1. Introduction
The process of selecting suitable
ingredients of concrete and determining their relative amounts with the
objective of producing a concrete of the required, strength, durability, and
workability as economically as possible, is termed the concrete mix design. The
proportioning of ingredient of concrete is governed by the required performance
of concrete in 2 states, namely the plastic and the hardened states. If the
plastic concrete is not workable, it cannot be properly placed and compacted.
The property of workability, therefore, becomes of vital importance. The
compressive strength of hardened concrete which is generally considered to be
an index of its other properties, depends upon many factors, e.g. quality and
quantity of cement, water and aggregates; batching and mixing; placing,
compaction and curing. The cost of concrete is made up of the cost of
materials, plant and labour. The variations in the cost of materials arise from
the fact that the cement is several times costly than the aggregate, thus the
aim is to produce as lean a mix as possible. From technical point of view the
rich mixes may lead to high shrinkage and cracking in the structural concrete,
and to evolution of high heat of hydration in mass concrete which may cause
cracking. The actual cost of concrete is related to the cost of materials
required for producing a minimum mean strength called characteristic strength
that is specified by the designer of the structure. This depends on the quality
control measures, but there is no doubt that the quality control adds to the
cost of concrete. The extent of quality control is often an economic
compromise, and depends on the size and type of job. The cost of labour depends
on the workability of mix, e.g., a concrete mix of inadequate workability may
result in a high cost of labour to obtain a degree of compaction with available
equipment.
2. Requirements of concrete mix
design
The requirements which form the basis of
selection and proportioning of mix ingredients are :
a ) The minimum compressive strength
required from structural consideration
b) The adequate workability necessary for
full compaction with the compacting equipment available. c) Maximum
water-cement ratio and/or maximum cement content to give adequate durability
for the particular site conditions
d) Maximum cement content to avoid
shrinkage cracking due to temperature cycle in mass concrete.
2.1 Types of Mixes
i. Nominal Mixes In the past the
specifications for concrete prescribed the proportions of cement, fine and coarse
aggregates. These mixes of fixed cement-aggregate ratio which ensures adequate
strength are termed nominal mixes. These offer simplicity and under normal
circumstances, have a margin of strength above that specified. However, due to
the variability of mix ingredients the nominal concrete for a given workability
varies widely in strength.
ii. Standard mixes The nominal mixes of
fixed cement-aggregate ratio (by volume) vary widely in strength and may result
in under- or over-rich mixes. For this reason, the minimum compressive strength
has been included in many specifications. These mixes are termed standard
mixes. IS 456-2000 has designated the concrete mixes into a number of grades as
M10, M15, M20, M25, M30, M35 and M40. In this designation the letter M refers
to the mix and the number to the specified 28 day cube strength of mix in
N/mm2. The mixes of grades M10, M15, M20 and M25 correspond approximately to
the mix proportions (1:3:6), (1:2:4), (1:1.5:3) and (1:1:2) respectively.
iii. Designed Mixes
In these mixes the performance of the
concrete is specified by the designer but the mix proportions are determined by
the producer of concrete, except that the minimum cement content can be laid
down. This is most rational approach to the selection of mix proportions with
specific materials in mind possessing more or less unique characteristics. The
approach results in the production of concrete with the appropriate properties
most economically. However, the designed mix does not serve as a guide since this
does not guarantee the correct mix proportions for the prescribed performance.
For the concrete with undemanding performance nominal or standard mixes
(prescribed in the codes by quantities of dry ingredients per cubic meter and
by slump) may be used only for very small jobs, when the 28-day strength of
concrete does not exceed 30 N/mm2. No control testing is necessary reliance
being placed on the masses of the ingredients.
3. Factors affecting the choice of mix
proportions
The various factors affecting the mix
design are:
3.1. Compressive strength It is one of the
most important properties of concrete and influences many other describable
properties of the hardened concrete. The mean compressive strength required at
a specific age, usually 28 days, determines the nominal water-cement ratio of
the mix. The other factor affecting the strength of concrete at a given age and
cured at a prescribed temperature is the degree of compaction. According to
Abraham‟s law the strength of fully compacted concrete is inversely
proportional to the water-cement ratio.
3.2. Workability The degree of workability
required depends on three factors. These are the size of the section to be
concreted, the amount of reinforcement, and the method of compaction to be
used. For the narrow and complicated section with numerous corners or
inaccessible parts, the concrete must have a high workability so that full
compaction can be achieved with a reasonable amount of effort. This also
applies to the embedded steel sections. The desired workability depends on the
compacting equipment available at the site.
3.3. Durability The durability of concrete
is its resistance to the aggressive environmental conditions. High strength
concrete is generally more durable than low strength concrete. In the
situations when the high strength is not necessary but the conditions of
exposure are such that high durability is vital, the durability requirement
will determine the water-cement ratio to be used.
3.4. Maximum nominal size of aggregate In
general, larger the maximum size of aggregate, smaller is the cement
requirement for a particular water-cement ratio, because the workability of
concrete increases with increase in maximum size of the aggregate. However, the
compressive strength tends to increase with the decrease in size of aggregate.
IS 456:2000 and IS 1343:1980 recommend that the nominal size of the aggregate
should be as large as possible.
3.5. Grading and type of aggregate
The grading of aggregate influences the mix proportions for a specified
workability and watercement ratio. Coarser the grading leaner will be mix which
can be used. Very lean mix is not desirable since it does not contain enough
finer material to make the concrete cohesive. The type of aggregate influences
strongly the aggregate-cement ratio for the desired workability and stipulated
water cement ratio. An important feature of a satisfactory aggregate is the
uniformity of the grading which can be achieved by mixing different size
fractions.
3.6. Quality Control The degree of control
can be estimated statistically by the variations in test results. The variation
in strength results from the variations in the properties of the mix
ingredients and lack of control of accuracy in batching, mixing, placing,
curing and testing. The lower the difference between the mean and minimum
strengths of the mix lower will be the cement-content required. The factor
controlling this difference is termed as quality control.
4 Mix Proportion designations
The common method of expressing the
proportions of ingredients of a concrete mix is in the terms of parts or ratios
of cement, fine and coarse aggregates. For e.g., a concrete mix of proportions
1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or
the mix contains one part of cement, two parts of fine aggregate and four parts
of coarse aggregate. The proportions are either by volume or by mass. The
water-cement ratio is usually expressed in mass.
4.1 Factors to be considered for mix design
• The grade designation giving the characteristic strength requirement of
concrete. • The type of cement influences the rate of development of
compressive strength of concrete. • Maximum nominal size of aggregates to be
used in concrete may be as large as possible within the limits prescribed by IS
456:2000. • The cement content is to be limited from shrinkage, cracking and
creep. • The workability of concrete for satisfactory placing and compaction is
related to the size and shape of section, quantity and spacing of reinforcement
and technique used for transportation, placing and compaction.
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