Life Cycle Costs Analysis (LCCA)

Life Cycle Costs Analysis (LCCA) is a method of analyzing infrastructure investment cost options over a design lifetime and includes the initial construction cost and the costs of downstream repairs adjusted back to a present value using a real discount rate (which accounts for both the nominal interest rate less the inflation rate).

The US Federal Highway Way Authority advocates the use of life cycle cost analysis in bridge design and material selection. Bridges are a significant target market segment for our company. For bridges, life cycle costs are computed from the time at which corrosion of the rebar starts to where patching and overlay of the deck surface is no longer viable, so that replacement of the deck is required.

In bridge infrastructure economics, a major cause of bridge maintenance costs relates to deck corrosion arising from of the rebar selected, which creates stresses in the concrete, because the volume of the corrosion (rust) is greater than that of the steel from which it is formed. When this occurs, local cracking, delamination and spalling of the concrete will be visible to the naked eye and eventually potholes will be formed on the bridge deck. When about 10% of the deck area has been patched, ride quality deteriorates sufficiently so that more serious and expensive rehabilitation (typically, installation of an overlay) must be undertaken to extend the life of the bridge. Eventually, if the design life is not reached, the deck and overlay deteriorate to such a degree that replacement of the deck is required.

Wiss, Janney, Elstner Associates, Inc. (WJE) have recently applied a sophisticated computer model to assess the service lives and associated life cycle costs for a bridge deck constructed using (1) black bar, (2) MMFX-II rebar, (3) epoxy coated rebar (ECR), (4) solid Type 304 stainless steel rebar, (5) NX Type 316L clad stainless rebar (NX-SCR^TM), and (6) solid Type 316L stainless steel rebar. The analysis is based on 100 years design life and considers the differing levels of corrosion resistance inherent with each of these alternative reinforcing bars. The model assumes severe chloride contamination of the bridge deck surface, using a surface chloride content of 26 lb/yd (based on measurements on 3 bridges in Iowa).

The major conclusions from the WJE study were:

• The initiation of corrosion and rate of damage accumulation are slowed for bars having higher chloride thresholds: when the threshold approaches the surface concentration, very durable performance is predicted.
• NX-SCR^TM provides the lowest annualised life cycle costs for real discount rates up to 6% p.a. depending on the life assumed for overlays installed when 10% damage has occurred.
• Even with favorable assumptions about their corrosion resistance, black bar, MMFX-II rebar, epoxy coated rebar (ECR), and solid Type 304 stainless steel rebar do not achieve 100 years life without costly bridge deck replacement and related disruption to traffic.
• NX-SCR^TM and solid Type 316L stainless steel are expected to exceed 100 years life without deck replacement.
• The model considers only the direct costs of repairs. If consideration is given to user costs, e.g. the costs associated with the disruption of traffic to the State economy, the relative position of NX-SCR^TM improves further.
• The model conservatively considers that the ends of NX-SCR^TM are not capped, resulting in localized corrosion performance similar to black bar, but shows that this has only a minimal effect on the predicted life compared with solid Type 316L stainless steel rebar. In practice NX-SCR^TM rebar is shipped and installed with end-caps.
• The first three materials are expected to suffer from corrosion damage for lower chloride concentrations ranging from 1.5 to 12 lb/yd, whereas NX-SCR^TM and solid Type 316 stainless steel are expected to remain corrosion free at, and possibly beyond, chloride concentrations of 15 lb/yd, and then only to corrode slowly above this level.
• The lifetimes predicted by the model for black bar and for ECR are in agreement with experience. There are only limited in-service performance data for the stainless steels or NX-SCR^TM for comparison.

Source: Company information, Monitor Group, "Corrosion Resistant Alloys for Reinforced Concrete": FHWA July 2007
1. 316 indicates the highest grade of stainless steel
2. Ranking from lowest to highest cost; Indipendent Research by Wiss, Janney, Elstner Associates, Inc.