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Steps of Concrete Mix Design

by Civil-Guy on May 12, 2010 in Materials Testing

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 requiredstrength and minimum cement content but leaving the grading and details of the concrete mix designto be work out.

Objective of Concrete Mix Design

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of 19Steps of Concrete Mix Design

11-10-2011http://www.civilcraftstructures.com/materials-testing/steps-of-concrete-mix-design/



Two main objectives for concrete mix design:

To determine the proportions of concrete mix constituents of; Cement, Fine aggregate (ornormally 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 theDepartment 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 valuesincluding;

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:

Concrete Mixer – Drum type 140L





Stage 1: Determining the Free Water/ Cement Ratio i) Specify the required characteristic strength at a specified age, f c

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 CCS 1).

iii) Calculate the target mean strength, f m

f m = f c + M ….. [ F2 ]

where;f m

= Target mean strength

f c

= The specified characteristic strength

iv) Given the type of cement and aggregate, use the table of CCS 1 to obtain the compressivestrength, at the specified age that corresponds to a free water/cement ratio of 0.5.

CCS 1: Approximate compressive strength (N/mm2) of concrete mixes madewith a free-water/cement ratio of 0.5





v) In figure CCS 4, follow the ‘starting line’ to locate the curve which passes through the point (thecompressive strength for water/cement ratio of 0.5). To obtain the required curve representing thestrength, it is necessary to interpolate between the two curves in the figure. At the target meanstrength draw horizontal line crossing the curve. From this point the required free water/cement ratiocan be determined.

Stage 2: Determining the Free-Water Content

CCS 4: Relationship between compressive strength and free-water/ cement ratio.





Given the Concrete Slump or Vebe time, determine the free water content from table CCS 2.

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 bespecified. If the calculated cement content from F3 is below a specified minimum, this minimumvalue must be adopted resulting in a reduced water/cement ratio and hence a higher strength than thetarget mean strength. If the calculated cement content is higher than a specified maximum, then thespecified strength and workability simultaneously be met with the selected materials; try to changethe 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 fromfigure CCS 5. This value depends upon the free-water content and the relative density of thecombined aggregate in the saturated surface-dry condition. If no information is available regardingthe relative density of the aggregate, an approximation can be made by assuming a value of 2.6 forun-crushed aggregate and 2.7 for crushed aggregate.

CCS 2: Approximate free-water contents (kg/m3) required to give various levelsof workability.





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 smallerthan 5 mm, i.e. the sand or fine aggregate content. The figure CCS 6 shows recommended values forthe 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 dependon the shape of the particular aggregate, the grading and the usage of the concrete.

CCS 5: Estimated wet density of fully compacted concrete.





The final calculation, equation F5, to determine the fine and coarse aggregate is made using theproportion of fine aggregate obtained from figure CCS 6 and the total aggregate content derivedfrom 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. Thevolume 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.

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 uppermostpart 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

CCS 6: Recommended proportions of fine aggregate according to percentage passing a 600µm sieve.

Slump Test apparatus for Concrete Workability





surfaces of the assembled mould with mould oil to prevent adhesion of concrete. Each mouldfilled 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 220Cand a relative humidity of not less than 90 %. The concrete all are covered with wet gunnysacks. After 24 hours, the mould is striped and the cubes are cured further by immersing them

in water at temperature 19 to 21o

C 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 thelaboratory. We have to fill in all particulars in the concrete mix design form with somecalculations…

Firstly, we specified 30 N/mm2 at 7 days for the characteristic strength. Then, we obtained the

standard deviation, s from the figure CCS 3. 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, f m is calculated as below:

Target mean strength, f m = f c + M= 30 + 13.12 = 43.12 N/mm2

CCS 3: Relationship between standard deviation and characteristic strength.





The type of cement is Ordinary Portland Cement (OPC). For the fine and course aggregate, thelaboratory’s fine aggregate is un-crushed and for coarse aggregate is crushed before producingconcrete.

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

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

After that, from the figure CCS 4, 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 usedin laboratory is 10 mm. For the specified above, we can obtained the free-water content from tableCCS 2 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 isestimated 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 the figure CCS 5 with

the free-water content 188.33 kg/m3, obtained that concrete density is 2450 kg/m3. The totalaggregate content can be calculated by:

Total Aggregate Content = D – C – W

= 2450 – 418.52 – 188.33 = 1843.15 kg/m3

The percentage passing 600 µm sieve for the grading of fine aggregate is about 60 %. The proportionof the fine aggregate can be obtained from the figure CCS 6, which is 38 %. Then, the fine andcourse 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 kgFine aggregate = 700.40 kgCoarse 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 kgWater = 0.71 kgFine aggregate = 2.61 kgCoarse 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 iscarried out. The results are as follows:

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

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

After 7 days of curing, the compressive strength of concrete cubes produced by the mix designmethod pass the specific strength requirements.

