CONCRETE BEAMSThe strongest concrete beams!

Cement/Concrete?What’s the Difference?

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.

Stronger and Harder

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.

We have a cure!

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.

Temperature 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 C ⁰during the curing period.

Concrete Structure

Concrete Structure

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.

Why Test Concrete?

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.

What are changes in matter? 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.

Chemical changes 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

The Law of Conservation of Mass

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.

Balancing chemical equations 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

Endothermic and Exothermic reactions

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.

Types of chemical change 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.

Combustion 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.

The Fire Triangle

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.

Photosynthesis

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.

Cellular respiration 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.

Activity

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