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Cold Concrete

American Municipal Power (AMP) is well into its fourth major hydroelectric upgrade project on several of its Ohio River locations at a site near St Mary’s, W.Va., known as Willow Island. The concrete portion of the project was started last summer and encompasses the upgrading of the hydropower generating station as part of AMP’s long-term sustainable power—hydro, wind and solar—upgrading program. Cleveland area-based The Ruhlin Company was awarded the project, which includes the production of high-performance concrete to be placed at a temperature of no more than 55°F (12.7°C).

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TURNKEY SOLUTION
The Ruhlin Company is best known for its concrete-intensive infrastructure projects throughout the Great Lakes region and Ohio River Valley. For the Willow Island hydroelectric project, the company chose to have a complete aggregate washing/re-screen and chilling system designed into an all-new concrete production facility.
The production and cooling system was designed by San Antonio-based Plant Architects and its cooling contractor firm Concrete Temperature Control (CTC/ConCool). The system install was under the construction management of Plant Architects, which partnered with The Ruhlin Company in selecting original equipment manufacturer’s (OEM) components and equipment, and then designing the pumps and valving routes, pipe work, and the thermal insulation, automation and temperature balance controls. 
Ruhlin Company Project Manager Doug Hartz reports that the entire design concept was based specifically on the project’s specifications and the owner’s requirement for the highest-quality concrete mixes, made to the coldest concrete temperature possible to ensure placement at no more than 55°F, which requires concrete be produced at 40°–45°F (4.4°–7.2°C).
“We worked together over many weeks with the entire group of Plant Architects’ designers, the CTC cooling specialists, engineers, industrial architects, project management and our construction trade leaders, to ensure the very best team approach. Our quality assurance and project managers, superintendents and the project’s estimators, all worked hand-in-hand with the Plant Architects team, which allowed a fast-tracked and efficient system design and selection of the equipment to be procured by Plant Architects and installed on a turnkey basis,” reports Hartz.

PRODUCTION METHODOLOGY
With temperature-critical concrete, there are a myriad of options for chilling product. The Ruhlin Company chose not to chill the concrete (taking hot concrete and cooling it down) but rather to produce cold concrete by controlling the temperature of all incoming constituent materials. The reason being that if super-chilled water (30°F/-1°C to 33°F/1°C), cooled rock, cooled sand and cooled bulk powders are used to make the fresh mixture, the concrete is already chilled and the Btu exchange (energy) required to chill existing hot and hydrating concrete en masse is avoided.
The production of pre-cooled concrete is not always chosen, however. The temperature control engineers at ConCool worked closely with Ruhlin technical and project management staffs to ensure that the sporadic pour schedules of both large and small quantities of concrete were looked at pragmatically.
Since chilling existing warm or hot concrete is sometimes desired over a pre-cooling system (based on the aforementioned idiosyncrasies of a particular project), all data are entered into worksheets and linear-traverse graphs tracking concrete heat rise and energy consumption. Then capital costs, personnel staffing and maintenance costs are also factored. In the case of this particular project, the extremely low specified concrete placement temperature at a moderate output of 200 cu. yd./hour was the deciding factor over utilizing large amounts of liquefied carbon dioxide, nitrogen gases or dry flake ice.

PRODUCTION SYSTEM
The Ruhlin Company and Plant Architects designed a precooled-concrete production system that contained the following components:
• Triple in-feed hoppers with variable-frequency-drive feed belts;
• Transfer conveyor for layered coarse aggregates from hoppers;
• Washing deck for incoming layered course aggregate with ambient water;
• 160-ft.-long (48.7-m) super-cold water Niagara wet belt for chilling the rock;
• Triple deck re-screen for dewatering and segregating the mixed rock;
• Three individual incline hoppers from the re-screen to the batch plant;
• Four bin batch plant—three coarse aggregates and one sand;
• 12-cu.-yd., twin-shaft compulsory mixer with nitrogen headers;
• Blue Northern Water/Air Chiller for cold-air recirculation in the plant;
• Fluidized-bed sand chiller for incoming sand;
• Series of air/water chillers for a 50,000-gal. (170,000-liter) storage pit;
• Self-contained instantaneous water heater for winter operations;
• Self-contained shaved ice water slush/cooler for batch water; and,
• Programmable logic controller customized for temperature balance.

