Damage on Concrete Highway Bridges: Causes, Symptoms, and Assessment

Theresa Steiner, summer semester 2021

This article gives an overview over typical causes of damage to highway bridges made from reinforced concrete and connects them with common consequences on the structure.

Highway bridges – as part of the road system – are subject to a constant load and have to withstand environmental influences without much protection. They consist of concrete, additionally reinforced with steel. Both materials are prone to different damage scenarios that can be equally fatal. To ensure the structure’s quality and the up-keeping of certain safety standards, the German Federal Highway Research Institute (Bundesanstalt für Straßenwesen – BASt) has released DIN 1076 ^[1], as well as several reports and guidelines for the inspection and testing of engineering structures in connection with roads.

There are several possibilities and criteria to classify the damage scenarios on highway bridges. The classification presented in Section 2 is based on literature ^[2] and investigates the causes of damages and their assessment further. Additionally, the detection and monitoring are briefly discussed.

Causes of Damage

The causes of damage to highway bridges can be manifold. This, in turn, means that usually more than one cause lies behind a damage and investigations are necessary to determine the root of the problem ^[2]. In general, damages can be categorized into three main groups:

• Damages due to environmental and operational conditions, as well as aging
• Damages due miscalculations during the planning and design stage
• Damages occurring due to improperly realized construction work ^[2]

Damage due to environmental and operational conditions, as well as aging, cannot be avoided. Therefore, a thorough structural health monitoring becomes even more important to detect them as early as possible and initiate appropriate countermeasures. The distinct origins of damages are also intricately connected, making it difficult to attribute one to a certain symptom and consequence. ^[2]

Carbonation

Carbonation is a reaction between the air-bound CO₂ and calcium hydroxide (Ca(OH)₂), which is dissolved in water. The reaction produces calcium carbonate (CaCO₃) and water. Additionally, it causes the concrete’s pH-value to drop, which in turn de-passivates the reinforcement and enables corrosion. Factors, such as CO₂ content of the ambient air and the concrete’s water content need to be taken into consideration to actively avoid carbonation. It can itself be sped up by water e.g., entering the concrete through cracks or defects in the concrete structure. ^[3]

Chlorine Penetration

Chlorine (Cl) can reach the structure through the application of road salt, sea water and marine air. It can also already be present due to the concrete’s composition. If the Cl around the reinforcement reaches a critical level, the passivating coating in the reinforcement steel can be damaged and gives room to corrosion. The chlorine penetration is influenced by the concrete composition and the amount of Cl or water available. A distinction of the different parts of the bridge are necessary since the Cl concentration varies. Namely, these are splash water areas, spray mist areas and others. Cracks or faults in the concrete microstructure can have a significant influence on its severance. ^[3]

Freeze-Thaw

Freeze-thaw proves especially problematic in colder climates, where there is a constant alternation between freezing and thawing, combined with the use of road salt. Water can enter the concrete through pre-existing micro-cracks. During frost periods, it increases its volume, causing cracks or cavities to grow and ultimately decreasing the strength and elasticity of the concrete. Additionally, more water can enter the structure through ever wider cracks, aiding corrosion. The road salt, moreover, facilitates chlorine penetration within the microstructure.  ^[3]

Alkali-Silica Reaction

This type of damage concentrates on the concrete. It is a chemical reaction between amorphous silica additives and alkali ions in the cement. The product is an amorphous gel that expands when absorbing water. In order realize the reaction, all three of the following factors must be fulfilled: sufficient material of both reactants and a high level of humidity. It is further accelerated by elevated temperature and the external introduction of more alkali, e.g. via road salt.  ^[5]

Stress Corrosion

The propagation of cracks in steel reinforcement driven by the simultaneous influence of corrosion and static tensile stress is called stress corrosion. Different to other forms of corrosion, its attack might not always be visible on the exterior. This can also be the case for crack propagation. This means that a detection of stress corrosion is difficult. The two types of corrosion – anodic and cathodic – described above can again be applied. ^[6] Investigation into reinforcements that failed due to this type of damage suggests, that the first seeds could already have been laid during the construction phase.  ^[6]

Humidity and Temperature

Humidity and the presence of water can have two effects. On one hand, it can be a driving force behind various sources of damage, such as carbonation or alkali-silica-reaction. On the other hand, the water can itself induce corrosion, especially on steel parts not enclosed by concrete or otherwise exposed to water and oxygen. Differences in temperature can also cause cracks and scaling, among other things during freeze-thaw ^[2].

