Masonry Magazine August 1976 Page. 15
MORTAR
In chimney construction, specify a mortar which contains the maximum lime compatible with other requirements. For chimney construction, 1:2:5 (cement:lime:sand) mortar is often specified.
VOLUME CHANGE
Volume changes in mortars can result from four causes: hardening, cyclic wetting and drying, temperature change, and unsound ingredients which chemically expand.
Volume changes caused by hardening and cyclic wetting and drying depend upon curing conditions, mix proportions, and water content. Mortars hardened in absorbent molds or in clay masonry exhibit considerably less volume change than those hardened in non-absorbent molds. An increase in water content will cause an increase in shrinkage during hardening of mortar. At one time these volume changes were believed to impair the rain resistance and durability of masonry. However, extensive tests and performance records do not substantiate this view.
Thermal volume change will lead to expansion and contraction in masonry. Available data indicate that differential thermal volume change between masonry materials has no important effect on performance. However, total volume change can be very significant. Thermal expansion and contraction of masonry and their accommodations are discussed in Technical Notes Series 18.
Volume changes due to unsound ingredients may be sufficient to cause disintegration of masonry. Materials conforming to ASTM standards should be free of detrimental constituents.
STRENGTH OF MORTAR
General. As with concrete, the compressive strength of mortar depends largely upon the cement content and the water-cement ratio. However, because compressive strength of masonry mortar is less important than bond strength, workability and water retentivity, the latter properties are normally given principal consideration in mortar specifications.
Accepted measures of strength are the compressive strength of 2-in. mortar cubes and the tensile strength of standard mortar briquets. Procedures for molding and testing cubes and briquets appear in ASTM Specifications C 109 and C 348, respectively. Because the tests are relatively simple and because they give consistent, reproducible results, strength is considered one basis for comparing mortars.
Effect of Proportions. Compressive strength increases with an increase in cement content of mortar. It decreases with an increase in water content and, thus, with increased flow. Sometimes air entrainment is introduced to obtain higher flows with lower water content. The reasoning here is that low water-cement ratios will provide high compressive strengths. However, this generally proves futile since compressive strength decreases with an increase in air content.
Effect of Retempering. Retempering will decrease mortar strength, the amount of decrease increasing with time after mixing. If mortar is retempered 3 hr after mixing, its compressive strength will be reduced approximately 25 percent. Mortar normally begins to harden about 2½ hr after mixing. Since it is mixing after initial set that lowers compressive strength, reduction in strength will be noticeably less if retempering occurs only 1 to 2 hr after mixing. It is frequently desirable to sacrifice some compressive strength in favor of improved bond, and retempering will definitely improve bond.
Working Stresses. Allowable compressive stresses permitted by building codes and design standards will depend upon mortar strength and the strength of individual masonry units. Table 3 indicates the typical range of allowable compressive stresses permitted in unreinforced masonry by most building codes.
TABLE 3
Allowable Compressive Stresses in Unreinforced Unit Masonry'
| Construction | Compressive Strength of Units in psi | Allowable Compressive Stresses Over Gross Cross-Sectional Area in psi (except as noted) |
|---|---|---|
| | | Type M Mortar | Type S Mortar | Type N Mortar | Type O Mortar |
| Solid masonry of solid brick | 8000 plus 45000 8000 2500 to 4500 1500 to 2500 | 400 250 175 125 | 350 225 140 115 | 300 200 140 100 | 250 150 110 75 |
| Masonry of hollow units | | 85 | 75 | 70 | - |
| Hollow walls cavity or masonry bonded | Solid units: 2500 plus 1500 to 2500 Hollow units | 140 100 70 | 130 90 60 | 110 80 55 | - - - |
'Adapted from American National Standard Building Cole Requirements for Masonry. American National Standard A41.1-1953 (R 1970).
"Permined stresses are for gross cross-sectional area minus the cavity area. These stresses assume that load bears on but one of the two wythes. For concentric loadings, increase allowable stresses by 25 perce
In the rational design of engineered brick masonry, the allowable compressive stresses are based on the assumed or actual (determined by prism tests) compressive strength of the masonry, fm. These strengths are also functions of unit strength and type of mortar, plus the level of workmanship. When the compressive strength of the brick masonry is not determined by prism tests, the allowable stresses may be based on an assumed