Rust Belt Structures
Figuring most of my readers are from the Philadelphia area, I assumed the topic of this post would be relevant. If you have ever glanced at the underside of a structure like an overpass while sitting at a light or in traffic, you’ve likely noticed something disheartening, rust. Hopefully the beam you saw wasn’t completely “orange”, but only showed signs of decay and some flaky paint. It might be pretty easy to imagine an “exposed” beam rusting due to long term environmental exposure. Sadly, concrete structures can suffer from this issue as well. But how can concrete rust, you may ask. It can’t, but the rebar within it can.
Rebar within concrete is meant to take the tension loads that the concrete cannot handle. Also, with “cross hatching”, the rebar resists shear forces within the concrete. Deterioration of the rebar within concrete can be as dangerous as a failing member. A large percentage of the structures in the US, as you may know from a previous article, are old to say the least. When construction was completed, standard steel (likely A-36) was used for reinforced concrete. This steel corrodes quite easily. But how does steel surrounded by concrete erode if it’s not exposed? Although small, the void ratio of concrete still exists. The void ration provides a value for the amount of “empty space” volume per amount of solid volume. Extremely small voids are present within concrete. Also, with thermal strains, concrete has the ability to crack, this exposes the rebar within to corrosion. For states using salt for road maintenance, as well as coastal states, a problem is obviously present.
With the dissolved salt-water compound penetrating the concrete, corrosion begins on the rebar within. The corrosion of rebar exposed to the compound occurs at a rate of several millimeters per year. Although this doesn’t sound too terrifying, consider the age of the the structure, as well as the possibility of a large amount of cracks and fractures within the concrete. My Technical Communications project involves a study and a report on a common engineering problem and possible solutions for it. This is the problem I have chosen. My possible solutions are quite simple, different types of rebar. Corrosion resistance, cost, as well as ultimate strength values are going to be investigated. The three types I am considering are Galvanized Steel rebar, Glass Reinforced Polymer rebar, and Stainless Steel rebar. Currently, from my knowledge, stainless steel is going to prove the most expensive and is not a cost effective solution. GFRP rebar is going to be more corrosion resistant, but may have a lower ultimate strength value. More information regarding the whole selection process and study will undoubtedly be shared at some point.
