Concrete Mix Design (Notes).pdf

8/9/2019 Concrete Mix Design (Notes).pdf

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Mix Design Appendix - 1 -

1.

MATERIAL TEST

Material used in reinforced concrete structures are subjected to preliminary material tests

according to specifications or standards for concrete steel. Material tests for concrete include

slump test and compression tests, whilst tests for steel reinforcement include tensile, bend and

steel fabric tests.and

1.1 Concrete

The keys to good quality concrete are the raw materials used to make concrete and the mix

design as specified in the project specifications.

1.1.1 Mix Design Process

The concrete mix design process consists of five stages and is explained according to the British

Standard (BS1881). Stage 1 consists of selection of target water/cement ratio. Figure 1 shows a

relationship between standard deviation and characteristic strength. If previous information

concerning the variability of the strength tests comprises less than 20 results, the standard

deviation to be adopted should be that obtained from line A. If previous information is available

consisting of 20 or more results, the standard deviation of such results may be used provided

that this value is not less than the appropriate value obtained from line B. In Table 1 the margincan then be derived from calculation Bl.

M = kxs (B1)

Where M = margin (Item 1.3)

k = a value appropriate to the 'percentage defectives' permitted below the characteristic

strength

s = standard deviation

The constant k is derived from the mathematics of the normal distribution and increaseas the proportion of the defective is decreased, thus:

k for 10 % defectives = 1.28

k for 5 % defectives = 1.64

k for 2.5 % defectives = 1.96

CHAPTER 2 - CONCRETE MIX DESIGN

At the end of this session, the students should be able to design the concrete mix with specific

requirement.


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Next, a value is obtained from Table 2 for the strength of a mix made with a free-water/cement

ratio of 0.5 according to the specified age, the type of cement and the aggregate to be used.

This strength value is then plotted on Figure 2 and a curve is drawn from this point and parallel

to the printed curves until it intercepts a horizontal line passing through the

ordinate representing the target mean strength. The corresponding value for the free-

water/cement ratio can then be read from the abscissa. This should be compared with any

maximum free-water/cement ratio that may be specified and the lower of these two values

used.

Note: When coarse and fine aggregates of different types are used, the free content is

estimated by the expression,

2 Wf + 1 Wc (B3)3 3

Where Wf = free water content appropriate to type of fine aggregate and Wc = free water

content appropriate to type of coarse aggregate


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Stage 3 is the determination of cement content where the resulting value should be checked

against any maximum or minimum value that may be specified. If the calculated cementcontent from in Table 3 is below a specified minimum, this minimum value must be adopted

and a modified free water/cement ratio calculated which would be less than that determined in

Stage 1. This will result in a concrete that has a mean strength somewhat higher than the target

mean strength. Alternatively, the free water/ cement ratio from Stage 1 is used resulting in

higher free water content and increased workability. On the other hand, if the design method

indicates a cement content that is higher than a specified maximum, then it is probable that

the specification cannot be met simultaneously on strength and workability requirements withthe selected materials. Consideration should then be given to changing the type of cement, the

type and maximum size of aggregate or the level of workability of the concrete, or to use water-

reducing admixture.


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


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Figure 3: Estimated wet density of fully compacted concrete

Figure 4: Recommended proportions of fine aggregate according to percentage passing a

600m sieve


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Figure 4: Recommended proportions of fine aggregate according to percentage passing a 600m sieve


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Stage 4 consists of determination of total aggregate which requires an estimate of the density

of the fully compacted concrete which is in Figure 3 depending upon the free water content and

the relative density of the combined aggregate in the saturated surface dry condition (SSD).

If no information is available regarding the relative density of the aggregate, an approximation

value of wet density may be deduced by assuming a value of 2.6 for uncrushed aggregate and

2.7) for crushed aggregate. From this estimafe3rdfengityvof the concrete, the total aggregate

content is determined from calculation B4,

Total aggregate content = D - C W (B4)

Where D = wet density of concrete (kg/m3)

C = cement content (kg/m3)

W = free water content (kg/m3)

Stage 5 consists of selection of fine and coarse aggregate which involves deciding how

much of the total aggregate should include materials smaller than 5 mm, i.e. the sand or fine

aggregate content. Figure 4 shows recommended values for the proportion of fine aggregate

depending on the maximum size of the aggregate, the workability level, the grading of the fine

aggregate (define by its percentage passing a 600m sieve.) and the free water/cement ratio.

The best proportion of fines to be used in a given mix will depend on the actual grading of

shape of the particular aggregate and the use to which the concrete is to be put. However,adoption of a proportion obtained from Figure 4 will generally give a satisfactory concrete in

the first trial mix that can then be adjusted as required for the exact conditions prevailing.

