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Concrete - Columbia University

Concrete 1. common cement is Type I. Type III cement is used Concrete if more rapid strength development is required. The A composite material that consists essentially of a other types are characterized by either lower heat binding medium, such as a mixture of portland ce- of hydration or better sulfate resistance than that of ment and water, within which are embedded parti- Type I cement. cles or fragments of aggregate, usually a combination Aggregate. The aggregate is a granular material, such of fine and coarse aggregate. as sand, gravel, crushed stone, or iron-blast furnace Concrete is by far the most versatile and most slag.

Concrete 1 Concrete A composite material that consists essentially of a binding medium, such as a mixture of portland ce-ment and water, within which are embedded parti-

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Transcription of Concrete - Columbia University

1 Concrete 1. common cement is Type I. Type III cement is used Concrete if more rapid strength development is required. The A composite material that consists essentially of a other types are characterized by either lower heat binding medium, such as a mixture of portland ce- of hydration or better sulfate resistance than that of ment and water, within which are embedded parti- Type I cement. cles or fragments of aggregate, usually a combination Aggregate. The aggregate is a granular material, such of fine and coarse aggregate. as sand, gravel, crushed stone, or iron-blast furnace Concrete is by far the most versatile and most slag.

2 It is graded by passing it through a set of sieves widely used construction material worldwide. It can with progressively smaller mesh sizes. All material be engineered to satisfy a wide range of performance that passes through sieve #4 [ in. ( mm). specifications, unlike other building materials, such openings] is conventionally referred to as fine ag- as natural stone or steel, which generally have to be gregate or sand, while all material that is retained used as they are. Because the tensile strength of con- on the #4 sieve is referred to as coarse aggregate, crete is much lower than its compressive strength, it gravel, or stone.

3 By carefully grading the material is typically reinforced with steel bars, in which case and selecting an optimal particle size distribution, it is known as reinforced Concrete . See REINFORCED a maximum packing density can be achieved, where Concrete . the smaller particles fill the void spaces between the Materials. A composite material is made up of var- larger particles. Such dense packing minimizes the ious constituents. The properties and characteristics amount of cement paste needed and generally leads of the composite are functions of the constituent to improved mechanical and durability properties of materials' properties as well as the various mix pro- the Concrete .

4 Portions. Before discussing the properties of the The aggregate constitutes typically 75% of the con- composite, it is necessary to discuss those of the crete volume, or more, and therefore its properties individual constituents as well as the effects of largely determine the properties of the Concrete . For the mix proportions and methods of production. See the Concrete to be of good quality, the aggregate has COMPOSITE MATERIAL. to be strong and durable and free of silts, organic mat- Cement. There are many different kinds of cements. ter, oils, and sugars. Otherwise, it should be washed In Concrete , the most commonly used is portland prior to use, because any of these impurities may cement, a hydraulic cement which sets and hardens slow or prevent the cement from hydrating or re- by chemical reaction with water and is capable of duce the bond between the cement paste and the doing so under water.

5 Cement is the glue that aggregate particles. binds the Concrete ingredients together and is instru- Admixtures. While aggregate, cement, and water are mental for the strength of the composite. Although the main ingredients of Concrete , there are a large cements and Concrete have been around for thou- number of mineral and chemical admixtures that may sands of years, modern portland cement was in- be added to the Concrete . The four most common vented in 1824 by Joseph Aspdin of Leeds, England. admixtures will be discussed. The name derives from its resemblance of the natu- 1. Air-entraining agents are chemicals that are ral building stone quarried in portland , England.

6 See added to Concrete to improve its freeze thaw re- CEMENT. sistance. Concrete typically contains a large num- portland cement is made up primarily of four ber of pores of different sizes, which may be par- mineral components (tricalcium silicate, dicalcium tially filled with water. If the Concrete is subjected silicate, tricalcium aluminate, and tetracalcium alu- to freezing temperatures, this water expands when minoferrite), each of which has its own hydration forming ice crystals and can easily fracture the ce- characteristics. By changing the relative proportions ment matrix, causing damage that increases with of these components, cement manufacturers can each freeze thaw cycle.

7 If the air voids created by control the properties of the product. the air-entraining agent are of the right size and av- The primary product of cement hydration is erage spacing, they give the freezing water enough a complex and poorly crystalline calcium-silicate- space to expand, thereby avoiding the damaging in- hydroxide gel (or CSH). A secondary product of hy- ternal stresses. dration is calcium hydroxide, a highly crystalline ma- 2. Water-reducing admixtures, also known as su- terial. A category of siliceous materials known as perplasticizers, are chemicals that lower the viscos- pozzolans have little or no cementitious value, but ity of Concrete in its liquid state, typically by creating in finely divided form and in the presence of mois- electrostatic surface charges on the cement and very ture will react chemically with calcium hydroxide to fine aggregate particles.

8 This causes the particles to form additional CSH. This secondary hydration pro- repel each other, thereby increasing the mix flowa- cess has a generally beneficial effect on the final con- bility, which allows the use of less water in the mix crete properties. Examples of pozzolans are fly ash, design and results in increased strength and durabil- ground granulated blast-furnace slag, and microsilica ity of the Concrete . See FLOW OF SOLIDS. or silica fume. 3. Retarding admixtures delay the setting time, The American Society for Testing and Materials which may be necessary in situations where delays (ASTM) defines five types of cement, specifying for in the placement of Concrete can be expected.

9 Ac- each the mineral composition and chemical and celerators shorten the period needed to initiate ce- physical characteristics such as fineness. The most ment hydration for example, in emergency repair 2 Concrete situations that call for the very rapid development of with steel or other high-strength material is known strength or rigidity. as reinforced Concrete . See MORTAR. 4. Color pigments in powder or liquid form may Production of Concrete . The properties of the end be added to the Concrete mix to produce colored product depend not only on the various constituent Concrete . These are usually used with white port- materials listed above but also on the way they are land cement to attain their full coloring potential.

10 Proportioned and mixed, as well as on the methods See PIGMENT (MATERIAL). of placing and curing the composite. Reinforcing steels. Because of Concrete 's relatively Mix design. It is not possible to predict the strength low tensile strength, it is typically reinforced with and other Concrete properties solely based on the steel bars (Fig. 1). These bars are produced in stan- properties and proportions of the mix components. dard sizes. In the United States, the identification Therefore, mixes are designed on an empirical basis, number of a reinforcing bar refers to the nominal di- often with the help of trial mixes.


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