Discussions Upon Concrete Mix Designs

Although our compressive strength passes the specific requirements, we still identified severalfactors which contribute to the lacking of compressive strength of concrete mixes produced in theexperiment. 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, excessivewater causes “honey-comb” effect in the concrete produced. The concrete cubes become porous, andhence its compressive strength is well below the design value. Other possible reasons include overcompaction, 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 fromprecipitation or other elements which may affect its physical properties.

The quantity of ingredients may be adjusted if necessary, theoretical values are not alwayssuitable. For example, if the aggregates are wet or saturated, less amount of water should beadded, vice versa.

Sample 1 2 3

Compressive Strength 32.37 33.54 35.70

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





Compaction should be done carefully, as either under or over-compaction will bringsignificant 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 toovercome 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

Dear readers, thank you for visiting and reading the above article. We lookingforward for your feedback and may re-tweet the post for sharing it to others. Wedo hope you will find the article to be useful, informative and enjoyable. Happy

reading and go engineering...

Thank You and Best Regards, Author: Civil-Guy | Blog: Civil Craft Structures | Subscribe: RSS or Email

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27 comments

{ 27 comments… read them below or add one }

A.parankunran June 29, 2010 at 8:15 pm

obviously thanks.

Reply

Vishwanu November 11, 2010 at 12:39 pm

I found ‘Steps in concrete mix design’ concise, simple, easy to read and very helpful.

Reply

Fathi December 14, 2010 at 5:12 am





Thanks alot

Reply

kachaala m. December 16, 2010 at 5:58 am

it’s interesting, keep it up

Reply

Srinivasa Rao January 5, 2011 at 4:22 pm

I got a good information, Thank you

Reply

nitin sharma January 5, 2011 at 8:31 pm

it is very helpful to me.thanks a lot.

Reply

Falak January 22, 2011 at 7:28 pm

It is very helpful to me

Reply

bpadmanabham January 23, 2011 at 4:15 pm

very nice website

Reply

Chito Dizon February 6, 2011 at 3:51 pm

Very informative. Big Thanks

Reply





walter allen February 11, 2011 at 7:29 pm

A VERY GOOD SUMMARY INDEED!!

Reply

vibhanshu March 12, 2011 at 8:18 pm

Thanks best procedure

Reply

Vincent March 15, 2011 at 8:31 pm

Im really pleased

Reply

nick karna April 7, 2011 at 5:42 pm

its reaallyyyyy so nice to have such infos online…………….

Reply

Gupta Hari Nepal April 18, 2011 at 2:20 pm

I have got good information

Reply

yogeswaran April 24, 2011 at 5:11 pm

it is very useful for me thanks….,

Reply

Wiley April 26, 2011 at 5:43 pm





Gret stuff

Reply

K.V.S.D. Jayamali May 2, 2011 at 1:56 pm

Thanks very much..Bt there is a mistake in calculation of The volume of trial mix for 3 cubes;It is mentioned here as follows= [(0.1 x 0.1 x 0.1) x 3] + [25% contingencies of trial mix volume]= 0.006 + 0.00075= 0.00375 m3

Bt it should be corrected as follows (in 2nd line it should 0.003 instead of 0.006)= [(0.1 x 0.1 x 0.1) x 3] + [25% contingencies of trial mix volume]

= 0.003 + 0.00075= 0.00375 m3

Bst Rgds

Reply

Civil-Guy May 14, 2011 at 7:09 pm

Dear Friend,

Firstly, so sorry for the significant mistakes. Thanks for correcting it in a nice and neatway.

Cheers to the Civil Engineering community…

Reply

Hailualemu May 19, 2011 at 5:07 am

10q ,nice procedure

Reply

PRAMOD CHAWADA May 26, 2011 at 9:11 pm

Dear Sir,Thanks a lot for very Good and Knowledgeable information.Hopes such information will receive further with new advance Technology.





With Thanks & Regards,Pramod Chawada

Reply

FRANK WAMBURA June 5, 2011 at 3:27 pm

I appreciate the way these steps have arranged to enhance the proper way of concrete mixdesign. But I wonder why the author didnot put the references of the tables and graphs/curvesused. I would like to ask the author and my fellow readers to assist me on knowing the sourcesof these tables and graphs whether Manuals or Books

Reply

nasrtullah June 6, 2011 at 4:50 pm

very nice really is helpful thnx and cheer up for civil guyssssssss

Reply

Mfinanga June 7, 2011 at 6:29 am

It is very useful, thanks for this information online

Reply

Boni Amin June 20, 2011 at 11:45 am

Its very helpful to the practicing structural engineers..Thanks

Reply

muhammad waqas July 5, 2011 at 3:41 am

it is very good for fresh graduates…………it helps to remind that topic……..

Reply

Nauman Akram July 13, 2011 at 3:35 pm





Thanks for the info. Nice informative site

Reply

albanch mokamba nyang'ara July 21, 2011 at 3:35 pm

Thanks for the information, it is real important to our daily concrete works.

Reply

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