Corrosion of steel rebar:
There has been some attention given to the use of steel reinforcement in the polymer concrete products. Sadly, the attention has been an attempt to portray our design philosophy in a negative fashion. These attempts are an effort to try and convince engineers and owners than steel reinforcement in polymer concrete will corrode when exposed to certain conditions. In particular, our competitors have produced a video showing steel reinforced polymer concrete in a beaker of sulfuric acid. In this video, hydrogen bubbles can be seen rising from the steel interface with the sulfuric acid. The reaction being witnessed is the formation of iron sulfate and hydrogen gas. The video shows a shiny exposed area of steel reinforcement. What the video doesn’t show is the formation of the iron sulfate layer. In the real world, the iron sulfate layer is what protects the steel from further exposure to the sulfuric acid. The same way one would see an oxidation layer on the exposed surface of rebar from non-acidic exposure as a form of protection to the material under that layer. When was the last time we saw shiny steel reinforcement? There are several factors that influence the effects of sulfuric acid on steel. These factors are agitation of acid, temperature, and concentration of acid. The agitation of the acid can cause erosion of the iron sulfate layer. By eroding the iron sulfate layer, more new steel is exposed and converted to iron sulfate. This is the same phenomenon that would occur with the formation of iron oxide. If these protective layers are removed and the steel is made shiny, a new protective layer would be formed, thus each time some material is lost. Given the exposure conditions in precast polymer concrete, and the static conditions of any acid that may be formed inside the environmental conditions of a sewer system, the likelihood of erosion is practically nonexistent. It has been estimated in EPA publications that the concentration of sulfuric acid in a sewer system can be 7% when the pH is 0.5.***This concentration is far lower than most of the research studies done of the exposure of steel to sulfuric acid.
Things to consider:
What is the type of exposure our reinforcement would experience? The reinforcement is encapsulated in an impermeable resin based concrete. The exposure would be from either the formation of a crack or from coring an opening in the structure. The exposure of a crack would permit the acid to possibly enter the crack. Depending on how far it travels, it could react with the steel reinforcement. Once this reaction takes place, the process will cease. The exception to this would be if the spent acid is removed from the crack, the steel is polished, and new acid is allow to enter the crack. If a cored opening exposes the ends of the reinforcing, the ends could react with the acid, the iron sulfate layer is created and the process will cease. As above, only if the steel is polished to remove the sulfate layer and re-exposed, would the process start again.
Why does USCP use steel reinforcement in their products? Yielding. USCP uses the well-established and long-standing stress block design methodology used for standard reinforced concrete designs. Our products are designed with more compression capacity than tensile capacity. This means the tensile reinforcement will begin to show over stress before the concrete does. Why is this important? Since steel will yield before it reaches failure, it permits a visual observation of an overstressed product before it fails. This allows for evaluation or repairs to be made. Steel reinforcement is also the standard reinforcement listed in ASTM C478. This applies to both the base slabs per ACI 318 and the barrel reinforcement.
Why doesn’t USCP use FRP reinforcement? It is true that FRP doesn’t require a protective layer to be formed from a reaction with acid. However, FRP does not yield. FRP will fail in a brittle mode. FRP is typically weaker in shear than steel and according to ACI 440.1R, alkaline exposure, UV, and moisture exposure will negatively affect the strength and stiffness of FRP bars. There is no standard for round utility structures using FRP reinforcement. While ACI 440.1R can be used for flat slabs, there is no direction or minimums given for using FRP in round barrel sections.