Masonry Magazine October 1987 Page. 24
Properties of Hardened Mortar
Properties of hardened mortar that affect the performance of the finished concrete masonry include compressive strength, bond strength, and durability.
These properties are difficult to measure in other than laboratory or field specimens prepared under controlled conditions.
Laboratory tests are performed using low water content mortars. Field tests, on the other hand, are performed on mortars containing high water contents. This often leads to conflict when comparing test results, since increased water adversely affects the compressive strength of mortar but at the same time enhances the bond strength.
The compressive strength of mortar increases as the cement content is increased, but decreases as lime and water are increased. Increases in mortar air content also reduce compressive strength to some degree. Compressive strength of masonry is an important factor when the structure is loaded in compression. Structural failures due to compressive loading are rare, however, and other properties such as bond strength are generally more critical.
Bond strength is possibly the most significant single property of hardened mortar. It is also the most capricious and unpredictable. Bond is the property of hardened mortar that holds the masonry units together. Since bond strength is considerably less than compressive strength, mortar joints subjected to relatively small tensile stresses are prone to failure.
For example, mortar having a laboratory compressive strength of 2500 psi develops a tensile strength equal to approximately 250 psi. Bond strength, being a function of the mortar tensile strength, may range from only 50 to 100 psi. Other factors being equal, bond strength will increase as the compressive strength of the mortar increases, although not in direct proportion.
As compressive strength is related to cement content, bond strength may therefore be improved by an increase in the cement content of the mortar. Bond may also be effectively increased through the use of properly designed mortars having water contents which provide good workability.
Stresses causing bond failure develop when masonry is subjected to eccentric gravity loads, loads from earth pressure, wind, and other lateral loads. Improper construction practices such as tapping or otherwise attempting to move a block once the mortar has begun to harden will also be highly detrimental to bond. The movement will break the bond between the mortar and the block, and the mortar will not be sufficiently plastic to re-establish adhesion with the masonry unit. A complete and intimate contact between the mortar and block surface is absolutely essential.
Durability of mortar is characterized primarily by its resistance to damage from repeated cycles of freezing and thawing.
High compressive strength mortars have been found to provide excellent durability as have air-entrained mortars. The billions of small, well distributed air bubbles in air-entrained mortars absorb the expansive forces of freezing water within the mortar itself. Development of sufficient early strength in mortar to resist damage from freezing is desirable, especially in construction during cold weather. For additional information on cold weather masonry, see NCMA TEK 16, "Cold Weather Concrete Masonry Construction."
NATIONAL CONCRETE MASONRY ASSOCIATION
P.O. Box 781, Herndon, Virginia 22070