Masonry Magazine December 1964 Page. 10
Shape
Factor
By Raymond J. Schutz
Vice President,
Research & Development,
Sika Chemical Corp.,
Passaic, N. J.
Part II
*Copyright, 1962, by American
Society of Civil Engineers.
Printed in U.S.A. by Rumford
in jointE
design
This is the second part of a valuable presentation by Raymond J. Schutz, Vice President, Research and Development, Sika Chemical Corp.
Joint size and shape
After the joint width has been determined, based on the anticipated movement, the type of sealant can be selected, based on service conditions and movement. The maximum strain that a particular sealant can endure can be determined in the laboratory by testing to failure at different temperatures. Reputable manufacturers will supply dependable data.
Knowing the width of the joint and the maximum strain the sealant can withstand, the shape factor required can be determined from the curves of Fig. 4 (These data are derived from the parabolic equation of Egon Tons, in his paper, "A Theoretical Approach to Design of a Road-Joint Seal," Highway Research Board, Bulletin 229, Washington 25, D. C., 1959.) The result can be checked against Fig. 1 to determine the maximum strain on the outer fiber with a predetermined shape factor. If the stress is found to be excessive, variations can be incorporated to improve the shape factor.
Selecting the sealant
It is possible today to formulate an elastomer that will satisfy any one or perhaps more than one critical service condition encountered in joints. The sealant can be modified to produce high physical values considered desirable in the building and construction industry, including hardness, extensibility, ability to withstand high compression, high adhesion and resistance to corrosion. But unfortunately not all of these advantages can be found in one material. Some features must be sacrificed at the expense of others.
When the extension of a joint has been determined, it is possible to select one or more appropriate sealants. The extension at which failure will occur at O deg F, as determined from tests on some commonly used thermosetting plastics and mastics, is shown in Fig. 5. These maximum extensions were determined in tests on Sika Colma (elastomeric) joint scaler or crack sealer, after curing for 48 hours, and on newly installed Igas (mastic) sealer. Test data were determined using the standard rate of extension-1/8 in. per hour on a joint 1 in. wide by 2 in. deep (AASHO Specification 206C).
Since all organic compounds, regardless of their composition, degrade as they age, it is recommended that a factor of safety of 4 be introduced when using Fig. 4. The maximum allowable strains indicated in the graph will not necessarily apply three to five years after testing. Not to be overlooked are other strength reducing factors. All elastomers exhibit a fatigue factor and the maximum allowable strains of the sealant should be reduced, depending on the number of cycles of extension or compression that are anticipated. Mastics also loose their integrity with cycles of expansion and compression as they pick up dirt, which alters their composition. In general, it is wise not to expect a sealant to perform at stresses beyond 75 psi in bond. If this rule is adhered to, other desirable design factors are usually satisfied as well.
The sealant should be chosen only after considering the conditions it is expected to withstand. In general, wherever nominal movement is expected, the more economical mastic sealants have proved their worth over the years. They do have limitations, which may be severe for some applications. They will not withstand foot traffic, solvents or poking. Also, they will assume the color of the local dust-laden atmosphere.
Where fast movements, or low or high temperatures are anticipated, the new elastomeric sealants are more suitable. They will withstand foot traffic, are non-tacky, and may be pigmented to match or harmonize with the color of the adjacent materials. However, their removal may prove very difficult.
Types of sealants
Mastics. Recommended limit for clongation at room temperature, 12 percent. The group of sealants classified as mastics are composed of viscous liquids rendered immobile by the addition of fibers and fillers. The vehicle in mastics may be a non-drying oil, a polybutene, a low-melting-point asphalt, or any combination of these materials. The filler used may be asbestos fiber, fibrous talc, or a finely divided calcareous or siliceous material.
Thermoplastics. Recommended limit for elongation at room temperature, 25 percent. Thermoplastics include such materials as asphalts, rubber asphalts, pitch and coal tar. They will become soft upon heating and will harden when cooled. For design purposes these are considered elastomers with limited extensibility.
Thermosetting plastics. Recom-
MASONRY December, 1964