- Published: Thursday, 17 November 2011 10:09
- Written by CP Staff
Cannelton Hydroelectric harnesses super-chilled concrete for sustainable infrastructure
American Municipal Power, Inc. (AMP), with headquarters in Columbus, Ohio, is a nonprofit corporation that owns and operates electric facilities providing power generation, transmission and electric supply distribution services to its several utility members. AMP is involved in sustainable electric power generation options, including the latest alternatives in development and practice. As a part of its sustainable energy program, both existing and new hydroelectric power-turbine generation facilities are undergoing studies, development, and expansion. Hydroelectric power requires significant infrastructure, of which concrete in mass quantities and structures is utilized.
Currently AMP is undertaking an aggressive hydroelectric expansion program on several of its generating plants along the Ohio River. These projects are the largest development of clean, renewable run-of-the-river hydroelectric generation in the country. The first is known as the Cannelton Hydroelectric Project, located in Hawesville, Ky., across the river from Cannelton, Ind., and was undertaken within the last year. It calls for a new power generating station and hydro-construction—requiring the supply of super-chilled mass concrete production and delivery.
Super-chilled mix spec
One of the more difficult requirements on the AMP hydroelectric projects entails production of mass concrete (+3-in. coarse aggregate concrete, placed in lifts in excess of 36 in. by overland conveyor or dump body chassis) at a maximum concrete temperature of 55°F at the time and point of placement. This requires concrete production in the 48°–50°F range dependent on prevailing climatic conditions.
The super-chilled concrete requirement of AMP calls for succinct production methodology involving selection of aggregate gradations and developing mix designs specifically for geothermal properties during heat of hydration of the mass—controlling the heat rise. The production equipment involved in the making of this type of temperature-controlled concrete includes varied methodologies, from which contractors can select. These span both passive and forced cooling measures.
Cooling methodology today
Normally passive measures, such as shading of coarse and fine aggregate stockpiles, or solar reflective enclosures for production and delivery equipment, are undertaken in the pre-production of chilled concrete. Forced cooling measures can include the chilling of 1) batch water to a few degrees above freezing, and is almost always a part of any concrete-cooling scheme; or, 2) coarse aggregates by dousing in chilled water recirculation systems in storage tanks, washing screen systems—commonly referred to as Shale Shakers—with chilled water, or using transfer belts, which are enclosed and equipped with perforated belting to allow for draining of the chilled water—often referred to as Wet Belts. These devices are all popular options. A rescreening with dewatering deck is normally required by specification when utilizing chilled water for aggregate precooling.
The fine aggregate can be cooled utilizing fluid bed dryers or asphalt rotary dryers modified with air chillers. The cooling of the fines is a major factor in obtaining higher outputs at the super-chilled temperature requirement. Lowering coarse and fine aggregate temperatures results in the majority of the mass of concrete volume starting well below the required mixed concrete temperature—in the lower to mid-40s F. By precooling the constituent materials, the concrete producer does not require additional steps in adjusting the final temperature with liquefied gases such as nitrogen or carbon dioxide.
Dry flake ice, once a mainstay in chilled concrete production, is rarely utilized today where coarse and fine aggregate chilling is employed. Low water-cement ratios and lower temperature requirements for these mix designs make displacement of chilled water with dry flake ice ineffective in attaining the required super-chilled temperatures.
The general infrastructure and concrete contractor on the Cannelton Hydroelectric Project is Walsh Construction of Chicago. For Cannelton, Walsh chose San Antonio-based concrete production and cooling specialty contractors Plant Outfitters and Concrete Temperature Control (CTC), divisions of Robert Ober & Associates, Inc., to assemble a system that would provide the super-chilled concrete at the mid-range (100–200 yd./hour) outputs required for this initial AMP upgrade project.
