Masonry Magazine January 1972 Page.20
All-Weather Masonry Construction
(Continued from page 19)
tion that allows measuring R.H. in masonry mortar joints. Developed by PCA, the instrument allows tracking the water loss from masonry when the humidity in the mortar is below 100.
portland cement and the masonry unit coupled with the sulfates from the masonry unit and the atmosphere contribute to this phenomenon. Climatic conditions-cold temperature, low relative humidity-greatly affect hydration and consequently influence the rate at which the salts can and do migrate toward the drying surface. Naturally or artificially applied water densifies the mortar, through more complete hydration, and reduces the efflorescence. Acid washing after the application of water to the masonry is recommended fewer burned joints will be experienced.
Illustration 16 shows relative humidity time relationships for a concrete brick masonry assemblage. These relationships are considered important as we know that when the relative humidity falls below 80%, hydration of the portland cement ceases. As will be noted, the degree of cement hydration is a function of depth from the drying surface. Wall areas in compression have undergone the most hydration (12 to 15 days) whereas wall areas in tension during wind loading have hydrated the least (approximately 3 days when exposed to 73°F. / 50% R.H.)
Illustration 17 returns us to our brick veneer where the moisture has equilibrated in the masonry and the collar joint. Evaporation to the atmosphere continues and may promote efflorescence. Additionally the moisture content of the masonry may be ideal for the first freeze-thaw destructive forces.
Illustration 19 on laboratory freeze and thaw tests shows that as the mortar air content increases the freeze and thaw durability increases. Test conditions-freezing from all sides of the masonry seldom are encountered in actual masonry, but the entrained air in mortars serves as insurance against this destructive force.
Illustration 20 shows the continued migration of moisture from masonry with the moisture-drying surface within the masonry. Although there exists some danger that water-soluble salts will be precipitated and cause expansion and failure as evidenced by spalling, failures caused by this phenomenon are extremely rare.
Illustration 18 on efflorescence shows the deposition of white crystals on masonry. Water-soluble salts from the
In summary, the performance of masonry materials in masonry construction must be understood singly and collectively. Concrete technology serves as a guide, but direct application of concrete technology to masonry procedures may confuse rather than enlighten us.