There are a multitude of factors that affect an individual brick structural strength.

These factors include forces, moments, as well as environmental impact, and material properties.

Torsion- twisting stress

Hardness and resistance to abrasion

Effect of wetting

Temperature

1. Plasticity- property by means of which clay forms a plastic mass when mixed with water and molded into a shape and retain that shape
2. Shrinkage
3. Tensile strength- the quality of the clay which resists the tendency of the clay to rupture
4. Fusibility- property of clay which allows it to become hard when heated

From what we have learned in class and through our own experiments, we have concluded that for structural reasons, soil alone cannot be used to create a brick. Something must be added to stabilize the mixture. There are a few different materials that we have read about which can be used to stabilize a soil, but the two that are most prevalent are Portland cement and fly ash. Although not yet tested and proven by us, sources have suggested that fly ash when mixed with concrete is a more beneficial concrete stabilizer than is Portland cement alone.

Portland cement is an artificial product manufactured from any number of raw materials that contain lime, iron, silica, and aluminum. The exact ingredients vary from readability of material in the area in which it is produced but mainly come from limestone, marble, marl, seashells, clay, and shale. The ingredients are sorted for size, crushed, blended, and heated. Gypsum is added during the cooling process and then the mixture is pulverized to a consistency of flour.

Fly ash is the non-combustible mineral residue produced by burning coal. Coal burning power plants capture the powder from their exhaust scrubbers for later use. It is a recycled material and has a positive effect on the environment. Fly ash is classified as either Class F produced from burning anthracite or bituminous coal or Class C, the result of burning lignite or sub-bituminous coal. Fly ash particles are almost spherical in shape, much more regular and finer than the particles of Portland cement. They are glassy and tend to act like a ball bearing creating a lubrication affect when the concrete is still in its plastic state. This benefits workability and once hardened, creates a better performing concrete.

Fly ash contributes to the workability of concrete. Because it has a lower weight than cement, it can take up more volume. With a larger percentage of ball bearing particles in the mixture, the better lubricated the aggregate becomes and the concrete flows much smoother. Pumping requires less energy and pumping distances can increase. Better flow also means that forms can be filled easier and will have a better finish when removed with fewer air pockets and blemishes.

One of the most beneficial aspects of fly ash is that it creates a stronger concrete than cement. Portland cement gains most of its strength in 28 days and then maintains this level. Fly ash on the other hand will be lower in strength than cement up until 28 days, at which point they are about the same. After this however, fly ash continues to react with lime gaining strength after 28 days becoming substantially higher in strength than concrete after one year.

During the hydration process that occurs as concrete hardens, lime is produced. Too much lime eventually creates efflorescence on the concrete after repeat wetting and drying. Fly ash however, reacts with lime produced by its hydration converting it into calcium silicate hydrate (CSH), the glue of the concrete. This results in a much less permeable concrete and reduces efflorescence. Decreased permeability reduces the ability for water to produce cracking from freezing and thawing and also protects any steel reinforcement from corrosion. The durability of the concrete is thus increased.

Fly ash is however, dependant on the lime created by the hydration process of cement and could never be used instead of cement entirely. It is possible though to replace a great amount of the normal cement used in concrete.

Types of assemblies

• Curtain and panel wall
• Parapet walls
• Copings
• Pilasters and buttresses
• Chases and recesses
• Sills and frames
• Solid walls
• Hollow walls
• Rolok-bak walls
• All-rolok common bond
• All-rolok Flemish bond
• Cavity walls

Factors affecting compressive strength:

• Strength of single unit
• Geometry of unit
• Strength of mortar
• Deformation characteristic of unit and mortar
• Joint thickness
• Suction of units
• Water retentively of mortar
• Brickwork bonding
• Workmanship

Brickwork is bonded into various arrangements in a wall, according to regular patterns, so that it is able to distribute applied loads throughout the length and thickness of the wall. Bonding thus ensures not only lateral and vertical stability but also that the work complies with other requirements of building codes. By systematically arranging the bricks loading will spread evenly through out the wall, which if not will cause point loads that can cause cracking and settling.