In addition to the above precooling, the group elected to install a liquefied nitrogen dispensing tank and tie it into the automated temperature balance system for touchup of any resulting concrete that, when produced, did not meet the super-low concrete temperature specification due to any malfunction of the system, or short-term increased production requirements, which surpass the cooling retention time required of the system.

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MAKING COLD CONCRETE
The Ruhlin Company decided to make cold concrete rather than attempting to remove Btu from already hot and hydrating concrete with only liquefied gas. ConCool’s design engineer, Jarrett Grindlay, explains, “The removal of energy or heat from a mass that is already hot and undergoing chemical reaction creating ever-increasing heat buildup rapidly during hydration is certainly more difficult to predict. We are also concerned with altering the hydration process or the possibility of creating frost spotting or flash freezing. The overall quality of the concrete for powerhouse and hydro structures is very closely monitored and with good reason: it is extremely expensive to replace freeze-damaged concrete within such massive poured structures.”
ConCool designers note that the use of high-intensity mixers within the batching plant reduces the chances of flash freezing as compared to low-intensity drum mixers utilizing liquefied gas. But overall, the use of liquefied gas as a sole means of concrete temperature alteration is normally not pursued because, in addition to the foregoing, the time required for dispensing and remixing, and the negative impact on hourly production, outstrip its benefits.

WHY NOT ICE?
ConCool designs customized dry flake ice plants of all sizes for weigh batching into the fresh concrete mixture as it is created in the batching plant. The use of super-cold ice, which is thin and flake-like, provides superior latent heat transfer within the concrete prior to initial hydration. Dry flake ice is effective in concrete temperature control. Ice is more perishable than even fresh concrete, however, and ice plants work best when continuous production requirements prevail.
For the Willow Island project, The Ruhlin Company saw their intermittent pour schedules as reason enough to steer clear of ice in this particular case. Too many small pours coupled with days in between larger pours made the decision in favor of alternative cooling methodology for the group. In the temperature balance worksheets developed, the team was able to see that it had two choices in addition to the wet belt and super-chilled water (ice bank) it had selected: dry flake ice or a sand chiller unit.

COLD AIR
The Ruhlin Company chose a fluid bed sand dryer that was converted to chill the sand more than dry the sand. This off-the-shelf dryer was converted to chill the sand to 50°F from temperatures that range from between 85° to 100°F. The sand chiller is made possible by chilling air in a heat-exchange system developed together with the air-to-water chillers used for the wet belt system.
A key to the success of the sand chiller system is to be able to operate the air chiller on a continuous basis. Since the intermittent pour schedule would impact this, the sand chiller cold air was piped to the batch plant by means of a series of automated valves in insulated ductwork, which allowed both the sand (already cooled and placed into the batch plant) and the rock (already chilled on the wet belt) to be kept cold and avoid temperature rise in the batch plant storage bins.

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TEMPERATURE BALANCE AUTOMATION
In the early 1980s, ConCool provided the first fully automated temperature balance computer control for concrete production, albeit far from what is used today. “Back in the 1980s, we developed a system that took an entire office to house,” says Robert Ober, CEO of Plant Architects + Plant Outfitters and a longtime partner and installer of ConCool systems and now the owner of the brand. “The original system utilized banks and banks of relays and relay logic control boards. Today, we simply use programmable logic controllers that are all solid-state.”
In fact, today, in automated temperature balance control, installation of thermocouples on all constituent material storage for each of the materials used in making the fresh concrete are utilized. These devices provide continuous readings of the current temperatures of all materials and the ambient conditions, and correlate to the prevailing specifications.
The temperature balance system calculates the required temperature of the concrete to be batched while calculating the current humidity and ambient temperature—as well as, the temperatures of the materials pre-chilled—and then doses any dry flake ice or liquefied gas automatically. An infrared temperature reading is taken of the combined mixture under way in the mixer and the resulting concrete produced to that specified temperature. Were any of the constituent materials not within a range to allow the making of the specified temperature concrete, the automation will not allow the batch to start. This is all controlled from a device in the batch room the size of a laptop.

CURRENT CONDITIONS
The summer has been another hot and humid season in this particular region of the Ohio River Valley, with concrete production at Willow Island on schedule and continuing to meet the specified, super-low placement temperature. The Ruhlin Company has earned accolades from project owners for their systematic approach to meeting the tough specifications with an entire production system that included cooling as part of the design from the outset. The state-of-the-art cold concrete production facility is setting new standards for the owner and owner’s representatives.

More information on Plant Architects or CTC concrete cooling and dry flake ice plants can be found at www.robertober.com.