In the age of advanced admixture technology that allows everything from ultra-high performance concretes to grout placement in high-velocity moving water, an environmentally friendly natural low-tech material is making concrete more durable and outperforming chemical admixtures. For the past few years, university researchers have investigated how soybean oil can improve surface durability.

Building owners, architects, and others expect and demand sustainable construction practices. Sustainability of concrete includes improving construction durability to maximize useful service life.

One key component of concrete durability is providing complete curing for increased hydrated cement paste density and performance.

Denser cement paste lowers permeability and helps to minimize unwanted penetration of water and aggressive ions into the cement paste matrix. Curing is commonly performed by applying a chemical curing compound to the surface of the fresh concrete.

Common curing compounds are chlorinated-vinyl rubbers. These work well, but they have the potential for stormwater and groundwater pollution.

The darker sections of this pervious concrete parking lot were not cured with soybean oil and experienced severe raveling. The lighter-colored sections were cured with soybean oil and are in good condition.
The darker sections of this pervious concrete parking lot were not cured with soybean oil and experienced severe raveling. The lighter-colored sections were cured with soybean oil and are in good condition.

Natural, plant-based curing compounds have been used successfully. Linseed oil/flaxseed oil were widely used in concrete curing until modern curing chemicals were introduced in the 1970s. Soybean oil, a newer curing mechanism, also has shown the potential. Soybean oil emulsion is currently marketed for pervious concrete curing and as a moisture repellent for wood decking.

Stormwater runoff

Pervious concrete is designed with a series of interconnected voids to help promote stormwater runoff. The large amount of void space means that fresh concrete mixtures are extremely susceptible to moisture evaporation and poor surface durability. Curing is achieved by leaving pervious concrete covered under plastic sheeting for at least seven days.

Iowa State University (ISU) researchers investigated the effect various curing methods had on strength and surface durability of pervious concrete. Results showed that mixtures cured with soybean oil had higher strength and better abrasion resistance than those cured with traditional curing chemicals.

Like other previously used vegetable and mineral oils for concrete, soybean oil provides a barrier between aggressive surface chemicals, such as deicing salts, and the concrete. Concrete has relatively high porosity at early ages, which makes concrete especially susceptible to freeze-thaw and deicer damage during the first winter. As state departments of transportation, municipalities, and commercial builders strive to become greener, traditional polyvinyl chloride-based curing compounds are limited due to environmental concerns.

ISU researchers showed that when a soybean oil emulsion is applied to fresh concrete, the water evaporates. This reduces surface drying while oil penetrates into the surface concrete pores up to 3 mm. Pervious concrete sections cured with soybean oil had higher strength and better abrasion resistance than sections cured with traditional surface-applied curing chemicals.

For pervious concrete, the soybean oil is typically applied after finishing operations and just before the section is covered with plastic. The soybean oil also helps prevent unwanted discoloration caused by the plastic resting on the surface.


Reduce evaporation

Additional research at the University of Missouri-Kansas City has shown that integrally mixed or surface-applied soybean oil reduces evaporation and allows early-age application of deicer salts, even to traditional concrete when up to 82% of cement is replaced with supplementary cementitious materials.

A paper recently published in the International Journal of Pavement Research and Technology showed that soybean oil applied to concrete containing 50% blast furnace slag eliminated deicer scaling. Mixtures were cured 14 days and then subjected to 50 cycles with a calcium chloride deicer solution pooled on the surface.

The slag mixtures had severe deicer scaling, while the sections cured with soybean oil performed well. Purdue University has investigated a soybean acrylic mixture which has shown potential for protecting sawed pavement joints from freeze-thaw damage.

So when your next job calls for an environmentally friendly solution for reducing evaporation and improving surface durability, you might consider soybean oil.

John T. Kevern is an assistant professor of Civil Engineering at the University of Missouri-Kansas City. He sits on several pervious concrete committees.

Jim Miller is the founder of C2 Products. He is an NRMCA-certified pervious craftsman in Indiana and is developing the decorative pervious concrete methods and market with C2 Construction. Visitwww.c2products.com, e-mailc2products@aol.com.