Project on Concrete Mix Design for various grades of Concrete
Aim of the Project:
1) To stipulate the mix design for grade M35 concrete with Fly ash as partial replacement of cement.
2) To stipulate the mix design for grade M50 concrete without Fly ash.
Introduction:
Concrete is a versatile agglomerate of building ingredients, constituted together to form a hard mass that is strong enough to resist different kinds of loading conditions throughout its useful life. Starting with the basic ingredients, concrete is a mix of cement, sand and aggregates with specific amount of water. The aggregates are mixed in prescribed or pre stipulated proportions which is also the main objective of this project.
Applications of concrete are numerous and nowadays every construction work from minor to major is completed successfully using concrete. As the applications varies the concrete varies too. For minor projects, projects of less importance or for plain cement concrete works usually the most basic grade of cconcrete is used. Talking about the grades, the concrete ranges from M5,M10,M15 to M50 and above where M denotes the mix and 5 denotes the characteristic strength developed after 28 days of proper curing.
Concrete grades are generally adopted based upon the structural importance or heavy duty applications. Like for a bridge or its component construction the minimum grade of concrete used is M30 due to its importance, heaviness and load bearing capacity. Similarly for plain cement concrete generally M5 or M10 are used. These grades and their ratios are generally prescribed in several standard codes Such as India Standards, American and British codes. In India we follow the IS codes and the basic code for concrete mix design and structural members such as beams and column is IS 456:2000. For design mix considerations a combination of IS 456:200 and IS 10262:2009 is used.
Methodology
Here we carry out the step by step procedure of the objectives to be completed. It involves the collected date based upon the different materials being used for the specific objectives.
1) To stipulate the mix design for grade M35 concrete with Fly ash as partial replacement of cement.
Materials required - Cement OPC 53 grade, Coarse aggregates (20 mm Nominal size) conforming to IS 456:2000 and IS 10262:2009, Fine Sand, Water (Clean), Fly ash class 'F', Superplasticizer Conforming to IS 9103.
Assumed Data- This data is generally taken by carrying out tests for various materials in accordance with the specific codes and is strictly recommended. But for this project demonstration we assume the standard values and specify the tests to be conducted before stipulation of mix design.
Tests to be conducted for materials before mix stipulations-
a) Consistency of cement
b) Fineness of Cement
c) Soundness of cement
d) Specific gravity of cement
e) Specific gravity of Coarse aggregates
f) Specific gravity of Coarse aggregates
g) Water absorption of coarse aggregates
f) Crushing value of coarse aggregates
g) Silt content of fine aggregates
h) Sieve Analysis of Coarse and Fine aggregates to find Fineness Modulus
e) Sieve Analysis of fine aggregates for conforming zone of sand.
so the assumed data we have as-
Gcem= 3.15, Gcoarse = 2.65 = Gfine, Gadm = 1.145, Gflyash = 2.2, Exposure Condition = Severe, Optimum Fly ash content = 21%, Slump Value = 50 to 75 mm, Platicizer effectiveness = 26%.
Step1 Target Mean Strength
fck' = fck + 1.65s
fck = 35 N/sqmm after 28 days of curing and s = Standard deviation = 5.0 for M30 and above
IS 10262 T-8
fck' = 35 + 8.25 = 43.25 N/sqmm.
Step 2 Water Cement Ratio
a) conforming to IS 456:2000, T-5 for severe exposure condition,
w/c = 0.45, and minm cement content 320 Kg/cum
b) Graph 1 from the curve indicating Strength Vs Wc ratio,
we have 0.36
c) Graph 2 consisting of different curves where our TMS lies between E values so accordingly w/c = 0.42
Now adopting the lowest of the three we have w/c = 0.36
Step 3 Water Content
T-2 IS 10262:2009 for 186 lts for slump 25 to 50 mm and an increment of 3% for every 25mm increase.
So for 50 t0 75mm we have w/c = 186 + (186*0.03) = 192lts.
since superplasticizer is being used so 192 - (192*0.26) = 142 lts respectively.
Step 4 Cement content
w/c = 0.36
c = 142/0.36 = 394.44 = 395 Kg/cum respectively.
Step 5 Aggregate Volume
As per IS 10262:2009 T-3
0.62 for w/c 0.50 but here for every decrement of 0.05 in w/c 0.01 is added to aggregate volume
so for 0.36 which is 0.14 less than 0.50 so for 0.14 difference
Agg Volume = 0.62 + 0.028 = 0.648.
Volume Fine = 1-0.648 = 0.352.
Step 6 Volume/cum
a) Concrete Volume = 1cum
b) Cement = Cement content/Gcem * 1/Gwater same formula for the following calculations
here since cement is replaced so = 395 - (395*0.21) = 312 Kg/cum
Fly ash content = 395 - 312 = 83 Kg/cum
c) water = 142/1000 = 0.142 cum
d) Admixture = 7.6/1.145*1000 = 0.006 cum
e) Fly ash = 83/2.2*1000 = 0.038 cum
f) Aggregate Volume = 1-0.099-0.142-0.006-0.0038 = 0.715
basically a - ( b+c+d+e)
g) Coarse aggregate = Gcoarse*f*Vcoarse*1000 =2.65*0.715*0.648*1000 = 1228 Kg/cum
here Vcoarse = 0.648
h) Fine aggregate = 2.65*0.715*0.352*1000 = 667 Kg/cum
Vfine = 0.352
Step 7 Stipulations
Cement = 312 Kg/cum
Sand = 667 Kg/cum
Coarse = 1228 Kg/cum
Water = 142 lts/cum
Adm = 7.6 Kg/cum
Fly ash = 83 Kg/cum
Step 9 Mix ratios
1: 667/312 : 1228/312
1: 2.15: 3.95
So the above stipulated ratios are to be used for M35 concrete with fly ash as partial replacement of cement.
