CE 413 Design of Concrete Structures Fall 2015

Homework Assignment 1 Due Wednesday, September 2nd

To help answer the questions and to prepare for Wednesday’s class read Chapter 1 (pp. 1 – 9), parts of Chapter 9 (pp. 153-157; pp. 169-172) and part of Chapter 12 (pp. 231 – 238) all of which are from the 15th edition of the Design and Control of Concrete Admixtures, Portland Concrete Association. These chapters are available on the ‘Sites’ course website to view/download. Also read part of Chapter 1 in the McCormac & Brown textbook (pp. 7-13; pp. 22-26) and the attached “How Cement is Made” from the cement.org website as part of this assignment.

Perform the following calculations:

Assume that unreinforced concrete weighs 145 lb/ft3  How much will one cubic yard of unreinforced concrete weigh? ______ What area will one cubic yard of concrete cover if poured 4” deep? ______ How much concrete do you need to mix to fill eight concrete test cylinders (each cylinder has a height of 8 inches and a diameter of four inches) in cubic yards? ______or in cubic feet? ______

We have a 4” diameter by 8” tall concrete cylinder test sample that we want to test to determine its maximum axial compressive strength. An axial load, P, is applied to the cylinder and the load is increased until the cylinder concrete crushes when the compressive stress created in the cylinder due to the axial load P reaches the concrete’s maximum compressive strength, f’c.  If the concrete’s f’c is 3000 psi, what load P is required to crush the concrete? ______ If the concrete’s f’c is 8000 psi, what load P is required to crush the concrete? ______

Assume that reinforced concrete weighs 150 lb/ft3 A building has overall floor dimensions of 60 feet by 80 feet. The floor slab that covers the entire floor area is 6” thick.  What is the self-weight of the floor slab (dead load) in lbs? ______in kips? ______ What is the area dead load of the floor slab in psf? ______psf (This is the area load used to calculate beam or column loads based on tributary area)

Answer the following questions based on the readings and other information:  What are the raw materials of cement?

 What are the raw materials of concrete?

 Concrete hardens through the process of drying. True or False? ______ Give three reasons why different types of Portland cement are produced.

You can either use a maximum coarse aggregate size (1”) or a maximum coarse aggregate size (1/2”).  Which coarse aggregate would require less cement paste per cubic yard of mixed concrete?

Your boss want to use 3” diameter aggregate in a concrete mix for a building’s beams and floor slabs.  What would concern you about using the 3” aggregate?

TURN OVER! CE 413 Design of Concrete Structures Fall 2015

 How many days does it take for concrete to reach its design strength? ______ Can this time be reduced?  What is permeability?

 When is a low permeability desired for concrete structures? When is a high permeability desired?

 Briefly describe what air-entraining a concrete mix means.

 Identify one construction situation in which it would be okay not to use air-entrained concrete and then identify one construction situation in which you would insist that air-entrained concrete be used.

 Sketch a graph that clearly shows the relationship between w/c ratio and concrete compressive strength for non air-entrained concrete and for air-entrained concrete. Summarize the effect that the w/c ratio has on concrete strength and also the effect that air entrainment has on concrete strength.

 List four benefits typically provided by a concrete with a low water-cement ratio (w/c).

The modulus of elasticity for A36, A992, and many other grades of steel is always E = 29,000 ksi. For concrete, the modulus of elasticity, Ec, depends on the compressive strength of the concrete, f’c.  How can you calculate the concrete modulus of elasticity, Ec?

 What is the Ec for a concrete with f’c = 4000 psi?

 What is the nominal diameter of a #4 reinforcing bar (rebar)? ______ What is the cross-sectional area of a #4 rebar? ______ What is the nominal diameter of a #9 rebar? ______ What is the cross-sectional area of a #9 rebar? ______

CE 413 Design of Concrete Structures Fall 2015

 What are the largest and smallest bar sizes available? ______

How Cement is Made http://www.cement.org/cement-concrete-basics/how-cement-is-made

Portland cement is the basic ingredient of concrete. Concrete is formed when portland cement creates a paste with water that binds with sand and rock to harden.

Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients.

Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore. These ingredients, when heated at high temperatures form a rock-like substance that is ground into the fine powder that we commonly think of as cement.

Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove. With this crude method, he laid the foundation for an industry that annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water.

Cement plant laboratories check each step in the manufacture of portland cement by frequent chemical and physical tests. The labs also analyze and test the finished product to ensure that it complies with all industry specifications.

The most common way to manufacture portland cement is through a dry method.

The first step is to quarry the principal raw materials, mainly limestone, clay, and other materials. After quarrying the rock is crushed. This involves several stages. The first crushing reduces the rock to a maximum size of about 6 inches. The rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller. The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed, and fed to a cement kiln.

The cement kiln heats all the ingredients to about 2,700 degrees Fahrenheit in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 12 feet in diameter—large enough to accommodate an automobile and longer in many instances than the height of a 40-story building. The large kilns are mounted with the axis inclined slightly from the horizontal.

The finely ground raw material or the slurry is fed into the higher end. At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil, alternative fuels, or gas under forced draft.

As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance called clinker. Clinker comes out of the kiln as grey balls, about the size of marbles. Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers. The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency.

After the clinker is cooled, cement plants grind it and mix it with small amounts of gypsum and limestone. Cement is so fine that 1 pound of cement contains 150 billion grains. The cement is now ready for transport to ready-mix concrete companies to be used in a variety of construction projects.

Although the dry process is the most modern and popular way to manufacture cement, some kilns in the United States use a wet process. The two processes are essentially alike except in the wet process, the raw materials are ground with water before being fed into the kiln.

Identify two durability issues of concrete and discuss what can be done with the concrete mix to improve the durability of the concrete with respect to each issue.

Read about Fly Ash in Ch. 3 (pp. 57 - 65). Focus on the sections discussing fly ash (not other additives like slag, silica fume, etc). What benefits can be gained for strength, workability, and durability by adding fly ash to a concrete mix?

Read about Admixtures in Ch. 6 (pp. 105 -112). Identify three effects on workability, durability, and/or strength that can be achieved depending on specific admixtures that are added to a concrete mix.

Visit the Concrete Thinker Website supported by the Portland Cement Association (PCA) at http://www.concretethinker.com/ From the information provided on the webiste, summarize two applications of concrete and also two ways in which concrete can be used to promote sustainability.

1. Given a normal weight concrete with a compressive strength of 4000 psi: a. What is the approximate tensile strength? 315-475 psi b. What is the approximate flexural strength? 475-630 psi c. What is the approximate modulus of elasticity? 3,600,000 psi

Cement manufacturers mine materials such as limestone, shale, iron ore, and clay, crushed and screened the rock, and place it in a cement kiln. After being heated to extremely high temperatures, these materials form a small ball called “clinker” that is very finely grounded to produce portland cement.

Lime and silica make up about 85 percent of the ingredients of cement. Other elements include alumina and iron oxide. The rotating kiln that cooks the materials resembles a large horizontal pipe with a diameter of 10 to 15 feet and a length of 300 feet or more. One end is raised slightly. The raw mix is placed in the high end and as the kiln rotates the materials move slowly toward the lower end. Flame jets at the lower end heat all the materials in the kiln to high temperatures that range between 2,700 and 3,000 degrees Fahrenheit. This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite). For each ton of material that goes into the feed end of the kiln, two thirds of a ton comes out the discharge end, called clinker. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce portland cement. Manufacturers often add gypsum and/or limestone during the grinding process. Portland cement is a hydraulic cement which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under water.