There are several bonds that are used for their structural impact more than economy or appearance these bonds are the Quetta (fig. 4), Raking, and Herring-bone. The Quetta bond was developed to increase resistance to seismic pressure; its cavities are filled with steel reinforcing rods cast into the concrete foundation. Raking and Herring-bone bonds both incorporate the idea of alternating the direction of the bricks establishing better bonding between the bricks, usually used for walls more than two bricks thick or more.

Like wise there are bonds that are used for economic value, these include the Dearne’s bond (fig. 5), Rat-trap bond (fig. 6), and Loudon’s hollow wall (fig. 7), just to name a few. Dearne’s bond consists of "alternate rows of headers laid flat on bed, and stretchers laid as brick on edge, giving and economical gain in height, with a continuous cavity between them" (Lynch). This type of bond is recommended for small houses and walls. The Rat-trap bond has the advantage of connected hollows cavities, but it has less strength than a solid wall but is more than strong enough for housing construction. Loudon’s wall is similar to Dearne’s wall; the advantage to all of these bonds is that they require less bricks to construct the wall than if it were a solid wall.

Cavity walls, stretcher type (fig. 4), consist of three main parts first, the outer leaf, which is the exterior part of the wall; second the cavity; and third the inner leaf, which is the interior part of the wall. The inner leaf is usually considered to carry nearly all of the load and the outer leaf almost none of the load, however most engineers believe that if designed correctly, the outer leaf could carry an equal amount of the load. In most construction the floor joist, beams, etc bear on only the inner leaf. In order to gain more structural strength out of the outer wall the wall must be constructed so that the load is carried partly by the outer wall. "Where timber floor joists are to be supported by a cavity wall it is necessary to conform with the following important rules: (1) Under no circumstances should a timber plate be built in the wall to form a bearing for the ends of the joist. Timber, being so very much weaker in resisting compression than the brickwork would cause the wall to be weekend. (2) The ends of the joists should either rest on a properly leveled mortar bed or on a 2" x _" in metal bearing bar"(Frost). The top of the cavity wall can be sealed with one brick, which also allows the roof load to be distributed to both leaves of the wall. Typically in cavity wall construction the sill of the widow rests on the outer leaf, and a wooden board extends across the cavity to seal the wall.

Mortar should have strength compatible with the bricks it binds in resistance to compressive loading it should never be harder than the bricks which will be bedded in the mortar, and should be weaker. This is because if the mortar is stronger than the brick the bricks will crack and they can’t be replaced. In a regular house the strength of the mortar is generally not going to be a factor since it is more than strong enough to withstand the loads.

There exist two successful ways to structurally enhance without changing the bonding these include piers and buttresses. A pier is used to provide vertical support for a wall, and a buttress is used to provide horizontal support. There are certain structures that do provide strength that occur within the wall, these include arches and lintels. Both of these structures are designed to provide space while distributing the load down the wall. Generally used over windows or door openings.

Objective: To design an entire structure of soil cement bricks, it seems critical to figure the approximate total load that the structure will have. Five to seven loads affect a structure. These are dead, live, snow (only in some locations), wind, earthquake (only in some locations), water and earth. While all of these loads can be found and accommodated for, it would be unrealistic to add up all of the individual loads that act upon a structure and design for this amount. It would result in a huge over design and not economical in the least. For example, you would not have 100 people in the house during a snowstorm that accumulated 25 lbs/sq. ft. of snow with 80 mph winds while at the same time an earthquake is occurring. Instead, you design for the worst-case scenario with specific combinations of loading; 100 guests during an earthquake would be acceptable.

Methodology: Using specific calculations and tables, the individual loads can be found or closely estimated.

Equipment: calculator, tables, known equations

Expected results: Using the given equations, each individual load can be approximated as accurately as possible. The maximum values of rational combinations of loads can be analyzed to determine

Potential problems: This is just an estimated assumption. Like all estimations, there will be a certain degree of error. There is no real way to find out if your calculation is correct or not. First, this is a theoretical structure. Second, even if it was built, you cannot weigh certain members to obtain exact data. It would also be impossible to consider every single load because some will surely arise that you did not think of. Another problem arises in obtaining data from a table. Sometimes the table does not list a value for your particular situation or characteristic. You must estimate between two and this will not yield and accurate amount.

