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Mix design

Concrete mix Design




 

 


1            INTRODUCTION

Mix Design M20 


The trials for the mix for concrete grade M20

 are undertaken and 28 days of results are awaited. Based on early-age strength, the following mixes are suggested for site trials. Detailed report follows 

Concrete mix design is the process of determining the proportions of cement, water, fine aggregate, and coarse aggregate for a specific strength and durability requirement.

 M20 grade of concrete refers to a mix design with a characteristic compressive strength of 20 MPa (Mega Pascals) at 28 days.

 The mix design for M20 grade concrete can be done using various methods such as the IS Method, the ACI Method, or the USBR Method. 

However, the most commonly used method in India is IS Method. The IS Method for mix design of M20 grade concrete includes the following steps: 

 Selection of water-cement ratio: A water-cement ratio of 0.55 is selected for M20 grade concrete as per IS 456:2000. Selection of nominal mix proportions: 

Nominal mix proportions of 1:1.5:3 (cement: fine aggregate: coarse aggregate) are selected for M20 grade of concrete as per IS 456:2000. 

 Selection of water content: The water content is determined based on the workability requirement, which is usually between 160-180 kg/m3. 

 Selection of fine aggregate: The fine aggregate should be clean and free from organic impurities and should conform to IS 383-1970. 

 Selection of coarse aggregate: The coarse aggregate should be clean and free from organic impurities and should conform to IS 383-1970.

 Mix design calculation: Once the above parameters are selected, the mix design calculation can be done to determine the proportions of cement, water, fine aggregate, and coarse aggregate. It's worth mentioning that the above steps are general guidelines and the mix design may vary depending on the specific project requirements, materials availability, and other factors. It's also important to note that the mix design is a laboratory process and the quality of the concrete in the field will depend on many factors such as proper mixing, curing, and protection from environmental conditions.


2) Mix Design of M25

Properties For M25

Mix Proportion (By Weight) :-    1 : 1.99 : 3.25 

Water Cement Ratio:-                 0.42 

Cement Content :-                        375 kg / cu.m 

Natural Sand Content:-               746 kg / cu.m 

Coarse Aggregates 20 mm :-        914 Kg / cu.m

 Coarse Aggregates 10 mm :-       305 Kg / cu.m

 Admixture SP-430 G8 of Fosroc Chemicals 250 ml/bag Water Content (W.rt.WCRatio) 157.5 Lit / cu.m 

Water Absorption of CA :-           1.0% 12.1 Lit / cu.m 

Water Absorption of FA :-            1.2% 8.9 Lit / cu.m 

Total Water Content :-                  178.5 Lit / cu.m 

 7 days strength :-                          22.6 Mpa

***

***


Note: The cement content recommended is based on early age strength. After confirmation of 28 days'  strength, cement content may reduce.

∙ The above water content is recommended considering dry aggregates; as such correction for absorption is made. In order to maintain workability, proportionate adjustment as above (water absorption), in the quantity of water shall be made, if the aggregates are wet. 

∙ The grading of the aggregates shall be checked regularly & should match the grading of aggregate received in the laboratory as this may have an effect on the strength & other characteristics of the concrete. If necessary adjustments/corrections in the proportion of fine aggregate shall be made. 

∙ Admixture used, shall be SP-430 G8 of Fosroc Chemicals. The quality of the admixture shall be checked regularly. If required dosage shall be adjusted according to site condition and workability requirement, keeping the same water-cement ratio as recommended above. 


∙ The above results are related only to the sample tested in this laboratory. 

∙ No report shall be reproduced, except in full, without the written approval of this laboratory. ∙ Any query about the report may be reported immediately.




 Mix Design M40

 

2.2        Sieve Analysis

 

 

Sieve Size

 

Percent Passing

 

mm.

31.5mm.

10 mm.

Natural Sand

31.5

100

100

100

26.5

97.3

100

100

19

67.2

100

100

9.5

15

78.8

93.3

4.75

2.8

0.5

75.4

0.6

0

0

26.9

0.15

0

0

0.9

0.075

0

0

0.4


 

 

  1. Properties of Aggregate

 

Sr.

