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LessonGrades 11 - 12

Repairing Cracked Steel Structures with Carbon Fiber Patches

A close-up photo shows a car spoiler made of curved carbon-fiber-reinforced polymer. It looks like a curvy black plastic material with a smooth but visible woven gray/black pattern.This car part made of carbon-fiber-reinforced polymers is composed of carbon filaments woven to meet the desired application.

Over several days, students learn about composites, including carbon-fiber-reinforced polymers, and their applications in modern life. This prepares students to be able to put data from an associated statistical analysis activity into context as they conduct meticulous statistical analyses to evaluate/determine the effectiveness of carbon fiber patches to repair steel. This lesson and its associated activity are suitable for use during the last six weeks of an AP Statistics course; see the topics and timing note for details. A PowerPoint® presentation and post-quiz are provided.

Maintenance and rehabilitation of concrete and steel structures is extremely important. A structure failure may lead to tragedies like the I-35W bridge in Minneapolis (August 2007, see Figure 1) or the Alexander Kielland Norwegian North Sea rig disaster (March 1980). Engineers have developed different methods to repair structures: crack welding, binding or cover plates, and stop holes, but these techniques require irreversible modifications to the original structures (see Figure 2). The best rehabilitations are the ones that do not produce irreversible changes in structures.

A photograph shows the scene at the I-35W Mississippi River Bridge after its August 2007 collapse. The bridge deck is in the water with a few cars on it, broken off from the rest of its steel truss components at the river’s edge.Figure 1. Structures rehabilitation is an extremely important engineering task since structure fatigue failures can have tragic consequences, such as this bridge collapse.

Three photographs show steel structure repair techniques for cracks. A man with a protective mask welds a large metal pipe. A bolted cover plate repair with a dashed line representing the crack beneath the doubler plate and the circle representing the hole drilled to remove the crack tip. A drawing of a cracked beam with drilled “stop holes” at each end of the crack.Figure 2. Conventional steel structure crack repair techniques—such as crack welding, crack cover plates and drilling crack stop holes—permanently alter the original structures.

Composites are mixtures made from two or more constituent materials with significantly different physical or chemical properties that remain separate and distinct within the finished structure, but result in a new material with enhanced properties (see Figure 3). Composites are carefully designed so that desired properties, such as strength, weight, durability, flexibility, conductivity and stability, are improved.

Two photographs show example composite materials. Rectangular block bricks composed of clay and straw dry on the ground in the sun. A wet mass of concrete composed of Portland cement and gravel.Figure 3. Composites consist of two parts: a matrix (base) and a reinforcement material.

In recent years, composites have been widely used to repair or reinforce old concrete and metallic structures; surprisingly, these components have not been combinations of standard construction materials like concrete or steel, but fiber reinforced polymers (FRP) bonded with resins or glues. This combination produces a new material that is stronger than steel or concrete, but lighter and non-corrosive.

Two photographs. Thousands of very thin filaments made of carbon crystals are combined into a bundle to make carbon-fiber-reinforced polymer (CFRP) thread, which can be woven into cloth, like this shiny woven black fabric. Embedded in an epoxy or other resin binder, the cloth is used to mold tubular forms and complex shapes. The resin (matrix) and cloth (reinforcement) mixture create a composite material with many applications, such as this bicycle chain ring.Figure 4. A fabric created by weaving thousands of carbon fibers. Combined with epoxy or resins, this cloth can be molded into different shapes, like this bicycle part.

One special class of FRP with an incredible strength-to-weight ratio and stiffness is carbon-fiber-reinforced polymers (CFRP; see Figure 4), which have been successfully used to rehabilitate cracked steel structures. CFRP patches provide a solution to avoid the propagation of cracks in steel and concrete structures (see Figure 5). Experimental data indicate that steel mean fatigue life improves substantially using CFRP. The main advantage of CRFP patches over conventional methods is that they require no irreversible modification of the underlying structure.

A photograph of the concrete ceiling of a parking garage shows an example of concrete structures being strengthened by CFRP reinforcement in the form of a three-sided wrap of six beams.Figure 5. Because of their strength, simple application and corrosion resistance, CFRP composites are used on interior and exterior structural members in almost all types of environments.

With the invention of CFRP, engineers have answered the question: Is there a material as strong as steel, but lighter, corrosion resistant, and simple to work with?

After this lesson, students should be able to:

  • Define composites and describe their properties.
  • Describe traditional methods of repairing cracked steel.
  • Describe carbon-fiber-reinforced polymers and their applications in modern life.
  • Describe the implications of cracked steel structures.

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