Masonry Magazine October 1989 Page. 23
accurately spaced so that they line up with the core spaces of the block. 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 1 inch horizontally per 6 inches of height.
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, laitance, cement dust, 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.
The first course of block on the foundation should have all webs and face shells set in a mortar bed for full bearing. Where the wall is to be grouted, mortar should be placed so that it does not project into the cores to be grouted, thereby permitting the grout to make direct contact with the top of the foundation.
Unless otherwise specified, hollow units are normally laid with mortar coverage on the horizontal and vertical face shells. Webs are mortared where they are adjacent to a cell or cavity which is to be grouted so that the grout will be confined to that area. If the wall is to be fully grouted, it is not necessary to mortar the cross webs since the grout will flow into these spaces.
As the masonry units are laid, the mortar for the bed joint is usually placed atop several units in advance of the units being laid. This mortar should not be permitted to stiffen to the point where the units being placed will not properly form a bond with the units below. The proper distance for stringing bed joints will vary, and depends upon temperature, humidity, wind and speed of laying.
Since all mortar is not used immediately after mixing, evaporation may require the addition of water, or "retempering" 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 22 hours after the original mixing.
If a block which has been laid in the wall is moved once the mortar has begun to stiffen, 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 occurs, the unit and mortar should be completely removed, the mortared area cleaned, and the unit reset in fresh mortar.
It is important that bearing walls be kept level and plumb, and in multi-story walls that they line up accurately one on top the other. Allowable tolerances for "engineered" concrete masonry construction based on actual dimensions are as follows: Bed joint: Thickness ± 1/8 in. Level ± 1/4 in. in 10 ft., 1/2 in. maximum Head joint: Thickness ± 1/8 in. Vertical alignment ± 1/4 in. in 10 ft., 1/2 in. maximum Multiple wythe wall: Thickness ± 1/4 in. Wall alignment: Vertical ± 1/4 in. in 10 ft., 1/2 in. maximum per floor, 1 in. maximum in total height Horizontal ± 1/4 in. in 10 ft., 1/2 in. maximum Top surface of bearing walls: Variation in level between adjacent floor elements 1/8 in. in 10 ft. Variation in level within width of a single unit 1/16 in.
For reinforced concrete masonry construction, vertical reinforcing steel should be located as shown on the drawings and held in position at not over 192 bar diameter on center, or frequently enough to prohibit displacement during grouting. Unless otherwise noted on the plans, vertical bars are placed at the centerline of the wall. They need not be located in the same core as a foundation dowel. The cores containing vertical wall steel are to be grouted to the full height of the bars; the cores containing foundation dowels only are to be grouted to the full height of the dowel.
The thickness of grout or mortar between masonry units and reinforcement should not be less than 1/4 inch except the 1/4 inch bars may be laid in 1/2 inch horizontal mortar joints and No. 6 gauge or smaller wires may be laid in 3/8 inch horizontal joints.
Grouting of hollow block construction requires that the units be aligned to form unobstructed, continuous vertical spaces within the wall.
When low lift grouting is to be used, cores to be grouted should have a minimum clear dimension of 2 inches and a clear area of 8 square inches. When high lift grouting is used, the minimum clear dimension should be 3 inches and the clear area 10 square inches.
Proper preparation of the grout space is very important. Mortar that protrudes more than about 3/8 inch into the grout space should be removed so that the grout will not bridge at these locations causing incomplete filling of the cores.
After the grouted spaces are checked for cleanliness and reinforcement positioned, clean out openings which have previously been provided for high lift grouting are closed by inserting face shells of masonry units, which had been removed, mortaring in units that were left out, or by forming over the openings to allow grouting to the plane of the wall. In hollow unit masonry, the mortar should be permitted to cure 24 hours before grouting to prevent mortar joints from being blown out by fluid pressure from the grout. When filling the cavity space between two wythes of masonry, a period of not less than 3 days is recommended to prevent "blowouts."
Procedures used in grouting are important. Low lift grouting consists of building the wall to scaffold height. After the appropriate curing period, grout is placed to a level of