Properties

Natural Sand

Coarse Aggregates

a.

Specific Gravity

2.64

2.85

b.

Water absorption

1.2 %

0.9 %

c.

Deleterious Content

Nil

Nil

 

  1. Tests on Cement and Flyash

 

a.

Consistency

29.0 %

b.

Specific Gravity

3.10

c.

Initial Setting time

145 min

d.

Final Setting time

210 min

e.

Soundness by Le’Chattelier Expansion

0.5 mm

f.

Specific Surface by Blain’s Air

284 sq.m / Kg

g.

3 Days Compressive Strength

37.5 Mpa

h.

7 Days Compressive Strength

51.5 Mpa

i.

28 Days Compressive Strength

58.0 Mpa

j.

Specific Gravity of Flyash

2.22

k.

Specific Surface by Blain’s Air of Flyash

348 sq.m / Kg

 

 

  1. MIX STIPULATION

The following design, stipulations are considered based on the site conditions and requirements of IRC-15 & MORT&H2013

Properties

M40 PQC

Type of concrete

Pavement Concrete

Degree of exposure

Moderate

Degree of quality control

Good

Degree of Workability

25 – 30 mm. slump.

Maximum size of aggregate

31.5 mm

Maximum Water Cement Ratio

0.45

Minimum cementitious content

310 kg./cu.m.

Retention time

60 min


 

 

  1. Target Mean Strength (ft)

As per IRC : 58/MORTH specifies Fcr = 4.5 Mpa (For M40 PQC) fcr = Characteristic Compressive strength in ,N/mm.sq

As per IRC: 44 Target average flexural strength is given as f ʹcr   =              fcr + 1.65 x Sf

f ʹcr = Target average flexural strength at 28 days, N/mm.sq.

fcr = Characteristic flexural strength (design strength) at 28 days,N/mm.sq.

Sf = Standard deviation of flexural strength, N/mm.sq. (Table 5 of IRC : 44 - 2017) Hence     f ʹcr =          4.42 + 1.65 x 0.40 = 5.08 Mpa

Compressive strength: As per MORTH target mean compressive strength for M40 grade is 52.0 Mpa

 

  1. RECOMMENDATIONS

 

The mix is designed as per the guidelines of IS:10262. Based on the results of a number of trials, the following mix is recommended for concrete. Details of trials taken are enclosed in the Appendix.

Properties

For M40 PQC

Ratio / proportion – weight  

1 : 1.99: 2.43

Water Cement Ratio

0.32

Cementitious Content

425 kg / cu.m

Cement Content

319 kg / cu.m

Flash Content

106 kg / cu.m

Natural Sand Content

845 kg / cu.m

Coarse Aggregates 31.5 mm

723 Kg / cu.m

Coarse Aggregates 10 mm

310 Kg / cu.m

Admixture - 300 Fosroc Chemicals (Auramix)

4.2 Kg / cu.m

Water required in liter

136.0 Lit / cu.m

Water Absorption of CA @ 0.9 %

9.2 Lit / cu.m

Water Absorption of FA @ 1.2 %

10.1 Lit / cu.m

Total Water Content

155.3 Lit / cu.m


 

 

  1. The above water content is recommended considering dry aggregates; as such correction for absorption is made. In order to maintain workability, proportionate adjustment as above (water absorption), in the quantity of water shall be made, if the aggregates received are wet.

 

 

  1. The admixture used shall be, Auramix 300 of Fosroc Chemicals. The quality of the admixture shall be checked regularly. 
  2. The density calculated for the above-recommended mix is based on a laboratory trial. If there is any variation in material gradation, proportions of ingredients shall be adjusted by measuring the actual density at the site, while maintaining the same cement content.

 

  1. The quality of fly ash shall be confirmed before use. The Blain’s air-specific surface shall not be less than 320 sq.m/kg.

