Superplasticizers belongs to a class of water reducer chemically different from the normal water reducers and capable of reducing water content by about 30%. The Superplasticizers are broadly classified into four groups: sulfonated melamine formaldehyde condensate (SMF), sulphonated naphthalene formaldehyde condensate (SNF), modified lignosulphonate (MLS) and others including sulphonic acid ester, polyacrylates, polystryrene sulphonates, etc. The benefits obtained by Superplasticizers in the reduction of water in the concrete mixes are best illustrated by the following examples.
MIX DESIGN DETAILS
1 | Grad of Concrete | : | M-40 |
2 | Cement | : | Three mixes are to be designed |
MIX-A
With PPC (Flyash based) conforming to IS:1489-part-I-1991. 7 days strength 38.5 N/mm^{2}. Specific Gravity : 3.00 | |||
MIX-B
With OPC-43- Grade conforming to IS: 8112-1989. 7 days strength 40.7 n/mm^{2}. Specific Gravity : 3.15 | |||
MIX-C
With OPC of Mix-B and Fly ash conforming to IS:3812 (Part-I)-2003 Specific Gravity : 2.25 | |||
Note: Requirements of all the three mixes are the same. Fine Aggregate, Coarse Aggregate and normal Super plasticizer are the same for all the three mixes. | |||
3 | Fly ash replacement | : | 30% Fly ash is required to be replaced with the total cementitious materials. |
4 | Maximum nominal size of aggregates | : | 20 mm Crushed aggregate |
5 | Fine aggregate | : | River sand of Zone-II as per IS:383-1970 |
6 | Minimum cement content | : | 360 kg/m^{3} including Fly ash |
7 | Maximum free W/C Ratio | : | 0.40 |
8 | Workability | : | 50 mm slump |
9 | Exposure condition | : | Extreme for RCC work |
10 | Method of placing | : | Site mixing |
11 | Degree of supervision | : | Good |
12 | Maximum of cement content (Fly ash not included) | : | 450 kg/m^{3} |
13 | Chemical admixture | : | Super plasticizer conforming to IS:9103-1999. With the given requirements and materials, the manufacturer of Normal Super plasticizer recommends dosages of 20 gm per kg of OPC, which will reduce 28% of water without loss of workability. For fly ash included cement dosages will be required to be adjusted by experience/ trials. |
TEST DATA FOR MATERIALS
1. The grading of fine aggregate, 10 and 20 mm aggregates are as given in Table. 1. Fine aggregate is of zone-II as per IS:383-1970. 10 and 20 mm crushed aggregate grading are single sized as per IS: 383-1970.
2. Properties of aggregates
Tests | Fine aggregate | 10 mm aggregate | 40 mm aggregate |
Specific Gravity | 2.65 | 2.65 | 2.65 |
Water Absorption % | 0.8 | 0.5 | 0.5 |
3. Target strength for all A, B and C mixes
fck = fck + 1.65 x S
40 + 1.65 x 5
= 48.3 N/mm^{2} at 28 days age
4. For Mix A and B free W/C ratio with crushed aggregate and required target strength of 48.3 N/mm^{2} at 28 days from Fig. 1 Curve D found to be 0.4 Taking into the consideration of water in admixture,let it be 0.39. This is lower than specified maximum W/C ratio value of 0.4
Note:
In absence of cement strength, but cement conforming to IS Codes, assume from Fig. 1 and Fig. 2.
Curve A and B – OPC 33 Grade
Curve C and D – OPC 43 Grade
Curve E and F – OPC 53 Grade
Take curves C and D for PPC, as PPC is being manufactured in minimum of 43 Grade of strength.
5. Other data’s: The Mixes are to be designed on the basis of saturated and surface dry aggregates. At the time of concreting, moisture content of site aggregates are to be determine. If it carries surface moisture this is to be deducted from the mixing water and if it is dry add in mixing water the quantity of water required for absorption. The weight of aggregates are also adjusted accordingly.
DESIGN OF MIX-A WITH PPC
a) Free W/C ratio for the target strength of 48.3 N/mm^{2} as worked out is 0.39.
b) Free water for 50 mm slump from Table 2 for 20 mm maximum size of aggregate.
2/3*180 + 1/3*210
= 190 kg/m^{3}
From trials also it is found that Normal Super plasticizer at a dosages of 21gm/kg of cement may reduce 28% water without loss of workability
Then water = 190 – (190 x 0.28) = 136.8 kg/m^{3}
for trials say 137 kg/m^{3}
c) PPC = 137/0.39 = 351 kg/m^{3} This is lower than minimum requirement of 360 kg/m^{3}. Then take 360 kg/m^{3}.
