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By: Bill O’Brien

August 26, 2020

During the 1800s, building fires became prevalent — most notably were the Great Chicago Fire of 1871 and the Great Boston Fire of 1872 — which forced the development of fire-rated building floor slabs. By the 1880s, various forms of brick and terra cotta tile arch floors were being used to facilitate the prevention of such large scale fires. The aforementioned types of building construction are not fireproof but can withstand some degree of exposure to fire without failure.

Although the cinder concrete slab system is no longer specified for use in new buildings, it remains relevant because it is a structural system utilized in many of our office buildings, residential buildings, school buildings, and industrial buildings in New York City and other urban areas. It is important that Engineers and Architects understand its origin, history, performance, and strengths and weaknesses, in order to properly plan renovations and repairs.

As the development of fireproof floors evolved, building department officials and insurance companies were forced to consider how floors were to be constructed. Eventually, the New York City Bureau of Buildings developed a fireproof slab that was implemented into the standards of the New York City Building Code at that time. The portion of the code that defined floor systems from 1891 to 1916 read: “All brick or stone arches, placed between iron floor beams, shall be at least four inches thick and have a rise of at least one and a quarter inches to each foot of span between the beams. Arches over a five feet span shall be properly increased in thickness…or the space may be filled in with sectional hollow brick of burnt clay or some equally good fire-proof material, having a depth of not less than one and one-quarter inches to each foot of span, a variable distance being allowed of not over six inches in the span between the beams.”

Another type of construction was brick/stone vaulting, which consists of creating an arch with brick or stone by using the weight of the material in compression to support the loading of the surface above. This was rarely used because of the heavy weight of the materials and the intensive labor required for construction. Since large steel-framed buildings were coming into the spotlight in the 1890s, designers and builders quickly became aware of the acute need for inexpensive, lightweight, and “fireproof” structural floors. In 1981, segmental terra cotta tile arches were starting to be utilized to reduce the weight of the materials, as well as the time and cost of labor. This system implements a prefabricated terra cotta section, that is set in wrought iron to create an arch, and is supported by the bottom flange of a steel beam. Although prefabrication terra cotta sections were a good alternative, there was a continued effort to improve building construction methods and reduce the weight of the structure.

In the winter of 1886/1887, Engineers Freytag & Heidschuch and G. A. Wayss & Cie, carried out extensive load tests on reinforced concrete structures in Berlin, Germany with some success, pushing more engineers to try. Eventually in 1899, Conrad Freitag developed and patented the first concrete drape floor slab. The drape floor slab was 1 of 406 patterns filed by Conrad and was called the “Metropolitan floor slab.” This slab consisted of an inverted arch of reinforcement, comprised of twisted pairs of wires individually strung across the building, and is anchored at slab edges, and drapes over the floor beams and under a hold-down bar at mid-span. Since the wires carry all loads, the slab serves only to provide a level surface and fireproofing. In many applications, wood or tile was used to create a unified walking surface.

In 1906, the cinder concrete draped slab was patented by two Engineers, Buel and Hill. Cinder is the by-product of burning coal, so recycling the cinder was an economical way to replace the more expensive aggregates. Additionally, cinder is lightweight and provided good fire resistance. The twisted pairs of steel wires used in this slab were replaced with a welded wire mesh that was first patented in 1901. Although this wire mesh had a variety of uses, it began to enter the building market in 1906, where rolls of wire mesh could be easily shipped and rolled out on a job site and used as a reinforcement in the structural slabs. The implementation of the wire mesh also provided a uniform structural loading.

During construction from the 1920s through 1960s, draped mesh cinder concrete slabs became popular as a lightweight, controlled-strength, and fireproofed concrete solution. These slabs can be found throughout New York City and other older urban areas where coal was burned throughout the city for heat and electricity. Over time, various modifications were made to the slab system in order to accommodate provisions for plumbing and steam piping, but the basic principal behind using mesh as the structural element of the concrete slab remained the same. Piping would be placed above the structural drape slab and covered with cinder fill, and then a wood or thin concrete topping was placed atop as a bearing surface.

The cinder that was utilized as an insulation in roofs has a poor R-value, but did provide minimal insulation to the steam piping that was concealed in the roof structure. Since cinder concrete is more porous than regular concrete, water can easily migrate through the concrete to the steel welded wire reinforcement, causing deterioration and structural integrity loss. Prolonged water exposure will cause corrosion and put outward pressure on the surrounding cinder concrete. The cinder slab will “sag” and crack, and eventually collapse from the severe stretching and corrosion of the steel wire mesh. Failure of the steel wire is generally called “ductile failure,” and can lead to extremely unsafe and disastrous conditions if left unaddressed.

Potential failures to a cinder concrete slab depend on many variables, such as the locations and types of slab terminations used during construction. It is very important for Engineers and Contractors to fully understand the construction of these structures so they properly plan and execute repairs. If water infiltration is present at a building with cinder concrete slabs, Sullivan Engineering highly recommends that the source of water infiltration be addressed and repaired, immediately. Water infiltration that damages the reinforcement may undermine the structural integrity and may require partial or total replacement of the cinder concrete slab. Fortunately, cinder concrete slabs have not been an industry normal for more than 40 years and are no longer installed in buildings because they are outdated and are costly to repair.

We recommend that property managers and building owners consult a qualified Engineer or Architect that can effectively assess the conditions of the cinder slabs in order to provide proven solutions for repairs and replacement.

If your building consists of cinder concrete slabs, repairs are inevitable, so make sure to hire an Engineer who understands the conditions and can perform the proper work.

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