UNIT 2

Characteristics of good concrete

 It should be hard
 It should be durable
 The compressive strength of concrete should not be less than 15.5 N/mm2
 The density of good concrete should be about 24 KN/m3
 It should be economical for the desired strength
 Resistance to abrasion
 Resistance to the effect of weathering agencies
 It should provide the required finish to the concrete structure
 It should have minimum thermal expansion

Workability

The workability of freshly mixed concrete is the property which determines the ease and
homogeneity with which it can be mixed, placed, compacted and finished

The concrete can be said to be workable if it can be easily mixed, placed and compacted
to make it strong and dense

Factors effecting the workability

1. Size of aggregate
2. Shape of aggregate
3. Surface texture of aggregate
4. Water cement ratio
5. Grading of aggregate
6. Use of admixtures
7. Environmental condition

1. Size of aggregate
Workability increases wit increase in maximum size of aggregate because total surface
area will be lower

2. Shape of aggregate
Workability increases with the use of aggregate which are rounded in shape because it
has less surface area and less voids than angular or flaky aggregate
 3. Surface texture of aggregate
Smooth or glassy textured aggregate will give better workability and rough textured
aggregate will show poor workability

4. Water cement ratio

Higher the water cement ratio, the higher will be the workability of concrete

5. Grading of aggregate
A well graded aggregate is the one which has least amount of voids and higher the
workability

6. Use of Admixtures
The workability may be considerably affected by the presence and nature of admixtures

7. Environmental conditions
The ambient temperature affects the workability of concrete. On a hot day it becomes
necessary to increase the water content of the concrete mix in order to maintain the
desired mix

Measurement of workability

The following test are commonly employed to measure workability

1. Slump test
2. Compaction factor test
3. Vee- Bee consist meter test
4. Flow test
5. Kelly ball test
6. Slump test
 The slump cone is to be placed on a water tight leveled platform and fresh concrete is
placed in it in four equal layer
 Each layer is to be tamped with twenty five strokes of the rounded end of the tamping rod
of steel 16 mm dia and 600 mm long
 After completely filling the cone with concrete , cone should be lifted vertically without
any jerk
 The difference between the height of the mould and that of the highest point of specimen
measured is known as slump.
The following slumps are recommended for different works
Vibrated concrete - 10 to 25 mm
 Mass concrete - 25 to 50 mm
Road work - 25 to 50 mm
Ordinary beams and slabs - 50 to 100 mm
Columns , retaining walls - 75 to 125 mm

Compaction factor test

 The apparatus consist essentially of two hoppers, each in the shape of frustum of cone
and one cylinder, the three being one over the other
 The upper hopper is filled with fresh concrete by placing gently without compaction
 The door at the bottom of the upper hopper is then released and the concrete falls into the
lower hopper
 Then the door of the lower hopper is released and the concrete falls into the cylinder
 Excess concrete is cut by a float across the top of the mould and net weight of concrete in
the cylinder is determined ( partially compacted concrete).
 Then the concrete is removed from the cylinder and is refilled in the cylinder in four
layers each being tamped for obtaining full compaction
 Then the net weight of fully compacted concrete is determined.
 The ratio of the weight of partially compacted concrete to the weight of fully compacted
concrete gives the compaction factor for the water cement ratio.

The values of compaction factors for different workabilities are given

Compaction factor Degree of workability

0.78 Very low

0.85 Low

0.92 Medium

0.95 High

Vee Bee test

 Place the slump cone in the cylindrical container of the consistometer. Fill the cone in
four layers, each approximately one quarter of the height of the cone. Tamp each layer
with twenty-five strokes of the rounded end of the tamping rod. The strokes are
distributed in a uniform manner over the cross-section of the cone and for the second and
subsequent layers the tamping bar should penetrate into the underlying layer. After the
top layer has been tamped, struck off level the concrete with a trowel making the cone
exactly filled.
  Move the glass disc attached to the swivel arm and place it just on the top of the slump
cone in the cylindrical container. Adjust the glass disc so as to touch the top of the
concrete cone, and note the initial reading on the graduated rod.
 Remove the cone from the concrete immediately by raising it slowly and carefully in the
vertical direction. Lower the transparent disc on the top of concrete. Note down the
reading on the graduated rod.
 Determine the slump by taking the difference between the readings on the graduated rod
 Switch on the electrical vibrations and start the stopwatch. Allow the concrete to remould
by spreading out in the cylindrical container. The vibrations are continued until the
concrete is completely remoulded, i.e, the surfaces becomes horizontal and the whole
concrete surface adheres uniformly to the transparent disc.
 Record the time required for complete remoulding seconds which measures the
workability expressed as number of Vee-Bee seconds.

Concrete Preparation

1. Batching
2. Mixing
3. transporting
4. placing
5. compacting
6. Finishing
7. curing

Properties of hardened concrete

 strength is defined as the resistance of a hardened concrete to rupture under different loadings
and is accordingly designated in different ways i.e., tensile strength, compressive strength,
flexural strength, etc.
 Tensile strength is the capacity of a material or structure to withstand loads tending to elongate,
as opposed to compressive strength, which withstands loads tending to reduce size
 The compressive strength of concrete is given in terms of the characteristic compressive
strength of 150 mm size cubes tested at 28 days (fck).
 Flexural strength, also known as modulus of rupture, or bend strength, or transverse rupture
strength is a material property, defined as the stress in a material just before it yields in a flexure
test. The flexural strength would be the same as the tensile strength if the material
were homogeneous

Following are the factors that affect the strength of concrete:

 Stiffness
 Poissons ratio
 Fatigue
 Elasticity
 Poissons ratio
 Creep
 Shrinkage
 Ductility
 Durability
 Bond strength
 Impact
 Modular ratio

Stiffness

It is the rigidity of an object — the extent to which it resists deformation in response to an

applied force. The complementary concept is flexibility or pliability: the more flexible an object
is, the less stiff it is

Poisson's ratio

It is the ratio of the relative contraction strain (transverse, lateral or radial strain) normal
to the applied load - to the relative extension strain (or axial strain) in the direction of the applied
load

fatigue

It refers to the phenomenon of rupture under repeated loadings, each of which is smaller
than a single static load that exceeds the strength of the material.

Elasticity.

Though hardened concrete is a brittle / material, it is desired that it should possess

adequate elasticity.

Poisson's Ratio

Poissons ratio can be expressed as the ratio of transverse strain to the longitudinal or axial
strain

μ = - εt / εl

Creep

Concrete creep is defined as deformation of structure under sustained load. Basically,

long term pressure or stress on concrete can make it change shape.
Shrinkage

Concrete is subjected to changes in volume either autogenous or induced. Volume change

of concrete affects the long-term strength and durability and causes cracks in concrete is called
shrinkage

Ductility

It is a measure of a material's ability to undergo significant plastic deformation before rupture,

which may be expressed as percent elongation or percent area reduction from a tensile test.

Durability

It is defined as the period of time up to which concrete in hardened state withstands the
weathering effects satisfactorily.

Bond strength

It is a measure of transfer of load between the concrete and reinforcement

Impact

It is the properties of a concrete which resistance to sudden shock or load

Modular ratio

It is defined as the ratio between Modulas of Elasticity of steel and Modulas of Elasticity
of concrete

Modulus of Elasticity

Modulas of Elasticity or Young's modulus is defined as the tendency of an object to

deform along an axis when opposing forces are applied along that axis;

It is defined as the ratio of tensile stress to tensile strain. It is often referred to simply as
the elastic modulus.

Testing of Hardened concrete

1. Compressive strength test – cube and cylinder
2. Tensile strength test
3. Flextural strength test