One of the most common masonry units is the concrete block. It consists of hardened cement and may be completely solid or contain single or multiple hollows. It is made from conventional cement mixes and various types of aggregate. These include sand, gravel, crushed stone, air-cooled slag, coal cinders, expanded shale or clay, expanded slag, volcanic cinders (pozzolan), pumice, and "scotia" (refuse obtained from metal ore reduction and smelting). The term "concrete block" was formerly limited to only hollow masonry units made with such aggregates as sand, gravel, and crushed stone. Today, the term covers all types of concrete block-both hollow and solidmade with any kind of aggregate. Concrete blocks are also available with applied glazed surfaces, various pierced designs, and a wide variety of surface textures.
Although concrete block is made in many sizes and shapes (figure 8-4) and in both modular and nonmodular dimensions, its most common unit size is 7 5/8 by 7 5/8 by 15 5/8 inches. This size is known as 8-by-8-by- 16-inch block nominal size. All concrete block must meet certain specifications covering size, type, weight, moisture content, compressive strength, and other characteristics. Properly designed and constructed, concrete masonry walls satisfy many building requirements, including fire prevention, safety, durability, economy, appearance, utility, comfort, and acoustics.
Figure 8-4.-Typical unit sizes and shapes of concrete masonry units.
Concrete blocks are used in all types of masonry construction. The following are just a few of many examples:
There are five main types of concrete masonry units:
Load-bearing blocks are available in two grades: N and S. Grade N is for general use, such as exterior walls both above and below grade that may or may not be exposed to moisture penetration or weather. Both grades are also used for backup and interior walls. Grade S is for above-grade exterior walls with a weather-protective coating and for interior walls. The grades are further subdivided into two types. Type I consists of moisture-controlled units for use in arid climates. Type II consists of nonmoisturecontrolled units.
Figure 8-4.-Typical unit sizes and shapes of concrete masonry unitsContinued.
BLOCK SIZES AND SHAPES
Concrete masonry units are available in many sizes and shapes to tit different construction needs. Both full- and half-length sizes are shown in figure 8-4. Because concrete block sizes usually refer to nominal dimensions, a unit actually measuring 7 5/8-by-7 5/8-by-15 5/8-inches is called an 8-by-8-by-16-inch block. When laid with 3/8-inch mortar joints, the unit should occupy a space exactly 8-by-8-by- 16 inches.
ASTM (American Society for Testing and Materials) specifications define a solid concrete block as having a core area not more than 25 percent of the gross cross-sectional area. Most concrete bricks are solid and sometimes have a recessed surface like the frogged brick shown in figure 8-4. In contrast, a hollow concrete block has a core area greater than 25 percent of its gross cross-sectional area-generally 40 percent to 50 percent.
Blocks are considered heavyweight or lightweight, depending on the aggregate used in their production. A hollow load-bearing concrete block 8-by-8-by- 16-inches nominal size weighs from 40 to 50 pounds when made with heavyweight aggregate, such as sand, gravel, crushed stone, or air-cooled slag. The same size block weighs only 25 to 35 pounds when made with coal cinders, expanded shale, clay, slag, volcanic cinders, or pumice. The choice of blocks depends on both the availability and requirements of the intended structure.
Blocks may be cut with a chisel. However, it is more convenient and accurate to use a power-driven masonry saw (figure 8-5). Be sure to follow the manufacturers manual for operation and maintenance, As with all electrically powered equipment, follow all safety guidelines.
Figure 8-5.-Masonry saw.
BLOCK MORTAR JOINTS
The sides and the recessed ends of a concrete block are called the shell. The material that forms the partitions between the cores is called the web. Each of the long sides of a block is called a face shell. Each of the recessed ends is called an end shell. The vertical ends of the face shells, on either side of the end shells, are called the edges.
Bed joints on first courses and bed joints in column construction are mortared by spreading a 1-inch layer of mortar. This procedure is referred to as "full mortar bedding." For most other bed joints, only the upper edges of the face shells need to be mortared. This is referred to as "face shell mortar bedding."
Head joints may be mortared by buttering both edges of the block being laid or by buttering one edge on the block being laid and the opposite edge on the block already in place.
