Class Note Concrete Mix Design

CLASS NOTES

ON

CONCRETE TECHNOLOGY

MIX DESIGN

Class notes on Concrete Technology Mix Design (P -1) Er. Sudip Karanjit

6.0 Mix Design of Portland Cement Concrete

Mix design is the process of determining relative quantity of concrete ingredients with the objective:

To ensure the desire workability of concrete at plastic stage (green stage).

To obtain the required strength, durability and surface finish at its hardened stage.

To achieve the economic mix.

6.1 Nominal & Design Mix of Concrete:

Nominal Mix: A concrete mix in which the proportions are adopted in generalized form irrespective

of its ingredient properties.

Design Mix: A concrete mix in which the proportions are adopted depending upon the ingredient

properties to obtain the desired workability, strength & durability of concrete.

6.2 Introduction to Nominal Mix:

Nominal mixes are suitable only upto the M20 grade concrete and desired strength / workability

cannot be guaranteed. But still useful if no any mix design data is available in site.

Table 6.1: Nominal Mix given by IS456:2000

Concrete Grade

Total dry Aggregate( in Kg )per 50Kg Cement

Fine: Coarse (Proportion)

Water per 50Kg of Cement

M5 800 Generally 1:2 but

subjected to an upper limit of 1:1.5 & lower

limit of 1:2.5

60 M7.5 625 45 M10 480 34 M15 330 32 M20 250 30

Example: For M15 grade concrete if we adopt, Cement=50Kg, Total Aggregate=330Kg, Water=32Kg

Fine:Coarse =1:2, Fine=330/(1+2)=110Kg, Coarse=2*330/(1+2)=220Kg

Cement: Fine: Coarse = 50: 110: 220 = 1: 2.2: 4.4, Water/Cement = 32/50 = 0.64

Nominal Mix = 1: 2.2: 4.4 and W/C =0.64

6.3 Mix Design of Concrete by DOE, ACI and Indian Standard Methods:

DOE Method

Developed by The Building Research Establishment Laboratory (BRE), Department of Environment (DOE), U.K. DOE method outlines the procedure for design of concrete mix with 28 days cube

strength as high as 75Mpa.

The proportion of fine aggregate is determined depending upon the max. size of aggregate, degree

of workability, grading of fine aggregate & W/C ratio.

The degree of workability is indicated by specific value of slump & Vee -Bee time.


Design Steps:

Step1: Determine target mean strength (Fmean) from specified characteristic strength(Fck):

Fmean = Fck+ * K

= Standerd deviation (IS456:2000 page23 For M10/15=3.5, M20/25=4.0 M30-50=5.0)

K= Himsworth constant (1.64 for 95% confidence level)

Figure 6.1: Normal Distribution curve.

Step2: Determine Minimum W/C ratio based on Target Strength and compare with W/C

requirement for Durability consideration.

Fig: 6.2: Compressive strength vs W/C ratio

Table 6.2: Approximate compressive strength of

concrete with free water cement ratio W/C 0.5

1 2


Table 6.3: Minimum Cement Content & Max. W/C for Durability:

Exposure PCC RCC Minimum Grade of concrete

Min. Cement Kg/m3

Max. free W/C

Min. Cement Kg/m3

Max. free W/C

PCC RCC

Mild 220 0.6 300 0.55 - M20 Moderate 240 0.6 300 0.5 M15 M25

Severe 250 0.5 320 0.45 M20 M30 Very severe 260 0.4 340 0.45 M20 M35

Extreme 280 0.4 360 0.4 M25 M40

Step3: Determine free water content depending upon max size & type of aggregate and degree of

workability. Table 6.4: Approximate Water content (Kg/m3) for various degree of Workability:

Max. Size of Aggregate

mm

Slump 0-10 10-30 30-60 60-180 Vee-Bee >12 6-12 3-6 0-3

Aggregate Type 10 Uncrushed 150 180 205 225

Crushed 180 205 230 250

20 Uncrushed 135 160 180 195 Crushed 170 190 210 225

40 Uncrushed 115 140 160 175 Crushed 155 175 190 205

Step4: Calculate cement content from W/C ratio & water content of mix determined previously. Compare value with min. / max. value of cement content of durability requirement (Table 6.3) & modify if required. Step5: Determine wet density of concrete depending upon free water content & relative density of combine aggregate. Calculate Total aggregate content knowing the Wet density of concrete.

Fig: 6.3: Wet density of mix vs Free water content.


Calculate total aggregate content (Saturated surface dry): o-c-w

o= Wet density of concrete (Kg/m3) c= Cement Content (Kg/m3) w= Free Water Content (Kg/m3) Step6: Determine Proportion of fine aggregate depending upon the W/C ratio, Max size of

aggregate, fine aggregate grading zone & workability level.

Fine Aggregate =Total Agg. * Proportion of Fine Agg. Coarse Aggregate= Total Agg. Fine Agg.

