Investigating Rust/Oxide Jacking Displacement of Chimneys
By Adam Cabral, P.E.
Forensic Engineer, DONAN
Does this sound familiar? The chimney is leaning and separating from the house. You observe separation between the chimney and the house that increases in width with height (i.e., there is no separation at the bottom and there is upwards of 2 inches of separation at the roofline) (Figure 1). The heavy self-weight of the chimney comprised of masonry components is prone to rotating away from the house due to settlement of the underlying soils. Further, due to the height of the chimney, a relatively small rotation of the chimney foundation could result in a significant separation toward the top of the chimney. Most commonly and rightfully so, the cause of the chimney displacement is settlement of the underlying soils supporting the chimney foundation.
Figure 1: Chimney Leaning Away from the House. Separation between Chimney and House Increases in Width with Height.
WAIT…what if? What if the chimney is supported by a foundation that is continuous with the house foundation, and there is no crack at the interface of the house and chimney foundations? For instance, the cast-in-place, concrete foundation wall of the house bumps out at the chimney to support the chimney, and there is no crack or separation along the corner where the basement foundation wall transitions to the side of the chimney (Figure 2). Certainly, if the leaning chimney is the result of soil settlement, then a vertical crack would be present at the interface of the foundations supporting the house and chimney.
Figure 2: Continuous Cast-In-Place Concrete Foundation Walls Bump out at the Chimney. No Crack at the Interface of the House and Chimney Foundation Walls.
Now What?… Upon further investigation, you notice horizontal and/or step cracks along the sides of the chimney that are at the approximate height of the top of the firebox inside the house (Figure 3). The cracks increase in width toward the house. The portion of the chimney below the cracks is plumb, but the remainder of the chimney above the cracks leans out away from the house. You have located the point of origin for the chimney displacement, but what is the cause?
Figure 3: Horizontal Crack in the Side of the Chimney. The Crack Increases in Width toward the House and is at the Approximate Height of the Lintel over the Firebox Inside the House.
Cause Revealed! Steel lintels are often used at the firebox and/or smoke chamber on the inside face of a chimney to provide the opening for access to the firebox and/or smoke chamber (Figure 4). Rust jacking or oxide jacking is the process in which steel and other metals expand as a result of rusting. The force resulting from this expansive corrosion can cause damage to structures made of stone, masonry, or concrete that the steel is embedded into. A common example of damage from rust/oxide jacking are step cracks that stem from lintel bearings for wall openings in brick veneer (Figure 5).
Figure 4: Common Chimney Section
Figure 5: Common Rust/Oxide Jacking at Brick Veneer Window Lintels
In some instances, water intrusion within a chimney and/or at the flashing at the interface of the chimney and roof can result in corrosion of a steel lintel on the inside face of the chimney. The expansive corrosion of the lintel can result in lifting of the portion of the chimney above the lintel at the inside face of the chimney. With the slight lifting at the inside face, the portion of the chimney above the lintel will lean out away from the house presenting a separation between the chimney and house that increases with height. When horizontal and/or step cracks that increase in width toward the house are identified along the sides of the chimney at the approximate height of a lintel location and the leaning of the chimney originates at the identified cracks, it is probable that the cause for the chimney displacement is rust/oxide jacking of the chimney from a steel lintel on the inside face of the chimney.
 Google Images, https://www.google.com/search?q=chimney+section&espv=2&biw=1093&bih=922&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjYmvqivKHSAhXm4IMKHZ74BJ4Q7AkILA#imgrc=_ (accessed 2/21/2017).
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