By: Rebecca Reilly
Building owners should be aware of the make-up of their buildingsa floor slabs, especially roofs, and even more so balcony slabs, since they are heavily exposed to rain and snow. Cinder concrete is a common material in balcony slabs. When cinder concrete is exposed to water, a chemical reaction occurs that can corrode the embedded structural steel and even the railing supports. As we all know, deteriorated railing supports put residents at risk of falling. Following the two balcony railing failures that lead to fatalities, the NYC DOB FISP (formerly Local Law 11/98) has a renewed concentration to include the inspection of balconies for structural integrity. When deteriorated steel is observed during FISP inspections or restoration projects, cinder concrete slabs are often discovered. Once the extent of damage is determined, owners can determine an appropriate repair.
Floor slabs that were concrete made with coal cinders as aggregate and reinforced with draped wire mesh were commonly used in the construction of steel frame buildings in the 1st half of the 20th century. The strength of the concrete used for draped wire mesh systems was not a priority because the system relies solely on the strength of the steel reinforcing. Cinders were chosen as an aggregate because they were lightweight and inexpensive. Cinders were also readily available since coal burning was the major source of heat at the time. After testing, cinder concrete was found to have a higher resistance to fire damage than stone concrete. Cinder concrete typically has approximately half of the compressive strength of standard stone concrete. Concrete quality control was often not a priority during the construction of these systems since the concrete is not meant to be structurally stressed. Additionally, the quality of the cinders was not a priority.
It was later discovered that, while these slabs are quite strong, they are extremely vulnerable to water infiltration. The concrete is very porous, and if not waterproofed, will allow surface water penetration. Low quality coal cinders contain sulfur, which in the presence of moisture, causes an acidic corrosion of the embedded steel reinforcing and the wire mesh. Since the system relies on the strength of the steel reinforcing to support the slab, if the steel reinforcing begins to fail, there is a risk that the entire slab will fail.
Draped-mesh slabs are very difficult to renovate because the mesh is extremely sensitive to alterations. It is sometimes easier and more cost effective to remove and replace an entire span of the draped mesh slab rather then attempt to repair one section. A repair to one section of the reinforcing could change the way the loads are transferred to the structure, causing further damage to the building. Contractors should be especially careful when removing the concrete surrounding the wire mesh to avoid damaging the wire. The draped mesh must remain continuous to provide sufficient structural support.
The type of aggregates used in slabs can be determined through core testing. If slabs contain cinder aggregate, confirm that the slabs are properly waterproofed to avoid water infiltration. If balcony slabs are not properly waterproofed and steel corrosion has occurred, itas best to keep residents off of the balconies until they are thoroughly inspected by a qualified professional and any necessary repairs are performed. Adding waterproofing or a coating to the existing cinder concrete slabs often does not sufficiently address the deteriorated slabs.
If balcony replacement is required to prevent further deterioration, all concrete should be removed from the existing slab and a new metal deck should be installed. New concrete is then poured over the metal deck to provide a walking surface. An appropriate waterproofing membrane is then installed over this structural deck with either a pedestrian traffic wearing surface or an optional concrete topping slab with an appropriate draining mat. Protection board can then be installed on top of the waterproofing membrane.