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Structural Concrete Frames 349
6.2 Concrete mixes
The strength and durability of concrete are affected by the voids in concrete caused by poor
grading of aggregate, incomplete compaction or excessive water in the mix.
Water/cement ratio
Workability
The materials used in concrete are mixed with water for two reasons: first, to enable the
reaction with the cement which causes setting and hardening to take place; and second, to
act as a lubricant to render the mix sufficiently plastic for placing and compaction.
About a quarter part by weight of water to one part by weight of cement is required for
the completion of the setting and hardening process. This proportion of water to cement
would result in a concrete mix far too stiff (dry) to be adequately placed and compacted.
About a half by weight of water to one part by weight of cement is required to make a
concrete mix workable. The greater the proportion of water to cement used in a concrete
mix, the weaker the ultimate strength of the concrete. The principal reason for this is that
the water, in excess of that required to complete the hardening of the cement, evaporates
and leaves voids in the concrete, which reduce its strength. It is usual practice, therefore,
to define a ratio of water to cement in concrete mixes to achieve a dense concrete. The
water/cement ratio is expressed as the ratio of water to cement by weight, and the limits
of this ratio for most concrete lie between 0.4 and 0.65. Outside these limits there is a great
loss of workability below the lower figure and a loss of strength of concrete above the upper
figure.
Water-reducing admixtures
The addition of 0.2% by weight of calcium lignosulphonate, commonly known as ‘lignin’,
to cement will reduce the amount of water required in concrete by 10% without loss of
workability. This allows the cement content of a concrete mix to be reduced for a given
water/cement ratio. Calcium lignosulphonate acts as a surface-active additive that disperses
the cement particles, which then need less water to lubricate and disperse them in concrete.
Water-reducing admixtures such as lignin are promoted by suppliers as densifiers, hardeners,
water proofers and plasticisers on the basis that the reduction of water content leads
to a denser concrete due to there being fewer voids after the evaporation of water. To ensure
that the use of these admixtures does not adversely affect the durability of a concrete, it is
usual practice to specify a minimum cement content.
Nominal mixes
Volume batching
The constituents of concrete may be measured by volume in batch boxes in which a
nominal volume of aggregate and a nominal volume of cement are measured for a nominal
mix, as for example, in a mix of 1 : 2 : 4 of cement : fine : coarse aggregate. A batch box usually
takes the form of an open top wooden box in which volumes of cement, fine and coarse
aggregate are measured separately for the selected nominal volume mix. For a mix such as
1 : 2 : 4, one batch box will suffice, the mix proportions being gauged by the number of fillings
of the box with each of the constituents of the mix.