General considerations for conducting repairs on bridges
When bridges are being repaired, the goal is to restore the bridge to a state in which it meets the established performance requirements by repairing the damaged elements, repairing the concrete, restoring the passivation of the reinforcement, and preventing pathogenic agents from entering through protective barriers.
For this, an investigation and further diagnosis (see chapters 5 and 6) must have been done before establishing a repair strategy to assess the condition of the structure and identify the causes of deterioration. Whether a repair succeeds or fails is heavily influenced by how well it is designed, for which the right data is essential.
Moreover, after the repair, it may be necessary to apply protection treatments to ensure the durability of the repair and the whole structure.
On the other hand, it should be noted that as part of the design of the repair, some logistical considerations that have little connection to the technical aspects of the repair will need to be accounted for. These are such matters as determining whether it is possible to close the bridge traffic to conduct the operations and then foreseeing detour routes. Or determining whether or not it is possible to close the traffic and then establishing the phases of the repair and the necessary measures (traffic lights, signal operators, etc.) to keep the bridge partially open and not close it.
What are the general methods of repair?
The repair methods applied to a bridge should be carefully designed by experienced bridge repair engineers on a case-by-case basis. However, to give an overview of the different available options, the most common methods for bridge repair are described below. In addition, for a more in-depth study of this subject, it is recommended to read the ‘Guide about technologies of repair and reinforcement of concrete structures’, also published by ACRP.
Pretreatment of concrete
The main goal of pre-treatment of concrete surfaces is to remove the remnants of older coatings, dust, algae, striking agents, surface pollution, remove damaged, carbonated or aggressive media-contaminated surface layers of grains concrete and achieve a load-bearing concrete base for applying the repair systems.
Briefly, the pre-treatment of concrete includes the rough removal of larger layers, fine roughening of the surface, including the opening of the porous system of cement sealant, removal of corrosion products from the reinforcement and in the last phase the perfect removal of any dust from the concrete surface and reinforcement.
The removal of damaged surface layers must take place in such a way that the quality and condition of the steel reinforcement are not endangered and the concrete in the core of the structural elements is not unnecessarily threatened.
There are several ways to perform this removal of deteriorated concrete:
Pretreatment and cleaning of the reinforcement
The main goal of pre-treatment and cleaning steel reinforcement is to remove corroded or otherwise damaged parts. To remove rust from the reinforcement (or any other harmful elements that may be adhered to it), the concrete covering the reinforcement must first be removed to expose it.
Once the reinforcement has been exposed, the rust that covers it must be cleaned. This cleaning can be conducted by manual brushing or by mechanical means, such as water jetting, sandblasting or metal blasting. The bars should be cleaned all around their perimeter.
It is important to note that if the oxidation processes are very advanced, there may be a significant loss of section in the reinforcement. In this case, it is necessary to assess whether it is enough to clean the surface rust or whether it is appropriate to replace the reinforcement to restore it to its original quantity.
It is also important to evaluate the reasons that led the original reinforcement to such an advanced state of corrosion in order to, to the extent possible, prevent it from happening again in the new reinforcement.
Reinforcement repair and protection technologies
A. REINFORCEMENT CORROSION PROTECTION AND PASSIVATION DURING REPAIR
One of the defensive layers against corrosion attacks in aggressive environments is the treatment of reinforcement with the right materials and agents. Reinforcement can be treated in several ways. For example, epoxy coatings and rust converting coatings or cement-based grouts can be applied as anticorrosive treatments.
There are also mortars based on polymers dispersed in water, cement binders, and corrosion inhibitors that can be applied to reinforcing rods to prevent corrosion. These treatment methods aim to prevent the reaction between aggressive substances such as chloride ions and steel reinforcement. As a result, the time needed to reach such harmful materials to steel bars increases, and the structure becomes significantly more durable.
B. CATHODIC PROTECTION:
Corrosion is a result of an electrochemical reaction driven by a potential difference between two locations on the surface of a metal, an anode, and a cathode, which are connected by an electronic path and immersed in the same electrolyte. The anodic site discharges current into the electrolyte, and the cathode receives current from the electrolyte. Cathodic protection involves providing current flow onto the structure through the electrolyte sufficient to overcome the natural corrosion current discharge from the anodic sites, making the entire structure a cathode or net recipient
of electric current.
The design of cathodic protection must be carried out in accordance with the standard EN 12696 by a specialist engineer, based on basic information about the structure (surface to be protected, geometry, the current degree of corrosion, degree of exposure, the resistivity of the concrete, continuity of the reinforcement, etc.) and with the cathodic protection products that best adapt to the protection requirements (maximum current intensity per anode, estimated current density depending on the degree of corrosion existing in the structure, estimated average life, the existence of
electrical supply, vandalism, available budget, etc.).
Cathodic protection systems fall into two categories, impressed current and galvanic.
There are advantages and disadvantages to selecting galvanic CP or ICCP. Selecting the right method for each application depends on the type of structure to be protected, the environment, and the needs of the owner.
C. CORROSION INHIBITORS:
A surface-applied corrosion inhibitor (SACI) treatment consists of the application of a low-viscosity, colourless liquid with concentrated corrosion inhibitors to the concrete’s surface. This fluid migrates through the porosity of the concrete to the reinforcing steel, stopping corrosion from progressing there, thereby preventing the development of future damage.
The inhibitor migrates through the concrete capillary structure, first by liquid diffusion via the moisture that is normally present in concrete, then by its high vapour pressure, and finally by following hairlines and microcracks.
The diffusion process requires time to reach the rebar’s surface and to form a protective layer. It is crucial to determine whether corrosion is present and its level to properly dose and apply these preventative treatments. Corrosion of reinforced concrete is a chemical process that evolves over time.
Prior to causing visible damage to a structure, it undergoes an invisible process that can last from a few months to several years before the effects of the corrosion reaction become apparent.
Both new and old concrete can be treated with organic corrosion inhibitors. Surface-applied corrosion inhibitors can be applied prior to performing patch repairs with repair mortars or after the mortar has been placed. Migrating corrosion inhibitors can also be admixed into repair mortar to introduce the inhibitor directly into the patch.
