Compressive strength of masonry depends to a great extent on workmanship and the completeness with which units are bedded. Tensile strength is a function of the adhesion of mortar to a unit and of the area of bonding (degree of completeness with which joints are filled). Hence, in specifying masonry work, it is important to call for a full bed of mortar, with each course well hammered down, and all joints completely filled with mortar. To minimize the entrance of water through a masonry wall, follow the practices recommended in Art. 3.4.2.
In particular, in filling head joints, a heavy buttering of mortar should be applied on one end of the masonry, and the unit should be pushed down into the bed joint into place so that the mortar squeezes out from the top and sides of the head joint.
Mortar should correspondingly cover the entire side of a unit before it is placed as a header. An attempt to fill head joints by slushing or dashing will not succeed in producing watertight joints. Partial filling of joints by ‘‘buttering’’ or ‘‘spotting’’ the vertical edge of the unit with mortar cut from the extruded bed joint is likewise ineffective and should be prohibited. Where closures are required, the opening should be filled with mortar so that insertion of the closure will extrude mortar both laterally and vertically.
Mortar joints usually range from 1⁄4 to 3⁄4 in in thickness.
Tooling of joints, if done properly can help to resist penetration of water; but it is not a substitute for complete filling, or a remedy for incomplete filling of joints.
A concave joint (Fig. 11.2) is recommended. Use of raked or other joints that provide horizontal water tables should be avoided. Mortar should not be too stiff at time of tooling, or compaction will not take place, nor should it be too fluid, or the units may move—and units should never be moved after initial contact with mortar. If a unit is out of line, it should be removed, mortar scraped off and fresh mortar applied before the unit is relaid.
The back face of exterior wythes should be back plastered, or parged, before backup units are laid. If the backup is laid first, the front of the backup should be parged. The mortar should be the same as that used for laying the masonry and should be applied from 1⁄4 to 3⁄8 in thick.
The rate of absorption of water by unit masonry at the time of laying is important in determining the strength and resistance to penetration of water of a mortared joint. This rate can be reduced by wetting the unit before laying. Medium absorptive units may need to be thoroughly soaked with water. Highly absorptive units may require total immersion in water for some time before unit ‘‘suction’’ is reduced to the low limits needed.
The test for the rate of absorption of brick is described in ASTM Standard C67.
For water-resistant masonry, the suction (rate of absorption) of the brick should not
exceed 0.35, 0.5, and 0.7 oz, respectively, for properly constructed all-brick walls
or facings of normal 4-, 8-, and 12-in thickness.
The amount of wetting that bricks will require to control rate of absorption properly when laid should be known or determined by measurements made before the bricks are used in the wall. Some medium absorptive bricks may require only frequent wetting in the pile; others may need to be totally immersed for an hour or more. While the immersion method is more costly than hosing in the pile, it ensures that all bricks are more or less saturated when removed from immersion.
A short time interval may be needed before the bricks are laid; but the bricks are likely to remain on the scaffold, in a suitable condition to lay, for some time. Bricks on the scaffold should be inspected and moisture condition checked several times a day.
In general, method of manufacture and surface texture of masonry do not greatly affect the permeability of walls. However, water-resistant joints may be difficult to obtain if the units are deeply scored, particularly if the mortar is of a dry consistency.
Loose sand should be brushed away or otherwise removed from units that are heavily sanded.
Mortar to be used in above-grade, water-resistant brick-faced and all-brick walls should be of as wet a consistency as can be handled by the mason and meet requirements of ASTM Standard C270, Type N. Water retention of the mortar should not be less than 75%, and preferably 80% or more. For laying absorptive brick that contain a considerable amount of absorbed water, the mortars having a water retention of 80% or more may be used without excessive ‘‘bleeding’’ at the joints and ‘‘floating’’ of the brick. The mortar may contain a masonry cement meeting the requirements of ASTM C91, except that water retention should not be less than 75 or 80%. Excellent mortar may also be made with portland cement and hydrated lime, mixed in the proportion of 1:1:6 parts by volume of cement, lime, and loose damp sand. The hydrated lime should be highly plastic. Type S lime conforming with the requirements of ASTM C207 is highly plastic, and mortar containing it, in equal parts by volume with cement, will probably have a water retention of 80% or more.
