concrete mix design steps

Steps of Concrete Mix Design

Concrete mix designs is best defined as a process in selecting suitable ingredients, which is

cement, aggregate, sand and water, and determining their relative proportions to give the

required strength, workability and durability. The mix designs, which is a performance

specification stating required strength and minimum cement content but leaving the grading and

details of the concrete mix design to be work out.

Objective of Concrete Mix Design

Two main objectives for concrete mix design:

• To determine the proportions of concrete mix constituents of; Cement, Fine aggregate (or

normally Sand), Coarse aggregate, and Water.

• To produce concrete of the specified properties.

• To produce a satisfactory of end product, such as beam, column or slab as economically

as possible.

Theory of Mix Designs

The Process of Concrete Mix Design

The method of concrete mix design applied here is in accordance to the method published by the

Department of Environment, United Kingdom (in year 1988).

There are two categories of initial information required:

1. Specified variables; the values that are usually found in specifications.

2. Additional information, the values normally available from the material supplier.

Reference data consists of published figures and tables is required to determine the design values

including;

• Mix parameters such as target mean strength, water-cement ratio and concrete density.

• Unit proportions such as the weight of materials.

The design process can be divided into 5 primary stages. Each stage deals with a particular

aspect of the concrete mix design:

Stage 1: Determining the Free Water/ Cement Ratio

i) Specify the required characteristic strength at a specified age, fc

ii) Calculate the margin, M.

M = k x s ….. [ F1 ]

where;

k = A value appropriate to the defect percentage permitted below the characteristic strength.

[ k = 1.64 for 5 % defect ]

s = The standard deviation (obtained from Figure 1).

Muhammad Mahamudul Hasan [email protected]

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Figure 1: Approximate compressive strength (N/mm2) of concrete mixes made with a free-

water/cement ratio of 0.5

iii) Calculate the target mean strength, fm

fm = fc + M ….. [ F2 ]

where;

fm = Target mean strength

fc = The specified characteristic strength

iv) Given the type of cement and aggregate, use the table of Figure 1 to obtain the compressive

strength, at the specified age that corresponds to a free water/cement ratio of 0.5.


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Figure 2: Relationship between compressive strength and free-water/ cement ratio.

v) In Figure 2, follow the ‘starting line’ to locate the curve which passes through the point (the

compressive strength for water/cement ratio of 0.5). To obtain the required curve representing

the strength, it is necessary to interpolate between the two curves in the figure. At the target

mean strength draw horizontal line crossing the curve. From this point the required free

water/cement ratio can be determined.


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Stage 2: Determining the Free-Water Content

Figure 3: Approximate free-water contents (kg/m3) required to give various levels of

workability.

Given the Concrete Slump or Vebe time, determine the free water content from table Figure 3.

Stage 3: Determining the Cement Content

Cement Content = Free Water Content / Free-water or Cement Ratio ….. [ F3 ]

The resulting value should be checked against any maximum or minimum value that may be

specified. If the calculated cement content from F3 is below a specified minimum, this minimum

value must be adopted resulting in a reduced water/cement ratio and hence a higher strength than

the target mean strength. If the calculated cement content is higher than a specified maximum,

then the specified strength and workability simultaneously be met with the selected materials; try

to change the type of cement, the type and maximum size of the aggregate.

Stage 4: Determining the Total Aggregate Content

This stage required the estimate of the density of fully compacted concrete which is obtained

from Figure 4. This value depends upon the free-water content and the relative density of the

combined aggregate in the saturated surface-dry condition. If no information is available

regarding the relative density of the aggregate, an approximation can be made by assuming a

value of 2.6 for un-crushed aggregate and 2.7 for crushed aggregate.


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FIgure 4 : Estimated wet density of fully compacted concrete.

With the estimate of the density of the concrete the total aggregate content is calculated using

equation F4:

Total Aggregate Content = D – C – W ….. [ F4 ]

Where;

D = The wet density of concrete ( in kg/m3)

C = The cement content (in kg/m3)

W = The free-water content (in kg/m3)

Stage 5: Determining of The Fine and Coarse Aggregate Contents

This stage involves deciding how much of the total aggregate should consist of materials smaller

than 5 mm, i.e. the sand or fine aggregate content. The Figure 5 shows recommended values for

the proportion of fine aggregate depending on the maximum size of aggregate, the workability

level, the grading of the fine aggregate (defined by the percentage passing a 600 µm sieve) and

the free-water/ cement ratio. The best proportion of fines to use in a given concrete mix design

will depend on the shape of the particular aggregate, the grading and the usage of the concrete.


