Reinforced concrete (RC) structures are durable but not impervious to decay. Deterioration of RC structures is a worldwide problem due to its excessive cost, environmental impact, and safety issues.
According to Dr. Farhad Nabavi, senior technical director at concrete durability specialist Xypex, there are few reports of failure of RC structures due to lack of intrinsic strength in modern construction industries. However, even in developed nations with stringent building regulations, concrete structures can fail without warning.
- In May 1980, Berlin’s Congress Hall’s external roof collapsed due to steel corrosion within concrete, resulting in one death.
- A reinforced concrete gallery in Wormerveer, Netherlands, collapsed in 1990 due to carbonisation-induced corrosion. The gallery’s main reinforcement was situated in the lower stressed zone, instead of the upper zone where the cracks were formed, enabling the penetration of salt.
- In Melle, Belgium, a prestressed bridge developed gradually widening cracks under large loads, enabling the penetration of chloride and corrosion of prestressed cables. A railcar driver was killed when the bridge collapsed.
- Quebec, Canada’s Concorde Boulevard Bridge collapsed suddenly in 2006, killing five people and injuring six others. Chloride-induced corrosion of the reinforcing steel members was identified as the cause.
Apart from these structural failures, Nabavi noted, it’s more common for RC structures to fail to meet their expected service lives due to premature deterioration. Such deterioration is hastened by several mechanisms such as electrochemical processes, chemical reactions, and physical processes. These processes cause degradation of the steel reinforcement and/or the cement matrix, which can lead to steel corrosion, expansion, concrete cracking, concrete spalling, mass loss, strength reduction, and a high risk of structural failure and collapse.
Reinforced concrete structure durability should be a consideration during “all stages of their creation—design, construction and maintenance,” Nabavi added, because “prediction of durability gives a clearer view on extending service life.”
The methodology of analysis of deterioration processes includes evaluation of the structure, its classification regarding the need for rehabilitation, and the extent of rehabilitation measures.
Assessing the condition of concrete structures on a regular schedule is crucial to predicting and maximising their service life. Nabavi looks at environmental exposure conditions and the practical effects on the structure of those conditions.
“For example, if the structure is exposed to a marine environment, chloride could diffuse into the concrete,” Nabavi said.
Australia’s coastal cities, a marine environment, are home to the vast majority of the population and concrete structures, emphasising the need for timely assessments. Chloride attacks the steel reinforcement, resulting in corrosion that makes the steel brittle and shrinks its diameter, breaks the bond between steel and concrete, and cracks the surrounding concrete.
“The durability assessment of the concrete structures has become a challenge being confronted by both practical engineers and academic scholars,” Nabavi noted.
The need for accurate service-life predictions has driven an increase in the number of studies on the topic, which Nabavi calls “one of the major trends of concrete durability research.”
Problems remain, however.
“Although much progress has been made in the past decades in understanding the mechanisms of concrete deterioration, completely satisfactory and reliable predictions of the expected or remaining service lives of concrete structures are still a matter of dispute,” Nabavi noted.
Despite some disputes, service life prediction of concrete is recognized as an approach needed to minimize the cost of maintenance and repair of existing concrete structures. Furthermore, accurately and rapidly assessing the rate of durability of concrete structures has been a problem in the concrete industry.
“Maintenance and methodology of the repair of the reinforced concrete structures, as well as quality control, needs rapid and precise assessment of the structure,” Nabavi said.
To meet that need, the industry has developed rapid, non-destructive testing (NDT) techniques that assess the current condition of the structure. After visual assessment, Nabavi noted, non-destructive test methods are used to measure operating strength, compressive strength, and corrosion level.
“With these tests, we can estimate the residual service life of the structure,” he said.