Timber, Concrete and Steel Substructures: What Fails in Each
By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Dana Whitcombe, technical reviewer.
Underwater Inspection | Commercial Diving Contractors Alabama I remember a pier cap on the Old Pascagoula River Bridge, about a decade back. The diver surfaced and held up a piece of concrete that had come away in his hand. It was the size of a dinner plate, and the rebar imprint was still in it. That was a moment when I realized that corrosion doesn't just crack—it uncorks. Since then, I've signed off on roughly 400 underwater inspections across the Gulf Coast. And I've seen the same three materials fail in the same three ways. Timber rots. Concrete spalls. Steel corrodes. But the why, and the timing, is all in the details. That's what this post is about—what to look for, and what it's trying to tell you.
Timber: Rot and Marine Borers
Timber was the original bridge substrate, and it's still common in older piling and for temporary supports. Most of what I see is creosote-treated southern yellow pine. It'll last a long time if it stays wet. But the term 'wet' is the catch—timber needs to be fully submerged or fully dry. It's the wet-dry cycle at the waterline that kills it. That's where the borers get a toehold and where the fungi find the oxygen they need. The typical failure chain starts with a crack in the shell. Borers—shipworms like Teredo navalis, or if you're in the Pacific Northwest, they're going to be gribbles, Limnoria lignorum—drill into the exposed wood and create tunnels you can't see from the outside. You'll have a pile that looks fine, weighs nothing when you lift it, and accepts a 6-inch spike with no resistance. I've seen piles tested that way on a barge deck. The sound tells you more than any tool. In river systems, you get less borer action but more decay. Bacterial and fungal decay needs a moisture content above 25% and temperatures above 40°F. So you get a slow softening at the mudline. You have to probe with a boring tool or a pocket knife. If you can sink the knife more than a quarter of an inch without heat, that pile is compromised. As a rule of thumb, if the cross-section loss is more than 20% at the mudline, I flag it for a load rating.
Concrete: Spalling and the Chloride Threshold
Concrete is the workhorse of modern substructures, but it fails by spalling, and spalling is a chloride problem. Chloride ions from deicing salts or marine saltwater penetrate the concrete cover and build up at the rebar level. When they get to about 1.2 to 1.5 pounds per cubic yard—call it 1.5 to be safe—the passive layer on the steel breaks down. The steel rusts, and rust takes up six to eight times the volume of the original steel. The pressure pops the concrete right off the rebar. I've seen a lot of 'Swiss cheese' columns. This is where the rebar is exposed and the concrete is falling away in sections the size of your hand. In a splash zone, that happens faster than you'd think. A column with 3 inches of cover built in 1975 might only have 1.5 inches now, and the rate is accelerating. We've had failures appear between annual inspections. That's the thing—concrete looks solid until it doesn't, and then it's a cascade. The fix isn't just patching. You have to clean the rebar, remove all chloride-contaminated concrete—which sometimes means knowing you can't get it all—and then you're talking about corrosion inhibitors or cathodic protection. I've used galvanic anodes from Cortec and impressed current systems from Vector. They work, but they have a service life, and you have to budget for replacement. And don't get me started on epoxy-coated rebar; it works in some places, but the cut ends and damaged spots are a classic site for concentrated attack.
Steel: Corrosion and Fatigue
Steel substructures are usually open-pile framing or caissons. They fail in two independent ways: corrosion and fatigue. The worst corrosion isn't underwater—it's in the splash zone, where there's oxygen and water. You get pitting that you can measure with a depth gauge. It's not hard to find a 3/8-inch plate that's been eaten down to paper-thin in 20 years. Underwater, the corrosion is slower, but it happens at the mudline where the coating is always abraded by sediment. Fatigue is the sneaky one. It doesn't need bulk corrosion. It needs a stress concentration and a few million cycles. I remember a truss bridge in the panhandle where a gusset plate lug had a crack that started at the edge of a bolt hole. It was a 1/16-inch wide crack. I had to put a strobe light on it to see the oil canning. It was clear that the plate was flexing. We drilled a stop-hole and it bought the owner time, but I put a 24-month limit on it. Coatings are your best defense. Steel that's been painted with a proper immersion-grade epoxy system can last 40 years in a marine environment. But once the coating breaks, the section loss accelerates. I've seen coating systems from Sherwin-Williams and Carboline do well, but only if the surface prep is right—SSPC-SP10 or better. You can't just slap paint on rust.
Repair, Retrofit, or Replace? What Rules Do I Use?
The condition ratings we use come from the AASHTO Manual for Bridge Evaluation. A '4' on a substructure means it's in poor condition, and that's when I'm looking at repair options. The threshold for action varies by element. For timber, if the loss is more than 25% of the cross-section, I'm thinking replacement. For concrete, if the spall depth exceeds the cover by 50% and the rebar section is reduced by more than 10%, then it's a repair-plus-cathodic-protection job. For steel, if you have a fatigue crack, you ask two questions: is it load-bearing? And is there a redundant load path? If the answer is 'yes' to the first and 'no' to the second, that's a replacement. But here's the thing—a lot of this is judgment. I'd rather be in the field hammering and probing than reading a spreadsheet. You get a feel for what's dead. There's a sound a hammer makes on sound concrete versus delaminated concrete—a sharp ring versus a dull thud. I know that's not a number, but you can't get that from a drone.
Frequently Asked Questions
How often should underwater inspections be performed? Every 24 months for most bridges, but it depends on the underwater complexity and the condition states. In the Gulf Coast, we go to 12 months if we have a '4' or lower. What's the most common cause of concrete substructure deterioration? Chloride intrusion leading to rebar corrosion. It's salt from deicing or marine environments. Once the chloride threshold is reached, spalling is inevitable. Can timber pile decay be stopped? You can slow it by controlling the water and oxygen boundary, but once a borers population is established, it's hard to stop. You can install a fiberglass jacket or a concrete encasement, but it's usually a holding action. How do you identify a failing steel substructure? Look for missing coating, rust staining, and cracked welds. But fatigue cracks are often hidden by paints. Do a dye penetrant or ultrasonic test if you suspect it.