Masonry has become increasingly important as a construction method for construction. The commonly accepted definition of masonry, or unit masonry as it is sometimes called, is a construction method made up of prefabricated masonry units (such as concrete block, brick, clay tile, and stone) laid in various ways and joined together by mortar.
This course covers the construction techniques of laying brick, structural clay tile, and stone, and the estimating procedures associated with concrete masonry units (CMUs).
When you have completed this course, you will be able to:
|1.0.0 Estimating Concrete Masonry Units|
Concrete masonry walls are laid out so as to make maximum use of full- and half-length units. This is called modular planning. Architectural and Engineering (A&E) firms and Builders strive to build modular structures because it minimizes cutting and fitting of units on the job, which in turn saves on labor and cost. Table1 lists the nominal lengths of concrete masonry walls by stretchers.
Table 1 Nominal Lengths of Concrete Masonry Walls in Stretchers.
Actual wall length is measured from outside edge to outside edge of units and equals the nominal length minus 3/8 (one mortar joint).
Table 2 lists nominal heights of concrete masonry walls by courses.
Table 2 Nominal Heights of Concrete Masonry Walls in Courses.
For concrete masonry units 7 5/8 and 3 5/8 in height laid with 3/8 mortar joints, height is measured from center to center of mortar joints.
Table 3 lists the average number of concrete masonry units by size and the approximate number of cubic feet of mortar required for every 100 square feet of a concrete masonry wall.
Table 3 Average Concrete Masonry Units and Mortar per 100 Square Feet of Wall
Mortar is based on 3/8 joint with a face-shell mortar bed and 10% allowance for waste.
As a builder, you might find yourself in the field without the tables handy. To solve that problem, we will cover two methods of estimating concrete masonry units (CMUs) without the tables.
Chasing the bond uses the 3/4 rule and the 3/2 rule. When estimating, always use outside measurements to calculate the number of blocks required per course. In most Seabee construction, 8 x 8 x 16 block is used.
Using the 3/4 rule (three full block [fb] per 4 feet in length) or .75, multiply the length of the wall by .75. For example, a retaining wall that is 100 feet in length (1,000 square feet) will require 75 block for the first course.
Length of course in feet x rule 3/4 = number of CMU per course
Using the 3/2 rule (three full block per 2 feet in height), multiply the height of the wall by 1.5. For example, the height of the retaining wall is 10 feet. Multiply 10 by the rule 3/2 (1.5), which will equal 15 block high (courses high).
Height of wall in feet x rule 3/2 = courses high
Then, to find the total number of full block in the retaining wall, multiply the number of block in length by the number of block in height, which in this example is 75 CMUs in length times 15 courses high, which equals 1,125 fb.
Lets take another example, using a building 20 feet long by 8 feet wide by 8 feet high.
.75 x 20 x 2 (sides) = 30 (8 x 8 x 16 block)
.75 x 8 x 2 (sides) = 12 (8 x 8 x 16 block)
Or you can find the total linear feet (lf) of the building and multiply by .75.
20 x 2 (sides) + 8 x 2 (sides) = 56 lf
56 x .75 = 42 fb
1.5 x 8 = 12 courses high
42 fb x 12 courses = 504 total fb
The square foot method is usually the quickest and simplest method but NOT the most accurate. As the estimator, however, you will use this method quite frequently. In the first example the retaining wall was 10 feet high and 100 feet long. To find square feet, all you do is use the equation L x H = SF; in this example, the answer is 1,000 square feet (sf). To find the number of 9 x 8 x16 block required, you must determine the square footage of one CMU, which is .89 sf per block. Next you divide 1,000 sf by 89 sf/CMU, which equals 1,124 fb. You calculated the block for 1,000 sf, and the difference is 1 less block figuring by the square foot method.