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Figure 5 : Recommended proportions of fine aggregate according to percentage passing a 600

µm sieve.

The final calculation, equation F5, to determine the fine and coarse aggregate is made using the

proportion of fine aggregate obtained from Figure 5 and the total aggregate content derived from

Stage 4.

Fine Aggregate Content = Total Aggregate Content x Proportion of Fines ….. [ F5 ]

Coarse Aggregate Content = Total Aggregate Content – Fine Aggregate

Procedures of Design Mixing

Production of Trial Mix Design

1. The volume of mix, which needs to make three cubes of size 100 mm is calculated. The

volume of mix is sufficient to produce 3 numbers of cube and to carry out the concrete

slump test.

2. The volume of mix is multiplied with the constituent contents obtained from the concrete

mix design process to get the batch weights for the trial mix.

3. The mixing of concrete is according to the procedures given in laboratory guidelines.

4. Firstly, cement, fine and course aggregate are mixed in a mixer for 1 minute.

5. Then, water added and the cement, fine and course aggregate and water mixed

approximately for another 1 minute.

6. When the mix is ready, the tests on mix are proceeding.

Slump Test apparatus for Concrete Workability


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Tests on Trial Mix Design

1. The slump tests are conducted to determine the workability of fresh concrete.

2. Concrete is placed and compacted in three layers by a tamping rod with 25 times, in a

firmly held slump cone. On the removal of the cone, the difference in height between the

uppermost part of the slumped concrete and the upturned cone is recorded in mm as the

slump.

3. Three cubes are prepared in 100 mm x 100 mm each. The cubes are cured before testing.

The procedures for making and curing are as given in laboratory guidelines. Thinly coat

the interior surfaces of the assembled mould with mould oil to prevent adhesion of

concrete. Each mould filled with two layers of concrete, each layer tamped 25 times with

a 25 mm square steel rod. The top surface finished with a trowel and the date of

manufacturing is recorded in the surface of the concrete. The cubes are stored

undisturbed for 24 hours at a temperature of 18 to 220C and a relative humidity of not

less than 90 %. The concrete all are covered with wet gunny sacks. After 24 hours, the

mould is striped and the cubes are cured further by immersing them in water at

temperature 19 to 21oC until the testing date.

4. Compressive strength tests are conducted on the cubes at the age of 7 days. Then, the

mean compressive strengths are calculated.

The Calculations

Here is one example of calculation from one of the concrete mix design obtained from the

laboratory. We have to fill in all particulars in the concrete mix design form with some

calculations…

Figure 6 : Relationship between standard deviation and characteristic strength.


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Firstly, we specified 30 N/mm2

at 7 days for the characteristic strength. Then, we obtained the

standard deviation, s from the Figure 6. So, s = 8 N/mm2.

From the formula F1, k = 1.64 for 5 % defect. The margin, M is calculated as below:

M = k x s = 1.64 x 8 = 13.12 N/mm2

With the formula F2, target mean strength, fm is calculated as below:

Target mean strength, fm = fc + M

= 30 + 13.12 = 43.12 N/mm2

The type of cement is Ordinary Portland Cement (OPC). For the fine and course aggregate, the

laboratory’s fine aggregate is un-crushed and for coarse aggregate is crushed before producing

concrete.

Then, we obtain the free-water/ cement ratio from table Figure 1. For OPC ( 7 days ) using

crushed aggregate, water/cement ratio = 36 N/mm2.

After that, from the Figure 2, the curve for 42 N/mm2 at 0.5 free-water ratio is plotted and

obtained the free-water ratio is 0.45 at the target mean strength 43.12 N/mm2.

