By Kyle Kerstiens, Assoc. AIA, LEED AP
The NPCA Sustainability Committee and NPCA staff are committed to helping members navigate the sea of “green” terminology that has kept some from embracing these new practices. As such, we will be including a sustainability page in each issue of Precast Inc. Since aggregates account for about 70 to 80% of concrete’s mass, substituting only 5% of a concrete mix with recycled aggregates can translate into positive environmental stewardship. Recycling reduces materials destined for landfills and saves energy required to remove raw materials from the earth. While recycling requires some modification to mix design, environmentally conscious producers are realizing the benefits of implementing this strategy.
Wausau Tile recycles glass
Wausau Tile, Wausau, Wis., understands the added value of using sustainable products. Terrazzo tile, the company’s most striking glass concrete product, features glass chips ranging in size from 1/16 in. to 3/8 in. (1.6 mm to 9.5 mm). These richly hued tiles have been used in schools, libraries and even art museums, because the recycled glass produces more vibrant color ranges well beyond that of traditional concrete. “Blues, reds, yellows, greens – vibrant colors that Mother Nature doesn’t make,” says Rodney Dombrowski, Wausau’s Terra Paving Division Manager.
Dombrowski sees glass concrete products as a way to make the recycling concept more accessible to the public. “With some products made from recycled materials, you wouldn’t know it,” he explains, “If it’s a stud in a wall, you’d never see it. With glass in terrazzo, it has a lot of aesthetic value.”
Of all the commonly collected post-consumer materials, glass has been among the most difficult to recycle. Broken, mixed-colored glass has proven virtually impossible to reuse cost-effectively. Mountains of used glass heap up around the country without a buyer. New York City, for example, collects more than 150,000 tons (136,050 tonnes) of glass per year, much of which ends up in landfills because it isn’t recycled.
Wausau Tile has been able to make glass aggregate versions of all its precast products, including site furnishings such as benches, tables, planters, pavers and terrazzo tile. “The glass in our products is 100% post-consumer/post-industrial recycled content and accounts for approximately 33% of our product’s weight,” notes Dombrowski, adding that it helps designers contribute to the U.S. Green Building Council’s (USGBC) LEED credits by promoting the use of recycled-content building materials.
According to a study conducted by the U.S. Federal Highway Administration in 2004, 11 states recycle concrete into new concrete. FHWA reports that the performance of concrete using recycled aggregates equals that of concrete with natural aggregates.
Contractors Precast Corp. finds profit in sustainability
Many precast manufacturers are recycling their products for use as bulk fills, road base construction, embankments or barriers. According to the FHWA study, 38 states recycle concrete as an aggregate base for roadways. NPCA-certified Contactors Precast Corp. of Davidsonville, Md., recycles all waste concrete and unused custom products. The concrete is first crushed and then screened on site using plant equipment. By using this strategy and selling the result to local contractors as base material, the company reduces material waste and increases profit at the same time. Scrap metal from the crushed concrete is sold to a local recycler. ACI 555R-01, “Removal and Reuse of Hardened Concrete,” provides guidance on practices for using recycled aggregates, in particular Chapter 5, “Production of Concrete from Recycled Concrete.”
Recycled aggregates affect mix design
The addition of recycled aggregates – both concrete and glass – require corrections to the mix design. Through careful monitoring of the mix design, a producer looking to increase recycled content should be able to make a sustainable process contribution without a cost increase. The rate of water absorption is higher for recycled concrete aggregates. Recycled concrete aggregates have a lower specific gravity than conventional aggregates and require more stringent moisture control. Concrete with recycled concrete aggregate will exhibit slightly higher drying and shrinkage creep.
Adherence to the gradation requirements of ASTM C33, “Standard Specification for Concrete Aggregates,” may be more difficult to achieve with recycled concrete aggregate because of the difficulty in controlling particle size during crushing. Producers must also consider the chloride content of recycled concrete aggregates.
Using recycled glass aggregates requires addressing inherent problems within standard concrete mixes. For example, it has long been widely accepted that glass reacts with hydrated lime in concrete causing the formation of a gel called calcium silicate hydrate that expands and can ultimately fracture concrete. This reaction is known as alkali-silica reaction (ASR). ASR can be overcome with the addition of a pozzolan know as high-reactivity metakaolin. Metakaolin helps produce both high-early strength and increases long-term strength.
LEED credits and the competitive edge
Recycled aggregates can help contribute to credits in various LEED rating systems. One such credit is in the LEED 2009 New Construction Rating System. The so-called “Credit MR 4, Materials and Resources-Recycled Content” is intended to promote the use of materials with recycled content. Glass aggregates and recycled concrete can help contribute toward this credit. NPCA has an online LEED Calculator to help precasters complete the necessary paperwork for this credit (visit www.leed.precast.org).
Economic downturns make finding additional work tough and highly competitive. Adding recycled aggregates can save money, give precasters a marketing edge over competitors, and establish greater public acceptance for environmentally friendly processes. Over the next few years, the practice of recycling aggregates will become more widely accepted as the precast concrete industry continues to lead the way toward sustainability in construction.
Kyle Kerstiens, Assoc. AIA, LEED AP, is NPCA’s director of Sustainability.