Concrete is a composite FOURTH class construction material, composed of cement (commonly Portland cement) and other cementious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate made of gravel or crushed rocks such as limestone, or granite, plus a fine aggregate such as sand), water and chemical admixtures.
Concrete is a composite material composed of various constituent materials: a binder, usually cement, aggregates, and water. Different mixes = different products. Cement + water = cement paste; Cement paste + sand = mortar; Mortar + aggregate = concrete. Each of these constituent materials can vary in its chemical makeup and performance characteristics, depending on where it was quarried, and the manufacturing methods and conditions used to produce it.
Concrete additives have been used since Roman and Egyptian times, when it was discovered that adding volcanic ash to the mix allowed it to set under water. Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened and adding blood made it more frost-resistant.
The chemical composition of water can affect concrete, but more important is the ratio of water-to-cement paste used in the mix. Cement needs only about 25% water for hydration to take place. To improve its workability, the ratio is commonly increased to around 45%. Increasing the ratio of water makes concrete more porous; as this extra water rises to the surface as bleed water and through evaporative drying, it leaves behind capillary voids. These capillary voids weaken concrete and make it more absorbent, increasing the chances of freeze damage and attack from liquid chemicals. It is not uncommon for the water-to-cement ratio to be well over 45%. If concrete starts to harden before it’s placed, water is sometimes added at the job site to maintain work-ability.
Recently the use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a by-product of coal-fired power plants. This use reduces the amount of quarrying and landfill space required as the ash acts as a cement replacement thus reducing the amount of cement required. In modern times, researchers have experimented with the addition of other materials to create concrete with improved properties, such as higher strength or electrical conductivity. Marconite is one example.
Movement of the sub-grade, such as settling or heaving, can crack concrete. This may be caused by changes in soil volume in response to changes in the soil’s moisture content, or it may be caused by subsidence. Subsidence is settling that can have a number of causes. Sub-surface mining, extraction of natural gas, the dissolution of limestone or conditions related to groundwater can all cause soil to settle. An example is when groundwater dissolves the carbonate cement holding sandstone particles together, and then carries away the particles, creating a void in the soil.
Concrete is porous; it can absorb water. When absorbed water freezes, it expands, and if no method is used to accommodate the expansion, it can lead to cracking, or flakes of concrete may break loose and separate from the surface. This is especially true if absorbed moisture freezes before the concrete has aged enough to gain significant strength. Concrete should be kept warm until it has had a chance to harden adequately. The appearance of concrete damaged by premature freezing will vary with environmental conditions, but it often appears as widespread spalling or delamination of the surface layer, since cracking is often parallel to the surface.
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