eprinted from GRAIN JOURNAL May/June 2020 Issue
This article is based on a presentation by Cheyenne Wohlford, president and chief executive officer of CCS Group, LLC, an engineering firm in Seward, NE (855-752-5047). He spoke in February at the annual convention of the Grain and Feed Association of Illinois in Springfield.
Most concrete grain elevators from the 1950s onward were built for an expected lifespan of about 30 years. Obviously, the majority of those have exceeded that lifespan considerably, a tribute to the skill and workmanship of the contractors who built them.
However, no grain elevator escapes wear and tear over time, no matter how well it was built. Among the culprits:
• Exposure to years of harsh and changing weather.
• Structural defects. They weren’t all well-built.
• Operational changes, as elevators have had to handle larger volumes of grain as yields increased.
• An increasing number of turns. Again, that’s attributable to higher volumes, as well as the advent of unit trains.
• A decrease in loading or unloading times. Elevators have increased the volume capacity of grain handling equipment.
Cheyenne Wohlford
Rail demurrage policies provide incentive to speed up the elevator.
• The addition of side-draw spouts for truck or rail loadout compromises a tank’s structural strength.
Facility Maintenance
The deterioration of concrete is slow in the beginning, but it speeds up over time. Deterioration can be slowed or halted with proper maintenance. Some reasons why this is a good investment are:
• Structural repairs may cost up to 10 times as much as regular maintenance.
• Regular preventive maintenance of a concrete tank involves no downtime, while structural repairs can take multiple weeks.
• Structural repairs have been associated with falling concrete. That can lead to an increase in the number of recordable injuries and higher workers’ compensation costs.
Site Inspection
A site inspection with a qualified engineer is an excellent way to see if there are any areas that need repairs beyond routine maintenance and to prioritize the work to be done.
• Gather as much information about the tank as possible. Try to piece together a complete structural history of the tank. What attracts your attention or causes you concern?
• If possible, find the original blueprint for the tank. This could prompt the inspection of an area that may be overlooked otherwise. It also may provide some explanation of suspect areas or some outright failures.
• There may be some useful information that is undocumented.
Final view of a steel-reinforced shotcrete repair to a concrete hopper.
This might include the number of years a tank has been in operation, the amount of utilization of specific components, turn rates, and the history of any tanks that were condemned in the past.
An exterior inspection of the tank should cover these items:
• Silo walls.
• Apex walls.
• Roofs.
• Beam pockets.
• Cornices.
• Any bulging areas.
• Wall-penetrating equipment such as sidedraw spouts, bobcat doors, or aeration fan ducting.
• Any noticeable changes over time.
Here are areas that should be covered in an interior inspection. As always, be sure to follow all safety guidelines for bin entry:
• Connection walls.
• Hopper slopes.
• Aeration hoppers.
• Discharge (sidedraw) spouts and holes.
• Roof beams and connections.
• Grain bridging and caulking. Be extremely careful with bin entry in situations where grain is bridged or hanging up on the walls.
• Moisture marks.
Concrete Crack Inspection
Any crack in the concrete should be inspected in terms of direction, width, and depth.
Active cracks are ones that are growing in width, depth, or direction. Dormant cracks are ones that do not grow.
Weather is an important factor in crack growth and concrete deterioration. Warm weather allows concrete to hydrate. During cold weather, water in concrete pores freezes and may cause spalling and other kinds of damage.
Water expands in volume by 9% during the freezing process. This exerts a force on concrete and creates tiny cracks. When the ice thaws, water then fills the new cracks. This process is repeated throughout the cold months.
Different types of cracks include:
• Offset cracks, that is, cracks at different levels on the tank.
• Capital H cracks, shaped like the letter in the alphabet.
• Straight, horizontal, and vertical cracks.
• Diagonal cracks combined with horizontal and vertical cracks.
Of these, the H-shaped cracks are of the most concern, since these usually indicate rapid deterioration of the concrete and the fastest route to tank failure.
Site Inspection Testing
During an inspection, a number of tests of the concrete may be employed. Nondestructive tests include:
• Ground penetrating radar (GPR) scans. These are available in two- or three-dimensional scans. They show spacing between rebar, concrete coverage, and wall thickness.
• PSI concrete tests measure the strength of the concrete in a given area.
• Concrete moisture tests measure concrete moisture content and are used to determine if and where moisture migration is occurring.
• Surface profiling measures the roughness of the concrete surface on a scale of one through 12, with 12 being the roughest. It determines whether the concrete is suitable for carbon fiber repairs.
• Drone and aerial photos.
The report from the site inspection contains two sections. In the first, findings include testing data, areas of concern, and requests for any further testing. The second section includes recommendations for immediate action, future needs, and a monitoring schedule.
Monitoring and repairs are recommended according to an alphabetical scale, taking into account reinforcement spacing, reinforcement concrete covering, PSI strength, concrete moisture, and exterior flaws:
A – Recommended monthly monitoring.
B – Recommended weekly monitorng.
C – Recommended installation of shotcrete or gunite liners.
D – Recommended only 50% tank capacity use until the installation of shotcrete or gunite liners is complete.
F – Recommended the tank be taken out of service until the installation of shotcrete or gunite liners is complete.
Exterior flaw data is presented on a scale from one to five, where one is a smooth surface, and five represents severe delamination, spalling, and cracking.
Sharing this report with a third-party engineer allows for independent verification and prevents unnecessary repairs.
Types of Repairs
Concrete crack injection. This is a technique that prevents cracks 1/16 inch in depth or larger from growing. It’s a three-step process:
• Grind out the crack to 1/4-inch-x-1/4 inch.
• Remove the debris with air or water.
• Inject the crack with a high-yield, non-shrink, elastomeric caulk.
Concrete spalling repair. This begins with a five-step procedure:
• Remove all delaminated concrete, including 1/4 inch around any exposed rebar.
• Clean the rebar to white metal.
• Right-angle cut the spalled area.
• Treat the rebar with a rust inhibitor.
• Patch with a high-strength, vertical-overhead concrete patch.
Applications for carbon fiber:
• Bean pocket repairs.
• Roof caps.
• Wall-penetrating equipment entry points.
• Cold joints.
• Structural reinforcement patching and liners.
Steel-reinforced shotcrete and gunite liners. This repair adds a new, structurally sound tank wall using the existing wall as a concrete form. The new liner is only 4 to 6 inches thick, resulting in minimal loss of storage capacity.
Ed Zdrojewski, editor
