Objective of Concrete Mix Design

8/3/2019 Objective of Concrete Mix Design

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Objective of Concrete Mix Design

Concrete Mixer Drum type 140L

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 DesignsThe Process of Concrete Mix Design The method of concrete mix design applied here is in accordance to the methodpublished 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 thedesign values including;

Mix parameters such as target mean strength, water-cement ratio and concretedensity.

Unit proportions such as the weight of materials.
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The design process can be divided into 5 primary stages. Each stage deals with aparticular aspect of the concrete mix design:

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 characteristicstrength. [ k = 1.64 for 5 % defect ]s = The standard deviation (obtained from CCS 1).

CCS 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, f m f m = f c + M .. [ F2 ] where;f m = Target mean strengthf c = The specified characteristic strengthiv) Given the type of cement and aggregate, use the table of CCS 1 to obtain thecompressive strength, at the specified age that corresponds to a free water/cement ratio

of 0.5.


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

v) In figure CCS 4, follow the starting line to locate the curve which passes through thepoint (the compressive strength for water/cement ratio of 0.5). To obtain the requiredcurve representing the strength, it is necessary to interpolate between the two curves inthe figure. At the target mean strength draw horizontal line crossing the curve. From thispoint the required free water/cement ratio can be determined.

Stage 2: Determining the Free-Water Content


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CCS 2: 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 tableCCS 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 thatmay be specified. If the calculated cement content from F3 is below a specifiedminimum, this minimum value must be adopted resulting in a reduced water/cementratio and hence a higher strength than the target mean strength. If the calculated cementcontent 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 isobtained from figure CCS 5. This value depends upon the free-water content and therelative density of the combined aggregate in the saturated surface-dry condition. If noinformation is available regarding the relative density of the aggregate, anapproximation can be made by assuming a value of 2.6 for un-crushed aggregate and 2.7for crushed aggregate.


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CCS 5: Estimated wet density of fully compacted concrete.

With the estimate of the density of the concrete the total aggregate content is calculatedusing equation F4:

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

where;D = The wet density of concrete ( in kg/m 3)C = The cement content (in kg/m 3) W = The free-water content (in kg/m 3)

Stage 5: Determining of The Fine and Coarse Aggregate Contents This stage involves deciding how much of the total aggregate should consist of materialssmaller than 5 mm, i.e. the sand or fine aggregate content. The figure CCS 6 showsrecommended values for the proportion of fine aggregate depending on the maximumsize of aggregate, the workability level, the grading of the fine aggregate (defined by thepercentage passing a 600 m sieve) and the free -water/ cement ratio. The bestproportion of fines to use in a given concrete mix design will depend on the shape of theparticular aggregate, the grading and the usage of the concrete.


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CCS 6: Recommended proportions of fine aggregate according to percentage passing a 600 m

sieve.

The final calculation, equation F5, to determine the fine and coarse aggregate is madeusing the proportion of fine aggregate obtained from figure CCS 6 and the total aggregatecontent 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 MixingProduction 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 theconcrete slump test.

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

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


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the uppermost part of the slumped concrete and the upturned cone is recorded inmm as the slump.

3. Three cubes are prepared in 100 mm x 100 mm each. The cubes are cured beforetesting. 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 preventadhesion of concrete. Each mould filled with two layers of concrete, each layertamped 25 times with a 25 mm square steel rod. The top surface finished with atrowel and the date of manufacturing is recorded in the surface of the concrete. Thecubes are stored undisturbed for 24 hours at a temperature of 18 to 22 0C and arelative 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 21 oC 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 CalculationsHere is one example of calculation from one of the concrete mix design obtained fromthe laboratory. We have to fill in all particulars in the concrete mix design form withsome calculations

CCS 3: Relationship between standard deviation and characteristic strength.

Firstly, we specified 30 N/mm 2 at 7 days for the characteristic strength. Then, we

obtained the standard deviation, s from the figure CCS 3. So, s = 8 N/mm2.


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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/mm 2 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/mm 2 The type of cement is Ordinary Portland Cement (OPC). For the fine and courseaggregate, the laboratorys fine aggregate is un -crushed and for coarse aggregate iscrushed before producing concrete.

Then, we obtain the free-water/ cement ratio from table CCS 1. For OPC ( 7 days ) usingcrushed aggregate, water/cement ratio = 36 N/mm 2. After that, from the figure CCS 4, the curve for 42 N/mm 2 at 0.5 free-water ratio isplotted and obtained the free-water ratio is 0.45 at the target mean strength 43.12

N/mm 2.Next, we specified the slump test for slump about 20 mm and the maximum aggregatesize we used in laboratory is 10 mm. For the specified above, we can obtained the free- water content from table CCS 2 at slump 10 30 mm and maximum size aggregate 10mm, the approximate free-water content for the un-crushed aggregates is 180 kg/m 3 andfor the crushed aggregates is 205 kg/m 3. 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 W f + 1/ 3 W c

= ( 2/ 3 x 180) + ( 1/ 3 x 205)= 188.33 kg/m 3 where, W f = Free-water content appropriate to type of fine aggregate W c = Free-water content appropriate to type of coarse aggregateCement 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/m 3 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/m 3, obtained that concrete density is 2450kg/m 3. The total aggregate content can be calculated by:Total Aggregate Content = D C W = 2450 418.52 188.33 = 1843.15 kg/m 3 The percentage passing 600 m sieve for the grading of fine aggregate is about 60 %. Theproportion of the fine aggregate can be obtained from the figure CCS 6, 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/m 3


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Coarse Aggregate Content = Total Aggregate Content Fine Aggregate= 1843.15 700.40 = 1142.75 kg/m 3 The quantity per m 3 can be obtained, which is;Cement = 418.52 kg

Water = 188.33 kgFine aggregate = 700.40 kgCoarse aggregate (10 mm) = 1142.75 kgThe 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 m 3 The quantities of trial mix = 0.00375 m 3, in which is;Cement = 1.57 kg

Water = 0.71 kgFine aggregate = 2.61 kgCoarse aggregate (10 mm) = 4.29 kgThe 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 compressivecube 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 targetmean strength.

= 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 mixdesign method pass the specific strength requirements.

Discussions upon Concrete Mix Designs Although our compressive strength passes the specific requirements, we still identifiedseveral factors which contribute to the lacking of compressive strength of concrete mixesproduced in the experiment. However, the main factor is the condition of aggregates whether it is exposed to sunlight or rainfall.


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When the free water/cement ration is high, workability of concrete isimproved. However, excessive water causes honey-comb effect in the concreteproduced. 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 suggestions upon several steps to avoid the problems previously faced:

All the raw materials, which is cement, aggregates, and sand should be protectedfrom precipitation or other elements which may affect its physical properties.

The quantity of ingredients may be adjusted if necessary, theoretical values are notalways 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 Conclusion1. 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 specifiedproportion.

2. The concrete produced did not fulfill the compressive strength requirements due toseveral reasons. Furthermore, some steps mentioned above should be taken intoconsideration 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
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Objective of Concrete Mix Design

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