Fig: 6.4 Recommended % of fine aggregate in total aggregate vs W/C ratio. (Nominal max. size 20mm)

Fig: 6.5 Recommended % of fine aggregate in total aggregate vs W/C ratio. (Nominal max. size

40mm)


EXAMPLE DOE METHOD: Design concrete mix for RCC work for the moderate exposure

environment. The characteristic strength required is 30Mpa. Max aggregate size = 20mm crushed.

Sieve analysis of fine aggregate shows 50% passing through 600 sieve. Average Sp. gr. of aggregate

is 2.65. Slump required 30-60 mm. OPC Cement (Type1) will be used.

Step1: Fmean = Fck+ * K = 30 + 1.64*5 = 38.2Mpa (For M30, =5 Ref; IS456:2000 page23)

Step2: Minimum W/C ratio based on Tar]\get Strength (Fig6.2 / Table 6.2) = 0.6

Minimum W/C ratio based on durability (Table 6.3) = 0.5

Adopt minimum W/C = 0.5

Step3: Free water content (Table 6.4) = 210kg/m3

Step4: Cement content (Step 2&3) = 210/0.5 = 420kg/m3 > min cement 300kg/m3 from Table 6.3

(OK)

Step5: Wet density of concrete for 210kg free water content & sp gr aggregate 2.65 (Fig 6.3) =

2375kg/m3

Total aggregate content = Total Concrete Cement Water = 2375 420 -210 = 1745kg/m3

Step6: % of fine aggregate (Fig 6.4 for slump 30-60 & W/C = 0.5 & FA 50% passing 600 sieve.) = 35%

of total aggregate

F A = 0.35 * 1745 = 610.75kg/m3

C A = 1745 610.75 = 1134.25kg/m3

Cement : Fine : Coarse = 420 : 610.75 : 1134.25 = 1 : 1.454 : 2.701


ACI METHOD

The American Concrete Institute mix design method is suggested by the ACI Committee 211.

One method is based on the estimated weight of the concrete per unit volume & the other method is based on calculation of the absolute volume occupied by concrete ingredient.

This method consider the requirement for workability, consistency, strength and durability of concrete.

Design Steps:

Collect the data required for mix design first:

- Fineness modulus of fine aggregate, Sp. Gravity of fine & coarse agg. , Unit wt. of dry rodded

coarse agg. & Sp. Gr. of cement.


Fmean = Fck+ * K

= Standerd deviation (IS456:2000 page23 For M10/15=3.5, M20/25=4.0 M30-50=5.0)

K= Himsworth constant (1.64 for 95% confidence level)

Step2: Determine Minimum W/C ratio based on Target Strength and compare with W/C requirement for Durability consideration. Table 6.5: Relation between W/C ratio & Av. Compressive strength (ACI 211.1:91)

38.2 0.45


Table 6.6: Relation between W/C ratio & Exposure conditions (ACI 318-89)

Step3: Determine free water content depending upon max size & type of aggregate and degree of workability.

Table 6.7: Water content determination depending upon slump & max. agg. size

Step4: Calculate cement content from W/C ratio & water content of mix determined previously. Step5: Determine Bulk volume of dry rodded C A depending upon Max. Agg. size & Fineness Modulus of fine aggregate (Table 6.8). Calculate Wt. of C A = Bulk Volume * Bulk Density


Table: 6.8 Bulk Volume of Dry rodded Coarse Aggregate depending upon FM & Max Agg. size

Step6: Determine Wt. of Fresh concrete (Table 6.9) per m3: Table 6.9:

Wm = 10A(lOO-A)+c(1-A/ C) -w(A -1)

where

Wm = weight of fresh concrete, kg/m3 A = weighted average specific gravity of combined fine and coarse aggregate

C = specific gravity of cement (= 3:15)

c = cement requirement, kg/m3

w = mixing water requirement, kg/m3

A = air content, percent

Step7: Determine Wt of F A required:

F. A. (wt.) = Total wt of concrete (step6) Cement (step4) Water (step3) C A (step5)

Now Mix proportion = Cement: FA: CA & W/C

OR


EXAMPLE ACI METHOD: Design concrete mix for RCC underground water tank. The characteristic

strength required is 30Mpa at 28days. Max aggregate size = 20mm crushed. Sieve analysis of fine

aggregate shows Fineness modulus 2.6. Dry rodded bulk density of CA = 1600kg/m3. Average Sp. gr.

of aggregate is 2.65. Slump required 30-60 mm. OPC Cement (Type1) will be used.

