Aggregate

Topics: Concrete, Construction aggregate, Asphalt concrete Pages: 9 (2910 words) Published: July 10, 2013
Discuss the advantages, disadvantages of types of aggregate being used in construction projects. Introduction:
Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and Portland cement. Aggregates are essential ingredient in concrete. For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete. Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinct categories - fine and coarse. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch (9.5-mm) sieve. Coarse aggregates are any particles greater than 0.19 inch (4.75 mm), but generally range between 3/8 and 1.5 inches (9.5 mm to 37.5 mm) in diameter. Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder. Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed. Crushed aggregate is produced by crushing quarry rock, boulders, cobbles, or large-size gravel. Once aggregates have been processed, the aggregates are handled and stored in a way that minimizes segregation and degradation and prevents contamination. Aggregates strongly influence concrete's freshly mixed and hardened properties, mixture proportions, and economy. Consequently, selection of aggregates is an important process. Although some variation in aggregate properties is expected, characteristics that are considered when selecting aggregate include grading, durability, particle shape and surface texture, abrasion and skid resistance, unit weights and voids and absorption and surface moisture.

Grading refers to the determination of the particle-size distribution for aggregate. Grading limits and maximum aggregate size are specified because grading and size affect the amount of aggregate used as well as cement and water requirements, workability, pump ability, and durability of concrete. Shape and Size Matter:

Particle shape and surface texture influence the properties of freshly mixed concrete more than the properties of hardened concrete. Rough-textured, angular, and elongated particles require more water to produce workable concrete than smooth, rounded compact aggregate. Consequently, the cement content must also be increased to maintain the water-cement ratio. Generally, flat and elongated particles are avoided or are limited to about 15 percentage by weight of the total aggregate. Unit-weight measures the volume that graded aggregate and the voids between them will occupy in concrete. The void content between particles affects the amount of cement paste required for the mix. Angular aggregate increase the void content. Larger sizes of well-graded aggregate and improved grading decrease the void content. Absorption and surface moisture of aggregate are measured when selecting aggregate because the internal structure of aggregate is made up of solid material and voids that may or may not contain water. The amount of water in the concrete mixture must be adjusted to include the moisture conditions of the aggregate. Abrasion and skid resistance of an aggregate are essential when the aggregate is to be used in concrete constantly subject to abrasion as in heavy-duty floors or pavements. Different minerals in the aggregate wear and polish at different rates. Harder aggregate can be selected in highly abrasive conditions to minimize wear. Advantages: Why We Use Aggregates in Concrete?

Aggregates are used in concrete for very specific purposes. The use of coarse and fine aggregates in concrete provides significant economic benefits for the final cost of concrete in place. Aggregates typically make up about 60% to 75% of the volume of a concrete mixture, and as they are the least expensive of the materials used in...

References: * http://www.aggregate-uk.com
* Young, J. F., Mindess, S., Gray, R. J., & Bentur, A. (1998). The science and technology of civil engineering materials. Saddle River, NJ: Prentice-Hall
* ACI Committee 221, “Guide for Use of Normal Weight Aggregates for Concrete (ACI 221R-96),” American Concrete Institute, Farmington Hills, Mich., 1996, 29 pp.
* http://www.concretebasics.org/articlesinfo/scc8.php
* M.L. Gambhir, Concrete Technology (Theory and Practice), fourth ed.
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