What is the Characteristics of Aggregates

Characteristics of Aggregates

The properties (characteristics) of aggregates greatly influence the properties of concrete in the fresh and hardened stage. In general, an aggregate to be used in concrete must be clean, hard, durable, inert, strong, properly shaped, and well graded. They should be free from deleterious material, which results in physical and chemical changes. The properties of aggregate which influence the properties of concrete↗️ are discussed below

1. Particle Size (Size of Aggregate)
2. The shape of Aggregates
3. Surface Texture
4. Specific gravity
5. Bulk density
6. Surface moisture content, Absorption, and Porosity
7. Bulking of Aggregate (Sand)
8. Deleterious material in aggregates
9. The soundness of Aggregates
10. Strength of Aggregate

1. Particle Size (Size of Aggregate)

The size of aggregates greatly influences the properties of concrete in the plastic↗️ and hardened stage↗️. It also governs the quantity of cement paste required and hence the economy of concrete. Larger the size of coarse aggregates, lesser is the surface area, hence lesser is the quantity of fine aggregate and cement required. 

The aggregate is generally designated by the maximum size of an individual particle in a lot. Size of an aggregate can be determined by means of sieves 80 mm, 63 mm, 50 mm, 40 mm, 20 mm, 16 mm, 12 mm, 10 mm, 6-3 mm, 4.75 mm, 2.36 mm, 1.18 mm, 600 microns, 300 microns, 150 microns, and 75 microns).

For example, - A particle of aggregate is said to be of 20 mm size, if it passes through 20 mm IS sieve and is retained on the next lower sieve in the series i.e., 16 mm IS Sieve.

Size of the aggregate is governed by the thickness of the member, the clear spacing between the reinforcement, etc. e.g. the aggregate of 40 mm nominal size are used for mass concreting works like dams, retaining walls, etc. 20 mm size aggregates are normally used for all types of RCC works like, beams, slabs, columns, etc. Similarly, 10 mm size aggregates are used for the construction of floors.


2. Shape of Aggregates

Aggregates both natural and crushed are available in all types of shapes. One can seldom find aggregates which are identical to each other in shape and size. The shapes of aggregates can be broadly classified as rounded, irregular, cubical, flaky, elongated. The rounded shape has a minimum surface area for the same mass than other shapes and therefore requires minimum cement paste for bonding as compared to other shapes. Flaky and elongated shapes have a larger surface area for the same mass as compared to rounded or cubical shapes. Concrete produced using flaky and elongated aggregates will be prone to segregation have poor surface and high cement and sand demand.


3. Surface Texture

Surface texture illustrates the nature of the surface of the aggregates. The surface texture influences the bond between the aggregate and cement. The rough texture of the aggregate provides a higher surface area for a given volume. Aggregates with smooth surfaces develop a poor bond. Crushed aggregates have a rough texture and give a good mechanical bond with cement. If the crushing value of the aggregates is high, the compressive strength will depend on the bond created by the cement paste between the aggregates. Rough texture reduces workability but is beneficial for obtaining high strength concrete. Based on the visual examination of the specimen, the aggregates may be classified as under

Aggregate with a rough, porous texture is preferred to one with a smooth and glossy surface as the former can increase the aggregate cement bond by 75% which in turn may increase the compressive strength of concrete by 20%.


4. Specific gravity

The specific gravity of the aggregate generally is indicative of its quality. A low specific gravity may indicate high porosity and therefore results in poor durability and low strength. It is very important that the aggregates have high specific gravity. The density of concrete will greatly depend upon the specific gravity. The range of specific gravity for aggregates is generally between 2:4 and 2:9. Some of the specific gravity values are given in the image.

There are two types of specific gravity are the following below.

• Apparent specific gravity
• Bulk specific gravity

(a) Apparent Specific gravity - The ratio of the weight of aggregate (oven-dried at 100 °C -110 °C for 24 hours) to the weight of water occupying the volume equal to that of solids of aggregate, excluding pores in aggregate is called apparent specific gravity.

