Winter 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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Page 38 of 51

S A F E T Y A N D S E R V I C E A B I L I T Y ASPIRE Winter 2019 | 37 I n 1948, the history of prestressed concrete bridges in the United States began when a Belgian professor named Gustave Magnel oversaw the design, prefabrication, and testing of 160-ft-long girders used to construct the first prestressed concrete girder bridge in the U.S., the Walnut Lane Memorial Bridge in Philadelphia, Pa. As I noted in my editorial for this issue of ASPIRE®, we can learn much about possible future improvements for the industry by reflecting on past achievements. As members of the bridge engineering community, we are part of a "long gray line" that connects what we do today, and what we want to do tomorrow, with the accomplishments of those who came before us. Industry Progress There are many ways to review the progression of our industry over the last seven decades. PCI issued a chronicle of the first 25 years compiled by industry pioneers titled Reflections on the Beginnings of Prestressed Concrete in America. 1 The American Society of Civil Engineers (ASCE) and other organizations periodically evaluate of the health of our infrastructure. These "report cards" help educate us and our communities about the need for infrastructure investments and the revenue streams to pay for them (such as tolls or gas taxes). (For the ASCE report card, see This article takes a different look at benchmarks of our industry's progress. The Federal Highway Administration's (FHWA's) National Bridge Inventory (NBI) (see nbi.cfm) is the source of the data used for comparisons. A bridge in the NBI database is classified by the material of its superstructure and the year it was built. So to start, let's use the period 1977 to 2006 as the base period for an analysis of recent trends. During the 30-year period, NBI data indicate that 476,417 bridges were built and 25 of 52 states (for our discussion, "states" will be taken to include the District of Columbia, and Puerto Rico) used some type of concrete (reinforced or prestressed) superstructure for at least 65% of the bridges built during the period. In contrast, during the most recent decade for which data are available, the period between 2007 and 2016, 123,158 bridges were built, 31 states met the 65% benchmark, and 35 states increased their use of concrete superstructure types. The states hitting the 65% benchmark in the 2007–2016 decade are colored in dark grey in Table 1 and Figure 1. As shown in the light-gray portions of the table, an additional seven regions are above 50% concrete utilization. Overall, the market share for all types of concrete superstructures has increased from 68.7% in the base period to 71.6% in the last decade (an increase of 2.9%). When examining the historical data, we gain further insight by looking at the types of crossings and the functional needs for the structures. In the base years of this study, shorter-span bridges (openings of 20 to 40 ft) were generally reinforced concrete whereas the long spans were typically constructed of prestressed concrete or steel. To better understand the progress of the prestressed concrete industry and its future growth potential, let's consider only steel and prestressed concrete bridges, which generally represent the longer spans. Table 2 and Figure 2 focus on these longer-span bridges by comparing the number of steel and prestressed (pretensioned or post-tensioned) concrete bridges built: 175,180 in the base period and 48,095 in the most recent decade. The table also presents the percentage of prestressed girder bridges based on the total number of these two bridge superstructure materials for each period. Between 2007 and 2016, 32 states built more than 50% of their longer- span bridges with prestressed concrete (those shown in dark gray or light gray) and 21 regions had an increase in use of prestressed concrete of over 5% in the last decade. This represents a 5.8% increase in prestressed concrete use in the last decade compared to the previous three decades. Looking Forward The data reviewed in this article demonstrate the notable growth of the concrete bridge industry in recent years—and that's good news. However, a few questions remain about the growth potential of prestressed concrete. For example, 13 of the 52 regions still select steel over prestressed concrete 65% of the time. Why do these jurisdictions prefer steel? Is there capacity to deliver concrete bridges in those areas? Understanding what is driving trends will help us expand the growth we have recently experienced. We need to choose benchmarks that help us move forward and improve. How shall we put other future priorities and technological innovations into historical context so we can effectively identify progress? For example, we know that new shallow concrete beam shapes can be used where span-to-depth ratios were previously seen as a de facto steel solution. We also can anticipate that accelerated bridge construction deliveries will need solutions that are constructible from off-the-shelf components, and prestressed concrete answers this need. Perhaps there are lessons to be learned from the long gray line of our seven decades as an industry that can help us toward this goal. Reference 1. Prestressed Concrete Institute (PCI). 1979. Reflections on the Beginnings of Prestressed Concrete in America. Chicago, IL: PCI. THE LONG GRAY LINE Reflections on concrete bridge progress by William Nickas EDITOR'S NOTE The data presented in this article were collected from the FHWA NBI portal in July 2018. The author thanks Hank Bonstedt for his diligence in the tedious task of analyzing the decades of data. Bonstedt presented a review of concrete bridges in the Spring 2017 issue of ASPIRE.

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