Ag.Mechanics Pd. 2,4,5,6

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

 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

 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

 Compounds:  Lime (calcium oxide)  Silica (silicon dioxide)  Alumina (aluminum oxide)

 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.

 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.

 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

 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.

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

 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.

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

 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)

 Moist curing improves the strength of concrete.  Surface is kept moist 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.

 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)

 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.

 Trowel

 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.

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

 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.

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

 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?

 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.

 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.