SURTREAT Structure Life Span Extension Calculation
Structure Life Span Extension Calculation Introduction
Ordinarily reinforced concrete structures derive about 70% of their strength from the embedded rebar. Corrosion reduces the amount of steel available to support a structure and is the leading cause of structure failure.
By stopping and preventing corrosion of the embedded reinforcement useful service life of any structure can be increased dramatically.
The following estimate (from 17 to 82 years) represents an existing structure life span extension calculation based on field studies over time.
This is a rebar specific calculation. Overall structure maintenance plays an important role in structure longevity.
By stopping and preventing corrosion of the embedded reinforcement useful service life of any structure can be increased dramatically.
The following estimate (from 17 to 82 years) represents an existing structure life span extension calculation based on field studies over time.
This is a rebar specific calculation. Overall structure maintenance plays an important role in structure longevity.
Structure Life Span Extension Data Presentation
The graph (above) describes an existing structure evaluation using a GalvaPulse (Germann Instruments, Inc.) measuring device. GlavaPulse uses a non-destructive polarization technique for evaluation of corrosion rates of the embedded reinforcement.
Structure Life Span Extension Calculation
It is generally assumed that a 25% loss in rebar steel would cause a structure to become unsafe for continued use. Unfortunately corrosion on rebar is not uniform so a critical rebar diameter condition can be reached at a much lower level of average diameter of steel loss. This is why it is important to look for these corrosion hot spots and to set a lower critical point for the average diameter loss. For example, in the bar graph shown above, a 28 data point corrosion current survey showed an average corrosion rate of 40 µM/cm² (corrosion current 3.5 µA/cm²). However, there were 6 measurement points averaging 100 µM/cm² (8.6 µA/cm²) or 2.5 times the average corrosion rate. With this in mind a K factor should be included in the equation calculating the time period for a present corrosion rate to reduce the rebar diameter to the critical point.
The loss of rebar diameter with respect to corrosion rate can be expressed by the following equation.
D= 2 x 10⁻³ x R(t) x K x T
Where:
D - Loss in rebar diameter in millimeters over time (t).
T - Number of years in time for loss in the measured rebar diameter.
R(t) - Average corrosion rate measured at time zero in terms of micrometers of steel lost per year as determined by multiplying the average corrosion current by 11.6 (Faraday’s Law).
K - Factor multiplying the average corrosion rate to account for the presence of corrosion hot spots.
If the average rebar diameter at time zero is 0.75 inches or 19 mm and the average corrosion current is 3.5 µA/cm² and the average hotspot current is 8.8 µA/cm² the time required for the loss of 15% ( a conservative point below the critical point) of rebar diameter can be calculated as follows.
3.5 µA/cm² = 40 µM/Yr.
K = 2 to account for the high hot spot average
D x 0.15 (15% loss) at future time = 2.85 mm
T = 2.85/2 x 10⁻³ ÷ (40 x 2) = 17.7 years
This assumes a starting point at time zero in the structures life, which is not the case. This can be corrected by actual inspection and measuring of the rebar diameter at the time of the corrosion rate measurement. Corrosion rate will vary with the ambient condition, particularly temperature, since it is an electro-chemical process. For example measurements made in the summer versus the winter could differ by a factor of 2, so some corrections should be made at the time of measurement for ambient conditions of temperature and moisture.
Once the life span, as determined by the condition of the rebar, is established the question of how to extend it can be answered.
Application of SURTREAT anodic type inorganic migratory corrosion inhibitor affects steel reinforced Portland cement concrete by combining chemically with the cement phase to:
In the example given, SURTREAT TPS II was applied to the examination area. After a sixty day period new results were obtained using the same GalvaPulse method (Germann Instruments, Inc.) following the identical grid pattern.
An examination of the corrosion rate bar graph for the after application measurements shows an average corrosion rate of 17.25 µM/Yr., and that the average of peak corrosion rates is in the same range, since the previous corrosion hot spots have now been pacified by the anodic inhibitor.
Using this average corrosion rate and a K factor of 1 the new life for the structure can be calculated as follows:
T = 2.85/2 x 10⁻³ ÷ 17.25 = 82 years
The forecasted structure life span as described by the rebar condition has been increased from 17 years to 82 years by the application of a migratory corrosion inhibitor.
The loss of rebar diameter with respect to corrosion rate can be expressed by the following equation.
D= 2 x 10⁻³ x R(t) x K x T
Where:
D - Loss in rebar diameter in millimeters over time (t).
T - Number of years in time for loss in the measured rebar diameter.
R(t) - Average corrosion rate measured at time zero in terms of micrometers of steel lost per year as determined by multiplying the average corrosion current by 11.6 (Faraday’s Law).
K - Factor multiplying the average corrosion rate to account for the presence of corrosion hot spots.
If the average rebar diameter at time zero is 0.75 inches or 19 mm and the average corrosion current is 3.5 µA/cm² and the average hotspot current is 8.8 µA/cm² the time required for the loss of 15% ( a conservative point below the critical point) of rebar diameter can be calculated as follows.
3.5 µA/cm² = 40 µM/Yr.
K = 2 to account for the high hot spot average
D x 0.15 (15% loss) at future time = 2.85 mm
T = 2.85/2 x 10⁻³ ÷ (40 x 2) = 17.7 years
This assumes a starting point at time zero in the structures life, which is not the case. This can be corrected by actual inspection and measuring of the rebar diameter at the time of the corrosion rate measurement. Corrosion rate will vary with the ambient condition, particularly temperature, since it is an electro-chemical process. For example measurements made in the summer versus the winter could differ by a factor of 2, so some corrections should be made at the time of measurement for ambient conditions of temperature and moisture.
Once the life span, as determined by the condition of the rebar, is established the question of how to extend it can be answered.
Application of SURTREAT anodic type inorganic migratory corrosion inhibitor affects steel reinforced Portland cement concrete by combining chemically with the cement phase to:
- Reduce concrete porosity thus reducing infusion of air, water and chlorides
- Reduce water soluble chloride content in the cement phase
- Buffer/elevate cement pH
- Increase concrete strength thus decreasing tendency to crack and spall
- Modify chemical condition on rebar surface to increase resistance to corrosion
In the example given, SURTREAT TPS II was applied to the examination area. After a sixty day period new results were obtained using the same GalvaPulse method (Germann Instruments, Inc.) following the identical grid pattern.
An examination of the corrosion rate bar graph for the after application measurements shows an average corrosion rate of 17.25 µM/Yr., and that the average of peak corrosion rates is in the same range, since the previous corrosion hot spots have now been pacified by the anodic inhibitor.
Using this average corrosion rate and a K factor of 1 the new life for the structure can be calculated as follows:
T = 2.85/2 x 10⁻³ ÷ 17.25 = 82 years
The forecasted structure life span as described by the rebar condition has been increased from 17 years to 82 years by the application of a migratory corrosion inhibitor.
Surtreat Solutions, LLC 437 Grant Street, Frick Building Suite 1210, Pittsburgh, PA 15219 USA, telephone 412.281.1202, fax 412.281.1282
Max Merzlikin, SURTREAT SOLUTIONS,LLC 2020
Max Merzlikin, SURTREAT SOLUTIONS,LLC 2020