2. High Volume Flyash Concrete for Pavements

April 2004 to March 2006
Central Road Research Institute, New Delhi (R)

Scope and Objective
The use of fly ash as a partial replacement of cement in conventional concrete has been limited to 25-30% only. Some people also think that larger volumes of Indian fly ashes might not be used in concrete. The major objective of this research project was to examine the effect of high volume of fly ash on various properties of paving concrete and also to evaluate the suitability of such concrete for paving projects. A suitable method of concrete mix design using high volume of fly ash, for highway paving project, was to be established.

Three control concrete mixtures having 400 kg/m3 of Ordinary Portland cement and water-cementitious material (w/cm) ratio of 0.30, 0.34 and 0.40 were prepared. Other concrete mixtures were then prepared by substituting 20, 30, 40, 50, and 60 percent fly ash in all control mixtures. Compressive and flexural strengths at the age of 7, 28, 90, 180, 256, and 365 days, drying shrinkage, and abrasion resistance of concrete were measured for all the mixtures.
The study has brought out following conclusion:

1. At 7 and 28 days, all the fly ash concrete mixtures with 0.40 w/cm ratio and containing fly ash showed lower compressive and flexural strength than the control mixture. The strength reduced with increasing amount of fly ash.

2. The 28-day flexural strength of the high volume fly ash concrete mixtures F50 (5.28 MPa) and F60 (4.75 MPa) at 0.30 w/cm ratio was more than the normally required strength (4-4.5 MPa) for the design of a concrete pavement.

3. At 90 days and beyond, the compressive strength of the mixtures with 20, 30, and 40 % fly ash was more, whereas, the strength of mixtures with 50 and 60 % fly ash was less than that of control mixtures at each w/cm ratio. The maximum strength developed in the mixture containing 60 % cement and 40 % fly ash. Similar trend was also observed for flexural strength of the mixtures.

4. All fly ash admixed mixtures exhibited lesser drying shrinkage as compared with the control mixtures. Drying shrinkage decreased with increasing amount of fly ash and decreasing w/cm ratio. The mixture F60 containing 60 % fly ash at 0.30 w/cm ratio showed the least shrinkage.

5. The abrasion resistance of all the fly ash admixed mixtures was less than that of the control mixtures at each w/cm ratio. It decreased with increasing fly ash content and increased with decreasing w/cm ratio of the mixture. In general, the abrasion resistance of concrete increased with increasing compressive strength.
High volume fly ash concrete mixture F60 with 0.30 w/cm ratio and containing 160 kg/m3 portland cement and 240 kg/m3 fly ash showed enough compressive and flexural strength and abrasion resistance at 28 days so as to make it suitable for concrete pavement construction. Its low drying shrinkage may provide either longer joint spacing or better aggregate interlocking and load transfer at joints in jointed concrete pavements. It is recommended that the construction of high volume fly ash concrete pavement must be carried out under very strict quality control and supervision of any qualified and experienced agency.