What makes a sustainable pavement?
LEED, GreenRoads, GreenLites, Green Credits and FHWA’s Sustainable Highways program among others are all bringing the green-building movement into pavements and highways. Most people consider sustainable pavements as those containing recycled materials. However, the Latin root of the word “sustainability” means to endure, so while recycling does make a pavement more sustainable, the ultimate goal is to make pavements last longer. For both cost and environmental life-cycle assessments, not reconstructing a pavement is better for the owner and the environment than frequent maintenance.
Concrete is a favorable choice when life cycle is considered, because it has the potential to last a long time, as 2,000 years of performance from Roman concretes has shown. Unfortunately, some concrete is being specified with higher-than-typical levels of recycled materials to meet LEED and other green-building requirements, without consideration to early-age effects on durability. Supplementary cementitious materials (SCMs), such as fly ash and blast furnace slag, increase long-term durability while decreasing the concrete’s carbon footprint, but slow the hydration densification process, making these pavements more susceptible to early-age durability distresses. In the age of advanced admixture technology for concrete, an environmentally friendly, natural, low-tech material is making concrete more durable and outperforming chemical admixtures. Research at the University of Missouri-Kansas City is showing that soybean oil can improve the surface durability of traditional and these more environmentally friendly concretes.
A better cure
The current socioeconomic climate expects and demands sustainable construction practices.
Sustainability of concrete includes both using “green” or recycled products, but also improving construction durability to maximize useful service life. One key component of concrete durability is providing moisture during 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 membrane-forming chemical compound to the surface of the fresh concrete. Curing compounds limit the surface evaporation and can be tinted to help keep the pavement cooler. Common curing compounds are wax-based or chlorinated-vinyl rubbers, or PVC, which work well, but can have the potential for storm-water and groundwater pollution and may contain high levels of volatile organic compounds.
Natural, plant-based curing compounds have been used successfully in the past. Linseed oil and flaxseed oil had wide-spread use in concrete curing until modern curing chemicals were introduced in the 1970s.
Soybean oil, a newer curing option, also has shown the potential for improved concrete curing. Soybeans are grown for many food and energy uses and widely available. The particular soybean-oil emulsion investigated here was first marketed as a moisture repellent for wood decking and currently used for pervious concrete curing.
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 (DOTs), municipalities and commercial builders strive to become greener, traditional PVC-based curing compounds are being limited due to environmental concerns.
Pervious concrete is designed with a series of interconnected voids to help infiltrate storm-water runoff. The large amount of void space means that fresh concrete mixtures are extremely susceptible to moisture evaporation and poor surface durability. Research shows that when a soybean-oil emulsion is applied to fresh concrete, the water evaporates, reducing surface drying, while the oil penetrates into the surface concrete pores up to 3 mm. The current curing method is to leave pervious concrete covered under plastic sheeting for at least seven days. A previous study performed at Iowa State University investigated the effect various curing methods had on strength and surface durability of pervious concrete. A widespread practice for pervious concrete is soybean oil applied before covering with plastic, where cover is maintained for seven days. The soybean oil prevents initial evaporation until the plastic is applied and prevents surface marking from the plastic sheeting.
Scaling the problem
Additional research at the University of Missouri-Kansas City has shown that integrally mixed or surface-applied soybean oil reduces evaporation on traditional, impervious concrete. Figure 1 shows a series of moisture-retention results of soybean oil and control concrete samples.
Moisture loss limits for a membrane-forming curing compound are 0.55 kg/sq meter according to ASTM C309 standards. The soybean oil reduces the moisture loss, nearly achieving the required retention.
Other testing investigated the effects on moisture loss of soybean oil mixed into the fresh concrete. Figure 2 shows that for several different amounts of soybean oil mixed into the fresh concrete, moisture loss was reduced by a third below the control, but was still more than that applied to the surface. The 1.3-mL samples represent an addition rate of 2 oz. soybean oil per 100 lb of cement. The 1% samples were by weight of cement and had 34 oz. per 100 lb of cement.
There was no effect on compressive strength for the samples with 2, 6 or 12 oz. of soybean oil per 100 lb of cement (1.3-, 3.9- and 7.8-mL samples), although there was not a significant decrease in moisture retention, either. In addition to improving curing, soybean oil was investigated for its ability to help prevent deicer scaling. Deicer salts penetrate concrete in an aqueous solution, dry and then crystallize and expand causing the surface of the pavement to flake off. The deicer salts also increase the level of saturation in the concrete, making it more susceptible to freezing and thawing damage. Concrete containing high levels of recycled SCMs cures slower and generally has higher permeability during the first winter than concrete with only cement. The higher porosity allows more salt solution and water to penetrate deeper into the concrete matrix. Mixtures containing high levels of SCMs tend to have more issues with surface durability during the first winter compared with mixtures containing only cement. Even though the mixtures containing SCMs are more environmentally friendly, many municipalities and state DOTs ban, or severely limit, SCMs because of durability concerns.
Soybean-oil deicer scaling results were recently presented at the FHWA International Conference on Sustainable Pavements and published in the International Journal of Pavement Research and Technology and show that soybean oil allows early-age application of deicer salts to concrete with up to 82% replacement of cement with SCMs. Traditionally, mixtures containing blast-furnace slag are the most susceptible to deicer scaling.
Deicer scaling results in the loss of the surface cement paste and large amounts of exposed aggregate. Soybean oil applied to concrete containing 50% to 75% blast-furnace slag helped to minimize deicer scaling. According to ASTM C672, mixtures were cured 14 days and then subjected to 50 freeze-thaw cycles with a calcium chloride deicer solution ponded on the surface. Uncured concrete containing only cement or 75% cement replaced with slag experienced severe deicer scaling, while the sections curing with soybean oil had good performance.
Silver nitrate testing on the sample profiles indicated that samples coated with soybean oil had less salt penetration than those without. Seventy-five percent replacement for cement with slag lowers the associated CO2 by around 70%, but finishing and deicer scaling are the primary concerns preventing more use.
When the soybean oil is applied to hardened concrete it soaks in and blocks surface pores. The concrete becomes slightly darker but, because the oil penetrates the surface, is not slippery. Penetration depth was the greatest on hardened dry concrete and less for moist or fresh concrete. An initial application of soybean oil to fresh concrete did not significantly improve deicer scaling, but reduced evaporation. Soybean oil applied to hardened concrete or reapplied to concrete initially cured with soybean oil had the best resistance to deicer scaling.
The rapid advancement of green building and sustainable infrastructure has all industries searching for more environmentally friendly products. Luckily for the concrete industry, a natural product, soybean oil, has the potential to improve curing and surface durability of currently utilized concrete mixtures and even those containing high levels of recycled/reused materials. Soybean oil is just one product in a long line of new products and techniques that will improve concrete durability and life span while having a smaller environmental footprint.
