High rise building failures and total or partial collapse of condominium complexes is a nightmare that hunts the construction industry. The loss of life, the financial devastation and anxiety experienced by the architects and tenants in nearby buildings makes sleepless nights. In this article we will discuss and address the aging high rises and the corrosion risk assessment in corrosive environments. How to determine if you have corrosion risks and low-cost engineering solution if the risks are captured on timely basis: fraction of that of replacement
Routine corrosion risk assessments performed by NACE certified corrosion experts are an invaluable asset to owners of aging concrete structures. Not only do they identify corrosion risk, but they can also quantify the severity of corrosion and offer remaining life estimates and corrosion mitigation strategies to prolong the life of these structures at much lower cost than total replacement. Based on years of our experience current codes for construction and inspection are not adequate from a corrosion engineering perspective since they do not consider qualitative corrosion risk assessments. The time period between major inspections and certifications should be based on the corrosivity of the environment and results from previous inspections.
Identifying corrosion is only the beginning, determining its severity and immediately responding can prevent disasters and save hundreds, or even thousands of lives. Recent collapse of Miami Surfside Champlain Towers South Condominium is a tragic example of what could happen if corrosion risks and quantification of risks is not considered
Following photo shows a core sample that was taken from a building that has been in service for many decades. Corroded rebar can be seen in the figure. The top piece of rebar has lost nearly half of its thickness. The next photo shows a close-up of the rebar. You can see the flat, thinned section of the rebar at the top.
Core sample from historic US building with corroded rebar.
Rebar corrodes in the presence of chlorides, which are common in sea water and de-icing salts. Corrosion of rebar can cause reduction in cross-sectional area as seen in the photos. This loss of thickness can weaken the rebar and cause collapse. Another way that corrosion of rebar can cause failure is through delamination. A delaminated piece of concrete can be seen in the following photo. Delamination is caused by the formation of iron oxide around the rebar which causes the concrete to expand. This eventually leads to cracking or sheets of concrete breaking away from the rebar. You can see where the iron oxide was building up at the brown/black line.
Close-up of the rebar inside core sample. Red Arrow indicates the thinned area of rebar.
Corrosion and Delamination Due to exposure to corrosive water
Corrosion mapping should be performed for condominium tower certifications in order to look at the thickness of the concrete around the rebars and load bearing members. Is especially true for buildings on the sea front which are at increased risk of such corrosion due to their proximity to the salt in the ocean.
This was not a one-night event or a sudden failure. Rather, it was a long time in the making because of the accelerated corrosion and loss of thickness. Once the thickness loss gets critical, the failure is catastrophic.
It seems rather apparent that current building regulations in Miami-Dade, as well as throughout Florida, do not adequately address the serious structural concerns that this tragedy has brought to the forefront. Matergenics’ and Dr. Zee’s specific inspection and condition assessment recommendations for nearby buildings of similar age are given here.
The on-site inspection and condition assessment includes a detailed investigation of all areas of the reinforced concrete structure and may require several hours to several days, depending upon the size of the structure. The inspection and condition assessment of the aging structures in ISO 12944-2: 2017 C5 coastal marine environments such as found in this area should include the following:
Concrete core samples should be retrieved from corroded areas (identified in corrosion mapping) for petrographic analysis to determine if the concrete is structurally sound or requires repair or replacement.
Based upon on-site surveying and laboratory analysis result, and employing sound materials and structural engineering principles, determination of extent of damage and remaining service life are undertaken. Critical questions to be answered here are the following.
Consideration should be given to materials selection for repair and/or replacement of the components of the reinforced concrete structure. In addition to concrete repair materials, this will include alternative materials to non-concrete auxiliary materials and maintenance coatings which may be applied to mitigate corrosion.
Specifically, it is imperative that future regulations require that both corrosion engineering experts, in addition to structural engineers, take part in building inspections to provide detailed reporting and an exact quantification of corrosion risk in their condition assessments. Condominium associations are also now on notice that they need to take immediate action with respect to corrosion assessment.
If accelerated corrosion is captured early on coating application and cathodic protection can mitigate the accelerated corrosion with low costs
Consideration should be given to coatings for repair prior to replacement of the components of the reinforced concrete structure. In addition to concrete repair materials, this will include alternative materials to non-concrete auxiliary materials and maintenance coatings which may be applied to mitigate corrosion.
Cathodic protection is a method wherein a sufficient amount of electric DC current is continuously supplied to a submerged or buried metallic structure to mitigate, slow down or temporarily stop the natural corrosion processes from occurring. The DC current corrodes a sacrificial anode when it is connected to a structure to be protected. There are two methods for supplying DC to cathodically protect a structure. They are the following:
The galvanic anode cathodic protection system generates DC as a result of the natural electrical potential difference (electrochemical reaction) between the metal to be protected (cathode) and another metal to be sacrificed (anode). The sacrificing metals such as magnesium (Mg), zinc (Zn) or aluminum (Al) all have a lower more negative electrical potential with respect to carbon steel reinforcement. The current output of this system is affected by factors such as:
The impressed current cathodic protection system comprises four main components which together constitute an electrical circuit. They are as follows:
Impressed current cathodic protection systems employ the use of electrically forced galvanic reactions to protect steel in an electrolyte (such as masonry). Anodes are installed in the masonry in strategic locations near the steel to be protected. A cathodic protection rectifier applies a DC voltage to the system with the positive lead connected to the anodes and the negative lead connected to the structural steel (cathode). Appendix B presents a more detailed mechanism and specification for impressed current cathodic protection. Appendix C presents two important case studies describing the application of impressed current cathodic protection to historical buildings, which is based on a pervious publication by the authors.
Please review case histories for corrosion risk assessment of aging buildings and corrosion mitigation in the following reference published by Dr, Zee, our expert in condition assessment.
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