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Concrete Technology II Chapter 7: Hardened Concrete

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45 Questions

What can lead to corrosion in concrete structures?

Plastic shrinkage cracks

Why is it essential to shelter the concrete surface from wind and sunshine during construction?

To reduce the rate of evaporation

Why is excessive water added to the concrete mix?

To improve the workability of concrete

What happens to the concrete volume as the excessive water evaporates?

It shrinks

What is the primary factor affecting the magnitude of the ultimate shrinkage?

Initial water content of the concrete

What percentage of the ultimate shrinkage occurs during the first year?

90%

Why does the shrinkage increase as the ratio of the surface to volume increases?

Because the larger surface area allows for faster moisture evaporation

What is the range of shrinkage strain values for plain concrete members?

0.0004 to 0.0007

How does reinforcement affect shrinkage in concrete members?

It reduces the shrinkage

What is the final state of concrete after the development of hydration?

Solid hardened state

What is the most important property of hardened concrete that is generally specified in construction design and quality control?

Compressive strength

What is the main reason for specifying compressive strength in construction design and quality control?

It is easy to measure

What is the typical failure mode of concrete under compression?

Compressive failure

What are the two types of specimens that can be used for uniaxial compression testing?

Cube and cylinder specimens

What is the standard size of the cube specimen in the cube specimen method?

150 × 150 × 150 mm

What is the definition of strength in the context of materials?

The ability of a material to resist stress without failure

How many times should each layer of the cube specimen be stroked with a hemispherical-tipped steel rod?

35 times

What is the primary reason for relating other properties to compressive strength?

It is believed that other properties can be deduced from strength data

What is the temperature range for curing the cube specimen?

20 ± 1°C

What is a common indicator of failure in concrete?

Cracking

What is the standard size of the cylinder specimen in the cylinder specimen method?

150 × 300 mm

How many times should each layer of the cylinder specimen be stroked with a hemispherical-tipped steel rod?

25 times

What is a characteristic of concrete that affects its failure modes under loading?

Presence of void spaces and microcracks

What is the length/diameter ratio of the cylinder specimen?

2

Why is grinding or capping necessary for the cylinder specimen?

To level and smooth the compression surface

What are the three categories of factors that affect the compressive strength of concrete?

Characteristics and proportions of materials, curing conditions, and testing parameters

In which region is the cube specimen method predominantly used?

Europe and China

What is the relationship between the increase in modulus of elasticity and compressive strength of concrete?

Modulus of elasticity increases approximately with the square root of the strength

What is the formula for modulus of elasticity of normal weight concrete according to ACI Building code 318-14?

Ec = 4.7√ƒ′𝑐

What is the typical range of Poisson's ratio for normal concrete?

0.15 to 0.20

What type of shrinkage occurs in concrete due to loss of mix water?

Plastic shrinkage

What is the main cause of plastic shrinkage in concrete?

Loss of mix water

What factor affects the rate of evaporation and plastic shrinkage?

All of the above

At what stage of concrete does plastic shrinkage occur?

Plastic stage

What is the range of density for normal weight concrete?

1500 to 2500 kg/m³

What is the alternative formula for modulus of elasticity according to ACI code?

Ec = 43 𝜌1.5√𝑓′𝑐 / 10^6

What are the two main causes of concrete deterioration?

Environmental factors and mechanical loading

What is the result of the coupling effect of environmental factors and loading on concrete structures?

Degradation of the structure

What is the common outcome of major durability problems in concrete structures?

Cracking and spalling of the concrete

What is the primary factor that affects the durability of concrete?

Permeability

What is permeability in concrete?

The property that governs the rate of flow of a fluid into a porous material under pressure

What is the effect of porosity on permeability in concrete?

It increases the permeability

What is the key difference between permeability and diffusivity in concrete?

Permeability is the rate of flow of water, while diffusivity is the rate of migration of ions or moisture

What is the condition under which permeability is measured in concrete?

When pores are completely filled with water

What is the condition under which diffusivity is measured in concrete?

Before saturation has been reached in the pores

Study Notes

Hardened Concrete

  • Hardened concrete is the final stage of concrete development, where it is ready to support external loads as a structural material.
  • The most important properties of hardened concrete include:
    • Various strengths
    • Dimension stability
    • Complete stress-strain relationship
    • Various moduli and Poisson's ratio
    • Durability

Strengths of Hardened Concrete

  • Strength is defined as the ability of a material to resist stress generated by an external force without failure.
  • Compressive strength is the most commonly specified property in construction design and quality control.
  • The reasons for this are:
    • It is relatively easy to measure
    • Other properties can be related to compressive strength and can be deduced from strength data

Compressive Strength and Corresponding Tests

  • There are two types of specimens used for uniaxial compression testing:
    • Cube specimen (used in Europe and China)
    • Cylinder specimen (used in North America)
  • The standard size of the cube specimen is 150 × 150 × 150 mm.
  • The standard cylinder size is 150 × 300 mm.
  • The cylinder specimen has a length/diameter ratio of 2.

Factors Affecting Compressive Strength

  • Factors affecting compressive strength can be categorized into three groups:
    • Characteristics and proportions of materials
    • Curing conditions
    • Testing parameters

Modulus of Elasticity

  • The modulus of elasticity of concrete increases approximately with the square root of the strength.
  • The ACI Building Code 318-14 gives the modulus of elasticity for normal weight concrete as:
    • Ec = 4.7√ƒ′c
  • The modulus of elasticity is also given by the ACI code as:
    • Ec = 43 ρ1.5√ƒ′c × 10^(-6)

Poisson's Ratio

  • Poisson's ratio is the ratio between lateral strain to longitudinal strain.
  • It is generally denoted by the letter μ.
  • For normal concrete, the value of Poisson's ratio lies in the range of 0.15 to 0.20.

Shrinkage

  • There are several types of shrinkage that can occur in concrete, including:
    • Thermal shrinkage
    • Plastic shrinkage
    • Autogenous shrinkage
    • Chemical shrinkage
    • Drying shrinkage
  • Plastic shrinkage occurs at a very early age of concrete, while the concrete underneath is still in the plastic stage.
  • The rate of evaporation depends on ambient conditions such as temperature, exposure to sun, wind speed, and relative humidity.
  • Drying shrinkage occurs when the concrete begins to dry and the excessive water that has not reacted with the cement begins to migrate from the interior of the concrete mass to the surface.

Durability

  • The durability of concrete depends on its permeability and diffusivity.
  • Permeability is defined as the property that governs the rate of flow of a fluid into a porous material under pressure.
  • Diffusivity is defined as the rate of migration of ions or moisture in concrete under the action of a higher concentration side to lower concentration side.
  • The common causes of concrete deterioration include:
    • Corrosion of the reinforcing steel
    • Freeze/thaw damage
    • Salt scaling
    • Alkali-aggregate reactions
    • Sulfate attack

This quiz covers the properties and states of concrete, from fluid to solid, as it undergoes hydration. It's designed for 2nd-year Civil Engineering students.

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