Masonry Magazine August 1976 Page. 14
Effect of Air Content
Mortar bond is reduced by high air content. Compressive strength is also associated with high air content, confirming the experience with mortars.
Effect of Flow
Although there is no discernible relationship between bond and compressive strengths of mortar, the relationship between mortar flow and tensile bond strength is quite apparent. For all mortars, and with minor exceptions for all brick suctions, bond strength increases as flow increases.
The time lapse between spreading mortar and placing brick will affect mortar flow, particularly when mortar is spread on high suction brick. In such a case, mortar will have less flow by the time brick are placed on it than it did when first spread. Conceivably, bond to brick placed on this mortar could be materially reduced. For highest bond strength, reduce this time interval to a minimum.
Because all mortar is not used immediately after mixing, some of its water may evaporate as it stands on the mortar board. The addition of water to mortar (retempering) is sometimes prohibited because the specification writer may have thought it to have detrimental effects. Although compressive strength will be reduced slightly if mortar is retempered, bond strength may be materially lowered if it is not. For this reason, permit retempering to replace water lost by evaporation. Because retempering is harmful only after mortar has begun to set, specify that all mortar be used within 2 hr after mixing.
Effect of Brick Suction
The laboratory-derived absorption rates of brick are important in that they indicate whether brick have relatively high, low, or medium suctions and whether brick suction must be altered before use. But it is the actual suction at the time of laying which influences bond strength.
In practically all cases, mortar bonds best to brick whose suctions are 5 to 20 g at the time of laying. If brick suction exceeds 60 g when laid, bond may be extremely poor. Where necessary, control brick suction by wetting brick prior to laying. Wetted brick should be surface dry.
Although wetting will lower suction of brick, there is no available means of increasing the absorption rate in very low suction brick. Fortunately, very few brick have suctions low enough to be significantly detrimental to bond strength. Sufficient control can usually be obtained by reducing water content of mortar.
Effect of Brick Texture
Mortar bond is somewhat less to smooth die skin or sanded surfaces of brick than it is to roughened surfaces; i.e., wire cut or textured surfaces. Usually, bond is less than 20 percent more for roughened surfaces. In normal use, few brick require mortar to bond to a die skin.
Effect of Movement
Tapping or attempting to otherwise move brick will be highly detrimental to bond, once mortar has begun to harden. At this time, movement will break bond, and mortar will not be sufficiently plastic to readhere well to masonry units. Do not permit the mason to realign, tap, or in any way move brick after initial placement.
Effect of Curing
Emphasizing the difference between mortar and concrete is the curious fact that, in masonry, wet curing seems to provide lower bond and compressive strengths than does dry curing. During wet curing, moisture apparently merely saturates masonry, decreasing its compressive strength and decreasing the adhesion between mortar and brick. Under normal conditions, it appears that there is little to gain by wet curing masonry. This observation is based largely upon extensive testing.
Effect of Proportions
Although inconclusive, available data indicate that, for maximum bond, the optimum lime-to-cement ratio lies between values of 1 and ¼. Lime characteristics and brick suctions are variables affecting this ratio. For lime putty or pressure-hydrated lime and for brick suctions less than 20 g, Type S mortars develop maximum bond.
DURABILITY
The durability of relatively dry masonry which resists water penetration is not a serious problem. It is generally conceded that masonry walls will stand 35 years and longer before requiring maintenance, an indication of mortar durability.
Although an increase in air content may increase the durability of masonry mortar, it will substantially decrease bond and other desirable properties. For this reason, and because mortar is normally quite durable without air entrainment, air-entraining ingredients to increase air content are not recommended.
EXTENSIBILITY AND PLASTIC FLOW
Extensibility is another term for maximum unit tensile strain at rupture. It reflects the maximum elongation possible under tensile forces. High lime mortars exhibit greater plastic flow than low lime mortars. Plastic flow acting with extensibility will impart some flexibility to the masonry, permitting slight movement without apparent joint opening.
For greater resiliency, increase the lime-cement ratio. Where very high resiliency is particularly