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Footings and Foundations

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1 Footings and Foundations
Working with various forms of Concrete and Brick Technology Education Mr. Welcome

2 Objectives By the end of this lesson, the students will know:
What concrete is and how to properly mix it What are various forms of Concrete What a Footing is What a foundation is and various forms

3 Concrete What is concrete? Answer:
Contrary to popular belief, concrete and cement are not the same thing; cement is actually just a component of concrete.

4 Concrete Concrete is made up of three basic components: water, aggregate (rock, sand, or gravel) and Portland cement. Cement, usually in powder form, acts as a binding agent when mixed with water and aggregates. This combination, or concrete mix, will be poured and harden into the durable material with which we are all familiar.

5 Concrete There are three basic ingredients in the concrete mix:
Portland Cement Water Aggregates (rock and sand) Portland Cement - The cement and water form a paste that coats the aggregate and sand in the mix. The paste hardens and binds the aggregates and sand together.

6 Concrete Water- Water is needed to chemically react with the cement (hydration) and to provide workability with the concrete. The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower the w/c ratio, the stronger the concrete. (higher strength, less permeability)

7 Concrete Aggregates- Sand is the fine aggregate. Gravel or crushed stone is the coarse aggregate in most mixes.

8 Desired Properties of Concrete
1. The concrete mix is workable. It can be placed and consolidated properly by yourself or your workmen. 2. Desired qualities of the hardened concrete are met: for example, resistance to freezing and thawing and deicing chemicals, water tightness (low permeability) , wear resistance, and strength. Know what you are trying to achieve with the concrete.

9 Desired Properties of Concrete
Economy. Since the quality depends mainly on the water to cement ratio, the water requirement should be minimized to reduce the cement requirement (and thus reduce the cost). Take these steps to reduce the water and cement requirements: use the stiffest mix possible use the largest size aggregate practical for the job. Use the optimum ratio of fine to coarse aggregate.

10 Footings In addition to providing a level platform for forms or masonry, footings spread out the weight of the house so the soil can carry the load. The load spreads out within the footing itself at about a 45-degree angle, and then spreads out in the soil at a steeper angle, more like 60-degrees from the horizontal.

11 Footings As the load under a footing spreads out, pressure on the soil diminishes. Soil directly under the footing takes the greatest load, and therefore should be thoroughly compacted

12 Footings Footings are usually made from concrete. They are classified as plain, reinforced, continuous, stepped, and isolated.

13 Footings Plain Footings carry light loads and are not reinforced with steel Steel is embedded in reinforced footings to make them stronger. Usually reinforcement is placed over weak spots in the soil such as where there have been excavations for sewer, gas, or other connections

14 Footings Continuous Footings are used to support a foundation wall or several columns in a row. Stepped Footings come in two forms. One way widens the base to provide added support. The other changes the levels to support a sloping grade

15 Footings Isolated Footings are not part of the foundation. They receive the loads of free standing columns or piers

16 Footings There are two rules of thumb when dealing with a footing:
The width of the footing should be twice the thickness of the wall The depth of the footing should be equal to the thickness of the wall or a minimum of 6”.

17 Footings A Ratt and Matt Foundation are concrete reinforced with enough steel to make them a single foundation/footing. A Matt foundation has the foundation slab on top of the footing walls. A Ratt foundation has the slab on the bottom with the walls coming up the sides

18 Footings Because the load spreads out, the pressure on the soil is greatest right beneath the footing. By the time we get down below the footing a distance equal to the footings width, the unit soil pressure has dropped by about half. Go down the same distance again, and the pressure has dropped by two-thirds. So it's the soil right under the footing that is the most critical and also, typically, the most abused.

19 Footings When we excavate for the footings, the teeth on the bucket stir up the soil and mix air into it, decreasing its density. Also, soil from the embankment may fall into the trench. Soil that loose has much less bearing capacity than the original soil. That's why it is so important to compact the trench bottom (use a vibrating plate compactor for sand or gravel soils, and a jumping jack compactor for silt or clay). If you don't compact that soil, you could get 1/2 inch of settlement in just the first 6 inches of soil which could shift your house.

