Got “Salty” Bridge Decks?

This installment of A Bridge Moment provides a summary of the basics on corrosion of reinforcing steel in concrete bridge decks that have a high salt diet. It also provides a summary of some ways to reduce and possibly eliminate the problems caused by rusting of bridge deck reinforcing steel on these “salty” bridge decks. Spoiler alert – don’t use carbon steel rebar for new bridge decks when your bridge is in a “salty” environment– use glass or carbon fiber reinforced polymer (FRP) bars.

For some bridge owners there is no way to avoid having “salty” bridge decks. In northern states, DOTs dump salt on the bridge decks in the winter which is the most cost-effective approach to meet their states “bare-road policy”.

In Florida and other coastal states, the bridge crossings near the coast have moist salty laden air and saltwater floating around them 24/7/365. Some of these bridges are adjacent to boat ramps and are constantly drenched with saltwater that drips off boat trailers after using the boat ramp. In addition, jet skis spray a “rooster tail” of salt water up on the bottom of bridge decks on low level bridges that provide recreational navigation channels under the bridge.

When embedded steel corrodes, the rust expands to a much greater volume more than the original steel which creates tensile expansion forces in the concrete. These expansion forces will eventually cause cracking, delamination and spalling in the concrete and the rusting causes loss of section of the steel rebar. See the photo below showing how the rusting (pack rust) of a steel connection angle caused the flange of the angle to be deformed which caused a rivet to fail in tension.

One of the primary factors affecting bridge deck performance is the penetration of water borne chloride ions from the salt into the concrete. Steel corrodes because it is not a naturally occurring material. Corrosion is an example of “mother nature” taking back what existed in nature by causing a man-made product (steel) to return to its natural oxidation state – iron oxide or rust. Rust forms when iron (an anode metal) is combined with oxygen and water (an electrolyte) and a red-brown rust is created (a cathode). The powerful electrolytes in the chloride ions in salt speeds up the corrosion process by allowing the metal (iron) to lose electrons more quickly than would be caused by just oxygen and water. ?

It is important to understand that the corrosion process will not start until the chloride concentration level reaches a certain threshold value. That is why if an older existing bridge deck may look to be in good shape and you are considering doing some maintenance repairs on the bridge, doing some chloride content testing to determine how close the concrete is to this threshold value is important.

In simple terms, the four ingredients needed for the initiation of corrosion of deck rebar are water, oxygen, salt (chlorides) and steel. The most effective way to significantly diminish the potential for initiation of the corrosion process is to take away one or more of these four ingredients. Therefore, if the bridge is in a salt free location you don’t have to do much more than follow the typical standard approach of providing at least 2 inches of concrete cover and specifying a dense concrete mix that is cured properly during installation to minimize drying shrinkage cracking.

If a bridge deck is going to get high doses of salt, increasing the concrete cover and improving concrete quality helps to slow down the salt intrusion. However, all concrete is porous to some degree and is usually cracked, especially bridge deck concrete. These approaches only delay the passage of salt water infused with chloride ions.

The following list provides the author’s opinions on how to solve this “salty” bridge deck problem:

1. For new concrete bridge decks, my preferred solution is to specify composite fiber reinforced polymer (FRP) reinforcing. Carbon and glass fibers are the most common materials used to manufacture this type of reinforcing and basalt will likely be available soon. No reinforcing steel – no deck problems. ?FDOT has included these materials in their standard specifications under Section 932-3 Fiber Reinforced Polymer (FRP) Reinforcing Bars. Some other states are dragging their feet on including these materials in their spec books and need to get on board soon.
2. For existing concrete bridge decks and on new decks in states where FRP reinforcing is not yet part of the DOTs standard specifications, the most reliable way to prevent salt intrusion is to seal the deck with one of the commercially available waterproofing sheet membranes. The membrane keeps out water and any additional chlorides from entering the deck and thereby stops the corrosion process or at least dramatically reduces the rate of corrosion. There are spray on liquid membrane systems but I prefer the sheet membranes especially on adjacent prestressed concrete slab beam bridges because the sheet membrane helps mitigate the potential for reflective cracking over the slab beam joints. Membrane manufacturers recommend a minimum thickness of 1.5” of asphalt but I prefer at least 2” or even 3” if the bridge superstructure has adequate capacity. The thicker layer improves the service life of the asphalt and the membrane. It also provides better protection for the membrane during the eventual maintenance cycle that includes milling of the top 1” of asphalt and resurfacing in the future. Using this approach, the membrane should last for 50 years or longer. To learn more about membranes, check out the NCHRP Synthesis 425 publication Waterproofing Membranes for Concrete Bridge Decks. See the photo of the installation of a sheet membrane below.
3. If the bridge does not have adequate capacity to allow for the dead load of an asphalt overlay or the reduction of the curb height is a concern, I recommend the application of a high molecular weight methacrylate sealer over the entire deck surface in accordance with FDOT Specification 0413-154. The FHWA publication Bridge Preservation Guide recommends a 3-to-5-year interval between future applications. The FHWA guide also recommends sweeping or washing the deck surface every 1 to 2 years. This approach is not cheap especially when you factor in the cost of traffic control (MOT) for these maintenance operations. Therefore, it could make more sense to do nothing and instead put the money you are not spending in a savings account until you can afford a deck replacement using FRP bars.
4. Thin polymer overlays help reduce the infiltration of water and chlorides in the same way a methacrylate sealer does and may last 2 times longer but for a much higher cost. I don’t think it is worth the additional investment when you factor in the higher risk involved with these applications delaminating from the deck and from new cracks forming in the deck that telegraph into the overlay.
5. Some states use epoxy coated or galvanized bars that will buy you some time before corrosion starts assuming the contractor doesn’t damage the coating during the storage, handling and installation of the bars. FDOT doesn’t approve using these coatings on their projects as a result of premature failures in a “salty” environment as a result of inadvertent coating damage during construction.
6. Some bridge engineers recommend a partial patching approach where they specify removals and patching at areas of visible damage. This is a temporary fix because the deck areas immediately adjacent to the patches will deteriorate at a faster rate after the patch job and you are back to square 1. I agree with doing some partial patching as long as a waterproofing membrane and asphalt overlay is installed over the entire deck after the patching is done.