CONCRETE MIX DESIGN

It is the process of selecting suitable ingredients of concrete and determining their

relative proportions with the object of producing concrete of certain maximum strength &
durability as economical as possible.

Variables in proportioning
In connection with specifying a concrete mix, four variable factors to be considered are:
1. Water- Cement ration,
2. Cement content or cement- aggregate ratio.
3. Gradation of aggregates,
4. Consistency.
Usually two pr three factors are specified, and the others are adjusted to give minimum
workability and economy
Water- Cement ratio expresses the dilution of the paste i.e., cement content varies directly
with the amount of paste.
Gradation of aggregate is controlled by varying the amount of given fine and coarse
aggregate.
Consistency is established by practical requirements of placing

Briefly, the effort in proportioning is to use minimum amount of paste ( and therefore
cement) that will lubricate the mass while fresh and after hardening will bind the
aggregate particles together and fill the space between them. Any excess of paste
involves greater cost, greater drying shrinkage, greater susceptibility to percolation of
water and therefore attack by aggressive waters and weathering action. This is achieved
by mi9nimzing the voids by good gradation.
ACI method of mix design
- It is the best and most popular method of rational mix design.

The following mix design procedure is based on the ACI manual of concrete of
concrete practice standard ACI 211.1-91, using metric units,

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Step 1 - Choose slump from Table 1-1a.
Step 2 – Choose the maximum size of the aggregate. The largest size that is
economically available in the area is usually chosen, subject to the following maximum:
 One- fifth of the width of the forms
 One –third of the depth of the slabs
 Three – fourths of the space between the reinforcing bars
Step 3 - estimate the amount of mixing water and air required from Table 1- 1b.
Admixtures, if used, are added to the water
Step 4 – Select the W?C ratio from Table 1 – 4a (for design strength), and from Table 1
-4b (for exposure ). Use the smaller.
Step 5 – Calculate the cement content from the W/C ration and water-content values.
Step 6 – Estimate the proportion of coarse aggregate from Table1-3, and its mass using
the dry- rodded density.
Step 7 – Estimate the mass of fine aggregate using the estimated total mass shown in
Table1-2.
Step 8 – Calculate the adjustments required for aggregate moisture. Usually the
aggregates are wet and therefore the mass of water that they contain, both absorbed and
surface water must be added to the required mass of aggregates, and the mass of surface
water subtracted from & the mass of absorption required will be added to the water
requirement.

Step 9- Make trial batches. Only sufficient water should be used to produce the desired
slump. Test for slump, air, and density, and cast cylinders for strength, if required. If
specifications are not met. New batches should be prepared with adjusted proportions. If
the mix is satisfactory, design quantities , corrected for moisture contents of the
aggregates, should be recorded for use.

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TABLES USED FOR MIX- DESIGN
Table 1- 1a: Recommended slumps for various types of construction.
Range of Slump, mm*
Minimum Maximum
Types of construction
Reinforced foundation walls and footings 20 80
Plain footings, caissons, & substructure walls 20 80
Beams and reinforced walls 20 100
Building columns 20 100
Pavements and slabs 20 80
Mass concrete 20 80

 The upper limit may be increased by 20mm for compaction made by hand .
Table 1- 1b: Approximate mixing water and air- content requirements.

Water Kg/m3, For Indicated Nominal Sizes of Aggregates ( mm)

Slump, mm 9.5 12.5 19 25 37.5 50 75 150
Non – Air –Entrained Concrete
20-50 207 199 190 179 166 154 130 113
80-100 228 216 205 193 181 169 145 124
150-180 242 118 216 202 190 178 160 -
Approximate 3 2.5 2 1.5 1 0.5 0.3 0.2
Entrapped air (%)
Air Entrained Concrete
20-50 181 175 168 160 150 142 122 107
80-100 202 193 184 175 165 157 133 119
150-180 216 205 197 184 174 166 145 -
Recommended air Content (%)
Mild exposure* 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0
Moderate 6.0 5.5 5.0 4.5 4.5 4.0 3.5 3.0
exposure**
Severe 7.5 7.0 6.0 6.0 5.5 5.0 4.5 4.0
exposure888
Note:
* Where air entrainment is not required for durability.
* Where concrete will not be continually exposed to water before freezing or to de-icing
agents (Na & Ca Cl2).
* Where de-icing or similar agents are used or where concrete may be highly saturated before

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freezing.

