3. CONCRETE MASONRY CONSTRUCTION
Learning Objective: Upon completing this section, you should be able to explain the elements of concrete masonry
Good workmanship is a very important factor in building masonry walls. You should make every effort to lay each masonry unit plumb and true. In the following paragraphs, we will discuss the basic steps in laying up masonry walls.
STEPS IN CONSTRUCTION
The first step in building a concrete masonry wall is to locate the corners of the structure. In locating the corners, you should also make sure the footing or slab formation is level so that each builder starts each section wall on a common plane. This also helps ensure that the bed joints are straight when the sections are connected. If the foundation is badly out of level, the entire first course should be laid before
The second step is to chase out bond, or lay out, by placing the first course of blocks without mortar (figure 8-7, view 1). Snap a chalk line to mark the footing and align the blocks accurately. Then, use a piece of material 3/8 inch thick to properly space the blocks. This helps you get an accurate measurement.
The third step is to replace the loose blocks with a full mortar bed, spreading and furrowing it with a trowel to ensure plenty of mortar under the bottom edges of the first course (figure 8-7, view 2). Carefully position and align the corner block first (view 3 of figure 8-7). Lay the remaining first-course blocks with the thicker end up to provide a larger mortar-bedding area. For the vertical joints, apply mortar only to the block ends by placing several blocks on end and buttering them all in one operation (view 4). Make the joints 3/8 inch thick. Then, place each block in its final position, and push the block down vertically into the mortar bed and against the previously laid block. This ensures a well-tilled vertical mortar joint (view 5). After laying three or four blocks, use a mason’s level as a straightedge to check correct block alignment (figure 8-8, view 1). Then, use the level to bring the blocks to proper grade and plumb by tapping with a trowel handle as shown in view 2. Always lay out the first course of concrete masonry carefully and make sure that you properly align, level, and plumb it. This assures that succeeding courses and the final wall are both straight and true.
Figure 8-7.-Laying first course of blocks for a wall.
Figure 8-8.-Leveling and plumbing first course of blocks for a wall.
The fourth step is to build up the corners of the wall, usually four or five courses high. This is also called laying up a lead. Step back each course one-half block. For the horizontal joints, apply mortar only to the tops of the blocks already laid. For the vertical joints, you can apply mortar either to the ends of the new block or the end of the block previously laid, or both, to ensure well-filled joints (figure 8-9). As you lay each course at the corner, check the course with a level for alignment (figure 8-10, view 1), for level (view 2), and for plumb (view 3). Carefully check each block with a level or straightedge to make sure that all the block faces are in the same plane. This ensures true, straight walls. A story or course pole, which is a board with markings 8 inches apart (figure 8-11), helps accurately place each masonry course. Also check the horizontal block spacing by placing a level diagonally across the corners of the blocks (figure 8-12).
Figure 8-9.-Vertical joints.
Figure 8-10.-Checking each course at the corner.
Figure 8-11.-Using a story or course pole.
Figure 8-12.-Checking horizontal block spacing.
When filling in the wall between the corners, first stretch a mason’s line along the extensor block edges from corner to corner for each course. Then lay the top outside edge of each new block to this line (figure 8-13). How you grip a block before laying is important. First, tip it slightly toward you so that you can see the edge of the course below. Then place the lower edge of the new block directly on the edges of the block below (figure 8-13). Make all position adjustments while the mortar is soft and plastic. Any adjustments you make after the mortar stiffens will break the mortar bond and allow water to penetrate. Level each block and align it to the mason’s line by tapping it lightly with a trowel handle.
Figure 8-13.-Filling in the wall between corners.
Fifth and last, before installing the closure block, butter both edges of the opening and all four vertical edges of the closure block with mortar. Then, lower the closure block carefully into place (figure 8-14). If any mortar falls out, leaving an open joint, remove the block and repeat the procedure.
To assure a good bond, do not spread mortar too far ahead when actually laying blocks. If you do, the mortar will stiffen and lose its plasticity. The recommended width of mortar joints for concrete masonry units is 3/8 inch. When properly made, these joints produce a weathertight, neat, and durable concrete masonry wall. As you lay each block, cut off excess mortar from the joints using a trowel (figure 8-15) and throw it back on the mortar board to rework into the fresh mortar. Do not, however, rework any mortar dropped on the scaffold or floor.
Figure 8-14.-Installing a closure block.
Figure 8-15.-Cutting off excess mortar from the joints.
