FIRE-EXPOSED CONCRETE BLOCK WALLS: REPAIR OR REPLACE?
Concrete block masonry is a common building component due to its simplicity of construction and economy. After a building fire, the question often arises, "Are the masonry walls structurally sound or should they be replaced?". Generally, concrete block masonry is very resistant to fire and the wall can be returned to its full structural integrity without replacement.
COLOUR AS INDICATOR
The colour of the exposed masonry is used to indicate the intensity of the fire attack. Soot remaining on the surface indicates relatively low fire temperatures. At higher temperatures soot burns away without leaving deposits. Normally, extremely high temperatures are indicated by a white chalky surface on the block. Mortar which has been subjected to heat becomes soft and decreases in strength. A hammer and chisel can be used to give a rough comparison of the strength of the mortar in fire-damaged and non-fire-damaged areas.
The wall may exhibit significant cracking on the non-fire-exposed surface of the block and only minor cracking on the fire-exposed surface. Vertical cracking may not greatly reduce the structural capacity of the wall in those areas. Some of the cracking in masonry may occur when the heated walls are quenched by cold water when the fire is extinguished. The mortar and block cannot withstand the contractive forces caused by the temperature decrease, and cracking can occur. Masonry has an excellent bridging property which redistributes the load around small areas of weakness.
Fire-damaged concrete suffers from adverse environmental deterioration unless properly protected. The heat causes the surface of the concrete to soften which reduces resistance to freeze/thaw and general breakdown. Therefore, it is recommended that the exterior surface of the wall be covered in a stucco finish to protect the masonry from the environment. The stucco is also structurally beneficial and aesthetically pleasing.
Therefore, by providing environmental protection and some structural strength through the application of a stucco finish, the fire-exposed wall in a house can be restored to an acceptable level. The cost of applying the stucco finish is much less than the cost to remove and replace the wall, especially when the wall is reinforced with steel bars.
In summary, it is generally wise to have fire-damaged masonry inspected by a qualified engineer because the savings can be significant. It should be remembered that reuse of structurally unsound masonry can be disastrous if structural collapse occurs.
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Walters Forensic Engineering | 277 Wellington
Street West, Suite 800 | Toronto, ON M5V 3H2