Problem: When my customers send back partial loads, I’ve been dumping it at the back of my yard. I need to get rid of a small mountain to make room for more waste. What are my options for dealing with this unused concrete?
Answer: Your situation is a problem that has plagued producers for decades. Until about 15 years ago, drivers could dump leftover material somewhere out of the way on the jobsite. Today, with land use requirements, antidumping laws, and water runoff laws, indiscriminant dumping is often no longer an option.
Many producers use returned concrete to make something of saleable value, such as parking bumpers or paving stones, or at their plant to pave the yard or make aggregate bin dividers. Others have tried recycling by washing and then splitting the concrete into aggregates and slurry; or dumping it and letting it harden, then crushing the material and selling it as base.
ASTM International Committee C09 on Concrete and Concrete Aggregates recently legitimized another recycling method: a little-known practice, called “top loaded concrete,” that has been used by some producers for a number of years. When a significant volume (typically less than 3 cubic yards to 4 cubic yards) was returned, the producer added fresh rock, sand, cement, and water and shipped it out as a new load. This “new” concrete mostly went to projects with low requirements, such as for fill, or mud slabs or for patios.
The practice may seem underhanded; however, producers often added extra cement to make the mix outperform loads made with all-virgin materials. Sometimes, if he knew he was receiving leftover concrete, the customer got a price break. Of course, there was also the environmental benefit of not having to dispose of the returned concrete.
Recognizing this, ASTM Subcommittee C09.40 on Ready-Mixed Concrete released ASTM C1798, “Standard Speciﬁcation for Returned Fresh Concrete for Use in a New Batch of Ready-Mixed Concrete” in 2016. The document outlined how the material must be treated to be used in a new load, and not all concrete was eligible. The producer had to know the returned concrete’s contents, quantity, and temperature. If it was too old, a hydration stabilizer had to be added.
The one thing the specification didn’t do was grant permission to use the material in a new load of concrete. The subcommittee rectified this in 2017. The change was approved by Committee C09 in December 2017 and is allowed under the International Building Code with certain restrictions, such as requiring purchaser approval. Even Caltrans, California’s transportation department, which used to vehemently oppose the use of returned fresh concrete, has recognized the environmental benefits of this practice and accepted it.
A number of specific requirements in the ASTM documents are intended to prevent inappropriate use of returned fresh concrete. If you are interested in this practice, get the newest versions of ASTM C94 and ASTM C1798 to avoid improper use of what may become an added source of cost reduction.