silica reaction (ASR) and so on may also need to be addressed at the same time and rehabilitation systems must be an integrated package of compatible, complementary systems. Some of the issues of compatibility are discussed later. Specifically the compatibility of patch repair systems and ASR with electrochemical treatments will be addressed. 

The undulatory extinction of strained quartz has become important in engineering geology as it serves as an indicator for aggregates to undergo Alkali-Silica Reaction (ASR). The amount of strained quartz found in the rock samples varied between 18 and 50% and their respective undulatory extinction angle varied from 16° to 36° Table 3. 

By replacing the cement Portland with groimd glass type II the expansion rate of the specimens decrease aroimd 15%. According to last researches materials with pozzolanic characteristics can suppress the alkali-silica reaction (ASR) and decrease the expansion... 

The characterisation of damage, e.g. in aggregates due to alkali-silica reaction (ASR) (this is discussed later)... 

AR = alumina ratio (alumina modulus). ASR = alkali silica reaction. LSF = lime saturation factor. SR = silica ratio (silica modulus). C, = analytical (total) concentration of x, irrespective of species, [x] = concentration of species x. x = activity of species x. RH = relative humidity. =... 

Side effects. During chloride extraction, hydroxyl ions are formed around the reinforcing steel, locally increasing the pH and sodium and potassium ions are enriched around the steel. These changes might stimulate aUcah-silica reaction (ASR, Section 3.4). In the framework of COST 521, the possibility of ASR was checked as a side-effect of chloride extraction [28,36,80,81]. The aggregates studied were reactive and the alkali content of the cement was just below the critical values. The results obtained with non-carbonated concrete showed that, under the worst conditions, chloride extraction induced concrete expansion, but no cracking was observed. 

It has been shown earlier that cathodic protection creates hydroxyl ions and will also attract positive ions such as sodium and potassium to the steel. This will increase the alkalinity around the reinforcing bar. In principle this could cause ASR or accelerate ASR in susceptible mixes. This has been demonstrated in the laboratory. However, there are no recorded cases of ASR being caused or accelerated by CP in field structures. In a review of field structures by SHRP, a structure with ASR showed no acceleration in the ASR in areas where CP was applied. The issue is also discussed in Mietz (1998). In the European standard on cathodic protection BSEN 12696(2000) Appendix A.5 states that cathodic protection applied in accordance with this standard has been demonstrated to have no influence on alkali silica reaction/alkali aggregate reaction (ASR/AAR). ... 

Handbook for the Identification of Alkali-Silica Reactivity in Higinvay Structures. Provides guidance for the field identification of alkali-silica reactivity (ASR) in Portland cement concrete structures such as highways and bridges. ASR development is assessed on two bases the occurrence and disposition of cracking and displacement of concrete, and the presence of reaction products from ASR. Colour photographs. 48 pages. SHRP-C-315, 10... 

One approach to preventing expansion due to ASR consists in lowering the alkali content in the concrete mix to sufficiently low concentrations. It is generally accepted that an alkali-silica/silicate reaction in concrete made with Portland cement will not occur if the content of equivalent Na20 (defined as Na2O =Na2O+0.66K2O) in the mix does not exceed 4 or even 3 kg/m. Such low alkali concentrations are usually not achievable with ordinary Portland cement, but may be achieved if a low-alkali Portland cement (see section 2.10) is used instead. 

Three conditions are essential to ASR development alkalis, reactive silica and sufficient moisture. If one of these conditions is not present, the reaction will not occur. Therefore, it is very important to evaluate the reactivity of these rocks before using them as aggregates, by taking preventive measures such as the use of puzzolanic materials. 

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