CONCRETE BASICSAg.Mechanics

Pd. 2,4,5,6

Bell Ringer

Name what the two aggregates are in the cement mixture.

What 3 things make up the cement mixture.

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

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

Making of Portland Cement

Compounds: Lime (calcium oxide)

Silica (silicon dioxide)

Alumina (aluminum oxide)

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.

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.

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

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.

Water

Water must be clean and free of dirt and oil.

Good rule is to use water you would drink.

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.

Hydration

The chemical reaction that occurs when cement is mixed with water.

What is the difference between hydration and dehydration?

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)

Moist - Curing

Moist curing improves the strength of concrete.

Surface is kept moist for @ 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.

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)

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.

Tools

Trowel

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.

Troubleshooting Concrete

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

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.

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.

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?

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.

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.