The final calculation B5 to determine the fine and coarse aggregate contents is made

using the proportion of fine aggregate obtained from Figure 4 and the total aggregate content

derived in Stage 4 Fine aggregate content = Total aggregate content x proportion of fines

Coarse aggregate content = Total aggregate content x fine aggregate content The coarse

aggregate content itself can be subdivided if single sized 10, 20 and 40 mm materials are to becombined. Again, the best proportions will depend on aggregate shape and concrete usage but

the following ratios are suggested as a general guide:

1: 2 for combination of 10 and 20 mm material

1:1.5:3 for combination of 10, 20 and 40 material


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EXAMPLE 1: CONCRETE MIXES DESIGN

The British method has applied to design the concrete. General process for designing concrete

mix is summarizes as follows:

STAGE DESCRIPTION

1 Deals with strength leading to the free-water/cement ratio

2 Deals with workability leading to the free-water content

3 Combines the results of Stages 1 and 2 to give the cement content.

4 Deals with the determination of the total aggregate content.

5 Deals with the selection of the fine and coarse aggregate contents.

The unrestricted design has been selected. The information of the requirement item on mix design

is shown in Table 3.1 and all available data for designing concrete mix are attached as Appendix.

You shall produce concrete mixes for Grade 30 concrete cubes. You need to prepare 3 concrete

test cubes (150x150x150 mm) for compression test at 7, and 28 days lifetime.

Table 3.1: The requirements and relevant i tem on the mix design form

No. Descriptions Specified

1 Characteristic compressive strength at 28 days 30N/mm2

2 Proportion defective 5% (k=1.64)

3 Cement type OPC

4 Maximum aggregate size 20mm

5 Aggregate type:

i. Coarse aggregate

ii.

Fine aggregate

Crushed

Uncrushed

6 Slump required 30 60 mm

7 Grading of fine aggregate - % passing 600m 54%

8 Relative density 2.70


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Cube

Calculation for Quantity of Concrete

Calculation of quantity required to cast concrete structure must be based on the volume of the

structure. In addition, extra quantity is required for cubes to enable compression strength tests to be

carried out. However, during mixing, an additional of 10% volume is recommended to

accommodate for waste. Table 3.3 shows the calculation in obtaining the concrete volume.

Table 3.3: Calculation of concrete Volume

i) The trial mix cubes mixing volume

= Total Volume + 10% additional

= (0.02025 + 0.002025) m3

= 0.022275 m3

CalculationT e of structure Total volume

6 cubes

= 6x0.003375

= 0.02025 m3

Size of mould 150mm x 150mm x

150mm Volume for one cube

= 0.15x0.15x0.15

= 0.003375 m3


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Table 3.2: Concrete Mix Design Form

Stage ItemReference or

calculationValues

1 1.1 Characteristic strength Specified 30 N/mm2 at 28 days

Specified Proportion defective 5 Percent

1.2 Standard deviation Figure A1.1 _____8_______ N/mm2 or no data - N/mm2

1.3 Margin C1(k = 1.64 ) 1.64 x 8 = 13N/mm2

1.4 Target mean strength C2 30 + 13 = 43 N/mm2

1.5 Cement type Specified OPC

1.6 Aggregate type: coarse Crushed

Aggregate type: fine Uncrushed

1.7 Free-water/cement ratioTable A1.3

Figure A1.20.54

1.8Maximum free water/ cement ratioSpecified

Use the lower value

2 2.1 Slump or Vebe time Specified Slump 30 60 mm or Vebe time s

2.2 Maximum aggregate size Specified 20 mm

2.3 Free-water content/cement ratio Table A1.4 190 kg/m3

3 3.1 Cement content C3 190 / 0.54 = 352 kg/m3

3.2Maximum cement contentSpecified

- kg/m3

3.3Minimum cement content Specified

- kg/m

Use 3.1, if < 3.2Use 3.3, if > 3.1

3.4 Modified free water/cement ratio 352 kg/m3

4 4.1 Relative density of aggregate(SSD)

2.7 known/assumed

4.2 Concrete density Figure A1.3 2445 kg/m3

4.3 Total aggregate content C4 2445 - 190 - 352 = 1903 kg/m3

5 5.1 Grading of fine aggregate% pass 600m

sieve54 %

5.2 Proportion of fine aggregate Figure A1.4 38 %

5.3 Fine aggregate content C5 1903 x 0.38 = 723 kg/m3

5.4 Coarse aggregate content C5 1903 - 723 = 1180 kg/m3

QuantitiesCement

(kg)

Water

(kg)

Fine aggregate

(kg)

Coarse aggregate (kg)

10 mm 20 mm 40 mm

Per m3(to nearest 5 kg) 355 190 725 395 785

Per trial mix of 0.022275 m3 7.91 4.23 16.15 8.80 17.49

Note: Design as according to BS 1881

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