Concrete is a hard material used in the construction of buildings, roads and other structures that need to be strong and durable. Concrete is created by the combination of a chemically inert mineral aggregate such as sand or gravel and a binder. Chemical additives and water then complete the mixture. As the concrete dries, it becomes harder and develops properties similar to stone.
The proportion that is usually followed in the manufacturing of concrete is one part cement to two fine aggregate to four part coarse mineral aggregate. However, this is not a standard formula in that it can be altered to adapt to the strength needed by the structure. Cement, which acts as a binder, is made from different materials such as limestone, silica, and clay or shale. The aggregates of concrete compose roughly 75 percent of its volume. It functions to improve the formation of the concrete as well as to increase strength.
The components of the cement are ground together in definite proportions to create a fine powder. The material is then preheated and calcined, which is a process that heats it to a high temperature such that impurities are burned away. After which, the material is burned again in a large, rotating kiln at a temperature of 2,550 degrees Fahrenheit. The purpose of this is to partially fuse the material together to form a substance called a clinker. The clinker is then cooled and ground to a fine consistency in a tube or ball mill.
The cement is then combined with the aggregates, fibers and water. Constant stirring motion is used to evenly coat the aggregates with the cement mixture and to blend all the materials completely. Fibers, such as glass or plastic, can be added to make the concrete more suitable for tensile loads. It is during the mixing process that admixtures, or substances that give the concrete additional characteristics, are also added.
After the mixture is transported to the intended site, the concrete must be placed and compacted. These processes happen almost at the same time in order to prevent the separation of the various ingredients as well as to avoid air bubbles. Compaction is usually done using an internal or external vibrator. An internal vibrator is placed inside the concrete to emit vibrations that will compact the material. Vibrating tables may also be used, wherein two shafts rotate in opposite directions to create a vertical vibration.
Once the concrete is in place, it must be cured to prevent it from drying too fast. The durability and strength of the material depends on the moisture levels at the time of its hardening. Take note that the concrete decreases in size as it dries. If it is unable to contract properly, weak points will develop in the concrete. To remedy this, the concrete is kept damp for several days as it hardens.
- Time Magazine; Building Materials: Cementing the Future; Peter Gumbel; December 2008
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