Masonry Magazine September 1975 Page. 14
Mortar Mixing
It is economical and practical to construct a box having a volume of one cubic foot and use this to batch the sand.
Mortar should be mixed in a paddle-type mechanical batch mixer designed for the purpose. The desirable sequence of mixing consists of loading approximately one-half of the sand and water into the operating mixer, next all the cementitious materials, and then the rest of the sand and water. Mixing should continue for at least three minutes after all of the ingredients have been added. Admixtures should not be used without permission of the specifying authority.
Mortar Workability and Adhesion
Workability is the property of mortar characterized by the smooth, plastic consistency which makes it easy to spread. Workable mortar holds the weight of concrete block when placed and makes leveling and alignment easy. It adheres readily to vertical masonry surfaces. Water affects the workability of a mix by influencing the consistency. A well graded aggregate improves workability. Mortar adhesion or bond can be checked by leaving a specimen of mortar on the wall for several minutes. When removed, it should adhere sufficiently well that it cannot be lifted off as a unit, leaving the block clean.
Mortar Aggregate
One of the primary causes of mortar problems is the aggregate. A lack of fine material (passing the No. 100 sieve) will cause the mix to be harsh and unworkable. An excessive amount of fines may cause low strength.
Mortar Inspection
The inspector does not have the authority to change the mortar from that specified; however, he should reject a mortar which is unworkable, and consult with the engineer or architect to find a solution. An experienced mason is the best judge of mortar; complaints from the mason concerning the mortar deserve an immediate investigation into the problem.
Mortar Retempering
Since all mortar is not used immediately after mixing, evaporation may require the addition of water, or "re-tempering" the mortar to restore it to its original consistency. Retempering is done on the mortar board by forming the unused mortar into a basin, adding water into the basin and thoroughly remixing. Mortar should be placed in final position within 2 1/2 hours after the original mixing. If a block which has been laid in the wall is moved once the mortar has begun to harden, 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 block. If this happens, the unit and mortar should be completely removed, the mortared area cleaned and the unit reset in fresh mortar.
Grout Mixing
Grout for use in concrete masonry walls is proportioned by volume in accordance with applicable specifications. Where significant amounts are required, grout is usually batched, mixed, and delivered in the same manner as transit-mixed concrete. When a batch mixer is used on the job site, all materials should be mixed thoroughly for at least 5 minutes. The consistency of grout should be as fluid as possible without segregation of the materials. If slump is measured the desired range is approximately 8 inches for units having low absorption and 10 inches for units having high absorption.
Grout Specifications
ASTM Specifications for grout does not provide for field control; it is a proportion specification. However, some building codes require that field tests be made. If field tests for compressive strength are required, the following procedure is used for making samples.
First, select a flat location where the block will remain undisturbed for 48 hours. Place a piece of wood 15/8 in. thick and 3 in, square on the level surface. Four masonry units with permeable paper, such as absorptive paper toweling, taped to one face shell are placed around the wood block to form the mold. The resulting mold is approximately 3 by 3 by 6 in. high.
Pour the grout into the 3 x 3 x 6 in. mold in two layers. Rod each layer 25 times with a 1 x 2 in, wood puddling stick to eliminate air bubbles. Puddle the bottom layer throughout its depth. Distribute the strokes uniformly over the cross-section of the mold. For the upper layer, allow the stick to penetrate about 1/2 in, into the underlying layer. After the second lift is puddled, level the top of the prism with a trowel and immediately cover the prism with wet burlap or similar material to keep it damp. The prisms are then protected against extreme changes in temperature, and after 48 hours, remove the masonry units and carefully pack them in damp sawdust or sand for transport to the laboratory where they will be stored in a moist room (100 percent relative humidity) until tested for compressive strength.
Construction
It is important that good bond be obtained between the foundation and the first course of masonry. For this reason, the top of the foundation should be level and clean; loose aggregate, cement dust, laitance and other materials that will prevent mortar and grout from forming a good bond with the concrete should be removed before the first course of block is laid, Foundation alignment must be accurate so that the centerline of the wall stays as close as possible to the centerline of the foundation. Where reinforced hollow block masonry construction is used, reinforcing dowels should be accurately spaced so that they line up with the core spaces of the block, and be of the specified size and length. If a foundation dowel does not line up with a vertical core, it may be bent if the maximum slope of the bent bar does not exceed one horizontal in six vertical.
The first course of block should be laid in a full mortar bed (webs and face shells). Where wall is to be grouted, mortar should be placed so that it does not protrude into the core.