AST 406 Concrete Beam

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AST 406 Concrete Beam

  1. 1. The strongest concrete beams!
  2. 2.  Cement is an ingredient of concrete.  Cement : Concrete = Flour : Cake  Concrete is basically a mixture of aggregates and paste.  Aggregates are sand and gravel or crushed stone.  The paste is water and portland cement.
  3. 3.  Cement comprises from 10 to 15 percent of the concrete mix, by volume.  Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass.  This hardening process continues for years meaning that concrete gets stronger as it gets older.
  4. 4.  Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength, hardness and durability.  Curing requires hydration: the chemical reaction between cement and water.  Hydration occurs only if water is available and if the concrete's temperature stays within a suitable range.  This can last from five to seven days.
  5. 5.  Temperature extremes make it difficult to properly cure concrete.  On hot days, too much water is lost by evaporation from newly placed concrete.  If the temperature drops too close to freezing, hydration slows to nearly a standstill.  Under these conditions, concrete ceases to gain strength and other desirable properties.  In general, the temperature of new concrete should not be allowed to fall below 10 ⁰C during the curing period.
  6. 6.  Concrete is strong in compression, but weak in tension.  To prevent the concrete from cracking when in tension, it is reinforced with steel rods (REBAR) or steel mesh.  The steel rods or mesh are strong in tension and prevent the concrete from cracking.
  7. 7.  Concrete is tested to ensure that the material that was specified and bought is the same material delivered to the job site.  Slump is a measure of consistency, or relative ability of the concrete to flow.  Air content measures the total air content in a sample of fresh concrete.  Density measures mass/volume.  Compressive strength is tested by measuring the force needed to break the concrete.
  8. 8.  All matter is subject to change either chemically, physically or through nuclear transformations.  Physical changes are changes in which only the state of the substance has changed: solid, liquid, gas. The atoms and molecules do not change – only their appearance.  Chemical changes involve new substances being created and change the nature of the matter. The bonds between atoms are rearranged and different molecules form. The characteristic properties are altered.  Nuclear transformations affect the nucleus of the atom where it splits to form new elements. The particles are rearranged.
  9. 9.  In these reactions, the chemical bonds between reactants are broken and new bonds formed.  New products form and have new characteristic properties.  Signs that a chemical change has occurred:  Gas bubbles form  Heat or light is given off  Change in colour  Precipitate forms  New substance is created  pH change  Change is difficult to reverse
  10. 10.  Antoine Lavoisier (1743-1794) observed that the masses of the products of a chemical reaction were the same as the mass of the reactants.  Mass of the reactants = Mass of the products  E.g. CH4 + 2 O2  CO2 + 2 H2O  16g + 64g 44g + 36 g  80g  80g  The nature of the atoms involved in the reaction are not altered but the bonds between the atoms are changed.
  11. 11.  We start by counting the number of atoms of each element based on the formula.  The reactant side must equal the product side in total # of individual atoms.  The only way we change the number is by using a large number called a coefficient which is placed at the front of the molecule. This number affects every individual atom.  The small # attached to the atoms is called the subscript # and it does not change.  E.g. Mg + O 2  Mg0  Mg - 1 Mg - 1  O - 2 O - 1  2 Mg + O 2  2 Mg0
  12. 12.  Chemical reactions are classified as:  1) Endothermic – absorbing energy; e.g. a cake baking absorbs heat from the oven.  2) Exothermic - releasing energy; e.g. a fire burns and the heat created is enough to continue the reaction.  Exothermic reactions release energy which is then absorbed by the environment often increasing temperatures while endothermic reactions absorb energy from the environment causing temperatures to decrease.  The amount of energy needed to break or form chemical bonds varies based upon its type.
  13. 13.  There are 5 basic ways in which chemical changes occur:  1. Synthesis is a reaction where 2 or more reactants combine to form a new product ( A + B  AB)  2. Decomposition is a reaction where a compound separates into two or more compounds or elements. (AB  A + B)  3. Precipitation  This occurs when two solutions are combined and an insoluble substance forms which is called a precipitate.  4. Acid Base Neutralization occurs when an acid and a base react to produce salt and water. (Acid +Base  Salt + Water)  5. Oxidation  Many substances oxidize when they react with the oxygen in the air or a substance with similar properties to oxygen.  Combustion is a form of oxidation that releases a large amount of energy.
  14. 14.  3 types:  Rapid: spectacular and in a short time, it releases a lot of energy in the form of heat or light;  E.g. candle, log fire  Spontaneous: rapid and without energy from outside source; unpredictable  E.g. dry wood catches fire by itself, hay heats  Slow: occurs over a long period of time and energy is released gradually into environment.  E.g. decomposition, fermentation, cellular respiration, metal corrosion.
  15. 15. Three conditions are needed for combustion to occur: fuel (something to burn); an oxidizing agent (causes fuel to react, e.g. oxygen) the ignition temperature.
  16. 16.  Is a chemical reaction  Is an endothermic reaction  Plants (producers) use the sun’s energy to make glucose and oxygen from carbon dioxide and water.  It releases oxygen and is needed for maintaining oxygen levels in the air.  Glucose is a source of energy.
  17. 17.  Is a chemical reaction where glucose and oxygen generate energy which also produces carbon dioxide and water.  Is a type of slow combustion  Exothermic reaction  Takes place in cells of living things.  Energy released disperses in the tissues in the form of heat; human body temperature is ~37⁰C  The energy allows cells to carry out tasks essential in sustaining life.
  18. 18.  Work on pages 1&2, 3 & 4 of the handout.  Concrete Temperature Lab  Concrete Boat Float Lab  Concrete Beam Lab  Page 132, Q 1-7

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