How to Produce Concrete Masonry Units

Concrete masonry units (CMUs) are produced in virtually every country in the world because of their high quality and the demand for economical building materials. A variety of local raw materials can be used to produce CMUs in thousands of different sizes and shapes. All these products can be made in the same plant by changing the mold. Despite the variables, the production process in a Besser plant is simple.

The main raw materials used to produce CMUs are cement, sand, and aggregate. These are normally delivered to the concrete products plant by truck or rail. Cement is pneumatically transferred to the storage silos equipped with dust collectors. When the sand and aggregates arrive they are normally stockpiled in the yard, then transferred as needed to storage bins. The most common method of transferring materials from stockpiles to bins is with a front-end loader. Some plants use labor-saving conveyor belts and stackers.

Aggregates used to produce normal weight concrete products include sand, gravel, and crushed stone. The sand and gravel may be natural or produced by crushing at the aggregate source. Aggregates used in lightweight concrete products include expanded shale that has been fired, clay, slate, and expanded blast furnace slag, sintered fly ash, coal cinders and natural material such as pumice, tuffa and scoria blended with sand.

Lightweight aggregates can reduce the weight of CMUs by 20% to 45% when compared to normal weight CMUs, without sacrificing structural properties. Lightweight CMUs retain superior fire ratings and insulation properties. Since lightweight CMUs use specialized aggregates, they generally cost more than normal weight block. However, this depends on the availability of lightweight aggregates and the proximity of the aggregate source to the plant.

Cementing ingredients are cement, fly ash and other pozzolanics. Some plants use more expensive "high-early strength" cements to reduce overall costs. Fly ash and pozzolanics are less expensive than cement and are used to improve the properties of the wet concrete mix. Because they are moisture-sensitive, cement and pozzolanics are transferred directly from trucks or rail cars to storage silos pneumatically using pipelines.

The raw materials in the storage bins and silos are transported to a weighing or batching system, as needed. Raw materials for each batch are weighed to ensure batch consistency.

The mixer is an oversized cylinder turned on its side, with mixing blades attached to a horizontal shaft extending into and through the drum. The inside of the mixer drum is lined with replaceable, hardened steel liners.

Raw materials are fed into the mixer at the top. Materials are dry mixed in the mixer for several minutes. Water is then added to the dry mix using an electronic water meter to assure consistency. Since CMUs are generally molded using zero slump concrete, only a relatively small amount of water is added to each batch. Admixtures, such as water repellents and coloring agents, are also added at this time. After some mixing, the consistency is automatically checked and if needed, additional water is added. The batch is then mixed for five to eight minutes. Once mixed, the concrete is dumped from the bottom of the mixer.

After a concrete batch is mixed, it is transferred to the concrete products machine, where it is fed into a mold. Molds consist of a mold frame and replaceable liners, cores and "shoes" that form the top surface of the masonry unit. The mold can be expensive, however, it has an unlimited life. Liners do require periodic replacement. The shape and hardness of the aggregate determine the longevity of liners. Not all parts of the mold need to be replaced at the same time. Some parts can tolerate greater amounts of wear before replacement.

Masonry units can be manufactured in almost any configuration, provided that the structural integrity of the unit is not compromised by walls or webs that are too thin or structurally unstable. A single mold box can make many different shapes by altering the combinations of parts that are assembled. Depending on the demand for the particular shape being produced, the mold may be changed after several hours or days. A mold change allows the company to make products as diverse as hollow plain products, brick, floor filler products, paving stones and sophisticated colored architectural products while using the same equipment in the same facility.

Once placed in the mold, the concrete is compacted and consolidated by a combination of pressure and controlled vibration. In Besser concrete products machines, the entire mold, the internal parts and the steel pallet forming the bottom of the products can be vibrated to maximize compaction, uniformity and strength. The mold is filled, compacted and stripped 6 to 11 times per minute. The concrete products machine uses molds capable of producing up to six standard size products at a time. Depending on the size and type of machine, more than 3240 (8" equivalent) concrete masonry units can be made each hour. Compacted products are pushed out of the mold onto a steel pallet. This differs from the multi-piece molds used in other industries, where mold sections are removed in opposite directions. At this point, the concrete products are referred to as "green" or uncured. As a pallet of "green" product leaves the concrete products machine, a rotating brush is sometimes used to remove loose aggregate from the top of the concrete products. A short blast of air is another way to remove loose pieces of concrete.

Once a curing rack is filled with steel pallets of "green" or uncured products, the entire rack is transported, usually on a rail mounted automated rack transporter system, to the curing environment. Non-automated plants may use a fork lift truck to transport the products.

A curing environment, referred to as a kiln or curing chamber, operates at atmospheric  pressure. Generally, the concrete products are held in the kiln for 12 to 24 hours.

Concrete products are normally cured at a maximum temperature of 120° to 180° F (55° to 75° C). Note: Curing CMUs requires a fraction of the energy used to produce clay brick, which are fired at a temperature as high as 800° F (425° C). When the temperature reaches its predetermined level, the steam, if used, is automatically shut off and the products are allowed to cure for an additional period.

Steam, if used, is turned on to maintain 100% humidity. In some climates, the heat generated by the chemical process, referred to as the heat of hydration, is sufficient to raise the temperature to desired levels without the use of steam.

The entire curing process normally takes 24 hours, but could be shortened by adjusting the mix design and curing temperatures. Products generally achieve 90% of their ultimate strength when 2 to 4 days old.

Cured concrete products are removed from the kilns and moved to a processing area where optional operations take place to create architectural units. The units are then "cubed" or palletized and placed in storage. Cubing consists of turning individual units and placing them in alternating layers to create an interlocking cube. 

Cubing is typically done by a piece of equipment that can be programmed to automatically create patterns. The cube of products can be placed on a wood pallet or handled by a fork lift using the cores of the bottom product as lifting locations. Some higher priced "value added" products are covered with plastic for protection. Cubes are generally stacked three to four high in the yard until they are delivered to a job site.