Engineering materials

1. Constituents of concrete-Coarse Aggregate
Presented by: H.M.A.Mahzuz

2. 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 Aggregate
Diameter > 4.75mm

3. 08/27/17 CE- Properties of Concrete 3
Coarse Aggregate
Acts as main filler, forms the main bulk mass
Binding material adheres on its surface
Imparts volumetric stability and durability to concrete
Increases crushing strength, resistance to wear and tear, and
water tightness of concrete
Economizes concrete since it is cheaper than cement

4. 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 size
5)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.

5. 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 Aggregate
The 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.

6. 6
Irregular or Partly Rounded Aggregate
The 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 Aggregate
The 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.

7. Flaky Aggregate
The 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 Aggregate
The aggregate is said to be elongated when its length is greater than 180% of its
mean dimension.
Flaky & Elongated Aggregate
Aggregate 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.

9. 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.

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

11. 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.

12. Example of a lab test result of coarse aggregate:

13. 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.

14. Thank you