Live Load

The load due to traffic has increased rapidly in the last 70 years. The result is an elevated mechanical load on the structure and cause concrete damage. The main portion of this can be attributed to more heavy haulage and special transportation. Future growth of traffic also has to be taken into consideration. Passenger cars only play a minor role. The consequences of the increased load are damages to the concrete in the form of cracks. In the end, this has an influence on the load bearing capacity, durability, and serviceability. ^[2]

Planning, Design and Execution

Damage can also occur due to mistakes during the planning stage of the bridge’s bearing structure, the design or construction phase. Especially miscalculations regarding the maximum load or inadequate pre-stressing of steel wires can be problematic. Furthermore, the reinforcement placement and its size are crucial. ^[2]

Concerning the building phase, common reasons for damages include the incorrect handling of the concrete at any stage of the construction and processing of the steel-tendons. The latter involves a deviation of their placement, the use of wrong, contaminated, or damaged tendons and possible pre-corrosion. Improving the quality of instructions, as well as the preparation of the persons involved, and their quality awareness are an active prevention in this case. ^[2]

In the end, the causes of damages due to planning, design and execution only account for less than 20% of damages on highway bridges. These mistakes and the resulting damages can – in combination with continuous monitoring – be avoided, at least to some extent. More examples and further explanations can be found in literature ^[2].

Possible Damage Scenarios

A single instance of one of the damages presented below is usually not instantly destructive. Their accumulation, however, and especially the appearance of more than one type of damage can be critical.

Damaged Concrete 

The sources of damage mentioned in Section 1 can have different effects on the state of the bridge’s concrete structure and microstructure and depend on the location on the bridge. Weathering and the accumulation of dirt might not directly influence the concrete but can hide other damages like cracks and scaling. Both can grow, making room for water to enter the concrete, enabling further damage mechanisms. 68% of cracks appear on prestressed concrete bridges rather than reinforced concrete bridges. It is necessary to both identify new cracks and monitor already existing ones via structural health monitoring ^[2]. Cracks as well as scaling usually appear on the bridge’s deck. This is also the case for moisture penetration and cavities. Exposed reinforcements pose another important damage on highway bridges and can be caused by scaling. They mostly appear on the bridge’s substructure. The exposure makes them vulnerable for corrosion, carbonatization and chloride penetration. ^[2]

Fatigue is a typical consequence of aging. The cyclic stress caused by environmental influences and traffic can damage the concrete microstructure significantly. In the long run, this results in fatigue failure of the materials. Fatigue failure is highly critical and should always be assessed with due care during the inspections. Dynamic fatigue testing is the method of choice to determine the materials behavior.
Fatigue failure can be characterized into three phases: first, the decrease of strength is initiated by the formation of micro-cracks. Then, those cracks propagate to an increase of stress at their tips. Lastly, this propagation is accelerated and ultimately leads to a brittle fracture.  ^[3]

Whilst the damages presented above can be attributed to aging and wear, damages due to construction mistakes can be problematic. These include the density and porosity of the concrete as well as the thickness of the concrete layer and gravel nests. The exposure of reinforcement can also happen here. ^[2]

Table 1: Summary of concrete damages ^[2]

Damage to the Reinforcement

Apart from their exposure, reinforcements and clampings are also subject to cracks. The steel is susceptible to (stress) corrosion or even fracturing. These failures can be hidden at first but ultimately lead to failure. This is also the case for defects on the anchors of cables and prestressing tendons. ^[2]

70% of damages can be attributed to the corrosion of the steel reinforcement, making it the main cause for reinforcement failure. Two subprocesses can be distinguished: on one hand, the anodic subprocess causes the dissolution of iron ions (Fe⁺⁺); on the other hand, during the cathodic subprocess, free electrons are absorbed by water and oxygen and start forming hydroxide ions (OH^-). Furthermore, the corrosion needs to be separated into an initiation phase and a damage phase. During the initiation phase, carbonation and Cl penetration de-passivate the reinforcement surface, as explained above, whilst during the damage phase, the steel reinforcements lose their cross section, and the concrete surface can crack and scale. Different types of corrosion are possible. The process, can, however, only take place if all three of the following are abundantly available: de-passivation, oxygen, moisture. ^[2] Finding these areas of corrosion is a large aspect of non-destructive testing. 

Again, looking at the construction phase, damages that are produced then can have an influence on the bridge’s structural safety. These include the wrong placement of reinforcements, the inappropriate choice of reinforcement bars (e.g. the wrong diameter) and the insufficient filling of cladding tubes. Any deviations from the original planning invalidate those calculations and jeopardize the use of the engineering structure. ^[2]

Table 2: Summary of reinforcement damage ^[2]

Structural Damage

Structural damages can accelerate other damage scenarios in the long run. A faulty sealing as well as mistakes in the drainage system aid the accumulation of water in and on the structure. Cracks in the pavement can not only be a danger to the structure but also to any passing vehicles. Furthermore, sagged, or torn expansion joints can have the same effect. Settlement can lead to unevenness which, in turn, causes stress in the concrete and reinforcement. ^[2]

Table 3: Summary of structural damage ^[2]

Other damages occurring on highway bridges might not have fatal effect on its structural health. These include graffiti on the pillars and walls of the support structure, as well as shrubbery closing in on the bridge. Their occurrence, however, is undesired since they might obscure more severe deteriorations or influence sensors. ^[2]

Assessment of Structures

Damages on highway bridges can occur on their surface (including the tarmac, as well as the raw concrete), the support structure (the steel reinforcement but also the pillars and bearings), or any additional features such as cables, markings and traffic signing ^[2]. As stated in DIN 1076, an examination as part of a main inspection takes place every six years ^[1]. During this, all parts of the structure, including ones difficult to reach, are assessed using specialized equipment. Three years after a main inspection, a simpler checkup is conducted. This usually comprises a visual inspection and other measures if appropriate. After special occurrences, such as fires or accidents, an unscheduled inspection is required, which does not interfere with the actual inspection agenda ^[2]. The results are documented in the bridge book as well as the “SIB-Bauwerke” database. In between inspections, the bridges can constantly be monitored using permanent sensors and structural health monitoring techniques.