Since capillary penetration of moisture through concrete and mortar is of minor importance, particularly in above-grade walls, the mortar need not contain an integral water repellent. However, if desired, water-repellent mortar may be advantageously used in a few courses at the grade line to reduce capillary rise of moisture from the ground into the masonry. The mortar should be of a type that does not stiffen rapidly on the board, except through loss of moisture by evaporation.
Mortar should be retempered frequently if necessary to maintain as wet a consistency as is practically possible for the mason to use. At air temperatures below 80F, mortar should be used or discarded within 31⁄2 hr after mixing; for air temperatures of 80F or higher, unused mortar should be discarded after 21⁄2 hr.
Cold-Weather Construction of Masonry Walls
Masonry should be protected against damage by freezing. The following special precautions should be taken:
Materials to be used should be kept dry. Tops of all walls not enclosed or sheltered should be covered whenever work stops. The protection should extend downward at least 2 ft.
Frozen materials must be thawed before use. Masonry units should be heated to at least 40F. Mortar temperature should be between 40 and 120F, and mortar should not be placed on a frozen surface. If necessary, the wall should be protected with heat and windbreaks for at least 48 hr. Use of mortars made with high-earlystrength cement may be advantageous for cold-weather masonry construction.
Bond between Wythes in Masonry Walls
When headers are used for bonding the facing and backing in solid masonry walls and faced walls, as shown in Fig. 11.3, not less than 4% of the wall surface of each face should be composed of headers, which should extend at least 4 in into the backing. These headers should not be more than 24 in apart vertically or horizontally (Fig. 11.3a and b). In walls in which a single bonder does not extend through the wall, headers from opposite sides should overlap at 4 in or should be covered with another bonder course overlapping headers below at least 4 in.
If metal ties (Figs. 11.3e and 11.4) are used for bonding, they should be corrosion- resistant. For bonding facing and backing of solid masonry walls and faced walls, there should be at least one metal tie for each 41⁄2 ft2 of wall area. Ties in alternate courses should be staggered, the maximum vertical distance between ties should not exceed 18 in, and the maximum horizontal distance should not be more than 36 in.
In walls composed of two or more thicknesses of hollow units, stretcher courses should be bonded by one of the following methods: At vertical intervals up to 34 in, there should be a course lapping units below at least 4 in (Fig. 11.3c). Or at vertical intervals up to 17 in, lapping should be accomplished with units at least 50% thicker than the units below (Fig. 11.3d). Or at least one metal tie should be incorporated for each 41⁄2 ft2 of wall area. Ties in alternate courses should be staggered;
the maximum vertical distance between ties should be 18 in and maximum horizontal distance, 36 in. Full mortar coverage should be provided in both horizontal and vertical joints at ends and edges of face shells of the hollow units.
In ashlar masonry, bond stones should be uniformly distributed throughout the wall and form at least 10% of the area of exposed faces.
In rubble stone masonry up to 24 in thick, bond stones should have a maximum spacing of 3 ft vertically and horizontally. In thicker walls, there should be at least one bond stone for each 6 ft2 of wall surface on both sides.
For bonding ashlar facing, the percentage of bond stones should be computed from the exposed face area of the wall. At least 10% of this area should be composed of uniformly distributed bond stones extending 4 in or more into the backup.
Every bond stone and, when alternate courses are not full bond courses, every stone should be securely anchored to the backup with corrosion-resistant metal anchors.
These should have a minimum cross section of 3⁄16 x 1 in. There should be at least one anchor to a stone and at least two anchors for stones more than 2 ft long or with a face area of more than 3 ft2. Larger facing stones should have at least one anchor per 4 ft2 of face area of the stone, but not less than two anchors.
Cavity-wall wythes should be bonded with 3⁄16-in-diameter steel rods or metal ties of equivalent stiffness embedded in horizontal joints. There should be at least one metal tie for each 41⁄2 ft2 of wall area. Ties in alternate courses should be staggered, the maximum vertical distance between ties should not exceed 18 in (Fig 11.3e), and the maximum horizontal distance, 36 in. Rods bent to rectangular shape should be used with hollow masonry units laid with cells vertical. In other walls, the ends of ties should be bent to 90 angles to provide hooks at least 2 in long.