Properly mixed and applied mortar is necessary for good workmanship and good masonry service because it must bond the masonry units into a strong, well-knit structure. The mortar that bonds concrete block, brick, or clay tile will be the weakest part of the masonry unless you mix and apply it properly. When masonry leaks, it is usually through the joints. Both the strength of masonry and its resistance to rain penetration depend largely on the strength of the bond between the masonry unit and the mortar. Various factors affect bond strength, including the type and quantity of the mortar, its plasticity and workability, its water retentivity, the surface texture of the mortar bed, and the quality of workmanship in laying the units. You can correct irregular brick dimensions and shape with a good mortar joint.
Workability of Mortar
Mortar must be plastic enough to work with a trowel. You obtain good plasticity and workability by using mortar having good water retentivity, using the proper grade of sand, and thorough mixing. You do not obtain good plasticity by using a lot of cementitious materials. Mortar properties depend largely upon the type of sand it contains. Clean, sharp sand produces excellent mortar, but too much sand causes mortar to segregate, drop off the trowel, and weather poorly.
Water retentivity is the mortar property that resists rapid loss of water to highly absorbent masonry units. Mortar must have water to develop the bond. If it does not contain enough water, the mortar will have poor plasticity and workability, and the bond will be weak and spotty. Sometimes, you must wet brick to control water absorption before applying mortar, but never wet concrete masonry units.
Mortar Strength and Durability
The type of service that the masonry must give determines the strength and durability requirements of mortar. For example, walls subject to severe stress or weathering must be laid with more durable, stronger mortar than walls for ordinary service. Table 8-1 gives mortar mix proportions that provide adequate mortar strength and durability for the conditions listed.
Table 8-1.-Recommended Mortar Mix Proportions by Unit Volume
Types of Mortar
The following mortar types are proportioned on a volume basis:
The manner in which mortar is mixed has a lot to do with the quality of the final product. In addition to machine and hand mixing, you need to know the requirements for introducing various additives, including water, to the mix in order to achieve optimum results.
Machine mixing refers to mixing large quantities of mortar in a drum-type mixer. Place all dry ingredients in the mixer first and mix them for 1 minute before adding the water. When adding water, you should always add it slowly. Minimum mixing time is 3 minutes. The mortar should be mixed until a completely uniform mixture is obtained.
Hand mixing involves mixing small amounts of mortar by hand in a mortar box or wheelbarrow. Take care to mix all ingredients thoroughly to obtain a uniform mixture. As in machine mixing, mix all dry materials together first before adding water. Keep a steel drum of water close at hand to use as the water supply. You should also keep all your masonry tools free of hardened mortar mix and dirt by immersing them in water when not in use.
You occasionally need to mix lime putty with mortar. When machine mixing, use a pail to measure the lime putty. Place the putty on top of the sand. When hand mixing, add the sand to the lime putty.
Wet pails before filling them with mortar and clean them immediately after emptying.
Mixing water for mortar must meet the same quality requirements as mixing water for concrete. Do not use water containing large amounts of dissolved salts. Salts weaken the mortars.
You can restore the workability of any mortar that stiffens on the mortar board due to evaporation by remixing it thoroughly. Add water as necessary, but discard any mortar stiffened by initial setting. Because it is difficult to determine the cause of stiffening, a practical guide is to use mortar within 2 1/2 hours after the original mixing. Discard any mortar you do not use within this time.
Do not use an antifreeze admixture to lower the freezing pint of mortars during winter construction. The quantity necessary to lower the freezing point to any appreciable degree is so large it will seriously impair the strength and other desirable properties of the mortar.
Do not add more than 2-percent calcium chloride (an accelerator) by weight of cement to mortar to accelerate its hardening rate and increase its early strength. Do not add more than 1-percent calcium chloride to masonry cements. Make a trial mix to find the percentage of calcium chloride that gives the desired hardening rate. Calcium chloride should not be used for steel-reinforced masonry. You can also obtain high early strength in mortars with high-early-strength portland cement.
Concrete masonry walls should be laid out to make maximum use of full- and half-length units. This minimizes cutting and fitting of units on the job. Length and height of walls, width and height of openings, and wall areas between doors, windows, and corners should be planned to use full-size and half-size units, which are usually available (figure 8-6). This procedure assumes that window and door frames are of modular dimensions which fit modular full- and half-size units. Then, all horizontal dimensions should be in multiples of nominal full-length masonry units.
