Masonry Magazine October 1995 Page. 20
Masonry Unit Influences
Units selected for the masonry segments will influence the bonding characteristics of the mortar.
An absorptive masonry unit, be it a clay brick or concrete masonry unit, will draw water out or the mortar joint into itself. The absorbed water is then lost through evaporation or returned to the mortar-if drier than the unit-promoting additional cement hydration. Emphasis on the initial rate of absorption (IRA) should be balanced with total absorption capacity of the unit.
Here again, it should be recalled that IRA is measured in the laboratory under controlled conditions, i.e., inch partial immersion for one minute. The unit, in situ, remains in contact with wet mortar for considerably longer. Likewise unit absorption is measured under arbitrary, controlled laboratory conditions. Data allows comparisons but does not necessarily reflect performance of unit within masonry.
Mortar joint "dry out" occurs because the initial mortar water content is rapidly lost to units and atmosphere, thus, slowing and abating the cement hydration process. Prewetting of absorptive, clay masonry units is considered good practice.
Moist curing of all completed masonry provides water to both the unit and mortar. This good practice should prevail whenever the ambient relative humidity is below 70%.
Admixture Influence
The effect of admixtures on bond strength of masonry has not been well documented. On the basis of past research, one can speculate that any admixture that increases the water demand of the mortar or accelerates cement hydration would be beneficial, providing workability is maintained.
If admixtures are selected for the mortar, comparative bond strength test results should be obtained under the conditions presumably requiring their use.
Bond Strength Test Methods
Before discussing bond test methods, the location of bond zones within the masonry wall should be identified to allow establishing the appropriateness of individual bond test methods.
Vertical (head) mortar jointsbond adjacent units in the same course. If concrete masonry units and volume change (shrinkage) occurs within the
eral guide lines are provided for the use of the mortar without directly stating that the requirement is related to bond strength of mortar to masonry unit. ASTM C 270 contains such insertions:
Mixing mortars requires a mix time to be between three and five minutes in a mechanical mixer.
Mixing mortars requires the maximum amount of water to produce a workable consistency.
The Guide for the Selection of Masonry Mortars, Table X1.1, includes recommended and alternative mortar types for specific applications. These specification insertions directly affect the bond strength of mortar to masonry unit.
The mixing time requirement was adopted to reduce the possibility of excessively high air contents in mortars containing air-entraining cementituous materials, be it an air entraining portland cement, masonry cement or lime.
High air content mortars are possible, especially during cold weather conditions and when mortars are mixed for prolonged periods-in excess of 5 minutes.
The maximum amount of water to produce a workable consistency was adopted to provide intimate and complete contact of masonry mortar at the interface with masonry unit, during unit bedment. The maximum amount of water to produce a workable consistency was adopted to satisfy both the unit's initial rate rate of absorption (IRA) and provide additional retained water in mortar to promote more complete hydration of portland cement.
The guide recognizes both the strength and workability characteristics of masonry mortars. Generally, avoid selecting high strength mortars and use maximum quantity of sand allowed to obtain best performing mortar.
These recommendations are in conflict with certain research studies, which show that increased bond strength is directly related to higher portland cement contents. These recommendations are supported by research that shows the extent of bond and the water retaining characteristics promote better bond strength.