If you’ve ever driven under a concrete bridge and seen exposed rebar rusting away on its underside, you may have wondered how it happened. There are many reasons why spalling occurs below a bridge. One common reason is the expansion of steel when it rusts or corrodes.
How does the corrosion process happen and why does concrete spall when the embedded metals corrode? Corrosion may be defined as the degradation of a material as a reaction to its environment. Because of moisture evaporation, concrete is a porous material. Water and oxygen molecules enter the pores of the concrete, and an electrochemical process occurs with the carbon steel bar. The iron in the steel is oxidized, which then produces rust. A buildup of rust products at the surface of the carbon steel bar exerts an expansive force on the concrete.
Based on the amount of oxidation, the rust products of steel can occupy more than six times the volume of the original steel, according to Marlou Rodriguez, P.E., anchor systems engineering manager at Simpson Strong-Tie. “Over time, further rust occurs and surface cracks will form. Eventually spalling will occur, exposing the rusted carbon steel bar.”
ANCHORAGE PRODUCTS
Just as with reinforcing bars below a concrete bridge, cracking and spalling can occur when a carbon steel anchor is used adjacent to a concrete edge. Simpson Strong-Tie has many anchorage products that can be used in these conditions to prevent cracking. One specific product is the new stainless steel Titen HD screw anchor. This new innovative screw anchor is made up of Type 316 stainless steel, which offers a high level of resistance.
“This makes the stainless steel Titen HD an excellent choice when it comes to an anchorage solution in corrosive environments,” says Rodriguez. “These environments include wastewater treatment plants, exterior handrails, exterior ledger attachments, stadium seating, central utility plants and kitchens just to name a few.”
Unlike expansion anchors, screw anchors require the leading threads to cut into pre-drilled holes. This can be easily achieved with hardened carbon steel cutting threads. Stainless steel is not hard enough to cut into concrete. The new innovative stainless steel Titen HD solves the problem by brazing heat-treated carbon steel cutting threads to the surface of the stainless steel tips of the screw anchor.
These carbon steel threads are hard enough to cut grooves into the surface of a predrilled hole, allowing the anchor to be installed with ease. The volume of the carbon steel cutting threads is less than 1% of the stainless steel, reducing the buildup of rust that eventually spalls the concrete edge. Other stainless steel screw anchor manufacturers in the market have a bimetal product that attaches a full carbon steel tip. These bimetal screw anchors contain up to 18% carbon steel, according to Rodriguez. “Such a large amount of carbon steel can expand up to six times its volume when it corrodes and can spall the concrete when used adjacent to an edge.”
When designing an anchorage solution for your next job in a corrosive environment, the stainless steel Titen HD can provide resistance to corrosion, and also give the ability to drive these anchors into the concrete with ease.
Simpson Strong-Tie has dedicated itself to creating structural solutions and technology to help people design and build safer, stronger homes and buildings for more than 60 years.
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