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Some Aspects of Investigation and Diagnosis of Reinforced Concrete Structures Placed in Aggressive Environments
Dr. Ioan PEPENAR
National Building Research Institute -INCERC, Bucharest, Romania
During the last decades, many reinforced concrete structures, especially the ones sheltering technological processes of a chemical nature, have suffered, significant deteriorations, sometimes severe ones, after a relatively short service time, as consequence of the action of aggressive environments.
The appearance and fast evolution of the damages generated by corrosion have imposed the necessity to determine their implications on the durability and service safety of the structure, in order to take the necessary measures of intervention.
The practice of a systematic investigation of the structure placed in aggressive environments permits both the detection of dangerous situations of damage, and the decision to undertake immediate measures of intervention with the aim to prevent casualities, important material losses, technological accidents or undesired ecological effects, as well as to stopp accelerated deterioration processes on the members/structures.
When making a diagnosis concerning intervention measures for the rehabilitation of the structures exposed to aggressive environments, one must consider not only the effect on the building materials (concrete, reinforcement), but also the on the broader issues of design and execution of intervention works for different categories of structures. The following concepts in this respect are worth mentioning:
- the construction has to be regarded and analysed as a whole, taking into account its initial design, as well as the evolution of its condition up to the moment of intervention;
- the causes of damage due to corrosion are multiple and have to be correctly and completely identified; one must also consider causes other than corrosion which damaged the structure, as well as the permanent danger of seismic actions;
- the damage generated by the action of aggressive environments occur, as a rule, on limited parts of the elements/construction, but the intervention solution must concern the construction as a whole; some local damages induced by corrosion may lead to altering the mechanisms of load distribution considered in the initial design;
- the investigation/diagnosis and design activities are mutually interconnected and intrinsically interdependent.
The investigation on the state of the reinforced concrete members/structures placed in aggressive environments must comprise the following main steps:
The aim of the analysis of the technical documentation is to know the biography of the structure/members, concerning: the description of the structure/members; design data; execution data; information about the service life of the structure (including the intervention measures to which it was subjected). This data is provided by the design engineer and the owner/user of the construction, is included in the materials specifications, quality certificates and the documents concluded between the design engineer, builder and owner/user, and is included in the Technical Book of the construction.
- the analysis of the technical documentation of the construction;
- the appreciation of the service conditions of the construction;
- the actual investigation of the damage state of the members/structure: directly on the members (in situ) or on samples taken from the members (in laboratory).
The service conditions of the construction are estimated by taking into account the characteristics of the aggressive environment (indoor and outdoor), the technological process going on inside the construction and the function of the technological installations, including the exhaust equipment (ventilation, etc.).
The investigation of the damage state of the members/structure in service are made by means of the following investigation methods:
The visual examination of the state of the structure/members, and of the anticorrosive protection applied on their surfaces, includes: the appreciation of aspect changes of the concrete and the (apparent) reinforcement surfaces; the presence of defects/degradation due to corrosion; the evaluation of the changes in condition of the anticorrosive protection. The results of the visual examination are registered in the form of surveys of defects/degradations on the lay-outs and sections of the construction, with details on the investigated members, indicating their position and the extent of the damage, in order to provide their identification on the structure. With this purpose, for each type of defect or damage, will be adopted a conventional mark. Any time possible, pictures of the defects or damages will be taken.
- the visual examination;
- the use of non-destructive testing methods;
- the use of methods that require the taking of samples, but which do not endanger the service safety of the members/structure.
Non-destructive methods of investigation are used for the control of the size of the members, the establishment of certain characteristics of the concrete (strength, density, module of elasticity, a.o.), verifying the position of the reinforcement and/or the presence of metallic pieces, embedded in the concrete, detection of hidden defects in the concrete or the reinforcement (segregations, cracks, etc.), and for determining the condition and risk of corrosion of the reinforcement embedded into concrete (by electrochemical methods).