(b) Bulk Specific gravity - The ratio of the weight of aggregate (oven-dried at 100 °C -110 °C for 24 hours) to the weight of water occupying the volume equal to that of solids of aggregate including pore in aggregates. This value is exactly known as specific gravity and is used to design a concrete mix and to calculate the void ratio for a given concrete mix. The specific gravity is required for the following given below the points.

• calculating the yield of concrete.
• getting valuable information about quality and properties.

Higher the specific gravity of an aggregate, the harder and stronger it will be. If the specific gravity is above or below the normally specified to a particular type of aggregate, it may indicate that the shape and grading of the aggregates have changed.

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5. Bulk density

The bulk density or unit weight of an aggregate provides important information regarding the shape and grading of the aggregate. The bulk density of aggregate is defined as the mass of the material in a given volume and is expressed in kg/l. It depends on how densely the aggregate is packed in the mold.
Bulk density can be of following below the two types.

• Loose bulk density
• Rodded bulk density.

(a) Loose bulk density - Loose bulk density can be used for converting weights of aggregate into the bulk volume or vice versa, for volume batching of aggregates during the manufacturing of concrete mix. The value of the bulk density of the aggregate depends upon the amount of effort used to fill the container. Some standard values of bulk densities are shown in Table 2.9. 

(b) Rodded bulk density - The bulk density determined by filling the container with aggregate in layers, properly compacted with the help of a rod, is known as rodded bulk density. This is useful in detecting changes in the uniformity of size and grading of aggregates. Bulk density can also be used for calculating the percentage of voids in aggregate as shown by the formula is the following given below

Percentage of voids = The specific gravity of aggregate - Bulk density / Specific gravity of aggregate X 100

Note - Bulk density of aggregate is of interest when we deal with lightweight aggregate and heavyweight aggregate.

Note - The higher the bulk density, the lower is the void content to be filled by sand and cement.


Factors affecting bulk density - The value of bulk density is governed by factors discussed below.

• Grading of aggregates - Well graded aggregates result in the least voids and hence more is the bulk density. Whereas poorly graded aggregates and single size aggregates have a lower value of bulk density.
• The shape of aggregates - The shape of aggregate greatly influences the bulk density. Rounded aggregates have lesser voids than angular aggregates. By applying the same comparative effort, the bulk density of rounded aggregate will be greater than that of the angular aggregates.

6. Surface moisture content, Absorption, and Porosity

Natural aggregates generally contain moisture. Moisture conditions are more meaningful in the concrete mix design. However, absorption tendency and porosity are equally important from the point of view of both qualities of the aggregate as well as the quality of concrete in which the aggregates are used. Generally, moisture is present in the following forms (As shown in image) 

• Moist or Surface wet
• Surface dry but saturated
• Air dry
• Oven dry

The moisture on the surface determines the free water which is to be considered while measuring the water-cement ratio. The absorbed water within the aggregate is not considered while measuring the water-cement ratio. Porous aggregates will absorb more moisture or water than dense aggregates. If absorption in aggregates is more, then the concrete will lose its workability at a much faster rate. As a precaution, it is also better to do concrete trials with saturated surface dry aggregates so that the free water requirement for the required workability is easily determined. 

Absorption can occur in both natural as well as in crushed aggregates. It is generally observed that, except in the rainy season, the surface moisture is present on natural aggregates that are dredged from the river or creeks or dug from pits.

For the above-mentioned moisture conditions, it is necessary to adjust the quantity of water in the mixer so that the net water available for cement reaction is equal to the desired water content as calculated from the water-cement ratio. Therefore, the following adjustments for water are to be made according to the different aggregate conditions.

1. Moist or Surface wet Aggregates = Desired water - Water available at the surface of aggregates.
2. Saturated Surface dry = Desired water.
3. Air dry = Desired water + water absorbed by aggregates in 10 to 15 minutes.
4. Oven dry = Desired water + Water absorbed by aggregates in 24 hours.

