When Your Project is on Life Support: Part 3 – Cage Match: Steel vs GFRP Rebar in Animal Habitat Construction
– By Jedd Pellerin, project architect
Details matter in the design of animal habitats. Even the details that guests will never see because they are buried in a four foot thick concrete slab. Rebar (or reinforcing bar) is a critical component of most modern, reinforced concrete construction. This includes the concrete walls and slabs that are often utilized to form exhibit pools and other animal habitats in zoos and aquariums. Deciding the size and spacing of rebar is a typical part of any of these projects and should be handled by a professional engineer. When it comes to the design and operational side of these projects, there are important choices to make about rebar as well.
Most common rebar is made from steel. It is readily available, contractors are familiar with how to install it, and it offers good value in terms of performance vs. cost under normal environmental conditions. GFRP (Glass Fiber Reinforced Polymer) is an alternative material that has some inherent properties that can, in some cases, offer better performance over time – generally at a higher initial cost. For typical concrete construction scenarios, steel rebar is an easy go-to choice. There are some instances in which GFRP may be a much better choice. Let’s look at a few of those:
- Corrosive Environments: Saltwater exhibit pools are a prime example of where GFRP can offer significant advantages over standard steel rebar. GFRP is much more resistant to corrosion/breakdown over time when exposed to saltwater than steel. In this regard, GFRP has the ability to extend the life of reinforced concrete components – sometimes to a degree that brings its overall lifecycle cost in-line with that of steel rebar.
- Electromagnetic Fields: GFRP is a non-conductive material, while steel is highly conductive. This can be an important factor when creating habitats for animals that are receptive and sensitive to electromagnetic fields. Sharks are a prime (and generally well-known) example of an animal that uses electroreceptors. They certainly aren’t the only ones though. Birds, bees, and a host of other animals are sensitive to electromagnetic fields. Therefore, careful consideration should be given to whether or not the use of steel rebar might cause an unwanted or dangerous electromagnetic field.
- Bend but Not Break: GFRP has incredible tensile strength and a higher yield strength than steel. This means that it can experience greater deflection before failure, meaning we can often be forewarned by seeing visible deflection before any catastrophic failure occurs. When dealing with large-scale aquarium exhibits, where there may be millions of gallons of water, tens of thousands of marine animals, and several thousand guests all inside the same facility at one time, the last thing anyone wants to see is an abrupt, critical structural failure.
So, why wouldn’t you use GFRP rebar?
While GFRP offers many benefits, including those listed above, there are many reasons why steel rebar is still used in most reinforced concrete animal habitat construction. First, while GFRP can sustain greater deflection before failing, it also begins to deflect under much less loading than a typical steel rebar of the same diameter. This is a major factor to consider when dealing with long spans (vertical or horizontal), and locations where deflection could cause failures in other components within the structure. A prime example of this is in concrete aquarium pool walls that have acrylic panels. Measurable deflection in the concrete wall around the opening for an acrylic panel could cause the watertight seal around the acrylic to fail. In this instance, steel rebar would typically be a preferred choice.
One of the major benefits of GFRP listed above, particularly when dealing with saltwater and other corrosive environments, is its corrosion resistance. However, there are also different types of steel rebar that can be utilized that make it more suitable for these conditions. The photo below shows a large, reinforced concrete aquarium pool wall under construction. You can see that some of the rebar is green and some is black. The black rebar is the standard type you would expect to see in most any commercial (or even residential) construction. The green rebar has an epoxy coating that increases its corrosion resistance. In this instance, the green rebar is used in close proximity to the openings for the acrylic panels in the aquarium pool wall and other areas where the concrete coverage may be less than what standard rebar would need to avoid being susceptible to corrosion. Stainless steel rebar would be yet another alternative that would offer a high degree of corrosion resistance, though the use of stainless steel is often cost-prohibitive.
Both steel and GFRP rebar have their place in the construction of animal habitats and exhibit spaces. Aside from the points mentioned above, weight, shipping costs, workability, and many other factors come into play when trying to decide which is right for a project. Giving careful consideration to all these points in order to determine which is best-suited for your particular application is the first step toward making your guests, animals, and overall project safer.