Concrete road repairs often require the contractor and engineer to develop a plan to manage the residue generated during sawing and diamond grinding operations, known as sawing slurry or concrete grinding residue (CGR).
How slurry is managed varies across the country. In many rural areas it’s spread along adjacent slopes as the operation moves down the roadway. In some rural areas and all areas with closed drainage systems, it’s collected and hauled to a landfill for disposal.
In 2012, Minnesota removed slurry from the list of regulated solid wastes, freeing contractors to dispose the material on site, provided they follow state-sanctioned procedures. The result of a concerted effort by industry and the state DOT, the exemption was based in part on the International Grooving & Grinding Association’s (IGGA) recently updated Slurry Best Management Practices guide.
Slurry spreading disposal
In rural areas that have vegetated slopes, the slurry can be deposited on the slopes as the grinding operation progresses down the road. As part of the contract documents, the engineer shall identify wetlands and other sensitive areas where slurry discharge operations are not permitted. The engineer and contractor shall make a site inspection before the start of grinding to identify sensitive areas. Spreading slurry should not take place through these sensitive areas.
The spreading start and stop points shall be clearly marked on the shoulder of the road. The slurry generated while grinding in unpermitted areas shall be picked up and hauled for disposal in non-sensitive areas on the job. Do not allow the slurry to flow across the road into adjacent lanes.
The diamond grinding equipment shall be equipped with a well-maintained vacuum system that is capable of removing all standing slurry, leaving the road in a damp condition after the grinder passes. Spread the vacuumed material evenly on the adjacent slopes by dragging a flexible hose or other approved device along the slope.
The spreading should not take place on the shoulder. Spreading should begin a minimum of 1 foot from the shoulder, with each pass of the grinder moving the spreading operation farther down the slope to ensure no build-up of grinding residue. The slurry shall not be spread within 100 feet of any natural stream or lake or within 3 feet of a water-filled ditch. Restrict the spreading operation to above the high-water line of the ditch.
Do not allow grinding residue to enter a closed drainage system. The contractor is responsible for providing suitable means to restrict the infiltration into the closed drain system.
Slurry collection and pond decanting
In urban and other areas with closed drainage systems, the slurry shall be collected in water-tight haul units and transported to settlement ponds constructed by the contractor. These ponds may be constructed within or outside the right-of-way. The engineer shall approve all locations. Construct the ponds to allow for the settlement of the solids and decanting of the water for reuse in the grinding operation.
At the completion of the grinding operation, the remaining water will be allowed to evaporate or may be used in a commercially useful manner such as dust control. After drying, the remaining solids may be used as a fill material, a component in recycled aggregate, or any other commercially useful application. The pond area shall be reclaimed to its original condition and vegetated to protect against erosion.
Slurry collection and plant processing
The slurry shall be collected and hauled as with pond processing. There are various plant designs that may be used, such as centrifuge and belt press. Prepare the plant site to control any stormwater runoff in accordance with state regulations.
The site should be restored and vegetated when operations are complete. The processed water and solids are to be handled in the same fashion as the settlement ponds. The site may be within or outside the right-of-way. The engineer should approve site locations.
PH control plan
With either the spreading or pickup operation, the contractor will monitor and control the pH of the slurry. The slurry pH should be below 12.5 and greater than 2.
When operations start, the contractor will test the pH at least once per hour to ensure it is within acceptable limits. The test equipment will be calibrated daily and approved by the engineer. Once the pH control plan is operational and producing consistent results, the testing frequency may be reduced to four tests per day.
The contractor shall log all test results and deliver a signed copy to the engineer weekly. At no time shall slurry containing a pH outside the above limits be deposited on the ground. The contractor shall determine the procedure to be used to maintain the slurry within the acceptable range. The engineer shall approve this procedure.
Best practices: A case study
In the 1980s, the Minnesota Department of Transportation allowed slurry to be deposited in roadside ditches and slopes. However, in the late 2000s contractors were required to vacuum all slurry and dispose it off site, adding significantly to project costs.
The goal was to control pollution, but because concrete slurry is very similar to the lime-treated water farmers use on crops the policy seemed unnecessary.
The Concrete Paving Association of Minnesota (CPAM) worked with state regulators for two to three years to alter the requirements. CPAM provided information from North Dakota State University, which had tested the slurry and its effects on the soils, as well as a California water quality study that reported no slurry-related impacts to groundwater. Procedures from other states were also presented.
After encountering an impasse, CPAM looked to state legislators for a ruling. During the process, Terry Kraemer, president of Diamond Surface Inc., Rogers, Minn., testified on behalf of the industry, outlining the best practices currently used throughout the nation. CPAM educated legislators on slurry and cited many studies that had been conducted on slurry management.
After weighing the facts, Minnesota’s legislature exempted slurry from being considered a regulated solid waste and therefore not subject to the same pollution controls. The new wording states that the definition of solid waste “does not include … concrete diamond grinding and saw slurry associated with the construction, improvement, or repair of a road when deposited on the road project site in a manner that is in compliance with best management practices and under rules of the agency.” Contractors could return to roadside disposal where conditions allow.
In response, Minnesota DOT developed the nation’s first best-practice guidelines for roadside disposal. Contractors may not dump slurry into fresh water and may treat the material to bring it within the correct pH range.
“This initiative has been very successful,” says CPAM Executive Director Matthew Zeller. “We spent a lot of time early in the process bringing legislators up to speed and making them familiar with the current research related to slurry disposal. We wanted to familiarize them with the slurry product and the disposal options that are available. The result now allows us to use slurry management methods that research has proven to be non-harmful to the environment while costing the DOTs, and ultimately the taxpayers, less for the project.”
By using IGGA’s best management procedures as a model, CPAM brought Minnesota’s slurry disposal procedures in line with those used throughout the nation.
“Unnecessary regulations add to project costs,” says IGGA executive director John Roberts. “Eliminating them in appropriate situations benefits agencies and taxpayers alike by reducing costs while providing a safe, smooth, and quiet roadway.”
Kari Moosmann is AEC senior editorial eirector, Constructive Communication Inc., Dublin, Ohio.