Then W/C = 137/360 = 0.38
d) Formula for calculation of fresh concrete weight in kg/m^{3}
U_{m}= 10 x G_{a} (100 – A) + CM(1 – G_{a}/G_{c}) – WM (G_{a} – 1)
Where,
U_{m} = Wight of fresh concrete kg/m^{3}
G_{a} = Weighted average specific gravity of combined fine
and coarse aggregate bulk, SSD
G_{c} = Specific gravity of cement. Determine actual value,
in absence assume 3.15 for OPC and 3.00 for PPC
(Fly ash based)
A = Air content, percent.
Assume entrapped air 1% for 40 mm maximum size of aggregate, 1.5% for 20 mm maximum size of aggregate and 2.5% for 10mm maximum size of aggregate.
There are always entrapped air in concrete. Therefore ignoring
entrapped air value as NIL will lead the calculation of higher value of density.
W_{m} = Mixing water required in kg/m^{3}
C_{m} = Cement required, kg/m^{3}
Note:- The exact density may be obtained by filling and fully compacting constant volume suitable metal container from the trial batches of calculated design mixes. The mix be altered with the actual obtained density of the mix.
U_{m} =10 x G_{a} (100 – A) + C_{m} (1 – G_{a}/G_{c}) – W_{m} (G_{a} – 1)
=10 x 2.65 (100 – 1.5) + 360(1- 2.65/3.00) – 137 (2.65 -1)
2426 kg/m^{3}
e)Aggregates = 2426 – 360 – 137 = 1929 kg/m^{3}
f) Fine aggregate = From Table 3 for zone-II Fine aggregate and
20 mm maximum size of aggregate, W/C ratio = 0.39, 50 mm slump found to be for trial 33%.
Fine aggregate = 1929 x 0.33 = 637 kg/m^{3}
Coarse aggregate = 1929 – 637 = 1292 kg/m^{3}
10 and 20 mm aggregate are single sized as per IS: 383-1970. Let they be combined in the ratio of 1.2:1.8 to get 20 mm graded aggregate as per IS: 383-1970
10 mm aggregate = 517 kg/m^{3}
20 mm aggregate = 775 kg/m^{3}
g) Thus for M-40 Grade of concrete quantity of materials per cu.m.
of concrete on the basis of saturated and surface dry aggregates:
Water=137 kg/m^{3}
PPC = 360 kg/m^{3}
Fine Aggregate (sand) = 637 kg/m^{3}
10 mm Aggregate = 517 kg/m^{3}
20 mm Aggregate = 775 kg/m^{3}
Normal Super Plasticizer = 7.560 kg/m^{3}
MIX- B WITH OPC
a) Water = 190 – (190 x 0.28) = 136.8 kg/m^{3} say 137 kg/m^{3}
b) OPC = 137/0.39 = 351 kg/m^{3}
Minimum content specified = 360 kg/m^{3}
Then take 360 kg/m^{3}
c) Density:
10 x 2.65 (100 – 1.5) + 360 (1 – 2.65/3.15) – 137 (2.65 – 1)
= 2442 kg/m^{3}
d) Total Aggregates = 2442 – 137 – 360 = 1945 kg/m^{3}
Fine Aggregate = 1945 x 0.33 = 642 kg/m^{3}
Coarse aggregate = 1945 – 642 = 1303 kg/m^{3}
10 mm Aggregate = 1303×1.2/3 = 521 kg/m^{3}
20 mm Aggregate = 1303×1.8/3 = 782 kg/m^{3}
e) Thus for M-40 Grade of concrete quantity of materials per cu.m of concrete on the basis of SSD aggregates are given below:
