Bridge Construction With Structural Lightweight Concrete: Material Properties, Performance, and Value

To view the figures and tables associated with this article, please refer to the flipbook above.

Data contained in the National Bridge Inventory (NBI), which is collected by the Federal Highway Administration (FHWA), reveals that just over 42,000 bridges, or nearly 7% of the total inventory of bridges in the U.S., as reported in 2024, are rated as being in “poor” condition. This rating may indicate that these bridges are candidates for repair or replacement.

Concerns about this relatively large number of bridges in “poor” condition are valid and have persisted for a number of years. To address this concern, the Infrastructure Investment and Jobs Act (IIJA) was passed in 2021, which allocated $27.5 billion through a formula bridge program, with $15.9 billion already distributed to many states in the first three years, as reported by a 2024 report by the American Road & Transportation Builders Association (ARTBA).

Furthermore, states have committed 46 percent of these funds, which is about $7.3 billion, to over 4,170 bridge projects.

The sheer volume and complexity of needed repair or replacement of bridges mean that the currently available resources, though significant, are not enough to address the problem if only conventional approaches are used. This situation calls for innovative cost- and time-efficient solutions offering long-term performance improvements. To meet the needs of the moment, some engineers have turned to precast concrete components, such as pretensioned and post-tensioned girders, slabs, and piles, as well as substructure elements, such as pier caps and columns, to accelerate bridge construction and to enhance the durability of concrete elements.
Factory-made precast concrete elements ensure precision and quality while enabling concurrent construction processes, which can significantly reduce project timelines in many cases.

However, one challenge with using precast concrete bridge elements on existing structures is they are typically made using normal weight concrete, which is very heavy and places additional loading demands on existing bridges. Plus, the use of precast concrete elements may increase transportation costs and logistical complications, risking delays, damage and budget overruns.
Structural lightweight concrete (SLC) made with lightweight aggregates offers an excellent solution by substantially reducing the weight of the deck when used in cast-in and precast concrete components through its lightweight properties. The reduced weight improves design efficiency and may contribute to accelerating project delivery—all without compromising strength, durability, or long-term performance.

SLC has been used in the building construction industry for nearly a century. The reduced density of concrete mixtures offers structural engineers significant opportunities to lower the load on their projects, especially in bridge applications. SLC mixtures that contain manufactured lightweight aggregates like expanded shales, clay, or slate (ESCS) typically have a density from 110 to 125 pounds per cubic foot (pcf). Taking it a step further, SLC mixtures can be designed to achieve a density as low as 90 pcf, which is nearly a 40 percent reduction in weight from typical normal weight concrete with a density between 140 pcf to 155 pcf.

The material properties of ESCS lightweight aggregates play a crucial role in the performance of SLC mixtures. This type of lightweight aggregate is produced by heating the raw materials (shale, clay or slate) in a rotary kiln to temperatures from 1900 to 2200 degrees Fahrenheit. This manufacturing process produces a high-quality, uniform, structural-grade lightweight aggregate with well-distributed, unconnected pores of moderate size (5 to 300 μm) surrounded by a strong, relatively crack-free vitreous ceramic matrix.

The internal composition of lightweight aggregates and the resultant reduced density of SLC mixtures offer significant benefits to bridge components, whether cast-in-place or precast.

As span length increases, dead load can become a more dominant component of bridge design loads. For engineers dealing with existing structural systems in bridge repair and rehabilitation projects, any reduction in the weight of a long-span bridge can offer efficiencies in design. The reduced density of SLC offers significant benefits in such applications.

Because SLC decks and girders are lighter than normal weight concrete components of the same size, structural engineers can design greater span lengths for a given bridge cross-section than is possible with conventional concrete members. Steel girders will also typically have an increased span capability with an SLC deck unless deflections govern the design. Furthermore, seismic loads on bridges are computed based on the mass of the structure. Therefore, reducing the mass of the structure by using SLC can lower applicable seismic loads for which the bridge is to be designed. Additionally, using SLC will reduce the loads on bearings, substructures, and foundations, simplifying structural design and limiting required modifications and investments when reusing existing or providing new substructure and foundation elements.

Such was the case of the I-5 bridge span replacement project over the Skagit River in Washington. SLC prestressed concrete deck bulb tee girders were used to permanently replace a steel portal frame truss span of the I-5 bridge that had collapsed after being struck by an over-height truck. Engineers selected SLC deck girders, which include a full-depth bridge deck cast monolithically with pretensioned girders, for the permanent replacement span to provide a lighter solution. In doing so, the designers kept the total weight of the new span below 918 tons, which allowed them to reuse the existing foundations without reanalysis or strengthening, saving significant cost and time.

