What is Structural Lightweight Concrete? And Why You Should Use It.

Why use Structural Lightweight Concrete Made with ESCS?

Structural lightweight concrete (SLC) has been a popular building material for over a century, effectively employed to solve weight and durability issues. Industry professionals now recognize SLC as a strong, low-density mixture of cement, water, and various combinations of normalweight aggregates and lightweight aggregates. When made with a combination of rotary-fired lightweight aggregates (LWA) such as expanded shales, clay or slate (ESCS), the lightweight concrete can be stronger and 30 percent lighter than normalweight concrete (NWC).

ESCS LWA’s network of unconnected internal pores helps lower SLC’s in-place density to 90-120 pounds per cubic foot (PCF) compared to NWC’s 140-150 PCF. Despite the lower density, SLC is designed for and expected to provide similar compressive strength and better durability for similar applications as NWC. In addition to these, the lighter concrete can provide significant benefits to bridge decks and high-rise construction projects, as elaborated below.  

Advanced concrete solutions reduce structural dead load

SLC’s reduced density minimizes dead load, which allows structural engineers to increase span lengths, decrease structural member depth and lower foundation material requirements. Additionally, SLC has a higher strength-to-weight ratio, allowing longer spans with less concrete volume. With all these properties, the advanced concrete solution helps reduce the quantity of reinforcements required to maintain the structural integrity of concrete components.

In fact, lower dead loads and higher concrete strength can enable structural teams to mitigate the impact of seismic forces. Lighter structures experience less seismic inertia than their heavier counterparts, which makes them more resilient to these events.

Mitigating concrete cracking with lightweight concrete

Concrete mixtures with high paste contents can lead to early-age concrete cracking. LWC has a lower modulus of elasticity, higher inelastic strains, a lower coefficient of thermal expansion, a more continuous contact zone between the aggregate and the paste, and more water in the pores of aggregates for continued internal curing when compared to NWC. Thus, when internally cured, lightweight aggregate concrete has improved permeability making it more resistant to aggressive liquid infiltration and sulfate attacks.

With these benefits in mind, building teams can achieve SLC mixes with a design strength of 20 to 35 MPa (2900 to 5000 psi) and even 70 MPa (10,000 psi). This means the concrete can carry a higher load more efficiently with less volume, which can help costs. That said, it is encouraged to consult with a lightweight aggregate supplier for guidance on mix design with desired strength and density.

Lightweight aggregate concrete offers enhanced fire performance

The fire resistance of SLC is generally greater than NWC for the same thickness. The superior performance is due to a combination of lower thermal conductivity and thermal expansion, both of which are influenced by ESCS LWA’s heat-conduction properties.

Along with the material’s lower heat conductivity, the heat-intensive processing required to manufacture ESCS provides inherent stability as the aggregate is already heated to high temperatures, and so is not as affected by fire as other materials. As a result, a two-hour fire-rated slab per standard regulatory requirements will use 38 percent less lightweight concrete than NWC. All these properties help realize stronger, lighter and more resilient concrete components.

Looking at the larger picture

A 25 percent reduction in density, comparable compressive strengths, enhanced fire-resistant properties and well-documented performance over decades demonstrates that SLC with ESCS can be considered as the reliable concrete choice for most applications. SLC is available from ready-mixed concrete suppliers and contractors across the country which can ease logistical issues and ensure material quality. To learn more about standard concrete specifications and testing protocols for your SLC mixtures, click here to refer to the specification guidelines on our website.