HOW AGGREGATE PROPERTIES AFFECTS CONCRETE PROPERTIES?

Aggregates influence the properties of concrete/mortar such as water requirement, cohesion and workability of the concrete in the plastic phase, while they influence the strength, density, durability, permeability, surface finish and color in the cured phase.

Aggregates are generally classified into two categories viz. natural and manufactured/processed.


Geologically, aggregates can be obtained from basalt, granite, limestone, quartzite, gabbro or shale etc. The main sources are igneous (granite or basalt) or sedimentary (limestone) rocks. In general, they are all suitable for concrete, depending on their degree of weathering density and shape.

The choice of aggregates depends on several factors. The first and most important factor is availability.

Sl No Factors Influence on concrete property
1 Specific Gravity / Porosity Strength / Absorption
2 Chemical stability Durability
3 Surface texture bond grip
4 to shape Water demand (strength)
5 Gradation or particle size distribution Water requirement (strength), cohesion, bleeding and segregation
6 Maximum size Power and water requirement
7 harmful material Water requirement, binding, cohesion and sustainability

Surface texture

Surface texture of aggregate affects the bond between aggregate and cement. A smooth surface such as that found on gravel will have poor adhesion. Broken aggregates have a rough texture and give good mechanical adhesion with cement. However, gravel or round aggregates have been used for high strength concrete without serious problems with poor bonding. If the gravel is clean and well washed, the changes from the bad bond are greatly reduced.

Shape of aggregate

The aggregate shape can be broadly classified in order of desirability as follows, such as rounded, irregularly rounded, cuboidal, flaky angular, elongated, flaky elongated.

rounded

Rounded shape has minimal surface area for the same mass as other molds, therefore it requires minimal cement paste for bonding compared to other molds. Therefore, for a given aggregate-cement ratio in a concrete mix, workability will be higher if round aggregates are used as compared to others.

Flaky

Flaky aggregate is aggregate whose smallest size is less than 3/5 of the average size, ie the average size of an aggregate passing through a 40mm sieve and held on a 20mm sieve is 40+20/2 = 30mm. Now, if the smallest dimension is less than 18mm (3/5 x 30 = 18mm), then the aggregate is flaky.

elongated

Elongated aggregates are the aggregates whose length is 1.8 times the mean size. For example, if the average size shown above is 30mm, then the aggregates for a length greater than 54mm (1.8x30mm) are classified as elongated.

Concrete produced using flaky and elongated aggregates will be prone to segregation, poor surface finish, high cement and sand requirements. In general, round, irregular round and cubical shapes are preferred in concrete mixes.


Size of aggregates

The nominal maximum size of coarse aggregate should be as large as possible within the specified limits, but in no case exceed 1/4 of the minimum thickness of the member.

20 mm aggregate is suitable for most works.

For the specified work, dimensions of 40 mm and 10 mm should be used. Plums over 160mm should be used, when specifically allowed in ordinary concrete with a maximum limit of 20% by volume and should not be closer than 150mm from the surface. For heavy RCC members such as main beam ribs, the nominal maximum size of the aggregate should usually be limited to 5 mm less than the minimum free distance between the main bars or 5 mm less than the minimum cover to the reinforcement, whichever is less .

Aggregates The grade should consist of naturally occurring (ground or unground) rocks, gravel and sand or a combination thereof. They must be hard, strong, dense, durable, clear and free from veining and adherent coatings, and free from harmful amounts of fragments, alkali, vegetable matter and other harmful substances. As for flaky and elongated pieces, pieces are avoided.

Visual inspection of gravel and natural sand is necessary for presence of clay lumps, clay coating, silt, grading and shape, while for crushed aggregates and sand inspection is necessary for rock dust, flake shape and grading. If clay dust, silt or mud is present and not removed/reduced by washing, it may produce lower strength concrete.

