Why Bolts Matter

Reprinted from GRAIN JOURNAL July/August 2019 Issue

This article is the second in a series based on a presentation by Michael Blough PE, project engineer at Clear Creek & Associates (CCA), Goshen, IN (574-537-9060). He spoke in March at GEAPS Exchange 2019 in New Orleans, LA.

The process of inspecting a grain facility may seem overwhelming, and a large facility may seem especially daunting. But taking things step by step, and looking at the issues a structural engineer typically looks for during concrete and steel inspections are ways to take on the challenge.

In the May/June issue of Grain Journal, we looked at safety practices while inspecting grain storage, then looked at specific issues including steel bin corrosion, bolted connections, loading and unloading, wind and snow damage, anchor bolts, concrete tanks, guy wires, and structural steel members.

In this issue, we continue with a look at inspection tools, simplified inspections, and repair procedures.

Inspection Tools

Steel analysis. Ultrasound technology can be used to gauge thickness on steel tanks. To determine thickness, a transducer sends an ultrasonic wave through the material, and thickness is measured by calculating the time it takes for the wave to return to the transducer. Measurements are made at the lap splice and the middle of the sheet to compare approximate original thickness with an area of the sheet exposed to interior grain material and moisture.

In steel tanks, the steel is exposed to the elements, and corrosion can occur quickly if the grain is not kept in quality condition. Corrosion removes material from steel components, and if enough material is lost, the structural component can fail. Tanks are not designed with a corrosion or section loss allowance. In addition, many older tanks were not designed using current typical grain loads. For that reason, structural components that experience material losses of 5-10% should be monitored closely. Those with material losses greater than 10% should be replaced.

Concrete tools. A laser level can be used to measure different amounts of settlement of concrete foundations. A tripod supports a spinning laser to emit a level plane of light. A moveable sensor attached to a staff positioned at points on a foundation can determine the elevation in relation to the level plane of light.

The naked eye is one of the simplest tools for inspecting a structure. Staff on-site can spot changes in a structure and alert qualified contractors or engineers to address problems.

Ground-penetrating radar produces an electromagnetic radio wave that passes through concrete until it detects a discontinuity. This lets the user locate rebar, determine diameter of rebar, evaluate concrete thickness, and evaluate levels of corrosion on the reinforcement.

A rebound hammer provides an approximate value of the compressive strength of concrete without the need for destructive compressive testing of concrete cores. This hammer, also known as a Swiss hammer or Schmidt hammer, measures the rebound of a spring-loaded mass after impacting the concrete surface.

The rebound value on the hammer correlates to the compressive strength of the concrete, as shown on a published chart from the manufacturer.

Simplified Inspections

The naked eye is one of the simplest tools for inspecting a structure. Staff onsite can spot changes in a structure and alert qualified contractors or engineers to address problems.

Calipers for steel and crack width rulers for concrete are other common, simple tools to monitor movement and changes of structures.

Two other tools are crack monitors and the delamination test.

Installing crack monitors involves fixing two separate components on either side of the crack. One component is a grid and the other a set of crosshairs. As the crack moves, the crosshairs move around the grid allowing crack movement trends to be observed.

The delamination test involves a hammer striking a concrete surface. The user listens to the sound the hammer makes. A solid sound means there is a good bond between rebar and concrete. A slapping or popping sound means delamination has occurred in that area, and the bond between concrete and rebar has been damaged. Both crack monitors and the delamination test are very inexpensive and non-obtrusive to the structure.

Crack evaluation. The biggest concern with concrete tanks is yielding of horizontal reinforcement of the walls. The primary indication of this is vertical cracking on the exterior wall. Inspectors evaluate the number of cracks, how pronounced they are, their location, and their length.

Horizontal cracking and spalling are a concern for maintenance of the tank but typically don’t indicate an immediate structural concern. Horizontal cracks typically are the result of horizontal rebar placed too close to the surface. Over time, water can infiltrate openings in the concrete and cause the concrete to break out, making the problem more severe. The reinforcing steel can start to corrode as a result, which eventually can create structural problems. This is especially an issue in areas with frequent freeze-thaw cycles.

