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 ...view middle of the document...
The amount and type of cement used must be compatible with the strength,durability and temperature requirements of the structure.
Admixtures are incorporated into the mix design as needed to obtain economy,
workability,or certain other desired objectives such as permitting placement over
extended period of time.Admixtures have varying effects on concrete and should be evaluated after thorough study of their effects.Most commonly used admixturees in the design of dam are accelerators,air-entraining agents,water reducing,self controlling admixtures and pozzolans.Calcium chloride which has been used a long time should not be used as an accelerator where aluminium or galvanized metal work is embedded.
When accelerators are used added care will be necessary to prevent cold joints during placing of concrete.Air entraining agents are used to increase the durability of the structure when it is exposed to cycles of freezing and thawing.Good quality pozzolans can be used as replacement for cement without sacrificing later age strength.Pozzolans are less expensive than cement and economically good to use.Since the properties of pozzolans vary widely if it has to be used in the construction of dam it is necessary obtain one that will not introduce adverse qualities into the concrete.Concrete containing pozzolans requires thorough curing to assure good resistance to freezing and thawing
The water used in concrete mix should be free of silt,organic matter,alkali,slats and other impurities.Water containing objectionable amount of chlorides and sulfates is particularly undesirable as they prevent the full development of the desired strength.
Inherently concrete is not a homogeneous material.An approach to homogeneous concrete is made by careful and control of batching and mixing operations which will
result in concrete of uniform quality throughout the structure Because of its on strength
water used in mixing is carefully controlled.This control should start in the stockpiles of
aggregate where an effort must be kept to obtain an uniform and stable moisture content..Water to be added to mix is done by some apparatus which will assure that correct...