Water = 137 kg/m^{3}
PPC = 360 kg/m^{3}
Fine Aggregate (sand) = 642 kg/m^{3}
10 mm Aggregate = 521 kg/m^{3}
20 mm Aggregate = 782 kg/m^{3}
Normal Super Plasticizer = 7.2 kg/m^{3}
MIX. C WITH OPC + FLYASH
With the given set of materials increase in cementitious materials = 12%
Total cementitious materials = 360 x 1.12 = 403 kg/m^{3}
Materials | Weight (kg/m^{3}) | Volume (m^{3}) |
OPC = 403 x 0.70 | 282/3150 | 0.0895 |
Flyash = 403 x 0.30 | 121/2250 | 0.0538 |
Free Water = 137 x 0.95 | 130/1000 | 0.13 |
Normal Super Plasticizer = 8.2kg | 8.2/1150 | 0.0071 |
Air = 1.5% | 0.015 | |
Total | 0.2954 | |
Total Aggregates = 1 – 0.2954 | 0.7046 | |
1.00 | ||
Coarse Aggregate | 1303/2650 | 0.4917 |
Fine Aggregate = 0.7046 – 0.4917 = 0.2129
= 0.2129 x 2650 = 564 kg
Note:-
1. Specific gravity of Normal Superplasticizer = 1.15
2. Addition of Flyash reduces 5% of water demand.
M-40 Grade of concrete quantity of material per cu.m of concrete
on the basis of saturated and surface dry aggregates of
Mix ‘A’, ‘B’ and ‘c’ are given below:
Materials | MIX. ‘A’ with PPC | Mix. ‘B’ with OPC | Mix. ‘C’ with OPC+Flyash |
Water kg/m^{3} | 137 | 137 | 130 |
PPC kg/m^{3} | 360 | — | — |
OPC kg/m^{3} | — | 360 | 282 |
Flyash kg/m^{3} | — | — | 121 |
Fine Agg. kg/m^{3} | 637 | 642 | 564 |
10mm Agg. kg/m^{3} | 517 | 521 | 521 |
20 mm Agg. kg/m^{3} | 775 | 782 | 782 |
Normal Super- plasticizer kg/m^{3} | 7.560 | 7.2 | 8.2 |
W/Cementations ratio | 0.38 | 0.38 | 0.323 |
Note:-
1. For exact W/C ratio the water in admixture should also be taken into account.
2. The W/C ratio of PPC and OPC is taken the same assuming that the strength properties of both are the same. If it is found that the PPC is giving the low strength then W/C ratio of PPC have to be reduce, which will increase the cement content. For getting early strength and in cold climate the W/C ratio of PPC shall also be required to be reduced.
3. PPC reduces 5% water demand. If this is found by trial then take reduce water for calculation.
4. If the trial mixes does not gives the required properties of the mix, it is then required to be altered accordingly. However, when the experiences grows with the particular set of materials and site conditions very few trials will be required, and a expert of such site very rarely will be required a 2nd trial.
Table. 1: Grading of Aggregates
IS Sieve Designation | Percentage Passing | ||
Fine Aggregate | Crushed Aggregate | ||
10 mm | 20 mm | ||
40 mm | — | — | 100 |
20 mm | — | — | 100 |
12.5 mm | — | 100 | — |
10 mm | 100 | 85 | 4 |
4.75 mm | 99 | 5 | 0 |
2.36 mm | 88 | 0 | |
1.18 mm | 74 | ||
600 Micron | 43 | ||
300 Micron | 24 | ||
150 Micron | 6 |
Table. 2: Approximate free-water content (kg/m3) required to give various levels of workability for non-air-entrained (with normal entrapped air) concrete.
Maximum size of aggregate (mm) | Type of aggregate | Slump (mm) Degree of workability | — vary low | 25-75 … Low | 50-100 … Medium | 100-180 … High |
10 | Uncrushed Crushed | 150 180 | 205 235 | 220 250 | 240 265 | |
20 | Uncrushed Crushed | 140 170 | 180 210 | 195 225 | 210 245 | |
40 | Uncrushed Crushed | 120 155 | 160 190 | 175 205 | 190 220 |
Note:- When coarse and fine aggregate of different types are used, the free water content is estimated by the expression.
2/3W_{f}+1/3W_{c}
Where,
W_{f}=Free water content appropriate to type of fine
Aggregate and W_{c}=Free water content appropriate to type of coarse aggregate.
Table. 3: Proportion of fine aggregate (percent) with 10mm and 20mm maximum sizes of aggregates and with different workability.
Table. 4: Proportion of fine aggregate (percent) with 40 mm maximum sizes of Aggregates and with different workability.
Grading Zone of F.A | W/C Ratio | 40 mm aggregate Workability | |||
VL | L | M | H | ||
I | 0.3 | 27-33 | 29-35 | 33-39 | 38-46 |
0.4 | 29-35 | 31-38 | 35-42 | 41-49 | |
0.5 | 31-38 | 33-41 | 37-44 | 43-52 | |
0.6 | 33-41 | 36-43 | 39-47 | 45-54 | |
0.7 | 36-44 | 38-46 | 42-50 | 47-57 | |
II | 0.3 | 22-27 | 23-29 | 27-33 | 31-28 |
0.4 | 24-29 | 25-31 | 28-35 | 32-41 | |
0.5 | 25-31 | 27-33 | 30-37 | 34-43 | |
0.6 | 27-33 | 29-36 | 32-39 | 36-45 | |
0.7 | 29-36 | 31-38 | 34-42 | 38-47 | |
III | 0.3 | 18-22 | 20-23 | 22-27 | 26-31 |
0.4 | 20-24 | 21-25 | 24-28 | 27-32 | |
0.5 | 21-25 | 23-27 | 25-30 | 29-34 | |
0.6 | 23-27 | 24-29 | 27-32 | 30-36 | |
0.7 | 24-29 | 26-31 | 29-34 | 32-36 | |
IV | 0.3 | 16-18 | 18-20 | 19-22 | 22-26 |
0.4 | 17-20 | 19-21 | 20-24 | 24-27 | |
0.5 | 18-21 | 20-23 | 22-25 | 25-29 | |
0.6 | 20-23 | 22-24 | 23-27 | 26-30 | |
0.7 | 21-24 | 23-26 | 25-29 | 28-32 |
VL = Very low workability.
L = Low workability – slump 25-75 mm
M = medium workability – slump 50-100 mm
H = High workability- slump 100-180 mm
Figures From Experimental Data
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