Table 1-2:First estimate of Mass of fresh concrete.

Nominal Maximum, Size of First Estimate of Concrete Unit Mass, Kg/m3

Non-Air Entrained Concrete Air-Entrained Concrete
Aggregate, mm
9.5 2285 2190
12.5 2315 2235
19 2355 2280
25 2375 2315
37.5 2420 2355
50 2445 2375
75 2465 2400
150 2505 2435

Table 1-3a: Dry Bulk Volume of Coarse Aggregate per Unit Volume of Concrete
Nominal Bulk Volume of Dry- Rodded Coarse per Unit Volume of Concrete For
Maximum Size 0 Fineness modulus of Fine Aggregate (sand of:
2.40 2.60 2.80 3.00
Aggregate, mm
9.5 0.50 0.48 0.46 0.44
12.5 0.59 0.57 0.55 0.53
19 0.66 0.64 0.62 0.60
25 0.71 0.69 0.67 0.65
37.5 0.75 0.73 0.71 0.69
50 0.78 0.76 0.74 0.72
75 0.82 0.80 0.78 0.76
150 0.87 0.85 0.83 0.81

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 Note: the values given will produce a mix with workability suitable for RC
construction. For less workable concrete, eg.that used in road construction, the values
may be increased by about 10%. For more workable concrete, such as may be
required for placing by pumping, the values may be reduces by up to 10%.

Table 1-3b: Factors to be applied to the volume of coarse aggregate calculated on the
basis of table 1-3a for mixes of consistency other than plastic.
Relative Factors for maximum size of aggregate (mm) of :
9.5 12.5 19 25 37.5
Consistency Slump water
(mm) Content
Extremely dry - 78 1.90 1.70 1.45 1.40 1.30
Very stiff - 83 1.60 1.45 1.30 1.25 1.25
Stiff 0-30 88 1.35 1.30 1.15 1.15 1.20
Stiff plastic 30-80 92 1.08 1.06 1.04 1.06 1.09
Plastic 80-130 100 1.00 1.00 1.00 1.00 1.00
(Reference)
Fluid 130-180 106 0.97 0.98 1.00 1.00 1.00

Table 1-4a: Relation between W/C ratio & Compressive strength of concrete.

Compressive strength, 28 W/C Ratio (By Mass)

Non- Air Entrained Concrete Air-Entrained Concrete
Days (MPa)
45 0.38 -
40 0.43 -
35 0.48 0.40
30 0.55 0.46
25 0.62 0.53
20 0.70 0.61
15 0.80 0.71

* Measured on standard cylinders (150mm.diam.& 300mm. height.)

Table 1-4b:
Maximum W/C Ratios for Concrete in Severe Exposures.

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 Exposure conditions
Structure continuously or Structure exposed to
frequently seawater or sulphates
Type of Structure
Wet &exposed to Freezing
& Thawing*
Thin sections (railings, curbs, sills,
ledges, Ornamental work) & 0.45 0.40**
section with less than 25mm,cover
over steel.
All other structures. 0.50 0.45**
 Air- entrained concrete should be used under all conditions involving severe
exposure.
 When Type II or type V cement is used, the maximum W/C ratio may be
increased by 0.05.
Example

(1) A trial mix is required for a 150mm pavement slab being constructed in an area
where exposure to seawater is expected. Strength requires is 20MPa. The
estimated design strength of the trial mix is 28.5MPa. For the sand, FM= 2.80 and
absorption is 1.2 % The absorption of the CA is 1.5% and its dry-rodded density
is 1730kg/m3. FA is dry and CA contains 3.0% water.
Solution

Step- 1 From Table 1-1a, slump allowed is 20-80mm for pavement slab.
Step – 2 Maximum aggregate size is 1/3 of depth of slab =1/3 *(150) = 50mm.But
size 37.5mm is chosen, as it is economically available in the area.
Step 3 From Table 1-1b, mixing water required is 150kg/m3for 20-50mm slump and
Air content requires is 5.5% for severe exposure.

