The stresses induced in concrete pavements are mainly flexural. Therefore flexural strength is more often specified than compressive strength in the design of concrete mixes for pavement construction. A simple method of concrete mix design based on flexural strength for normal weight concrete mixes is described in the paper.
Usual criterion for the strength of concrete in the building industry is the compressive strength, which is considered as a measure of quality concrete. However, in pavement constructions, such as highway and airport runway, the flexural strength of concrete is considered more important, as the stresses induced in concrete pavements are mainly flexural. Therefore, flexural strength is more often specified than compressive strength in the design of concrete mixes for pavement construction. It is not perfectly reliable to predict flexural strength from compressive strength. Further, various codes of the world specified that the paving concrete mixes should preferably be designed in the laboratory and controlled in the field on the basis of its flexural strength. Therefore, there is a need to design concrete mixes based on flexural strength.
The type of aggregate can have a predominant effect, crushed rock aggregate resulting in concrete with higher flexural strength than uncrushed (gravel) aggregates for comparable mixes, assuming that sound materials are used. The strength of cement influences the compressive and flexural strength of concrete i.e. with the same water-cement ratio, higher strength cement will produce concrete of higher compressive and flexural strength.
IRC: 15-2011 specified that for concrete roads OPC should be used. This code also allowed PPC as per IS: 1489 (Part-1) with flyash content not more than 20 percent by weight of PPC. Accordingly OPC + fly ash may be used in concrete roads. Flyash shall be not more than 20 percent by weight of cementitious material. However, IS: 456-2000 specified that fly ash conforming to grade-1 of IS” 3812 may be used as part replacement of OPC provided uniform blended with cement is essential. The construction sites where batching plants are used this may be practicable. In ordinary sites where mixer or hand mixing are done uniform blending of fly ash with cement is not practicable. At such construction sites, PPC may be used. PPC should be used with caution where rapid construction methods like slip form is being used. Joints cutting also need early strength.
3. Target average flexural strength for all A, B and C mixes
S = S+ Zq
=4.5 + 1.96 x 0.40
= 5.3 N/mm2 at 28 days age
4. For Mix A, B and C free W/C ratio with crushed aggregate and required average flexural target strength of 5.3 N/mm2 at 28 days from Fig. 1 Curve D found to be 0.42. This is lower than specified maximum W/C ratio value of 0.45 for OPC and 0.50 for PPC.
Note:
In absence of cement strength, but cement conforming to IS Codes, assume from Fig. 1
Curve C and D for OPC 43 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 flexural strength of 5.3 N/mm2 as worked out is 0.42 for first trial.
b) Free water for 40 mm slump from Table 2 for 31.5 mm maximum size of aggregate.
2/3×170 + 1/3×200= 180 kg/m3
From trials it is found that Retarder Super plasticizer at a dosages of 1.3% bw of PPC may reduce 15% water without loss of workability
Then water = 180 – (180 x 0.15) = 153 kg/m3
c) PPC = 153/0.42 = 364 kg/m3 (Required minimum PPC is 425 kg/m3)
d) Formula for calculation of fresh concrete weight in kg/m3
Um = 10 x Ga (100 – A) + Cm(1 – Ga/Gc) – Wm (Ga – 1)
Where,
Um=Weight of fresh concrete kg/m3
Ga=Weighted average specific gravity of combined fine and coarse aggregate bulk, SSD
Gc=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 for trial entrapped air 1.5%
For 31.5 mm maximum size of aggregate
There is always entrapped air in concrete. Therefore ignoring entrapped air value as NIL will lead the calculation of higher value of density. Take exact value of air as obtained in the test
Wm=Mixing water required in kg/m3
Cm=Cement required, kg/m3
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.
