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.

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.

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 and the resulting qualities, SLC can contribute to longer-lasting bridge decks by reducing the potential for water penetration and corrosion.

Resisting Freeze-Thaw Cycles with SLC

When rainwater or snow melt collects on bridge decks in colder climates, water freezes inside the cracks and widens them. In time, the cracks increase in size and accelerate corrosion even in normal conditions. In addition, many states’ Department of Transportation (DOT) can use deicers containing chlorides to melt snow from roads and bridges. With existing cracks, the chemicals can penetrate the material more easily and cause greater structural damage by corroding the reinforcements. 

In laboratory freeze-thaw testing, ESCS aggregate is subjected to 300 cycles of freezing and thawing. Being composed of vitrified silicates, the aggregate has unconnected gaps or voids that allow air entrainment, providing space for water to expand upon freezing. As a result, SLC mixes typically have durability factors between 95 and 100 after 300 cycles of freezing and thawing, indicating that the three concrete mixes were relatively unaffected by the test. 

On the contrary, for typical NWC mixtures, the relative dynamic modulus falls below 50 percent of its initial value after 95 to 100 cycles of freezing and thawing. Hence, testing of these mixtures is terminated after these cycles in accordance with ASTM C 666. A SLC mixture containing ESCS is less prone to cracking and can better protect the structural integrity of a deck, even in snow-prone areas, serving as an asset for bridge deck durability. 

Further, when compared to NWC, SLC bridge decks have a lower modulus of elasticity that provides the deck flexibility to accommodate movements and return to its original form under load. The lightweight mixture also has a lower coefficient of thermal expansion that reduces the contraction and expansion of material with changes in temperature, further minimizing the dimensional change in a bridge deck. These properties, along with a stronger contact zone between the aggregate and the cementitious paste, enable SLC to efficiently offset concrete’s brittleness in the mixture and resist early-age cracking in bridge deck replacements and repairs. 

Reduced Loads for Bridge Decks

To increase the efficiency of bridge deck repairs and reduce road closure periods, engineers often turn to precast, prestressed concrete. Not only does this type of concrete ensure higher quality products due to factory-controlled curing environments, but it also contributes to more accurate concrete forms. 

Precast, prestressed NWC bridge deck slabs and girders can easily exceed truck load limits, raising the number of shipments required to finish a repair. This can increase construction time, project cost and road congestion. SLC is 25-30 percent lighter than NWC, sidelining this issue. Depending on the nature of the renovation, the use of SLC often increases the load-carrying capacity for older bridge structures, meeting higher load rating specifications. 

In areas that need seismically resilient bridges, SLC’s reduced dead loads can also weaken the magnitude of seismic forces that act on a structure. Because lighter structures experience less seismic inertia during an earthquake and so exert less pressure on foundation systems, they make more resilient and longer-lasting infrastructure.

Economic and Environmental Sustainability

While SLC made with ESCS can initially cost more than NWC (with exact cost differences depending on a site’s distance from an ESCS production plant), it can reduce costs across the entire structure for significant net savings. This is true even if the material needs to be shipped from distant production sites. The reduction in material due to fewer piles, smaller footings, less reinforcing steel, and smaller supporting members can often shrink the project cost for repair and renovation.

In addition, ESCS’s low density facilitates more quantities of material to be transported per truck to the site. Fewer trucks reduce the cost of transport, further shrinking the total project cost and the consequent environmental impact of transportation. As such, lower material quantity requirements mean lower embodied carbon over the life of a bridge repair project. The reduction in the carbon footprint of the overall project contributes to the engineering building team’s sustainability goals.

Structural lightweight concrete for efficient bridge repair

Structural lightweight concrete produced with expanded shale, clay or slate aggregate achieves significant goals: resistance to cracking, lower dead load, reduced cost of the entire project and long-term environmental benefits. The combination of these factors make SLC a powerful material in bridge repairs and replacements when compared to NWC. This lighter, more durable concrete can help civil and structural engineers provide holistic, long-term solutions for bridge structures that adequately address both expansion and economic issues while meeting sustainability goals.

Ken Harmon is Territory Manager/Director of Engineering Resources for STALITE Lightweight Aggregate Company.  He is a registered Professional Engineer in Georgia, a member of ACI, ASCE, ASTM, and is Chairman of the Expanded Shale, Clay, and Slate Institute (ESCSI) Structural Committee.