20 Footings If you dig too deep and replace the soil to recover the grade, you are adding back soil that has expanded by as much as 50%. Under load, it will reconsolidate and cause settling. So when you replace material in the trench, compact it thoroughly, or else use large gravel. One-inch-and-a-half or larger gravel is virtually self-compacting as you place it. Under the weight of a wood house, it won't settle to any significant degree.

21 There are 3 types of Foundations:
T-Shaped A traditional foundation method to support a structure in an area where the ground freezes. A footing is placed below the frost line and then the walls are added on top. The footing is wider than the wall, providing extra support at the base of the foundation. A T-shaped foundation is placed and allowed to cure; second, the walls are constructed; and finally, the slab is poured between the walls.

22 Foundations

23 Foundations In summary:
T-shaped foundations are used in areas where the ground freezes. First, the footing is placed. Second, the walls are constructed and poured. Lastly, the slab is placed.

24 Foundations Slab-on-grade foundation
As the name suggests, a slab is a single layer of concrete, several inches thick. The slab is poured thicker at the edges, to form an integral footing; reinforcing rods strengthen the thickened edge. The slab normally rests on a bed of crushed gravel to improve drainage.

25 Foundations Casting a wire mesh in the concrete reduces the chance of cracking. A slab on grade is suitable in areas where the ground doesn't freeze, but it can also be adapted with insulation to prevent it from being affected by the frost heaves.

26 Foundations

27 Foundations In summary:
Slab on grade used in areas where ground does not freeze. The edges of the slab-on-grade are thicker than the interior of the slab. The slab-on-grade is monolithic (poured all at one time).

28 Foundations Frost Protected
This method only works with a heated structure. It relies on the use of two sheets of rigid, polystyrene insulation—one on the outside of the foundation wall and the other laid flat on a bed of gravel at the base of the wall—to prevent freezing, which is a problem with slab-on grade foundations in areas with frost. The insulation holds heat from the structure in the ground under the footings and prevents heat loss from the edge of the slab. This heat keeps the ground temperature around the footings above freezing.

29 Foundations

30 Foundations In summary: Only works with a heated structure.
Has the benefits of a the slab-on-grade method (concrete poured monolithically) in areas subject to frost. Concrete is poured in one operation, versus 3 pours required for T-shaped foundations

31 How Foundations Start The following slides will brief you on foundation prep.

32 Foundations Subgrade is ground on which concrete is poured. Forms should be in place and level. Subgrades should be smooth and moist. Moistening the subgrade prevents rapid loss of water from the concrete when pavements, floors, and similar flatwork are being placed.

33 Foundations Foundation walls are load transferring elements that must:
Support the weight of the building above Resist pressures from the ground Provide anchorage for the superstructure of the building Be durable and resist moisture penetration

34 Lewis S. Mills High School Mission Statement
The mission of Lewis S. Mills High School is to foster academic excellence, personal growth, and respect for diversity. Within a safe learning environment, we share the collective responsibility with students, parents, and the community to cultivate the skills, knowledge and attitudes that help our students to realize their full potential as learners and responsible citizens, and to experience success beyond high school. Student Performance Expectations: 1. The LSM graduate comprehends written, visual, and auditory texts and communicates with clarity in a variety of modes. 2. The LSM graduate effectively employs critical thinking in the problem solving process.

35 3. The LSM graduate demonstrates effective use of information and technology to enhance learning.
4. The LSM graduate demonstrates knowledge and skills to improve personal wellness. 5. The LSM graduate acquires and applies knowledge within and across the curriculum to develop a global perspective. 6. The LSM graduate is a responsible member of the community demonstrating respect for self and others. 7. The LSM graduate demonstrates ethical behavior and assumes responsibility for his/her actions. 8. The LSM graduate is aware of the importance of the arts in expressing the human experience.


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