Step -4 Maximum W/C ratio for strength is 0.48 (Table 1-4a) and is 0.45 for
exposure ( Table 1-4b). Use the smaller value 0.45.

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 Step -5 Cement content= water requires ( step-3) / (W/C) =150/0.45=333.3kg/m3.
Step – 6 From Table 1-3a, volume of dry-rodded CA per unit volume of concrete is
0.71.
Step -7 Materials per m 3 considered ( Estimation only as variation in aggregate
densities and water absorption not considered ) for estimation of FA content
CA = 0.71* 1730=1228.3k
Water = 150.0kg
Cement = 333.3kg
Total 1712.0kg

From Table 1-2 estimate total mass of concrete is 2355kg/m3.

 Dry mass of FA =2355-1712 =643kg.
Step -8 Adjustment for moisture contents of aggregates.

1. Extra water required for FA =1.2% of 643 = 1.28 643/100=7.7kg (absorption)

2. Extra water required for CA =1.5% of 1228.3kg = 18.4kg (absorption).
3. Total water of CA (both absorption % surface water ) = 3.0% of 1228.3kg
=36.8kg
From (2) & (3) , Surplus water on CA = (3.0%-1.5% ) of 1228.3kg = 18.4kg

Corrected mix proportions:

Cement =333.3kg
Water =150+7.7 -18.4 = 139.3kg
FA (dry) = 643.0kg
CA = 1228.3 + 3.0% of 1228.3 = 1265.0kg

Step -9 Make trial batches & conduct different tests to check its suitability to satisfy the
requirements. Then calculate the corrected batch quantities.

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Properties of Hardened Concrete

The properties of fresh concrete are important only in the first few hours of its history
where as the properties of hardened concrete assumes an importance which is retained for
the remainder of the life of the concrete.

 The important properties of hardened concrete are:

Strength
Durability
Dimensional stability
Deformation under load
Permeability and shrinkage

1. STRENGTH

DefN : It is the maximum load (stress) the concrete can carry.

Concrete compressive strength is commonly used in the construction industry for the
purpose of specification and quality control.
Compressive strengths up to 80N/mm 2 can be achieved by selective use of the type of
cement, mix proportions, method of compaction and curing conditions.
 Concrete is a comparatively brittle material which is relatively weak in tension. So,
concrete structures except for road pavements are normally designed on the basis that
concrete is capable of resisting only compression, the tension being carried by steel
reinforcement.

Factors affecting Strength

1. Type of cement
 Despite all other factors same, strength for cements stored for longer time
is lower.

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  Cements with higher proportions of C3A produce higher strength
concrete, at least in earlier stages.
 The finer the particle size of the cement, the higher the ultimate
compressive strength.
2. Nature of aggregates
 Sharp edged, rough textured and clean surfaced aggregates result in a good
bond with the cement showing higher strength than smooth & rounded
aggregates.
 Aggregates with poor strength should be avoided for concrete making (ex
.Chalk-very soft limestone is weak in compressive strength.)
3. Water - Cement ratio
 In general, other things being constant, increase in W/C ratio decreases
compressive strength. Hence, when minimum water has been used just to
insure complete hydration of the cement, the resulting concrete will give
maximum strength on proper compaction.
4. Curing Conditions
Good curing condition makes the concrete to be stronger, more durable, denser &
more resistant to abrasion.
Incomplete curing & intermittent drying of concrete in the curing period may
cause a loss in the compressive strength to the extent of 40% or more.
5. Weather Conditions
The same concrete placed in different weathers, extremely cold & dry-hot, may
develop different strength values. The cause is related to incomplete hydration of the
cement in the concrete.
6. Methods of preparation
In order to get concrete with good quality, it adequately be mixed into a consistent &
homogeneous mass. Care must also be taken during transporting, placing &
compaction to minimize the probability of occurrence of bleeding segregation &
honeycombing. This is all determined by the skill of the workmanship.
Note: An incompetent worker may spoil the entire work despite being given the best
designed concrete mix.

