Under Construction: Concrete

By Gregory Havel

Buildings under construction do not always give a clear indication of what they will actually look like or how they are constructed when completed. They also do not indicate how they will behave under fire conditions, how combustible they are, the location or absence of fire-rated wall and floor-ceiling assemblies, or the presence or absence of concealed spaces.

Photo 1 shows wooden form work supported by “falsework” of wood and steel in place for the second floor (first-floor ceiling) of a multistory building of poured-in-place concrete. Wood beams and girders support the plywood deck on which the reinforced concrete will be poured. The beams and girders are supported by adjustable steel and wood columns. After several applications of form release [a chemical sprayed on concrete forms to prevent the concrete from sticking as it cures. It is frequently kerosene-based, but it may also be a water-based plastic emulsion], the plywood forms, and the wood supports become saturated with form release; it will burn furiously if ignited.

(1) Photos by author.

 

At the lower left in photo 1 is the architect’s rendering, showing what the building will look like when it is complete. From the exterior (or from the finished interior), you will be unable to tell the type of construction or its fire resistance. You must take notes on your preincident plans for this building; whatever you know about it based on photos and notes made during its construction.

Photo 2 shows the three lower floors of the same building after completion and the forms and falsework in place for pouring the concrete roof. This photo also shows clearly from the forms that are still in place that the building’s frame is poured-in-place reinforced concrete. From the thin cross-section of the floor in the foreground, it is likely that the floors in this building are posttensioned concrete rather than simply reinforced. This also deserves a note on your preincident plan.

(2)

 

In Photo 2, also note that the falsework is in place not just to support the fresh concrete on the roof deck. Temporary support columns are visible to support the floor below the roof and, again, to support the floor below that.

Concrete takes time to cure and to gain its designed strength; this often takes 30 days or more and is tested by pouring test cylinders at the same time as the concrete floor or roof. These test cylinders are cured under the same conditions as the concrete structure and are broken at specific intervals with a hydraulic press in a laboratory to determine what percentage of design strength they have achieved. The breaking of test cylinders is also used to determine when tension should be applied to the reinforcing tendons in a posttensioned building.

The arrangement of the temporary columns suggests that the second-floor slab (with no temporary supports below) has achieved its design strength and the two floor slabs above have not since they still have temporary columns in place. This arrangement can speed up construction and help contractors reach deadlines. However, this procedure can give firefighters a false sense of security if a fire breaks out at this construction site since the uncured concrete is weak and unstable.

The presence of concrete forms and falsework on upper floors of a building should be a sign to firefighters performing size-up at an incident that the structure is unstable. Another sign of instability is the presence of temporary columns and falsework on the floors below the floor with the concrete forms.

Any building under construction offers different hazards than a completed and occupied building. When a building is under construction, there may be hazards present that will not be evident when the building is complete. Interior division walls and fire-rated walls and floor-ceiling assemblies may not yet be in place and automatic fire alarm systems and fire sprinkler systems may not yet be functioning. A firefighter who is not alert and thinking while responding to a building under construction may be surprised when the fire behaves in an abnormal way.

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Gregory Havel is a member of the Town of Burlington (WI) Fire Department; retired deputy chief and training officer; and a 30-year veteran of the fire service. He is a Wisconsin-certified fire instructor II, fire officer II, and fire inspector; an adjunct instructor in fire service programs at Gateway Technical College; and safety director for Scherrer Construction Co., Inc. Havel has a bachelor's degree from St. Norbert College; has more than 30 years of experience in facilities management and building construction; and has presented classes at FDIC.

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