Structures
Structural Lightweight Concrete: A Game-Changer for Longevity and Sustainability in Bridge Deck Repairs
By Ken Harmon
Bridges are integral to a country’s connectivity and development. Yet, according to the American Society of Civil Engineers’ (ASCE) 2021 Infrastructure Report Card, 42 percent of all bridges in the United States are at least 50 years old, out of which, about 4600 of them are considered structurally deficient. These deficiencies may cause bridges to be posted for load or speed restrictions that limit transportation options and pose a risk of traffic disruption and congestion. But more importantly, their weakened structural integrity increases the risk of collapse, which can lead to severe accidents, injuries, and loss of life. However, this is changing. In recent years, all levels of government have prioritized bridge repairs through the Bipartisan Infrastructure Law. This act invests approximately $40 billion for the repair and replacement of bridges with additional funding streams to advance major and rural-focused bridge repair. The initiative opens an opportunity and a challenge: In bettering existing infrastructure, how can civil and structural engineers create bridges that will outlast their predecessors? With the number of bridges in need of repair or replacement, it is important to choose a material that is not only robust but also efficient. Ordinary normal weight concrete (NWC) can crack due to early-age plastic drying shrinkage, which reduces durability. The dead load of NWC limits span length and increases substructure requirements, all of which work against the goals of building long-lasting and efficient infrastructure. Structural lightweight concrete (SLC) sidelines these issues. Because lightweight concrete has lower weight, it reduces seismic forces, allows longer spans, and requires less reinforcing, prestressing, and structural steel. In fact, it increases a bridge’s live load capacity, often allowing bridge upgrades and expansion without replacing or adding support foundations, thereby maximizing returns on investment. Consequently, the material can provide considerable advantages to facilitate repairs when compared to NWC.
Cracking, a Cause of Concern Early-age cracking is a major and expensive problem for bridge decks. It often accelerates corrosion, increases maintenance costs, and shortens the service life of the deck. High-performance NWC and supplemental cementitious materials used together in bridges can often fail to fully hydrate or react with each other. This leads to shrinkage and the corresponding stresses that develop inside the concrete. If the stress gets high enough, the concrete cracks, first as micro cracks then as visible cracks. Reports by Virginia Transportation Research Council (VTRC) suggest that effective control of early-age cracking can help limit later-age cracking. It suggests that careful material selection can minimize the risk of cracks throughout the service life of a bridge. When SLC is made with expanded shale, clay, or slate (ESCS) aggregates, it cures from the inside out in a process known as internal curing. Internal curing not only reduces the chances of early-age cracking but also increases the strength of the interfacial transition zone (ITZ), resulting in a better bond between the aggregate and cement paste. Through this process What is ESCS? Fired in a rotary kiln at 2000 degrees Fahrenheit, ESCS develops a network of unconnected internal voids, which, when a part of a concrete mix, act like tiny reservoirs. These voids absorb water and steadily release it into the concrete mixture from within as it sets. Further, these voids increase the bonding surface between the aggregate and the cement paste to improve the strength of the concrete. Both qualities of ESCS aggregate improve bridge deck slabs’ durability and ability to resist microcracking for a longer service life.
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csengineermag.com
October 2023
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