Page 248 - Construction Waterproofing Handbook

SEALANTS 5.7

FIGURE 5.6 Design of sealant joint.

Coefficients of thermal expansion are usually expressed as inch per inch per degree
Fahrenheit. To determine expected movement and resulting joint size, the coefficient of
linear expansion is multiplied by temperature range, span length of material, and appro-
priate safety factor (usually at least a factor of 4).
The following is a typical calculation for joint design. As an example using 10-ft con-
crete precast panels with a coefficient of 0.000007 in/in/°F, the following would be the rec-
ommended joint design width:

1
150°F 0.000007 in/in/°F 10 ft 12 in/ft 4 (safety factor) 0.504 2 in
In determining the joint size necessary for moving joints located at different material
intersections, materials with highest coefficients of expansion are used in calculations.
However, if a material with a lower movement coefficient is spanning a greater width,
these data may present a larger joint size. Therefore, it is necessary to calculate all possi-
ble combinations to determine the largest joint size necessary. Table 5.1 summarizes coef-
ficients of thermal expansion for several common building materials.

BACKING SYSTEMS

No sealant joint can be properly installed without an appropriately sized backing system
installed first. Backing materials are as important to successfully installing a joint as the
sealant material itself. Unfortunately, backing systems are misused as much as sealants.