What are Properties of Hardened Concrete|Governing Factors

Properties of Hardened Concrete

Properties of hardened concrete are explained on the basis of its Strength and Performance under various types of force actions and conditions. Main parameters that are used to define the strength of concrete are

Bridge Pier Substructure

Bridge Pier Substructure


Compressive Strength: Compressive strength of concrete is usually defined as characteristic compressive strength (fcu / fck). Which is obtained by cube test or cylinder test of concrete specimen after 28 days of curing. Compressive strength of concrete varies from 10N/mm2 to 500N/mm2 for ultra high strength concrete (2000-8000 psi). This is the main dimension of the material which a design engineer is concerned of.

Tensile Strength: Concrete is very weak in tension, it is so weak that the tensile strength of concrete is usually ignored in the design of Reinforced concrete structures. It is utilized in the design of prestressed concrete design. Tensile strength of concrete is usually calculated using the formula 0.7 sqrt (fck or fcu). The determination of tensile strength of concrete is necessary to determine the load at which the concrete members may crack. The cracking is a form of tension failure. Tensile strength of concrete ranges from 1N/mm2 to 10 N/mm2

Factors Affecting Strength

Various factors that affect the strength properties of hardened concrete are as follows

  1. Curing conditions, humidity
  2. Temperature
  3. w/c , (inversely related) Abram’s law
  4. Air content, (inversely related), short and long term
  5. Aggregate characteristics, roughness,grading, mineralogical.
  6. Cement type, composition, fineness, type I vs. type III
  7. Cement content (directly related)
  8. Strength porosity relationship
  9. Mixing water

Hardened Concrete Properties

Factors Affecting Strength of Concrete in Service Stage

  1. Resistance to freezing and thawing
  2. Cracking
  3. Internal Problems
  4. Rebar Corrosion

Resistance to freezing and thawing-Major factors

  • Air Entrainment
  • w/c, low water -cement ratio/ water content
  • volume stability, stiff aggregates with low coefficient of thermal expansion

Air Entrainment

  1. Water gains 9% in volume upon freezing
  2. nighttime freezing followed by daytime thawing,
  3. approximately 40 cycles per year, average. max of 200 cycles per year.
  4. fatigue loading of ice formation within pores
  5. Air Entraining Admixture (AEA) Must provide:
  • Pore size
  • Pore spacing
  • Pore specific surface area

Durability-Cracking

path for harmful material to get into concrete

sulfates- soils

  • cause severe expansion, and deterioration

chlorides -deicing salts

  • initiate corrosion

Excessive shrinkage

Corrosion Protection

  • Proper cover of at least 2”
  • lower w/c
  • denser concrete
  • avoid using chlorides

Strength

Durability

Water Tightness

  • water cement ratio
  • Permeability

Volume Stability

  • shrinkage deformation with no load applied
  • creep deformation under sustained loading

Consequences of creep

  • Loss in pre-stress
  • possibility of excessive deflection
  • stressing of non load bearing members

Economy

Cement Content

  • 50-60$/ton

Aggregates

  • 5-6 $/ton

minimum cement required at the minimum water cement ratio, with the maximum strength and durability

 

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