Masonry Magazine November 1964 Page. 15
Mastic
IX
Concrete
1x2 joint
894%
1x1 joint
2x1 joint
862%
8max-32%
FIG. 1. Strain on the extreme fiber of a sealant for a -in. extension of different joint designs.
reverse many traditional joint design concepts in wide use. They reveal that there are three basic causes behind failure:
1. Poor joint design. Some joints are doomed to failure regardless of the sealants that may be used.
2. Use of an inappropriate sealant for the service conditions to be encountered. Physical properties of sealants cannot be exceeded without some degree of joint failure.
3. Poor workmanship in preparing surfaces or applying sealants.
Three factors in joint design
The main elements to consider before designing joints are their spacing, their size and their shape. The extent and frequency of movement has to be calculated fairly accurately from anticipated temperature variations, dynamic loadings and possibly from uneven settlement of the structure. The selection of the proper sealant to withstand the anticipated service conditions is the next step, followed by specifications that will satisfy these requirements, their conditions of installation and quality of workmanship.
Although materials expand uniformly as temperature rises, structures do not. A structure will tend to deform or buckle until restraint is overcome. When restraint is overcome, the structure may expand with a jerk. Sealants that have become brittle may be radically affected. This jerky movement is frequently audible as radiant energy heats up curtain-wall buildings and large roof decks.
Often the type of structure determines its own service conditions. Structures such as tunnels usually undergo small and infrequent movements and as a result experience few joint difficulties. Lasting joints can also be expected in the large majority of water tanks. Major movement occurs only during loading and unloading of the structure and from relatively small variations in temperature (from 80 to 40 deg F).
More flexible structures (and their components, such as floors, roofs, decks, and in many cases walls) experience frequent joint difficulties. The temperature range for some of these may be 160 deg F (from 130 deg F when exposed to the sun down to -30 deg F). Large movement and the accompanying strain imposed on the sealants works against the longevity of joints. The tendency toward lighter and more limber structures is also partially responsible for the increasing number of joint failures. Structures with smaller masses respond more rapidly to changes in the ambient temperature. Non-masonry structures and some components (such as metal curtain walls) are subjected to wide temperature variations during a 24-hour period.
Highly flexible structures, such as bridge decks, present the most challenging of all joint design and maintenance problems. Not only does the structure move from wide temperature variations, but the impact from heavily loaded, high-speed trucks often strains joints beyond the physical properties of the sealant.
Part 11, "Shape Factor In Joint Design" will appear in next month's masonry.
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15