Corrosion – concrete bridge cancer

How to avoid aging concrete bridge failures?

Concrete bridge collapses due to steel corrosion have occurred in several cases providing important lessons on the vulnerabilities of such structures. Corrosion in prestressed and post-tensioned tendons is a leading cause of structural failure in concrete bridges.

This article discuss about causes for steel corrosion and highlights the critical importance of ensuring proper protection against corrosion for steel bars and tendons. Whether it's the material used (like Sigma Oval), the environmental exposure leading to stress corrosion cracking, or failures due to improper grouting, the consequences of neglect can be severe, resulting in sudden and catastrophic collapses. Enhanced inspection, better quality control in construction, and using corrosion-resistant materials can mitigate these risks.

Improper grouting in tendon ducts or improper steel bar protection

Improper grouting is one of the most significant causes of tendon corrosion in prestressed and post-tensioned structures. In many projects, incomplete or poorly executed grouting allows air, water, and chlorides to penetrate the tendon ducts. This exposure accelerates corrosion, particularly in humid or marine environments.

One prominent case occurred in the Varina-Enon Bridge in Virginia, USA, where improper grouting of post-tensioned tendons led to severe corrosion and tendon failure within 20 years of service. The grout used in the tendon ducts either contained voids or was improperly mixed, allowing water to seep in, causing localized corrosion of the steel tendons. The ingress of chlorides from de-icing salts further accelerated the degradation process, eventually leading to tendon rupture.

In Germany, studies have shown that improper grouting in tendon ducts has been a recurring problem, leading to significant maintenance efforts on aging bridges. Some bridges experienced tendon corrosion due to grout segregation, which allowed chloride to penetrate the ducts and compromise the steel tendons. While these bridge did not collapse, extensive retrofitting was required to ensure structural integrity. And we don’t know clear reason for Bridge collapse in Dresden 2024, do we?

The most notable collapse occurred in Genoa, Italy, with the failure of the Ponte Morandi in 2018. The main cause of this disaster was steel corrosion. The bridge, built in the 1960s, faced several environmental challenges, which increased chloride exposure. Over time, the bridge's steel cables, inside concrete, were severely corroded. It was later discovered that the construction defects, combined with environmental exposure, had allowed chloride ingress to accelerate the degradation of the steel strands, leading to the combination of chloride-induced corrosion and undetected tendon damage led to a catastrophic failure.

Steel tendons and corrosion vulnerabilities

The Sigma Oval steel tendons were used primarily during the 1960s to early 1980s in various bridge and infrastructure projects. This period marked the widespread adoption of prestressed concrete technologies, including the use of these specialized steel tendons. One of the challenges that later emerged with Sigma Oval tendons during this time was their vulnerability to corrosion. These tendons were manufactured by several steel production companies across Europe, including facilities in Germany and Italy, which specialized in prestressing steel production. Several projects that utilized Sigma Oval tendons have reported corrosion-related issues.

One example involving Sigma Oval steel tendons was seen in certain European bridges where chloride ingress from road salt and rainwater runoff caused deterioration. While no specific bridge projects of collapse due to Sigma Oval tendons alone are clearly documented, these tendons share similar corrosion or even higher risks with other prestressed steel tendons in terms of exposure to chlorides and poor protective measures.

An example of prestressed concrete failure outside of bridges was the collapse of the Lohja Water Tower in Finland in 1981. This structure used stressed tendons for its cylindrical design. Corrosion in the post-tensioned tendons, exacerbated by exposure to the elements and improper maintenance, led to a complete collapse. It was discovered post-collapse that the grout in the tendon ducts had been inadequately applied, leaving voids where water had accumulated, leading to severe corrosion and tendon failure.

Stress corrosion cracking in steel wires

Stress corrosion cracking (SCC) is another critical issue, particularly affecting steel wires used in prestressing. SCC occurs when steel under tensile stress is exposed to a corrosive environment. It leads to brittle cracking, which can propagate rapidly and cause sudden failure without significant visible warning.

One well-documented case involving stress corrosion cracking occurred in the failure of the Mid-Bay Bridge in Florida, where post-tensioned tendons experienced cracking due to stress corrosion. The combined effects of environmental exposure, stress from tension, and poor maintenance led to cracking in the steel tendons. Though this example is from the U.S., similar failures have been observed in Europe where SCC was a contributing factor in the degradation of tendon integrity.

Cold climate challenges

Cold weather conditions pose a unique set of challenges. The use of de-icing salts during the winter months contributes significantly to chloride-induced corrosion in rebar, also prestressing tendons in concrete bridges. Repeated freeze-thaw cycles exacerbate the ingress of chlorides into the concrete, initiating corrosion of the internal steel elements. Finland has not witnessed high-profile bridge collapses like Italy's Morandi Bridge, but numerous structures show advanced signs of corrosion due to these conditions, which compromise long-term durability. Recent efforts have focused on more advanced corrosion-resistant materials and improved maintenance to prevent such failures.

In Germany, bridge failures related to rebar corrosion have also raised concerns. Corrosion in bridges is typically caused by chloride infiltration from de-icing salts, which accelerates the degradation of steel reinforcements embedded in the concrete. In regions with heavy freeze-thaw cycles and frequent use of road salts, the protective concrete layer around the rebar is compromised, allowing chloride to reach the steel, causing localized corrosion pits and weakening the structure. Studies in Germany have shown that once rebar corrosion starts, it induces cracking and spalling of the concrete cover, further exposing the steel and accelerating the damage process. While no specific major bridge collapse has been attributed solely to these factors, the corrosion of rebars is a leading cause of concern for the aging infrastructure across the country.

In all these cases, the key issue revolves around chloride-induced corrosion, which weakens steel reinforcements and tendons, leading to structural failure if not properly managed through regular inspections and repairs. The lessons from bridge collapses have been particularly influential in reshaping maintenance protocols and monitoring practices for prestressed and reinforced concrete bridges worldwide.