Masonry Magazine November 1964 Page. 14
Shape Factor
in joint design
It is possible to design a satisfactory
joint system for almost any type of
structure one that will perform with
little or no maintenance for long pe-
riods of time. Properly designed joints
can serve for as long as some parts of
a structure so that maintenance is
never required.
All joint sealants are solids at ambi-
ent temperatures (mastics, thermoplas-
tics and thermosetting plastics) and
alter their shape but not their volume
as the joint opens and closes. When a
joint slot becomes wider, the sealant
necks down. It will bulge up as the
slot becomes smaller.
Obviously, there can be no one
joint design or material that will work
equally well for every joint in all
structures. But surprisingly, only one
formula (but not one sealant) is re-
quired to design joints that will func-
tion properly for any structure. It is
based on a geometric relationship of
changing sealant cross-section during
extension and compression. The major
factor in joint design is the depth-to-
width proportion, referred to as the
shape factor. A shallow joint is su-
perior to a deep one and is much more
By Raymond J. Schutz
Vice President,
Research & Development,
Sika Chemical Corp..
Passaic, N. J.
Part I
"Copyright, 1962, by American
Society of Civil Engineers.
Printed in U.S.A. by Rumford
Press.
economical. It can withstand volu-
metric changes with less strain on the
sealant.
The shape factor of the joint (depth
to width) has a critical effect on a
sealant's capacity to withstand exten-
sion and compression. As the cross-
section of the sealant adjusts to the
new size of the opening, internal
strains are imposed that are often se-
vere. Actually the strain is largely de-
termined by the depth-to-width ratio,
and the strain on the extreme fiber is
highly significant, as shown in Figs. 1
and 2.
Some examples will illustrate the
importance ance of the shape factor. A
joint 1 in. wide by 2 in. deep will in-
crease the length of the outer fiber 94
percent when extended ½ in. Tests
have shown that the increase in length.
of this fiber is directly proportional
to the increase in strain. By simply
reducing the depth of the joint to 1 in.
(making it a 1-in. x 1-in. joint), the
strain on the outer fiber will be only
62 percent for the same extension-
about a third less. Doubling the width
of the joint to 2 in. (that is, making
it a 2-in. x 1-in. joint) will reduce the
strain on the outer fiber to 32 percent,
a 68 percent reduction in strain as
compared to the first example. The
cross-sectional areas of joint filler used
in the first and third examples are the
same, that is, 3 sq. in.
Strains imposed on sealants from
joint compression are shown in Fig. 2.
These data are for joints that are free
to move on the bottom, since they are
not bonded. In fact, a bond breaker
was used. If the bottoms were bonded,
the strains would be 100 percent great-
er, as shown in Fig. 3. Good bond
breakers include polyethelene, wax pa-
per, aluminum foil and polyethelene
tape.
A five-year study
This new approach to joint design
is a result of a five-year study of joints
that had failed in all types of struc
tures. The study was made by the Re-
search and Development Section of
Sika Chemical Corporation under the
direction of the writer. Field observa-
tion was backed by extensive research
and experimentation to remedy an
increasing number of joint failures in
many types of structures. The findings
Joint failure occurs when extensibility of sealant is exceeded. Al-in, opening in the sealant is
caused by excessive extension and necking down of sealant of almost 1 in.
14
MASONRY
November, 1964