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strength should be thoroughly mixed and placed with as little ramming as possible to
produce a porous concrete.
(f) In proportioning concrete or mortar which is to be subjected to freezing temperatures
shortly after placement, a minimum amount of water and a quick-setting cement should
be used.
(g) Concrete for road construction should be made from a carefully graded, hard tough
aggregate bound together with as small a proportion of rich mortar as is consistent with
the required workability, strength, and imperviousness. In locations where resistance to
freezing and thawing is required, the concrete should have 3–6 per cent of entrained air.
The principal methods used in scientific proportioning of mixes are based upon relationships
between properties and ratio of cement to voids in the mix, or on the relationship between
properties and the ratio of water to cement in the mix.
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Before the engineer can begin to design a concrete mix the following information form the site
of work is required:
(i) Grade of concrete: The grade M 20, M 25 connotes characteristic strength, fck of 20 N/mm^2 ,
25 N/mm^2 , respectively, and standard deviation based on the degree of control to be
exercised on site.
(ii) Type of cement: The grade of Ordinary Portland Cement (OPC) such as 33, 43, or 53 grade.
Portland Pozzolana Cement (PPC) to relevant IS specifications.
(iii) Type and size of aggregate: Natural sand, crushed stone, gravel etc. conforming to
IS:383–1970, quoting the source of supply.
(iv) Nominal maximum size aggregate (MSA): 40 mm, 20 mm, 10 mm, as per IS:383–1970.
(v) Maximum/minimum cement content (kg/m^3 ): This is required for durability/considerations.
(vi) Type of mixing and curing water: Whether fresh potable water, seawater, ground water is
to be used.
(vii) Maximum free water-cement ratio by weight: This is required for considerations of strength
and/or durability for different exposures and to meet appearance and other requirements,
Figs 11.1 and 11.2, Tables 11.1, 11.2 and 11.3, respectively.
(viii)Degree of workability of concrete: This is dependent on placing and compaction conditions.
Table 10.6.
(ix) Air content: This is inclusive of entrained air.
(x) Type of admixture used.
(xi) Maximum/ minimum density of concrete.
(xii) Maximum/minimum temperature of fresh concrete.
The mix should be designed to obtain the concrete having the desired durability (Tables 11.l,
11.2 and 11.3); workability (Table 10.6); and characteristic compressive of strength fck not less
than that for the particular grade.
The curves shown in Fig. 11.1 show 7-days cement strengths versus 28-days concrete strengths
for 6 different water-cement ratios using Indian cements, the band (ranging 1 N/mm^2 ) imply
that the relationship is not precise but is only of a general natural to be adopted as a guideline.
These bands are also applicable to good quality PPC.