Properties of concrete aggregates

Cement

Aggregates

Admixtures

Mixture Design

Fresh Concrete

Hardened Concrete

Dimensional Stability

Durability

 

 

A majority of the volume of concrete (60 – 80%) is occupied by aggregate. In fact, for most conventional concretes, more than 75% of the volume is aggregate. Aggregates are essential in concrete from the point of view of economy (since cement is expensive), dimensional stability (aggregates do not easily creep or shrink), stiffness, and abrasion resistance.

Classification of aggregates

Typically, coarse aggregate sizes are larger than 4.75 mm (5 mm in British code), while fine aggregates form the portion below 4.75 mm. A maximum size up to 40 mm is used for coarse aggregate in most structural applications, while for mass concreting purposes such as dams, sizes up to 150 mm may be used. Fine aggregates, on the other hand, have particles up to a minimum size of 0.075 mm. Typical particle size analyses of coarse and fine aggregates are shown in Figure 1.

 

Figure 1. Particle size distribution of aggregates by sieve analysis

The grading of the aggregate can be distinguished based on the sieve analysis. Aggregates that predominantly show size fractions in a limited range of sizes (such as the 20 mm and 10 mm aggregate in Figure 1) are called ‘uniformly’ graded, while aggregates that show a continuous gradation of size (sand in Figure 1) are called ‘well’ graded or continuously graded. When an aggregate shows size fractions in two or more well defined and well separated ranges, it is called ‘gap’ graded. The overall objective is to use a combination of coarse and fine aggregate in concrete in such as way as to get a continuous gradation of sizes, to ascertain the best packing of the aggregate. This issue will be addressed later in the Mixture Design link.

Based on the source, aggregates may be classified as natural and manufactured. Natural aggregates may again be classified into those that may be used as is (such as river gravel), and those which are obtained from crushing of rock. On the other hand, manufactured aggregates are those prepared artificially, such as slag, glass, fly ash (see Figure 2 for lightweight palletized-sintered fly ash aggregate), etc.

Aggregates can also be classified based upon their density as: lightweight, normal weight and heavyweight. Lightweight aggregates such as pumice and tuff (natural pyroclastic igneous) or vermiculite and perlite (synthetic) have densities in the range of 800 – 1000 kg/m3, compared to normal weight aggregates such as limestone or granite that range from 2500 – 2900 kg/m3 and heavyweight aggregates such as hematite and barite, which have densities as high as 5400 kg/m3.

Figure 2. Pelletized and sintered fly ash makes a good lightweight aggregate

Particle shape and texture

  • Rounded Vs. Angular: The roundness of the aggregate will affect its packing properties as well as the interlock obtained between aggregates. Angular aggregates result in better packing and interlock. On the other hand, rounder aggregates typically need less water for the same workability. Roundness may be defined by the sphericity of aggregate, or, from the other viewpoint, as the angularity of the aggregate. Because of weathering, river gravel (or sand) gives rounded aggregate, while angular aggregates are obtained using crushed stone.
  • Flakiness or elongation of the aggregate can result in anisotropic packing, poor compaction, and lowered concrete strengths. As discussed earlier, flakiness and elongation could result from the geological nature of the aggregate (aggregates from rocks showing directional properties would tend to be flaky and elongated).
  • Texture of the aggregate is rough or smooth, and dictates the strength of the paste-aggregate bond. Rougher aggregates show better bond with paste, but also cause an increase in the water demand. Weathered aggregates are smooth, while crushed aggregates are rough.

Aggregate strength, modulus, and toughness

The strength of aggregate tested independently is generally always higher than that of concrete. However, aggregate in concrete is stronger than the concrete for conventional concrete, while the aggregate strength is lower than concrete strength for high strength concrete, where the cementitious matrix is extremely strong. Aggregate strength depends on its parent rock composition, texture and structure.

The modulus of elasticity is related to strength of the aggregate. It is very important for a number of reasons:

  • E values of aggregate very different from paste will cause a mismatch at the interface, and this region can thus get prone to cracking. Figure 3 shows a typical pattern of crack development in concrete subjected to compressive loading. The interface cracks form first, and then grow outwards into the mortar. Thus the best concrete aggregates are those with E values in the range of the paste.
  • Stiffness of the aggregate will affect the deformation of concrete due to creep and shrinkage. Stiffer the aggregate, lesser is the overall deformation.

Figure 3. Crack development pattern of concrete in compression

Other aggregate qualities related to its strength are its hardness (which will affect abrasion resistance), impact resistance (related to toughness), and crushing value, or resistance to pulverization.

Specific gravity and bulking

Aggregates are porous, and posses a number of voids, some of which are penetrable and some impenetrable. This nature of aggregate makes it difficult to describe one value for its specific gravity. Based on the test methods used, three types of specific gravity are calculated – Bulk, Saturated surface dry, and Apparent. Determination of the true specific gravity of the aggregate is not possible (unless it is crushed to the smallest possible dimension) because the true volume of the aggregate can never be determined (as some pores are totally inaccessible).

Fine aggregate is susceptible to increases in volume due to the presence of moisture. This phenomenon is called bulking. Bulking can lead to problems during volume batching of the aggregates and causes harsh mixes with compaction problems. Crushed sand is more susceptible to this problem compared to natural sand.

Deleterious substances in aggregate

  • Impurities, such as organic matter, which may interfere with hydration
  • Coatings, such as clay, which may affect the quality of the paste-aggregate bond
  • Weak and unsound particles, such as salt, and low density porous aggregate which cause a high degree of water absorption and loss of strength

Soundness

Soundness of the aggregate is its ability to resist excessive changes in volume as a result of the change in physical conditions. The absorption and porosity of the aggregate primarily dictates its soundness. Aggregates for concrete are tested for soundness by subjecting them to wetting and drying cycles (using sodium sulphate solution as wetting fluid) and measuring the weight loss due to salt crystallization and associated cracking.

 

 

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