Glossary of terms relating to cement concrete, Part 11: Prestressed concrete

IS 6461 (Part 11) - Scope Summary

• Scope: This part of IS 6461 specifically deals with prestressed concrete (Part XI of the glossary on cement concrete).
• It covers definitions, materials, structural aspects, and testing relevant to prestressed concrete.
• The standard is part of a 12-part series covering comprehensive cement concrete topics (aggregates, reinforcement, formwork, properties, etc.).

Key Points on Scope:

• Applies to prestressed concrete design, materials, and construction.
• Includes definitions related to prestressing such as wobble friction (Clause 2.82).
• Supports structural engineers in applying prestressing techniques per Indian standards.

Reference Table: IS 6461 Series Parts

Typical Formulae in Prestressed Concrete (General reference):

• Prestressing Force, P:
  [ P = A_s \times f_{ps} ] where
  (A_s) = area of prestressing steel,
  (f_{ps}) = stress in prestressing steel after losses.

• Losses in Prestress:
  Include elastic shortening, creep, shrinkage, relaxation, friction.

Diagram: Prestressed Concrete Concept

flowchart LR
    A[Prestressing Steel] --> B[Apply Initial Stress]
    B --> C[Transfer to Concrete]
    C --> D[Concrete in Compression]
    D --> E[Counteracts Tensile Stresses]

For detailed design formulas, losses, and testing procedures, refer to the full IS 6461 Part XI document.

IS 6461 Part 11: Definitions and Terminology (Key Points)

• Scope: Clause 2.0 specifies that definitions in this standard apply to cement concrete terminology.

• Glossary Grouping: The glossary is divided into 12 parts covering all aspects of cement concrete:

  Part No.	Subject
  I	Concrete aggregates
  II	Materials (except cement & agg.)
  III	Concrete reinforcement
  IV	Types of concrete
  V	Formwork for concrete
  VI	Equipment, tools, and plant
  VII	Mixing, laying, curing, etc.
  VIII	Properties of concrete
  IX	Structural aspects
  X	Tests and testing apparatus
  XI	Prestressed concrete
  XII	Miscellaneous

• Reference Standards for Terminology:

  • IS 4305-1967 (Pozzolanic materials)
  • IS 4845-1968 (Hydraulic cement)
  • BS 2787:1956 (Concrete & reinforced concrete terms)
  • BS 4340:1968 (Formwork terms)
  • ASTM C125 (Concrete aggregate terms)
  • ACI SP-19 (Cement & concrete terminology)
  • ACI 617-1968 (Concrete formwork practices)

Summary

This part of IS 6461 provides a comprehensive glossary aligned with international standards, ensuring uniform understanding of terms related to cement concrete materials, processes, and structural aspects. It does not contain formulas but serves as a key reference for consistent terminology.

flowchart LR
    A[IS 6461 Part 11] --> B[Glossary of Terms]
    B --> C[12 Parts of Concrete Terminology]
    C --> D[Aggregates]
    C --> E[Materials]
    C --> F[Reinforcement]
    C --> G[Types of Concrete]
    C --> H[Formwork]
    C --> I[Equipment & Tools]
    C --> J[Mixing & Curing]
    C --> K[Properties]
    C --> L[Structural Aspects]
    C --> M[Tests]
    C --> N[

IS 6461 Part 11: Prestressing Tendons and Anchorage Devices

Key Definitions

• Tendon (2.67): Steel wire, cable, bar, rod, or strand tensioned to impart prestress.
• End Anchorage (2.26): Mechanical device transferring prestress force to concrete.
• Unbonded Member (2.77): Tendons free to move, tension applied only at ends.

Important Specifications & Formulas

1. Tendon Stress Limits:

   • Initial stress ( f_{pi} ) typically 0.7 to 0.8 of ultimate tensile strength ( f_{pu} ).
   • Final prestress ( f_{pf} ) after losses should be ≥ 0.55 ( f_{pu} ).

2. Losses in Prestress:

   • Immediate losses: elastic shortening, anchorage slip.
   • Time-dependent losses: creep, shrinkage, relaxation.

3. Anchorages:

   • Must safely transfer tendon force without damage.
   • Design per IS 6461 Part 11 includes bearing area and wedge capacity.

Typical Tendon Properties Table (Example)

Formula for Prestressing Force:

[ P = f_p \times A_p ]

Where:

• ( P ) = Prestressing force (N)
• ( f_p ) = Stress in tendon (MPa)
• ( A_p ) = Cross-sectional area of tendon (mm²)

flowchart LR
    Tendon -->|Tensioned| Prestressed_Concrete
    End_Anchorage -->|Transfers Force| Concrete
    Tendon -->|Anchored by| End_Anchorage

For detailed design, refer to IS 6461 Part 11 clauses on anchorage dimensions, wedge design, and tendon specifications.

