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3/22/2016 Mix Design For Concrete Roads As Per IRC:15-2011http://www.engineeringcivil.com/mix-design-for-concrete-roads-as-per-irc15-2011.html 1/11
engineeringcivil.com
http://www.engineeringcivil.com/mix-design-for-concrete-roads-as-per-irc15-2011.html
Mix Design For Concrete Roads As Per IRC:15-2011
By Kaushal Kishore, Materials Engineer, Roorkee
ABSTRACT:
The stresses induced in concrete pavements are mainly flexural. Therefore flexural strength is more oftenspecified than compressive strength in the design of concrete mixes for pavement construction. A simplemethod of concrete mix design based on flexural strength for normal weight concrete mixes is described inthe paper.
INTRODUCTION:
Usual criterion for the strength of concrete in the building industry is the compressive strength, which isconsidered as a measure of quality concrete. However, in pavement constructions, such as highway andairport runway, the flexural strength of concrete is considered more important, as the stresses induced inconcrete pavements are mainly flexural. Therefore, flexural strength is more often specified thancompressive strength in the design of concrete mixes for pavement construction. It is not perfectly reliable topredict flexural strength from compressive strength. Further, various codes of the world specified that thepaving concrete mixes should preferably be designed in the laboratory and controlled in the field on thebasis 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 withhigher flexural strength than uncrushed (gravel) aggregates for comparable mixes, assuming that soundmaterials 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 andflexural strength.
MIX DESIGN DETAILS
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 maybe 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 partreplacement of OPC provided uniform blended with cement is essential. The construction sites wherebatching plants are used this may be practicable. In ordinary sites where mixer or hand mixing are doneuniform blending of fly ash with cement is not practicable. At such construction sites, PPC may be used. PPCshould be used with caution where rapid construction methods like slip form is being used. Joints cutting alsoneed early strength.1 CharacteristicFlexuralStrength at 28days: 4.5 N/mm
2
2 Cement : Three mixes are to be designed
3/22/2016 Mix Design For Concrete Roads As Per IRC:15-2011http://www.engineeringcivil.com/mix-design-for-concrete-roads-as-per-irc15-2011.html 2/11
MIX-A
With PPC (Flyash 18 percent based) conforming to IS:1489-part-I-1991. 7 daysstrength 37.5 N/mm
2
. Specific Gravity : 3.00
MIX-B
With OPC-43- Grade conforming to IS: 8112-1989. 7 days strength 40.5 n/mm
2
.Specific Gravity : 3.15
MIX-C
With OPC of Mix-B and Fly ash conforming to IS:3812 (Part-I)-2003 SpecificGravity : 2.20
Note:
Requirements of all the three mixes are the same. Fine Aggregate, Coarse Aggregate and Retarder Super plasticizer are the same for all the three mixes.3 Fly ashreplacement: 20% Fly ash is required to be replaced with the total cementitious materials.4 Maximumnominal sizeof aggregates: 31.5 mm Crushed aggregate5 Fineaggregateand coarseaggregategrading: Given in Table 16 Minimumcementcontent for 4.5 N/mm2characteristicflexuralstrength:: (a) OPC shall not be less than 360 kg/m
3
.(b) PPC shall not be less than 425 kg/m
3
. Fly ash in it 20% maximum by weightof total cementitious materials(c) OPC + Fly ash mix OPC shall not be less than 340 kg/m
3
. Fly ash 20%maximum by weight of cementations material7 Maximumfree W/CRatio: (a) For OPC 0.45(b) For PPC 0.508 Workability : 40 mm slump at pour the concrete will be transported from central batching plantthrough transit mixer, at a distance of 15 Km during June, July months. Theaverage temperature last year during these months was 30
0
C.
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9 Exposurecondition: Moderate10 Method of placing: Fully mechanised construction11 Degree of supervision: Good12 Maximum of cementcontent: (a) OPC 425 kg/m
3
(b) PPC 425 kg/m
3
13 Chemicaladmixture: Retarder Super plasticizer conforming to IS:9103-1999. With the givenrequirements and materials, the manufacturer of Retarder Super plasticizer recommends dosages of 1% bw of OPC, which will reduce 15% of water withoutloss of workability. For fly ash included cement dosages will be required to beadjusted by experience/ trials.2% maximum by weight of cementitious material14 Values of Z x (for NationalHighway): 1.96 x 0.40
TEST DATA FOR MATERIALS AND OTHER DETAILS
1. The grading of fine aggregate, 1 & 2 aggregates are as given in Table. 1. 2. Properties of aggregatesTests Fine aggregate Aggregate 1 Aggregate 2Specific Gravity2.65 2.65 2.65Water Absorption %0.8 0.5 0.53. Target average flexural strength for all A, B and C mixes S = S+ Zq=4.5 + 1.96 x 0.40= 5.3 N/mm
2
at 28 days age4. For Mix A, B and C free W/C ratio with crushed aggregate and required average flexural target strength of 5.3 N/mm
2
at 28 days from Fig. 1 Curve D found to be 0.42. This is lower than specified maximum W/C ratiovalue 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
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Curve C and D for OPC 43 GradeTake 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 thetime of concreting, moisture content of site aggregates are to be determine. If it carries surface moisture thisis 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/mm
2
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/m
3
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 workabilityThen water = 180 – (180 x 0.15) = 153 kg/m
3
c) PPC = 153/0.42 = 364 kg/m
3
(Required minimum PPC is 425 kg/m
3
)d) Formula for calculation of fresh concrete weight in kg/m
3
U
m
= 10 x G
a
(100 – A) + C
m
(1 – G
a
/G
c
) – W
m
(Ga – 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 for trial entrapped air 1.5%For 31.5 mm maximum size of aggregateThere is always entrapped air in concrete. Therefore ignoring entrapped air value as NIL will lead thecalculation of higher value of density. Take exact value of air as obtained in the test 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 metalcontainer from the trial batches of calculated design mixes. The mix be altered with the actual obtaineddensity 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) + 425(1- 2.65/3.00) – 153 (2.65 -1)=2409 kg/m
3
e) Aggregates = 2409 – 425 – 153 = 1831 kg/m
3
f) Fine aggregate = 1831 x 0.45 = 824 kg/m
3
Aggregate 1 = 1831 x 0.29 = 531 kg/m
3
Aggregate 2 = 1831 x 0.26 = 476 kg/m
3

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