CONCRETE MIX DESIGN
CONTENTS
1. ABSTRACT
2. SCOPE AND LIMITATIONS
3. INTRODUCTION
4. MIX DESIGN METHODS
5. DESIGN PROCEDURE
6. DATA FOR MIX DESIGN
7. DESIGN ILLUSTRATION
8. CONCLUSION
9. GRAPHS AND TABLES
10. BIBLIOGRAPHY
ABSTRACT
Concrete Mix Design is an art of fixing the proportions of the various ingredients of concrete namely, fine
aggregate, coarse aggregate and cement and water. It is a trial and error method of setting the various
values in order to attain the maximum strength with the given proportions. This paper has followed the
Indian Standards for the mix design and the mix design has been done for M50 concrete. The references to
the IS code have been mentioned as and where used. This also includes design of normal concrete mixes,
both for medium and high strength concrete.
SCOPE
The Indian Standard lays down the recommended procedure for designing concrete mixes for general types
of construction using the concrete materials normally available. The design is carried out for a desired
compressive strength and workability of concrete, using continuously graded aggregate.
LIMITATIONS
·This standard does not include the design of concrete mixes for flexural strength and when gap-graded
aggregates or various admixtures and pozzolana are to be used.
·The characteristic strength of the given concrete mix cannot exceed 52 N/mm2
INTRODUCTION
The design of the concrete mix involves the determination of the most rational proportions of the ingredients
of concrete to achieve a concrete that is workable in its plastic state and will develop the required qualities
when hardened. A properly designed concrete should have minimum possible cement content without
sacrificing the concrete quality in order to make it an economical mix. It is essential to know the complete
details under which freshen and hardened concrete will be used. The proportions of materials will differ with
different requirements. In general a fresh concrete must be workable and hardened concrete must be
durable and have the desired strength and appearance. If fresh concrete is not properly workable it will not
be possible to achieve full compaction with the result strength and durability of hardened concrete will be
significantly hampered. Experience has shown that water cement ratio is the most critical factor in concrete
mix design. Other important factors are: the aggregate-cement ratio, grading of aggregate, aggregate
particle shape and texture and amount of entrained air. During the past few decades considerable have
been made to examine the effects of these factors on the properties of concrete. Hence design of concrete
remains more of an art than a science.
MIX DESIGN METHODS
1. Minimum voids method.
2. Maximum density method.
3. Fineness modulus method.
4. British Mix Design (DOE) method.
5. American Concrete Institute method (ACI method).
6. Indian Standard method.
TERMINOLOGY
CEMENT CONCRETE
A homogeneous mixture of cement, sand, gravel and water is very strong in carrying compressive forces
and hence is gaining increasing importance as a building material throughout the world. It is weak in
resisting tensile forces.
PROPORTIONING OF CONCRETE
The process of selection of relative proportions of cement, sand, coarse aggregate, water so as to obtain a
concrete of desired quality.
CONCRETE MIX DESIGN
Mix design is a process of selecting suitable ingredients of concrete and determining their relative quantities
with the object of producing an economical concrete having certain minimum properties, that is, workability,
strength and durability.
CHARACTERISTIC STRENGTH
It is the value of the strength of the material below which not more than 5% of the test results are expected
to fall.
WORKABILITY
It is used to describe the ease or difficulty with which the concrete is handled, transported and placed
between the forms with minimum loss of homogeneity.
It is also defined as the amount of useful internal work necessary to produce full compaction.
DURABILITY
Durability is defined as the ability of concrete to withstand the environmental conditions to which it is
exposed. It is necessary to emphasize durability in the design and construction of concrete structures.
STANDARD DEVIATION
It is defined as the value of deviation of a given value from the mean calculated from the actual test results.
TARGET STRENGTH
In order that not more than the specified proportions of the test results are likely to fall below the
characteristic strength, the concrete mix has to be designed for a somewhat higher compressive strength.
This strength is called target strength.
