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Sustainable Aggregate


Most people are familiar with the importance of recycling traditional household items, but environment- and budget-conscious construction companies and

PIERCE FLANIGAN AND WILLIAM ADAMS

Most people are familiar with the importance of recycling traditional household items, but environment- and budget-conscious construction companies and developers have been making use of more unlikely recycled materials ó concrete and asphalt ó for years. In fact, asphalt recycling was a common practice starting in the 1970s, long before recycling became fashionable. Recycling concrete, on the other hand, has caught on during the past 10 to 20 years due to greater environmental awareness and ever-rising fuel costs.

P. Flanigan and Sons, a Baltimore-based transportation and infrastructure construction company, has been producing sustainable aggregate products since January 2006 and recycles approximately 250,000 tons of concrete each year. The company continues to improve its capacity to produce sustainable construction material and recently opened a new plant in West Baltimore ó the Winchester Recycling Facility, devoted solely to recycling concrete debris. This innovative facility, converted from an old iron foundry, produces recycled aggregate that meets the rigorous specifications of the Maryland State Highway Administration for virgin aggregates.

NEW VERSUS RECYCLED AGGREGATES

A common misconception of recycled concrete products is that they are sub-par to virgin products, which (by definition) have never been used. While there is the potential for foreign objects remaining in the stone from its prior use, plants that produce high-quality recycled concrete take many precautionary steps to control the purity of their product. These steps include investing in equipment that ensures top-quality recycled concrete.

Virgin concrete is a finished product composed of cement, coarse aggregates, water and chemical admixtures that can be used for pavement in building roads, bridges, overpasses and parking structures. Recycled concrete is not old concrete re-used for a new purpose; instead, it is a crushed aggregate that can be used as-is for base filler under roads and other foundations and as a component in the process of manufacturing new concrete.

Sustainable aggregates, such as reclaimed concrete, are produced using resources as efficiently as possible. Any environment that has undergone redevelopment, reconstruction or significant repair should be looked at as the most convenient quarry possible. Such sites often have a substantial amount of natural resources ready to be ìminedî from the aging and deteriorating infrastructure that often get sent to the local landfill. Excavating sites not only involves clearing the way for new construction, but also reclaiming materials and transforming them for use in new construction.

According to the National Asphalt Pavement Association, hot-mix asphalt, the most common reclaimed aggregate product, is currently the most recycled product in the nation. In fact, 80% of asphalt taken up annually is reused in roadway projects, and concrete is slowly joining the trend. National initiatives are dedicated to encourage recycling practices among contractors to make even greater use of reclaimed aggregate products ó including concrete.

WHY CHOOSE SUSTAINABLE?

While new concrete products certainly can be manufactured at rates that keep up with demand, there are plenty of good reasons to consider sustainable aggregate material over virgin aggregate. Environmental benefits include reducing the industry's need for freshly mined aggregates. Gravel mining expends energy and increases carbon emissions through the transportation of equipment, product and personnel, which could be avoided by recycling material that already is in a project's geographic region.

Recycling concrete also reduces the strain that large-scale construction projects place on landfills. Disposal of recyclable materials not only wastes the material, it takes up a great deal of dwindling landfill space. Taking into account landfill, transportation and gravel mining issues, recycling aggregate produces considerably less by-product and pollution than using new materials. In addition, the quality control performed on a recycled product is equally as strenuous as the standards used for virgin material.

Perhaps endorsing environmentally friendly practices is the most obvious reason for choosing sustainable aggregates, but using recycled concrete also provides economic incentives that can lead to significant cost savings for construction companies. Landfill disposal itself can needlessly drain a project's budget due to the simultaneously rising cost and decreasing availability of landfill dumping. Producing and using recycled asphalt and concrete can translate into a more profitable bottom-line. Companies can preserve landfill space as well as save on trucking and fueling expenses.

However, companies do not need to produce the recycled aggregate themselves to reap the benefits. With diesel fuel costs on the rise and landfill fees expensive, an increasing number of contractors are purchasing recycled concrete, which is approximately half the cost of virgin materials.

