Lightweight Concrete Masonry Block: Building Efficient Buildings Efficiently

Words: Jody Wall

In the construction industry, masonry is a labor-intensive trade. Each day, masons handle thousands of concrete masonry unit (CMU) that form a building’s structure. Because normal weight (NWT) CMU weigh about 50 pounds (lbs), repetitive lifting can strain a worker’s back. An injured team can slow project timelines—especially with the current workforce shortages in construction.

Lightweight concrete masonry unit (LWT CMU) made from expanded shale, clay or slate (ESCS) aggregates offer a lighter, more efficient alternative. While they seem slightly more expensive upfront, these innovative block provide contractors, architects, and developers with advantages that far outweigh their initial costs.

Not only do they support faster, safer construction, but they also contribute to projects’ energy efficiency and carbon reduction goals. In terms of design, CMU’s embodied carbon emissions are closely evaluated as projects aim for sustainability. Not only do LWT CMU sequester carbon from the environment, but an insulated LWT CMU wall has excellent thermal resistance and improves energy efficiency.

Increased Profitability and Efficiency for Mason Contractors

Masonry is an intensely physical trade. Estimating a mason will lift 150 3-web, 12” block per day weighing about 50 lbs each, by the end of the week, this mason will have lifted a total of around 3.8 tons – about the weight of seven Ford F350 trucks. This repetitive heavy lifting can lead to injury, potentially adding to workers’ compensation claims, project delays, and labor shortages.

LWT CMU are approximately 46% lighter than NWT CMU. This reduced weight allows masons to lay three additional blocks per hour, totaling 25 more per day – a productivity increase of about 17%. As a result, while lighter block has a slightly higher price than standard block, a LWT CMU wall proves to be more cost-effective than a NWT CMU wall when factoring in the boost in productivity, time taken to build it, and reduction in workplace injuries.

Using lightweight CMU doesn’t just improve the speed of installation; it can also make projects more profitable for contractors. Completing a job more quickly translates to more projects over time, resulting in better cash flow. Additionally, LWT CMU make it easier for contractors to meet demanding project deadlines, positioning them as more competitive players in the market.

The next-generation block made with ESCS LWA take this efficiency a step further. With fewer and thinner webs than traditional CMU, next-generation block are typically ‘A-shaped’ or ‘H-shaped.’ This reduction makes a 12-inch block 25% lighter in addition to the 46% reduction in weight when comparing a traditional three-web NWT CMU of the same size. As a result, next-generation block enable masons to work more comfortably and efficiently.

LWT CMU Sequester More Carbon than Dry-cast Concrete

The benefits of LWT CMU extend beyond masons and mason contractors. Typically, the fire resistance, acoustic properties, and improved seismic performance of CMU hold significant appeal for design teams. Additionally, LWT CMU mitigates carbon emissions by carbon sequestration and thermal efficiency—key benefits for architects to consider.

CMU naturally sequesters carbon dioxide (CO₂) over time, which helps recoup its carbon footprint from the manufacturing process. From the moment a concrete component (block, slabs, walls) is cast, it begins absorbing CO₂ from its environment, a process that continues indefinitely as long as hydrated cement is exposed to CO₂. This fundamental characteristic of concrete, due to the natural presence of hydrated cement and inevitable exposure to CO₂, contributes to ongoing carbon sequestration.

Dry-cast concrete, commonly used in block production, contains interconnected voids that allow CO₂ to penetrate more easily than wet-cast concrete, which has a denser structure. The open configuration of CMU, with thin face shells and webs, further accelerates its natural carbonation rate, resulting in significantly higher CO₂ uptake compared to wet-cast concrete.

Research shows that wet-cast concrete absorbs about 2 kilograms per cubic meter (kg/m3) of CO₂ at 28 days. After two years, it sequesters around 8 kg/m3, which is less than 3% of the block’s total embodied carbon (greenhouse gas emissions arising from the manufacturing process). Meanwhile, a study by the Concrete Masonry and Hardscapes Association (CMHA) found that dry-cast CMU sequesters approximately 19 kg/m3 of CO₂ at 28 days and reaches 42 kg/m3 after two years, which is nearly 49% of the block’s embodied carbon. This difference underscores the natural advantage of dry-cast concrete in carbon sequestration, making it a valuable material in sustainable building practices.

Insulated SmartWall Enhances Energy Efficiency

In any kind of wall construction, the R-value—or thermal resistance—is a crucial factor for reducing energy consumption for heating and cooling. In the past, building teams aimed to save energy by adding more insulation to exterior walls. However, the Law of Diminishing Returns proves that extra insulation does not yield proportional energy savings beyond a certain point. Sidelining this dilemma, the thermal resistance provided by lightweight concrete in LWT CMU can significantly boost energy performance without packing excessive insulation layers.

LWT CMU offers more resistance to heat flow than regular NWT CMU. A wall made with next-generation block, called SmartWall, when insulated with injection foam, can have two to three times the thermal performance than what is possible with insulated NWT CMU. This is why some even say that LWT CMU with ESCS qualifies as the concrete world’s version of insulation.

The University of Louisville conducted a study to test the performance of a single wythe wall in a secondary school in different climate zones. The three alternative wall configurations of the same size were compared, including one made with 12” 3-web NWT CMU, another made with 12” 3-web LWT CMU, and a SmartWall made with 12-inch 2-web next-generation block. The evaluation for yearly energy use showed SmartWall configurations outperformed standard concrete masonry in terms of thermal resistance, translating to reduced energy costs over the building's life cycle.

Additional Environmental Benefits of Lightweight CMU

Beyond reducing CO₂ emissions, LWT CMU and next-generation block offer other environmental benefits, particularly in transportation and long-term maintenance, due to their reduced weight. More blocks can be transported per load, reducing the number of trips needed to deliver materials to the job site. This means lower fuel consumption and fewer greenhouse gas emissions, further contributing to the project’s overall sustainability.

Moreover, the durability and fire resistance of LWT CMU make them inherently resilient, especially when used in load-bearing applications. These walls are also resistant to mold, termites, and rot, which eliminates the need for frequent repairs or replacements and reduces waste over the building’s lifecycle. In contrast to wood-framed structures, masonry buildings do not emit additional CO₂ when burned in a fire or decomposed in a landfill. These qualities make masonry structures both sustainable and resilient, able to withstand the test of time and reduce environmental impact.

A Well-rounded Solution for All

In addition to productivity and carbon reduction potential, LWT CMU made with ESCS lightweight aggregates ensure safer job sites. The block contains significantly less crystalline silica than NWT CMU, with levels about 83% lower than granite-based aggregates. When saw-cutting LWT CMU, the amount of silica dust is typically below OSHA’s action limit, making the jobsite environment safer for all.

Understanding the advantages of LWT CMU enables contractors to not only boost their productivity but also to actively support architects’ and engineers’ efforts to design low-carbon, high-performance buildings. From faster installation to reduced carbon footprints, LWT CMU represents a win-win solution that benefits all parties involved in the project, paving the way for more successful and sustainable outcomes in masonry construction.

About the Author

A member of the Expanded Shales, Clay, and Slate Institute (ESCSI), Jody Wall, P.E., LEED AP has been involved in many lightweight innovations since joining Stalite over 25 years ago. Jody has served as Chairman of the Board of the ESCSI, Chairman of ASTM Subcommittee C9.21, Chairman of the National Concrete Masonry Association Acceptable Workmanship Committee, and Chairman of the Board of Directors of the Southeast Concrete Masonry Association. Jody’s areas of interest include energy efficiency, structural design, and production optimization.

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