INFLUENCE OF AGGREGATE PROPERTIES ON CONCRETE
Aggregates influence the properties of concrete/mortar such as water requirement, cohesiveness and workability of the concrete in plastic stage, while they influence strength, density, durability, permeability, surface finish and colour in hardened stage.
Aggregates are generally classified into two categories viz. natural and manufactured/processed.
Geologically aggregates may be obtained from basalt, granite, limestone, quartzite, gabbro or schist rock etc. Main resources are igneous (granite or basalt) or sedimentary (limestone) rock. Generally all are suitable for concrete depending on their degree of weathering density and shape.
CHOICE OF AGGREGATE
Choice of aggregates depends on several factors. The first and the most important factor is availability.
|Sl No||Factors||Influence on concrete property|
|1||Specific gravity / Porosity||Strength / Absorption|
|3||Surface texture||Bond grip|
|4||shape||Water demand (strength)|
|5||Gradation or particle size distribution||Water demand (strength), cohesion, bleeding and segregation|
|6||Maximum size||Strength and water demand|
|7||Deleterious material||Water demand, bond , cohesion and durability|
Surface texture of aggregate influences the bond between the aggregate and cement. Smooth surface such as that found on gravels will have a poor bond. Crushed aggregates have a rough texture and give a good mechanical bond with cement. However, gravel or rounded aggregates have been used for high strength concrete without any serious problems of poor bond. If the gravel is clean, and well washed the changes of the poor bond is considerably reduced.
SHAPE OF AGGREGATE
Shape of aggregate can be broadly classified as follows in order of desirability like Rounded, Irregular rounded, Cubical, Flaky angular, Elongated, Flaky elongated.
Rounded shape has minimum surface area for the same mass than other shapes therefore it requires minimum cement paste for bonding as compared to other shapes. Hence, for a given aggregate-cement ratio in a concrete mix, workability will be higher, if rounded aggregates are used as compared to others.
Flaky aggregate are the aggregates whose least dimension is less than 3/5th of its mean dimension i.e. the mean dimension of an aggregate passing through 40 mm sieve and retained on 20 mm sieve is 40+20/2 = 30 mm. Now, if the least dimension is less than 18 mm (3/5 x 30 = 18 mm) then the aggregate is flaky.
Elongated aggregates are the aggregates whose length is 1.8 times of its mean dimension. If the mean dimension as shown above is say 30 mm then for a length above 54 mm (1.8x30mm) the aggregates will be classified as elongated.
Concrete produced using flaky and elongated aggregates will be prone to segregation, poor surface finish, high cement and sand demand. Generally rounded, irregular rounded 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 limits specified, but in no case greater than 1/4th of the min. thickness of the member.
For most works, 20 mm aggregate is suitable.
40 mm and 10 mm size should be used for work specified. Plums above 160 mm should be used, when specifically permitted in plain concrete with a max limit of 20% by volume and should not be closer than 150 mm from the surface. For heavily RCC members like ribs of main beams, the nominal maximum size of the aggregate should usually be restricted to 5 mm less than the minimum clear distance between the main bars or 5 mm less than the minimum cover to the reinforcement whichever is smaller.
Aggregates Quality should consist of naturally occurring (crushed or uncrushed) stones, gravel and sand or combination thereof. They should be hard, strong, dense, durable, clear and free from veins and adherent coating, and free from injurious amounts of disintegrated pieces, alkali, vegetable matter and other deleterious substances. As for as possible, 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 stone dust, flaky shape and grading. If clay dust silt or mud is present and not removed/reduce through washing, it may produce lower strength concrete.
The maximum quantity of deleterious materials in the coarse and fine aggregates (crushed or natural) should not exceed the limits as specified in the table—
The presence of mica should be investigated in fine aggregate and should also make suitable allowances for the possible reduction in the strength of concrete or mortar. The aggregate should not contain harmful organic impurities, excess to the limit specified.
Aggregate Crushing Value (ACV)
Aggregate crushing value gives a relative measure of the resistance of an aggregate to crushing under a gradually applied compressive load. The aggregate crushing value should not exceed 45% for aggregate used for concrete other than for wearing surfaces, and 30 % for concrete for wearing surfaces, such as runways, roads for pavements.
Aggregate Impact Value (AIV)
Aggregate impact value gives a relative measure of the resistance of an aggregate to sudden shock or impact, As an alternative to crushing value, the aggregate impact value should not exceed 45% by weight for aggregates used for concrete other than for wearing surfaces and 30% by weight for concrete for wearing surfaces, such as runways, roads for pavements
Specific Gravity (G)
Specific gravity, a low specified gravity indicates to high porosity, which results in poor durability and low strength. The concrete density is greatly depending on the specific gravity. A porous nature (i.e. those which increase in weight exceeding 10% of dry weight after immersion for 24 hours) should not be permitted, unless specified. The range of specific gravity for aggregates should be between 2.4 and 2.9.
Soundness of Aggregate
Soundness of aggregate -for concrete liable to be exposed the action of frost, coarse and fine aggregates should pass a sodium or magnesium sulphate accelerated soundness test.
GRADING OF AGGREGATES:
Grading of aggregates is an important aspect as they can influence the various properties of concrete such as cohesion, water demand, workability and strength. Aggregates should be well graded and consistent in their grading. Fine aggregates are the aggregates, most of which passes through 4.75 mm IS sieve and contained only so much coarser material, which is permitted as per specification. Coarse Aggregates are the aggregates, most of which retained on 4.75 mm IS sieve and contained only so much finer material, which is permitted for the various types described in the specification.
SAND OR FINE AGGREGATE:
The sand should consist of natural sand, crushed stone sand or crushed gravel sand, or a combination of any these. The sand should be hard, durable, chemically inert, clean and free from adherent coatings organic matter and harmful impurities e.g. iron pyrites, alkalis, salts, laminated or other materials in such form or in such quantities as to affect adversely the hardening, the strength, the durability or the appearance of the mortar. The sand is graded into four zones as per table given below:
A close look at the various ranges for each sieve size shows that zone 1 sand is the coarsest and zone 4 sand is finest whereas sand in zones 2 and 3 are moderate.
- Fine aggregate conforming to grading zone 4 should not be used in reinforced concrete.
- When the grading falls outside the percentage limits prescribed for the sieves other than 600 micron and 300 micron (IS) Sieves by not more than 5%, it should be re-graded as falling within this zone. The portion of the aggregate below 600 micron size are finer fines and they greatly influence the cohesiveness, water demand, workability and permeability as their specific surface per unit volume is much greater than other aggregate sizes.
- If a particular sieve size is not present in aggregates they are called gap graded. Sand, which has gap grading, is generally not preferred to the sand, which is uniformly graded. Gap grading can create deficiency in cohesiveness, permeability and surface finish.
- Organic matters in fine aggregates (sand) are usually found due to presence of vegetable matter. Even vary small fraction of organic matter will delay or prevent the hardening of concrete.
- Chlorides, if present in fine aggregates will not be harmful to concrete or mortar but will be harmful to the reinforcement or other steel embedment in concrete will accelerate rusting. Chlorides may also be present in cement, water and concrete additives. The total chloride content for long span bridges and pre-stressed structures should be limited to 0.10% by weight of cement and for reinforced concrete structures it should be limited to 0.15% by weight of cement.
- Alkali reactivity: If the aggregates proposed are alkali reactive then the presence of alkali oxides (Nao+0.658 K2O) in cement have to be carefully examined. Alkali oxides in cement should not be beyond permissible limits i.e. more than 1 %, which may cause cracking and disintegration of concrete in longer term.