1. How water -cement ratio affects:. Durability of concrete 2. Strength of concr

ID: 2997310 • Letter: 1

Question

1. How water -cement ratio affects:. Durability of concrete 2. Strength of concrete 3. Consistency of concrete.What is the use of slump test? Specify ASTM Code values. 2. What is air -entrainment test? Where air -entrainment is needed? How does it affect the strength od concrete? 3. What are Type 1, 11,111,IV and V of concrete? 4. What is reinforced concrete? 5. What is pre -stressed concrete? 6. What are admixtures , normally used for resistance of concrete to freezing and thawing? 7. What are the main ingredients of concrete? 8. What is the difference between fine and coarse aggregates? 9. What range of fineness modulus you like to see in a 25-MPa strength concrete?

Explanation / Answer

A lower water-cement ratio leads to higher strength and durability, but may make the mix more difficult to place.

Too much water will result in segregation of the sand and aggregate components from the cement paste. Also, water that is not consumed by the hydration reaction may leave the concrete as it hardens, resulting in microscopic pores (bleeding) that will reduce the final strength of the concrete. A mix with too much water will experience more shrinkage as the excess water leaves, resulting in internal cracks and visible fractures (particularly around inside corners) which again will reduce the final strength.

Consistency increases with water to cement ratio. But we need standard consistency which can we get when vicat plunger penetrates mould to 5-7mm from bottom.

The concrete slump test is an empirical test that measures the workability of fresh concrete.

Standards specify a slump cone height of 300 mm, a bottom diameter of 200 mm and a top diameter of 100 mm. The slump measured should be recorded in mm of subsidence of the specimen during the test.

5 classes of common cement that comprise Portland cement as a main constituent. These classes differ from the ASTM classes.

Portland cement

Comprising Portland cement and up to 5% of minor additional constituents

Portland-composite cement

Portland cement and up to 35% of other single constituents

III

Blast furnace cement

Portland cement and higher percentages of blast furnace slag

Pozzolanic cement

Portland cement and up to 55% of pozzolanic constituents(volcanic ash)

Composite cement

Portland cement, blast furnace slag or fly ash and pozzolana

Reinforced concrete is a composite material in which concrete's relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength and/or ductility. The reinforcement is usually, though not necessarily, steel reinforcing bars (rebar) and is usually embedded passively in the concrete before the concrete sets. Reinforcing schemes are generally designed to resist tensile stresses in particular regions of the concrete that might cause unacceptable cracking and/or structural failure.

Prestressed concrete is a method for overcoming concrete's natural weakness in tension. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebars, inside poured concrete. Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded or unbonded post-tensioned concrete.

Ingredient of concrete