Additional bonding ties should be provided at all openings. These ties should be spaced not more than 3 ft apart around the perimeter and within 12 in of the opening.
When two bearing walls intersect and the courses are built up together, the intersections should be bonded by laying in true bond at least 50% of the units at the intersection. When the courses are carried up separately, the intersecting walls should be regularly toothed or blocked with 8-in maximum offsets. The joints should be provided with metal anchors having a minimum section of 1⁄4 x 1 1⁄2 in with ends bent up at least 2 in or with cross pins to form an anchorage. Such anchors should be at least 2 ft long and spaced not more than 4 ft apart.
Construction of walls requiring two or more wythes of brick or solid concrete block, similar to the wall shown in Fig. 11.3a, may be speeded by pouring grout between the two outer wythes, to fill the interior joints. Building codes usually require that, for the wythes, the mortar be type M or S, consisting of portland cement, lime, and aggregate (Art. 4.16). Also, they may require that, when laid, burned-clay brick and sand-lime units should have a rate of absorption of not more than 0.025 oz/ in2 over a 1-mm period in the standard absorption test (ASTM C67). All units in the two outer wythes should be laid with full head and bed joints.
Low-Lift Grouting. The vertical spaces between wythes that are to be grouted should be at least 3⁄4 in wide. Masonry headers should not project into the gap.
One of the outer wythes may be carried up 18 in before grout is poured. The other outer wythe is restricted to a height up to 6 times the grouting space, but not more than 8 in, before grout is poured. Thus, in this type of construction, grout is poured in lifts not exceeding 8 in. The grout should be puddled with a grout stick immediately after it has been poured. If work has to be stopped for an hour or more, horizontal construction joints should be formed by raising all wythes to the same level and leaving the grout 1 in below the top. A suitable grout for this type of construction consists of 1 part portland cement, 0.1 part hydrated lime or lime putty, and 21⁄4 to 3 parts sand.
High-Lift Grouting. This type of construction is often used where steel reinforcement is to be inserted in the vertical spaces between wythes; for example, in the cavity of the wall shown in Fig. 11.3e. Grout is poured continuously in lifts up to 6 ft high and up to 30 ft long in the vertical spaces. (Vertical barriers, or dams, of solid masonry may be built in the grout space to control the horizontal flow of grout.) Building codes may require each lift to be completed within one day. The grout should be consolidated by puddling or mechanical vibrating as it is placed and reconsolidated after excess moisture has been absorbed but before plasticity has been lost. A suitable grout for gaps 2 or more inches wide consists of 1 part portland cement, 0.1 part hydrated lime or lime putty, 2 to 3 parts sand, and not more than 2 parts gravel, by volume.
In construction of the wall, the wythes should be kept at about the same level.
No wythe should lay behind the others more than 16 in in height. The masonry should be allowed to cure for at least 3 days, to gain strength, before grout is poured. The grout space should be at least 2 in wide. If, however, horizontal reinforcement is to be placed in the gap, it should be wide enough to provide 1⁄4 in clearance around the steel, but not less than 3 in wide.
Cleanouts should be provided for every pour. This may be done by omitting every other unit in the bottom course of the wall section being poured. Before grout is placed, excess mortar, mortar fins, and other foreign matter should be removed from the grout space. A high-pressure water jet may be used for the purpose. After inspection but before placement of grout, the cleanout holes should be plugged with masonry units, which should then be braced to resist the grout pressure.
Wire ties should be inserted in the mortar joints between masonry courses and span across each grout space, to bond the wythes (Fig. 11.5). The ties should be formed into rectangles, 4 in wide and with a length 2 in less than the distance between outer faces of the wythes being bonded. The wire size should not be less than No.9. Spacing of ties should not exceed 24 in horizontally. For running-bond masonry (Fig. 11.3ƒ), vertical tie spacing should not exceed 16 in, and for stackbond masonry (Fig. 11.3j), 12 in.
