Masonry Magazine December 1995 Page. 18
Sand
Sand is often added to the mixer using a shovel. However, some positive control should be established to assure the proper amount of sand is used. This can be accomplished by periodically checking the required shovel count with a cubic foot (0.28 m³) box, by using a cubic foot measuring box to add the sand to the mixer, or by using other containers of known volume to measure the sand. When a cubic foot box is used, a hinged attachment to the mortar mixer can be constructed to facilitate one-man operation.
Fig. 4 Sand piles should be covered to assure consistent mortar quality.
Mixing Water
Water to be used for mixing mortar should be clean and free of deleterious amounts of acids, alkalis, and organic materials. Generally, water that is potable can be used to mix mortar. Since water has the ability to store a significant amount of energy as heat, controlling the temperature of water used to mix mortar is an important consideration in both hot and cold weather masonry construction. Often, the most effective means of controlling water temperature is to store the water in large barrels located adjacent to the mixer. Evaporation may provide sufficient cooling for uncovered, shaded barrels in hot weather. Provisions can easily be made to heat water contained in barrels during cold weather.
Accessories
Arrangements must be made to store accessories (flashing materials, joint reinforcement, metal ties, metal anchors, control joint materials, expansion joint materials, etc.) such that they are protected from damage and are readily available for delivery to working areas. Delays associated with not having the right accessories available when and where needed are costly and adversely affect the quality of the masonry construction. Unfortunately, unacceptable alternatives such as omission of specified accessories or improvisation with unsuitable materials have sometimes been implemented in order to avoid delaying construction, measures that are even more costly and detrimental to quality than waiting for delivery of the proper accessories. These situations can be avoided by assuring that accessories are available when and where needed through careful planning, preparation, and coordination.
Mixing and Proportioning Mortar
Mortar materials mixed at the job site are commonly proportioned by volume. A bag of portland, masonry, or mortar cement is one cubic foot (approximately 0.28 m³) by volume. ASTM C 270 defines the bulk density of hydrated lime as 40 lb/ft³ (650 kg/m³). Thus, a 50 lb (22.7 kg) bag of hydrated lime is 1-and-1/4 cubic foot (approximately 0.35 m³) by volume. As previously noted, sand should be damp and loose. Proportions of mortar ingredients must be consistent with the project specifications and the preconstruction submittals.
Fig. 5 A cubic foot box can be used to verify sand volumes added to mixer.
Mechanical mixing of mortar is recommended. The mixer should be conveniently located next to the sand pile and water. Conventional mortar mixers are of rotating-spiral or paddle-blade design with tilting drum. After all batched materials are together, they should be mixed from 3 to 5 minutes. Less mixing time may result in non uniformity, poor workability, low water retention, and less than optimum air content. Extended mixing times may adversely affect the air contents of mortars containing air-entraining cements, particularly during cool or cold weather. Extended mixing times may also reduce the strength of the mortar.
Batching procedures will vary with individual preferences. Experience has shown that good results can be obtained when about three-fourths of the required water, one-half of the sand, and all the cementitious materials are briefly mixed together. The balance of the sand is then charged and the remaining water added to bring the mortar to optimum working consistency. The amount of water added should be the maximum that is consistent with satisfactory workability. This statement assumes that determination of satisfactory workability includes consideration of unit absorption characteristics and weather.