Predict Reinforced Concrete Structure Life Time (Part I)

One can find that the predicting the life time of the structure is very complicated subject. As after collecting all the required data about the strength and loads still the time of the structure failure is varies from structure to another depending on many factors.

According to many researchers are interesting to focus about the buildings life time as they found that there are some materials and structures have a longer life time than the other.
Throughout history, service-life predictions of structures, equipment, and other components were generally qualitative and empirical. The first step it is required to understand the causes of many degradation processes and identifying the reasons is the main step  for making quantitative predictions of the service life of concrete structures and its components. In addition to actual or potential structural failure, many other factors can govern the service life of a concrete structure, for example, excessive operating costs or major repair cost can lead to a structure's replacement.

In most text books usually used expression durability and service life as the terms "durability" and "service life" are often erroneously interchanged. The distinction between the two terms is evident when their definitions, as given in ASTM E 632, are compared and defined the durability as, its capability of maintaining the serviceability of a building, component, assembly, or construction over a specified time. Serviceability is viewed as the capacity of the above to perform the function(s) for which they are de­signed and constructed.

The definition of Service life  is defined by by three types of service life have been defined which are:

·         Technical service life is the time in which the building in service until shown  unacceptable state is reached, such as spalling of concrete, safety level below acceptable, or failure of elements.
·         Functional service life is the time in service until the structure no longer fulfills the functional requirements or becomes obsolete due to change in functional requirements, such as the needs for increase clearance, higher axle and wheel loads, or road widening in case of bridges.
·         Economic service life is the time in service until replacement of the structure (or part of it) is economically more advantageous than keeping it in service or repairing or strengthening it.

The service-life methodologies have been included  in the design stage of a structure-where certain parameters  are established and this is clear in EC2. The some of the parameters that affect by defining the service life of the structure as selection of water-cementitious materials ratios (w/c), concrete cover, and admixtures-and in the operation phase where inspection and maintenance strategies can be developed in support of life-cycle cost analyses.

According to  CEBIRILEM 1986 service-life design includes the architectural and structural design, selection and design of materials, maintenance plans, and quality assurance and quality control plans for a future structure. Based on mixture proportioning, including selection of concrete components, known material properties, expected service environment, structural detailing such as concrete cover, construction methods, the loading history, and the definition of end-of-life, the service life can be predicted and concrete with a reasonable assurance can achieve the design service life as discussed by Elreedy MA (2012).

There are many trials to develop a method to predict the service life of existing concrete structures. To predict the service life of existing concrete structures, information is required on the present condition of concrete, rates of degradation, past and future loading, and definition of the end-of-life as mention that Clifton (1991). Based on remaining life predictions, economic decisions which is usually the responsibility to the owner to take the decision  on whether or not a structure should be repaired, rehabilitated, or replaced.

It is worth to mention that, all decisions concerning the definition of end of-life are combined with human safety and economic considerations. In most cases, the condition, appearance, or capacity of a structure can be upgraded to an acceptable level; however, costs associated with the upgrade can be prohibitive.

The service life of new and existing concrete structures is influenced by measures taken during design and construction to resist degradation from imposed loads and environmental conditions (for example, the degree of durability). Durability brings the time element into the design of reinforced concrete structures and should be given equal importance to that given to strength. It is worth to mention that, design and construction currently consist of seven which are as follow:

1) Design loads and actions;

2) Performance criteria;

3) Factors of safety, or reliability;

4) Design and detailing;

5) Material specifications;

6) Workmanship and construction practices

7) Minimum levels of maintenance.

Provisions for durability in the past have primarily been addressed under Items 5 and 6. It is important to mention that the design for durability requires essentially an improved understanding of the degradation mechanisms, improved characterization of service environments, data on materials, the development of advanced models, and the development of standards and guidelines for the use of design methods and acceptance for durability predictions.

To predict the concrete structure life time, it needs a complete evaluation for the whole building by visual inspection, collecting data and provide some experimental test if needed.

The data which will be collect and evaluate will be in three directions which are the loads, the environmental condition surrounding the building and the structural strength.

