Evaluation and repair of old concrete structures
Evaluation and repair of old concrete structures is an essential job to do. Today, many bridges and other civil engineering structures are approaching a century in age. When a structure ages, it needs to be evaluated in order to make sure about its safety. Some parameters that fuel the need of evaluation are the possibility of change of use, increased stresses or different load paths. Additionally, in some cases the structure need restoration because of deterioration of reinforcement or concrete over time. The original quality of the materials is going to decrease during the time.
Evaluation of existing concrete structures
Some standards, such as ACI 562 (ACI, 2016), provide guidelines to evaluate an existing concrete structure. However this document has not yet been a part of the building code but it is a useful guide for professionals to evaluate existing and historic reinforced concrete structures. the evaluation process must be planned based on several factors such as the extent of deteriorated conditions, and the impact of changes to the load path.
Evaluation of a structure generally includes sampling and visual inspection. Visual observation techniques alongside mechanical sounding can identify locations of underlying shallow concrete distress, such as corrosion of the underlying reinforcement. Concrete cores are usually obtained and tested in compression. ACI 562 gives information about minimum values of concrete compressive strengths, minimum tensile yield and ultimate strength of reinforcing bars.
For historic concrete structures several non-destructive tests can be applied to identify potentially problematic conditions. This includes carbonation testing, chloride testing, and petrographic analysis. Carbonation testing can be used to assess the potential for accelerated corrosion of the reinforcement. Carbonation is when concrete reacts with carbon dioxide in the presence of moisture . This process occurs slowly.Petrographic analysis is another approach to identify many types of concrete material distress including alkali-aggregate reactions and sulfate attack.
Common problems of aged concrete structures
The most common issues in concrete construction can fuel the long-term issues caused by aging. Some of them are listed below:
- Corrosion of the reinforcement
- Use of inappropriate concrete mixture
- Lack of sufficient vibration in placing the concrete
- Excessive use of construction joints
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