The maximum amount of harmful substances in the coarse and fine aggregates (ground or natural) must not exceed the limits indicated in the table—

harmful material in total

The presence of mica should be investigated in fine aggregate and the possible reduction in the strength of concrete or mortar should also be taken into account. The aggregate must not contain harmful organic impurities above the specified limit.

Total crushing value (ACV)

The aggregate breaking value provides a relative measure of an aggregate’s resistance to crushing under a progressively applied compressive load. The breaking value of the aggregate shall not exceed 45% for aggregate used for concrete other than for wearing surfaces, and 30% for concrete for wearing surfaces such as runways, sidewalk roads.

Total Impact Value (AIV)

The total impact value provides a relative measure of an aggregate’s resistance to sudden shocks or shocks. As an alternative to the crush value, the total impact value should not exceed 45% by weight for aggregates used for concrete other than for wear surfaces and 30% by weight for concrete for wear surfaces such as runways, pavement roads

Specific gravity (G)

Specific gravity, low specific gravity indicates high porosity, resulting in poor durability and low strength. The concrete density is strongly dependent on the specific weight. A porous nature (ie those which increase in weight to more than 10% of the dry weight after 24 hours of immersion) should not be allowed unless specified. The specific gravity range for aggregates should be between 2.4 and 2.9.

Solidity of aggregate

Aggregate Solidity – For concrete subject to frost exposure, coarse and fine aggregates must pass an accelerated solidity test with sodium or magnesium sulphate.


Classification of aggregates:

The sorting of aggregates is an important aspect because they can influence the various properties of concrete, such as cohesion, water requirement, workability and strength. Aggregates must be properly classified and consistent in their layout. Fine aggregates are the aggregates, most of which pass through a 4.75mm IS sieve and contain only as much coarser material as is allowed by specification. Coarse aggregates are the aggregates, most of which were held on a 4.75mm IS screen and contained only so much finer material allowed for the various types described in the specification. sorting coarse aggregate

The sand should consist of natural sand, crushed stone, or crushed stone, or a combination of these. The sand must be hard, durable, chemically inert, clean and free from adherent coatings, organic matter and harmful impurities, e.g. iron pyrite, alkalis, salts, laminates or other materials in such form or quantities as to allow the curing, strength adversely affect the durability or appearance of the mortar. The sand is divided into four zones according to the table below:

sorting fine aggregate

A close look at the different ranges for each sieve size shows that sand in zone 1 is the coarsest and sand in zone 4 is the finest, while sand in zones 2 and 3 is moderate.

  • Fine aggregate complying with gradation zone 4 should not be used in reinforced concrete.
  • When the grading does not fall more than 5% outside the percentage limits for the sieves other than 600 microns and 300 microns (IS) sieves, it shall be reclassified as falling within this zone. The part of the aggregate less than 600 microns in size is finer and has a major impact on coherence, water requirement, processability and permeability, as their surface area per unit volume is much greater than that of other aggregate sizes.
  • If a certain sieve size is not present in aggregates, they are called gap graded. Sand that has a gap layout is generally not preferred over the sand that is uniformly graded. Gap grading can lead to a lack of cohesion, permeability and surface finish.

Other impurities

  • Organic substances in fine aggregates (sand) are usually found due to the presence of plant material. Even a small fraction of the organic material can delay or prevent the hardening of concrete.
  • Chlorides, when present in fine aggregates, are not harmful to concrete or mortar, but are harmful to reinforcement or other steel embedding in concrete and will accelerate rusting. Chlorides can also be present in cement, water and concrete additives. The total chloride content for long span bridges and prestressed structures should be limited to 0.10 wt% cement and for reinforced concrete structures to 0.15 wt% cement.
  • Alkali reactivity: If the proposed aggregates are alkali reactive, the presence of alkali oxides (Nao+0.658 K2O) in cement should be carefully investigated. Alkali oxides in cement should not exceed the permitted limits, ie more than 1%, which in the longer term can lead to cracking and disintegration of concrete.

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