The knowledge gathered during the inspection is summarized in a specialized form and transferred to the database. In it, the damages are evaluated in terms of their influence on safety (Standsicherheit, S), serviceability (Verkehrssicherheit, V) and durability (Dauerhaftigkeit, D) ^[7]. Each of these categories is assessed on a scale from 0 to 4, where 0 means no influence and 4 indicates crucially influence on the structure and requires immediate action ^[2]. The critical point for repairs outside the usual maintenance is reached at a value of 2. Additionally, the time of the first discovery and size are noted for future reference. In the end, an overall assessment is calculated by forming a total condition rating ^[7].

The safety S of a structure regards its ability to carry the load it was designed for without damage. Most damages mentioned in Section 2 do not have a big influence on a bridge’s safety. In fact, more than 90% have no impact at all ^[2]. This is similarly the case for the serviceability V, which concerns the proper and intended use without detectable or undetectable danger to users and machinery ^[2]. The durability D refers to a structure’s resistance against exposure to realize a long period of use, as well as the necessary safety and serviceability. Carbonation and chloride penetration are only relevant for durability. At some point, repairs and renewals will become necessary to ensure safe use, or rather, any use at all. ^[2]

Table 4: Examples for damage assessment ^[9] (part of ^[8])

An exemplary catalogue for the assessment and classification of damages ^[9] as well as a completed example assessment can be found in the reference literature ^[8].

Detection of Damages

Similar to the division of the causes of damages into categories, the detection of these damages can also be carried out in two distinct ways. With structural health monitoring, any damages and failure occurring during the structure’s service – i.e. those due to environmental and operational conditions, as well as aging – can be detected early on. Non-destructive testing, on the other hand, can identify previous damages, like those resulting from construction.

Structural Health Monitoring

Structural health monitoring (SHM) plays an integral part in the early detection of possible damage scenarios. Many of the effects described in Section 2 can be prevented from being a danger to the bridge’s durability by taking quick action. The most common sensors applied are wire strain gages, fiber optic sensors, capacitive sensors, acoustic emissions sensors, piezo sensors, and laser vibrometers. These detect the necessary data regarding traffic, temperature, humidity, crack propagation, corrosion etc. ^[2]

Non-destructive Testing

For non-destructive testing (NDT) during the scheduled inspection, the typical NDT procedures can be used, e.g. visual and acoustic inspection, electromagnetic and -chemical methods, and spectroscopy. The focus, however, should be on SHM, as it is highly effective and more important to detect the damages early.

Literature

1. DIN Deutsches Institut für Normung e.V. Ingenieurbauwerke im Zuge von Straßen und Wegen: Überwachung und Prüfung;93.010(DIN1076). Berlin: Beuth Verlag GmbH; 1999.
2. Schnellenbach-Held M, Peeters M, Miedzinski G. Intelligente Brücke - Schädigungsrelevante Einwirkungen und Schädigungspotenziale von Brückenbauwerken aus Beton. Bergisch Gladbach; 2015.
3. Florian Dier. Zur zuverlässigkeitsbasierten Bauwerksprüfung unter Berücksichtigung von Spannstahlausfällen infolge von Spannungsrisskorrosion. Dissertation. München; 2012.

4. Kessler S, Thiel C, Grosse CU, Gehlen C. Effect of freeze–thaw damage on chloride ingress into concrete. Materials and Structures 2016;2017(50).

5. Pan JW, Feng YT, Wang JT, Sun QC, Zhang CH, Owen DRJ. Modeling of alkali-silica reaction in concrete: a review. Frontiers of Structural and Civil Engineering 2012;6(1):1–18.

6. Jan Lingemann. Zum Ankündigungsverhalten von älteren Brückenbauwerken bei Spannstahlausfällen infolge von Spannungsrisskorrosion. Dissertation. München; 2009.

7. Bundesministerium für Verkehr, Bau und Stadtentwicklung. Richtlinien für die Erhaltung von Ingenieurbauen (RI-ERH-ING): Leitfaden Objektbezogene Schadensanalyse (OSA).

8. Bundesministerium für Verkehr und digitale Infrastruktur. Richtlinien für die Erhaltung von Ingenieurbauten (RI-ERH-ING): Richtlinie zur einheitlichen Erfassung, Bewertung, Aufzeichnung und Auswertung von Ergebnissen der Bauwerksprüfungen nach DIN 1076 (RI-EBW-PRÜF); 2017.

9. Bundesministerium für Verkehr und digitale Infrastruktur. Schadensbeispiele nach RI-EBW-PRÜF 2017; 2017.