Concrete Constituent and Mix Design - Essay Example

Natural heavyweight aggregates include materials such as goethite, limonite, barite, illmenite, magnetite, and hematite, with specific gravities (SGs) ranging from about 3.5 up to about 5.3, leading to concretes with unit weights up to about 4100 kg/m3 . For higher unit weight concretes, synthetic materials such as Ferro phosphorous (SG 5.8 to 6.8) or scrap iron and steel punching (SG 7.8) can be used, with resulting concrete unit weights of up to 6100 kg/m3. High-density aggregates are good attenuators of gamma rays and of fast neutrons, hence their use in radiation shielding.

Although heavyweight concretes can be proportioned in much the same way as ordinary concretes, the aggregates tend to be harsh and have a tendency to segregate from the rest of the mix. As a result, both higher than usual cement contents and a higher ratio of fine to coarse aggregates are recommended. Normal Weight Aggregates Aggregates make up about 75% of the volume of concrete, so their properties have a large in?uence on the properties of the concrete (Alexander and Mindess, 2005). Aggregates are granular materials, most commonly natural gravels and sands or crushed stone.

The role of the aggregate is to provide much better dimensional stability and wear resistance; without aggregates, large castings of neat cement paste would essentially self-destruct upon drying. Also, because they are less expensive than Portland cement, aggregates lead to the production of more economical concretes. These aggregates are used to make ordinary concrete having unit weight of 2300 kg/m3. In general, aggregates are much stronger than the cement paste, so their exact mechanical properties are not considered to be of much importance (except for very high-strength concretes).

Similarly, they are also assumed to be completely inert in a cement matrix, although this is not always true, as will be seen in the discussion on the alkali–aggregate reaction. Light Weight Aggregates Light weight aggregates, which can be either natural or synthetic materials, are characterized by a high internal porosity. Ordinary concrete has a unit weight of about 2300 kg/m3, but lightweight concretes with unit weights as low as 120 kg/m3 can be produced, although they are accompanied by a significant decrease in concrete strength.

Natural lightweight aggregates include pumice, scoria, and tuff; however, most light- weight aggregates are synthetically produced. The most common such lightweight aggregates are made from expanded clay, shale, or slate. The raw material is either crushed to the desired size or ground or then pelletized; it is then heated to 1000 to 1200°C. At these temperatures, the material bloats (or puffs up) due to the rapid generation of gas produced by the combustion of the small amounts of organic material that these particles generally contain.

(The process is similar to that of popping popcorn.) Other materials, such as volcanic glass (perlite), calcium silicate glasses (slag), or vermiculite, can similarly be bloated. Lightweight aggregates tend to be angular and irregular in shape and can be quite variable. b) Select one physical property and one mechanical property of aggregate and describe in detail the role and influence of these properties on the overall performance of the concrete. Particle Grading The particle-size distribution in a sample of aggregate, referred to as the grading, is