This paper discusses a short study on life cycle cost analysis (LCCA) on corrosion remedial measures for concrete bridges and marine structures, which are subjected to carbonation or ingress of sodium chloride from sea water and other sources. Life cycle costing software, Bridge LCC 2.0, was used to perform life cycle cost analyses on three case studies, based on net present value method. The analysis of the results showed that LCCA is capable of assisting engineers or transportation agencies to evaluate optimum maintenance decisions in corrosion-related problems. It can be used as an engineering economic analysis tool that helps in quantifying the differential costs and choosing the most cost-effective corrosion remedial measures. Life cycle costs for the remedial measures are influenced by many costing variables such as initial costs, periodic maintenance costs, frequency years and analysis period. The best practice of LCCA should not only consider agency expenditures but also user costs and sensitivity analysis throughout the service life of a remedial measure.
The cost elements used in life-cycle costing for corrosion remedial case studies:
Initial cost
made up of a number of cost elements that do not recur after an activity
is initiated such as surface preparation, removal of defective concrete
Disposal cost
the cost of disposing the structure when it is non-repairable.
Discount rate
real discount rates reflect the true time value for money with
no inflation premium. FHWA recommends using a real discount rate in the
range of 3% to 5%.
Maintenance cost
group of costs experienced continually over the useful life of the activity
such as re-applying surface coating, replacement of anodes in CP etc.)
Analysis period
time used to evaluate the total cost required for a remedial measure, typically 75 to 100 years for bridges.
Inflation rate
measures the change in the prices of goods and services from one year to the next
The relationship between various cost elements is diagrammatically shown in a typical expenditure stream diagram for LCCA as in Figure 1. In this project, disposal cost is neglected due to its remoteness from the life cycle and thus tends to be small after discounting. The input data are obtained from three case studies:
Case Study I
Chosen a preventive option in bridges which involves: coating, silane, cathodic protection, waterproofing membranes, painting.
Case Study II
Choose repair and maintenance techniques in a wharf structure which include chloride extraction, cathodic protection and patch repair.
Case Study III
Choose anode systems in impressed-current cathodic protection for concrete bridges.
The input data for the three case studies are summarized in Table 1, 2, and 3.
Table 1 Input data for analysis in Case Study I
Corrosion preventive techniques
Analysis period: 75 years Base year: 2003
Real discount rate: 3.2 % Inflation rate: 2.3%
Alternatives Costs (US Dollars, $)
Painting Initial repair cost = $448,000
On-going cost = $45,000 (repeat at 10 years)
Waterproofing membranes Initial repair cost = $450,000
On-going cost = $43,000 (repeat at 25 years)
Coating Initial repair cost = $443,000
On-going cost = $40,000 (repeat at 25 years)
Silane Initial repair cost = $440,000
On-going cost = $36,000 (repeat at 10 years)
Cathodic protection Initial repair cost = $1,100,000
On-going cost = $240,000 (repeat at 8 years)
Table 2 Input data for analysis in Case Study II
Corrosion repair/stopping techniques
Analysis period : 20 years Base year : 1990
Real discount rate : 14 % Inflation rate : 10 %
Alternatives Costs (US Dollars, $)
Patch repair Initial repair cost = $ 280,000
On-going cost = $ 280,000 (repeat at 5 years)
Cathodic protection Initial repair cost = $ 474,000
On-going cost = $ 17,700 (repeat at 5 years)
Chloride extraction Initial repair cost = $ 306,000
On-going cost = $ 58,000 (repeat at 10 years)
Table 3 Input data for analysis in Case Study III
Anode systems
Analysis period : 75 years Base year : 2002
Real discount rate : 3.2 % Inflation rate : 2.2%
Alternatives Costs (US Dollars, $)
Catalyzed Ti-Mesh Initial repair cost = $ 155,000
On-going cost = $ 7,800 (repeat at 75 years)
Conductive paints Initial repair cost = $ 235,000
On-going cost = $ 11,800 (repeat at 14 years)
Thermal-sprayed Zn-coating Initial repair cost = $ 220,000
On-going cost = $ 10,000 (repeat at 27 years)
Thermal-sprayed Ti-coating Initial repair cost = $ 279,000
On-going cost = $ 13,800 (repeat at 30 years)
Results
Table 4,5,6 is the result of life-cycle costing reflect by the case studies I,II,III.
Life cycle costs by life cycle periods are shown in Figures 3, 5, and 7.
Figures 2, 4, and 6 show the cumulative life cycle costs, in net present value, for the competing alternatives in case I, case II and case III.
