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Concrete

1)      A composition material which consist Portland cement, aggregate, sand, pebbles, shale with water and allowing the mixture to harden by hydration. When the materials mixed together, it will result in a chemical action that will set and harden into rock-like mass. Then, concrete is the oldest manmade building material. The chemical reaction can be shown as below.

PORTLAND CEMENT + H₂O + ROCK = HARDENED CONCRETE + ENERGY (HEAT)

2)      There are three basic ingredients in the concrete mix:

1.      Portland Cement

2.      Water

3.      Aggregates 

i)                    Course aggregates (gravel/rock)

ii)                  Fine aggregates (sand)

Properties of the basic ingredients

Portland cement

- is a mixture of processed limestone, shales, and clays which contain the following compounds: CaO (lime), Al₂O₃ (Alumina),SiO₂(silica) and iron oxides. Properties of the cement will vary depending on the relative amounts of these compounds.

- The cement and water form a paste that coats the aggregate and sand in the mix. The paste hardens and binds the aggregates and sand together.

- Production of cement can be separated into two manufacturing processes. 

Two different processes, "dry" and "wet," are used in the manufacture of Portland cement.

When rock is the principal raw material, the first step after quarrying in both processes is the primary crushing. Mountains of rock are fed through crushers capable of handling pieces as large as an oil drum. The first crushing reduces the rock to a maximum size of about 6 inches. The rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller.

In the wet process, the raw materials, properly proportioned, are then ground with water, thoroughly mixed and fed into the kiln in the form of a "slurry" (containing enough water to make it fluid). In the dry process, raw materials are ground, mixed, and fed to the kiln in a dry state. In other respects, the two processes are essentially alike.

The raw material is heated to about 2,700 degrees F in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 12 feet in diameter large enough to accommodate an automobile and longer in many instances than the height of a 40-story building. Kilns are mounted with the axis inclined slightly from the horizontal. The finely ground raw material or the slurry is fed into the higher end. At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil or gas under forced draft.

As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance with new physical and chemical characteristics. The new substance, called clinker, is formed in pieces about the size of marbles.

Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers. The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency.

- Portland Cement can be differentiated into several types such as Ordinary Portland Cement (OPC), Rapid-hardening Portland Cement (RHPC), Ultra-high early strength Portland cement, Low-heat Portland Cement, Sulphate resisting Portland cement, white Portland cement, Portland blast-furnace cement, Pozzolanic cement.

Water

- Water is needed to chemically react with the cement (hydration) and too provide workability with the concrete. The amount of water in the mix in pounds compared with the amount of cement is called the water/cement ratio. The lower ratio of water over cement, the stronger the concrete will be. This meant that the higher strength, the less permeability of the mixtures.

Aggregates

- Sand is the fine aggregate. Gravel or crushed stone is the coarse aggregate in most mixes. The reasons of using aggregates in concrete are because of aggregates able to reduce cost, reduce the heat output, reduce thermal stress, reduce shrinkage of concrete, and they help to produce satisfactory plastic properties.

-  For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete. Aggregates strongly influence concrete's freshly mixed and hardened properties, mixture proportions, and economy. Consequently, selection of aggregates is an important process. Although some variation in aggregate properties is expected, characteristics that are considered when selecting aggregate include:

• grading
• durability
• particle shape and surface texture
• abrasion and skid resistance
• unit weights and voids
• absorption and surface moisture

Grading refers to the determination of the particle-size distribution for aggregate. Grading limits and maximum aggregate size are specified because grading and size affect the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability of concrete. In general, if the water-cement ratio is chosen correctly, a wide range in grading can be used without a major effect on strength. When gap-graded aggregate are specified, certain particle sizes of aggregate are omitted from the size continuum. Gap-graded aggregate are used to obtain uniform textures in exposed aggregate concrete. Close control of mix proportions is necessary to avoid segregation.