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Summary of Texas Recycled Concrete Aggregate Review

April 20-25, 2003

As a member of a resource-intensive industry, Texas DOT (TxDOT) realizes that while most of the raw materials for roadway construction are available in sufficient quantities to develop and maintain a high-quality transportation system, many native materials, including aggregate, are not evenly distributed throughout the state. TxDOT further realizes that materials being removed from their right-of-way, such as the removal of existing pavement, can benefit the TxDOT if they are reused. TxDOT is a large user of RCA materials. However, private industry and municipalities consume over 60% of the RCA currently produced in Texas.

Although the use of recycled concrete aggregate (RCA), or other recycled-concrete materials or products, is not specifically mentioned in TxDOT's strategic plan, they have created the conditions that allow for use on their projects. Through research, implementation, and competition, TxDOT has found that using RCA, like many other recycled materials, provides engineering, economic, and environmental benefits.

Engineering:

  • RCA in new concrete decreases the resilient modulus and increases the creep. However, it should be noted that these changes are only benefits in specific applications.
  • RCA that originated as concrete with rounded aggregate yields a new product with particles having fractured angular shapes for increased paste bond.

Economic:

  • RCA is bid as an option in construction contracts for non-structural concrete (Coarse 100%, fines 20%) and as a base material (100%). Because of the current economic benefits of RCA, the total generated waste flow in Houston is being consumed.
  • Decreased haul distance because the waste stream of RCA primarily originates and is consumed within the same urban areas.

Environmental:

  • Eliminates the development of waste piles of concrete.
  • There is a decreased haul distance because the waste stream of RCA usually originates and is consumed within the same urban area. This decreases energy consumption and helps improve air quality through reduced mobile source emissions.

TxDOT has been using RCA for the last 10 years and has overcome many hurdles. A summary of TxDOT's experiences is provided below:

  • The use of RCA in new concrete initially created problems with mix workability. The problem was associated with the absorbency of the aggregate and the difficulty maintaining a consistent and uniform saturated surface dry condition of RCA aggregate.
    • This hurdle was overcome by the contractor through their process control program. Their process control program heightened their awareness of the need to water stockpiles and to conduct more frequent testing of aggregate for moisture content.
  • Research has identified an increase in creep and shrinkage when RCA is incorporated into new concrete. This can be a major issue when RCA is used in structural concrete.
    • TxDOT does not currently use RCA in structural concrete because of the possible issues with creep and shrinkage. However, TxDOT has used RCA in some structural applications and is monitoring them.
  • Initially, there was a general perception that RCA is a waste product and thus substandard material.
    • TxDOT has used RCA where the risk is minimal and with a high potential for performance. They have also performed training and continually present information to their Districts concerning the performance of the projects they have completed around the state.
  • TxDOT initially experienced lower compressive strength and workability issues.
    • Research linked the use of RCA fines in the concrete to the lower compressive strengths and workability. At that time it was determined that 20% was the maximum amount of RCA fines that would be allowed in the concrete.
  • Initially, there was a lack of information on the cost benefits and performance of RCA.
    • The lack of quantitative data continues, however, with over 10 years experience TxDOT believes RCA provides a cost benefit in specific applications. The RCA is bid as an option, so the economics of the low bid system drives the use of RCA.
  • There was a perception that environmental impacts would rise from using recycled materials.
    • TxDOT has educated itself through several years of using RCA. TxDOT has also established mechanisms, such as waste stream documentation, to identify the source of the materials.
  • Benefits could only be realized where there is an adequate supply of quality RCA.
    • The market place, in some areas, assures that a constant waste stream is present within specific geographical regions.
  • The placement of RCA base material has provided some hurdles in grading and compacting.
    • Excessive working of the RCA base will segregate the base materials. Minimum shaping of the RCA base material should occur.
    • Compaction of the RCA base should be in a saturated state to aid in the migration of fines throughout the mix.

TxDOT still is looking to gain additional experience or information in the following areas:

  • An acceptable test procedure for assuring durability of the RCA in concrete, asphalt, and base materials.
  • A better method of evaluating and accepting RCA that has contaminates (gypsum, wood, fiberglass, etc).
  • Crushing facility relationships with neighbors in an urban environment. This includes dust production, noise pollution, and truck traffic.
  • Determination of allowable thresholds of other materials, such as lead and asbestos, in the RCA.
  • Establishment of environmental testing designated to evaluate the impacts of using RCA.
  • Better understanding of the relationship of RCA in structural concrete and its performance.
  • Conduct research and educate the public on the social economic impacts of crushing.
  • Conduct research and educate consumers on the social economic impacts of recycling concrete aggregate.
  • Indirect barriers of RCA being perceived as a lower quality product continue to exist.
  • Some standard specifications continue to exist that infer that recycled materials cannot be used (TxDOT Specification 247 and 421)

People in attendance at discussion groups

Review Team

Pavement & Materials

Environmental and Planning

Project Administration

Industry

Leadership

Materials Received during the review

  • General Notes for Item 262, 263, and 276
  • Current DMS-11000, Guidelines for Evaluating and Using Non-hazardous Recyclable Materials (NRMs) in TxDOT Projects.
  • Proposed DMS-11000, Guidelines for Evaluating and Using Non-hazardous Recyclable Materials in Department Projects.
  • Texas Natural Resource Conservation Commission, Air Quality Standard Permit for Temporary Rock Crushers.
  • Texas Functional Organizational Chart
  • Some project files for US 59 - In Houston
  • Some project files for IM-10-7(361)762

Research Reports

  • TX-02/4954-1, Characterization of crushed concrete materials for paving and non-paving applications.

Sites Visited

Projects

  • US 59 - In Houston, this project added a lane to an existing bridge structure. The lane addition contained RCA in the decking concrete and structural piers.
  • IM-10-7(361)762, This project was the removal of CRCP pavement and replacement with 14 " of CRCP with RCA as the coarse and fine aggregate in the mix.

Crushing/Recycling facilities

  • Williams Brothers recycling facility. We observed a stock pile of concrete to be recycled, crushing facilities, washing facilities, stockpiles, and a closed wash and rain water containment system. This facility only accepts concrete rubble from highway projects. The washed aggregate is used in new concrete.
  • Southern Crushed Concrete recycling facility. We observed concrete to be recycled, crushing facilities, stockpiles, and a pug-mill mixer for producing cement stabilized base. This facility accepts concrete ruble from all sources. The RCA produced at this facility is only produced and sold as base or fill material.
 
Updated: 04/07/2011
 

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