Now moving on to the next objective we have,
2) To stipulate the mix design for grade M50 concrete without Fly ash.
Materials required - Cement OPC 53 grade, Coarse aggregates (20 mm Nominal size) conforming to IS 456:2000 and IS 10262:2009, Fine Sand, Water (Clean), Superplasticizer Conforming to IS 9103.
Assumed Data- This data is generally taken by carrying out tests for various materials in accordance with the specific codes and is strictly recommended. But for this project demonstration we assume the standard values and specify the tests to be conducted before stipulation of mix design.
Tests to be conducted for materials before mix stipulations-
a) Consistency of cement
b) Fineness of Cement
c) Soundness of cement
d) Specific gravity of cement
e) Specific gravity of Coarse aggregates
f) Specific gravity of Coarse aggregates
g) Water absorption of coarse aggregates
f) Crushing value of coarse aggregates
g) Silt content of fine aggregates
h) Sieve Analysis of Coarse and Fine aggregates to find Fineness Modulus
e) Sieve Analysis of fine aggregates for conforming zone of sand.
so the assumed data we have as-
Gcem= 3.15, Gcoarse = 2.65 = Gfine, Gadm = 1.145, Gflyash = 2.2, Exposure Condition = Very Severe, Slump Value = 50 to 75 mm, Platicizer effectiveness = 20%.
Step1 Target Mean Strength
fck' = fck + 1.65s
fck = 50 N/sqmm after 28 days of curing and s = Standard deviation = 5.0 for M30 and above
IS 10262 T-8
fck' = 50 + 8.25 = 58.25 N/sqmm.
Step 2 Water Cement Ratio
a) conforming to IS 456:2000, T-5 for very severe exposure condition,
w/c = 0.45, and minm cement content 340 Kg/cum
b) Graph 1 from the curve indicating Strength Vs Wc ratio,
we have 0.30
c) Graph 2 consisting of different curves where our TMS lies above F values so accordingly w/c = 0.35
Now adopting the lowest of the three we have w/c = 0.30
Step 3 Water Content
T-2 IS 10262:2009 for 186 lts for slump 25 to 50 mm and an increment of 3% for every 25mm increase.
So for 50 t0 75mm we have w/c = 186 + (186*0.03) = 192lts.
since superplasticizer is being used so 192 - (192*0.20) = 157.6 lts respectively.
Step 4 Cement content
w/c = 0.30
c = 157.6/0.30 = 448 = 448 Kg/cum respectively. which is less than 450Kg/cum so OK.
Step 5 Aggregate Volume
As per IS 10262:2009 T-3
0.62 for w/c 0.50 but here for every decrement of 0.05 in w/c 0.01 is added to aggregate volume
so for 0.30 which is 0.20 less than 0.50 so for this difference
Agg Volume = 0.62 + 0.03 = 0.65.
Volume Fine = 1-0.648 = 0.352.
Step 6 Volume/cum
a) Concrete Volume = 1cum
b) Cement = Cement content/Gcem * 1/Gwater same formula for the following calculations
= 448/3.15*1000 = 0.142
c) water = 157/1000 = 0.157 cum
d) Admixture = 7.6/1.145*1000 = 0.006 cum
e) Aggregate Volume = 1-0.142-0.157-0.006 = 0.695
basically a - ( b+c+d+e)
f) Coarse aggregate = Gcoarse*f*Vcoarse*1000 =2.65*0.695*0.65*1000 = 1197 Kg/cum
here Vcoarse = 0.65
h) Fine aggregate = 2.65*0.695*0.35*1000 = 645 Kg/cum
Vfine = 0.35
Step 7 Stipulations
Cement = 448 Kg/cum
Sand = 645 Kg/cum
Coarse = 1197 Kg/cum
Water = 157 lts/cum
Adm = 7.6 Kg/cum
Step 9 Mix ratios
1: 645/448 : 1197/448
1: 1.45: 2.67
So these are the stipulated ratios to be used for M50 grade concrete.
Important tests after mix design stipulations for on and off field are
a) Slump test
b) Compaction Factor test
c) Compressive Strength test after 7 and 28 days curing
d) Kelly Ball test
e) Vee Bee test
f) Flow Table Test
Conclusion
In this project we learned how to design a specific grade concrete mix with an understanding of the different materials used, as the materials are strictly tested for their actual properties and accordingly their values are used in the mix calculations.
The tests are done both prior and after stipulation of the mix so as to check the quality of the mix designed. The design mix method as observed provide us the highest degree of control but at the same time requires precision. Tests are important as they validate the results and if in any case the test results are not satisfactory then the mix has tobe redesigned.
With the methodology used we obtained the ratio values for the M35 mix as 1: 2.15: 3.95
and for the M50 mix as 1: 1.45: 2.67.