When start to predict the life time of the structure it is very important to know the loads that have been affect the building from its construction until the time of evaluation and the expecting loads that affecting the structure for upcoming years.

The most important design parameter is the definition of structural loads. Minimum design loads and load combinations are prescribed by legally adopted building codes for example, ACI 318, or ASCE7. There is a balance between selection of a design to meet minimum loading conditions and the design responsibility is to provide a conservative design that results in higher initial price but can provide lower life-cycle cost.

If the design is go through another approach by provide the least cost building with minimum load mention in the codes but this can design approach will need a higher life cycle cost and this can be more susceptible to degradation than the more conservative design approach.

Predicting service life of new concrete , in most cases the selection of the concrete materials and design mix of the concrete based o the laboratory test and its expectation for its behavior after pouring on site. But this approach is based on the concrete is durable materials regardless the surrounded environmental condition and it has a service life according to the codes which in most cases around 25 years. Now a days due to enhanced the materials of concrete and there are a new materials developed in the market such as additives, silica fume , slag , fly ash and others so we have the opportunity to choose the materials additives that will enhance the properties of concrete in harsh environment and as a result of that the concrete can reach the required service life  without maintenance cost during its life time.

There are many service-life prediction methods focus on the effect of one degradation process. Experience, however, has shown that degradation results when one or more degradation processes are operative or if there is a combination between the environmental condition and the loads.

• Predictions based on experience-Semiquantitative predictions
• Comparison performance approach
• Accelerated testing
• Mathematical models-Mathematical

This combination effect complicates the prediction of the new concrete structures service-life prediction for both new concrete structures where environmental factors and loads may have not been well defined, and existing structures where the contribution to degradation by various influences is difficult to assess. The main factors that affect the service life of reinforced concrete structures include the presence of chlorides, carbonation, aggressive chemicals, such as acids and sulfates, freezing-and thawing cycling, and mechanical loads, such as fatigue, vibration, and local overloads. Typically, only one primary factor limits the service life and is the focus of service-life prediction. As limited information is available on the synergistic effect when more than one factor is operative.

Mohamed A. El-Reedy, Ph.D

www.elreedyman.com

Industrial Projects Charactristics

The development of any country depends on the rate of industrial growth. Currently, there is a race in industrial projects worldwide. The development of the industry depends on the development of the energy reserve by investment in projects of oil and gas exploration, onshore and offshore, which require new facilities or rehabilitation of existing facilities. At the same time, there are projects that are running in parallel to deliver electricity from electrical power stations or through nuclear power plants.

The term industrial structures means all the reinforced concrete and steel structures from a small factory to a nuclear plant. My book (www.amazon.com/author/elreedyma) provides  an overview of industrial project management, design, construction, and eventually providing a maintenance plan. Industrial projects, in most cases, are huge and can cost a billion dollars for one project, so the client, engineering firm, and contractor are in the same boat until they achieve project success through a strong management system and technical competence.

In these types of projects, all the engineering disciplines are working together, but, unfortunately, the structural or civil engineers are usually the last ones to obtain the exact data from the other disciplines and the first ones to start on site. Therefore, it is a challenge for the structural engineers to work fast and efficiently in this type of project. So it is required a strategy plan and professionalism in dealing with these projects.

It is important to regular familiar ourselves with up-to-date methodology and industry technical practice and guidelines to design, construct, and maintain the reinforced concrete and steel structures in these industrial projects. The other challenge that faces structural engineers is that most of the undergraduate courses they studied in college focused mainly on real estate projects and housing. However, the characteristics of industrial projects are different, as it needs from the project and construction manager to lead the project and to successfully achieve the owner’s requirements.

On the other hand, the structure engineer should be  familiar with the codes and standards that are usually used in industrial projects which is really few and is not famous rather normal building codes and standard and the most applicable methods used in the design of the steel and reinforced concrete structures that serve the static equipment, tanks and towers, and vibrating equipment.

Dr. Mohamed A. El-Reedy

www.elreedyman.com