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Ever-tightening environmental constraints are putting more and more emphasis on controlling of wastes from concrete production. Current regulations require producers to address both waste concrete and wastewater. To protect groundwater supplies, some municipal regulators are enacting zero-discharge policies for process water.Controlling wastewater has become the regulator's new point of emphasis. It's ironic that conserving aggregate resources by reclaiming waste concrete actually generates additional water to be processed.Process water is really a slurry with varying amounts of suspended solids composed of cement in some state of hydration, fines from aggregates and possibly chemical admixtures.To many observers inside and outside the industry, the most obvious use for the slurry is in batching concrete. Yet many local and state specifications prohibit such use by citing the requirements that mixing water must be clean and potable. State DOT specifications that prohibit or limit the use of slurry can be a major stumbling block for producers who do a lot of state work. ACI 301, "Standard Specifications for Structural Concrete," a very widely cited document, defers its requirements to ASTM C 94, "Standard Specification for Ready-Mixed Concrete." Both standards include a total solids content of less than 50,000 parts per million. A recent proposal to allow for use of slurry with a solids content higher than 50,000 ppm was defeated because the majority of subcommittee members were not convinced that we know enough about the effects of higher slurry solid contents on the short- and long-term performance of our concrete.To address this question, a major study is underway. Preliminary data indicate that the subject is much more complex than was anticipated going in. There is a lot more to the use of slurry than just developing a method to determine the solids content and specific gravity reliably. Many concerns have surfaced.According to Dick Gaynor, NRMCA's former chief engineer who is working with the association on this study, good progress is being made on this complex problem. An interim report is expected within the next few months. Additional portions of the study will be reported as the data becomes available. Gaynor believes that new standardized test methods for characterizing slurry will be developed, and changes to C 94 may result. Regardless of its complexity, this problem must be addressed and resolved. Ready-mixed concrete producers, recycling equipment manufacturers and admixture suppliers can address this important issue with three action plans:

  • Support the necessary independent testing needed to develop the data required to back up proposed changes to current industry standards and specifications
  • Become directly involved in the ACI and ASTM committees governing this area
  • Work with the regulatory agencies to make them aware that simply mandating zero discharges is not a workable solution given the complexity of the situation
Cadman Inc., a major producer in Washington state, has been one of the strongest advocates for changing the limits on the amount of slurry water that can be used. The firm has made a significant investment in testing and quality control for successful use of increased solids in slurry for day-to-day concrete production.

Keywords: reuse, slurry, recycle, process water, limit, environment, waste, wastewater, zero-discharge, reclaim, ASTM C 94, washwater, NRMCA, Cadman, groundwater, solids, set