Masonry Magazine August 1976 Page. 13
Mix mortars with the maximum amount of water consistent with workability to provide maximum tensile bond strength within the capacity of the mortar. Permit retempering, but only to replace water lost by evaporation. This can usually be controlled satisfactorily by requiring that all mortar be used within 2 hr after mixing.
WORKABILITY
A mortar is workable if its consistency allows it to be spread with little effort and if it will readily adhere to vertical masonry surfaces. Although experienced masons are good judges of the workability of the mortars, there is no standard laboratory test for measuring this property.
Water retentivity, flow and resistance to segregation affect workability. In turn, these are affected by properties of the mortar ingredients. Because of this complex relationship, quantitative estimates of workability are difficult to obtain. Until a test is developed, the requirements for water retentivity and aggregate gradation must be relied upon to insure satisfactory workability.
FLOW AND WATER RETENTIVITY
Mortar flow is determined by a simple laboratory test. A truncated cone of mortar is formed on a flow table, a testing apparatus which is mechanically raised and dropped 25 times in 15 sec. Under this treatment, mortar will flow, increasing the diameter of the test specimen. Flow is the ratio of the increase in diameter to the original 4-in. cone base diameter, expressed in percent. Test procedures for determining flow are given in Standard Method of Test for Compressive Strength of Hydraulic Cement Mortars, ASTM Designation C 109.
A similar test, flow after suction, indicates theoretical mortar flow after loss of water to an absorbent masonry unit. Flow of a mortar is measured and then, for I min, the mortar is placed into a device which removes some water by a vacuum. This mortar is then again tested for flow. Test procedures for determining flow after suction can be found in Sections 25 to 27 of ASTM C 91, Standard Specification for Masonry Cement.
These two laboratory tests give an indication of a mortar's water retentivity. Essentially, water retentivity is mortar's ability to hold water when placed in contact with absorbent masonry units. A mortar which has low water retentivity will bleed moisture, creating a thin layer of water between mortar and masonry unit, materially lowering bond. The laboratory value of water retentivity is the ratio of flow after suction to initial flow, expressed in percent.
Do not confuse laboratory standards with job requirements. Mortar standards require 70 percent flow after suction, based on an initial flow of only 105 to 115 percent. Construction mortars normally have initial flows in the range of 130 to 150 percent in order to produce a workability satisfactory to the mason. The lower initial flow requirements for laboratory mortars were set because they will yield fairly consistent test results on most available laboratory equipment.
BOND STRENGTH
General. Bond strength is perhaps the most important single physical property of hardened mortar. Because many variables affect bond, it is difficult to devise a single laboratory test which will consistently yield reproducible results and which will approximate construction results. These variables include air content, elapsed time between spreading mortar and laying brick, suction of brick, water retentivity of mortar, pressure applied to masonry joint during forming, texture of brick, and others. The effects of these variables on bond are discussed in subsequent paragraphs.
The crossed-brick couplet test, as prescribed in Standard Method of Test for Bond Strength of Mortar to Masonry Units, ASTM Designation E 149, is, at present, the only standard method of test for evaluating this property of mortar. Another method of test that is, perhaps, more meaningful and that is rapidly becoming more acceptable by laboratories is patterned after ASTM C 78, Standard Method of Test for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading). ASTM Committee E-6 on Performance of Building Constructions is currently (1972) considering the adoption of such a standard method of test for masonry assemblages (Fig. 1).