Total sf divided by sf/CMU = total number of CMU
Now calculate the 20 x 20 x 8 building:
20 x 8 = 160 sf x 2 (sides) = 320 sf
8 x 8 = 54 sf x 2 (sides) = 128 sf
Total = 448 sf
448 sf / .89 sf/CMU 503.4 or 504 total fb
Or, you can multiply the square footage of the building times the number of block per square foot (1.125 CMU/sf).
448 SF x 1.125 CMU/SF = 504 CMU
If you were planning a modular building, you would use the square foot method for quicker estimating, but there is an additional step you need to take, calculating the duplicating factor, which takes into account that every course will have a half block at each corner. For example, you estimated 504 fb for this building. To estimate the fb accurately, you would deduct two f /course or multiply 12 courses x .5 (half block [hb]) x four corners = 24 fb. Then deduct the 24 fb from the total fb as shown in the following formula:
12 courses x .5 x 4 corners = 24 fb
504 fb - 24 fb = 480 fb
When you estimate CMUs, usually the window and door openings are designed to be modular and the window and door frames are of the same mode. If the design is NOT modular, you can expect a lot of cutting time. When you estimate for openings, just calculate the area of the opening, and then subtract the area of the opening(s) from the overall area of the wall or building to get the net area. Finally, multiply the number of CMU per square foot by the net area.
Builders have found that it takes about 38 cubic feet of raw materials to make 1 cubic yard of mortar. Therefore, you can use rule 38 for calculating the raw material needed to mix 1 cubic yard of mortar without having to do a great deal of paper work. However, this rule does not 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 may use rule 38 to make a quick estimate of the quantities of raw materials required.
Here is how you use 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 specification calls for a 1:3 mix for mortar, 1 + 3 = 4. Since 38 ๗ 4 = 9 1/2, you need 9 1/2 sacks, or 9 1/2 cubic feet, of cement. To calculate the amount of fine aggregate (sand), you multiply 9 1/2 by 3. The product (28 1/2 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 previous example, 9 1/2 sacks of cement plus 28 1/2 cubic feet of sand equal 38.
Table 3 shows that it takes 8.5 cubic feet (cf) of mortar to lay 100 sf of 8" x 8" x 16" block. In the previous example, you estimated the building at 480 sf of wall area. To calculate the amount of mortar to lay the CMU:
First convert the 480 sf to units.
480 sf ๗ 100 sf = 4.8 units
Then multiply the units by the number of cubic feet of mortar:
4.8 units x 8.5 cf = 40.8 cf of mortar
To calculate the ingredients needed to make 40.80 cf of mortar with a 1:1/4:3 mix, the 1/4 being hydrated lime, first calculate the amount of cement using rule 38. Remember the formula: 9 1/2 sacks of cement (94 lb/sk) per cubic yard. Use the following procedure:
First, convert cubic feet of mortar to cubic yards:
40.8 cf ๗ 27 cf/cy = 1.51 cubic yard
Cement: (1) x 9 1/2 cf = 0 1/2 (sacks) x 1.51 cd = 14 sks (cf)
Sand: 3/4 x 9 1/2 = 28 1/2 / 38 (cf) x 1.51 cd = 43 cf
Lime: (1/4) x 9 1/2 cf = 2.5 (cf) x 1.51 cd = 4 cf
Total: 61 cf
Lets briefly cover the mixing time it will take to mix mortar. A typical mortar mixer has a capacity of mixing 4 to 7 cubic feet per batch, and each batch must be mixed for a minimum of 3 minutes. In the most recent example, we calculated a total of 61 cubic feet of raw materials needed to construct this building. Now just divide the number of cubic feet per batch by the total number of cubic feet of raw materials, and then multiply that number by the number of minutes per batch.
61 cf / 4 cf/batch = 15 batches
15 batches x 3 minutes/batch = 45 minutes
The time indicates only the required continuous mixing time and does not include the cleaning, staging, or transporting time of the material or the time required for you to lay the CMU. Batching procedures will vary with individual preference. Experience is the key to good results in obtaining the desired mix.
