The Progreso Pier is a saltwater pier located in Yucatan, Mexico. It consists of two segments: the original pier, which is 2.1 km in length and was built in 1941, and a roughly 4.4 km additional segment, which was built in the 1980s. Together, the construction stretches roughly 6.5 km from the shore and is the longest pier in the world. The subject of this analysis is the original 2.1-km pier, excluding the later addition. The addition is excluded due to lack of construction and inspection data (referenced reports are specific to the original span), as well as to simplify the analysis to a single construction project.
The initial scope of the study was a comparison of the Progreso Pier against a neighbouring pier. The unnamed neighbouring pier was constructed in 1969 and used carbon steel reinforcement; conversely, the Progreso Pier was initially completed in 1941 and extended in 1988, built with stainless steel reinforcements (18–8 Cr-Ni SAE 304). The carbon steel pier has since collapsed due to saltwater erosion, with only the piles remaining, while the Progreso Pier has remained functional (Fig. 1).
Saltwater erosion (exacerbated by humid climate and as well as wave and tidal action) is the most important factor in determining the life of these concrete-and-steel piers. The effectively identical environmental conditions (same high heat and humidity, seasonal hurricane-force winds, wave and tidal patterns and corresponding saltwater exposure) presented the rare opportunity to compare differing structures under matching environmental stresses. Both structures had undergone an equivalent maintenance schedule.
Yet, despite the compelling similarity in use-phase conditions, the Progreso Pier and the neighbouring pier were found to be non-comparable for the purposes of this study. Differences in function and size, as well as dissimilarities in other aspects of the pier designs, meant that functional equivalence between the two piers could not be ascertained.
For this reason, this study compares the Progreso Pier, constructed with stainless steel rebar, against an alternate, hypothetical carbon steel case study: a pier with size and function equivalent to the Progreso Pier built with carbon steel rather than stainless steel rebar. Within this report, the pier built with stainless steel rebar is referred to as the “as-built design”; the alternative with carbon steel rebar is referred to as the “alternative design.”
Functional unit and reference flows
Ideally, the functional unit would describe the function of the pier, such as its ability to move goods, using a metric expressed as magnitude, unit, duration and level of quality (Cooper 2003; Koffler et al. 2014). However, given that the functions provided by the pier do not easily lend themselves to such a simplistic approach, the function of the Progreso Pier is determined by its intended use as defined by the designers.
The original 2-km pier finalized in 1941 was designed to accommodate two railway lines and one roadway running parallel across the full length of the pier. The pier design supports live loads associated with the operation of two 85-t, 1.5 m wide trains, as well as 39-t, 1.5 m wide, 3-axle trucks along the length of the pier. The pier platform was designed to support a continuous distributed load of 4 t/m2. The piles were designed for a total allowable stress of 15 kg/cm2.
The pier is composed of 175 12-m spans with massive columns and arches. The structure is concrete throughout, with steel reinforcements in structural locations under tensile or mixed load. The total volume of concrete used is 72,500 m3, with a mass of reinforcing steel of 220 t. The functional performance of carbon steel and stainless steel rebar is equivalent in the chosen application, with the exception of corrosion resistance. Because the structural properties are equivalent, the material content of the two piers is the same (i.e. equal amounts of steel and concrete), with the only difference being the type of steel used (stainless or carbon steel). Over the life cycle, the differences in corrosion resistance lead to more frequent maintenance and reconstruction for the alternative design.
In summary, even though there is no clear-cut quantified functional unit as such, the above approach ensured that the two pier designs were functionally equivalent and therefore fully comparable. In addition, the duration of the functional unit was chosen to be 79 years. This period is chosen to capture both the past performance (beginning in 1941) and the expected performance into the near future (through 2020). The analysis period does not correspond to expected service lives of the alternative or as-built design; rather, it is an arbitrary period that is chosen for communication purposes. The arbitrariness of the analysis period does not have a significant impact on the results due to the consideration of remaining service at the end of the analysis period. Both the LCA and LCC offer a credit at the end of life for this remaining service life, as explained in the respective sections.
Service life and maintenance
For the purposes of LCA, the maintenance schedule defines the materials and activity required over the lifetime of the pier and can be a substantial contribution to the inventory of the structure. The expected service life and the maintenance requirements of designs are based on analysis conducted by CTL Group (CTL 2013) and displayed in Table 1. The service life follows the definition per US Navy’s engineering command (NAVFAC), as the number of years before major restoration is necessary, given minimal maintenance to the structure during its life. Major restoration is further defined as “extensive areas that require extensive repairs using a jack hammer or other destructive means to prepare the concrete for rehabilitation” (US Navy 2012).
The maintenance schedule is based on the time for corrosion initiation and propagation of the steel. Over time, chloride ions diffuse through the concrete cover (in accordance with Fick’s second law of diffusion) to the depth of the reinforcing steel. Once a critical concentration is reached, corrosion initiates and continues to propagate until repairs are required. Time to initiation and propagation is modelled using the Life-365 software (www.life-365.org). More details, including input variables, are reported in the engineering report (CTL 2013), which is available upon request from the authors of this study. For the alternative design, repairs are first required at year 10; for the as-built design, repairs are first required at year 44.Footnote 1
The system under study includes the materials, the maintenance over the life cycle, the transport of materials and the end of life of the pier. The system boundaries were selected in order to enable the effective comparison of the two designs (Fig. 2). The power consumption to operate the pier (i.e. street lighting) was excluded as it was deemed to be of little relevance and because there was no reliable data available. Due to the same reasons, temporary construction materials and processes were excluded from the system boundary. Construction-related impacts will differ between the two designs in the case of reconstruction, which will be shown to occur during the analysis period for the alternative design, but not for the as-built design. Reliable estimates for economic and environmental impacts are difficult to ascertain for construction and are thus excluded from the baseline analysis. However, due to their potential influence on the results and conclusions, sensitivity analyses are performed for construction impacts.
Co-product allocation was not relevant, as co-products do not occur in pier construction. End-of-life (EOL) allocation is used to account for recycling of steel scrap at the end of life. The “value of scrap” approach is applied, which is essentially an avoided burden/EOL recycling allocation method that has been endorsed by the metals industry (Atherton et al. 2007). At the end of the analysis period, both the as-built and alternative designs have remaining structural service life. For both the LCC and LCA, this service life is credited back to the system in proportion to the service life remaining divided by the total service life.