Concrete is seen everywhere in transportation construction. Concrete is made by combining Portland cement, sand, water, and coarse aggregate. Portland cement is the “glue” that holds concrete together and gives it its structural strength. However, as demand for Portland cement increases in our steadily developing world, its price continues to increase. One way of reducing the cost of Portland cement needed to make concrete while maintaining the necessary strength is to replace part of the cement with materials called pozzolans. One such material is the fly ash that is produced by burning coal, for example, in power plants. In Florida, fly ash is the principal pozzolan used to replace some of the Portland cement in concrete mixes – the Florida Department of Transportation allows replacement of up to 50% of cement with fly ash, depending on how the resulting concrete will be used. Considering the amount of concrete that is used in transportation projects, this can result in significant savings. However, the availability of coal fly ash, once cheap and plentiful, has been dwindling because of the reduction in coal use due to alternative energy sources, the availability of inexpensive natural gas, and environmental regulations.

To lessen the impact from this shortage, Dr. Chris Ferraro of the Department of Civil and Coastal Engineering, an affiliate of the UFTI, conducted a Florida Department of Transportation (FDOT) grant-funded project to test alternative materials that could replace some or all of the fly ash currently required by the FDOT in concrete mixes.  The use of alternative materials, or those that are pozzolanic in nature, have benefits: costs are lowered, while maintaining standards of strength and durability; the environment is protected by reducing greenhouse gas production; and their use reduces the amount that end up in landfills.

“We are currently working with a number of different industries within Florida, and the Southern United States, to provide a sustainable and cost-effective alternative to traditional pozzolans used to supplement concrete,” Ferraro said. “We are confident that we will be able to alleviate the pressure on the portland cement concrete industry borne from the shortage of coal-fly ash, which is prevalent throughout the Florida and the other states.”

The researchers studied several materials as possible partial replacements for Portland cement. Many were recycled: class C coal fly ash (from coal-fired power plants), pulverized glass (from glass recycling), wood ash (from wood-fired electricity production), sugarcane bagasse ash and rice husk ash (from agricultural production). Using the alternative pozzolans, the researchers made a total of 55 mortar mixes (cement + sand + water) and 22 concrete mixes (mortar + aggregate). Each mixture was tested for compressive strength, modulus of elasticity, splitting tensile strength, flexural strength, surface and bulk resistance to cracking, bulk resistivity, coefficient of thermal expansion, and heat generation.

Based on test results, researchers produced recommendations that can be found in the final report to the Florida Department of Transportation at http://www.fdot.gov/research/Completed_Proj/Summary_SMO/FDOT-BDV31-977-06-rpt.pdf.

For more information, contact Dr. Chris Ferraro at ferraro@ce.ufl.edu.