This section briefly covers labor estimates for concrete masonry units according to the Planners and Estimators Handbook, P-405. Table 4 shows the labor table from the P-405 on how to estimate labor.
Table 4 Labor Chart for Masonry.
When using this table, you will see that 8" x 8" x 16" block takes one person (skilled labor) 160 man hours to lay 1,000 square feet of CMUs. If you were to break this labor down into how many CMUs are laid in an 8-hour period, it would be calculated as follows:
1000 sf of wall area = 1,125 CMU
160 man hours ๗ 8 hour days = 20 duration days
1125 CMU ๗ 20 days= 56.25 CMU/day
Lets return to the building example. How many man hours (MH) will it take with a crew of one skilled and three non-skilled laborers? This is the ratio/proportion part of this calculation.
If 160 MH equals 1,000 sf of wall area (P-405), then, X (MH) equals the square footage of the wall area.
160 (MH) :1000 sf :: x (MH) : 448 sf =
160 x 448 :: 1000 x
71680 ๗ 1000 x = 71.68
X = 72 MH
Another method you may use to calculate this number is as follows:
x/160 = 448/1000
In this equation, you simply cross multiply the following:
160 x 448 = 71680
X times 1000 = 1000 x
71680 ๗ 1000 x
X = 71.68 or 72 MH
In this example, it takes 72 man hours to lay 448 sf or 504 CMUs. Now divide the number of MH by 8-hour days. It would equal 9 duration days. To see how close the estimate is, one person (skilled) lays 56.25 CMU/day and you calculated 9 days. Then multiply 56.25 times 9 which equals 506 block. There is a two block difference, which is not much in this example, but it could be if you were estimating thousands of square feet of CMU.
- To Table of Contents -
Brick masonry is masonry construction in which units of baked clay or shale of uniform size, small enough to be placed with one hand, are laid in courses with mortar joints to form walls. Bricks are kiln-baked from various clay and shale mixtures. The chemical and physical characteristics of the ingredients vary considerably. These characteristics and the kiln temperatures combine to produce brick in a variety of colors and hardnesses. In some regions, individual pits yield clay or shale which, when ground and moistened, can be formed and baked into durable brick. In other regions, clay or shale from several pits must be mixed.
A finished brick structure contains face brick, brick placed on the exposed face of the structure, and backup brick, brick placed behind the face brick. The face brick is often of higher quality than the backup brick; however, the entire wall may be built of common brick. Common brick is made from pit-run clay with no attempt at color control and no special surface treatment like glazing or enameling. Most common brick is red.
Although any surface brick is a face brick as distinguished from a backup brick, the term face brick is also used to distinguish high quality brick from brick of common quality or less. Applying this criterion, face brick is more uniform in color than common brick and may be obtained in a variety of colors as well. It may be specifically finished on the surface, and, in any case, it has a better surface appearance than common brick. It may also be more durable as a result of the use of select clay and other materials or as a result of special manufacturing methods.
Backup brick may consist of brick that is inferior in quality even to common brick. Brick that has been underburned or overburned, or brick made with inferior clay or by inferior methods, is often used for backup brick.
Still another type of classification divides brick into grades according to the probable climatic conditions to which they are to be exposed. These are as follows:
Table 5 Quantities of Material Required for Brick Walls.
Quantities of brick include the thickness of the mortar joint with no allowance for waste.
The following example shows the square foot method of estimating the number of bricks for a 4 inch wall measuring 8 feet high and 14 feet long. Specifications call for the use of U.S. standard brick with a 1/2 inch mortar joint. The brick face with its mortar joints measures 2 3/4 inches high by 8 1/2 inches long. The correct steps to follow are these:
2 3/4" x 8 1/2 = 2.75 x 8.50 = 23.38 square inches per brick
Here the number of bricks is 6.16 per square foot for a 4 inch wall.
