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Mortar for below grade foundation

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  • Mortar for below grade foundation

    I'm putting in a block foundation below grade. What is the best type of Mortar to use for this application? I have two books from the library, one says to use Type S, the other says to use Type M. Does it matter?
    Mike - Saginaw, MI

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  • #2
    Re: Mortar for below grade foundation

    Here's a good summary of the various mortar types and their properties: link

    Either type M or S should work for your foundation wall, but S might be best. This is from the mason contractors website:

    "It should be remembered that stronger is not necessarily better when selecting mortar for unit masonry. For example, it is not typically necessary to use Type M mortar for high-strength masonry. Type S will provide comparable strength of masonry, and in fact the Masonry Standards Joint Committee's design standard ACI 530/ASCE 5/TMS 402 does not distinguish between the structural properties of masonry constructed using Type S mortar from that constructed using Type M mortar. Moreover, Type S and Type N generally have better workability, board life, and water retention. As a rule of thumb, the specifier should not require use of a mortar of higher compressive strength than necessary to meet structural design criteria."

    What is that strength?

    "The five typical mortar mixes designated types M,S,N,O and K are labeled so because each is an alternate letter in the term MASON WORK in descending psi strength....

    M 2,500 psi
    S 1,800 psi
    N 750 psi

    O 350 psi
    K 75 psi
    Last edited by dbhansen; 05-06-2008, 08:34 PM. Reason: added link
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    • #3
      Re: Mortar for below grade foundation

      Good info there! Strength is a relative term though. Compressive, tensile, shear...
      Everyone makes mistakes. The trick is to make mistakes when nobody is looking.