Figure 8-6.-Planning concrete masonry wall openings.
Both horizontal and vertical dimensions should be designed to be in multiples of 8 inches. Table 8-2 lists nominal length of concrete masonry walls by stretchers. Table 8-3 lists nominal height of concrete masonry walls by courses. When 8-by-4-by-16 units are used, the horizontal dimensions should be planned in multiples of 8 inches (half-length units) and the vertical dimensions in multiples of 4 inches. If the thickness of the wall is greater or less than the length of a half unit, a special-length unit is required at each corner in each course. Table 8-4 lists the average number of concrete masonry units by size and approximate number of cubic feet of mortar required for every 100 square feet of concrete masonry wall. Table 8-5 lists the number of 16-inch blocks per course for any wall.
Table 8-2.-Nominal Lengths of Concrete Masonry Walls in Stretchers
Table 8-3.-Nominal Heights of Modular Concrete Masonry Walls in Courses
Table 8-4.-Average Concrete Masonry Units and Mortar per 100 sq. ft. of Wall
Table 8-5.-Number of 16-Inch Blocks per Course
You should always use outside measurements when calculating the number of blocks required per course. For example, a basement 22 feet by 32 feet should require 79 blocks for one complete course. Multiply 79 by the number of courses needed. Thus, a one-course basement requires a total of 790 blocks for a solid wall, from which deductions should be made for windows and doors. If any dimension is an odd number, use the nearest smaller size listed in the table. For example, for a 22-foot by 31-foot enclosure, use 22 feet by 30 feet and add one-half block per row.
As a Builder, you might find yourself in the field without the tables handy, so here is another method. Use 3/4 times the length and 3/2 times the height for figuring how many 8-by-8-by-16-inch blocks you need for a wall. Lets take an example:
Given: A wall 20 ft long x 8 ft high
You can use "rule 38" for calculating the raw material needed to mix 1 yard of mortar without a great deal of paperwork. This rule does not, however, accurately calculate the required raw materials for large masonry construction jobs. For larger jobs, use the absolute volume or weight formula. In most cases, though, and particularly in advanced base construction, you can use rule 38 to quickly estimate the quantities of the required raw materials.
Builders have found that it takes about 38 cubic feet of raw materials to make 1 cubic yard of mortar. In using rule 38 for calculating mortar, take the rule number and divide it by the sum of the quantity figures specified in the mix. For example, lets assume that the building specifications call for a 1:3 mix for mortar, 1 + 3 = 4. Since 38 ÷ 4 = 9½, youll need 9½ sacks, or 9½ cubic feet, of cement. To calculate the amount of fine aggregate (sand), you multiply 9½ by 3. The product (28½ cubic feet) is the amount of sand you need to mix 1 cubic yard of mortar using a 1:3 mix. The sum of the two required quantities should always equal 38. This is how you can check whether you are using the correct amounts. In the above example, 9½ sacks of cement plus 28½ cubic feet of sand equal 38.
SAFE HANDLING OF MATERIAL
When you handle cement or lime bags, wear goggles and snug-fitting neckbands and wristbands. Always practice good personal cleanliness and never wear clothing that has become stiff with cement. Cement-impregnated clothing irritates the skin and may cause serious infection. Any susceptibility of the skin to cement and lime burns should be reported. Personnel who are allergic to cement or lime should be transferred to other jobs.
Bags of cement or lime should not be piled more than 10 bags high on a pallet. The only exception is when storage is in bins or enclosures built for such storage. The bags around the outside of the pallet should be placed with the mouths of the bags facing the center, The first five tiers of bags each way from any corner must be cross piled. A setback starting with the sixth tier should be made to prevent piled bags from falling outward. If you have to pile bags above 10 tiers, another setback must be made. The back tier, when not resting against an interior wall of
sufficient strength to withstand the pressure, should be set back one bag every five tiers, the same as the end tiers. During unpiling, the entire top of the pile should be kept level and the necessary setbacks maintained.
Lime and cement must be stored in a dry place. This helps prevent lime from crumbling and the cement from hydrating before it is used.
|David L. Heiserman, Editor||
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Revised: May 18, 2013