Aggregates {Indian Standard 456:2000}
Aggregates :-
Aggregates shall comply with the requirements of Indian Standard 383. As far as possible preference shall be given to natural aggregates.
1. Other type of aggregates such as slag and crushed overburnt brick or tile, which may be found suitable with regard to strength, durability of concrete and freedom from harmful effects may be used for plain concrete members, but such aggregates should not contain more than 0.5 % of sulphates as SO3 and should not absorb more than 10% of their own mass of water.
2. Heavy weight aggregates or light weight aggregates such as bloated clay aggregates and sintered fly ash aggregates may also be used provided the authority is satisfied with the data on the properties of concrete made with them.
3. Size of aggregates :- The nominal maximum size of coarse aggregate should be as large as possible within the limit specification but in no case greater than 1/4 of the minimum thickness of the member, provided that the concrete can be placed without difficulty so as to surround all reinforcement thoroughly and fill the corners of the form, For most work, 20 mm aggregates is suitable. Where there is no restriction to the flow of concrete into sections, 40 mm or larger size may be permitted. In concrete elements with thin section, closely spaced reinforcement or small cover, consideration should be given to the use of 10 mm nominal size. Plums above 160 mm and up to any reasonable size may be used in plain concrete work up to a maximum limit of 20 % by volume of concrete when specifically permitted by the authority. The plums shall be distributed evenly and shall be not closer than 150 mm from the surface.
4. For heavily reinforced concrete members as in the case of ribs of main beams, the nominal maximum size of the aggregate should usually be restricted to 5 mm less than the maximum clear distance between the main bars or 5 mm less than the minimum cover of the reinforcement whichever is smaller.
5. Coarse and fine aggregate shall be batched separately. All in aggregate may be used only where specifically permitted by the authority.
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