ABSTRACT

Concrete is the most widely used construction material and can be durable for hundreds of years; however, the largest cause of concrete deterioration is due to corrosion of reinforcing steel. Steel is thermodynamically unstable and will eventually oxidize to a lower energy state. The rate of deterioration in existing structures can be modeled to predict the service life of a given structure based on the service environment, concrete quality, chloride ingress, carbonation depth, corrosion rate measurements, and other factors to various degrees of success. Service life is defined as the remaining useful time for a structure based on the current deterioration rate without repairs. Through the assistance of commercialized software and models, the service life under the influence of corrosion can be estimated for maintenance to achieve the most economical solution. This paper will provide an overview of many of the service life models in use and industrial standards regarding structural deterioration.

INTRODUCTION

The durability of concrete has been studied for decades seeking for improvement from various approaches such as supplemental cementitious materials, alternate reinforcement, cathodic protection, life cycle costing and maintenance planning, and service life prediction models.[1] Corrosion of reinforcing steel is the major cause of deterioration.[2] In a service environment, carbonation or chloride ion diffusion will occur with time until corrosion initiates. To delay the corrosion initiation, admixed corrosion inhibitors can be added to postpone corrosion initiation, or maintenance such as surface sealers can be applied to form a hydrophobic layer and keep the moisture out of the concrete. However, there is no solution that can last forever. Proactive maintenance can reduce the overall cost of maintaining a structure. Once corrosion has initiated, prompt action can still be taken if the structure was monitored and maintained properly, based on the available budget. Field monitoring techniques are well covered in the literature to provide a timely alert to the structure owners, while a maintenance plan should be part of the design according to the materials selected and the service environment.[3-6] As maintenance is deferred, any repairs cost much more since the repair area increases as well as the severity of the corrosion. Therefore, the challenge has been determining the timeline of corrosion and how and when a maintenance or repair should take place for the most economical outcome. Service life prediction is to estimate the onset of corrosion and the rate of propagation depending on the service environment using various tools.[7] Before the aide of computer technology, service life prediction was deemed-to-satisfy due to prescriptive designs and specification being used by the industry. The traditional service life prediction is usually based on structure survey to obtain the condition rating of an existing structure.[8] The remaining service life is then estimated empirically by the subject matter expert or by following industrial standards.[9-11] In contrary, the emerging service life prediction tools utilize theoretical equations such as Fick’s law, probabilistic models, and other computer-based models, i.e., finite element analysis built upon theoretical assumptions or performance measurements such as lab and field testing data. Distinct from prescriptive approach, the new tools can integrate the information from performance testing and in-situ field monitoring, which provide a rational way to predict service life. In addition to the computer programs, data mining and machine learning for corrosion prediction have become popular techniques for service life prediction, but the challenges remain from noise or the inconsistent data.[12] In this paper, corrosion-related service life prediction models and standards are reviewed and discussed. There are many research papers using different modeling techniques and testing methods. For example, Sagues et al. published a prediction of existing structures using specialized software to calculate the diffusion coefficient from field cores and the solution to the error function to evaluate marine piles of two structures and validated later.[13, 14] However, to cover the general concepts utilized by the industry concisely, this work considers only the commercialized tools or software, or standards published with certain extent of recognition.

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