Remaining Service Life of Existing Structures

When designing a structure, engineers take into account the desired life span. Most standards provide a service life of 50 years; however some infrastructures such as dams, bridges, and skyscrapers are designed for 100 years or longer. But in reality, estimating the remaining service life of the structure, or planning for the optimal time to conduct maintenance and repair work, requires a lot more than simple arithmetic. Due to the uncontrolled nature of environmental loads acting over a long period of time, a structure’s life-span could be reduced significantly. Examples of environmental loads are a sulfur-rich soil attacking the concrete deep foundation, or high concentrations of Chloride in the concrete mix leading to corrosion of rebars. Thus, time-dependent induced damages are underestimated and the designed service life is often overestimated.

Figure-1: A concrete bridge in dire need for repair

[endif]--A key aspect for prolonging the life of a structure is to enhance the durability of its concrete mixture. Portland Concrete Association (PCA) defines the durability of concrete as the ability to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Different structures require different degrees of durability depending on the exposure environment and properties desired. For example, concrete columns exposed to tidal seawater such in a jetty, will have different requirements than an indoor concrete slab. Many factors affect the durability of reinforced concrete structures. This may be either a mechanical load such in a load increase, or it may be an environmental load as for instance carbonation, chloride penetration, or frost damage. Here is list of environmental factors affecting the durability of concrete:

Freeze-Thaw Resistance
Chemical Resistance
Resistance to Sulfate Attack
Seawater Exposure
Chloride Resistance and Steel Corrosion
Resistance to Alkali-Silica Reaction (ASR)
Abrasion Resistance

In the past 10 – 15 years, many prediction models have been developed to estimate the remaining service life cycle of reinforced concrete structures. Some models focus on concrete elements exposed to chloride environment, others on carbonation penetration, and some on the level of corrosion of steel rebars. These models rely heavily on the initial and current material properties and configuration, found from field and laboratory experimental data. The following are some properties and parameters required to estimate the service life cycle:

Local Temperature and Humidity Cycles
Concrete Cover
Concrete Mixture Properties (Aggregates, Moisture Content, W/C Ratio, Etc.)
Depth of Carbonation
Rate of Carbonation
Chloride Penetration and Concentration
Concrete Compressive Strength
Diffusion Coefficient
Corrosion Initiation Time and Rate of Steel Rebars (Corrosion Rate)
Location, Type, and Width of Cracks

Keep in mind that an effective life-time management of buildings and civil infrastructures needs methods for verifying the compliance of their service-life with the design value. And in the case of non-compliance, methods are needed for predicting the time-evolution of degradation, thus enabling the owner to plan economical and efficient maintenance and repair actions.

Figure-2: The performance and economic service life of a concrete structure

References

F. H. Wittmann, T. Zhao, P. Zhang, and F. Jiang. “Service Life of Reinforced Concrete Structures Under Combined Mechanical And Environmental Loads”. 2nd International Symposium on Service Life Design for Infrastructure 4-6 October 2010, Delft, The Netherlands.
Tiziano Teruzzi. “Estimating the service-life of concrete structures subjected to carbonation on the basis of the air permeability of the concrete cover”. University of Applied Sciences and Arts of Southern Switzerland, Switzerland.
Estimating Residual Service Life of Deteriorated Reinforced Concrete Structures - Scientific Figure on ResearchGate. https://www.researchgate.net/flow-chart-for-estimating-residual-service-life-of-structures_fig6_275515428 [viewed on August 29, 2018].
Portland Cement Association. “Durability.” https://www.cement.org/learn/concrete-technology/durability [viewed on September 17, 2018].
Figure 1: concordia.ca
Figure 2: researchgate.net