Lecture 4, constituents of concrete-coarse aggregate

Constituents of concrete-Coarse Aggregate

Presented by: H.M.A.Mahzuz

Aggregates

08/27/17 CE- Properties of Concrete 2

Granular materials, such as sand, gravel, crushed stone used with cementing medium to form concrete or mortar.

Aggregates act as a relatively inexpensive inert filler, providing stability against volume changes and influencing strength.

Fine Aggregate Diameter< 4.75mm

Coarse AggregateDiameter > 4.75mm

08/27/17 CE- Properties of Concrete 3

Coarse AggregateActs as main filler, forms the main bulk massBinding material adheres on its surfaceImparts volumetric stability and durability to concreteIncreases crushing strength, resistance to wear and tear, and water tightness of concreteEconomizes concrete since it is cheaper than cement

Expected stone quality for concrete: 1)It should be sufficient compressive strength, toughness against impact load, 2)Should have uniform colour.3)free from dirt, organic content, salt.4)20 mm downgraded , should have different size5)Non reactive to cement.6)water absorption should not exceed 3%, impermeable, very low-electrical conductivity, resistant to abrasion. 7)Unit weight should be around 1500 kg/m3.

According to shape the aggregate is classified as•Rounded aggregate•Irregular or partly rounded aggregate•Angular aggregate•Flaky aggregate•Elongated aggregate•Flaky and elongated aggregate

Rounded AggregateThe aggregate with rounded shape has the minimum percentage of voids ranging from 32 to 33%. It gives minimum ratio of surface area to given volume and hence requires minimum water for lubrication. It gives good workability for the given amount of water and hence needs less cement for a given water cement ratio. The only disadvantages is that the interlocking between its particles is less and hence the development of bond is poor. This is why rounded aggregate is not suitable for high strength concrete and for pavements subjected to tension.

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Irregular or Partly Rounded AggregateThe aggregate with irregular shape has higher percentage of voids ranging from 35 to 37%. It gives lesser workability than rounded aggregate for the given water content. Water requirement is higher and hence more cement is needed for constant water cement ratio. The interlocking between aggregate particles is better than rounded aggregate but not adequate to be used for high strength concrete and pavements subjected to tension.

Angular AggregateThe aggregate with angular shape has the maximum percentage of void ranging from 38 to 45%. It requires more water for lubrication and hence it gives least workability for the given water cement ratio. For constant water cement ratio and workability the requirement of cement increase. The interlocking between the aggregate particles is the best and hence the development of bond is very good. This is why angular aggregate is very suitable for high strength concrete and for pavements subjected to tension.

Flaky AggregateThe aggregate is said to be flaky when its least dimension is less than 3/5th (or 60%) of its mean dimension. Mean dimension is the average size through which th particles pass and the sieve size on which these are retained. For example, mean size of the particles passing through 25 mm sieve and retained on 20 mm sieve is (20+25)/2=22.5 mm. if the least dimension is less than 3/5 x (22.5) = 13.5 mm, then the material is classified as flaky. Flaky aggregate tends to be oriented in one plane which affects the durability.

Elongated AggregateThe aggregate is said to be elongated when its length is greater than 180% of its mean dimension.

Flaky & Elongated AggregateAggregate is said to be flaky and elongated when it satisfies both the above conditions. Generally elongated or flaky particles in excess of 10 to 15% are not desirable.

Angular aggregate are superior to rounded aggregate from the following two points of view:

•Angular aggregate exhibit a better interlocking effect in concrete, which property makes it superior in concrete used for roads and pavements.

•The total surface area of rough textured angular aggregate is more than smooth rounded aggregate for the given volume. By having greater surface area, the angular aggregate may show higher bond strength than rounded aggregate.

•The higher surface area of angular aggregate with rough texture requires more water for a given workability than rounded aggregate. This means that for a given set of conditions from the point of view of water-cement ratio and consequent strength, rounded aggregate gives higher strength.

•As a conclusion we can say that:-•For a water-cement ratio below 0.4, the use of crushed aggregate has resulted in strength upto 38% higher than rounded aggregate. With an increase in water-cement ratio the influence of roughness of surface of the aggregate gets reduced, presumably because the strength of the paste itself becomes paramount, and at a water-cement ratio of 0.65, no difference in strength of concrete made with angular aggregate on rounded aggregate has been observed.

Stone availability in Bangladesh

In Bangladesh quality stone is found in:1)Jaflolg2)Volagonj (20 Km West of jaflong)3)Small quantities of stone is found in Chittagong hill tracts, Mymensing.4)1st class brick chips.

Recycled coarse aggregate:

Concrete aggregate collected from demolished concrete is put through a is called recycled coarse aggregate. It can be used in concrete, pavement construction, etc.

These must be made free from the cement and mortar normally attaching the surface.

Example of a lab test result of coarse aggregate:

Brick chips as coarse aggregate:•In concrete good quality Brick chips can be used.

•High strength is hard to achieve.

•Absorbs water in concrete which can harm reinforcement.

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