Next, we specified the slump test for slump about 20 mm and the maximum aggregate size we

used in laboratory is 10 mm. For the specified above, we can obtained the free-water content

from table Figure 3 at slump 10 – 30 mm and maximum size aggregate 10 mm, the approximate

free-water content for the un-crushed aggregates is 180 kg/m3

and for the crushed aggregates is

205 kg/m3. Because of the coarse and fine aggregates of different types are used, the free-water

content is estimated by the expression:

Free-water Content, W

= 2/3 Wf +

1/3 Wc

= (2/3 x 180) + (

1/3 x 205)

= 188.33 kg/m3

where,

Wf = Free-water content appropriate to type of fine aggregate

Wc = Free-water content appropriate to type of coarse aggregate

Cement content also can obtained from the calculation with the expression at F3:

Cement Content, C = Free Water Content / Free-water or Cement Ratio

= 188.33 / 0.45 = 418.52 kg/m3

We assumed that the relative density of aggregate (SDD) is 2.7. Then, from Figure 4 with the

free-water content 188.33 kg/m3, obtained that concrete density is 2450 kg/m

3. The total

aggregate content can be calculated by:

Total Aggregate Content = D – C – W

= 2450 – 418.52 – 188.33 = 1843.15 kg/m3


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The percentage passing 600 µm sieve for the grading of fine aggregate is about 60 %. The

proportion of the fine aggregate can be obtained from the Figure 5, which is 38 %. Then, the fine

and course aggregate content can be obtained by calculation:

Fine Aggregate Content

= Total Aggregate Content x Proportion of Fines

= 1868.74 x 0.38 = 700.40 kg/m3

Coarse Aggregate Content = Total Aggregate Content – Fine Aggregate

= 1843.15 – 700.40 = 1142.75 kg/m3

The quantity per m3 can be obtained, which is;

Cement = 418.52 kg

Water = 188.33 kg

Fine aggregate = 700.40 kg

Coarse aggregate (10 mm) = 1142.75 kg

The volume of trial mix for 3 cubes

= [(0.1 x 0.1 x 0.1) x 3] + [25% contingencies of trial mix volume]

= 0.006 + 0.00075

= 0.00375 m3

The quantities of trial mix = 0.00375 m3, in which is;

Cement = 1.57 kg

Water = 0.71 kg

Fine aggregate = 2.61 kg

Coarse aggregate (10 mm) = 4.29 kg

The Results of Mix Design

Slump Test = True Slump of 55 mm…

All the 3 concrete cubes produced were then cured for 7 days. After that, the compressive cube

test is carried out. The results are as follows:

Sample 1 2 3

Compressive Strength 32.37 33.54 35.70

Average (32.37 + 33.54 + 35.70) / 3 = 33.87

For cubes after 7 days of curing, compressive strength should not be less than 2/3 target mean

strength.

= 2/3 × 43.12 = 28.75 N/mm2 < 33.9 N/mm

2

After 7 days of curing, the compressive strength of concrete cubes produced by the mix design

method pass the specific strength requirements.


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Discussions Upon Concrete Mix Designs

Although our compressive strength passes the specific requirements, we still identified several

factors which contribute to the lacking of compressive strength of concrete mixes produced in

the experiment. However, the main factor is the condition of aggregates whether it is exposed to

sunlight or rainfall.

When the free water/cement ration is high, workability of concrete is improved. However,

excessive water causes “honey-comb” effect in the concrete produced. The concrete cubes

become porous, and hence its compressive strength is well below the design value. Other

possible reasons include over compaction, improper mixing methods and some calculation

errors.

Few suggestion upon several steps to avoid the problems previously faced:

• All the raw materials, which is cement, aggregates, and sand should be protected from

precipitation or other elements which may affect its physical properties.

• The quantity of ingredients may be adjusted if necessary, theoretical values are not

always suitable. For example, if the aggregates are wet or saturated, less amount of water

should be added, vice versa.

• Compaction should be done carefully, as either under or over-compaction will bring

significant negative effect on the concrete produced.

The Conclusion

1. By using the concrete mix design method, we have calculated the quantities of all

ingredients, that is water, cement, fine and coarse aggregate according to specified

proportion.

2. The concrete produced did not fulfill the compressive strength requirements due to

several reasons. Furthermore, some steps mentioned above should be taken into

consideration to overcome this problem.

Standard reference for the concrete mix design is as accordance to British Standard;

BS 5328: 1981 : Methods of Specifying Concrete including Ready-Mixed Concrete

concrete mix design steps

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