Effect of porosity
- It is a primary factor that governs the strength of concrete.
- Strength of concrete decreases with the increase in porosity.
- Permeability of concrete increase with increase in porosity of concrete.
- Different factors such as water to cementitious materials ratio, degree of hydration, air content, consolidation, mineral admixtures, aggregate, reaction between aggregate and cement paste, pozzolanic material admixture, etc. affect concrete porosity.
- Increase in water to cementitious materials ratio increases porosity.
- For achieving high durability, concrete porosity should be kept low so as to reduce its permeability.
- The magnitude of porosity is such that, for the usual range of w/c ratios, the cement paste is only about ‘half solid’. For instance, at a w/c ratio of 0.6, the total volume of pores is between 47 and 60 percent of the total volume of the cement paste, depending on the degree of hydration.
Effect of water cement ratio
- The W/c ratio is an index of the strength of concrete. The strength of cement paste increases with an increase in cement concrete and decreases with water content and air content.
- For the fully compacted concrete at a given age and thermal temperature, its strength is inversely proportional to the w/c ratio.
- In the figure below, the strength vs. w/c ratio shows that a lower w/c ratio could be used when the concrete is vibrated to get higher strength whereas a comparatively higher w/c ratio is required when concrete is hand compacted or manually compacted.
- In both cases, when the w/c ratio is below the practical limit, the strength of concrete falls rapidly due to the introduction of air voids.
- The relation between the compressive strength of concrete and the w/c ratio is introduced by Abram’s law.
Abram’s law
He states that ‘’with given materials and condition of test, the quantity of mixing water to the quantity of cement alone determines the strength of concrete and it is independent of aggregate cement ratio, so long as the mix is of workable plasticity.
Mathematically,
C = A/Bx
Where, C = Compressive strength of concrete
A,B = Empirical constants
x = Water cement ratio
Effect of aggregate size
- Aggregate is an important factor Affecting concrete strength. The most important properties of concrete are shape, texture, and size of aggregate.
- Maximum size affects the strength in several ways. Larger particles reduce the specific surface area of aggregate which leads to a reduction in bond. Also, large particles tend to more strain volume change in cement paste and induce more internal stress which weakens the concrete. The aggregate effects can be offset by reducing the water content. In general, at a constant w/c ratio, higher strength can be obtained by using the linear mixture. If constant workability is maintained, strength will increase with cement content.
- High strength concrete or rich mix concrete is adversely affected by the use of large size aggregates. When large size aggregate is used due to internal bleeding in the transition zone, it will become much weak by developing micro cracks with lower compressive strength.
I hope this article on “Effect of water cement ratio on compressive strength of concrete” remains helpful for you.
Happy Learning – Civil Concept
Contributed by,
Civil Engineer – Ranjeet Sahani
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