While dead loads are one aspect of design, structural engineers must also consider the compressive strength of a concrete member. The same concrete compressive strengths specified for design with conventional normal weight concrete are typically achieved with SLC. Concrete compressive design strengths above 5,000 psi (35 MPa) are often used in bridge projects and these strengths can also be achieved using SLC. In fact, the SLC concrete used for the prestressed concrete girders for the Skagit River Bridge span replacement used a design compressive strength of 9,000 psi, which was consistently achieved.

Along with density and design compressive strength, the modulus of elasticity is a critical parameter for structural engineers, particularly in bridge projects, as it reflects the stiffness of concrete. A higher modulus of elasticity contributes to greater stiffness, slightly reducing deflections under traffic loads and elastic shortening prestress losses. However, in some cases, such as seismic design, potential long-term deformations, or situations where differential settlement must be considered, a lower modulus of elasticity can be beneficial to provide flexibility and reduce loads on the structure.

For SLC, the pores in the ESCS particles reduce the aggregate stiffness and provide a closer match between the elastic properties of the aggregate and mortar. Comparing values of the modulus of elasticity computed using the equation in the latest edition of the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications (BDS) shows that SLC, with a density of 115 pcf, is expected to have a modulus of elasticity of approximately 63 percent of the value for normal weight concrete with a density of 145 pcf, when both types of concrete have the same compressive strengths. Making the same comparison for SLC with a density of 100 pcf, the computed value of the modulus of elasticity is estimated to be 48 percent of the value for normal weight concrete.

The difference in performance is because, in normal weight concrete, the interaction between coarse aggregate (more than 35 percent by volume) and the mortar phase (approximately 65 percent by volume) yields a modulus of elasticity that is intermediate between the two components. For SLC, the pores in the lightweight aggregate particles reduce their stiffness and result in a closer match of elastic properties between the aggregate and mortar. This convergence of the properties of aggregate and mortar fractions also minimizes the strength penalty associated with air entrainment in SLC members compared to normal weight concrete members, where typically stiff normal weight aggregates can create a greater elastic mismatch, potentially leading to cracking around article particles.

Creep and shrinkage are other critical properties in projects where long-term deformation and volume changes can impact the structural performance of bridge infrastructure. Factors such as aggregate properties, paste volume, and curing conditions influence both quantities. Along with these aspects, the stress-to-strength ratio also impacts creep. A common misconception is that SLC exhibits greater creep and shrinkage than normal weight concrete; however, studies suggest otherwise. For context, creep, which is the long-term deformation of a material under sustained load, affects critical aspects of bridge design, including prestress losses, design of bearings and expansion joints, and force redistribution in composite structures.

A paper by Concrete Technology Corporation and Castrodale Engineering Consultants on three SLC bridges in Washington State suggests that higher-strength SLC exhibits creep behavior similar to, or even less than, comparable normal weight concrete mixes. For example, a lightweight concrete production mix with a design compressive strength of 9 ksi for a prestressed girder (used for the Skagit River Bridge span replacement) had less creep (and shrinkage) than a comparable normal weight concrete mix.

The same paper reported that the SLC girder mixture had less shrinkage than the comparable normal weight concrete. This behavior was unexpected because the SLC mixture had a total cementitious materials content of 935 lb/yd3. In comparison, the normal weight concrete mixture had a total cementitious materials content of only 752 lb/yd3. The SLC mixture, with a 24% higher total cementitious materials content compared to the NWC mixture, would typically be expected to have a significantly greater shrinkage. However, tests of both concrete mixtures demonstrated that the SLC mixture has a significantly lower shrinkage than the NWC mixture. With both a reduced shrinkage and modulus of elasticity, which are typical features of SLC, it is expected that SLC would experience reduced and/or delayed cracking, in applications where the concrete is restrained from freely shrinking.

Permeability in concrete components facilitates the ingress of aggressive agents and increases vulnerability to chloride attack, which accelerates the corrosion of embedded steel reinforcement. Though many of the mechanical properties of precast SLC bridge members are comparable to those made with normal weight concrete, SLC exhibits superior durability, particularly in permeability resistance.

In concrete bridge components, elevated permeability often arises from a weak interfacial transition zone (ITZ) at the aggregate surface, which can contribute to microcracking in the ITZ that further increases permeability. However, components made with SLC benefit from the superior quality of the cementitious paste and internal curing, both of which are facilitated by using ESCS lightweight aggregates. The internal curing provided by moisture being released from prewetted lightweight aggregate results in a more complete reaction of cementitious materials, reducing permeability, shrinkage, and cracking. In contrast, NWC exhibits an abrupt transition at the dense aggregate/porous cementitious matrix interface, which can exacerbate the risk of increased cracking and permeability.

