Execution and Installation
decomposed rock, gypsum and coal are not
suitable because of inferior quality. These rocks
tend to reduce the strength and hardening
qualities of concrete and the quality of weather
resistance as well. The aggregates can be sourced
from river deposits. However, coarse aggregates
of the right quality may not be readily available in
required quantities from the river deposits. Purity
of aggregates is of prime importance to the quality
of concrete. Clayey and loamy materials cling
to the aggregate thereby reducing the bonding
strength of the concrete.
Sand
The sand must not contain more than 3 per
cent of dust and organic impurities like humus,
peat, plant remains, wood, and coal. For a
quick check, a sample when squeezed together
in the hand should crush and flow freely when
let go. If the sample sticks together or leaves a
powdery trace in hand, then it is an indication
of contamination with loam or it contains too
many fine-grained components. The presence
of organic impurities may be checked in the
laboratory.
Water
Water acts as catalyst to the concrete chemical
reactions as well as ensures the workability of the
concrete mix. The points to be noted for water to
be used are as follows:
Suitability: Water which contains organic
matter should not be used. Mineral water
may jeopardize the bonding and the
stability of the concrete. Water containing
sulphates, chlorides, sugar, organic
impurities, oil and grease reduce the
quality of concrete and therefore should
be avoided. Generally, water, which is
suitable for drinking, is also suitable for
concrete.
Water content: The water content of the
concrete mix is the combined sum of the
mixing water and the moisture content
of the aggregates. The water content is
normally specified from the water ratio for
a particular concrete grade.
Consistency and workability: The
consistency of concrete is the degree of
density and liquidity and the workability
is based on the density and liquidity. The
workability is the ease with which the
concrete can be made to fill the formwork
shapes. It depends on the water content
and is controlled from the slump test.
7.5.3 Mixing of concrete
Weigh batching of constituents
The various concrete constituents may be mixed
in proportions defined by their masses. The ratios
of the constituents are given in the standard mix
design in mass per sack mix (50 kg) of cement on
the basis of required strength. The proportions of
sand and aggregates required will then be weighed
using scales for the standard mix proportions. The
mass of the water may also be weighed.
Volume batching of constituents
For remote sites, it may be difficult to use weigh
batching of constituents. In that case, the mix
proportions may be specified per cubic metre of
concrete in which case the mix proportions are
given as volumes to make up specified target
strength. This method is not ideal because of
the imprecision in calculating the exact volume
of constituents, especially with the type of
equipments that are used at site for volume
batching i.e. wooden boxes, shovels, and
wheelbarrows. The "levelling off" and "heaping"
specifications for each volume batch are at the
discretion and judgment of the operator. With this
method, concrete of varying qualities are likely
to be produced. River sand shows a very high
degree of bulking with moisture content. Thus,
the result of concrete with the volume of sand
used in volume batching very much depends on
the sand moisture content.
Concrete mixing methods
The quality and consistency of concrete will depend
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