8 feet x 14 feet = 112 square feet
112 x 6.16 or 690 bricks plus 10% waste which equals 760 bricks
If there are windows, doors, and other openings on the wall, you subtract the area of these openings from the overall area of the wall to get the net area. Then in Step 4, you multiply the number of bricks per square foot by the net area.
In finding how much mortar is required to build this wall, divide the number of bricks by 1,000, then multiply the result by the factor given in Table 5 and allow 20% for waste.
760 ๗ 1000 = .76
.76 x 11.7 (cubic feet of mortar per 1000 bricks) = 8.90 cf of mortar
8.90 x 20% (waste) = 10.68 or 10.7 cubic feet of mortar
Therefore, to construct this wall with U.S. standard brick with a 1/2 inch mortar joint, you require 760 bricks and 10.7 cubic feet of mortar.
- To Table of Contents -
Hollow masonry units made of burned clay or shale are variously called structural tiles, hollow tiles, structural clay tiles, structural clay hollow tiles, and structural clay hollow building tiles, but they are most commonly called building tile. In building tile manufacture, plastic clay is pushed through a die, and the shape that emerges is cut off into units. The units are then burned much as bricks are burned.
The apertures in a building tile, which correspond to the cores in a brick or a concrete block, are called cells. The solid sides of a tile are called the shell, and the perforated material enclosed by the shell is called the web. A tile that is laid on one of its shell faces is called a side construction tile; several sizes and shapes are shown in Figure 4- 1.
Figure 4-1 Standard shapes of side construction building tiles.
A tile that is laid on one of its web faces is called an end construction tile; several sizes and shapes are shown in Figure 4-2.
Figure 4-2 Standard shapes of end construction building tiles.
Special shapes for use at corners and openings or for use as closures are also available.
The compressive strength of the individual tile depends on the materials used and the method of manufacture in addition to the thickness of the shells and webs. A minimum compressive strength of tile masonry of 300 pounds per square inch based on the gross section may be expected. The tensile strength of structural clay tile masonry is small. In most cases, it is less than 10 percent of the compressive strength.
The abrasion resistance of clay tile depends primarily upon its compressive strength. The stronger the tile, the greater is its resistance to wearing. The abrasion resistance decreases as the amount of water absorbed increases.
Structural clay facing tile has excellent resistance to weathering. Freezing and thawing action produces almost no deterioration. Tile that will absorb no more than 16 percent of its weight of water has never given unsatisfactory performance in resisting the effect of freezing and thawing action. Only Portland cement lime mortar or mortar prepared from masonry cement should be used if the masonry is exposed to the weather.
Walls containing structural clay tile have better heat-insulating qualities than walls composed of solid units because of the dead air space that exists in tile walls. The resistance to sound penetration of this type of masonry compares favorably with that of the resistance of solid masonry walls, but it is somewhat less.
The fire resistance of tile walls is considerably less than the fire resistance of solid masonry walls. It can be improved by the application of a coat of plaster to the surface of the wall. Partition walls of structural clay tile 6 inches thick will resist a fire for 1 hour provided the fire produces a temperature of not more than 1700ฐF.
The solid material in structural clay tile weighs about 125 pounds per cubic foot. Since the tile contains hollow cells of various sizes, the weight of the tile varies depending upon the manufacturer and type. A 6 inch tile wall weighs approximately 30 pounds per square foot, while a 12 inch tile wall weighs approximately 45 pounds per square foot.
Structural clay tile may be used for the exterior walls of either the load bearing or non-load bearing type. It is suitable for both below grade and above grade construction.
Structural load bearing tile is made from 4 to 12 inch thicknesses with various face dimensions. The use of these tiles is restricted by building codes and specifications, so consult the project specification.
Non-load bearing partition walls from the 4 to 12 inch thicknesses are frequently made of structural clay tile. These walls are easily built, light in weight, and have good heat and sound insulating properties.
Figure 4-3 shows the use of structural clay tile as a back unit for a brick wall.