Some horizontal cracks can be identified as cold joints. These are planes of weakness in concrete caused by an interruption or delay during construction. In general, these areas need to be monitored for movement or development of further cracking.

“H” cracking refers to two large vertical cracks connected by a horizontal crack, forming the letter “H.” This predicts the start of spalling and delamination.

Concrete structures typically deteriorate progressively and require corrective retrofits or structural stiffening to prevent the tank’s condition from becoming worse.

Repair Procedures

Steel tanks. The typical repair process for steel tanks involves removing and replacing the damaged bin component. Common damages found during inspections include damaged stiffeners and sheets from unloading, roof damage, or damage around the door opening. Damaged components are replaced with like members according to manufacturers’ recommendations.

If bolts are removed, replace them with new bolts to avoid issues with bolted connections.

A variety of repair methods can work on steel structures. When modifying a structure, replacement of components may be the right solution. For damaged flanges, reinforcing the area with plate steel may be the best. In areas of interference, redesigning bracing to allow clearance is a reasonable solution.

In almost all situations, the help of a qualified engineer is needed, because a level of structural analysis is required.

Concrete repairs. Two main types of concrete repair are surface repair and structural repair. Surface repair includes processes like patch repair and crack injection. These are not a structural fix but may prevent further damage.

If damage or cracking returns, a structural repair may be required. This includes processes like Shotcrete or Gunite liners and carbon fiber. This type of repair restores strength to the deteriorating concrete so the storage usually can be returned to normal use.

Repairing an area of damaged concrete is a multistep process. Usually, it is a reinforcement of the existing failing structure.

First, remove the concrete damage. Remove loose, delaminated concrete or debris in the affected area. Corroded reinforcing steel may need to be undercut, if the concrete is delaminated behind the reinforcement.

Areas of delamination and spalling should be cut out perpendicular to the surface. When cutting around the delamination to remove the concrete damage, make the cut approximately six inches outside of the delaminated zone with a simple shape around all the smaller damaged areas (see illustration on page 89). Re-entrant corners should be minimized or avoided, since they are susceptible to cracking.

Next, the steel reinforcement must be prepared. Review the steel to determine if the reinforcement still will meet the original diameter specification after cleaning the bar of rust. If it is suspected that it won’t meet the original diameter specification, contact a structural engineer to replace or reinforce the steel. Remove the rust from the steel with a wire wheel or sandblasting.

Next, prepare the concrete surface. Use a wire wheel grinder to remove dust and debris to allow proper bonding. For cracking, grind out and open the cracks.

Finally, repair the concrete and steel. Treat rebar with a rust inhibitor and apply a bonding agent to the concrete surface. Patch small concrete areas with a high-yield, non-shrink, cement grout. Patch large areas with Shotcrete or Gunite. Inject high-yield, non-shrink caulk into cracks with a foam backer rod.

Life Expectancy

Life expectancy for steel tanks is around 40 to 50 years even with proper maintenance. Repair methods like replacing components often cost as much or more than replacing an older tank. It is important to consider the cost benefit of repair vs. replacement.

For inspection purposes, older tanks that are 30 to 50 years old are more likely to have issues with corrosion or section loss. Newer tanks less than 15 years old are more likely to have issues from operation during loading and unloading. Sidedraw damage is very common.

With proper maintenance, concrete tanks can last 70 years or more. Repair methods like liners can extend this life. Geographical conditions greatly affect the structure’s life expectancy.

For inspection purposes, older concrete 30 to 70 years old is more likely to develop cracking from long-term settlement, moisture infiltration, and delamination. Newer concrete tanks less than 15 years old are more likely to develop cracking from inadequate reinforcement, initial settlement, new loading, or openings made in the structure.

- Ed Zdrojewski, editor