Alkali Reactive Aggregates. What Happens When Alkali Cement is Low Essay

Alkali Reactive Aggregates. What Happens When Alkali Cement is Low Essay

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Considerable bad experience has been encountered when alkali reactive aggregates have been used.Low alkali cement is normally required to protect against disruptive
Expansion of concrete which occurs generally due to alkali-aggregate reaction.(a
chemical reaction between alkali’s in the cement and reactive aggregates.Another
means of of controlling alkali-aggregate reactions is by the use of suitable fly ash
or natural pozzolan.If a highly reactive aggregate have to be used then it is necessary
to use a low alkali cement and a pozzolan.

Another design recommendation is the type of the cement to be used.Type-2 cement is
normally used by the Bureau of Reclamation in mass concrete dams.Limitations on the
Heat of hydration of this cement are specified when determined necessary to minimize cracking in the concrete structure.Use of Type-2 cement will generally reduce the heat of
hydration of cement to an acceptable level, particularly when type-2 cement is used in
conjunction with other methods of heat reduction.These include use of lower cement content and using pozzolan as part of the cementitious material,use of pipe cooling system and use of a specified maximum temperature for placing of concrete,which may be as low as 50 degF.Use of all or some of this methods usually reduce or eliminate the need for stringent limitations on heat of hydration of concrete.However a limitation
of 58% on Tricalcium Aluminate plus Tricalcium Silicate content of Type-2 required where
Heat of hydration must be kept low.Further limitation on heat of hydration,if more stringent control of heat is needed can be obtained with a Type-2 cement by providing a maximum limitation on the cement of 70 calories per gram at 7 days or 80 calories per gram at 28...

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Conveying concrete whose aggregate size is less than 7.5 cm.Pumping of concrete is to be done in such way that concrete without air pockets or interruptions is delivered.

Placing of Concrete:

Concrete has to be placed at a uniform rate through out the placing of the concrete.
Each layer is compacted well before placing of the other layer.Generally thickness of
layers shall not exceed the following limits.

Reinforced Concrete-25 cm.
Vibrated Concrete-45 cm.
Hand compacted mass concrete-30 cm.

Concrete is placed vertically above the other layer so as to avoid damage,segregation
and air pockets.Compaction should be done as quickly as possible after placing.When
concrete is placed in the forms temperature may go high as suggested by some previous studies so measures have to be taken in order to reduce such effects.They are

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