Aggregates (Coarse and Fine)

Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and portland cement, are an 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 sieve. Coarse aggregates are any particles greater than 0.19 inches but generally range between 3/8 and 1.5 inches 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. Recycled concrete is a viable source of aggregate and has been satisfactorily used in granular subbases, soil-cement, and new concrete.

After harvesting, aggregate is processed:  crushed, screened, and washed to obtain proper cleanliness and gradation. 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, the selection of aggregates is an important process. Although some variation in aggregate properties is expected, characteristics that are considered include:

• grading
• durability
• particle shape and surface texture
• abrasion and skid resistance
• unit weights and voids
• 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 these properties affect the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability of concrete. In general, if the water-cement ratio is chosen correctly, a wide range in grading can be used without a major effect on strength. When gap-graded aggregate is specified, certain particle sizes of aggregate are omitted from the size continuum. Gap-graded aggregate is used to obtain uniform textures in exposed aggregate concrete. Close control of mixed proportions is necessary to avoid segregation.

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 the 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 percent by the 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 aggregates increase the void content. Larger sizes of well-graded aggregate and improved grading decrease the void content. Absorption and surface moisture of aggregate is 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 is 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.

AGGREGATES PROPERTIES BY TESTS:

10% Fine Value:

All aggregates are required to meet a minimum strength valve, as defined by the 10% value test. This and other specifications ensure that only the highest quality materials are being used on construction projects. Test Method: IS: 2386 (P-4)1963

Alkali Aggregate Reactivity:

Alkali-aggregate reaction is the term mainly referred to as a reaction that occurs over time in concrete between the highly alkaline cement paste and non-crystalline silicon dioxide, which is found in many common aggregates. This reaction can cause expansion of the altered aggregate, leading to spalling and loss of strength of the concrete.
Test Method: IS: 2386 (P-7) 1963

Chloride Content:

The total chloride content of aggregate is usually measured to assess whether the aggregate’s contribution to the total chloride content of a concrete mix will be low enough to prevent the early onset of corrosion of any embedded steel reinforcement.
Test Method: IS: 4032-1985

Clay Lumps and friable Particles:

This test helps in determining the percentage of clay lumps and friable particles in aggregate. Clay lumps in aggregate shall be defined as any particles or aggregation of particles which when thoroughly wet can be distorted when squeezed between the thumb and forefinger, or will disintegrate into individual grain sizes when immersed for a short period in water. Friable particles are defined as particles that vary from the basic aggregate particles such, that they may either readily disintegrate under normal handling and mixing pressures, imposed upon them by construction procedures, or break down after being incorporated into the work.
Test Method: IS: 2386 (P-2) 1963

Clay Slit and Dust passing/ Sieve Analysis:

This test helps in determining the size of a granular material. The size distribution is often of critical importance to the way the material performs in use. A sieve analysis can be performed on any type of non-organic or organic granular material including sand, crushed rock, clay, granite, fled-spars, coal, and soil, a wide range of manufactured powders, grain, and seeds, down to a minimum size depending on the exact method. Being such a simple technique of particle sizing, it is probably the most commonly used to date.
Test Method: IS: 2386 (P-2) 1963

Compaction Fraction:

This test helps in determining the compaction percentage of an aggregate when loose compared to the same aggregate compacted in a standard manner. It is useful in assessing an aggregate when placed loosely, for example, as a pipe surround material. Aggregate suitable for use as pipe bedding would display a low compaction fraction, indicating it reaches a state of near full compaction under loose placement.
Test Method:

Crushing Value:

The Aggregate Crushing Value offers a related measure of the resistance of an average to crushing under a compressive load that is gradually applied.
Test Method: IS: 9376-1979, IS: 2386 (P-4)1963

Drying Shrinkage:

Drying shrinkage is defined as the contracting of a hardened concrete mixture due to the loss of capillary water. This shrinkage causes an increase in tensile stress, which may lead to cracking, internal warping, and external deflection before the concrete is subjected to any kind of loading.

Flakiness and Elongation Index:

Flakey is the term applied to aggregate or chippings that are flat and thin with respect to their length or width. Aggregate particles are said to be flakey when their thickness is less than 0.6 of their mean size. The flakiness index is found by expressing the weight of the flakey aggregate as a percentage of the aggregate tested.
Test Method: IS: 2386 (P-1) 1963

Impact Value:

This test helps in determining the value which indicates the ability of an aggregate to resist crushing. The lower the figure the stronger the aggregate, i.e. the greater its ability to resist crushing.
Test Method: IS: 2386 (P-4) 1963, ASTMC 131-2006

Lightweight Pieces:

This test helps in determining the approximate percentage of lightweight pieces in aggregate by means of sink-float separation, in a heavy liquid of suitable specific gravity. This method may be used to identify porous aggregate particles in research activities or in petrographic analysis.

Los Angeles Abrasion Test:

Los Angeles (L.A.) Abrasion Test (Figure 1) is a common test method used to indicate aggregate toughness and abrasion characteristics. Aggregate abrasion characteristics are important because the constituent aggregate in HMA (Hot Mix Asphalt) samples should resist crushing, degradation and disintegration in order to produce a high-quality HMA.
Test Method: IS: 2386 (P-4) 1963

Effect of Organic Impurities on Mortar Strength:

This test helps in determining the rapid assessment of organic impurities by indicating their presence. Comparison is made between compressive strengths of mortar made with washed and unwashed fine aggregate.
Test Method: ASTM C40

Organic Impurities:

This test helps in determining the presence of organic compounds aggregated to use in cement mortar or concrete. The test provides a quick, relative measure to determine if further tests of the fine aggregate are necessary before approval for use.
Test Method: IS: 2386 (P-2)1963

Particle Size Distribution:

This test helps in determining the percentile quantity of particles of known diameter within a sample. The specimen can be either passed through a set of standard sieves in its natural state or if a significant amount of binding material is present, such as clay, then the sample can first be washed over a small aperture sieve to remove the binding material.
Test Method: IS: 2386 (P-1) 1963, ASTM C136-2006, IS: 383-1970

Sand Equivalent Value:

The sand equivalent test is a rapid field test to show the relative proportions of fine dust or clay-like materials in fine aggregate (or granular soils).
Test Method: MORTM, IS: 2720 (Pt-37)1976, ASTM D 2419-2009

Soundness:

This test helps in determining the resistance to disintegration by freezing and thawing. It furnishes information helpful in judging the soundness of aggregates subjected to weathering, particularly when adequate information is not available from service records of the behavior of the aggregate.
Test Method: IS: 2386 (P-5)1963

Specific gravity and water absorption:

The specific gravity of an aggregate is considered to be a measure of the strength or quality of the material. The specific gravity test helps in the identification of stones. Water absorption gives an idea of the strength of aggregate. Aggregates having more water absorption are more porous in nature and are generally considered unsuitable unless they are found to be acceptable based on strength, impact, and hardness tests.
Test Method: IS: 2386 (P-3)1963, ASTM C 127, 128-2007

Sulphate Content:

The total sulfate content of aggregate is usually measured to assess whether the aggregate’s contribution to the total sulfate content of a concrete mix will be low enough to prevent any deleterious effects.
Test Method: IS: 4032-1985