IS 6461 Part 11: Stress Losses in Prestressed Concrete

Key Definitions:

• Shrinkage Loss (Clause 2.60): Stress loss due to concrete shrinkage, reducing prestressing steel tension.
• Loss of Prestress (Clause 2.43): Combined reduction from steel creep, concrete creep & shrinkage, excluding friction but possibly including elastic shortening.
• Final Stress (Clause 2.29): Stress in prestressing steel after all losses.

Important Formulas for Stress Losses:

1. Shrinkage Loss: [ \Delta f_{sh} = E_p \times \varepsilon_{sh} ]

   • (E_p): Modulus of elasticity of prestressing steel
   • (\varepsilon_{sh}): Shrinkage strain of concrete

2. Loss due to Creep: [ \Delta f_{cr} = f_{pi} \times \frac{C_{cr}}{1 + C_{cr}} ]

   • (f_{pi}): Initial prestress
   • (C_{cr}): Creep coefficient of concrete

3. Elastic Shortening Loss: [ \Delta f_{es} = f_{ci} \times \frac{E_p}{E_c} ]

   • (f_{ci}): Concrete stress at transfer
   • (E_c): Modulus of elasticity of concrete

Typical Losses Summary Table:

Notes:

• Total losses typically range 15% to 25% of initial prestress.
• Use creep and shrinkage values from IS 456 or IS 1343 for concrete properties.
• Always consider time-dependent effects for accurate final stress estimation.

flowchart LR
    A[Initial Prestress] --> B[Elastic Shortening Loss]
    B

IS 6461 (Part 11) - Key Points on Bonded and Unbonded Tendons

Definitions

• Bonded Tendon (Clause 2.11): Tendon bonded to concrete via direct contact or grout, ensuring force transfer along its length.
• Unbonded Tendon (Clause 2.79): Tendon free to move inside the concrete, tension applied only at anchorages.
• Unbonded Member (Clause 2.77): Post-tensioned concrete element with tendons free to move; prestress force acts only at ends.

Key Specifications & Differences

Important Formulae

• Prestress Losses in Bonded Tendons:

[ \Delta P = \Delta P_{\text{elastic}} + \Delta P_{\text{creep}} + \Delta P_{\text{shrinkage}} + \Delta P_{\text{relaxation}} + \Delta P_{\text{friction}} ]

• Friction Loss for Bonded Tendons:

[ P_x = P_0 e^{-(\mu \alpha + k x)} ]

Where:

• (P_x) = force at distance (x)
• (P_0) = initial prestress force
• (\mu) = coefficient of friction
• (\alpha) = cumulative angle of curvature
• (k) = wobble coefficient
• (x) = length along tendon

Summary

• Bonded tendons provide better crack control and lower prestress losses but are permanent.
• Unbonded tendons allow easier replacement but have higher losses and less crack control.

flowchart LR
    A[Tendon

IS 6461 Part 11 (1973) - Post-Tensioning & Pretensioning: Key Points

Definitions (Clauses)

• Pretensioning (2.54): Tendons tensioned before concrete casting.
• Post-Tensioning (2.51): Tendons tensioned after concrete hardening.
• Pre-Post-Tensioning (2.52): Combination of both methods.
• Unbonded Member (2.77): Post-tensioned element with free-moving tendons inside.

Key Formulas

Typical Specifications

• Tendon Area (A_p): As per design, typically high tensile steel wires or strands.
• Initial Stress (f_pi): Usually 0.7 to 0.8 of ultimate tensile strength.
• Losses: Follow IS 1343 for detailed loss calculations.
• Unbonded Tendons: Must be greased and sheathed to allow free movement.

Summary Table: Tendon Stress Stages

flowchart LR
    Pretensioning -->|Tendons tensioned before casting| ConcreteCast
    ConcreteCast -->|Concrete hardens| Release

Friction and Wobble Effects in Post-Tensioning (IS 6461 Part 11)

Key Definitions:

• Curvature Friction (Clause 2.17): Friction due to bends/curves in the tendon profile.
• Wobble Friction (Clause 2.82): Friction caused by unintended variations (secondary curvature) in the tendon duct or sheath.
• Wobble Coefficient (Clause 2.81): A coefficient used to quantify friction loss due to wobble.

Friction Loss Formula

The total friction loss ( \Delta P ) in prestressing tendons is given by:

[ \Delta P = P_0 \times (e^{\mu \alpha + k x} - 1) ]

Where:

• ( P_0 ) = Initial prestressing force
• ( \mu ) = Coefficient of friction (curvature friction)
• ( \alpha ) = Total angular change of tendon profile (radians)
• ( k ) = Wobble coefficient (accounts for secondary curvature)
• ( x ) = Length of the tendon (m)

Typical Values (As per IS 6461 Part 11 or common practice):

Notes:

• Curvature friction depends on the number and sharpness of bends.
• Wobble friction accounts for small irregularities in duct alignment.
• Losses are cumulative and exponential with length and bends.
• For design, use conservative values of ( \mu ) and ( k ) to ensure safety.

flowchart LR
    A[Initial Force \(P_0\)] --> B[Curvature Friction Loss \( \mu \alpha \)]
    A --> C[Wobble Friction Loss \( k x \)]
    B & C --> D[Total Friction Loss \( \Delta P \)]

Summary:
Use the exponential friction loss formula incorporating both curvature and wob

IS 6461 Part 11: Elastic Shortening & Shrinkage Effects in Prestressed Concrete

Key Definitions:

• Elastic Shortening (2.25): Immediate shortening of concrete member due to prestressing force.
• Shrinkage Loss (2.60): Stress loss in prestressing steel from concrete shrinkage.
• Stress Relaxation (2.65): Stress reduction in steel under constant strain.
• Loss of Prestress (2.43): Combined prestress loss from creep, shrinkage, and elastic deformation.