7. Bulking of Aggregate (Sand)

Bulking (Swelling) takes place more in fine aggregates (sand) than coarse aggregates. When sand is damp, the water coating the surface of each sand particle causes separation of particles from one another due to surface tension. This causes the sand to bulk. Bulked sand occupies more volume and hence if the batching is done by volume, then the correction for bulking should be applied.

The volume of sand goes on increasing as its moisture content increases up to about 5 - 6% of its weight. Beyond this value, the extra moisture destroys the surface tension.

If the sand contains moisture content up to 12 - 20%, then it occupies the same volume as when it was dry. With the addition of moisture content up to 5%, the sand may bulk up to 20 - 40% depending upon the fineness of sand. Bulking is more in the case of fine sands as shown in images.

Importance of bulking of sand - The bulking of sand is important when the batching of aggregates is done by volume. Bulked sand occupies more volume as compared to dry sand hence it should be compensated by adding extra sand. If the correction for the bulking of sand is not applied, then the concrete will be deficient is sand and will have the tendency to segregate, also the yield of concrete will be less.

Note

• The effect of the bulking of sand can be reduced when batching is done by weight.
• Dry sand occupies the same volume as fully saturated sand.

Determination of percentage bulking

The following procedure may be adopted for calculating the percentage of bulking are given below.

• Fill loosely filled sand into a cylinder up to 2/3rd height.
• Level the top surface of sand and measure its height by passing a steel ruler vertically down through the sand. Let the height = H. 
• Transfer the sand into another cylinder.
• Fill the cylinder with water to 1/4th of the volume occupied by sand.
• Put back about half the sand and compact it with the help of a steel rod so that its volume reduces to a minimum.
• Add the balance sand and compact it is discussed above.
• Again level the top surface of compacted sand and measure its height. Let the height = h.
• Bulking of sand can be calculated from the following relation = H-h / h X 100

8. Deleterious material in aggregates

The materials which may adversely affect the properties of concrete in the fresh and hardened stage are known as deleterious materials. These materials affect the strength, workability, and long term performance of concrete-like durability, etc.

Such materials are undesirable in aggregates because of their intrinsic weakness, harmful chemical characteristics, etc. Depending upon their action, the deleterious materials present in aggregates may be divided into three groups are the following given below.

(i) Organic impurities - Such impurities are not desirable in aggregates as they may interfere with the process of hydration of types of cement.

For example - Decayed vegetable matter and organic loam. 

(ii) Unsound particles - If weak and unsound particles are present in excess, then it is possible that they might react with cement. If unsound particles are present in large quantities (more than 1%), they decrease the durability of concrete.

For example - Clay lumps, Shale, etc.

(iii) Silt and Clay - Silt and clay coatings on aggregates prevent good bond strength between particles of concrete mix. Clay and other fine materials such as silt and crusher dust are commonly seen in aggregates. The soft or loosely adherent coatings can be removed by washing. An aggregate with chemically reactive coatings can lead to serious problems in a mix.

For example - Silt in fine aggregates, clay coating, and crusher dust in coarse aggregates.


9. The soundness of Aggregates

The soundness may be defined as the ability of the aggregate to resist excessive changes in volume due to change in environmental conditions like freezing and thawing, alternate wetting and drying and thermal changes.

Aggregate is said to be unsound when the volume changes result in the deterioration of concrete. Unsound aggregates may result in local scaling to extensive surface cracking or may seriously disintegration of hardened concrete mix.

Testing of Aggregates for Soundness

• Weigh the sample of aggregates and immerse them in a solution of sodium sulfate (Na2SO4) or magnesium sulfate (MgSO4) for a specified time.
• Take out the aggregates and dry them.
• Repeat this cycle 10 times.
• After 10 cycles the weight of the aggregate is again taken.
• The average loss of weight after 10 cycles should not exceed 12% when tested with sodium sulfate and this loss should not exceed 18% when tested with magnesium sulfate.

10. Strength of Aggregate

The resistance of an aggregate to compressive load may be defined as the crushing strength of aggregates. The compressive strength of the concrete mix is greatly influenced by the strength of aggregate. To have an idea about the crushing strength of aggregates, the crushing strength test may be carried out.