The methods of investigation using concrete samples - under condition that they do not endanger the service safety of the members - include the measurement of the concrete cover, the appreciation of the depth of the concrete that suffered changes under the action of corrosive agents, the examination of the state of the reinforcement, the determination of the type of damage induced by corrosion, as well as for performing laboratory tests. Under circumstances of extreme necessity (for instance in presence of stress corrosion cracking), samples are taken of the reinforcement itself, provided that measures to protect the integrity and the service safety of the investigated member/construction are taken.
The tests and determinations performed in laboratory concern the physico-chemical, electro-chemical and physico-mechanical tests, performed on the concrete and reinforcement samples taken off from the structures and consist mainly of : the measurement of the mechanical characteristics, the humidity, the water absorption and the porosity of the concrete; the determination of the alkalinity of the concrete (pH of the aqueous solution) and the concentration of corrosive ions (Cl-, SO42-, a.o) in the concrete mass; measurement of the mechanical and physico-chemical characteristics of the reinforcement steel (composition, metallographic examination, a.o.).
The corrosion state of the reinforcement steel is evaluated in accordance with the following main parameters: the characteristic of corrosion (general, uniform, local, pitting, stress corrosion cracking etc); the nature, structure and thickness of corrosion products; the physico-mechanical characteristics of the reinforcement; the structure of the steel. In the case of diagnosing general and local corrosion phenomena of the reinforcement steel, the use of a conventional corrosion degree estimation system is recommended.
We draw attention to the phenomenon of stress corrosion cracking of high strength steels, in which case complex investigations become necessary, with the aim to determine the chemical composition, the purity, the microstructure of the steel, as well as the presence of intergranullar/transgranullar cracks, of defects or local modifications of structure, etc. Such investigations are performed by specialized laboratories, using specific metallographic analysis methods, X -rays diffraction, electronic diffraction, electro-chemical and acoustic methods, etc.
The diagnosis is the result of investigation (the complexity of which depends on the degree of damage to the members/structure) and represents the basis for adopting intervention measures.
The diagnosis concerns first evaluation of the damage state induced by corrosion, specific to the analysed environments, and second, the initial defects and the degradations induced by other causes (earthquakes, fire, shocks, landslopes, etc.).
The main steps which lead to the establishment of a diagnosis are:
All the information above concerning the investigation, analysis and diagnosis will be centralized in a written report drawn up by the specialists/experts who performed these activities.
- The preliminary examination of the construction and its project, consisting mainly of the visual analysis of the bearing members of the construction and the engineer evaluation of their state in the context of the knowledge of their general structural construction. As result of the preliminary examination conclusions can be drawn up for temporary measures, in order to prevent imminent or potential accidents/breakdowns.
- The detailed investigation by means of measurements and both "in situ" and laboratory tests, aimed to point out: (initial) defects and damages, other than the ones induced by corrosion, with an explanation of their causes; defects of the technological installations, which are one of the major causes of corrosion induced degradations; other defects, on ventilation, exhaust and waste water sewage equipment, waterproofing, a.o.
- Data processing and analysis of the safety degree of the construction by interpreting the data, estimation of loads on the service time of the construction, a.o.
- The establishment of the diagnosis and proposals concerning intervention measures.
4. Intervention measures
The intervention measures on the bearing structure of the construction are proposed in correlation to the nature and degree of the damage, presented by the members of the investigated construction:
Taking into account the particular character of the investigation, diagnosis and of some intervention measures, it is recommended that these activities would be performed by pluri-disciplinary teams of corrosion and bearing structures specialists or with specialists in other fields (according to the case).
- for damages that do not affect the safety and the stability of the construction, the measures consist of interventions for repairing/maintaining the anticorrosive protection;
- for damages which affect the durability of certain members, without imminently endangering the resistance and stability of the construction, the measures consist of interventions for repairing the members and providing the necessary anticorrosive protection;
- for damages which may dangerously affect the resistance and stability, the measures consist either on entirely or partially disaffecting and demolishing the construction or in interventions for repairing/strengthening its bearing structure and providing subsequently the necessary anticorrosive protection measures.
Based of the research results, and the author's 30 years of experience in this field, national technical regulations stipulating the conditions and the ability to perform investigation and diagnosis on reinforced concrete structures placed in aggressive environments have been achieved.