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Strength of concrete (for civil engineering)
- 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