High-Performance Masonry Units: Elevating Structural Integrity and Aesthetics

Words: Peter Roberts
Photos: Stephen R. LiVoti, Best Block

Manufactured concrete masonry units have been present in the construction marketplace for over 120 years. They are ubiquitous and are made in virtually every country on earth. The development of high-performance masonry units currently available across the United States and throughout the world is a testament to the rigorous standards, precision, innovation, and craftsmanship of the concrete block manufacturing industry. High-performance masonry units include standard concrete blocks used for wall construction, insulated masonry units, architectural precast concrete, engineered stone (mentions non-masonry materials), retaining walls, and more. These products provide the construction industry and its customers with advanced structural benefits, aesthetic versatility, and installation efficiencies unavailable at the scale, cost, and convenience of other materials and methods.

This article delves into the technical aspects of high-performance concrete block masonry units, highlighting the processes, materials, and benefits of working with high-strength CMUs.

Defining High-Strength Concrete Block Masonry Units
Concrete Masonry Units, as defined by ASTM C90, are well-known products that have been successfully produced and commercially developed for over 120 years. CMUs are renowned for their versatility, aesthetics, strength, and durability, making them a favored choice for architects and builders seeking a reliable, tried, and true method for building vertical walls. The benefits of concrete block masonry construction extend to insulated CMUs, architectural precast concrete, engineered stone (mentions non-masonry materials), retaining walls, and roofing units.

A typical concrete block is primarily specified for exterior vertical walls, trim, and ornamental elements. Typical applications include load-bearing structural walls, corners, columns, lintels, and interior applications, including non-load bearing walls and designs as specified by ASTM C129, including mantels, replicating and replacing deteriorated elements in historic structures, and landscaping features, including retaining walls.

High Strength
The CMU is a building block composed of concrete that is linked together with mortar for the construction of walls. Instead of bricks used as a construction material, high-strength concrete blocks are often preferred to be used as masonry units. Brickwork for construction has been largely replaced by concrete masonry units. There are several reasons why it has become a popular building material in recent years. It is strong and durable, meaning it can withstand severe weather events, wildfires, and other similar threats. It is also easy to work with, making it a popular choice for building structures that can last over long spans.

Additionally, the concrete block is fire-resistant and insect-resistant, making it a low-maintenance option for both homes and other structures. Plus, the concrete block is a sustainable building material, as it can be made from recycled materials. But most importantly, one of the key factors in its success is its compressive strength.

Typical CMUs can be expected to have a compressive strength of at least 2,500 psi or 17.5 N/mm². Strengths are often higher and are commonly as high as 4,000 psi and even higher. This is quite incredible compressive strength for a material widely available at affordable prices across the United States and around the world.

Anisotropy
Isotropic material has the same property in all directions: strength (for example), would be measured with the same value whether it was measured along the x, y, or z axis in a square cube. Manufactured concrete blocks are anisotropic and have different strengths depending on which direction it is measured. This is due to how CMUs are manufactured: the concrete mix is violently vibrated and shaken within the mold as the material is pressed, which results in the orientation of aggregate such that the longer axis of the aggregate is oriented normal (at 90°) to the applied force of compaction as blocks are made.

This results in an intimate interlocking of “tip-to-face” contact with the aggregate in the concrete mix, and the corresponding high compressive strength in the axis of compaction as blocks are made. In a typical CMU used to assemble walls, this high-strength axis is oriented vertically, as you would want the high strength oriented for a vertical wall.

In concrete masonry units used to assemble arched roofs, the high-strength axis is oriented radially toward the outside of the structure, as you would want the high-strength axis oriented for an arched roof, to better handle impacts in extreme weather events such as hurricanes, tornadoes, etc. Testing by the author has shown that the difference in anisotropic compressive strength of a typical CMU displays 70% higher strength in the high-strength axis of manufactured concrete block, which is quite significant. The images below show the experimental process used to measure anisotropy in CMUs and the aggregate shown in CMUs using optical microscopy in research done by the author with his student, Steven LiVoti.

Experimental Process to Determine Anisotropy in CMU

Optical Microscopy of Aggregate within Anisotropic Concrete

Insulated Concrete Masonry Units
Concrete blocks have the benefit of high thermal mass, which is the ability to store thermal energy in the masonry building envelope. This high thermal mass delays the time for the heat or cold of the outside to penetrate the building and make it hot or cold on the inside (especially compared to other types of buildings, especially wood frame construction (mentions non-masonry material),and necessary to either heat or cool the inside. The big advantage of this thermal delay is that it reduces the peak load of a given energy grid and thus reduces the peak load requirements of the energy grid. This is especially important because any energy grid is defined by its peak load requirements: by reducing the peak load, the energy grid does not have to have such a high capacity.

Another important feature of high-strength concrete masonry units is the development of insulating concrete blocks. Insulating block typically uses an insert of expanded polystyrene (XPS) within the inner and outer walls of the concrete block. Some CMUs use an interlocking feature of the XPS with the block to prevent any thermal bridging of the concrete since concrete is not a great thermal insulator. Some structural applications require the high strength of having the inner and outer walls of CMUs to be connected as a continuous piece of concrete, so both types of insulating block are available: either interlocking XPS or webs of concrete connecting the outside wall to the inside wall, as with a conventional CMU. It should also be noted that insulating concrete block has the added benefit of providing acoustic insulation, in addition to thermal insulation. This helps to greatly reduce noise from the outside.

Concrete Block Segmented Retaining Walls
Segmented retaining walls (SRWs) have become a very popular and commercially successful component of the concrete block industry. SRWs provide an affordable, easy-to-install method of providing retaining walls for landscaping and hardscaping applications. There are several great SRW products available: they typically use an interlocking feature, which may be a part of the block, or may include an independent plastic pin that helps lock subsequent courses of SRW together vertically. The locking features (pins, keys, etc.) are typically complemented by being anchored with a geotextile fabric. This geotextile goes behind the assembled SRW wall and is covered by an overburden of dirt or soil to help lock and anchor the wall into place. (Highlights mention of non-masonry materials and methods). SRW walls are available in several shapes, textures (split-faced, polished, etc.), and colors for an attractive appearance in providing the end-user with a number of aesthetic options which showcase the beauty of block.

Concrete Block Roofing
Spherical Block LLC has recently developed a roofing system using high-strength manufactured concrete block to extend the benefits of concrete block masonry to the top of the roof of a building. Roofs can be assembled as arches and domes from these specialty CMUs. This provides a complete building envelope made from high-strength CMUs, with all the benefits of CMU construction, including high strength, fire safety, energy efficiency, affordability, and the beauty of the block. This technology was described in greater detail in the recent issue of Masonry Magazine.

High-strength Concrete Masonry Units provide customers with an excellent option for their building needs. This highly developed technology is well-known, tried over time, affordable, and creates safe, efficient, beautiful buildings and structures.

High-Performance Masonry Units: Elevating Structural Integrity and Aesthetics

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