Conveyor Innovation Speeds Concrete Placement

By Kirk A. McWilliams, P.E. | September 28, 2010

Getting concrete from the mixer to the forms in the quickest manner possible is always a primary concern for those in the concrete construction business. When placement sizes vary from 300 cubic yards to 800 cubic yards at a time, the logistics of this material-handling dilemma are magnified even more. Special concretes used for underwater tremie placements include many chemical admixtures to delay setting of the concrete and to provide maximum flowability. In addition, when concrete placements are as massive as stated above, concrete temperatures become a factor that must be dealt with. Both situations demand that the concrete be in its final place of disposition as quickly as possible.

To properly address this situation in the construction of the new River Wall at Charleroi Locks and Dam on the Monongahela River in Southwestern Pennsylvania, the U.S. Army Corps of Engineers Pittsburgh District's contractor, Trumbull Corporation and Brayman Construction Corp., a joint venture, developed a concrete delivery system that employs high-speed conveyor belts from the mixer drum to the point of placement.

A 12-cubic-yard batch of concrete takes approximately four minutes to travel the nearly 2,100-linear-foot distance, at a speed of 750 feet per minute. This rate of placement, coupled with a state-of-the-art concrete batch plant designed to produce 150 cubic yards per hour, assures that tremie and mass concrete is delivered as quickly and efficiently as possible.

The conveyor system is manufactured by Rotec Industries, Elmhurst, Ill., and employs up to 8-inch- to 24-inch-wide main feed belts, seven transfer points and a post-mounted swinger. The post-mounted swinger is a rotating, telescoping conveyor boom that is operated remotely by an operator, giving the operator the ability to discharge concrete at any location within the placement.

Although using conveyor belts to move concrete is nothing new, using them in this capacity is a first for the Pittsburgh District Corps of Engineers. Typically, concrete is transported by a combination of conveyor, mixer truck, concrete bucket, or pump involving a substantial supporting labor crew. These delivery systems require a longer period of time to get the concrete to its final destination and involve the transfer of concrete from one system to the next. As a result, the concrete must be batched with higher slump and air content values to account for a decrease in these properties caused by the physical handling of the concrete along the way.

Also, the longer the concrete sits in the transport device on warm days, the more likely it will begin to tighten up and increase in temperature, contributing to concrete not meeting specifications at the point of placement.

The high-speed conveyor system at Charleroi Locks and Dam has alleviated all of these concerns. Test results have proven that concrete slump and slump flow are virtually the same when measured at the point of placement as compared to the batch plant, due to very little disturbance to the concrete during its delivery. Air content is also being preserved with little or no change. This allows for concrete meeting specifications to be produced at the batch plant, and provides for a much more consistent history of compressive strength results when compared to more traditional methods of concrete delivery.

Concrete temperature gain while on the belt is also a common concern. This has been addressed by installing metal covers over the belt line, and painting the topside white to further reduce heat gain from the direct rays of the sun. Temperature differentials between the point of placement and the batch plant only vary by approximately 4 degrees Fahrenheit on 85-degree days.

A rapid delivery of concrete provides a continuous and uninterrupted flow to the forms that maximizes the time in which the tremie concrete remains flowable. This helps to guarantee that the tremie concrete will flow around and through the reinforcing steel and other embedded items in a homogeneous mass without creating voids.

Conveyor belt operations also reduce the size of the supporting labor crew required to transport concrete to the point of placement when compared to more traditional methods. To address environmental issues, the conveyors are equipped with catch trays that contain any concrete or cement paste that might otherwise fall into the river, assuring that environmental concerns associated with using conveyor belts over water are mitigated. Each catch tray drains back to a transfer point, where collection boxes capture all wash water. Submersible pumps move this water back to the batch plant's sedimentation basins for proper treatment, leaving the solids behind for proper disposal as well.

The concrete delivery system employed at Charleroi Locks and Dam has proven that innovation is still possible in heavy construction, and that concrete delivery can be designed to meet the demanding requirements imposed by new in-the-wet construction techniques.