Historic Building Restoration & Rehabilitation
MATCO Services can offer assistance to you as corrosion consultants in the restoration of historic buildings and structures. In order to accomplish this, we first require an on-site condition evaluation and survey. It is necessary to visually observe and make measurements of the corrosion activity, geometry, concrete and steel materials, associated construction materials, etc., in addition to the corrosion which the building has already suffered. After these observations and measurements, and assessment of potential rehabilitation techniques we will be able to recommend to you what measures and materials, including the use of cathodic protection, to take to mitigate further corrosion deterioration of the building.
On-Site and Laboratory Corrosion Investigation
A detailed on-site condition evaluation and survey of the building is undertaken. The evaluation and survey will consist of implementation of several analysis techniques, including the following.
Visual examination – to identify surface defects
Petrography – to determine concrete condition
Chloride content – to identify chloride corrosion
Half cell potential mapping – to determine corrosion risk (map corrosion hot areas)
Continuity – to determine continuity/connectedness of rebar
Stray current identification – to determine stray current corrosion risk
If possible, concrete core samples will be marked and retrieved for petrographic analysis. They will be taken in areas of rusting of the rebar reinforcement. The client is responsible for the coring of the sample.
After the on-site evaluation, laboratory analysis of petrographic and corrosion product samples is undertaken by an experienced petrographic specialist. Petrographic evaluation will determine aggregate type and size, depth of carbonation, air void content and mode of deterioration if present. These will determine integrity of concrete and the best means of rehabilitation and corrosion protection.
Based on the on-site and laboratory work, the extent of damage is determined. Critical questions to be answered here are the following.
Design of Suitable Corrosion Protection Measures
Preliminary technical assessment will be undertaken of a suitable repair system for the building including specification of a cathodic protection system. Cathodic protection (CP) is a method in which a sufficient amount of electric direct current (DC) is continuously supplied to a corroding metallic structure to mitigate, slow down or temporarily stop the natural corrosion processes. 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 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. The current output of this system is affected by factors such as:
Driving voltage difference between the anode and the cathode
Resistivity of the electrolyte (environment)
Natural or man made environmental chemistry and/or contaminants
The impressed current cathodic protection system includes four main components together constituting an electrical circuit. They are:
A controllable DC power source – usually a transformer rectifier
An applied anode – a material placed onto or into the concrete or surrounding electrolyte to enable current flow
An electrolyte – normally the pore water present within the concrete, or in the case
Remote anodes, also the water, soil or mud in which the anodes are placed
A return electrical path – normally the electrically continuous reinforcement steel to be protected
The CP transformer rectifier can be powered by external power sources, such as alternating current (AC). The CP rectifier converts the input power source into DC. DC is discharged from impressed current anodes made of metals such as steel, high silicon cast iron, graphite, platinum and titanium mixed metal oxide. The potential current output of an impressed current CP system is limited by factors such as available AC power, rectifier size, anode material, anode size and anode backfill material. The current output of an impressed current cathodic protection system is far greater than the current output of a galvanic anode cathodic protection system. However, higher maintenance during service is required and short circuiting of anode and rebar should be taken into consideration in design and implementation of this system.
It is important to determine the condition of the steel beams and current requirements for the design of a reliable cathodic protection system.
A 90 year old state capitol building, is undergoing a renovation and seismic upgrade in order to extend its life by another century. Concrete in the dome serves as a base for terra cotta panels. Spalling, corrosion, efflorescence, and other problems were occurring in the steel-reinforced concrete in interior portions of the dome, itself shrouded with an exterior copper roof.
MATCO's principal corrosion scientist was called upon to conduct an on-site corrosion survey and audit of the interior of the dome. The on-site investigation and preliminary laboratory evaluation revealed that visible internal cracking and steel reinforcement core (rebar) corrosion starts at a level just below where the external copper roof ends. The concrete is porous and carbonation is present, as confirmed by petrographic analysis. Chloride ions were found by energy dispersive x-ray (EDS) analysis in the corrosion products collected on site.
Electrical potential measurements showed the active corrosion of the rebar in many locations. However, continuity testing for the rebar was positive, meaning that a cathodic protection system is viable for this structure.
In the second phase of this project, electrochemical studies were performed by MATCO on the dome concrete and steel reinforcement cores to identify the most suitable cathodic protection technique for corrosion control of the renovated dome. Current requirements were also determined.
After the optimum cathodic protection technique, titanium mesh anodes, and current for the dome renovation was identified, MATCO developed and established the specifications for its installation and operation. Provision was made for a corrosion sensor or reference electrode to monitor corrosion.
MATCO has extensive experience as corrosion consultants for reinforced concrete structures. When approaching the problem of a deteriorating reinforced concrete structure, it is necessary to visually observe and make measurements of the corrosion activity, geometry, concrete and steel materials, associated construction materials, etc., in addition to the corrosion which the structure has already suffered. After such observations and measurements, assessment of potential rehabilitation techniques and, as required, corrosion analysis, MATCO can recommend measures and materials to consider which will help to mitigate further corrosion deterioration of the structure for its expected life using cathodic protection and/or coatings. The methodology employed is described below.
On-Site Condition Assessment
The on-site condition evaluation survey 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. Several analysis techniques are implemented during this on-site evaluation survey, including but not limited to the following.
- Visual examination - to identify surface defects
- Petrography - to determine concrete condition
- Hammer/chain- to detect delaminations
- Phenolphthalein - to determine carbonation depth
- Chloride content - to identify chloride corrosion
- Half cell potential mapping - to determine corrosion risk (map corrosion hot areas)
- Linear polarization - to determine corrosion rate
- Continuity - to determine continuity/connectedness of rebar
- Stray current identification - to determine stray current corrosion risk
- Resistivity - to determine concrete resistivity and corrosion risk
Concrete core samples are marked and retrieved for petrographic analysis. At least one will be from an area of rusting of the rebar reinforcement or concrete. At least one will be from an undamaged and uncorroded area of the concrete structure.
Laboratory Petrographic and Corrosion Analysis
Laboratory analysis of petrographic and corrosion product samples are conducted by experienced petrographic and corrosion specialists. Petrographic evaluation will determine aggregate type and size, depth of carbonation, air void content and mode of deterioration if present. These will determine integrity of concrete and the best means of rehabilitation and corrosion protection.
Determination of Extent of Damage and Remaining Service Life
Based upon on-site surveying and laboratory analysis results, 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.
Materials Selection and Coatings Application
Consideration is 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.
Assessment of Repair and Cathodic Protection Systems
Assessment is undertaken of suitable repair systems, and of suitable cathodic protection systems for the reinforced concrete structure. Cathodic protection (CP) is a method wherein a sufficient amount of electric or impressed current (DC) is continuously supplied to a submerged or buried metallic structure to mitigate, slow down or stop altogether the natural corrosion processes from occurring. This technique is routinely used in service of submerged units or structures in brine and seawater environments.
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 zinc (Zn) or aluminum (Al) all have a lower more negative electrical potential. The current output of this system is affected by factors such as the driving voltage difference between the anode and the cathode, the resistivity of the electrolyte (concrete), pH, and salinity.