Addressing key differences in mix design
When designing concrete mixes for bridges and other structures, project engineers have multiple options—including choosing between traditional, normal weight concrete (NWC) or structural lightweight concrete (SLC) with aggregate meeting ASTM C330, Standard Specification for Lightweight Aggregates for Structural Concrete performance. Depending on a project’s goals, SLC can create stronger, more resilient and cost-effective structures.
But there may be some questions about what makes SLC different and how this category of concrete contributes these benefits to load-bearing construction. I will address common questions here to help start the conversation on designing an SLC mix.
What is the difference between an SLC and a traditional concrete mix design?
Primarily, the difference lies in the type of aggregate used. SLC mixes incorporate ASTM C330-grade lightweight aggregates, such as expanded clay, shale, and slate (ESCS). They typically have a dry-loose density between 49-65 lb/ft³ (785-1,040 kg/m³), whereas conventional aggregate density ranges from 90-125 lb/ft³ (1,450-2,000 kg/m³).
The reduction in aggregate density can contribute to concrete that is between 25 percent and 35 percent lighter than NWC—without significantly compromising compressive strength. SLC advantages include optimizing structural efficiency, lowering transportation requirements, improving durability and extending the structure’s service life. This makes ASTM C330 aggregate ideal for applications where reducing the weight of the structure is required due to seismic and other important constraints normally found in high-rise buildings or bridges.
Does SLC meet strength requirements for bridge decks and structural elements?
Able to achieve compressive strengths between 3,000-8,000 psi (20.6-55.1 MPa), SLC bearing ASTM C330-grade aggregate—when designed properly with suitable admixtures and mix proportions—can deliver adequate strength for multiple bridge components. Discussing project goals and mix types with a lightweight aggregate producer can help clarify options, particularly when a producer is knowledgeable of multiple mix designs and their ability to impact project results.
How might lightweight aggregates impact the structural characteristics of a concrete mix?
First, the use of ASTM C330 lightweight aggregate reduces the overall dead load of a structure. This can contribute to less material used due to reductions in foundation, beam and column dimensions. It can also enable longer spans for increased design flexibility. For projects in seismic zones, lighter concrete components can also reduce seismic inertia for more resilient bridges and other infrastructure.
Equally important, when an SLC or NWC mix incorporates prewetted fine ESCS aggregates, engineers can factor criteria indicated in ASTM C1761, Standard Specification for Lightweight Aggregate for Internal Curing of Concrete. Internal curing increases resistance to early matrix cracking, allowing the production of higher-performance concrete that may last more than 75 years. Shrinkage cracking is a key limiting factor in achieving long-term performance in concrete structures.
What are the cost differences between SLC and normal weight concrete?
The use of ESCS lightweight aggregates generally results in a more expensive mix due to production processes. However, reduced unit weight results in a smaller carbon footprint relating to smaller structural components, footing and foundation excavation, insulation, and lower concrete volume. Thus, SLC has the potential to reduce the overall cost of a project.
Working with an ESCS producer well-versed in lightweight aggregates can help create a more robust cost-benefit analysis of using a SLC mix, including how this type of mix can contribute to structural improvements.
Why SLC? Reduced density with comparable strength improves project design
Using SLC in a design reduces the dead load of the concrete, which has many advantages in economics, performance and durability. This weight reduction allows the structural designer to reduce the size of columns, footings and other load bearing elements.
SLC mixtures also allow more efficient strength-to-weight ratio in structural elements for project savings due to less reinforcing steel and reduced volumes of concrete. When used in bridge construction and other infrastructure projects, it can support design improvements and increased lifespans.
For engineers trying to determine if specifying SLC designed with ASTM C330 aggregate would support their project goals, the Expanded Shale, Clay and Slate Institute (ESCSI) has assets posted under the Educational Resources, Applications and Sustainability tabs at www.escsi.org. Further, producers associated with ESCSI can work with engineers to determine which options might best meet their needs.
Darren Medeiros is a Regional Technical Salesperson at Holcim Lightweight Aggregates – Utelite Plant. He has two decades of experience working in the lightweight aggregate industry and is active in Expanded Shale, Clay and Slate Institute committees involving lightweight concrete use.