Weathertight joints and the neat appearance of concrete masonry walls depend on proper striking (tooling). After laying a section of the wall, tool the mortar joint when the mortar becomes “thumb print” hard. Tooling compacts the mortar and forces it tightly against the masonry on each side of the joint. Use either concave or V-shaped tooling on all joints (figure 8-16). Tool horizontal joints (figure 8-17, view 1) with a long jointer first, followed by tooling the vertical joints (view 2). Trim off mortar burrs from the tooling flush with the wall face using a trowel, soft bristle brush, or by rubbing with a burlap bag.
Figure 8-16.-Tooled mortar joints for weathertight exterior walls.
Figure 8-17.-Tooling mortar joints.
A procedure known as pointing may be required after jointing. Pointing is the process of inserting mortar into horizontal and vertical joints after the unit has been laid. Basically, pointing is done to restore or replace deteriorated surface mortar in old work. Pointing of this nature is called tuck pointing. However, even in freshly laid masonry, pointing may be necessary for filling holes or correcting defective joints.
You must prepare in advance for installing wood plates with anchor bolts on top of hollow concrete masonry walls. To do this, place pieces of metal lath in the second horizontal mortar joint from the top of the wall under the cores that will contain the bolts (figure 8-18, view 1). Use anchor bolts 1/2 inch in diameter and 18 inches long. Space them not more than 4 feet apart. Then, when you complete the top course, insert the bolts into the cores of the top two courses and till the cores with concrete or mortar. The metal lath underneath holds the concrete or mortar filling in place. The threaded end of the bolt should extend above the top of the wall (view 2).
Figure 8-18.-Installing anchor bolts for wood plates.
Control joints (figure 8-19) are continuous vertical joints that permit a masonry wall to move slightly under unusual stress without cracking. There are a number of types of control joints built into a concrete masonry wall.
Figure 8-19.-Control joints.
The most preferred control joint is the Michigan type made with roofing felt. A strip of felt is curled into the end core, covering the end of the block on one side of the joint (figure 8-20, view 1). As the other side of the joint is laid, the core is filled with mortar. The filling bonds to one block, but the paper prevents bond to the block on the other side of the control joint.
Figure 8-20.-Making control joints.
View 2 of figure 8-20 shows the tongue-andgroove type of control joint. The special units are manufactured in sets consisting of full and half blocks. The tongue of one unit fits into the groove of another unit or into the open end of a regular flanged stretcher. The units are laid in mortar exactly the same as any other masonry units, including mortar in the head joint. Part of the mortar is allowed to remain in the vertical joint to form a backing against which the caulking can be packed.
View 3 shows a control joint that may be built with regular full- and half-length stretcher blocks with a Z-shaped bar across the joint or a 10- or 12-inch pencil rod (1/4-inch smooth bar) across each face shell. If a pencil rod is used, it must be greased on one side of the joint to prevent bond. These rods should be placed every other course. Lay up control joints in mortar just as any other joint. However, if they are exposed to either the weather or to view, caulk them as well. After the mortar is stiff, rake it out to a depth of about 3/4-inch to make a recess for the caulking compound. Use a thin, flat caulking trowel to force the compound into the joint (figure 8-21).
Figure 8-21.-Making a control joint.
The location of control joints is established by the architectural engineer and should be noted in the plans and specifications.
Walls are differentiated into two types: load bearing and non-load bearing. Load-bearing walls not only separate spaces, but also provide structural support for whatever is above them. Nonload bearing walls function solely as partitions between spaces.
Do not join intersecting concrete block load-bearing walls with a masonry bond, except at the corners. Instead, terminate one wall at the face of the second wall with a control joint. Then, tie the intersecting walls together with Z-shaped metal tie bars 1/4-by-1/4-by-28 inches in size, having 2-inch right-angle bends on each end (figure 8-22, view 1).
Figure 8-22.-Tying intersecting bearing walls.
Space the tie bars no more than 4 feet apart vertically and place pieces of metal lath under the block cores that will contain the tie bars ends (figure 8-18, view 1). Embed the right-angle bends in the cores by filling them with mortar or concrete (figure 8-22, view 2).
To join intersecting non-load-bearing block walls, terminate one wall at the face of the second with a control joint. Then, place strips of metal lath of 1/4-inch mesh galvanized hardware cloth across the joint between the two walls (figure 8-23, view 1) in alternate courses. Insert one-half of the metal stops into one wall as you build it, and then tie the other halves into the mortar joints as you lay the second wall (view 2).
Figure 8-23.-Tying intersecting nonbearing walls.
BOND BEAMS, LINTELS, AND SILLS
Bond beams are reinforced courses of block that bond and integrate a concrete masonry wall into a stronger unit. They increase the bending strength of the wall and are particularly needed to resist the high winds of hurricanes and earthquake forces. In addition, they exert restraint against wall movement, reducing the formation of cracks.
