A Look At How CMU Is Produced

Words:

Words: Steve Hansen 
Photo: CatLane

This might seem a bit basic for masonry professionals, but do you actually know how CMU is made? It’s fascinating to learn how the product that we all know well—block—has evolved in manufacturing. Materials have seen more of an evolution in the time that manufacturing experienced a revolution. Back in the day, masons used to cast small batches onsite in small molds. Now, of course, huge trucks with lifts drop materials at the job site and off you go. Let’s take a look at the process and more.

First, there’s more than one type of block. 

  • Standard stretcher: this is the everyday block with two face shells and three webs.
  • Corner or double square end: used for general purpose and for forming corners.
  • Two-web unit: used to accommodate reinforcement; reduces the number of cross-webs in an assembly; can reduce material usage and can be used to increase energy efficiency.
  • Single-web or “H-block”: used in walls with ample reinforcing steel; also eliminates cross webs for greater energy efficiency.

These are standard units available always and everywhere, for the most part, but anything can be cast to specifications. CMU may be designed for a specific application to achieve the desired appearance, size, or performance. That’s one of the advantages of concrete—it’s not fussy about how it’s used.

Other configurations are common

Other typical configurations include segmental retaining wall units (SRW). You see these everywhere in a variety of sizes and typically dry stacked in retaining walls. Pavers of all shapes and sizes are available, and grid paving units are used for erosion control and allow grass to grow inside the cells. Finally, ACB or articulating concrete block is used for erosion control and revetment systems.

The standard configuration of CMU is two face shells, which are the long sides, and three webs, or the short sides and middle web. Two cells can be used for running ductwork, conduit, piping, and rebar. All CMU dimensions and materials are stipulated by ASTM requirements to ensure minimum impact resistance, strength, and robustness.

Applications and materials

Materials and methods used to produce all these different units are generally the same, though details can be tweaked to meet different standards. Most CMU is used in vertical wall construction and may be either load-bearing or non-loadbearing. Each application may call for a different standard, such as units used in load-bearing applications being required to meet ASTM C90. These standards, of course, ensure compressive strength, dimensional tolerance limitations, water absorption, and permissible constituent materials. SRW and paving units may need to meet different standards, as they’ll typically be dry stacked.

Materials

All concrete products will contain some type of cementitious material, aggregates, and water. Some requirements call for admixtures and/or pigments. Depending on the properties desired, aggregates will make up 75-85% of the mix, cementitious materials will be at 10-15%, water will be 5-10%, and any admixtures and pigments just 1% or less. Compared to other concrete building methods such as tilt-up, poured-in-place, and ICF, CMU walls may use up to 25% less cement.

CMU can be made from standard portland and hydraulic cements, and also with supplementary cementitious materials (SCM) like silica fume, slag cement, and fly ash. These SCM act as a binder in the mix, providing strength and durability. They may replace cement or may be filler. The water/cement ratio is also a strong contributor to the overall strength and durability of the end product.

Portland cement and other cementitious materials

Portland cement is a specific type of cement that complies with the ASTM C150 standard. Types I and II are the most common, Type III is used when high early strength is needed, and Type V is used when high sulfate resistance is required.

Adding ground limestone to C150 cement yields block cement, which has been used for many years, but less so recently with the advent of ASTM C595. This newer standard allows larger amounts of limestone additions.

Blended hydraulic cements add one of the supplementary cementitious materials (SCM) mentioned above, as required to comply with ASTM C595 to yield the desired properties.

Hydraulic cement complies with ASTM C1157, which is a performance standard and does not specify materials. Rather, it’s focused on characteristics such as strength, set time, and permeability.

Pozzolans are another category of SCM that include fly ash produced in coal plants and natural pozzolans. These additives can increase strength and comply with ASTM C618. Besides fly ash, ground blast furnace slag is another waste product that’s useful in cement production; ASTM C989 covers this one, while ASTM C1240 covers silica fume, a byproduct of electric arc furnaces.

Aggregates

Providing many qualities including bulk and strength, a variety of aggregates are used in manufacturing CMU. ASTM C33 covers normal-weight aggregates, while ASTM C331 covers lightweight aggregates. Lightweight density is considered as 105 pounds/cubic foot or less.

