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Mix Design of Concrete

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PROPERTIES OF CONCRETE

A good concrete should have three basic qualities : a) Strength b) Workability c) Durability

Table 1: Nominal Mixes As Per IS: 456-2000(Fine Aggregate of Zone II As Per IS: 383- 1970)Grade of As per Mix ratio by wt concrete IS:383-1970 Cement Fine Coarse Max size of aggregat aggregat graded e e coarse aggregate M20 M20 M20 10 20 40 1 1 1 : : : 1.8 1.5 1.3 : : : 2.7 3.0 3.2 Max Max W/C cement ratio aggregate ratio by mass 0.60 0.60 0.60 1:5 1:5 1:5

Proportions by weight can be converted to proportions by volume, by dividing with the bulk density of the materials available for use at site. The bulk density of sand may be taken 1.44 Kg/lit

Table 2: Grades Of Concrete As Per IS:456-2000Group Grade Designation Specified characteristic compressive strength of 150mm cube at 28 days N/mm10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

Ordinary Concrete Standard Concrete

M10 M15 M20 M25 M30 M35 M40 M45 M50 M55 M60 M65 M70 M75 M80

High Strength Concrete

DURABILITY OF CONCRETE A durable concrete is one that performs satisfactorily in the working environment under its anticipated exposure conditions during service. Main characteristics influencing the durability of concrete is its permeability to ingress of water, oxygen, carbon dioxide, chloride, sulphate and other potentially deleterious materials. Factors influencing durability includea) b) c) d) e) f) The environment The cover to embedded steel The type and quality of constituent materials The cement content and water/cement ratio of the concrete Workmanship, to obtain full compaction and efficient curing The shape and size of the member

STATISTICAL PRINCIPLEFor assessing the acceptance criteria of concrete on statistical principles a large number of cube test results are needed. At a given stage of construction, let the cube strength of n samples be available. The strength of each sample is the average strength of 3 samples. The sampling is also done in a random manner. If the strength of ith sample Mean strength, fm Deviation of each sample from mean Standard deviation = fi = fi / n = (fm fi ) = (fm fi )2/(n-1)

The value of standard deviation will be low in case of good control and high in case of lax control. The acceptability of concrete depends upon achieving a desired mean strength and keeping the deviation small.

Acceptance criteria as per old codea) The grade of concrete is specified. M25 concrete means concrete with a characteristic strength of 25 MPa at 28 days curing. A target mean strength is fixed. ft = fck + k x s ( The value of k for calculating ft is taken as 1.65, which gives a probability of not more than 5% of the samples tested will fall below fck) Each sample is checked for acceptability. Test strength of each sample will be the average of three specimen with individual variation being not more than 15% of the average.

b)

c)

Clause 15.1 of IS: 456-1978 a) Accept all samples with fi fck b) For samples with strength less than fck two checks are applied. Both checks must be satisfied First check : fi should not be less than the larger of two values.(i) fck 1.35 x s (ii) 0.8 x fck

Second check : Average strength of all the samples is not less than fck + 1.65 [ 1-(1/n)] s Clause 15.2 of IS: 456-1978 a) Reject the concrete if fi is less than the greater of(i) fck 1.35 x s (ii) 0.8 x fck

b) Reject the concrete also if fi < fck + [1.65 (3/n)] s Note : concrete which does not meet the requirement of Clause 15.1 but has a strength greater than required by clause 15.2 can be accepted at the discretion of the designer.

Acceptance criteria as per new codeThe acceptance criteria as per new code is : a) fm fck + 0.825 x s or (fck +4) MPa , whichever is greater AND b) fi (fck 4) MPa Where, fm = Mean of four results in a group fi = Strength of any individual sample s = Established standard deviation rounded off to nearest 0.5 MPa Note :- When sufficient number of samples have not accumulated, a standard deviation of 4.0 MPa is assumed for M20 & M25 concrete. While 5.0 MPa is taken for M30 to M50. The calculated value of s is used as soon as 30 results are available.