Um =10 x Ga (100 – A) + Cm (1 – Ga/Gc) – Wm (Ga – 1)
=10 x 2.65 (100 – 1.5) + 425(1- 2.65/3.00) – 153 (2.65 -1)
=2409 kg/m3
e) Aggregates = 2409 – 425 – 153 = 1831 kg/m3
f) Fine aggregate = 1831 x 0.45 = 824 kg/m3
Aggregate 1 = 1831 x 0.29 = 531 kg/m3
Aggregate 2 = 1831 x 0.26 = 476 kg/m3
g) Thus for 4.5 N/mm2 flexural strength quantity of materials per cu.m. of concrete on the basis of saturated and surface dry aggregates:
Water = 153 kg/m3
PPC = 425 kg/m3
Fine Aggregate (sand) = 824 kg/m3
Aggregate (1) = 531 kg/m3
Aggregate (2) = 476 kg/m3
Retarder Super Plasticizer 1.3% bw of PPC = 5.525 kg/m3
MIX- B WITH OPC
a) Water = 180 – (180 x 0.15) = 153 kg/m3
b) OPC = 153/0.42 = 364 kg/m3
c) Density: 10 x 2.65 (100 – 1.5) + 364 (1 – 2.65/3.15) – 153 (2.65 – 1)= 2416 kg/m3
d) Total Aggregates = 2416 – 364 – 153 = 1899 kg/m3
Aggregate 1 = 1899 x 0.29 = 551 kg/m3
Aggregate 2 = 1899 x 0.26 = 494 kg/m3
Fine Aggregate = 1899 x 0.45 = 854 kg/m3
e) Thus for 4.5 N/mm2 flexural strength quantity of materials per cu.m of concrete on the basis of SSD aggregates are given below:
Water = 153 kg/m3
OPC = 364 kg/m3
Fine Aggregate (sand) = 854 kg/m3
Aggregate (1) = 551 kg/m3
Aggregate (2) = 494 kg/m3
Retarder Super Plasticizer 1% bw OPC = 3.640 kg/m3
MIX-C WITH OPC + FLY ASH
With a total cementitious material of 430 kg/m3,
OPC = 430 x 0.80 = 344 kg/m3
Fly ash = 430 x 0.20 = 86 kg/m3
Mix on the basis of SSD Aggregates,
(1) Water as worked out earlier = 153 kg/m3
(2) OPC = 344 kg/m3
(3) Fly ash = 86 kg/m3
Density = 10 x 2.65 (100 – 1.5) + 430 (1 – 2.65/3.00) – 153 (2.65 – 1) = 2410 kg/m3
Total Aggregates = 2410 – 153 – 344 – 86 = 1827 kg/m3
(4) Fine aggregate 0.45 x 1827 = 822 kg/m3
(5) Aggregate (1) 0.29 x 1827 = 530 kg/m3
(6) Aggregate (2) 0.26 x 1827 = 475 kg/m3
(7) Retarder super plasticizer 1.5% bw of cementitious material = 6.450 kg/m3
Note:
(1) Cementitious material worked out as per IRC : 15-2011, which specified: In case fly ash (as per IS: 3912 Part 1) is blended at site, the quantity of fly ash shall be restricted to 20 percent by weight of cementitious material and the quantity of OPC in such a blend shall not be less than 340 kg/m3 .
(2) After the first trial mix, its actual density is to be determined, as specified elase where in this paper. The mix proportions shall then be worked out accordingly including the water content, the dosages of Retarder SP for required workability keeping the free w/c ratio with in the permissible limits and adjusting it according to the required flexural strength.
(3) The mix proportions given in this paper are for first trial and to be adjusted as per actual site materials, conditions and requirements.
For 4.5 N/mm2 flexural strength 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:
Note:-
1. For exact W/C ratio the water in admixture should also be taken into account.
2. PPC reduces 5% water demand. If this is found by trial then take reduce water for calculation.
3. 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.
CONCLUSION
1. For 4.5 N/mm2 flexural strength concrete having same material and requirement, but without water reducer, the OPC required will be 180/0.42 = 429 kg/m3
2. With the use of superplasticizer the saving in OPC is 65 kg/m3 and water 27 lit/m3.
3. In the financial year 2009-2010 India has produces 200 million tonnes of cement. In India one kg of cement produce emitted 0.93 kg of CO2. Thus the production of 200 million tonnes of cement had emitted 200 x 0.93 = 186 million tonnes of CO2 to the atmosphere.
4. If 50 million tonnes cement in making concrete uses Water Reducers 7500000 tonnes of cement can be saved. 3750000 KL of potable water will be saved and the saving of Rs. 3300 crores per year to the construction Industry. 6975000 tonnes of CO2 will be prevented to be emitted to the atmosphere. The benefits in the uses of water reducers not limited to this. When water reduces shrinkage and porosity of concrete are reduces which provides the durability to concrete structures.
5. India is facing serious air, water, soil, food and noise pollution problems. Every efforts therefore are necessary to prevent pollution on top priority basis.
6. As the stress induced in concrete pavements are mainly flexural, it is desirable that their design is based on the flexural strength of concrete. The quality of concrete is normally assessed by measuring its compressive strength. For pavings, however, it is the flexural strength rather than the compression strength of concrete which determine the degree of cracking and thus the performance of road, and it is imperative to control the quality on the basis of flexural strength.
7. As per IRC: 15-2011, in case of small size projects, where facilities for testing beams with three print loading are not available, in such cases, the mix design may be carried out by using compressive strength values and there after flexural strength will be determined as per correlation between flexural strength with compressive strength given the following equation.
Where fcr is the flexural strength in MPa or N/mm2 and fck is the characteristic compressive strength in MPa or N/mm2 as per IS: 456-2000.