Support Conditions for Walls
Provision should be made to distribute concentrated loads safely on masonry walls and piers. Heavily loaded members should have steel bearing plates under the ends to distribute the load to the masonry within allowable bearing stresses. Length of bearing should be at least 3 in. Lightly loaded members may be supported directly on the masonry if the bearing stresses in the masonry are within permissible limits and if length of bearing is 3 in or more.
Masonry should not be supported on wood construction.
Where a solid masonry wall 12 in or more thick must be increased in thickness above a specific level, the increase should be achieved gradually by corbeling. In this method, successive courses are projected from the face of the wall, as indicated in Fig. 11.3f.
The maximum corbeled horizontal projection beyond the face of a wall should not exceed one-third the wall thickness for walls supporting structural members. In any case, projection of any course of masonry should not exceed 1 in.
Chimneys generally may not be corbeled more than 6 in from the face of the wall. In the second story of two-story dwellings, however, corbeling of chimneys on the exterior of enclosing walls may equal the wall thickness.
Openings, Chases, and Recesses in Masonry Walls
Masonry above openings should be supported by arches or lintels of metal or reinforced masonry, which should bear on the wall at each end at least 4 in. Stone or other nonreinforced masonry lintels should not be used unless supplemented on the inside of the wall with structural steel lintels, suitable masonry arches, or reinforced- masonry lintels carrying the masonry backing. Lintels should be stiff enough to carry the superimposed load with a deflection of less than 1⁄720 of the clear span.
In plain concrete walls, reinforcement arranged symmetrically in the thickness of the wall should be placed not less than 1 in above and 2 in below openings. It should extend at least 24 in on each side of the opening or be equivalently developed with hooks. Minimum reinforcement that should be used is one No.5 bar for each 6 in of wall thickness.
In structures other than low residences, masonry walls should not have chases and recesses deeper than one-third the wall thickness, or longer than 4 ft horizontally or in horizontal projection. There should be at least 8 in of masonry in back of chases and recesses, and between adjacent chases or recesses and the jambs of openings.
Chases and recesses should not be cut in walls of hollow masonry units or in hollow walls but may be built in. They should not be allowed within the required area of a pier.
The aggregate area of recesses and chases in any wall should not exceed onefourth of the whole area of the face of the wall in any story.
In dwellings not more than two stories high, vertical chases may be built in 8- in walls if the chases are not more than 4 in deep and occupy less than 4 ft2 of wall area. However, recesses below windows may extend from floor to sill and may be the width of the opening above. Masonry above chases or recesses wider than 12 in should be supported on lintels.
Recesses may be left in walls for stairways and elevators, but the walls should not be reduced in thickness to less than 12 in unless reinforced in some approved manner. Recesses for alcoves and similar purposes should have at least 8 in of masonry at the back. They should be less than 8 ft wide and should be arched over or spanned with lintels.
If the strength of a wall will not be impaired, pipe or conduit may be passed horizontally or vertically through the masonry in a sleeve. Sleeves, however, should not be placed closer than three diameters center to center.
Flashing in Masonry Walls
Flashing should be used to divert to the exterior of a building water that may penetrate or condense on the interior face of masonry walls. Accordingly, flashing should be installed in exterior walls at horizontal surfaces, such as roofs, parapets, and floors, depending on type of construction; at shelf angles; at openings, such as doors and windows (Fig. 11.6a and b); and at the bases of walls just above grade (Fig. 11.6c and e). The flashing should extend through a mortar joint to the outside face of the wall, where it should turn down to form a drip.
Flashing in tooled mortar joints, however, would trap water unless some means is provided to drain it to the outside. Consequently, flashing should be used in conjunction with weep holes, which should be formed in head joints immediately above the flashing (Fig. 11.6d). When the weep holes are left open, spacing should not exceed 24 in c to c. If wicks of glass-fiber or nylon rope, cotton sash cord, or similar materials are left in the holes, spacing should not exceed 16 in c to c.
Materials used for flashing include sheet copper, bituminous fabrics, plastics, or a combination of these. Copper may be selected for its durability, but cost may be greater than for other materials. Combinations of materials, such as cold-formed steel and plastic or bituminous coating, may yield a durable flashing at lower cost.