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Shotcrete on URM

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Drew and Marlou:

When you apply a thin reinforced concrete layer to a much thicker
unreinforced masonry wall, shear deformations predominate because of the
behavior of the unreinforced masonry. If you fail to make the stiffness
calculation, you will usually underestimate the amount of reinforced
concrete needed to protect the lower allowable stress material. It is easy
to assign 100 percent of the demand to a new reinforced concrete wall
because you can add steel to make your numbers work. It is also a bad idea
to take off a wythe of brick for a four inch thick reinforced shotcrete
replacement layer(which is commonly done because it computes so easily
while not changing the thickness of the wall). I have seen many perfectly
fine brick walls damaged by severing the header bricks prior to applying
the shotcrete. I take exception to the notion that this concept is
theoretical. The stiffness calculation is simply adding the rigidity of
another element. The E value for brick comes from experience with a lot of
flat jack and prism tests and was posted for your use.

Michael Krakower SE