REFERENCES
| 1 |
IS : 383-1970
Specifications for coarse and fine aggregates from natural sources for
concrete (second revision) BIS, New Delhi |
| 2 |
IS: 456-2000 Code of
practice for plain and reinforced concrete (fourth revision), BIS, New Delhi |
| 3 |
IS: 9103-1999
Specification for admixtures for concrete (first revision) BIS, New Delhi |
| 4 |
IS: 8112-1989
Specifications for 43 Grade ordinary portland cement (first revision) BIS,
New Delhi |
| 5 |
IS: 2386 (Part-III) 1963
method of test for aggregate for concrete. Specific gravity, density, voids,
absorption and bulking, BIS, New Delhi |
| 6 |
IS: 3812 (Part-I) 2003
Specification for pulverized fuel ash: Part-I for use as pozzolana in cement,
cement mortar and concrete (second revision) BIS, New Delhi |
| 7 |
IS: 1489-Part-I 1991
Specifications for portland pozzolana cement (Part-I) Flyash based. (Third
revision), BIS, New Delhi |
| 8 |
IRC: 15-2011 – Standard
specifications and code of practice for construction of concrete road (Fourth
revision) |
| 9 |
Kishore Kaushal,
“Concrete Mix Design Based on Flexural strength for Air-Entrained Concrete”,
Proceeding of 13th Conference on
our World in Concrete and Structures, 25-26, August, 1988, Singapore. |
| 10 |
Kishore Kaushal, “Method
of Concrete Mix Design Based on Flexural Strength”, Proceeding of the
International Conference on Road and Road Transport Problems ICORT, 12-15
December, 1988, New Delhi, pp. 296-305. |
| 11 |
Kishore Kaushal, “Mix
Design Based on Flexural Strength of Air-Entrained Concrete”. The Indian
Concrete Journal, February, 1989, pp. 93-97. |
| 12 |
Kishore Kaushal,
“Concrete Mix Design Containing Chemical Admixtures”, Journal of the National
Building Organization, April, 1990, pp. 1-12. |
| 13 |
Kishore Kaushal,
“Concrete Mix Design for Road Bridges”, INDIAN HIGHWAYS, Vol. 19, No. 11,
November, 1991, pp. 31-37 |
| 14 |
Kishore Kaushal, “ Mix
Design for Pumped Concrete”, Journal of Central Board of Irrigation and
Power, Vol. 49, No.2, April, 1992, pp. 81-92 |
| 15 |
Kishore Kaushal,
“Concrete Mix Design with Fly Ash”, Indian Construction, January, 1995, pp.
16-17 |
| 16 |
Kishore Kaushal,
“High-Strength Concrete”, Bulletin of Indian Concrete Institute No. 51,
April-June, 1995, pp. 29-31 |
| 17 |
Kishore Kaushal,
“Concrete Mix Design Simplified”, Indian Concrete Institute Bulletin No. 56,
July-September, 1996, pp.25-30. |
| 18 |
Kishore Kaushal,
“Concrete Mix Design with Fly Ash & Superplasticizer”, ICI Bulletin No.
59, April-June 1997, pp. 29-30 |
| 19 |
Kishore Kaushal. “Mix
Design for Pumped Concrete”, CE & CR October, 2006, pp. 44-50. |
Table. 1: Grading of Aggregates
| IS Sieve
Designation |
Percentage of passing by
mass |
| Fine aggregate from river |
Crushed aggregate |
Combined grading of mix |
IRC: 15-2011 recommended
grading of combined aggregates for pavement quality concrete (PQC) |
| 45% |
-1 |
-2 |
| |
29% |
26% |
| 31.50 mm |
100 |
100 |
100 |
100 |
100 |
| 26.50 mm |
100 |
100 |
98 |
99 |
85 – 95 |
| 19.0 mm |
100 |
100 |
25 |
81 |
68 – 88 |
| 9.50 mm |
100 |
46 |
0 |
58 |
45 – 65 |
| 4.75 mm |
94 |
5 |
|
44 |
30 – 55 |
| 600 micron |
42 |
0 |
|
19 |
8 – 30 |
| 150 micron |
10 |
|
|
5 |
5 – 15 |
| 75 micron |
2 |
|
|
1 |
0 – 5 |
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) |
25 + 10 |
40 + 1031.5 |
| |
Uncrushed |
|
160 |
170 |
| Crushed |
190 |
200 |
Note:- When coarse and fine aggregate of different types are used, the free water content is estimated by the expression.
2/3Wf+1/3Wc
Where,
Wfsub>= Free water content appropriate to type of fine Aggregate
And Wc= Free water content appropriate to type of coarse aggregate.
 |
| Concrete Mix Design For Concrete Roads |