AGGREGATE/CEMENT RATIO
It is ratio of aggregate to cement. The various factors involved in selecting the aggregate/cement ratio of a
mix are the desired workability, size, shape, texture and overall grading of the aggregates.
INDIAN STANDARD MIX DESIGN
PROCEDURE
The National Council of cement and building materials, New Delhi formerly Cement Research Institute of
India (CRI) has developed a method of mix design based on extensive tests on Indian materials and
cements.
This method is applicable to design of normal concrete mixes (non-air entrained) for different grades of
cement based on their 28-day strengths.
The following basic data are required for design namely,
1.Characteristic compressive strength (fck) at 28 days.
(ie. Below which only a specified proportion of
test results are allowed to fall)
2.Degree of workability desired.
3.Limitations of water cement ratio and the minimum cement content to ensure adequate durability for the
type of exposure.
4.Type and maximum size of the aggregate to be used.
5.Standard deviation for the compressive strength of concrete.
The step by step procedure of mix proportioning is as follows.
Step 1:
Determine the target or design laboratory strength based on 28-day characteristic strength and standard
deviation referring to IS: 10262-1982 table 1.
The corresponding formula is
ft = fck + K. S
where,
ft = target mean compressive strength at 28 days.
fck = characteristic compressive strength at 28 days.
K = a statistical value depending upon the accepted proportion of low results and
the number of tests.
S = standard deviation
Step 2:
The water cement ratio for the target mean strength is chosen from Fig 1 .The water cement ratio so chosen
is checked against the limiting water cement ratio for the requirements of durability considerations from
appendix A in IS:456-1978 and the lower of the two values is adopted. Fig.1 is based on a large number of
results under Indian conditions but on a given situation, may need slight modifications depending upon the
characteristics of cement available. As such, it is used more as a guide and actual water cement ratio is
determined by means of trial mixes. More precise estimate of the preliminary water cement ratio
corresponding to the target average strength may be made from the relationships shown in Fig.2, using
curves corresponding to the 28 days compressive strength of cement. It is to be noted that cements have
been characterized by its 28 day strength in Fig.2 rather than upon its 7 days strength because 28 days
strength of concrete is found to be better related to the 28 day strength of cement rather than at earlier ages
more so for blended cements. The relationship in Fig.1 is really a mean curve through Fig.2.
Step 3:
The air content (entrapped air amount) is estimated from table 3 from maximum size of aggregate used.
Step 4:
The water content and percentage of sand in total aggregate by absolute volume are next selected from
tables 4 and 5 for medium and high strength concretes respectively for the following standard reference
condition:
Crushed (angular) coarse aggregate, fine aggregate consisting of natural sand conforming to grading zone II
of table 4, IS 383-1970, in saturated surface dry conditions,
Water cement ratio of 0.60 and 0.35 for medium and high strength concretes respectively, and workability
corresponding to compacting factor of 0.80.
Step 5:
For other conditions of workability, water cement ratio, grading of fine aggregate and for rounded
aggregates, adjustments in water content and percentage are made as per Table 6.
Step 6:
The cement content is calculated from the water cement ratio and the final water content arrived after
adjustment. The cement content so calculated is checked against the minimum cement content from the
requirements of durability (appendix A IS 456-1978) and the greater of the two values adopted.