ASTM C33 includes sand, gravel, stone, crushed hydraulic-cement concrete, and air-cooled blast furnace slag. Organic impurities, the presence of clay lumps, friable particles are all regulated.

The lightweight standard ASTM C331 includes natural aggregates such as tuff, scoria, and pumice, as well as manufactured materials like blast-furnace slag and fly ash and by-products like those produced in coal and coke combustion. This standard also includes mandates for the presence of iron-staining materials and pop-out materials, in addition to density.

Another standard, ASTM C90, differs from ASTM C33 and C331 in that it exempts aggregates for CMU production grading requirements, allowing CMU producers to use the aggregates best suited for their production and desired properties, rather than be required to meet specific requirements for aggregates.

Water

As with all concrete production, water is essential for CMU manufacturing. Water is part of the chemical process that turns raw materials into concrete. Though potable water is most often used, it’s not required as long as harmful impurities are not present.

Admixtures

Though not used in all CMU, admixtures are common in achieving various properties. These admixtures typically enhance certain properties of the final product or improve the production process in some way. Three common admixtures include plasticizers, integral water repellents, and efflorescence control.

Plasticizers can act as a lubricant in a low-water “dry-cast” concrete mix and help to disburse cement particles. Integral water repellents can be used with other moisture-management tactics such as weeps and flashing as part of a dry-wall system. Efflorescence, the whitish deposit that sometimes forms on the surface of CMU as soluble salts move to the face, can also be managed with admixtures.

Pigments

For an easy aesthetic touch, pigments are available in liquid, powder, and granulated forms, and comply with ASTM C979.

Mixing and curing

Concrete manufacturing is a lot like baking: following the recipe yields the desired results. A major factor in optimal results is the moisture content of the mix; too much water results in an excessive slump and the CMU won’t hold its shape. Too little moisture may be good for production efficiency in the block machine but results in less-optimal finished properties. Curing after forming is also important, as a moisture-rich curing environment maximizes cement hydration. 

Compared to ready-mix concrete, a CMU mix uses smaller aggregates—about ⅜” compared to ¾”—and is not pourable. CMU mix is vibrated into forms, and the smaller aggregates ensure that the mix reaches all areas of the form. Typical slump for the ready mix is 3-6”, while CMU mix is zero slump, meaning the mix will not slump under its own weight.

History of CMU

In the not-so-good old days, masons made blocks on-site as needed using a manual device, or form. Advertisements from that time claim a production output of 300 blocks per day, compared to output of 30,000 blocks per day today with equivalent manpower. 

Modern CMU production

Making CMU today is a partially automated system that includes:

  • Raw material handling
  • Material mixing
  • Concrete forming
  • “Wet-side” handling into kilns
  • Curing
  • “Dry-side” handling
  • Optional finishing like splitting
  • Cubing and transport

The raw materials, mainly aggregates and cement, arrive via truck, rail, or boat. While aggregates can be stored outside in bins, cement must be protected from moisture and is typically stored in silos.

From their storage areas, the raw materials are loaded onto a conveyor belt and then into a weigh hopper. The materials are added to the mix by weight and controlled by the computerized batching system. This system adds the materials required for the desired mix and can measure and compensate for moisture in the materials.

Next, it’s off to the mixer, which does what you would expect, then the concrete mix travels to the block machine. The wet mix is loaded into the block mold, which creates the size and shape of the finished block. The compaction head drops down and compacts the material while the machine vibrates the mix to fill any voids and achieve the desired density.

The units come out of the mold and onto a conveyor belt, get hit with a de-burring brush to remove any loose materials, and then go onto specialized racks and into the kiln for curing. The curing process is crucial for developing strength by introducing moisture to the dry mix in the presence of mildly elevated temperatures of 120-140°F. Moisture comes from steam generation, water misters, or vaporization systems, while heat may come from heaters or from the curing process itself within the closed kiln. 

Coming out of the kiln, the units go into the “dry-side” processes of palletizing and wrapping for transport, then await shipment. CMU slated for finishing operations will go-to machines for various architectural finishes like splitting, grinding, tumbling, or prefacing, then to palletizing and wrapping.

Finally, CMU is loaded and transported to the wholesale yard, retailer, or job site.


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