Alkali-Aggregate reaction

Introductuon. 

The aggregates were considered to be inert material till 1940. Considerable trouble has been experienced through extensive pop-outs and cracking in a fairly close pattern, of concrete work probably a year or more after the concrete has been cast.

The trouble is due to reaction between silica in aggregate and alkalis in the cement. In some cases alkalis, mainly from the cement supplemented by alkalis in the aggregate, react with carbonates in the aggregate to produce similar result. The types of rocks which contain reactive constituents include traps, andesites, rhyolites, siliceous limestone and certain types of sandstones. 

The reactive components may be in the form of opals, cherts, chalcedony, volcanic glass (excepting basaltic glasses), zeolites, and tridmite.

Mechanism. 

The precise causes and behaviour of the phenomenon are, however, still a little obscure.  A reactive aggregate, if in finely ground state, will inhibit the action. The dividing line between aggregate which will cause trouble and that which will not, appears to be retained on and passing a 150  m sieve. 

The precise explanation of this is not quite clear but the action is probably akin to that of lime, the presence of which if fine and thoroughly mixed with the cement has no harmful effect on the concrete, but if present in large lumps it subsequently slakes, if it becomes damp, with disruptive effect. Reaction between cement and aggregate can be of two types: reaction between the alkalis of the cement and either silicas or carbonates in the aggregate. The former appears to be more common. The alkali-aggregate reaction takes place only in the presence of water or water vapour. 

The water forms strong caustic solute with alkalis of cement. This caustic liquid attacks reactive silica to form alkali-silica-gel of unlimited swelling type. When the conditions are congenial, progressive manifestation of concrete, particularly in thin concrete sections, results cracks and eventual failure of concrete structures. Conspicuous effect may not be notified in mass concrete sections. 

The formation of pattern cracks result in subsequent loss in strength and elasticity because of stresses induced by the growth of silica gel. 

Many destructive forces become operative on the concrete disrupted by alkali-aggregate reaction which further hasten the total disintegration of concrete. 

The carbonate rocks which are reactive range from pure limestones to pure dolomites and those rocks which have excess of Mg or Ca ions over the ideal proportion are likely to give trouble.  

Apparently there are several different types of alkali-carbonate reactions not all of which are expansive and harmful. The reaction zones are up to 2 mm wide around the aggregate particles and are visible on polished surface in thin petrographic sections.

Factors affecting the alkali- aggregate reactions. 

1. Reactive type of aggregated. 

Reactive material have been found to have serious effects if present in small quantities but not if it constitutes the whole of the aggregate.

2. High alkali content cement. 

 If the cement contains less than 0.4 per cent alkalis (computed as Na2O) no expansion or disruptive effect is likely even with a quite highly reactive aggregate, but due to difficulties of manufacture it is not usual to specify an alkali content of less than 0.6 per cent.

3. Availability of moisture. 

Progress of the alkali-aggregate reaction takes place only in the presence of water. That is why this destructive effect is not observed in the interior of mass concrete. 

4. Temperature conditions. 

The favourable temperature for the reaction is 10-38°C.  

Control of alkali-aggregate reaction. 

1. By selecting non reactive aggregate. 

 Aggregate can be identified by petrographic examination. The mortar bar test and the chemical test are used.

2. By using low alkali cements. 

Cements with alkali less than 0.6 per cent should be used.

3. By controlling moisture. 

Old concrete should not be allowed to come in contact with water. The best way is to apply mortar with water proofing agents on concrete surface.

4. By puzzolanas. 

The aggregates are found to be reactive when they contain silica in a particular proportion and fineness.

 When fly-ash or surkhi or crushed stone dust is added this optimum condition of silica being in particular proportion and fineness is disturbed and the aggregates turn to be innocuous.

5. By air entraining agents. 

 The alkali-silica-gel imparts osmotic pressure over the set cement gel and this is mainly responsible for formation of cracks. When air entraining agents are added they absorb the osmotic pressure and control the expansion.

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