Life-Cycle Management of Concrete Structures

    Hiroshi Yokota, Professor, Faculty of Engineering, Hokkaido University, Sapporo, Japan

    A concrete structure is required to maintain its functions and performance during its design service life. However, serious damages have been sometimes found, which may be caused by physical and chemical actions. Such damages lead to structural performance degradation, and even structural collapse. Damages are attributed to insufficient durability design and/or by lack of sufficient maintenance work after construction of the structure.

    Life Cycle Management

    The service life of a structure is the result of planning, basic and detailed design, execution including material selection, production and construction, maintenance, and decommissioning stages. The life-cycle management (LCM) is the overall strategy with the aim of ensuring that the structure meets the associated performance requirements defined at the time of design and may be subsequently modified during the service life of the structure. LCM also contributes to realize a sustainable society through structures. Sustainability is defined in terms of environmental, economical, and social aspects. During the life-cycle of structures, sustainability is generally considered with one or a few sustainability indicators. The article briefly introduces the concept of LCM focusing on ensuring structural performance and sustainability of concrete structure.

    Concept of Life-Cycle Management

    A concrete structure passes through different stages during its life, which implies that it is essential to coordinate the stages and to transfer important information from one stage to another in an appropriate form. LCM is an integrated concept to assist in activities managing the total life-cycle of structure based on management of each stage to ensure structural function and performance and to achieve sustainability. In other words, LCM can provide an overall strategy to be used in ensuring that a structure meets the associated performance requirements defined at the time of design or as may be subsequently modified. The overall framework of LCM is presented in Fig. 1.

    For a new structure, an LCM scenario should be formulated during or after the planning stage of the structure. The scenario includes the fundamental strategy on how the structure will be managed in terms of structural performance and sustainability aspects. The structure is generally designed to keep its structural function and performance without major remedial measures; however, planned remedial measures can be included in the scenario if they are required. The scenario mediates between the stages of the structural life-cycle.

    Design will be carried out to satisfy the scenario initially formulated. When the design outputs do not satisfy the scenario, either the scenario is modified to be consistent with design outputs and/or design is carried out again. After the execution, initial assessment is carried out to check the conditions of the structure. When any defect is found from the assessment, remedial measures should be taken as required. Then, it will be judged whether the scenario is suitable for the subsequent life-cycle of the structure or not. When the scenario is found to be unsuitable, the scenario should be updated.

    During the use stage, the structure is periodically assessed for its condition and performance, and the above procedure should be repeated. When the scenario was updated, the updated scenario should be reflected on subsequent management. If it is judged that remedial measures should not be taken from the sustainability evaluation mentioned later, the structure goes to the life-of-end stage. For an existing structure, the assessment should be carried out before starting the LCM procedure. The scenario is created according to the result of the first assessment. When the assessment results conclude that remedial measures are difficult to take to recover structural performance, the structure goes to the life-of-end stage; otherwise, the same procedure as that for a new structure can be followed.

    Life Cycle Management

    The LCM scenario has to be evaluated with the PDCA (plan-do-check-act) process. Figure 2 shows the conceptual framework of the PDCA process. It is considered that well planned and organized inspection and evaluation at the use stage are necessary to enhance the maintenance cycle. For the maintenance procedure, it is important to identify the mismatch or differences of design assumption and real situation (Check), which may be applied for improvements on future maintenance, design and/or execution methods (Action).

    Accordingly, for the LCM cycle, it is important that information from the maintenance cycle is fed back into the design and execution stages. When inspection data as well as technical knowledge and experience are accumulated, the more realistic optimum scenario can be created.

    Ensuring Structural Performance

    Life Cycle Management
    Figure 3: Flow of sustainability evaluation
    The fundamental concept of how structural performance should be ensured has to be well considered in the design stage based on conditions, design service life, structural characteristics, material properties, difficulties in assessment and remedial measures, social and economic importance, etc. The durability design is a process to predict the performance degradation during the life-cycle of the structure. For a concrete structure, corrosion of steel rebar embedded in concrete is a principal cause of performance degradation to be considered at the design stage. Prediction at the design stage is done based on theoretical models or specific values determined by investigating existing structures or experimental findings. Otherwise, corrosion protection has been generally applied as an avoidance of deterioration approach, such as surface coating and cathodic protection.

    Serious deterioration may be caused by insufficient durability design with optimistic assumptions against materials’ deterioration and by lack of proper maintenance after construction of the concrete structure. Based on the performance-based design concept, it is necessary to provide the methods to ensure the structural performance requirements over the minimum limits during the design and maintenance stages. The output from the durability design has to be verified with the maintenance work because progress of deterioration would not follow the design assumptions. The progress of deterioration differs widely by its location because of inhomogeneous characteristics of materials and diversity of environmental conditions. By using the real inspection data, the scenario should be updated as mentioned earlier.

    Information accumulated at each stage of planning, design, execution, use, and end of life in the life-cycle of a structure should be transferred within and across the stages. For realizing appropriate LCM of each stage, transferring and sharing the information include basic philosophy of implementation of LCM as well as basic data at each stage.

    Sustainability Considerations

    Sustainability is defined as a concept based on the environmental, economic, and social aspects, and is one of the key issues in the construction sector to be well considered in the 21st century. During the life-cycle of a concrete structure, sustainability is generally considered with one or a few of these viewpoints, but it is not so easy to find the best solution among alternatives because no comprehensive sustainability indicator exists. For example, when the margin of safety (safety redundancy) is taken more, more resources and energy may be needed for construction and higher construction cost will be the consequences. In other words, a sufficient balance among each sustainability indicator should be achieved.

    It is easy to understand that structural collapse impairs the sustainability because the treatment of debris produced by destruction of infrastructure needs huge energy, and reconstruction of infrastructure and buildings requires an additional amount of resources and energy. In terms of social sustainability, many people were killed or injured, and employment and production bases were temporally unavailable. In terms of economic sustainability, structures were destroyed, which required a huge amount of reconstruction costs. In terms of environmental sustainability, nature and environments were destroyed. Engineers must keep it in mind what might happen by phenomena that is not covered by the design. Thus, the safety margin or safety redundancy that represents the resistance of structure directly links the local sustainability.

    Figure 3 shows the flow of sustainability evaluation for the structure at the initial design stage and repair design stage. Not only structural performance but also sustainability indicators should be verified in the sustainability evaluation. The structure factor gi, is a kind of an overall safety margin that represents safety redundancy. Durability is directly related to structural performance such as safety and serviceability, while resilience and robustness are related to the safety margin and the mechanism of failure.

    concrete Bridge

    The sustainability evaluation is systemized to consider in a comprehensive manner: safety and serviceability under the social aspect, cost under the economic aspect, and resources and energy under the environmental aspect. The system allows for designers to find a good balance between social, economic and environmental indicators. Even when it is difficult to set and quantify indicators for qualities such as adaptability, comport, cultural values, and social contribution, these should be considered by a social-scientific manner. The appropriate scenario should be selected from among multiple scenarios set at each stage of the life-cycle of the structure, in consideration of the balance among sustainability indicators.

    Conclusion

    A concrete structure inherently has a long life when it is well designed, executed, and maintained. It can achieve longer life with the proper LCM system, which will result in realizing sustainability.

    NBM&CW November 2018

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