Poorly poured bin foundations can lead to structural problems. The time lapse between concrete plant and completed pour should be short as possible. A quicker pour process shortens that gap.
To help reduce the cost of a new bin, pouring grain bin foundations is often undertaken by farmers themselves. However, this can compromise the foundation’s quality and lead to expensive repairs down the road, according to a discussion paper produced by Seth Ulmer, a representative with Curb Roller Manufacturing in Kansas. Much of the information in this story comes from Ulmer’s paper.
The time taken to complete the pour on a 54-foot diameter foundation can be significantly reduced if the operators use a motor driven tubular screed that’s easily switched back and forth between clockwise and counter-clockwise. Ulmer says Curb Roller is one of the few companies manufacturing such a machine.
Conventional screeds — Circular concrete pours start with a pivot located at the exact centre of the circle, allowing the operator to attach one end of the screed to the centre stake and control the screed from the other end. This pivot is typically used to determine the circumference of the concrete forms.
Any number of different concrete screed devices can attach to the pivot. One of the most common is the A-frame screed, which looks like a 28-foot section from a spray boom. The other common unit is the roller screed.
Hand screeding uses a long aluminum or heavy wooden board extended to the centre stake and pulled by hand around the circular form. This is a strenuous, time consuming task and may result in structural flaws due to a warped board, uneven concrete dispersion and cold joints where fresh cement is poured adjacent to concrete that’s already curing.
Ulmer says A-frame screeds consist of multiple sections that can be cumbersome. A-frames can weigh 200 pounds if they’re large enough to reach the full radius of a big grain bin pad. Connecting sections can leave dozens of seams and gaps for concrete to seep in, further complicating cleaning and maintenance.
One benefit is that fins and vibrating attachments hang off the back of the main bar to provide a compact smooth finish as the screed pulls, vibrates and spreads concrete. However, this design limits screeding to one direction.
On the other hand, a roller screed consists of a drive head and pipe that spins in the opposite direction it is being pulled to level concrete. Roller screeds can be electric, hydraulic or powered by a gas motor. They are typically lighter and easier to manoeuvre than an A-frame. The power head is always located at the outer end of the aluminum tube.
A gas motor without a gearbox can be a limiting factor, restricting the direction of spin, a shortcoming similar to the A-frame’s.
Avoid cold joints — Screeding in one direction causes cold joints, a major problem. Unidirectional screeding is acceptable for conventional concrete flatwork, but problematic with circular pours. A circular concrete pad can take several hours to pour. Unidirectional screeds are committed to screeding one direction for the duration of the pour. When the circle approaches the 360 degree mark at the end of the project, that final concrete pour will butt up against concrete that has been curing for hours.
In this one-direction screeding scenario, the pad is poured in continuous sections with the first being from the 12 to 2 o’clock positions. The crew screeds that section and then the next is poured from 2 to 4 o’clock positions. This pattern continues around the 360 degrees until the crew gets back to the 12 o’clock position.
Once the pour is nearing completion and arrives back to the first dump, the crew is now pouring and screeding wet concrete next to curing concrete. This creates a cold joint, which can cause a division in the uniformity of the concrete pad, preventing it from curing as one solid, smooth, level foundation. The irregularity from a cold joint is a structural flaw that can result in costly repairs or total grain bin failure.
Multidirectional screeding — The best method to avoid a cold joint is to alternate left side to right sides of the circle.
If the first section is poured at the 12 to 2 o’clock position, the next section can be poured from 2 to 4 o’clock position. But the third section should be poured in the 10 to 12 o’clock position because that pour still has a relatively fresh face.
This alternating pattern ensures every section cures for the shortest amount of time before fresh concrete is poured next to it, leading to a stronger joint at 360 degrees.
Roller screeds are designed to have the pipe spinning opposite the direction the pipe is moving. The ability to change spin direction at the push of a button allows operators to screed clockwise or counterclockwise, making these rollers capable of performing the alternating left-right pattern. This means operators can alternate screeding sections rather than working in a full rotation and risking cold joints where the fresh dump meets the curing concrete.
A number of manufacturers offer directional switching on their screeds, but making the switch isn’t always easy. Some require the entire drive head to be removed, rotated and reattached to the other side to change direction. The alternating pattern method depends on screeds with an easily changeable spin direction.
Corded electric roller screeds are a convenient option and provide constant power without needing gas, batteries or hydraulics. However, limited power supply in remote areas requires a portable generator.
Hydraulic screeds are another option. They have the power to spin the largest custom pipes to tackle heavy duty applications but depend on extra equipment for hydraulic power.
Ulmer says, “battery-powered roller screeds have no cords or hoses and can be easily operated in remote environments without an external power supply. Look for a screed that utilizes common 60-volt lithium-ion batteries like those found in standard battery-powered drills.
“Some screeds utilizing such batteries can screed up to 2,500 sq. feet of concrete on one charge, more than enough to screed-off any standard grain bin pad. Compared to A-frame, cordless roller screeds lower the learning curve and operational requirements.”
The Curb Roller website lists the following suggested retail prices in U.S. dollars: The Eel corded electric lists for US$1,763. The Batt 6000 cordless lists for $2,755. The Hydra HS300 hydraulic lists for $4,409.