C&D World: Road Warriors

Recycled aggregates can be used successfully in a number of highway applications, according to Charles Gaskin, director of construction for the Houston District of the Texas Department of Transportation (TxDOT), who shared his district’s success stories with the material at a session at the C&D World Exhibition & Conference held recently in San Antonio.

Gaskin said the Houston District of TxDOT has had success using a variety of recycled aggregate materials, including recycled concrete aggregate, asphalt pavement, tires, asphalt shingles and industry/foundry sands. He said the district uses a large amount of recycled concrete because of the lack of native course aggregates, the costs of importing aggregates and a large steady supply of recyclable concrete.

The district first experimented with using recycled crushed concrete as cement treated base in the late 1980s. As a coarse material with very little binder, it easily segregates and wicks moisture. Because of these traits, crushed concrete works better in dry areas and is not recommended for use in wet conditions or under flexible pavement, Gaskin said. He added the material worked well under continuously reinforced concrete pavement.

The Houston District expanded the use of crushed concrete into Type A dry rip rap in the early 1990s, and first used the material for concrete paving and structures in 1996 on a rehabilitation project on a 5.8-mile stretch of Interstate Highway 10.

The concrete mix used on the rehab project included 6.5 sacks of cement, 1.5 inch recycled coarse concrete, 30 percent fly ash, 5 percent entrained air and recycled fine aggregate. According to Gaskin, this project yielded both positive and negative lessons. Due to higher creep values, recycled concrete aggregates are no longer allowed in structural concrete. In addition, the project showed recycled fine aggregate has an adverse effect on flexural strength and is not limited to 20 percent of the fine aggregate portion of the mix. The district also learned that the material is highly absorbent, making moisture control of recycled aggregate critical, Gaskin said. He recommended being selective in what is crushed, maintaining uniformity of material types in stockpiles and aggressively monitoring moisture for success in using recycled aggregate in concrete paving applications.

Gaskin added that recycled crushed concrete can also be used successfully as concrete traffic barriers, fines for asphalt pavement mix, shouldering up roadway edges and fill.

According to Gaskin, the Houston District has also had success using recycled asphalt pavement (RAP). He said the district uses a large amount of RAP because of high costs of importing aggregates. He added that local contractors have developed a high level of experience using the material and most of them have equipment capable of using recycled asphalt materials.

TxDOT specs allow 20 percent RAP in dense graded applications (non-surface hot mixes) and 30 percent in all other non-surface hot mixes. The Houston District allows 10 percent in surface mixes, 20 percent in dense graded (non-surface mix), 30 percent in all other non-surface mixes and 100 percent in cement stabilized base.

Gaskin said RAP is firm, but porous, making it well-suited for use in shouldering next to pavement edges, lining ditches, providing temporary or permanent driveways and providing temporary side streets on construction projects. Using RAP produces several benefits, including reduction of the amount of new aggregate required, reduction in the amount of new oil required and an overall reduction of the cost of new asphalt mixes, according to Gaskin.

Gaskin added that the Houston District has successfully used crumb rubber and tire chips in highway applications. He said crumb rubber is particularly suited to use in hot mix asphalt, especially in high-accident areas because it creates good friction.

When considering the use of recycled products, Gaskin advised potential users to consider the cost of collection and processing, the cost of manipulation to meet given specifications and location, as hauling becomes a major factor in pricing.

Additional information on TxDOT’s recycling efforts is available at www.txdot.gov/recycling.