The principles of maintenance of structures made of concrete

Lack of maintenance is not the only reason for concrete deterioration. Degradation can be caused by a number of factors, such as high water content in the mix, incorrect concrete composition design, errors in structural calculations, poor workmanship during construction, carbonation, corrosion of the reinforcement, or overloading of the structure.

Concrete structure repair involves restoring or replacing the concrete or the coating of the structure’s reinforcement after a sufficient amount of time has passed since the initial casting. These interventions are costly and require construction experience. It should be noted that the cost increases significantly when their quality is not satisfactory, resulting in their premature failure and the need to repeat them in a short period of time. Usually, the goal of a repair is to restore the service level of the structure and ensure that the repaired element or section will be maintained for the expected lifetime of the entire structure. At present, there are no standardised procedures for checking whether a repair is successful. In addition, after the scaffolding has been removed, it is practically impossible to carry out checks, and the project owner is usually left with the impression that the repair has been done well and that there are no failures or imminent failures. However, after a certain period of time (not necessarily long), local flaking begins or signs of corrosion of the reinforcement become apparent.

It is therefore clear that quality control (QC) and quality assurance (QA) procedures are required to ensure the reliability of the repair. Of course, QC and QA procedures alone do not guarantee reliability. The problem with repairs to concrete structures is that they often fail to meet the objectives set. Signs of repair failure include cracking of the repair material, rust stains, and detachment from the substrate. Cracks and detachments can occur even a few hours after the material has been applied, or/and after months, and can be attributed to poor workmanship, inadequate curing, strong winds, drying shrinkage, etc. Perhaps the most fundamental reason for the failure of concrete repairs (patching) shortly after application is that the underlying problem has not been properly addressed. A repair failure can also be considered when the repair material is not compatible with the texture and color of the old concrete.

Definition of maintenance

Maintenance could be defined as the process of preserving parts or components of a structure or restoring them in such a way that the structure retains its functionality. This process includes: Checks, Tests, Inspections, Adjustments or Alignments, Removal, Replacement, Reinstallation, Problem Detection/Investigation, Repairs, Modifications, Reconstruction, Remedies, etc. Although both concrete and repair materials can be highly durable, all of the above are necessary for both concrete and repair materials. Maintenance can be divided into two basic categories: Corrective maintenance:

Maintenance, which is required so that in the event of failure, the structure can be restored to its intended or desired level of functionality. It includes determining the cause of the failure, locating the problem (intervention site), removing, replacing, or repairing the component/member, and verifying the result.

Interventions for the maintenance of a structure must be planned and managed by technicians with sufficient experience, who will take into account the following factors: Age of the structure. Characteristics of the natural environment of the structure.

Use of aggressive substances in the structure. Previous maintenance work. Cost of interventions. Reliability requirements for restoration interventions. The human factor plays a decisive role in construction maintenance procedures. The maintenance program defines the methods and procedures for supporting the system (or structure) throughout its lifetime. It includes the specification and use of all materials required for the ongoing support of the system/structure.

In general, it should include: Description of the anticipated level of maintenance.

Those responsible for maintenance (manufacturer/producer – final beneficiary). Criteria for securing the various materials/components.

Factors affecting the effectiveness of interventions. Reliability requirements. The conditions under which maintenance will be performed. The main objective of a maintenance plan is to ensure the reliability of the repair/restoration work carried out on the structure. The term reliability is directly related to the following factors: The system (structure). The probabilities. Performance. The target level of functionality set in each case. The prevailing conditions. A specific period of time. It is therefore essential to identify the root causes of failure or malfunction of a structure, i.e., technical uncertainties and probabilities. Reliability assessment, as a process, consists of collecting (at intervals) data on the technical condition of the structure (input data) and determining the additional costs required to achieve a specific level of functionality of the structure. Quality control and assurance refer to a specific time, demonstrating compliance or non-compliance with specific requirements, the existence or non-existence of specific characteristics of the structure.