( BASED ON IS:456 2000)

METHOD OF CONCRETE MIX DESIGNStep 1 :Work out the Target mean strength of concrete from the relation:

fck = fck +t x sWhere,

fck =fck =s t

Target average compressive strength at 28 days characteristic compressive strength at 28 days

= Standard Deviation = 1.65 (as per IS:456-2000 clause 9.2.2 )

Table 3: Assumed Standard Deviation For Three Strength GroupsGrade of Concrete Assumed standard deviation MPa3.50 4.00

M10 M15 M20 M25 M30 M35 M40 M45 M50

5.00

The above values correspond to the site control having proper storage of cement; Weigh batching of all materials; controlled addition of water; regular checking of all materials, aggregate grading and moisture content; and periodic checking of workability and strength. Where there is deviation from the above, the values given in the above table shall be increased by 1 N/mm

Step 2 :Find out the value of W/C ratio against the target mean strength from the curves available for the purpose. Select the appropriate curve for the type of coarse aggregate and the type of cement being used. Step 3 :For durability consideration check the obtained free watercement ratio from appropriate table of IS:456-2000. If required use lower value of water cement ratio. Step 4 :As per clause 7 page 17 of IS:456-2000 find out the degree of workability required for the job. Step 5 :From table 4, find out the quantity of free water in Kg/m for the type and maximum size of aggregate being used and for the required workability and normal- air entrained concrete.

Step 6 :Find out the cement content in Kg/m from the information of step 2 and step 5.Cement content = Free water content Water cement ratio

For durability consideration, check the minimum cement content , if required take higher value. Step 7 :Find out the density of concrete in Kg/m for different maximum sizes of coarse aggregate corresponding to particular value of combined specific gravity of aggregates from the given tables 5,6 &7. Step 8 :Find out the aggregates content per cum of concrete. Total aggre = conc density Free water cement content(Kg/m ) (Kg/m ) (Kg/m ) (Kg/m )

Step 9 :Find out the proportions of sand (percent) of the total aggregate from table 8 for particular maximum size of coarse aggregate, required workability, zone of sand and water- cement ratio. Find out the quantity of fine aggregates in Kg/m from total aggregate calculated in step 8 above. Step 10 :Find out the quantity of coarse aggregate in Kg/m by subtracting the quantity of fine aggregate from total aggregates. Step 11 :If the coarse aggregates are in more than one fraction , they shall be combined in a suitable proportion to get a graded coarse aggregate as per IS: 383-1970. In general, if the coarse aggregates are in single size as per IS: 383-1970, they shall be combined as follows for obtaining a combined graded aggregate. (a) 10 & 20 mm aggregate in ratio 1:2 (b) 10, 20 & 40 mm aggregate in ratio 1:1.5:3

IMPORTANT NOTETHE CALCULATED MIX PROPORTIONS MUST BE CHECKED BY TRIAL MIXES FOR REQUIRED SPECIFICATIONS, AND AS PER REQUIREMENTS SUITABLE ADJUSTMENT IN THE MIX PROPORTION SHOULD BE DONE.

COMPRESSIVE STR VS W/C RATIO(Uncrushed coarse aggregate)80.0 70.028 DAY COMPR STR OF CONCRETE (N/mm)

A=21.6 25.0 N/mm B=25.0 29.8 N/mm C=29.8 35.0 N/mm D=35.0 41.5 N/mm E=41.5 48.0 N/mm F=48.0 53.4 N/mm

60.0 50.0 40.0 30.0 20.0 10.0 0.0 0.30

F ED C B A

(Values given above correspond to the 7- day strength of cement tested according to IS: 4031-1968)

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

WATER CEMENT RATIO

COMPRESSIVE STR VS W/C RATIO(Crushed coarse aggregate)80.0 70.028 DAY COMPR STR OF CONCRETE (N/mm)F

A=21.6 25.0 N/mm B=25.0 29.8 N/mm C=29.8 35.0 N/mm D=35.0 41.5 N/mm

E

E=41.5 48.0 N/mm F=48.0 53.4 N/mm (Values given above correspond to the 7- day strength of cement tested according to IS: 4031-1968)

60.0 50.0 40.0 30.0 20.0 10.0 0.0 0.30

D CB A

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

WATER CEMENT RATIO

Water cement ratio & cement contents for DurabilityType of Exposure Plain Cement Concrete Min cement Kg/m Mild 220 Max Free W/C 0.6 Min grade of concrete Reinforced Cement Concrete Min cement Kg/m 300 Max Free W/C 0.55 Min grade of concrete M20

Moderate

240

0.6

M15

300

0.5

M25

Severe

250

0.5

M20

320

0.45

M30

Very Severe Extremely Severe

260

0.45

M20

340

0.45

M35

280

0.40

M25

360

0.4

M40

Note :- Min cement content value applies to 20mm max size of agg

Adjustments to minimum cement content for aggregates (other than 20mm max size of agg)Sl No. Nominal max aggregate size Adjustments to minimum cement content +40 0 -30

1. 2. 3.

10 20 40

Note :a) b) Cement content prescribed in the table is irrespective of the grades of cement and is inclusive of mineral admixtures like fly ash, silica fume etc. The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement content and water cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolona and slag specified in IS 1