Fine Aggregate And Coarse Aggregate Pdf
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Aggregates are essential components of concrete. They act as inert material in concrete. Fine aggregate and coarse aggregate are two main types of aggregate for concrete. As the name indicates, they are basically classified depending on the sizes of aggregate particles.
Classification of Aggregates
Fine aggregates Fig. Coarse aggregates Fig. Some natural aggregate deposits, called pit-run gravel, consist of gravel and sand that can be readily used in concrete after minimal processing. Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed. Crushed stone is produced by crushing quarry rock, boulders, cobbles, or large-size gravel. Crushed air-cooled blast-furnace slag is also used as fine or coarse aggregate The aggregates are usually washed and graded at the pit or plant.
Some variation in the type, quality, cleanliness, grading, moisture content, and other properties is expected. Close to half of the coarse aggregates used in Portland cement concrete in North America are gravels; most of the remainder are crushed stones. Recycled concrete, or crushed waste concrete, is a feasible source of aggregates and an economic reality, especially where good aggregates are scarce. Conventional stone crushing equipment can be used, and new equipment has been developed to reduce noise and dust.
Aggregate particles that are friable or capable of being split are undesirable. Identification of the constituents of an aggregate cannot alone provide a basis for predicting the behavior of aggregates in service. Visual inspection will often disclose weaknesses in coarse aggregates. Service records are invaluable in evaluating aggregates. In the absence of a performance record, the aggregates should be tested before they are used in concrete.
Aggregates of expanded shale, clay, slate, and slag Fig. Rounded aggregate, such as gravel, requires slightly lower values, while crushed aggregate requires slightly higher values. The aggregate particle size is determined by using wire-mesh sieves with square openings. The 13 standard sieves for coarse aggregate have openings ranging from 1. The grading and grading limits are usually expressed as the percentage of material passing each sieve. Curves indicate the limits specified in ASTM C 33 for fine aggregate and for one commonly used size number grading size of coarse aggregate.
There are several reasons for specifying grading limits and nominal maximum aggregate size; they affect relative aggregate proportions as well as cement and water requirements, workability, pumpability, economy, porosity, shrinkage, and durability of concrete. Variations in grading can seriously affect the uniformity of concrete from batch to batch. Very fine sands are often uneconomical; very coarse sands and coarse aggregate can produce harsh, unworkable mixtures.
In general, aggregates that do not have a large deficiency or excess of any size and give a smooth grading curve will produce the most satisfactory results. The effect of a collection of various sizes in reducing the total volume of voids between aggregates is illustrated by the simple method shown in Fig.
The beaker on the left is filled with large aggregate particles of uniform size and shape; the middle beaker is filled with an equal volume of small aggregate particles of uniform size and shape; and the beaker on the right is filled with particles of both sizes.
Below each beaker is a graduate with the amount of water required to fill the voids in that beaker. Note that when the beakers are filled with one particle size of equal volume, the void content is constant, regardless of the particle size.
When the two aggregate sizes are combined, the void content is decreased. If this operation were repeated with several additional sizes, a further reduction in voids would occur. The cement paste requirement for concrete is related to the void content of the combined aggregates. The level of liquid in the graduates, representing voids, is constant for equal absolute volumes of aggregates of uniform but different size.
When different sizes are combined, the void-content decreases. The illustration is not to scale. In reality, the amount of cement paste required in concrete is greater than the volume of voids between the aggregates.
This is illustrated in Fig. Sketch A represents large aggregates alone, with all particles in contact. Sketch B represents the dispersal of aggregates in a matrix of paste.
B A Fig. Illustration of the dispersion of aggregates in cohesive concrete mixtures. Fine-Aggregate Grading Requirements of ASTM C 33 permit a relatively wide range in fine- aggregate gradation, but specifications by other organizations are sometimes more restrictive.
The most desirable fine-aggregate grading depends on the type of work, the richness of the mixture, and the maximum size of coarse aggregate. In leaner mixtures, or when small-size coarse aggregates are used, a grading that approaches the maximum recommended percentage passing each sieve is desirable for workability.
In general, if the water-cement ratio is kept constant and the ratio of fine-to-coarse aggregate is chosen correctly, a wide range in grading can be used without measurable effect on strength. However, the best economy will sometimes be achieved by adjusting the concrete mixture to suit the gradation of the local aggregates. The fineness modulus must be not less than 2. If this value is exceeded, the fine aggregate should be rejected unless suitable adjustments are made in proportions of fine and coarse aggregate.
The lower limit may be sufficient for easy placing conditions or where concrete is mechanically finished, such as in pavements.
Fineness Modulus. The fineness modulus FM of either fine or coarse aggregate according to ASTM C is calculated by adding the cumulative percentages by mass retained on each of a specified series of sieves and dividing the sum by FM is an index of the fineness of an aggregate—the higher the FM, the coarser the aggregate. Different aggregate grading may have the same FM.