A Request For Proposal (RFP) was called for a new concrete batch plant. Walsh enlisted Plant Outfitters in selecting a standard paving plant that could be modified to the requirements of the project, but reverted easily for standard paving operations later. Vince Hagan Co. was selected to supply a standard model, which was set atop specially designed modular skid frames fabricated to Robert Ober & Associates designs.
A triple compartment volumetric coarse aggregate hopper is used in the primary feed system sourced from Kentucky-based Stephens Manufacturing. Walsh, foregoing the primary washing of the coarse aggregates ahead of the wet belt, undertakes the washing of the coarse sizes on the wet belt itself. The wet belt was sourced from CTC affiliate Coldcrete, Inc. The triple hopper utilizes volumetric feeder belts, which allow the proper amounts of each coarse aggregate to be layered into the wet belt for cooling and washing. The CTC-supplied programmable logic controller (PLC) automatically proportions three coarse aggregate materials to within the requirements of the mix design being batched, thereby ensuring the plant is kept recharged with these same aggregates accordingly.
The 180-ft.-long, 48-in.-wide, variable-speed perforated wet belt is used to cool the coarse aggregate layered onto the flow-through belt, down to a temperature of 39°–45° F dependent on the prevailing ambient temperature and humidity. Three 250-ton air-to-water chillers supplying a 44,000-gallon, in-ground insulated storage pit supply the chilled water for both the wet belt and for batching operations. Water is pumped from this pit to the spray system on the wet belt in a recirculation pump system. Return water containing silt from the incoming larger unwashed aggregate sizes is weir-pit-settled to allow for minimal fines buildup in the in-ground insulated and covered storage pit.
A triple-deck 15-degree incline rescreen with a bottom tray for waste fines and silts is used to feed three coarse aggregate in-feed conveyors on the batching plant at a rate commensurate with the required coarse aggregate replenishment of the plant. The chilled aggregates are not refrigerated in the plant with Walsh opting instead for another more practical solution where high-outputs (of up to 600 cu. yds./hour) are not required, by means of minimal in-plant storage as not to allow heat gain of the precooled aggregates.
Complimenting the Hagan plant is a specially designed, Sicoma 11-yd. compulsory mixer equipped for liquefied gas dispensing rigging; special mixing tools, and liners. The 400-hp Sicoma is the largest high-intensity, multipurpose mixer manufactured, and suits roller compacted concrete (RCC), paving mixes, and ready-mixed concrete production. Walsh chose the model for its rapid mixing cycle of just 30 seconds, capped with dispensing of the liquefied gas into the fresh concrete mixture for cooling.
The mixer bears on a Robert Ober & Associates-designed, skid-mounted frame similar to the plant frames supplied. No anchor bolts are used and the entire plant and mixer are set atop a simple raft slab. The seismic activity in the region made this design of particular interest: the mixer is accessed via an observation deck built into the control room structures made of sea containers, allowing a safe distance from the transfer of liquefied nitrogen, which is reconstituted/gasified in the mixer.
Walsh chose not to chill the fine aggregate, but rather use liquid nitrogen injection in the twin-shaft compulsory mixer to cool the resulting fresh concrete temperature. Robert Ober & Associates had calculated such an approach might require subsequent cooling of as much as 28°F under the most challenging weather conditions. Normally liquefied gases are used for touchup of 1°–7°F. The cooling of the possible 28°F differential is the largest temperature adjustment by liquefied gas undertaken in the country to date.
To assist Walsh in the higher onsite costs involved in turnkey construction of the concrete production system, Plant Outfitters pre-assembled the entire batching plant and mixing system in Dallas, for complete setup, commissioning and test run. This allowed for total prefabrication, prewiring and pre-plumbing in Texas, with a high level of quality control and quality assurance—and a fast, one-week onsite installation.
The cooling equipment was staged offsite awaiting the installation of the batching and mixing plant. Late spring arrival, coupled with heavy flooding, caused delays for Walsh onsite with regard to preparing the site—which proved out the approach of the plant mockup.