 

 

APPENDIX – I

1.0        MIX TRIALS

Based on the test results of different ingredients and considering the design stipulations, the initial mix is designed as per the guidelines of IS:10262 as indicated below

 

  1. MIX AS PER IS 10262: 2019 (Trial 1)

 

Mix as per IS10262


Water cement ratio

Design calculation for concrete




 

  1. VERIFICATION OF MIX AS PER IS:10262

 

The weights determined above (i.e. as per IS:10262) are converted into proportions and a trial was taken. The observations are as under –

Properties

Trial 1

Observations

Mix 

Proportion

1 : 1.63: 2.89

Mix prepared indicated 
higher workability.
 
The sand content in the mix is 36%, 
and segregation was observed.
 
The actual density is lower
 then the theoretical one, 
resulting in a higher yield of concrete. 
This must be matched to avoid variation
goog_144801757 in the quantity of concrete delivered at the site.

Water Cement

 Ratio

0.35

Cementitious 

content

425 Kg/cu.m

Cement content

319 Kg/cu.m

Flash content

106 Kg/cu.m

Admixture 

Dosage

@1.0%

Slump 

Observed after 

60 min

120 mm

Actual Density

2466 kg/cu.m.

Correction in 

Mix required

Yes

 

 

Mix prepared as per IS indicated deficiencies. Workability achieved is higher, desired workability can be achieved by reducing the water-cement ratio from 0.35 to 0.32. Sand cement increased from 36% to 45%. Fines in the mix are meager, This not only leads to segregating mix but also the combined grading does not fit-in grading as per IRC- 15:2017, MORT&H2013. Also, there is a significant difference between theoretical and actual density. This need to be matched to avoid a difference in yield. Considering all points, the mix is revised.

 

With actual density as a base, and cementitious content is as per IS trial i.e 425 kg/cu.m. mix proportion is revised. Fines are increased not only to achieve cohesiveness but also to meet IRC-15: 2017, MORTH2013 grading requirements. The proportion of aggregates are finalized as –

 

 

Natural Sand                  :

45.0 %

31.5 mm.                        :

38.5 %

10 mm.                            :

16.5 %


 


                   

 

  1. Trial 2

 

With the re-proportioned aggregates and cementitious content being 425 kg/cu.m., this trial was conducted with WCR 0.32 and an admixture dosage of 1.0%. The details of the mix are given below

Properties

Trial 2

Remarks

Mix Proportion

1 : 1.99: 2.43

  1. This trial was carried out with 
  2. cementitious content of 425 Kg/cu.m.
  3. With 45% sand content, the mix indicated good cohesiveness and texture
  4. The     strengths      achieved are

satisfactory for M40

Water Cement Ratio

0.32

Cementitious Content

425 Kg/cu.m

Cement Content

319 Kg/cu.m

Flyash Content

106 Kg/cu.m

Admixture

@1.0%

Initial Slump observed

165 mm

Slump observed after 60 min

60 mm

 

7 Days of compressive strength

36.3 Mpa

 

7 Days Flexural strength

3.34 Mpa

 

28 Days compressive strength

52.8 Mpa

 

28 Days Flexural strength

5.25 Mpa

 


 

 

 

  1. Trial 3

 

In the previous mix, the strength achieved is satisfactory for M40. One more trial was carried out with cementitious content of 450 kg./cu.m. The results are presented below –

 

Properties

Trial 3

Remarks

Mix Proportion

1 : 1.85 : 2.26

This trial was carried out with 
cementitious content of 450 Kg/cu.m.
With 45% sand content, 
mix indicated good cohesiveness and texture
The strengths achieved is higher for M40

Water Cement Ratio

0.30

Cementitious Content

450 Kg/cu.m

Cement Content

338 kg./cu.m

Flyash Content

112 kg./cu.m

Admixture

@1.0%

Initial slump

160 mm

Slump observed 

after 60 min

55 mm

7 Days of 

compressive strength

40.8 Mpa

7 Days Flexural strength

3.86 Mpa

28 Days 

compressive strength

55.3 Mpa

28 Days Flexural strength

5.62 Mpa

 

Based on the above trials mix with cementitious content of 425 kg./cu.m. is finalized for M40 PQC.

 


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