In order to incorporate these characteristics into a reliability assessment system, they must be correlated with factors on which reliability depends (linking quality control results with reliability). Unreliable systems or structures are almost certain to fail, and failure has a number of causes, such as: Normal wear and tear. Environmental factors. A defective component/part of the structure.

Low quality (in terms of one or more criteria). Faulty design. Reduced reliability entails additional costs: The cost of failure (damage, downtime, restoration). The cost of upgrading the reliability of the system/structure. General approach to assessing the current condition of the structure. Nowadays, many industrial sectors have established strategies for assessing the condition of their existing facilities and structures and extending their service life. The usual approach is a step-by-step assessment, starting with a general visual inspection and ending with a detailed analysis of structural reliability. The methodology is included in ISO 2394 and ISO 1382 standards. The methodology for collecting information, assessing existing load-bearing capacity through static analysis, and selecting measures for restoration and reinforcement constitute a decision-making process aimed at identifying the most effective investigations and modifications necessary to meet new requirements for the use of the structure and to remove any doubts regarding the condition and future performance of the structure.

Steps of the assessment

1.Reason for assessment
(Assessment initiator).
2.Review system, actions, and conditions.
3.Is assessment reasonable?
4.Refine the system, actions, and reactions based on the information from step (2) (Refine system, actions, and reactions).
5. Identification of uncertainties and/or targeted levels of safety (refine uncertainties and/or target safety levels).
6.Numerical assessment.
7.Laboratory assessment (Experimental assessment).
8.Demolish object.
9.Reduction of the level of environmental exposure (Reduction of exposure level).
10.Restoration.
11.Preservation.
12.Cost-benefit analysis Laboratory and computational assessment methods. It is recommended that the analysis begin at a basic (simplified) level and be scaled up. The usual computational estimates, from the simplest to the most complex, are as follows: Linear analysis and control of individual elements (in accordance with applicable regulations). Calculation of actions and reactions with greater accuracy (Refined). Linear elastic redundancy analysis. Nonlinear analysis and control of elements (Plastic analysis). Bayesian structural reliability analysis (Structural reliability analysis, using Bayesian event updating). Obviously, not all of these methods are suitable for all cases. Often, additional laboratory test data is required for higher-level analysis.

By securing more data, the target safety level, uncertainties in exposure conditions, reactions, and assumptions in the system analysis model can be weighted more accurately. Laboratory tests and checks are basically divided into the following sections:

Inspections

Simple visual inspections and measurements, often performed during maintenance procedures. Destructive testing/inspections DT, destructive techniques.

Non-destructive testing/NDT

Another classification can be made based on the characteristics of the structure being assessed: Resistance level, e.g., of materials. Exposure level, e.g., measurement of wind intensity or wave height.

Relationship between strength and level of exposure E.g. maximum loads of operation or failure. Reasons time and economic make necessary the good coordination between the analytical and laboratory methods of evaluation. In every case it is advisable to answer questions such as:

What is expected from laboratory tests/trials? Will the results obtained be useful for the intended analysis?

What data is required for the next step in the computational estimates? What are the most appropriate tests/checks for the intended purpose?

Do the expected results justify the cost of laboratory testing? The life cycle cost of an acceptable level of reliability can be achieved at a reasonable cost, but high reliability costs more. This issue is directly related to the life cycle cost of the system/structure. From this perspective, when a system has reduced reliability, it will be more expensive, precisely for this reason!

The increase in the design/construction costs of a structure is justified when it offsets costs that would arise from factors such as: Shortening the life span of the structure. High maintenance costs. The impact on the safety and health of users. The cost of restoration. Justifying the increased cost of implementation (cost effectiveness) requires an assessment of these factors. Consequently, the degree of reliability with which a structure must be implemented is related to the economic impact of a structure with lower reliability. To improve the reliability of a system, the root causes of failures must be identified, addressed, or mitigated. This process is called failure analysis, and its basic steps are as follows:

Identification of potential failures. Collection of data on these failures. Visual and physical inspection (using simple portable equipment).

Determination of failure. Laboratory tests