Deliverable products: Using the estimated maximum values obtained for each load, rational combinations can be analyzed to determine the amount of loading the structure could possibly have during a given scenario. This data would most certainly aid in design of the structure.

Results: We figured that a brick can be arranged in an assembly in two different manners. First is as a stretcher, fig. 1 and the second as standing on its long skinny face fig. 2. So, for every situation, we found results A for the stretcher and results B for the other. We had to design an arbitrary structure to make all of these calculations for. It is a two story residential structure of our soil cement bricks 18 feet tall measuring 55 feet by 55 feet. Internal columns are placed 18 feet on center. The roof is wood shingles with a 30 degree pitch, fig 3. Each individual brick weighs 13 pounds and its dimensions are 12 inches by 6 inches by 3 inches.

1. Dead load of brick wall itself- in all of these scenarios we are designing for the worst case possible, so for these calculations it is for the brick at the very bottom of the assembly.

1. 55*72= 3960 bricks*13 lbs= 51980 lbs/55= 936 lbs/72 sq.in= 13 psi
2. 55*36= 1980 bricks*13 lbs= 25740 lbs/55= 468 lbs/36 sq.in= 13 psi

1. Dead load of 2^nd floor- the first floor will not rest upon the bricks, thus we figured we did not have to count it as a dead load. The second floor, however, will be supported by the brick wall in some method. From Table 2.1 in Statics, Stregths, and Structures for Architects, the weights for common building materials are given:

495 sq.ft*10 lb/sq.ft (plywood+carpet+joists)= 4950lbs/55 bricks= 90 lbs

90lbs/surface area of brick= psi on bottom brick

1. 90lbs/72 sq.in= 1.25 psi
2. 90lbs/36 sq.in= 2.5 psi

1. Live load of 2^nd floor- same reason as above to only calculate the 2^nd floor. Live load approximation was again given by Statics . . . in Table 2.2 as 40 lbs/sq.ft. We then rounded up slightly to 60lbs/sq.ft for our calculations.

1. 540 lbs/72 sq.in= 7.5 psi
2. 540 lbs/36 sq.in= 15 psi

1. Dead load of roof- the roof is supported directly by the second floor wall which then takes the load down into the ground. From Table 2.1 again are the weights of the various roofing materials:

Equation: 10.39 ft*55ft= 571.58 sq.ft(area of roof that puts force on a wall)

1. 184.98 lbs/72 sq.in= 2.57 psi
2. 184.98 lbs/36 sq.in= 5.14 psi

1. Snow load on roof- because our structure is to be built in Kansas, a snow load must be taken into consideration. Given by Fig 2.1, for the Lawrence area a 20-25 lb/sq.ft load must be accommodated. Snow loads above 20 lbs/sq.ft may be reduced for each degree of pitch over 20 degrees. For our 30 degree pitch we may reduce the load by the result of the given equation.

1. 246.82 lbs/72 sq.in= 3.43 psi
2. 246.82 lbs/36 sq.in= 6.86 psi

1. Wind load- wind load as we’ve found seems pretty negligible to calculate for just one brick, but could none the less be calculated. For Lawrence, Kansas the maximum velocity to design for is given in Fig 2.2 as 80 mph. Using the formula in Statics . . . the basic wind pressure is found and used to find the design wind pressure.

Conclusion:

1. Total sum of loads= 13+1.25+7.5+2.57+3.43= 27.75 psi
2. Total sum of loads= 13+2.5+15+5.15+6.86= 42.51 psi

We were very surprised at how small the total psi turned out to be. When we tested our bricks in class we were dealing with a much larger psi factor and even talking about failing bricks in class brought discussions of several thousand psi. Having a relatively small psi to design for gave a lot more confidence that a brick could be made simply from soil.

Frost, William. Modern Practical Brickwork. London: B.T. Batsford, 1954.

Hendry, Arnold W. Structural Brickwork. London: MacMillan Press, 1981.

Hibbeler, Robert C. Structural Analysis. New Jersey: Prentice, 1999.

http://www.flyash.com/

http://www.geocities.com/CapeCanaveral/Launchpad/2095/flyash.html

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