Ce 209 hw#5 - homework assignment PDF

Preview

Summary

homework assignment...

Description

Review Questions of “Fundamentals of Building Construction”:

Chapter 13: 1. What is the difference between cement and concrete? Cement is a rocklike material produced by mixing coarse and fine aggregates, Portland cement, and water and allowing the mixture to harden. Coarse aggregate is normally gravel or crushed stone, and fine aggregate is sand. Portland cement, referred to simply as cement, is a fine gray powder. During the hardening, or curing, of concrete, the cement combines chemically with water to form strong crystals that bind the aggregates together, a process called hydration.

2. List the conditions that must be met to make a satisfactory concrete mix. A satisfactory concrete mix is made of fresh cement; clean, well-graded aggregates; and clean water. Admixtures are sometimes added to the mix to regulate its properties or the properties of the finished concrete. The concrete must be thoroughly blended in the correct proportions, with special attention given to avoiding excessive water content. To avoid segregation of its constituents, it must be handled without moving or dropping it excessive distances and compacted in the forms without excessive agitation.

3. List the precautions that should be taken to cure concrete properly. How do these change in very hot, very windy, and very cold weather? It’s essential that concrete be kept moist until its required strength is achieved. The curing reaction takes place over a very long period of time, but concrete is normally considered fully cured after 28 days. In hot weather, you may have to cool the ingredients in the concrete, such as adding ice in the place of some of the mixing water to prevent premature curing during the placement. In heavy winds, concrete slabs may need to be protected from wind during the pouring and finishing. In cold weather, to achieve full strength, it’s important that concrete be protected from low temperatures or freezing until it’s adequately cured.

4. What problems are likely to occur if concrete has too low a slump? Too high a slump? How can the slump be increased without increasing the water content of the concrete mixture? If the slump is too low, not enough water, this creates weakness in cured concrete. If it is too high, too much water, the concrete becomes hard to work with. Without increasing the water content, add air entraining or water reducing admixtures to increase workability.

5. Explain how steel reinforcing bars work in concrete.

Steel reinforcing bars are placed in the regions of reinforced concrete components where tensile forces will be experienced. They are placed there to resist these forces and control the cracking of concrete.

6. Explain the role of stirrups in beams. They act to resist the diagonal tension forces that occur near the ends of concrete beams.

7. Explain the role of ties in columns. Column ties are discrete, closely spaced hoops, individually wired in place. They are used mostly to the rectangular arrangements of vertical bars. Column ties prevent the column bars from buckling under compressive loads.

9. Explain the differences between reinforcing and prestressing and the relative advantages and disadvantages of each. Reinforcing is where tensile forces are expected to be to prevent them from happening. Steel can stretch under tension and concrete around it can crack and lose efficiency. Prestressing places the entire concrete slab in compression by stretching the reinforcing bars before the beam is loaded. This is susceptible to creep and steel strands stretch and loses some pre-stress force.

10. Under what circumstances would you use pretensioning, and under what circumstances would you use posttensioning? Pretensioning is used with precast concrete members. Posttensioning is mostly done in place on the building site.

11. Explain the advantages of using higher strength reinforcing bars in concrete that requires very heavy reinforcing. Using higher strength reinforcing bars in concrete that requires very heavy reinforcing gives the concrete strength in needed areas.

Chapter 14:

2. What are control joints and isolation joints? Explain the purpose and typical locations for each in a concrete slab.

Control joints are intentionally weakened sections through a concrete slab. They resist tensile forces caused by concrete drying without disfiguring the slab. Isolation joints completely separate the slab from adjacent elements. They relieve potential stresses. Isolation joints are commonly provided where the edge of a concrete slab abuts adjacent walls or curbs, as well as around elements, such as columns or loadbearing walls, which pass through the slab within its perimeter.

3. List the steps that are followed in forming and pouring a concrete wall.

Forming: 1) concrete wall is made like concrete slab. 2) On top of slab, a groove is made for mechanical connection. 3) steel bars installed vertically in footing. Pouring: 1) Two concrete bars are poured at an interval in such a way that the axis of the future wall will intersect their midpoint at a right angle 2) trench is dug extending through the bars 3) In the trench, reinforcement is provided with a transverse partition 4) The concrete is poured in the trench between the two transverse partitions.

4. Distinguish one-way concrete framing systems from two-way systems. Are steel and wood framing systems one-way or two-way? Is one-way construction more efficient structurally than two-way construction? A one-way slab rests across parallel line of support furnished by walls or beams. The formwork of one-way slab is erected only after pouring of walls and columns. The forms for girder and beams are built along with the forms of slabs as a single piece. Two-way concrete framing systems are more economical. Columns can be spaced in bays. Used for heavily loaded floors. Generally used to accommodate the varying stresses in different zones of the slab with in a uniform thickness of concrete. Furthermore, steel and wood are one-way systems. One-way systems are more efficient.

5. List the common one-way and two-way concrete framing systems and indicate the possibilities and limitations of each. One-way Concrete Framing Systems: solid slab/ribbed slab system

One-way solid slab: Flexible but limited in span One-way concrete joist system: Allows longer spans Skip-joist system: Allows longer spans and uses less concrete than one-way joist system

Two-way Concrete Framing Systems: flat slab/ waffle slab system Flat slab: Useful where loads are very heavy Two-way flat slab: Good for heavy loadings Two-way flat plate: Flexible in column locations Waffle slab: Good for longer spans

6. Why posttension a concrete structure rather than merely reinforce it? Because posttensioning places the entire slab under sufficient compressive stress so that it will never experience tensile stress under any anticipated conditions. External tension/bending loads can be applied to structure many times stronger.

Chapter 15:

1. Under what circumstances might a designer choose a precast concrete framing system over a sitecast system? Under what circumstances might a sitecast system be favored?

Precast concrete framing might be chosen in cases where rapid erection, reduced dead weight, all-weather construction capability, quality control of concrete, or slenderness of structure are important considerations. Sitecast framing might be chosen where two-way action is desired, unusual shapes must be spanned, sculptural forms must be created, or highly customized surface textures poured.

2. Why are precast concrete structural elements usually cured with steam? Steam curing of precast elements develops the full strength of the concrete many times faster than curing at ambient temperature, which allows the precasting plant to reuse its casting beds on a 24-hour pouring cycle. This permits a casting bed to be many times as productive as if ambient-temperature curing were used, leading to lower product costs.

3. Explain several methods of producing hollow-core slabs. Hollow-core slabs may be produced by direct extrusion, by casting the concrete in two layers with dry aggregate introduced as a filler to create the voids (after which it is poured out for reuse), or by using inflatable tubes to form the voids.

6. Explain the construction process for a filigree precast concrete system. What are this system’s unique advantages over other concrete systems? In the filigree, precast concrete system, relatively thin precast elements that are either conventionally reinforced or prestressed are used as the formwork for site casting of beams and slabs. Once the process is complete, composite structural action between the site cast concrete and precast units results in a unified, structurally efficient system. Because the precast units remain in place as part of the finished system, formwork costs are much less in comparison to conventional site cast concrete construction methods....