In the United States, concrete masonry units (which are also referred to as concrete block, cement block and foundation block) is a large, rectangular brick used in construction. These building blocks are made from concrete. The concrete is typically composed of Portland cement and aggregate, usually sand and fine gravel in high-density block. Lower density block may use aggregates that include industrial wastes, such as fly ash or cinders. From the historical use of such materials the term “cinder block” arose.
The building and construction world were largely devoid of materials testing and code as we know them today. As the threat of fire influenced the drive toward “fire proof” construction, and large cities created code to protect life and property, the focus tended to be toward commercial, institutional and industrial construction. The realm of residential construction where concrete block was born was largely unregulated. Most testing was performed by the owners of proprietary products and systems as a part of their marketing strategy. Nevertheless, testing and standards (discussed later) evolved in the process.
Like most advances in technology, invention and creativity was driven by a profit motive. Foundations were typically constructed of stone, and built by stone masons. While the materials used were typically those indigenous to the region, few areas offered stone that was easily workable in a building capacity. From the granite boulders used in Boston cellars, to the bluestone used in the basements of Schenectady, working with stone is a laborious process performed by skilled men. Compare that to a similar tradesman of the late nineteenth century, the bricklayer. It is no coincidence that the International Union of Bricklayers and Allied Craftworkers provides separate designations for members who perform stone masonry and those who lay brick; and the bricklayers also lay the block.
Bricklayers erect masonry walls using relatively standard materials that are uniform in size. Brick manufacturing, by comparison to the quarrying and dressing of stone, is a relatively simple (and mobile) operation. Once the brick are ready for installation, little more must be done but to lay the brick level and straight between two plumb points. Block work on the same principles, but with much larger units. Foundation walls, for example, can be laid up rapidly, with much less labor and material, and conform with the ideals of standardized, modular construction that were firming up in the latter part of the nineteenth century. It is also easier to teach apprentices to lay brick and block than it easy to train stone masons.
Several companies, including Sears Roebuck and Montgomery Ward, sold machines that allowed the “average man” to make his own block. In essence, the machine was a stand with a form that the block material was packed into. A wide variety of shapes and sizes were available; one of the more popular plates allowed for the creation of ‘rock faced’ block that created the illusion of stone. The use of aggregate was wide and varied, and allowed for the use of cinders, giving rise to the term “cinder block.” Manuals instructed block makers to use greater portions of Portland cement toward the outer face of the block which would be exposed to the elements. Such a lack of standards often back fired, with negative consequences.
At this time, Sears and others were marketing kit homes in the United States. These machines, and the blocks they produced, became integral parts of many home packages. In fact, many kits were sold to make entire block houses. Many extant structures are still found from the turn of the century. Many homes and buildings (including a church in Hudson, NY, with a block arch) were erected with this system. An example that was proffered as an early successful use of the technology was the Allen G. Thurman house (ca. 1885) in Columbus, Ohio.
In addition to the materials used in manufacturing, the curing process was critical to the creation of good blocks. Only 50% strength was achieved after a week of curing, and a mere 65% at 30 days, yet testing took place at 28 days, as that was the approximate time in which they would be used. 95% strength is achieved after 6 months, and 100% was predicted at one year. The earliest testing methods were simple but effective.
The test of transverse strength measured the modulus of rupture on three blocks at 28 days. The average required was 150 pounds, with no single unit failing below 100 pounds. The compression test required an average strength of 1000 psi between the three units tested, with no single unit failing below 700 psi. The absorption test was calculated by measuring the percentage of absorption (weight of water absorbed, divided the weight of the dry sample.) The test was deemed a failure if the average absorption exceeded 15% or any single unit exceeded 22%. Later testing for freezing and against extreme heat was introduced, varying somewhat wider than the three relatively standard tests described.
The introduction of Portland cement in US building and trades after 1872 spurred technological advances in masonry greater than any the trade had seen in its millennia of history. That’s not as grand a claim as it might first appear when one considers how relatively similar masonry practices and materials remained up until the nineteenth century. Portland cement allowed for the creation of concrete, cement-based mortars and concrete block; these were masonry’s contributions to building technology as iron and steel allowed structures to grow taller and larger than ever before.
“Cinder block fire test,” Concrete. Vol. 21, No. 1, pp. 123-5. New York, 1922.Concrete masonry unit, http://en.wikipedia.org/wiki/Concrete_masonry_unit
“Practical concrete block making,” by Charles Palliser, Industrial Publications Company. New York, 1908.Concrete block, http://www.madehow.com/Volume-3/Concrete-Block.html