Lightweight Concrete Masonry Units 101

By Kevin Cavanaugh
Techical Marketing Consultant & Sales Team Leader – Midwest/Great Lakes Region, Arcosa Lightweight

High-performing lightweight aggregates (LWAs) like expanded shales, clays or slates (ESCS),  are key ingredients to producing excellent lightweight concrete masonry units ( LWT CMU). A typical LWT CMU  weighs 30-40 percent less than normalweight (NWT) CMU and meets all the requirements of ASTM C90 (Standard Specification for Loadbearing Concrete Masonry Units).

ESCS aggregates add significant value to LWT CMU in addition to reduced weight. The ceramic-like ESCS lightweight aggregate is made by expanding selected raw shale, clay or slate minerals in a rotary kiln at temperatures over 2000°F. The expansion or bloating process makes the finished ESSC aggregate structurally strong, durable, inert and lightweight. All of these in turn benefit CMU. Consequently, lightweight CMU aids both the design, construction and operation of buildings made with concrete masonry walls.

Masonry blocks with superior fire resistance  

All CMU have very good fire resistance ratings and perform exactly as designed in a real-life fire situation. LWT CMU perform even better because the ESCS LWAs in them contain trapped air bubbles, which act as an insulator. As a result, less concrete is required to achieve the fire resistance ratings when making LWT CMU versus normal weight CMU. In fact, LWT CMU can easily and economically achieve two, three and four-hour fire ratings based on the minimum equivalent thickness requirement specified in building codes. 

Because ESCS LWA in LWT CMU is an insulator, less concrete is required to withstand fire condition temperature increases. Additionally, an increased quantity LWT CMU can be stacked on a single pallet and consequently, in a truckload. As a result, fewer truckloads are required to transport the blocks to the site, in turn reducing the amount of fossil fuel used, which benefits the environment.

SmartWall LWT CMU – Higher Compressive Strength and even Lighter LWT CMU

As per ASTM C90, generic or regular LWT CMU made with concrete weighing no more than 105 pounds per cubic foot (pcf) are required to have a minimum compressive strength of 2,000 pounds per square inch (psi) . Ultra LWT and Ultra strong LWT CMU, also known as ‘SmartWall’ blocks exceed both the minimum ASTM C90 density and strength requirements.

SmartWall units have a concrete density less than or equal go <93 pcf, a minimum 2500 psi compressive strength and are made using an optimized gradation of ESCS LWAs combined with regular aggregates. A higher strength allows structural engineers to design even more efficient load-bearing structures. Additionally, these even lighter CMU lead to improved mason productivity and morale which can shorten construction schedules.

LWT CMU Walls can Easily Meet Energy Codes even in Cold Climates

Depending on the number of cross webs and web thicknesses, insulated LWT CMU have R-Values that are 2-3 times higher than an identical NWT CMU. Consequently, these higher R-Values make meeting energy codes easier. That said, it is recommended to use the COMChecksoftware to determine energy code compliance for commercial buildings. In the same vein, whole building computer modeling studies have shown that LWT CMU, both the traditional 3-web and newer 2-webs units perform very well in all eight climates zones in three Department of Energy (DOE) prototype buildings, including schools, supermarkets and big box retail stores.

LWT CMU Facilitates Mason Safety and Faster Construction.

While using lightweight CMU has advantages for the design team, masons and subcontractors also benefit. Not only does the 30-40 percent lighter LWT CMU speed up the construction of CMU walls, but it also reduces back and musculoskeletal injuries. Reducing job site injuries greatly reduces Worker Compensation Claims, which in turn reduces the mason contractors’ insurance premiums and wall costs to the owner. 

In most areas of the US, wages for masons and mason tenders have reached the level that only two more blocks per hour are needed to be placed into the wall to pay for the LWT CMU’s higher cost. The project owner is paying for a finished wall, and if the finished wall’s cost is the same or less when using LWT CMU, why not use LWT CMU?  After all, you get a better CMU product and the mason can build more walls (and complete more projects in a given time frame) all while laying a lighter, easier to handle CMU that preserves the mason’s health and wellbeing. As a result, mason contractors are able to deliver more work and bid even more work.

Best of Both Worlds LWT CMU made with ESCS LWA benefits all stakeholders involved in the design, manufacturing, construction and operational phases of a building’s life. Click here to see how lightweight CMU were used in successful projects to gain more insight. Find answers to commonly asked questions about LWT CMU here.

By ESCSI | December 19, 2023 | Articles