Masonry Magazine August 1970 Page. 18
100 psi. Stresses causing oond ranure
develop when masonry is subjected to eccentric gravity loads, loads from earth pressure, winds, and other lateral loads. A lack of bond at the interface of mortar and concrete block may lead to moisture penetration through the unbonded areas. Other factors being equal, bond strength of mortar will increase as compressive strength increases although not in direct proportion. Bond strength can therefore be improved by increasing compressive strength. A complete and intimate contact between mortar and block surface is essential for good bond and can be achieved through use of mortar having good workability.
Workmanship is paramount in affecting bond strength. The time lapse between spreading mortar and placing block should be kept to a minimum because the flow will be reduced through suction of the block on which it is first placed. If too much time elapses before another block is placed upon it the bond could be reduced.
Since all mortar is not used immediately after mixing, evaporation may require the addition of water or retempering the mortar to restore it to its original consistency. Retempering is not harmful so long as it is done prior to hardening of the mortar. Mortar should be placed in final position within 24 hours after the original mixing. This is required by ASTM C-270. Tapping or otherwise attempting to move block once the mortar has begun to harden will 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 adherence to the masonry unit.
Dularumy of mortalis character-
ized primarily by its resistance to damage from repeated cycles of freezing and thawing. Mortars of high compressive strength have been found to give good 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 construction see NCMA TEK 16, "Cold Weather Concrete Masonry Construction."
Additional Considerations
White mortar can be made from white portland cement, lime, and white sand. Colored mortar is produced through the use of colored aggregate or mortar pigments. Pigments must be sufficiently fine for thorough disposal throughout the mix. Dry pigments mixed with cement in large controlled quantities assure a more uniform color than obtainable by mixing small batches. Only the minimum amount of pigment necessary should be used. As a rule, color pigment additions should be of mineral oxide composition in amounts not exceeding 10% of the portland cement content. Carbon black may be used as a coloring agent if it does not exceed 3%. A good practice is to use only those coloring agents which have been found acceptable by testing or experience.
Emotescense is sure witis церозн
on exposed surfaces of masonry caused by leaching of soluble salts from within. Efflorescense can occur only if there is a source of soluble salts, such as sodium or potassium sulphate, and a means of leaching them from within. Portland cements low in alkali content are less of a potential source of efflorescense than are high alkali cements. Since lime contains fewer salts than portland cement, substituting it for cement will tend to reduce efflorescense, but at the expense of reduced mortar strength. Achievement of good bond between block and mortar to prevent the entrance of rainwater from acting as a leaching agent will also help reduce efflorescense.
Admixtures for masonry mortars are available in a wide variety. Since they are commercially prepared products, their compositions are not generally disclosed in full. Many substances have been employed as admixtures such as tallows, salts of wood resins, and various chemicals. Admixtures are functionally classified as agents promoting air entrainment, water retentivity, workability, accelerated set, and so on. Calcium chloride in an amount not over 2% of the portland cement content may be used as an accelerator as provided by ASTM C-270. Little data have been published regarding the effect of proprietary admixtures on mortar bond or strength, but field experience indicates that detrimental results have occured in some cases, For this reason admixtures of unidentified composition should be used only after it has been established by test or experience that they do not materially impair the mortar performance.
NATIONAL CONCRETE MASONRY ASSOCIATION
P.O. Box 9185, Rosslyn Station, Arlington, Virginia 22209