Building Materials, Third Edition

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passing a 150 mm 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.

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2
2e—X2Reactive material have been found to have serious effects if
present in small quantities but not if it constitutes the whole of the aggregate.

r2e—2g 2gX2If the cement contains less than 0.4 per cent alkalis (computed
as Na 2 O) 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.

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2
2w : 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.

„—2g  X2The favourable temperature for the reaction is 10-38°C.

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2ƒ™2x E—™2e—X2Aggregate can be identified by petrographic examination.
The mortar bar test and the chemical test are used.

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2…2v $2e—2gX2Cements with alkali less than 0.6 per cent should be used.

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2g  2w X2Old 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.

f
2€&& ——X2The 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