Step1: Fmean = Fck+ * K = 30 + 1.64*5 = 38.2Mpa (For M30, =5 Ref; IS456:2000 page23)

Step2: Minimum W/C ratio based on Target Strength (Table 6.5) = 0.45



Step3: Free water content (Table 6.4) = 185kg/m3

Step4: Cement content (Step 2&3) = 185/0.45 = 411kg/m3

Step5: Bulk volume of dry rodded C A = 0.64 {depending upon Max. Agg. Size 20mm & Fineness

Modulus of fine aggregate 2.6 (Table 6.8)}

Calculate Wt. of C A = Bulk Volume * Bulk Density = 0.64m3 * 1600kg/m3 = 1024 kg/m3

Step6: Wet density of concrete for Max 20mm aggregate & Normal Concrete (Table 6.9) =

2355kg/m3

Step7: Determine Wt of F A required:

F A required: = F. A. (wt.) = Total wt of concrete (step6) Cement (step4) Water (step3) C A

(step5)

= 2355 411 - 185 1024 = 735kg/m3

Cement : Fine : Coarse = 411 : 735 : 1024 = 1 : 1.788 : 2.491

W/C = 0.45


IS METHOD OF MIX DESIGN

This method is recommended by the Bureau of Indian standards covered in IS10262: 1982. The method given can be applied from medium to high strength concrete. As the procedure developed in 1982 & since then lots of modification seen in concrete technology, code revision seems to be required. Design Steps:


Fmean = Fck+ 1.65 * S

S = Standerd deviation (IS456:2000 page23 For M10/15=3.5, M20/25=4.0 M30-50=5.0)

Step2: Determine Minimum W/C ratio based on Target Strength & Cement type, and compare

with W/C requirement for Durability consideration.

Fig: 6.6: Compressive strength vs W/C ratio

IS10262:82

Fig 6.7: Compressive strength of concrete various

cement type IS10262:82


Fig 6.7: Compressive strength of concrete various cement type (Germany)

Table 6.10: Minimum Cement Content & Max. W/C for Durability (IS456:2000):

Exposure PCC RCC Minimum Grade of concrete

Min. Cement Kg/m3

Max. free W/C

Min. Cement Kg/m3

Max. free W/C

PCC RCC

Mild 220 0.6 300 0.55 - M20

Moderate 240 0.6 300 0.5 M15 M25 Severe 250 0.5 320 0.45 M20 M30

Very severe 260 0.4 340 0.45 M20 M35 Extreme 280 0.4 360 0.4 M25 M40

Step3: Determine free water content & Fine to total Agg. ratio depending upon max size of

aggregate & concrete Strength for standard condition.

Make the adjustment depending upon Ingredient properties.

Table 6.11: Approximate Sand & Water content (Kg/m3) for Compaction Factor CF =0.8

(approximate slump 30mm), Angular coarse aggregate & Sand zone II as per IS383:1970

Upto M35 W/C - 0.6

CF =0.8

> M35 W/C 0.35

CF =0.8


Table 6.12: Adjustment table for water content & % sand in total aggregate.

Step4: Calculate cement content from W/C ratio & water content of mix determined previously. Compare value with min. / max. value of cement content of durability requirement (Table 6.10) & modify if required. Step5: Calculate Aggregate content:

Step6: The mix proportion above obtained by the assumption that the aggregates are saturated &

surface dry if any deviation, make adjustment in water & aggregate content.


EXAMPLE IS METHOD: Design concrete mix for RCC Silo. The characteristic strength required is

30Mpa at 28days. Max aggregate size = 20mm crushed. Sieve analysis of fine aggregate shows Sand

zone III. Workability required compaction factor 0.9. Exposure condition moderate. Cement Type E -

53Mpa will be used. Sp. gr. of CA = 2.7 / FA = 2.6 / Cement = 3.15.

Step1: Fmean = Fck+ 1.65*S = 30 + 1.65*5 = 38.25Mpa (For M30, =5 Ref; IS456:2000 page23)

Step2: Minimum W/C ratio based on Target Strength (Fig 6.7) = 0.45



Step3: Free water content (Table 6.11) = 186kg/m3 & F A = 35% of Total Agg.

Adjustments (Table 6.12):

Condition Change Water Content % adjustment Sand content % Adjustment

Sand confirming Zone III 0 - 1.5%

Increase in Compaction Factor

(0.9-0.8) = 0.1 + 3% 0

Decrease in W/C ratio (0.45-0.6)

= -0.15 0

- (0.15/0.05)

= - 3%

Total +3% -4.5%

F A content = 35 -4.5 = 30.5%

Water content =186 +3% *186 = 191.6kg

Step4: Cement content (Step 2&3) = 191.6/0.45 = 425.8kg/m3 > 240kg ok

Step5: Calculate Aggregate content:

(1 0.02) = (191.6+425.8

3.15+

.3052.6)

1

1000

fa = 518kg/m3

=10.305

0.305 518

2.7

2.6

Ca= 1225.8kg/m3

Cement : Fine : Coarse = 425.8 : 518 : 1225.8 = 1 : 1.217 : 2.879 & W/C = 0.45

Air content

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Class Note Concrete Mix Design

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