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Historic Preservation - Technical Procedures

Spectitle:

Criteria For Selecting Masonry Joint Sealants

Procedure code:

0790001S

Source:

Outdoor Sculpture Manual - Center For Public Buildings

Division:

Thermal And Moisture Protection

Section:

Joint Sealers

Last Modified:

02/24/2012

Details:

Criteria For Selecting Masonry Joint Sealants



CRITERIA FOR SELECTING MASONRY JOINT SEALANTS


THE USE OF CAULKS AND SEALANTS IN THE MASONRY JOINTS OF HISTORIC
STRUCTURES, IS ONLY AN APPROPRIATE MAINTENANCE TREATMENT WHEN IT IS
INTENDED TO REPLACE CAULKING OR SEALANTS USED AS PART OF THE
ORIGINAL DESIGN.  


This standard includes guidance on selecting joint sealants for
masonry, based on composition, elasticity and durability.  For
guidance on installing joint sealants, see 07900-01-P "Sealing
Masonry Joints to Make Them Airtight and Watertight".

-    Sealants may be used for expansion joints, at intersections of
    differing materials and to infill gaps where differential
    settlement/movement is still active.

-    Sealants should NOT be used as a substitute for proper masonry
    repointing (i.e. facade surfaces, etc at vertical and
    horizontal joints).

-    The performance of sealants is affected by several factors:

    1.   Physical properties such as porosity, texture and
         elasticity.

    2.   Chemical properties such as composition and salts.

    3.   Environmental factors affecting durability.

    4.   Joint design affecting performance (i.e. the width to
         depth is directly related to sealant performance).

-    Sealants in satisfactory condition should be:

    1.   be pliable enough to conform to masonry joints, cracks,
         etc.

    2.   be able to withstand the anticipated joint movement.

    3.   be insensitive to moisture.

    4.   be resistant to alkalinity which is often present in
         stone.

    5.   NOT stain or mar the appearance of the masonry.

-    Masonry joints to receive sealants should be clean and free of
    dirt, dust, grease, or oil, salts, chalk or lime and other
    debris.

-    In their 1987 report, "The Use of Sealants in Masonry Joints",
    The National Institute of Standards and Technology (NIST) -
    formerly known as The National Bureau of Standards (NBS) -
    listed the following factors and properties of stone which can
    affect sealant performance:

    1.   Porosity of masonry and stone

    2.   Permeability of masonry and stone

    3.   Moisture content of masonry and stone in presence of
         freezing and thawing conditions

    4.   Salt crystallization within masonry and stone pores

    5.   Thermal and moisture expansion of joints

    6.   Surface texture of masonry and stone

-    Sealants may be classified by composition, elasticity and
    durability.

    1.   Chemical or composition types of sealants in 1 and 2-part
         formulations include:

         a.   Oil based
         b.   Acrylic
         c.   Butyl
         d.   Polysulfide
         e.   Urethane
         f.   Silicone
         g.   Polymercaptan

    2.   Classification according to elasticity or joint movement
         includes three levels as follows:

         a.   Low Movement - Up to 5% movement; this group is
              generally composed of oil and resin based sealants.

         b.   Medium Movement - From 5 to 12.5% movement; this
              group is typically comprised of polymer based
              acrylic types and some urethanes.  (Acrylics are
              not generally recommended for outdoor use.)

         c.   High Movement - Up to 25% movement; this class
              generally includes the urethanes, silicones and
              polysulfides.

    3.   Classification according to durability is as follows,
         according to the (NIST) study of sealants:

         a.   Five-Year Durability

              1)   Oil based
              2)   Polybutanes

         b.   Ten-Year Durability

              1)   Butyl
              2)   Acrylics
              3)   Hypolon  
              4)   Nuprene

         c.   Fifteen-to-Twenty-Year Durability

              1)   Polysulphides
              2)   Silicones
              3)   Polyurethanes
              4)   Mercaptors

                             END OF SECTION