Properties of aggregates

• Introduction• Strength of aggregate• Particle shape and texture• Grading• Moisture content• Classification of aggregate

Aggregates

• Aggregates commonly considered as inert filler material which occupy 60 to 80 percent of the volume of concrete/70 to 85 percent of the weight of concrete.

• Sand, gravel and crushed stone are the primary aggregates used.

• All aggregates must be essentially free of silt (size 60 - 2 m) and/or organic matter.

Aggregates

• Coarse aggregates > 5 mm

• Fine aggregates < 5 mm

• The physical and mineralogical properties of aggregates must be known before mixing concrete to obtain a desirable mixture.

Fine aggregateFine aggregate

Natural sand, crushed stone dust or manufactured sand by Natural sand, crushed stone dust or manufactured sand by

crushing rockcrushing rock

Material passing 75 µm sieve should not be 4% by weightMaterial passing 75 µm sieve should not be 4% by weight

Sand (sea, river, lake or pit) should be properly sieved, washed Sand (sea, river, lake or pit) should be properly sieved, washed

and tested for total percentage of clay, silt, salt etc.,and tested for total percentage of clay, silt, salt etc.,

Coarse aggregateCoarse aggregate

Maximum size 200 mm for mass concreteMaximum size 200 mm for mass concrete

Nominal size of 20 mm for normal concreteNominal size of 20 mm for normal concrete

Moisture content 0.25 % - 4%Moisture content 0.25 % - 4%

Bulk density 1500 – 1600 kg/mBulk density 1500 – 1600 kg/m33

Properties

• Strength – Compressive strength• Tests Aggregate crushing value Aggregate impact value 10% fines value (12.5 – 10 mm size)

• Soundness: Reaction of aggregates with sulfate.• Bulk density kg/m3

Mechanical properties

• Toughness – Resistance of aggregate to failure by impact (AIV, 10% fines)

• Hardness – Resistance to wear (aggregate abrasion value test (Los Angeles test))

• AAV< 30% for wearing surfaces • AAV<50% for non-wearing surfaces

Properties

Mechanical properties BS 882:1992

Type of concrete

10% fines (min.) kN

AIV (max.) %

Heavy duty concrete floors

150 25

Pavement wearing surfaces

100 30

Others 50 45

Particle shape and texture• It will affects the properties of fresh concrete

• Natural sands and gravel from river beds are smooth and rounded in shape, hence concrete will be high workable.

• Crushed stone --- rough angular or elongated -- low workability

• High surface to volume ratio, better bond characteristics but require more cement paste to produce workable mixture.

Classification of particle shape

• As far as coarse aggregate concerned, flaky and elongated particles are undesirable.

• The mass of flaky particles expressed as a percentage of the mass of the sample is called the flakiness index.

• Shall not exceed 60 for uncrushed gravel and 40 for crushed rock or gravel

Grading of aggregate• It determines the paste requirement for workable concrete.

• This paste requirement is the factor controlling the cost since cement is most expensive component

• Therefore, it is desirable to minimize the amount of paste consistent with the production of concrete that can be handled, compacted, and finish while providing the necessary strength and durability.

• Required cement paste ---- amount of void space filled and total surface area to be covered.

• When uniform sizes are used, the spacing is the greatest

• When range of sizes are used, void spaces are filled and paste required is lowered but less workable concrete.

• Hence, compromise between workability and economy is necessary.

Sieve analysis of sand – limits (BS 882:1992)Sieve analysis of sand – limits (BS 882:1992)

Sieve analysis of sand – BS 882 (coarse)Sieve analysis of sand – BS 882 (coarse)

Sieve analysis of sand – BS 882 (medium)Sieve analysis of sand – BS 882 (medium)

Sieve analysis of sand – BS 882 (fine)Sieve analysis of sand – BS 882 (fine)

Sieve analysis of coarse aggregate – limits Sieve analysis of coarse aggregate – limits (BS 882:1992)(BS 882:1992)

For example: a 20 mm single size aggregates means an aggregate most of which passes through a 20 mm sieve and major portion is retained on 10 mm sieve.

Moisture content

• Important factor when developing the proper W/C ratio.

• All the aggregates contain some moisture based on the porosity of the particles and the moisture condition of the storage area (1% - 40%).

Moisture states

• Oven dry (OD) – all moisture is removed from the aggregate by heating in an oven 105 C to constant weight.

• Air dry (AD) – all moisture removed from surface, but internal pores partially full.

• Saturated surface dry (SSD) – all pores filled with water, but no film of water on the surface.

• Wet – All pores completely filled with water with a water film on the surface.

Classification of aggregates

• Based on bulk density

• Light weight, normal weight and heavy weight

• Normal concrete (1520 – 1680 kg/m3)

• Lightweight concrete (less than 1100 kg/m3)

• Heavyweight concrete (more than 2080 kg/m3)

Concrete admixtures Concrete admixtures When concrete is made, it is possible to add various substances that could provide desirable properties. They are called admixtures that could improve

Workability (Super plasticizes)

Reduce the amount of water needed for a given workability (water–reducing admixes)

Increase the time required for setting of cement (retarders)

Reduce the time required for setting by increasing the rate of hydration of cement while generating additional heat (accelerates)

Admixtures

• Admixtures are ingredients other than portland cement, water, and aggregates.

• Admixtures are added to the concrete mixture immediately before or during mixing.

Retarding admixtures: • are used to slow the rate of concrete

hardening.

• They are useful for concrete that is placed during hot weather.

Accelerating admixtures

• such as calcium chloride, are used to increase the rate of hardening--usually during cold weather.

WaterWater

Water to be used in the concrete work should have the following

properties:

It should be free from injurious amounts of acids or alkalis or other

such organic or inorganic impurities.

It should be fit for drinking purposes