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1. STRENGTH OF
CONCRETE

2. WATER CEMENT RATIO
• Water/cement law (also known Abram law) – according to this
law, strength of cement can be calculated by - S=𝐴/𝐵 𝑥
• Where, x= water/cement ratio by volume
and, A and B are constant 14000 lbs/sq. in. And 7
respectively.
• It states that the strength of concrete is only dependent upon
water/cement ratio provided the mix is workable.
• This law was given by duff abram.

3. • Strength of concrete depends upon the strength of cement paste and
strength of cement paste depends upon dilution of paste or in simple
terms.
• Strength of cement paste increases with cement content and
decreases with air and water content.
• In this graph, when water
cement ratio is below
practical limit the strength
of concrete falls rapidly
due to introduction of air
voids.
• It can be seen that lower
water cement ratio can be
used when concrete is
vibrated to achieve higher
strength, whereas
comparatively higher water
cement ratio is required
when concrete is hand
compacted.

4. GAIN OF STRENGTH WITH AGE
• Concrete develops strength with continued hydration. Rate of
gain of strength is faster to start with and rate get reduced with
age.
• It is customary to assume the 28 days strength as full strength
of concrete.
• But actually, concrete develops strength beyond 28 days also.
Earlier codes have not permitted to consider this increase of
strength beyond 28 days for design purpose.

5. • Increase in strength beyond 28 days used to get immersed with
factor of safety.
• With better understanding of material, progressive designer
have been trying to reduce factor of safety and make structure
more economical.
• In this, the increase in strength beyond 28 day is taken into
account.

6. RELATION BETWEEN COMPRESSIVE AND
TENSILE STRENGTH
• It is seen that strength of concrete is compression and
tensile(both direct and flexural ) closely related, but the
relationship is not direct proportionality.
• The ratio of two strength depends on general level of strength of
concrete.
• For higher compressive strength concrete shows higher
compressive strength, but the rate of increase of tensile strength
is of decreasing order.
• In reinforced concrete construction the strength of concrete in

7. • But the design of concrete pavement slabs in often based on
the flexural strength of concrete.
• Hence, it is necessary to access the flexural strength of
concrete either from compressive strength or independently.
• The type of coarse aggregate influences this relation – Crushed
aggregate gives relatively higher flexural strength than
compressive strength.
• Use of pozzolanic material increases the tensile strength of
concrete.
The relation between compressive strength and tensile strength
can be calculated by - flexural strength = 0.7 𝑓𝐶𝑘
Where 𝑓𝐶𝑘 is the characteristic compression strength of concrete

8. BOND STRENGTH
• The bond strength of concrete can be seen from two different
angle-
1. Bond strength between paste and steel reinforcement
2. Bond strength between paste and aggregate

9. BOND STRENGTH BETWEEN PASTE AND
STEEL REINFORCEMENT
• Bond strength arises primarily from the friction and adhesion
between concrete and steel.
• The roughness of the steel surface is also one of the factor
applying bond strength.
• The bond strength of concrete is function of the compression
strength and is approx. proportional to a compressive strength
upto 20MPa .

10. • Bond strength is also function of specific surface of gel.
• Cement which consist of higher percentage of C2S will give
higher bond strength. On other hand, concrete containing more
C3S (or concrete curved at
high temperature) results in smaller specific surface of gel
which gives lower bond strength

11. AGGREGATE-CEMENT BOND STRENGTH
• The strength of concrete of a whole is depending upon the
strength (bond strength ) of the hydrated hardened cement
paste (HCP)
• The strength of HCP is depending upon water/cement ratio
which determines the quality continuity density porosity of the
product of hydration in particular the C-S-H gel