Concrete Mix Design
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.
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.
Types of Mixes
1. 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.
2. 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.
3. 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.
Factors affecting the choice of mix proportions
The various factors affecting the mix design are:
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.
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. 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.
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.
5. Grading and type of aggregate
The grading of
aggregate influences the mix proportions for a specified workability and
water-cement 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.
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.
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
Factors to be considered for mix design
ð The grade designation giving the characteristic strength requirement of concrete.
ð The type of cement influences the rate of development of compressive strength of concrete.
ð Maximum
nominal size of aggregates to be used in concrete may be as large as
possible within the limits prescribed by IS 456:2000.
ð The cement content is to be limited from shrinkage, cracking and creep.
ð 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.
Procedure
1. Determine the mean target strength ft from the specified characteristic compressive strength at 28-day fck and the level of quality control.
ft = fck + 1.65 S
where S is the standard deviation obtained from the Table of approximate contents given after the design mix.
2. Obtain
the water cement ratio for the desired mean target using the emperical
relationship between compressive strength and water cement ratio so
chosen is checked against the limiting water cement ratio. The water
cement ratio so chosen is checked against the limiting water cement
ratio for the requirements of durability given in table and adopts the
lower of the two values.
3. Estimate the amount of entrapped air for maximum nominal size of the aggregate from the table.
4. Select
the water content, for the required workability and maximum size of
aggregates (for aggregates in saturated surface dry condition) from
table.
5. Determine
the percentage of fine aggregate in total aggregate by absolute volume
from table for the concrete using crushed coarse aggregate.
6. Adjust
the values of water content and percentage of sand as provided in the
table for any difference in workability, water cement ratio, grading of
fine aggregate and for rounded aggregate the values are given in table.
7. Calculate
the cement content form the water-cement ratio and the final water
content as arrived after adjustment. Check the cement against the
minimum cement content from the requirements of the durability, and
greater of the two values is adopted.
8. From
the quantities of water and cement per unit volume of concrete and the
percentage of sand already determined in steps 6 and 7 above, calculate
the content of coarse and fine aggregates per unit volume of concrete
from the following relations:
where V = absolute volume of concrete
= gross volume (1m3) minus the volume of entrapped air
Sc = specific gravity of cement
W = Mass of water per cubic metre of concrete, kg
C = mass of cement per cubic metre of concrete, kg
p = ratio of fine aggregate to total aggregate by absolute volume
fa, Ca = total masses of fine and coarse aggregates, per cubic metre of concrete, respectively, kg, and
Sfa, Sca = specific gravities of saturated surface dry fine and coarse aggregates, respectively
9. Determine the concrete mix proportions for the first trial mix.
10.
Prepare the concrete using the calculated proportions and cast three
cubes of 150 mm size and test them wet after 28-days moist curing and
check for the strength.
11. Prepare trial mixes with suitable adjustments till the final mix proportions are arrived at.
1. REQUIREMENTS
a) Specified minimum strength = 20 N/Sq mm
b) Durability requirements
