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Concrete Masonry Products
1) History Of Concrete Block
2) Manufacturing Process
3) CMU Today
4) Properties & Characteristics
5) Types Of Concrete Masonry Units
6) CMU Shapes
1. History Of Concrete Block
Concrete Masonry Units are commonly referred to today as "CMU's" or Concrete Blocks" by most writers of specifications, architects, engineers, and builders. The modular masonry building material associates with other concrete building materials because the end product is a hardened material primarily from portland cement, graded aggregates, and water. Its modular size and product characteristics have found modular concrete units a home with stone and clay materials used widely in masonry construction.
Since 1882 when the first concrete block was molded in a manual process, the industry has incorporated a high degree of automation. Harmon S. Palmer is generally credited with the development of the first commercial process for manufacture of concrete block in the United States. Palmer patented the basic principle of the hollow concrete building block machine with removable cores and adjustable sides in 1900. Palmer's concrete blocks were poured at the job site to save cost of transportation and eliminate breakage in handling. The blocks were quite large in size measuring 30' x 8" x 10" made from cement, sand, and water. The units were so large and heavy they had to be set in the wall with the aid of a hand cranked derrick.
Block machine at the turn of the century
A few years following Palmer's process the first hand tamp block machine (circa 1904) was developed. The equipment consisted of vertically placed cores and collapsible sides for block removal. Cement and aggregates were hand mixed, then shoveled into the mold and hand tamped around the core. The CMU varied in consistency and quality from unit to unit and generally measured 24" x 12" x 8". Three men working at top speed could turn out 200 block in a 10 hour day on this first machine.
In the early 1900s high costs and scarcity of competing materials made it natural for the concrete masonry industry to develop. One concrete block in 1906 replaced 28 common brick as an alternative product. Builders turned to this new material as a partial solution to their problems. Domestic concrete block manufacturers had several cost advantages over competing materials. Freight charges on lumber, stone, and usually on brick were greater than block, which was locally manufactured utilizing readily available materials. Growth of the concrete industry closely paralleled that of the Portland Cement industry during the first few years following 1900 thus creating reasonable prices for cement.
In 1904, Herman Besser used a hand manufacture block machine. His son designed and engineered their first automated block machine. The Besser Company is considered a pioneer in marketing concrete block manufacturing equipment and their developments are viewed as key milestones in the history of manufacturing concrete masonry units.
A breakthrough came in 1909, when a machine was developed that featured power tamping and self discharging mixer and skip loader, eliminating the laborious job of hand tamping.
In 1939, Besser introduced a radically new production method of "vibration under pressure," a system which eliminated costly wear on the facing liner of the machine associated with tamping.
In summary, the milestones mentioned above are essential for one to fully appreciate the manufacturing technology incorporated into our modern day automated concrete products manufacturing facilities. In the Pacific Northwest, block producers have fallen favor to two equipment manufacturers: The Besser Company in Alpena, Michigan, and Columbia Machine, Inc., Vancouver, Washington.
Handling of aggregates in a modern CMU plant
Production has changed from large cumbersome units produced one at a time in crude metal forms at the job site requiring large quantities of labor, to smaller high quality modules manufactured in large volume at highly automated facilities. Portland cements used in concrete have been greatly improved since the early years and quality of aggregates along with their gradation improvements have enhanced the concrete masonry industry.
2. Manufacturing Process
Concrete masonry units are made mainly of portland cement, graded aggregates, and water. Depending upon specific requirements, the concrete mixtures may also contain other suitable ingredients such as an air-entraining agent, coloring pigment, and siliceous and pozzolanic materials.
Manufacturing process diagram
Mass production has contributed to the relatively low cost of quality concrete masonry units. In many production plants some phases of the manufacturing process are completely automated.
Briefly, the manufacturing process involves the machine-molding of very dry, no-slump concrete in to the desired shapes, which are then subjected to an accelerated curing procedure. This is generally followed by a storage or drying phase so the moisture content of the units may be reduced to the specified moisture limits prior to shipment. The concrete mixtures must be carefully proportioned and their consistency controlled so that texture, color, dimensional tolerances, and other desired physical properties are obtained, High-strength units have concrete with higher cement contents and more water, but still have no slump, Automatic machines consolidate, mold, and compact these concretes by vibration and pressure.
