Concrete

Concrete: Different types of Concrete are Categorized according to their method of Installation. Ready or Pre Mixed Concrete is batched and Mixed at a central Plant before it is delivered to a Site. Because this type of Concrete is sometimes transported in an agitator truck, it is also known as Transit Mixed Concrete. Shrink Mixed Concrete is partially Mixed at the central Plant, and its Mixing is then completed en route to the Site.

Raw Materials: Structural Concrete normally contains one part Cement to two parts fine Mineral aggregate to four parts coarse Mineral Aggregate, though these Proportions are often varied to achieve the Strength and Flexibility required in a particular setting.

Concrete contains: a wide range of chemicals that imbue it with the characteristics desired for specific applications. Portland Cement, the kind most often used in Concrete, is made from a combination of a Calcareous Material usually Limestone and of Silica and Alumina found as Clay or Shale. It can also contain iron Oxide and Magnesia.

Aggregates classified according to the type of rock they consist of:
Basalt, Flint, and Granite, Among others. Another type of aggregate is Pozzolana, a Siliceous and Aluminous Material often derived from Volcanic ash. Reacting Chemically with Limestone and Moisture, it forms the Calcium Silicate Hydrates that are the basis of Cement. Pozzolana is commonly added to Portland Cement Paste to Enhance its Densification.

Mixing: One type of Volcanic Mineral, an Aluminum Silicate, has been combined with Siliceous Minerals to form a CompoSite that reduces weight and improves Bonding between Concrete and Steel Surfaces. Its applications have included precast Concrete Shapes and Asphalt, Concrete Pavement for highways.

Fly ash: a Coal burning Power Plant byproduct that contains an AluminoSilicate and small amounts of Lime, is also being tested as a Possible Pozzolanic Material for Cement. Combining fly ash with Lime (CaO) in a Hydrothermal Process also Produces Cement.

The Manufacturing Process: The Manufacture of Concrete is fairly simple. First, the Cement is prepared. Other Ingredients aggregates, Admixtures, any necessary Fibers, and Water are Mixed Together with the Cement to form Concrete. The Concrete is then shipped to the work Site and Placed, Compacted, and Cured.

Preparing Portland Cement: The Limestone, Silica, and Alumina that make up Portland Cement are Dry Ground into a very fine Powder, Mixed Together in Predetermined Proportions, Preheated, and Calcined. Next the Material is Burned in a Large Rotary Kiln at 2,550 degrees Fahrenheit. At this temperature, the Material Partially fuses into a substance known as Clinker. A modern Kiln can produce as much as 6,200 tons of Clinker a day.

Mixing: The Cement is then Mixed with the other Ingredients, Aggregates, Admixtures, Fibers, and Water. Aggregates are Pre blended or added at the Ready mix Concrete Plant under normal operating conditions. The mixing operation uses Rotation or stirring to coat the surface of the aggregate with Cement Paste and to blend the other Ingredients uniformly. A variety of batch or Continuous Mixers are used.

Fibers: If desired, can be added by a variety of Methods including direct Spraying, Premixing, Impregnating, or hand laying up. Silica fume is often used as a Dispersing or Densifying Agent.

Transport to work Site: Many Methods of Transporting Concrete, including Wheelbarrows, Buckets, Belt Conveyors, The first step in making Concrete is to prepare the Cement. One type of Cement, Pordand Cement, is considered superior to Natural Cement because it is Stronger, more Durable, and of a more Consistent Quality. Raw Materials are Crushed and Ground into a fine Powder and Mixed Together.

Calcining: The Material undergoes two heating steps Calcining and Burning. In Calcining, the Materials are Heated to a high Temperature but do not fuse Together. In Burning, however, the Materials Partially Fuse Together, Forming a Substance known as "Clinker". The Clinker is then Ground in a Ball Mill a Rotating Steel Drum Filled with Steel Balls that Pulverize the Material.

Special trucks and pumping: Pumping Transports large Quantities of Concrete over large distances through Pipelines using a system consisting of a Hopper, a Pump, and the Pipes. Pumps come in several types the Horizontal Piston Pump with Semi Rotary Valves and small Portable Pumps called Squeeze Pumps. A Vacuum Provides a continuous flow of Concrete, with two Rotating Rollers Squeezing a Flexible Pipe to move the Concrete into the Delivery Pipe.

Placing and Compacting: Once at the Site, the Concrete must be placed and compacted. These two operations are performed almost simultaneously. Placing must be done so that Segregation of the Various Ingredients is avoided and full compaction with all air bubbles eliminated can be achieved.

Whether chutes or buggies are used: Position is important in achieving these goals. The rates of placing and of compaction should be equal; the latter is usually accomplished using internal or external vibrators. An internal Vibrator uses a Poker housing a Motor driven Shaft. When the Poker is inserted into the Concrete, Controlled Vibration occurs to Compact the Concrete.

Curing: Once it is Placed and Compacted, the Concrete must Cured before it is Finished to make Sure that it does not Dry too Quickly. Concrete Strength is influenced by its Moisture level during the Hardening process as the Cement Solidifies, the Concrete shrinks. If Site Constraints Prevent the Concrete from Contracting, Tensile stresses will develop, weakening the Concrete. To minimize this problem, Concrete must be kept Damp during the several days it requires to set and harden.

Quality Control: At the Cement Production Factory, the Proportions of the various Raw Materials that go into Cement must be checked to achieve a consistent Kiln Feed, and samples of the Mix are Frequently examined using X-ray Fluorescence analysis.

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