Concrete Basics

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Concrete Basics

• Ag.Mechanics
• Pd. 2,4,5,6

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Bell Ringer

• Name what the two aggregates are in the cement
  mixture.
• What 3 things make up the cement mixture.

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Portland Cement

• Roman builders obtained natural cement from
  pumice.
• Pumice is a mineral deposit on the slopes of
  volcanoes
• When mixed with water it becomes a hard and
  durable substance

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Portland Cement

• The cement used in modern concrete is called
  Portland cement.
• It is a manufactured substance that is created
  using heat.
• Got its name from being similar in color to
  Portland stone, which is an English limestone

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Making of Portland Cement

• Compounds
• Lime (calcium oxide)
• Silica (silicon dioxide)
• Alumina (aluminum oxide)

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Materials

• Raw materials are crushed and then ground into a
  powder.
• Mixed in various proportions, based on the
  desired characteristics of the end product.
• The mixture is heated in a large kiln to 2,700
  degrees F
• The heating changes the chemical composition of
  the ingredients and they form small lumps called
  clinker.

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Making of cement

• A small amount of gypsum no more than 5 is added
  to the clinker
• The resulting mixture is then pounded into the
  fine powder we call cement.

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5 Basic Types of Portland Cement

• Type 1- (standard) most general construction
  purposes
• Type 2-(modified) most general construction
  purposes
• Type 3- (high strength) used where forms must be
  removed quickly or concrete must be put in
  service quickly
• Type 4-(low heat) used only on very large
  concrete projects, such as dams
• Type 5- (sulfate-resistant) used where concrete
  will be exposed to highly alkaline conditions and
  sulfates

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Specialty Cements

• Self-leveling
• -often poured on floors, like a syrup
• Hydraulic
• - expands when mixed with water and hardens in
  minutes, plugs holes or cracks in foundations
• Anchor
• -fast setting, used to secure railings and
  hardware, higher compressive strength
• Resurfacing
• -fine aggregate allows to spread thin, used to
  repair damaged surfaces.

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Water

• Water must be clean and free of dirt and oil.
• Good rule is to use water you would drink.

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Ratio of Water to Cement

• This is extremely important.
• This determines the strength of the cement.
• The more water added , compressive and tensile
  strength decreases.
• Tensile strength resistance to forces that bend
  and pull.

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Hydration

• The chemical reaction that occurs when cement is
  mixed with water.
• What is the difference between hydration and
  dehydration?

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Bell Ringer

• Explain in your own words how hydration works.
• (ie. think of materials that need water added in
  order to work correctly)
• Explain how dehydration works.
• (ie. Think of material that begin as a liquid and
  dry hard)

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Moist - Curing

• Moist curing improves the strength of concrete.
• Surface is kept moist for _at_ several days after
  placement.
• Also be done by spraying a material that retains
  moisture or with water to slow evaporation.
• Concrete gains most of its strength in the 28-day
  period after it has been placed.
• Concrete gains strength for many years after.

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Admixtures

• Admixture is an ingredient other than cement,
  aggregate, or water that is added to a concrete
  mix to change its physical or chemical
  characteristics.
• (I.E. air-entraining mixtures, retarding
  mixtures, accelerating mixtures)

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Colorants

• Color or pigment is sometimes added.
• 1. Uncolored layer, and a colored layer is added
  over the top.
• 2. Dust colored power over the surface of wet
  concrete.

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Tools

• Trowel

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Troubleshooting Concrete

• Crazing is the appearance of fine cracks that
  appear in irregular patterns over the surface of
  the concrete.
• Plastic shrinkage cracks occur mostly in
  concrete slabs.

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Troubleshooting Concrete

• Efflorescence is a whitish crystalline deposit
  that sometimes appear on the surface of concrete
  or mortar.

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Trouble Shooting Concrete

• Cracks that extend through the concrete can
  significantly reduce its strength and long-term
  durability.
• Chalking is a term that describes the formation
  of loose powder on the surface of hardened
  concrete. Sometimes called dusting.

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Trouble Shooting in Concrete

• Scale is a term that describes widespread
  flaking of a hardened concrete surface. Often a
  result of exposing concrete to freeze/thaw
  extremes.

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Self Check

• 1. If a material contains fine aggregate, cement,
  and water, but not coarse aggregate, what is the
  material called?
• 2. What is clinker?
• 3. How are plastic shrinkage cracks formed?
• 4. What is an admixture?

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Safety when working with Concrete

• Protect your skin and eyes
• Concrete is very alkaline, it can irritate the
  skin and eyes.
• Wear gloves, rubber boots, and eye protection
  with mixing and placing concrete.

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Working with Concrete

• Concrete is measured by the cubic yard. One cubic
  yard contains 27 cubic feet.
• Dry ingredients are mixed together first.
• Water is than poured into the dry ingredients,
  this helps the formation of lumps.

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