Step 7:
With the quantities of water and cement per unit volume of concrete and the percentage of sand in the total
aggregate already determined, the coarse and fine aggregates content per unit volume of concrete are
calculated from the following equations:
V = W + C + 1 . fa x 1
Sc p Sfa 1000
V = W + C + 1 . fa x 1
Sc 1-p Sfa 1000
Where,
V = absolute volume of fresh concrete
= Gross volume (1m3) minus the volume of entrapped air
Sc= specific gravity of cement,
W = mass of water (kg) per m3 of concrete,
P = ratio of fine aggregate to total aggregate by absolute volume,
fa, Ca = total masses of fine aggregate and coarse aggregate,(kg) per m3 of concrete respectively, and
Sfa, Sca = specific gravities of saturated surface dry fine aggregate and coarse aggregate respectively
DESIGN EXAMPLE
(AN ILLUSTRATIVE EXAMPLE ON CONCRETE MIX DESIGN)
a.DESIGN STIPULATIONS:
1) Characteristic compressive strength required at 28 days = 30 N/mm²
2) Maximum size of aggregates = 20 mm
3) Degree of workability = 0.90 (Compacting factor)
4) Degree of quality control = Good
5) Type of exposure = Mild
b.TEST DATA FOR MATERIALS:
1.Cement used – OPC as per (IS: 269-1976)
2.Specific gravity of cement = 3.15
3. i) Specific gravity of coarse aggregates = 2.60
ii) Specific gravity of fine aggregates = 2.60
4.Water absorption
i) Coarse aggregate = 0.5%
ii) Fine aggregate = 1.0%
5.Free (surface) moisture
i) Coarse aggregate = NIL
ii) Fine aggregate = 2.0%
DESIGN
Step 1:
TARGET MEAN STRENGTH OF CONCRETE
For a tolerance factor of 1.65 and using table 1, target mean strength for the specified characteristic cube
strength = 30 + 1.65 x 6
= 39.9 N/mm2.
Step 2:
SELECTION OF WATER CEMENT RATIO
From fig 1., the free water-cement ratio required for the target mean strength of 39.9 N/mm2 is
0.375(approx). This is lower than the maximum value of 0.65 prescribed for mild exposure.
Step 3:
SELECTION OF WATER AND SAND CONTENT
From table 4, for 20 mm nominal maximum size aggregate and sand conforming to grade II, water content
per meter of concrete = 186 kg and sand content as percentage of total aggregate by absolute volume = 35
%.
For change in values in water cement ratio, compacting factor and sand belonging to zone II, the following
adjustment is required:
ADJUSTMENTS
Change in conditionWater content (%)sand in total aggregate ( % )
For decrease in water cement ratio by (0.6- 0.375) ie. 0.225 0 -4.5
For increase in compacting factor (0.9-0.8) i.e. 0.1 +3 0
+3 percent-4.5 percent
Therefore, required sand content as percentage of total aggregate by absolute volume = 35 – 4.5
= 30.5 %
Required water content = 186 + 186 x 3 / 100 = 186 + 5.58
= 191.6 l/m3
Step 4:
DETERMINATION OF CEMENT CONTENT
Water cement ratio = 0.375
Water = 191.6 litres
=> Cement = 191.6 / 0.375
= 510.93 kg/m3
This cement content is adequate for mild exposure condition, according to appendix 1 of IS: 456-1978
Step 5:
DETERMINATION OF COARSE AND FINE AGGREGATE CONTENT
From table 3, for the specified maximum size of the aggregate of 20mm, the amount of entrapped air in the
wet concrete is 2 %. Taking this into account and applying equations from 3.5.1,
fa = {(.98 x 1000)- 191.6 – (510.93/3.15)}0.305 x 2.60
ca = {(.98 x 1000)- 191.6 – (510.93/3.15)}0.695 x 2.60
·fa = 496.576 kg/m3
·ca = 1131.54 kg/m3
Step 6:
MIX PROPORTION
WATER CEMENT FINE AGGREGATE COARSE AGGREGATE
191.6 lit. 510.93 kg 496.58 kg 1131.54 kg
.375 1 0.97 2.22
CONCLUSION
The concrete mix for various grades of concrete was designed with a good degree of accuracy under the
guidance of INDIAN STANDARD Method of design.
The same was also put forward in the form of mix design software, which gave highly satisfactory results for
all the mix designs ranging from M20 to M40.
BIBLIOGRAPHY
1.INDIAN STANDARD recommended guidelines for concrete mix design IS: 10262-1982, Indian Standards
Institution, New Delhi 1983
2.N.Krishna Raju, DESIGN OF CONCRETE MIXES
3.Neville, PROPERTIES OF CONCRETE, second edition
4.M.S.Shetty, CONCRETE TECHNOLOGY
5.SP23-1982
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