SLC components have good resistance to freezing and thawing cycles when the SLC mixture is properly proportioned, batched, and placed. Once the lightweight aggregate is prewetted, moisture is released into the concrete as internal curing, improving the density of the ITZ and other concrete properties. The aggregate pores also act like little “relief valves” by providing additional space for the expansion of freezing water, if permeable concrete has allowed water to penetrate and freeze. This way, including ESCS lightweight aggregate helps reduce microcracking and permeability in bridge decks and improves resistance to freeze-thaw cycles. Because the benefits to SLC from internal curing and an improved ITZ depend on the lightweight aggregate being properly saturated, the concrete mix demands a higher degree of quality control than is typically required for normal weight concrete.

Field studies and laboratory tests underscore these advantages. Reports that evaluate the performance of bridge decks in Virginia demonstrate that SLC decks exhibit less cracking compared to decks constructed with normal weight concrete, which has resulted in the inclusion of SLC mixtures as one of two approved options for low-shrinkage deck concrete in the Virginia DOT Road and Bridge Specifications since the 2016 edition.

The reduced permeability, as well as improved resistance to microcracking and freeze-thaw cycles, enhance the durability of SLC bridge members. Using SLC members can help structural engineers provide better protection for embedded reinforcement against corrosion, which in turn, safeguards the long-term structural integrity of our country’s bridge infrastructure.

Even after structural engineers educate project teams about the benefits of SLC components in long-term performance, some engineers may be skeptical about using SLC components due to the slightly higher cost. Evaluating the economics of SLC members in bridge repair and rehabilitation needs to go beyond just the cost of the constituent materials and components, and instead focus on the value that the use of SLC brings to the project over its extended service life.

A report by the FHWA has estimated the cost effect of using SLC or normal weight concrete in a precast segmental box girder bridge. It reported savings in the schedule due to the concurrent construction nature of the members as well as the cost. In application, the reduction in dead loads provided by SLC leads to more efficient designs, allowing for extended span ranges, wider girder spacings and shallower girder sections. Such design optimizations can significantly reduce material usage and construction costs while maintaining or improving long-term performance.

For example, the Wabash River Bridge was an early project in Indiana that used precast lightweight concrete girders with both pretensioning and post-tensioning to extend the span length to 175 ft. Using SLC components resulted in savings of $1.7 million on the $9.4 million project. In a review of SLC use in California, Thompson and Wu compared a typical two-span overpass design for a box girder superstructure that uses normal weight concrete versus SLC. Results showed that the footing size of the SLC bridge was smaller, with fewer piles, resulting in a 20 percent savings in overall foundation cost, including the column, footing, and piles.

The savings on structural systems when using SLC components are particularly advantageous for rehabilitation projects. As mentioned earlier in the article, the ability to reuse existing substructure elements can eliminate the need for extensive demolition and reconstruction, significantly reducing costs and construction timelines. In cases of bridge widening or load rating improvements, the use of SLC may facilitate these upgrades without necessitating modifications to the existing superstructure or substructure. This adaptability can be especially valuable when updating aging infrastructure, which is often heavily traveled, enabling enhancements to be constructed without requiring modification of the existing structure, while also minimizing disruption to the users of the facility.

The reduced design loads on bearings, substructure elements and foundations further streamline logistics and improve overall efficiency. The reduced weight of SLC components may allow a greater number of small precast components to be transported in each truckload. The longer segments possible due to lighter weight minimize the number of fabricated pieces required for a project, accelerating project delivery and reducing the number of joints in the structure. Fewer joints translate to improved durability and reduced maintenance over the bridge’s lifecycle.

This approach also results in fewer loads being delivered to the construction site, improving safety for both construction personnel and the traveling public. The reduction in transportation leads to not only lower costs associated with traffic control but also lessens traffic congestion. The reduced weight of SLC components enables the handling of larger precast pieces without exceeding the capacity of existing hauling or lifting equipment, which further optimizes delivery schedules and enhances construction efficiency.

As structural engineers continue to navigate performance requirements with budget constraints in bridge repair and rehabilitation projects, SLC emerges as a forward-thinking solution that delivers both immediate construction advantages and long-term performance benefits. The proven track record of SLC use in projects across the country and its comprehensive technical advantages make it an important consideration for bridge infrastructure projects where efficiency, durability, and cost-effectiveness are primary objectives. ■

About the Author

Michael Robinson is the Senior Technical Consultant to the sales department at Stalite Lightweight Aggregate, who is a member of the Expanded Shale, Clay and Slate Institute (ESCSI)—an international trade association for manufacturers of rotary kiln-produced expanded shale, clay and slate lightweight aggregate.

Special thanks to Reid Castrodale, PhD, PE, Director of Engineering at ESCSI, for his valuable expertise and insights.