huangyhg

cracks in cmu parapet wall
hello.
i've been inspecting a damaged cmu parapet wall. the damaged seems to cause by lack of vertical expansion joints in the wall. there are horizontal cracks at the bottom of the wall. also, diagonal cracks are at the end of the wall.
does anyone have suggestions for possible repair?
thanks.
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is it possible to cut a vertical joint in the cmu parapet wall at each end of the wall or in the middle of the wall, but not cuting through the reinforcing to relieve the stress?
i will also provide connections to allow for horizontal movement and still resist load perpendicular to the face of the wall.
does this solution sound reasonable?
cutting vertical joints in excess of the spacing of the joints below (in the main wall) is our usual practice.
care should be taken to ensure that the joint is properly waterproofed with flashing, sealants, etc.
there should be a bond beam at the course aligned with the roof (although in an existing building this may not be true or may be difficult to verify).
the bond beam at the top of the parapet can be cut through.
structural01,
is this a new building or an older one?
a common cause of the horizontal crack at the base of the parapet in buildings that have been around for awhile is differential vertical expansion due to moisture absorption and radiant heat (since there is no load on the masonry to clamp the tendency to crack). this is the reason that the horizontal crack usually occurs at the joint that is level with the bearing course for the roof framing.
the horizontal cracking is always worse on the walls that are exposed to the sunny sides of the building. the front face of the parapet heats up by the sun's rays,and expands.
to exacerbate the situation, additional expansion occurs as a result of moisture absorption. add freeze-thaw activity and gusting wind forces, and you can see that the crack is almost inevitable.
the cracks are usually larger when the wall is composed of two wythes, especially when the outer wythe is clay brick, and the back-up wythe is concrete masonry (due to the difference in expansion and contraction of the two materials).
the diagonal cracking near the ends is likely a result of not only horizontal expansion, but the inward movement of the parapet as well. on older buildings, the parapet can be tilted backward so far that it almost falls over.
above all, do not just assume that vertical joints in the parapet will cure the condition, until you rule out the above mechanism.
jae and epr,
thank you very for your respond.
the building i inspected is an older building. i believe it was built in the early 70's.
since the crack in the parapet wall is caused mainly by differential expansion, is it reasonable to cut only vertical expansion joint starting at the bottom of the parapet wall up?
what about the horizontal crack? how can i repair the horizontal crack that is already there?
structural01,
the structural configuration of the parapet governs any options with regard to how to repair the horizontal crack. in some instances the parapet has needed complete reconstruction.
in others, it may be suitable to construct bracing from the roof onto the rear face of the parapet and simply caulk the horizontal crack. (when supported with rear bracing, i usually run the roofing up the sloping bracing to the coping.)
there is no single answer for this. use your creativity.
epr,
i was hoping that i wouldn't have to provide additional bracing. but, maybe this is the only thing to do. thanks.
structural01,
structural01,
i perform infrared surveys on cmu walls and have found many cracks originate at voids in grouted columns or cells.
i'm not saying this is your problem, but you might want to confirm the wall was constructed properly.
greg

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