This report presents a summary of the research conducted by the research team of the CRC project 2002-005-C, "Decision Support Tools for Concrete Infrastructure Rehabilitation". The project scope, objectives, significance and innovation and the research methodology are outlined in the introduction, which is followed by five chapters covering different aspects of the completed research. Key findings from a literature review conducted covering both the use of fiber reinforced polymer composites in the rehabilitation of concrete bridge structures and decision support frameworks in civil infrastructure asset management are presented in chapter two.
The decision support software tool developed to compare different treatment options based on reliability-based total cost will also be briefly described in this chapter. Strengthening or rehabilitation of existing bridge structures requires careful analysis of the capacity of the existing structure, identification of the deficiencies and then selection of the most effective solution for treating the bridge. The project has addressed two main questions in support of decision-making in connection with the rehabilitation of concrete infrastructure.
Design guidelines for the use of fibre-reinforced polymer composites in the rehabilitation of concrete bridge structures have been developed, combining the results of published research work and the provisions and expectations of the Austroads Code for Bridge Design and the Australian Concrete Code. A decision tool was developed to compare the full life cycle costs of different treatment options, taking into account different cost elements for the road authority and the user, as well as the risk of failure probability and corresponding costs.
Objectives
In Australia, more than 60% of local road bridges are over 50 years old and about 55% of highway bridges are over 20 years old (Stewart 2001). When choosing a treatment option, the decision-maker has a number of accepted solutions as well as emerging solutions. Innovative and emerging solutions tend to have low perceived reliability compared to proven solutions, which should be taken into account when making decisions.
Significance and Innovation
The decision support software tool developed for the calculation of entire life cycle costs is extremely innovative, both in methodology and in application. The lifetime cost of a given treatment option is determined using a probabilistic evaluation of the net present value of a given scenario, which is then combined with the costs if the risk of a given option is not taken into account.
Research methodology
Decision Support using Life Cycle Costing
The case study was selected following a number of discussions with the Queensland Department of Main Roads, which identified that the fixed head of reinforced and prestressed concrete bridge structures is currently the weak link, requiring reinforcement to meet the current demands of traffic and other taxes. .
Preliminary structural assessment
The bridge design code of Austroads (1992) was used for bridge evaluation to determine the bridge capacity. The following assumptions form the basis for calculating the ultimate strength of the reinforced concrete member in flexure. Design calculations are based on the actual dimensions, internal arrangement of reinforcing steel and material properties of the existing part.
QDMR has decided to use post tension strengthening techniques to increase the shear and flexural capacity of the Tenthill Bridge head. These issues can be solved by encasing the reinforcement system in concrete, painted channels or other protective methods. For post-tensioning the bridge head, wire strands or bars can be used as pre-stressing tendons.
In the post-cracking behavior, the prestressing steel in the tensile region causes a significant contribution to the moment capacity of the section. It has been found that the recommendations of ACI can be easily adapted to the provisions of the AustRoads Bridge Design Code as well as the AS3600 Concrete Construction Code. It differs between systems and even within a system depending on the state of the structure.
The status of the structure to be strengthened should be examined and repairs carried out if necessary. A key feature of the user manual is worked design examples that cover the typical reinforcement scenarios mentioned in 4.4. The analysis results can be presented in different ways, but the most commonly used indicator in road asset management is the net present value of the investment option.
Where Blifecycle is the benefit that can be obtained from the existence of the bridge after rehabilitation and Clifecycle is the cost associated with the bridge during its lifetime. To account for such uncertainties, the probabilistic behavior of input costs must be included. The expected repair costs, discounted to time t=0, are the sum of the individual repair costs.
Similarly, for all input cost values, the sensitivity of the NPV can be determined and compared. Thoft-Christensen (2000) defined the risk for a failure mode as the product of the failure cost and the associated probability. 2004 f) “A case study of the application of FRP composites in strengthening the reinforced concrete head of a bridge structure”.
Proceedings of the Second International Conference on FRP Composites in Civil Engineering, December 8-10, 2004 - Adelaide, Australia.
Structural analysis
Existing Capacity of the headstock
The positive and negative flexural and shear capacities of the section were calculated in accordance with Australian Standard AS 3600. Degradation due to corrosion of steel and creep and shrinkage of concrete were ignored. This method is effective and economical for long-span beams due to the negligible additional weight of the repair system and the use of existing material.
A question is raised about sliding failure of the beam along the shear crack at high prestressing forces. The bending strength of the top stem can be increased from 3800 kN-m to 5854 kN-m by gluing four FRP unidirectional strips of 120 x 1.4 mm to the tension face of the beam part (bottom fiber) of the headstock with fibers oriented along the length of the element (Figure 3-4). Details of the analysis of the section and the rationale for design decisions are provided in CRC Report 2002-005-C-02.
Proper design, detailing and application of it in a particular structure should guarantee the overall structural behavior and safety of the reinforced element. WLCCA evaluates each alternative by estimating the costs and the timing of the costs over a selected analysis period and converting these costs into economically comparable values, taking into account the time value of money over the predicted entire life cycle. The likely behavior (means and standard deviations) of any of the input costs must be entered into the respective cells of the spreadsheet relative to the distribution function considered.
Finally, the decision analysis should be subjected to a sensitivity analysis to ensure that the decision is not unreasonably influenced by the associated cost uncertainties. LCCA estimates should be checked for sensitivity to uncertain analysis parameters such as analysis period, discount rate, traffic growth rates, traffic speed, capital costs, and accident forecasts. The risk analysis approach uses random samples from uncertain probability-based input variables (initial cost, future cost, discount rate, and payback year) to generate a probabilistic description of the production outcome, NPV.
However, both design and practical implementation require careful analysis of the existing structure, surface preparation, FRP materials and design concepts. The use of one international guideline is not appropriate when designing an enhancement plan that meets the provisions of the Austroads Code. 2004 c) “Development of a User Friendly Guide for Rehabilitation or Strengthening of Bridge Structures Using Fiber Reinforced Polymer Composites” Proceedings of the Clients Driving Innovation International Conference, October 2004, Surfers Paradise, Queensland, Australia, pp.
Solution for Rehabilitation of Existing Bridge Structures” Proceedings of the Clients Driving Innovation International Conference, October 2004, Surfers Paradise, Queensland, Australia, pp. 2004 e) “A comparison of external post-tensioning and use of FRP composites in strengthening reinforced concrete Bridge Headstock or Tenthill Bridge in Queensland” Proceedings of 18th Australasian Conference on the Mechanics of Structures and Materials, 1-3 December 2004, Perth, Australia.
FRP Strengthening of the headstock
DISCOUNT RATE AND INFLATION
The interest rate used for discounting is a rate that reflects the investor's opportunity cost of money over time. The discount rate is defined as “an interest rate that reflects the investor's time value of money (Mearig et al. 1999). The interest rate is what would make the investor feel the same way about receiving a payment now or a large payment sometime in the future.
Current dollar analyzes include the general rate of inflation in all costs, discount rates, and price escalation rates. It is obvious that the discount rates are usually influenced by the economic, social and political factors.
FORMULATION OF WHOLE OF LIFE CYCLE COST
Modeling of the initial cost
Modeling of the maintenance (repair) cost
Modeling of user cost