Bond beams are constructed with special-shape masonry units (beam and lintel block) filled with concrete or grout and reinforced with embedded steel bars. These beams are usually located at the top of walks to stiffen them. Since bond beams have appreciable structural strength, they can be located to serve as lintels over doors and windows. Figure 8-24 shows the use of lintel blocks to place a lintel over a metal door, using the door case for support. Lintels should have a minimum bearing of 6 inches at each end. A rule of thumb is to provide 1 inch of bearing for every foot of clear space. When bond beams are located just above the floor, they act to distribute the wall weight (making the wall a deep beam) and thus help avoid wall cracks if the floor sags. Bond beams may also be located below a window sill.
Figure 8-24.-Lintel made from blocks.
Modular door and window openings usually require lintels to support the blocks over the openings. You can use precast concrete lintels (figure 8-25, view 1) that contain an offset on the underside (view 2) to fit the modular openings. You can also use steel lintel angles that you install with an offset on the underside (view 3) to fit modular openings. In either case, place a non-corroding metal plate under the lintel ends at the control joints to allow the lintel to slip and the control joints to function properly. Apply a full bed of mortar over the metal plate to uniformly distribute the lintel load.
Figure 8-25.-Installing precast concrete lintels without end with steel angles.
You usually install precast concrete sills (figure 8-26) following wall construction. Fill the joints tightly at the ends of the sills with mortar or a caulking compound.
Figure 8-26.-Installed precast concrete sills.
PIERS AND PILASTERS
Piers are isolated columns of masonry, whereas pilasters are columns or thickened wall sections built contiguous to and forming part of a masonry wall.
Both piers and pilasters are used to support heavy, concentrated vertical roof or floor loads. They also provide lateral support to the walls. Piers and pilasters offer an economic advantage by permitting construction of higher and thinner walls. They may be constructed of special concrete masonry units (figure 8-27) or standard units.
Figure 8-27.—Pilaster masonry units.
REINFORCED BLOCK WALLS
Block walls may be reinforced vertically or horizontally. To reinforce vertically, place reinforcing rods (called rebar) into the cores at the specified spacing and till the cores with a relatively high-slump concrete. Rebar should be placed at each corner and at troth sides of each opening. Vertical rebar should be spaced a maximum of 32 inches on center in walls. Where splices are required, the bars should be lapped 40 times the bar diameter. The concrete should be placed in one continuous pour from foundation to plate line. A cleanout block maybe placed in the first course at every rebar stud for cleaning out excess mortar and to ensure proper alignment and laps of rebars.
Practical experience indicates that control of cracking and wall flexibility can be achieved with the use of horizontal joint reinforcing. The amount of joint reinforcement depends largely upon the type of construction. Horizontal joint reinforcing, where required, should consist of not less than two deformed longitudinal No. 9 or heavier cold-drawn steel wires. Truss-type cross wires should be 1/8-inch diameter (or heavier) of the same quality. Figure 8-28 shows joint reinforcement on 16-inch vertical spacing. The location and details of bond beams, control joints, and joint reinforcing should all be shown on the drawings.
Figure 8-28.-Masonry wall horizontal joint reinforcement.
PATCHING AND CLEANING BLOCK WALLS
Always fill holes made by nails or line pins with fresh mortar and patch mortar joints. When laying concrete masonry walls, be careful not to smear mortar on the block surfaces. Once they harden, these smears cannot be removed, even with an acid
wash, nor will paint cover them. Allow droppings to dry and harden. You can then chip off most of the mortar with a small piece of broken concrete block (figure 8-29, view 1) or with a trowel (view 2). A final brushing of the spot removes practically all the mortar (view 3).
Figure 8-29.-Cleaning mortar droppings from a concrete block wall.
The purpose of a retaining wall is to hold back a mass of soil or other material. As a result, concrete masonry retaining walls must have the structural strength to resist imposed vertical and lateral loads. The footing of a retaining wall should be large enough to support the wall and the load of the material that the wall is to retain. The reinforcing must be properly located as specified in the plans. Provisions to prevent the accumulation of water behind retaining walls should be made. This includes the installation of drain tiles or weep holes, or both.
PAINTING CONCRETE MASONRY
Several finishes are possible with concrete masonry construction. The finish to use in any specific situation should be governed by the type of structure in which the walls will be used and the climatic conditions to which they will be exposed.
Paints now commonly used on concrete masonry walls include portland cement paint, latex paint, oil-based paint, and rubber-based paint. For proper application and preparation of the different types of paint, refer to the plans, specifications, or manufacturer’s instructions.