i) Exposure Moderate
ii) Minimum Cement Content = 300 Kgs/cum
c) Cement
(Refer Table No. 5 of IS:456-2000)
i) Make Chetak (Birla)
ii) Type OPC
iii) Grade 43
d) Workability
i) compacting factor = 0.7
e) Degree of quality control Good
2. TEST DATA FOR MATERIALS SUPPLIED
a) CEMENT
i) Specific gravity = 3.05
ii) Avg. comp. strength 7 days = 46.5 more than 33.0 OK
28 days = 55.0 more than 43.0 OK
b) COARSE AGGREGATE
i) 20mm Graded
Type Crushed stone aggregate
Specific gravity = 2.68
Water absorption = 1.46
Free (surface) moisture = 0
c) FINE AGGREGATE (Coarse sand)
i) Type Natural (Ghaggar)
Specific gravity = 2.6
Water absorption = 0.5
Free (surface) moisture = 1.4
3. TARGET MEAN STRENGTH (TMS)
a) Statistical constant K = 1.65
b) Standard deviation S = 4.6
Thus, TMS = 27.59 N/Sqmm
4. SELECTION OF W/C RATIO
a) As required for TMS = 0.5
b) As required for ‘Moderate’ Exposure = 0.55
Assume W/c ratio of 0.5
5. DETERMINATION OF WATER & SAND CONTENT
For W/C = 0.6
C.F. = 0.8
Max. Agg. Size of 20 mm
a) Water content = 186 Kg/cum
b) Sand as percentage of total aggregate by absolute volume = 35 %
Thus,
Net water content = 180.42 Kg/cum
Net sand percentage = 33 %
6. DETERMINATION OF CEMENT CONTENT
W/c ratio = 0.5
Water content = 180.42 Kg/cum
Thus, Cement content = 360.84 Kg/cum Adequate for moderate exposure Say 360 Kg/cum
7. DETERMINATION OF COARSE AND FINE AGGREGATE CONTENT
Assume entrapped air as 2 %
Thus,
0.98 cum = [180.42+360/3.05 + {1/0.33}*{fa/2.6}]/1000
& 0.98 cum = [180.42+360/3.05 + {1/0.67}*{Ca/2.68}]/1000
Hence,
fa = 584 Kg/cum
Ca = 1223.8 Kg/cum
The final mix proportions of M-20 grade of concrete become:-
a) Specified minimum strength = 20 N/Sq mm
b) Durability requirements
i) Exposure Moderate
ii) Minimum Cement Content = 300 Kgs/cum
c) Cement
(Refer Table No. 5 of IS:456-2000)
i) Make Chetak (Birla)
ii) Type OPC
iii) Grade 43
d) Workability
i) compacting factor = 0.7
e) Degree of quality control Good
2. TEST DATA FOR MATERIALS SUPPLIED
a) CEMENT
i) Specific gravity = 3.05
ii) Avg. comp. strength 7 days = 46.5 more than 33.0 OK
28 days = 55.0 more than 43.0 OK
b) COARSE AGGREGATE
i) 20mm Graded
Type Crushed stone aggregate
Specific gravity = 2.68
Water absorption = 1.46
Free (surface) moisture = 0
i) Type Natural (Ghaggar)
Specific gravity = 2.6
Water absorption = 0.5
Free (surface) moisture = 1.4
a) Statistical constant K = 1.65
b) Standard deviation S = 4.6
Thus, TMS = 27.59 N/Sqmm
4. SELECTION OF W/C RATIO
a) As required for TMS = 0.5
b) As required for ‘Moderate’ Exposure = 0.55
Assume W/c ratio of 0.5
5. DETERMINATION OF WATER & SAND CONTENT
For W/C = 0.6
C.F. = 0.8
Max. Agg. Size of 20 mm
a) Water content = 186 Kg/cum
b) Sand as percentage of total aggregate by absolute volume = 35 %
Thus,
Net water content = 180.42 Kg/cum
Net sand percentage = 33 %
W/c ratio = 0.5
Water content = 180.42 Kg/cum
Thus, Cement content = 360.84 Kg/cum Adequate for moderate exposure Say 360 Kg/cum
7. DETERMINATION OF COARSE AND FINE AGGREGATE CONTENT
Assume entrapped air as 2 %
Thus,
0.98 cum = [180.42+360/3.05 + {1/0.33}*{fa/2.6}]/1000
& 0.98 cum = [180.42+360/3.05 + {1/0.67}*{Ca/2.68}]/1000
Hence,
fa = 584 Kg/cum
Ca = 1223.8 Kg/cum
The final mix proportions of M-20 grade of concrete become:-
Note: 1 The above recommended mix design must be verified, by actual cube tests.
2 The mix design is based on the quality and grading of the materials actually supplied, by the client.
Any variation in quality and gradation will result in changes in the mix design.
2 The mix design is based on the quality and grading of the materials actually supplied, by the client.
Any variation in quality and gradation will result in changes in the mix design.
This mix design was submitted by a regular contributor to this site. We are thankful to him for his excellent service.
About The Author:-
Sp.Aswinpalaniappan M.E.,*
Member of American Concrete Institute
Sri Raaja Raajan College of Engineering and Technology
Karaikudi, Tamil Nadu 630301
Member of American Concrete Institute
Sri Raaja Raajan College of Engineering and Technology
Karaikudi, Tamil Nadu 630301
