As people seek more sustainable and environmentally friendly ways to live, industries are working to find easily renewable resources that can take the place of limited resources such as petroleum. They are also investigating beneficial uses for industrial byproducts that once would have gone to waste in landfills. The concrete industry has made significant progress in this area, from the use of alternative fuels in cement manufacturing to the inclusion of fly ash, blast-furnace slag, and recycled aggregate in modern concrete mixtures.
This trend now extends to some of the products used to treat concrete or enhance its performance. Sustainable soybean-based products are already used as protective coatings and curing agents, and some promising research suggests they may be effective for sealing pavement joints as well. When using greener products can improve concrete’s durability and extend its service life, it promotes truly sustainable construction.
Soybean oil in a water-based emulsion can serve as an all-natural alternative to acrylic sealers for concrete driveways, sidewalks, patios, garage floors, and other flatwork. One such product, called The Bean, is marketed for this purpose by C2 Products Inc., Arcadia, Ind. The company’s founder, Jim Miller, says eco-friendliness is the sealer’s strongest selling point, “People are looking at greener alternatives for all sorts of products, and it’s become more of an issue for concrete sealers as VOC regulations have gotten stronger.”
Steve Hostettler, who runs Hostettler Concrete Construction LLC, Pinesville, Ind., often uses The Bean as a sealer, especially when his customer wants a matte finish. “I like to use it when we want to avoid shine and slipperiness, such as on a garage floor. It goes on as easily and works just as well as acrylic sealers. Because it has the oil in it, it also works as a form release agent, and it’s convenient to be able to use the same product and sprayer for both purposes,” Hostettler says.
Proper curing is an important part of quality concrete work. It promotes durability by increasing the density of the hydrated cement paste. Denser, more hydrated paste at the concrete surface provides greater resistance to water and chloride penetration, and thus combats a major cause of deterioration.
Curing is especially crucial for modern concrete mixes that contain significant amounts of supplementary cementitious materials such as fly ash and blast-furnace slag. These ingredients serve to increase concrete durability over the long term, but they slow the hydration of cement paste. Without sufficient attention to curing, they can make concrete more susceptible to early-age distress.
Membrane-forming curing agents are typically applied to fresh concrete to slow evaporation, keeping water available for hydration. Many such agents are composed of chlorinated-vinyl rubbers, which work well but have the potential to pollute ground water and stormwater runoff. As state DOTs and other owners require more environmentally friendly construction, soybean oil emulsions are being considered as an alternative.
Before the introduction of chemical curing agents in the 1970s, plant-based products, such as linseed oil and flaxseed oil, often were applied to help cure fresh concrete. Soybeans, now widely grown for food and fuel production, provide another natural source of oil that can be similarly used.
Pervious concrete is designed with an open structure, produced through the use of gap-graded coarse aggregate and no fines. With approximately 15% to 25% void space, pervious concrete allows rainwater to flow through, while filtering out pollutants. Because of its open structure though, fresh pervious concrete must be cured carefully to prevent moisture evaporation and ensure surface durability.
It has become common to use soybean oil emulsions to promote proper curing of pervious pavements. The material is sprayed on the pavement surface after screeding, and then covered with plastic sheeting for seven days or more. The oil penetrates into the surface pores, and within about 24 hours, dries to form a sticky, pliable coating.
According to Miller, the product imparts three main benefits in pervious pavement applications: “First, it retards the evaporation rate to leave moisture available for cement hydration. Second, it helps prevent condensation from marking the concrete surface when the plastic is removed. Third, the sticky film adds some abrasion resistance to the surface. The bean oil will also carry pigment, so it can serve as a decorative stain as well.”
Research at the University of Missouri-Kansas City has shown that soybean oil emulsions also can enhance curing of impervious concrete. Tests showed that soybean oil, when spray-applied to hardened concrete, significantly reduced early moisture loss compared to an untreated control sample. When mixed into fresh concrete, soybean oil also improved its moisture retention, but to a lesser extent than spray application.
Research results published in the International Journal of Pavement Research and Technology in 2010 have shown that soybean oil applied as a curing agent to hardened concrete can help to reduce chloride penetration and prevent deicer scaling. This effect has been demonstrated both on concrete made with only portland cement and on concrete with up to 75% supplementary cementitious materials (SCMs), such as blast furnace slag. Because it slows hydration, SCM replacement typically makes concrete more susceptible to surface scaling and freeze/thaw deterioration in its first winter. Such concerns about surface durability have discouraged the use of recycled slag in concrete pavement, despite its environmental benefits.
Soybean oil can penetrate hardened concrete and block surface pores, which reduces its ability to absorb deicer salt solution. In accelerated freeze/thaw testing, concrete samples treated with soybean oil showed significantly less deicer scaling than untreated samples, whether or not the concrete contained blast-furnace slag.
Researchers at Purdue University’s Pankow Materials Laboratory, led by Dr. Jason Weiss, are investigating another potential soybean-based product application for concrete. They are working with a material that combines soy methyl esters (SME), a byproduct of soy biodiesel production, with fillers derived from waste polystyrene. It is a green product that beneficially reuses waste and contains no VOCs. SMEs have solvent properties, so that polymers can be easily incorporated into SME solutions to modify the viscosity of the fluid.
Applied as a liquid, the material penetrates concrete surface pores and then dries to a somewhat “gelatinous” condition. In a laboratory setting, SME-PS solutions seem to be effective in reducing water and chloride ion infiltration in concrete and improving its freeze/thaw durability.
A field testing program is underway to determine its performance in real-world conditions. Weiss says the tests are designed to assess the SME-PS material as a joint sealant rather than as a total surface coating, because pavement joints are the site of most pavement distress.
Funded by the Indiana Soybean Alliance, the field tests began in August 2011 and will run through three winters. One test site is a stretch of U.S. Route 231 south of Lafayette, Ind., where 500 lineal feet of longitudinal joints in 10-year-old concrete pavement were treated with SME-PS after any existing joint sealant was removed. Another stretch of pavement was left untreated but marked off to serve as a control. The second site is in the town of Fishers, Ind., where about 7000 lineal feet of both transverse and longitudinal joints, were treated. The Fishers’ test site includes both newly paved and older sections. In the new pavement, the SME-PS sealant was applied just after the joints were cut.
The material was applied in two passes, using a fan-tipped backpack sprayer. For the first pass, the spray tip was lowered into the joint; for the second pass, the tip was raised 1 to 2 inches to spray a 2- to 4-inch band on either side of the sawcut. The application along Route 231 was done by Indiana DOT personnel, and at the Fishers site by municipal crews and DOT staff.
Over the testing period, researchers will drill holes at designated distances from the joints and collect the powder generated by the drilling. Then the powder will be titrated and analyzed for the presence of both chlorides and the sealant material.
“This past winter was so mild that the sealant wasn’t exposed very severely,” says Ryan West, the Soybean Alliance’s director of new use innovation. “So far, there was relatively little snow and little deicer used, but upcoming winters will likely be more of a test.”
By studying and using naturally green products, such as soybean oils, to extend the service life of concrete, the industry can make an environmentally friendly material even more sustainable.
Kenneth A. Hooker is a freelance writer based in Oak Park, Ill.