Since 1988, Master Builders Inc., of Cleveland, and General Electric Co.'s Plastics Div., of Pittsfield, MA, have been working together to develop plastic aggregates that can replace conventional mineral aggregates in concrete. After experimenting with several plastic formations, researchers found that an engineering thermoplastic called polybutylene terephthalate (PBT) had great potential. PBT is has a high mechanical strength, low moisture absorption, and good dimensional stability, making it an excellent aggregate. In laboratory tests, concrete containing an aggregate mix of virgin and recycled PBT had a 28-day compressive strength of 5500 psi. Concrete made with PBT aggregate also shows marked increases in flexural strength and impact resistance, as well as improved resistance to freeze/thaw damage. Other advantages to PBT aggregate concrete include: availability of nearly any color desired, availability of recycled plastic as an aggregate, it is lightweight, and it can be sandblasted, polished or textured just like mineral aggregates. The major drawbacks to plastic aggregate is cost--it is several times more expensive than mineral aggregate--and a lack of in-field performance testing. Because the product is so new, there have been few installations to date.