Mix Design For Concrete Roads As Per IRC-15-2002

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By
Er. Kaushal Kishore ,
Materials Engineer, Roorkee

Check out the Mix Design For Concrete Roads As Per IRC:15-2011

ABSTRACT:
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.

INTRODUCTION:
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.

MIX DESIGN DETAILS
IRC: 15-2002 specified that for concrete roads OPC should be used. This code also allowed PPC as per IS: 1489 may also be used. Accordingly OPC + fly ash may be used in concrete roads. 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.

TEST DATA FOR MATERIALS AND OTHER DETAILS
1. The grading of fine aggregate, 10 and 20 mm aggregates are as given in Table. 1 ( given in the end). 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

3. Target average flexural strength for all A, B and C mixes
S    = S^’+ J_ao-
= 4.5 + 1.65 x 0.5
= 5.3 N/mm² at 28 days age

4. For Mix A and B 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 ( Figure shown in the end) found to be 0.42. This is lower than specified maximum W/C ratio value of 0.50
Note: In absence of cement strength, but cement conforming to IS Codes, assume from Fig. 1

Curve A and B – For OPC 33 Grade
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

b)     Free water for 30 mm slump from Table 2 for 20 mm maximum size of aggregate.
2/3*165 + 1/3*195
= 175 kg/m³

From trials it is found that Retarder Super plasticizer at a dosages of 15gm/kg of cement may reduce 15% water without loss of workability
Then water  =  175 – (175 x 0.15)  =  148.75 kg/m³
For trials say 149 kg/m³

c) PPC = 149/0.42 = 355 kg/m³
This is higher than minimum requirement of 350 kg/m³

d)     Formula for calculation of fresh concrete weight in kg/m³

U_M = 10 x G_a (100 – A) + C_M(1 – G_a/G_c) – W_M (G_a – 1)
Where,
U_m =  Weight of fresh concrete kg/m³

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 1.5% 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/m³

C_m = 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.

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) + 355(1- 2.65/3.00) – 149 (2.65 -1)
= 2405.9 kg/m³
Say  2405 kg/m³

e) Aggregates  =  2405 – 355 – 149  =  1901 kg/m³

f) Fine aggregate = From Table 3 for zone-II Fine aggregate and
20 mm maximum size of aggregate, W/C ratio = 0.42, 30 mm slump found to be 35%.

Fine aggregate  =  1901 x 0.35  =  665 kg/m³
Coarse aggregate =  1901 – 665  =  1236 kg/m³

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  = 1236×1.2/3  = 494 kg/m³
20 mm aggregate  =  1236×1.8/3 = 742 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 = 149 kg/m³
PPC = 355 kg/m³
Fine Aggregate (sand) = 665 kg/m³
10 mm Aggregate = 494 kg/m³
20 mm Aggregate = 742 kg/m³
Retarder Super Plasticizer = 5.325 kg/m³

MIX- B WITH OPC
a) Water = 175 – (175 x 0.15) = 149 kg/m³ say 149 kg/m³

b) OPC = 149/0.42 = say 355 kg/m³

c) Density: 10 x 2.65 (100 – 1.5) + 355 (1 – 2.65/3.15) – 149 (2.65 – 1)
=  2420.8 kg/m³ say 2420 kg/m³

d) Total Aggregates  =  2420 – 355 – 149 = 1916 kg/m³
Fine Aggregate = 1916 x 0.35 = say 670 kg/m³
Coarse aggregate = 1916 – 670 = 1246 kg/m³
10 mm Aggregate = 1246×1.2/3 = 498 kg/m³
20 mm Aggregate = 1246×1.8/3 = 748 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 = 149 kg/m³
OPC = 355 kg/m³
Fine Aggregate (sand) = 670 kg/m³
10 mm Aggregate = 498 kg/m³
20 mm Aggregate = 748 kg/m³
Retarder Super Plasticizer = 3.550 kg/m³

MIX. C WITH OPC + FLYASH
With the given set of materials increase in cementitious materials = 10.7%
Total cementitious materials = 355×1.107 = 393 kg/m³

Fine Aggregate = 0.6994 – 0.4702  = 0.2292
=  0.2292 x 2650 = 607 kg

Note:-
1. Specific gravity of Normal Superplasticizer = 1.15
2. Addition of Flyash reduces 5% of water demand.

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. 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 2^nd trial.

5. It may be noted that, for the fly ash concrete the total cementation material is greater but the OP cement content is smaller, the coarse aggregate content is deliberately, the same, the water is reduced and the density is reduced, because of the lower density of fly ash compared with OPC.

CONCLUSION
1. For 4.5 N/mm² flexural strength concrete having same material and requirement, but without water reducer, the PPC and OPC required will be 175/0.42 = 417kg/m³

2. With the use of superplasticizer the saving in cement is 62 kg/m³ and water 26 lit/m³ for PPC and OPC.

3. In the Fly ash concrete the saving in cement is 122 kg/m³ and water 33 lit/m³ including utilization of 98 kg/m³ of fly ash witch is a waste material.

4. 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.

5. 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.

6. India is facing serious air, water, soil, food and noise pollution problems. Every efforts therefore are necessary to prevent pollution on  top priority basis.

7. 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.

REFERENCES:

Table. 1: Grading of Aggregates

Table. 2: Approximate free-water content (kg/m³) required to give various levels of workability for non-air-entrained (with normal entrapped air) concrete.

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
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 slump 15-45 mm.

I am thankful to Sir Kaushal Kishore for publishing his research work here on engineeringcivil.com. I am sure, this research paper will help many civil engineers around the world in understanding how to do mix design for concrete roads as per IRC-15-2002.

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