Figure 4-3 Structural tile used as a backing for bricks.
Figure 4-4 shows the use of 8 x 5 x 12 inch tile in wall construction. Exposure of the open end of the tile can be avoided by the application of a thin tile, called a soap, at the corner.
Figure 4-4 Eight inch structural clay tile wall.
- To Table of Contents -
Stone masonry units consist of natural stone. In rubble stone masonry, the stones are left in their natural state without any kind of shaping. In ashlar masonry, the faces of stones that are to be placed in surface positions are squared so that the surfaces of the finished structure will be more or less continuous plane surfaces. Both rubble and ashlar work may be either random or coursed.
Random rubble is the crudest of all types of stonework. Little attention is paid to laying the stones in courses, as shown in Figure 4-5. Ashlar coursed masonry is at the opposite end of the stone masonry spectrum, having structured courses and squared stone faces, as shown in Figure 4-6.
Figure 4-5 Random rubble stone masonry.
Figure 4-6 Coursed ashlar stone masonry.
Each layer must contain bonding stones that extend through the wall, as shown in Figure 4-7. This produces a wall that is well tied together. The bed joints should be horizontal for stability, but the builds or head joints may run in any direction.
Figure 4-7 Layers of bond in random stone masonry.
Coursed rubble consists of roughly squared stones assembled in such a manner as to produce approximately continuous horizontal bed joints, as shown in Figure 4-8.
Figure 4-8 Coursed rubble masonry.
The stone used in stone masonry should be strong and durable. Durability and strength depend upon the chemical composition and physical structure of the stone. Some of the more commonly found stones that are suitable are limestone, sandstone, granite, and slate. Unsquared stones obtained from nearby ledges or quarries, or even fieldstones may be used. The size of the stone should be such that two people can easily handle it. Using a variety of sizes avoids using large quantities of mortar.
The mortar used in stone masonry may be composed of Portland cement and sand in the proportions of 1 part cement to 3 parts sand by volume. Such mortar shrinks excessively and does not work well with the trowel. A better mortar to use is Portland cement lime mortar. Mortar made with ordinary Portland cement will stain most types of stone. If staining must be prevented, non-staining white Portland cement should be used in making the mortar. Lime does not usually stain the stone.
- To Table of Contents -
You have learned how to estimate material and labor for concrete masonry units according to P-405. You also learned to estimate materials for brick construction, and are now able to identify the components, requirements, and construction techniques for laying structural clay tile and stone masonry.
- To Table of Contents -
1. Masonry is a construction method made up of prefabricated masonry units laid together in various ways and joined together by what type of mix?
2. What type of planning ensures concrete masonry walls are laid out so maximum use is made of full-length and half-length masonry units?
3. Which one of the two methods used by Builders to estimate CMUs is the quickest, but NOT the most accurate?
4. Building specifications call for a 1:2 mortar mix. Using rule 38, how many sacks of cement are required to make up a 2 cubic yard mix?
5. It takes one person (skilled labor) a total of how many man hours to lay 1,000 square feet of 8" x 8" x 16" concrete block?
Use this table to answer Question 6:
6. How many man hours are required to construct 1,500 square feet of wall area using 8 inch by 8 inch by 16 inch CMU?
7. What type of brick is designed to withstand exposure to below freezing temperatures in a moist climate?
8. Tile masonry has a compressive strength of how many pounds per square inch?
9. Partition walls of clay tile 6 inches thick can resist for 1 hour a fire that produces heat not exceeding what temperature, in degrees Fahrenheit?
10. The use of structural load bearing tiles is restricted by building codes.
11. Stone masonry units are classified into what two types?
12. What type of rubble stonework is the crudest of all types?
13. What type of rubble consists of roughly squared stones assembled in such a manner as to produce approximately horizontal bed joints?
14. The mortar used in stone masonry should be composed of what ratio of cement to sand?
- To Table of Contents -
Copyright ฉ David L.
All Rights Reserved