Table 4 Life-cycle costs for case study I
Cost category Water-
USD$)
Painting
Water-proofing membranes
Coating
Silane
CAthodic protection
Agency cost initial cost
448,000
450,000
443,000
444,000
1,100,000
Maintenance cost
62,000
18,000
18,000
50,000
420,000
Total cost(net present value)
510,000
468,000
461,000
494,000
1,520,000
Table 5 Life cycle costs for Case Study II
Cost category
Patch repair
Cathodic protection
Chloride extraction
Agency cost
Initial cost
280,000
474,000
306,000
Maintenance cost
440,000
28,000
34,000
Disposal cost
0
0
0
Total cost (Net present value)
720,000
502,000
340,000
Table 6 Life cycle costs for Case Study III
Cost category
Catalyzed TI-Mesh
Conductive paints
Thermal-sprayed Zn-coating
Thermal-sprayed Ti-coating
Agency cost
Initial cost
155,000
235,000
220,000
279,000
Maintenance cost
750
20,000
7,000
7,700
Disposal cost
0
0
0
0
Total agency(Net present value)
155,750
255,000
227,000
286,000
DISCUSSION
Economic Evaluation
From the case study I (Table 4), the most cost-effective preventive technique among the alternatives is the coating that is $461,000. The most expensive used for prevention of corrosion is cathodic protection because of the high initial and maintenance costs.(Figures 3). The cost of coating and waterproofing membranes are lower than cathodic prevention, painting and silane because coating and waterproofing have longer frequency years for periodic maintenance and cathodic prevention, painting and silane only have shorter frequency years for periodic maintenance.(Figure 2).
Figure 2 Cumulative life cycle costs, in present value, for each corrosion preventive techniques in Case Study I
Figure 3 Life cycle costs by life cycle periods for Case Study I
From the case study II, the most effective repairing technique of the chloride extraction is causes by it has the lowest life cycle cost over the analysis period. (Table 5). In figure 5 shows that the longer frequency year is chloride extraction. In contrary, patch repair need much expenses for periodic maintenance and causes it more costly if compare to Cathodic protection and Cathodic extraction.
Figure 4 Cumulative life cycle costs, in present value, for each corrosion maintenance techniques in Case Study II
Figure 5 Life cycle costs by life cycle periods for Case Study II
From the case study III, the most cost effective is catalysed Ti-mesh because of their initial cost are lower if compare to others. The reason of the catalysed has a lower cost is because the service life for the anodes is long and cause the periodic maintenance costs to be far away in the life cycle, therefore it incline to small after discounting the present value (Figure 7). So, we can see the effect on the life cycle costs is small in Figure 6.
Figure 6 Cumulative life cycle costs, in present value, for each anode used in impressed-current cathodic protection in Case Study III
Figure 7 Life cycle costs by life cycle periods for Case Study III
Costing Variable
As a result, costing variables such as frequency years, initial costs analysis period and periodic maintenance costs will affect the life-cycle costing. If the analysis period and the frequency year are long, the impact of the periodic maintenance costs on the life-cycle costing will be small
5.4 Shortcomings in LCCA
The fundamental problem associated with the application of life cycle costing in practice is the requirement to forecast a long time ahead in predicting the related future events. While some of these events can at least be considered, analysed, and evaluated, there are other aspects that cannot even be imagined today. These therefore, remain outside the scope of prediction and probability, and cannot be assessed in the analysis. Besides that, the results of LCCA are highly dependent on the input variables. Many times these inputs are only best estimates. This is due to the difficulty in identifying definite cost information as this varies from job to job, and country to country.
6.0 CONCLUSIONS
The case studies have demonstrated the useful application of Life Cycle Cost Analysis (LCCA) as a decision support tool in analysing investment decision making of repairing corrosion-induced damage and determining optimum maintenance strategies for concrete bridges and wharf structures. LCCA has been shown to be useful in assisting engineers or transportation agencies to evaluate optimum maintenance decisions in corrosion-related problems. It can be used as an engineering economic analysis tool that helps in quantifying the differential costs and choosing the most cost-effective corrosion remedial measures. Life cycle cost is influenced by many costing variables
such as initial costs, periodic maintenance costs, frequency years, and analysis period. The analysis of the results in the case studies showed that initial costs should not be the only criteria in selecting remedial measures. Input variables such as periodic maintenance costs and frequency years should be taken into consideration by discounting to the net present value in LCCA. In order to obtain more reliable analysis, the best practice of LCCA should not only consider agency expenditures but also the user costs and sensitivity analysis throughout the service life of a remedial measure
