Er. Kaushal Kishore ,
Materials Engineer, Roorkee
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.
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.
|1||Characteristic Flexural Strength at 28 days||4.5N/mm2|
|2||Cement||Three mixes are to be designed|
With PPC (Flyash based) conforming to IS:1489-part-I-1991. 7 days strength 37.5N/mm2. Specific Gravity: 3.00
With OPC-43- Grade conforming to IS: 8112-1989. 7 days strength 40.5N/mm2. Specific Gravity : 3.15
With OPC of Mix-B and Fly ash conforming to IS:3812 (Part-I)-2003 Specific Gravity : 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 ash replacement||25% 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||350 kg/m3 including Fly ash|
|7||Maximum free W/C Ratio||0.50|
|8||Workability||30 mm slump at pour the concrete will be transported from central batching plant through transit mixer, at a distance of 20 Km during June, July months. The average temperature last year during these months was 400C.|
|10||Method of placing||Fully mechanized construction|
|11||Degree of supervision||Good|
|12||Maximum of cement content (Fly ash not included)||425 kg/m3|
|13||Chemical admixture||Retarder Super plasticizer conforming to IS:9103-1999. With the given requirements and materials, the manufacturer of Retarder Super plasticizer recommends dosages of 10 gm per kg of OPC, which will reduce 15% of water without loss of workability. For fly ash included cement dosages will be required to be adjusted by experience/ trials.|
|14||Values of Jaxo-||1.65 x 0.5 N/mm2|