FM of fine aggregate is useful in estimating proportions of fine and coarse aggregates in concrete mixtures. An example of how the FM of a fine aggregate is determined with an assumed sieve analysis is shown in Table Table The grading for a given maximum-size coarse aggregate can be varied over a moderate range without appreciable effect on cement and water requirement of a mixture if the proportion of fine aggregate to total aggregate produces concrete of good workability.
Mixture proportions should be changed to produce workable concrete if wide variations occur in the coarse-aggregate grading. The maximum size of coarse aggregate used in concrete has a bearing on the economy of concrete. Usually more water and cement is required for small-size aggregates than for large sizes, due to an increase in total aggregate surface area. Furthermore, aggregates of different maximum sizes may give slightly different concrete strengths for the same water-cement ratio.
In some instances, at the same water-cement ratio, concrete with a smaller maximum-size aggregate could have higher compressive strength. This is especially true for high-strength concrete. The optimum maximum size of coarse aggregate for higher strength depends on factors such as relative strength of the cement paste, cement aggregate bond, and strength of the aggregate particles.
The terminology used to specify size of coarse aggregate must be chosen carefully. Particle size is determined by size of sieve and applies to the aggregate passing that sieve and not passing the next smaller sieve. When speaking of an assortment of particle sizes, the size number or grading size of the gradation is used.
The size number applies to the collective amount of aggregate that passes through an assortment of sieves. As shown in Table 5- 5, the amount of aggregate passing the respective sieves is given in percentages; it is called a sieve analysis. Because of past usage, there is sometimes confusion about what is meant by the maximum size of aggregate.
ASTM C defines this term and distinguish it from nominal maximum size of aggregate. The maximum size of an aggregate is the smallest sieve that all of a particular aggregate must pass through. The nominal maximum size of an aggregate is the smallest sieve size through which the major portion of the aggregate must pass. For example, aggregate size number 67 has a maximum size of 25 mm 1 in.
The maximum size of aggregate that can be used generally depends on the size and shape of the concrete member and the amount and distribution of reinforcing steel. The maximum size of aggregate particles generally should not exceed: 1. One-fifth the narrowest dimension of a concrete member 2. Three-quarters the clear spacing between reinforcing bars and between the reinforcing bars and forms 3. One-third the depth of slabs These requirements may be waived if, in the judgment of the engineer, the mixture possesses sufficient workability that the concrete can be properly placed without honeycomb or voids.
Combined Aggregate Grading Aggregate is sometimes analyzed using the combined grading of fine and coarse aggregate together, as they exist in a concrete mixture. This provides a more thorough analysis of how the aggregates will perform in concrete.
Sometimes mid-sized aggregate, around the 9. Strength and durability may also be affected. Optimum combined aggregate grading for concrete. Gap-Graded Aggregates In gap-graded aggregates certain particle sizes are intentionally omitted. Gap-graded mixes are used in architectural concrete to obtain uniform textures in exposed-aggregate finishes. They can also be used in normal structural concrete because of possible improvements in some concrete properties, and to permit the use of local aggregate gradations.
Care must be taken in choosing the percentage of fine aggregate in a gap- graded mixture.
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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. Recycled concrete is a viable source of aggregate and has been satisfactorily used in granular subbases, soil-cement, and in new concrete. If necessary, a benefaction process such as jigging or heavy media separation can be used to upgrade the quality. Once processed, the aggregates are handled and stored to minimize segregation and degradation and prevent contamination. Aggregates strongly influence concrete's freshly mixed and hardened properties, mixture proportions, and economy. Consequently, selection of aggregates is an important process.
PDF | The effects of using crushed concrete as coarse and fine aggregates upon strength and deformation of concrete are reported.
The Fineness Modulus FM is an empirical figure obtained by adding the total percentage of the sample of an aggregate retained on each of a specified series of sieves , and dividing the sum by The same value of fineness modulus may therefore be obtained from several different particle size distributions. In general, however, a smaller value indicates a finer aggregate. Fine aggregates range from a FM of 2.
It includes gravel, crushed stone, sand, slag, recycled concrete and geosynthetic aggregates. Aggregate may be natural, manufactured or recycled. They provide compressive strength and bulk to concrete. The usual range employed is between 9. A larger size, 40mm, is more common in mass concrete.
Posted on 29th March by Hintons. Aggregates form an essential part of many construction projects, from large-scale commercial to smaller domestic works. Whether you need aggregates to form a sub-base for foundations or paving, decorative aggregates for driveways and footpaths — or simply need something to fill in unsightly holes — you should know which kind of aggregates will work best. Read about Our Aggregates.
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