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 2. DURABILITY

Defn : The capacity of hardened concrete to withstand all the forces of deterioration
that are likely to act on it , after setting in a given environment.

Among all forces, the following are more likely to act: - Frost action
- Chemical action
- Abrasive forces
- Alkali- aggregate reaction

1. Frost action
It is major cause of deterioration of concrete in cold climates. This is due to the
Cyclic freezing & thawing of ice crystals with in the pores (gel, capillary & cavities
due to entrained air ) which are responsible for development of cracks of various
matures in the concrete.
Methods used to minimize the development of this cracks:
- Using low W/C ratio to minimize the formation of pores due to surplus water
- Using good drainage & covering methods for removing any surplus water from
staying in the concrete during curing process.
2. Chemical action

Concrete is liable to attack to various degrees by acids, alkalis & salt solutions. So,
we have to give attention for concrete structures constructed near areas with these
chemicals.
3. Abrasion
- Open concrete surfaces are exposed to strong wing action.

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 - Concrete used for flooring & pavements has to resist abrasive actions of a
greater magnitude. Wearing problem after sometime, so structures has to be designed
for those situations.
- In general, the higher the crushing strength of the concrete, the higher is its
abrasive resistance.

4. Alkali-aggregate reaction
Alkalis (K2O & Na2O) of cement are capable of reacting with silica which is the
common component of many coarse aggregates & is called alkali – aggregate
reaction. It may create harmful effects on the cement concrete.
To avoid this reaction:
- Keep the percentage of alkalis (K2O & Na2O) in PC to be very low (as in the low-
alkali cements)
- Take a great care for selecting aggregates free of amorphous silica like opal. Chert,
cristobalite etc.
3. DIMENSIONAL STABILITY

It is about three important terms: creep, shrinkage & expansion.

(a) Creep: It defines the behavior of concrete under loads.

Generally, concrete is elasto-plastic material, when loaded it deforms:
(1) Partly as a result of elastic strain (which is recovered on the removal of the
load&
(2) Partly as a result of plastic strain (a permanent deformation).
 The non- elastic (plastic ) time dependent deformation is a typical property of
concrete & is commonly referred as CREEP.
The rate of creep is profoundly influences by:
- The proportion of mix &
- The type of cement used in concrete.
Note: Concrete mixes with higher W/C ratio, will show higher creep rates.

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 Creep can be beneficial or harmful in different cases. For ex. In RC structures,
it Helps in transferring stresses to the reinforcement & their by reduce chances of failure
by stress concentration.
(b) Shrinkage of concrete:
It is caused by the settlement of solids & the loss of free water from the plastic
concrete (plastic shrinkage), by the chemical combination of cement with water
(autogenous shrinkage), by the drying of concrete (drying shrinkage) & by fall in
temperature of concrete (thermal shrinkage).
Where movement of the concrete is restrained, shrinkage will produce tensile tresses
with in the concrete which may cause cracking.
 The effect of drying shrinkage should be minimized by providing movement
joints.

(C) Expansion:

Concrete may expand on hardening due to :

(1) Thermal expansion – In massive concrete works, when the upper layers are laid
before the lower layers have completely set it will result in the development of
thermal expansion in the lower layers. This is because the heat of hydration gets
accumulated in those layers & may magnitudes beyond acceptable limits.
(2) Chemical reactions – this is due to alkali- aggregate reactions. It results in the
formation of crystalline structures of bigger volumes which is harmful because of
its instability & weakness in strength than reaction between CaO & SiO 2 .It
become the cause of cracking & quicker deterioration of concrete,

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