Acceleration curing is utilized by the concrete masonry industry, with variations according to local plant requirements and raw materials used, The common type of curing provides for heating the block in a steam kiln at atmospheric pressure to temperatures ranging from 120 to 180 degrees F. for periods up to 18 hours, Atmospheric pressure methods may require subsequent accelerated drying treatment of a period of natural drying in the storage yard under protective cover. A variation of this low-pressure curing is the carbonation stage, which is added to reduce the shrinkage characteristics of the masonry units.
Following steam curing and gas drying the units are placed onto wooden pallets and stored for future shipping.
2.1. Handling and Storage of Aggregates. Proximity of a production facility in relationship to its aggregate source is very important to the success of a block manufacturing company. Both sands and gravels are trucked into the plant and stored separately. Pumice aggregates are brought into the Northwest either by truck or rail. Availability of plentiful, highly uniform, graded materials is essential, While aggregate materials are brought in directly from the pit any contained moisture is accommodated for through meter readings during the batching process.
Cements are trucked into the production plant and stored in silos separate from other materials and moisture to reduce possibility of contamination.
2.2. Batching and Mixing. Metering and weighing systems for batching concrete vary widely depending on the equipment manufacturer. Batching the right proportions of sand, gravel, pumice, cement, and water may be semi or fully automated and must be given recognition for the role it plays in relationship to quality of the end product. Integral colored concrete masonry units are manufactured by adding coloring oxides during the mixing process.
CMU plant batching and mixing
2.3. Handling Wet Mixes. This element is mentioned only to point out there is a variety of equipment available for handling concrete while in its plastic stage. Some facilities operate in a vertical fashion where the mixer is mounted high off the ground placed directly above the block machine hopper, In most cases the mixer is located near ground level wherein various conveyors or bucket type of skip elevators are employed.
2.4. Block Machines, Molds and Dies. The work horse of the concrete unit manufacturing facility is the block machine. In the Northwest either "Columbia" or "Besser" machines are used. Many plants have only one block machine on line, however, because they last many years and are quite expensive some plants have modified their facility to accommodate more than one machine when equipment is upgraded.
Block machine in operation
Block machines are designed to employ the use of molds and dies in forming and shaping modular sized units, A wet no slump mix enters into the machine wherein it is vibrated and compressed into two or more units during each machine cycle. Block machines making three units per pass have become the industry's standard over the years, however, machines producing larger quantities are available.
Molds and dies have been developed to accommodate a multitude of shapes and sizes, Companies tend to make readily available those units where volume justifies tooling and die costs. Customer demand plays a large part in regionalizing manufacturer product lines throughout the U.S.
2.5. Transfer Equipment and Kilns.
Automation has contributed substantially to improvements in concrete masonry unit quality. Constant high quality products are moved by transfer equipment from the block machine into the steam and drying kilns and out of kilns ready for palletizing.
While mixes, block machines, and molds and dies play important roles in the physical properties and characteristics of CMU's, the kiln curing and drying processes are of vital importance. Following ASTM requirements for unit and composite strengths along with dimensional tolerances and moisture content standards, the block manufacturer creates atmospheric conditions inside of kilns using steam and temperature combinations conducive for accelerating concrete curing.
2.6. Palletizing and Storage. Steel pallets large enough to handle three 8x8x16 standard size c.m.u.'s are used to transfer block throughout the curing process. At the end of the automation line c.m.u.'s are placed either manually or mechanically onto wood pallets. An automated machine for palletizing is called a cuber and is operated by one person, versus manual stacking by two or more people.
An important item to point out in storing c.m.u. s is the need for light weight block to be kept dry so that ASTM moisture content specifications are met.
The NW Masonry Guide Table of Contents
Masonry Institute of Washington
Washington State Conference of Mason Contractors
