The document discusses the strength of concrete and factors that influence it. It states that concrete is strong in compression but weak in tension. The principal factors that affect the strength of concrete are the water-cement ratio, quality of materials used, degree of compaction, and curing process. Non-destructive testing methods like rebound hammer, ultrasonic pulse velocity, and covermeter tests can be used to evaluate the in-place strength of hardened concrete without damaging it.

2. Strength of concrete
 Concrete is not strong in tension – you can pull it apart easily.
 Strong in compression – when you push it, its relatively stronger.
 Strength of concrete – emphasize on the compressive strength of concrete. We also discuss on the
tensile strength of concrete.
 Variation in strength of concrete:
 Variation in the quality of the materials used
 Variation in the mix proportions due to the batching process
 Variation due to sampling and testing

3. Strength of concrete : General Outline
 Cube Strength
 Porosity
 Role of w/c
Experiment results indicate the principal sources of strength variation:
• Water Cement Ratio
• Moisture Content
• Quality of cement
• Storage of cement
• Degree of Compaction
• Curing of Concrete
• Moulding Temperature
• Curing Temperature
• The moisture content at the time of testing
• Rate of application of load
• Duration of Load
• Degree of lateral restraint
• Compression machine and operation factors

4. Cube Strength
 Strength of concrete is required for structural design – determined through specimen testing
 Cube strength is a relative measure of the mix that we are using.
Water-Cement Ratio
 Control of Water Cement Ratio involves adjustments in cement and water content.
 Errors relating to Water Cement Ratio will be reflected in the strength and non-uniformity of the
mix.
 Water Cement Ratio strength relationship is different for different types of cement.

5. Porosity
 Strength of concrete increases as porosity and pore size decreases.
 Strength is a function of water cement ratio
 Abram’s Law
 Pores in cement based material
 Gel pores – micro pores – shrinkage and creep
 Capillary meso pores – mechanical and durability
 Macro pores due to air entrainment and poor compaction.
 w/c ratio is the fundamental factor that governs the strength of concrete. Higher w/c ratio – increase
in capillary pores – increase in pore size – crack will start and propagate faster.
 High strength concrete – reduce water cement ratio

6. Strength of Concrete: Aggregate Contribution
 Aggregate paste bond is very important.
 ITZ – Interfacial Transition Zone -the region of the cement paste around the aggregate particles, which
is perturbed by the presence of the aggregate.
 ITZ governs the concrete strength.
 Roughness of the surface of the aggregate.
 Pozzolanic aggregate – Fly ash modified. Silica fume
 w/c ratio
 Mechanical, Physical or Chemical Bond

7. Factors Affecting Strength of Concrete

8. Non-Destructive Testing in Concrete
 Non destructive test is a method of testing existing concrete structures to assess the strength and
durability.
 In the non destructive method of testing, without loading the specimen to failure i.e. without
destructing the concrete we can measure the different parameters.
 Inspect or measure without doing any harm.
 Objectives of NDT in concrete:
 Strength of concrete
 Cracks in concrete
 Delamination or voids behind a concrete surface.

9. Non-Destructive Testing in Concrete
 Non destructive test is a method of testing existing concrete structures to assess the strength and
durability.
 In the non destructive method of testing, without loading the specimen to failure i.e. without
destructing the concrete we can measure the different parameters.
 Inspect or measure without doing any harm.
 Objectives of NDT in concrete:
 Strength of concrete
 Cracks in concrete
 Delamination or voids behind a concrete surface.
 Thickness of a concrete layer.
 Permeability of concrete.
 Corrosion of reinforcement in concrete.
 Alkali aggregate reaction in concrete.

10. Non-Destructive Testing in Concrete

11. Assessing the likely compressive strength of concrete with the help of suitable correlations between
rebound number and compressive strength.
(IS 13311( Part 2 ) : 1992)

12. Principle of working :
When the plunger of rebound
hammer is pressed against the
surface of the concrete, the spring-
controlled mass rebounds and the
extent of such rebound depends
upon the surface hardness of
concrete.
The surface hardness and
therefore the rebound is taken to be
related to the compressive strength
of the concrete. The rebound is read
off along a graduated scale and is
designated as the rebound number
or rebound index.

15. (IS 13311( Part 1 ) : 1992)

16.  An ultrasonic wave is passed through the concrete surface and the time it takes to pass through this
distance, let us say, d, this gives some velocity which is d by t and this velocity is a measure of the
quality of concrete or the strength of concrete.
 Technique of measuring pulse velocity
through concrete
1. Direct transmission
2. Indirect transmission
3. Surface transmission
 Factors affecting the measurement of pulse
velocity
1.Smoothness of contact surface under test
2. Influence of Path length on pulse velocity
3.Temperature of concrete
4.Moisture condition of concrete
5.Presence of reinforcing steel

17. A pullout test measures the force required to pull out specially shaped rod whose enlarged end has been cast
in to the concrete. The force required to pull out denotes the strength of concrete.

18. A modified version of the
pull out test.
Not involving any pre-
embedded probes.

19. Concrete Cover Test
 COVERMETER is a device used to determine the precise concrete cover depth and to pinpoint the
exact location of the rebars in the concrete.
 Used to locate steel reinforcing bar in concrete and to estimate the thickness of the concrete cover
over the reinforcement.
 Eddy-current principle with pulse-induction
The presence of reinforcing bars(rebars)
will results in the development of more
amount of magnetic flux and thus the
rebars can be located.

20. Measuring Crack Width
 Recording crack width using a calibrated magnifying glass.
 Accurate measurement.
 Tedious to carry out and require close access to the structure.
 Glass strips can be fixed across cracks to identify live or dead cracks.

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