Extending Service Life of Reinforced Concrete Structures

10.31.16   Hayder H. Al-Hilal, PE, SMSI, CIP-1, CCI-2, Level 1 Thermographer, ICRI CSMT | More by this Author

Extending Service Life of Reinforced Concrete Structures

It is human nature to delay a maintenance expenditure until a situation becomes urgent, but informed owners know they can save over the long term and avoid years of frustration by planning ahead. Regardless of how well designed and constructed, concrete structures must be occasionally and properly assessed and maintained to extend their service life and reduce the need for costly major repairs. Understanding the most common causes of deterioration, the phases of service life, and the kinds of repairs and typical costs associated with them is important for making sound and cost effective maintenance and repair decisions.

Deterioration – What’s the Biggest Contributor?
In the northern United States, corrosion of embedded metal in concrete, primarily reinforcing steel, is the leading cause of concrete deterioration, especially for exterior concrete structures such as parking decks and bridges. These structures are punished by year-round exposure to the elements combined with the corrosive impacts of acid rain and de-icing salts, freeze-thaw cycles, occasionally poor construction and design flaws, and deferred maintenance.

Deterioration of reinforced concrete resulting from corrosion of embedded reinforcement.

 Without proper maintenance, corrosion is inevitable. However, there are practical ways to delay it and mitigate its effects.

How does corrosion affect service life of reinforced concrete structures? Can one predict remaining service life, and what can be done to extend it? Tuutti’s (1982) and Song’s (2007) concrete service life models address these questions and can be used to predict concrete service life based on degree of steel corrosion, and clearly illustrate the benefit of active maintenance and timely repairs.

The Phases of Concrete Service Life – Tuutti’s and Song’s Models

In Tuutti’s model, the service life of reinforced concrete structures is divided into two phases: initiation and propagation. The initiation phase is the span of time it takes for reinforcing steel corrosion to initiate. The rate of carbonation and chloride migration through concrete determines the length of the initiation phase. During this phase there is a loss of the passive layer that covers and protects embedded reinforcing steel by greatly reducing the rate of corrosion to a very small amount, which occurs when carbonation reaches the reinforcing steel surface.

Once the passive layer breaks down as a result of the pH lowering effect of carbonation, corrosion of embedded reinforcing steel will occur more readily especially in the presence of chloride ions (chloride-induced corrosion), leading to the propagation phase. Chloride-induced corrosion occurs when the value of chloride ion concentration at the reinforcing steel surface exceeds a certain amount known as the critical threshold level (CTL). Proper design, construction, and maintenance of reinforced concrete structures that consider minimizing the effects of carbonation and chloride migration will significantly lengthen the initiation phase. During the propagation phase, accelerated corrosion and volume expansion (rust accumulation) occurs, leading to high concrete tensile stresses that can crack and delaminate surrounding concrete and cause spalls, i.e. detachment of concrete material. These conditions give way to the acceleration and deterioration phases later identified in Song’s model. The resulting delamination and spalling tends to impair concrete serviceability and if left unrepaired, can affect strength leading to structural failure.

Repairs Made During Initiation Phase vs Propagation, Acceleration, and Deterioration Phases

Repairs during the initiation phase typically consist of topical surface treatments, such as the application of a surface sealer and/or coating along with topical crack treatment. This differs from repairs during the propagation, acceleration, and deterioration phases, that typically involve concrete removal to expose and clean corroded reinforcement and replacement of carbonated, chloride-rich concrete.

The cost of topical surface treatment typically ranges from $2 to $7 per square foot of concrete surface area. The cost of repairs during the propagation, acceleration, and deterioration phases typically ranges from $25 to $200 plus per square foot. Therefore, it is clearly more economical to initiate preventive maintenance and minor repair activities during the initiation phase, before concrete life cycle reaches its propagation phase. The four phases of concrete service life with estimated maintenance costs are illustrated.

Illustration of initiation, propagation, acceleration, and deterioration phases for reinforced concrete service life (based on Tutti's and Song's models).

Extending the service life of reinforced concrete structures in a cost effective manner is possible when one understands the primary causes of deterioration, understands their influence on service life, and takes proactive maintenance and repair steps during reinforced concrete service life when repair costs are relatively low. Implementation of an active monitoring and maintenance program so that the condition of reinforced concrete elements can be reviewed at regular intervals, can minimize repair costs by detecting and addressing problems can be detected before they become expensive repairs and prospective safety issues.

If you are concerned about costly repairs to your reinforced concrete structures? Have more questions about how to alleviate problems? SME’s engineers have assessed and produced rehabilitation specifications for concrete structures worldwide. We can help!

For more information, contact Hayder Al-Hilal.

TAGS: Concrete

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