Despite the obvious advantages, there is some hesitation to embrace recycled aggregate within the industry. The reasons include a lack of available recycled aggregate, a lack of processing technology on-site, state specifications and past (negative) experience, according to a recent study conducted by the National Cooperative Highway Research Program, which is administered by the Transportation Research Board and sponsored by individual state departments of transportation.

Availability largely is determined by geographic location, with an excess of reclaimed aggregate available in urban areas and depleted supplies in more rural areas. Changes in practice and technology have evolved to accommodate the use of recycled aggregate; state guidelines and personal judgments are sure to follow.

THE RECYCLING PROCESS

Recycling concrete mainly requires crushing equipment, much like the equipment used at a quarry. Crushers for reclaimed aggregate are modified to crush concrete that contains steel. Magnets are used to extract the metal, which is sold to steel companies for recycling. The material is crushed small enough to release the foreign materials and purify the aggregate. The crushed concrete can be used as filler in transportation infrastructure or in manufacturing ìnewî concrete.

As recycling asphalt and concrete has become more desirable, the recycling process naturally has evolved to meet changing standards and become more efficient. In the past, concrete facilities were primarily batch plants. Such facilities produced a single batch at a time ó heating the aggregate, sorting it by size, and finally sending it to a pugmill to blend the asphalt cement and aggregate. High upfront costs and expensive maintenance were a negative factor, and such plants did not lend themselves to processing a large percentage of recycled aggregate.

With the advent of reclaimed asphalt pavement in the early 1970s, drum plants became a viable alternative to batch plants. Drum plant facilities are able to produce asphalt cement using a larger proportion of recycled aggregate. However, early drum plants used a parallel flow design that lacked air pollution controls and tended to overheat the aggregate, which emitted smoke instead of steam.

Today, modern drum plants typically feature a counter flow design. Along with other engineering innovations, this design allows the recycled product to enter the drum mixer after virgin aggregates, and flow counter to the heat source that controls temperature and decreases pollutants. Drum plants ensure that the greatest content of recycled aggregate (allowed by the state) in hot-mix asphalt is used.

LEADING BY EXAMPLE

Flanigan's plants produce approximately 750,000 tons of aggregate annually, of which 250,000 tons contain recycled content. This includes crushed concrete material, which is used as filler in transportation infrastructure. Recycled concrete generally runs about half the price of virgin concrete. Contractors taking their ìwasteî to recycling facilities save on landfill fees, transportation and the product itself, if they choose to purchase it after processing. Taking concrete material from a construction site to a recycling plant generally involves a tipping fee, which is around half the cost of dumping.

Recycling asphalt and concrete ó a practice that started because it made good business sense ó is now becoming increasingly popular as the industry adopts more environmentally friendly practices and products. The ìgreen buildingî movement already is underway, and continued development and improvement of recycled products will help the industry by enabling it to change and adapt to practices that will eventually be the norm.

Pierce Flanigan IV is vice president of plants and equipment and William Adams is plant manager for P. Flanigan & Sons Inc., a Baltimore-based heavy construction and infrastructure builder.

WHAT IS FOAMED ASPHALT?

Foamed asphalt is produced using a specialized manifold to force water and air through conventional PG 64-22 liquid asphalt. The high temperature of the asphalt causes the cool water to expand rapidly. This expansion causes the asphalt to form miniscule air bubbles or ìfoam.î During blending, the foamed asphalt's volume is 10 times the size of the liquid asphalt alone.

The benefit of the foam formation is that the liquid asphalt has a much greater surface area and much lower viscosity. This causes it to bind more readily with aggregates. Another benefit is that the mix will not compact until the air bubbles have been driven from the liquid. This allows stockpiling of the material for up to 40 days.

This product can replace both a stone base course and the asphalt base course. It is comprised of 40% screened millings and 60% crushed concrete blended in a pugmill. No heating or drying of the aggregate is required. In the pugmill, 7% water and 3% foamed asphalt are introduced into the mix, which produces a very stable and extremely strong base material. Due to 14% air voids in the mix, it is not suitable as a surface course; however, foamed asphalt base can be placed in a conventional paving operation. The joints are nearly impossible to detect. Its stability is similar to asphalt, and its load bearing is similar to concrete.