Key Formulas:

1. Elastic Shortening Strain, ε_es: [ \varepsilon_{es} = \frac{\sigma_c}{E_c} ]

   • (\sigma_c) = compressive stress in concrete due to prestress
   • (E_c) = modulus of elasticity of concrete

2. Shrinkage Loss in Prestress Steel: [ \Delta f_{sh} = E_p \times \varepsilon_{sh} ]

   • (E_p) = modulus of elasticity of prestressing steel
   • (\varepsilon_{sh}) = shrinkage strain of concrete (from IS 456 or IS 1343)

3. Stress Relaxation Loss: [ \Delta f_{relax} = f_{pi} \times R(t) ]

   • (f_{pi}) = initial prestress
   • (R(t)) = relaxation factor (time-dependent, from IS 1343)

Typical Values / Specifications:

Summary Diagram:

flowchart LR
    Prestressing Force -->|Immediate| Elastic_Shortening
    Elastic_Shortening -->|Concrete Shortens| Stress_Loss_in_Steel
    Concrete -->|Over Time| Shrinkage
    Shrinkage -->|Steel Stress Decreases| Shrinkage_Loss
    Steel -->|Constant Length|

IS 6461 Part 11 primarily provides glossary and definitions related to prestressed concrete, with limited direct info on equipment/tools.

Key Equipment & Tools for Prestressing (General Practice):

• Prestressing Jacks: Hydraulic jacks to apply tension on tendons.
• Anchorage Devices: Wedges or clamps to hold prestressing strands.
• Tendon Sheaths: PVC or steel duct for tendon protection.
• Tension Measuring Devices: Load cells or pressure gauges.
• Grouting Equipment: Pumps for injecting cement grout into ducts.
• Cutting and Threading Tools: For prestressing strands.

Typical Formulas (from IS 1343 & related):

• Prestressing Force, P:
  [ P = A_p \times f_{pu} \times \eta ]
  where,
  ( A_p ) = area of prestressing steel,
  ( f_{pu} ) = ultimate tensile strength,
  ( \eta ) = efficiency factor.

• Losses in Prestress (approximate):
  [ \text{Total Loss} = \text{Elastic} + \text{Creep} + \text{Shrinkage} + \text{Relaxation} ]

Reference Tables (from IS 1343):

For detailed specs on equipment, IS 1343 and manufacturer catalogs are recommended.

flowchart LR
    A[Prestressing Jack] --> B[Tensioning Tendon]
    B --> C[Anchorage Device]
    C --> D[Concrete Member]
    B --> E[Load Cell (Measure Force)]
    D --> F[Grouting Equipment]

IS 6461 Part 11: Tendon Trajectories & Eccentric Tendons

Key Definitions

• Eccentric Tendon (2.23): Tendon path not aligned with the concrete member's gravity axis.
• Tendon Profile (2.68): The actual path the tendon follows.
• Deflected Tendons (2.20): Curved or bent tendon paths.
• Concentric Tendons (2.15): Tendons aligned with the gravity axis.

Important Specifications:

• Eccentricity (e): Distance between tendon centroidal axis and concrete member gravity axis.
• Tendon profiles must be designed to control stresses and deflections, ensuring no undue bending or shear.

Typical Formulas:

1. Eccentricity, e: [ e = y_t - y_g ] where:

   • ( y_t ) = tendon centroid coordinate
   • ( y_g ) = gravity axis coordinate

2. Stress due to eccentricity: [ \sigma = \frac{P}{A} \pm \frac{P \times e}{Z} ] where:

   • ( P ) = prestressing force
   • ( A ) = cross-sectional area
   • ( Z ) = section modulus about the axis of bending

Tendon Trajectory Design Guidelines:

• Limit eccentricity to avoid excessive bending.
• Provide smooth tendon curvature to reduce stress concentrations.
• Follow IS 1343 for detailed tendon profile limits and permissible eccentricities.

Summary Table: Tendon Types

graph LR
A[Concrete Member Gravity Axis] --> B[Concentric Tendon]
A --> C[Eccentric Tendon]
C --> D[Deflected Tendon (Curved Path)]

Note: For detailed tendon profile limits, curvature radii, and stress checks, refer to IS 1343 and IS 6461

IS 6461 Part 11: Multistage Stressing Key Points

Definition

• Multistage Stressing (Clause 2.45): Applying prestress in stages during construction to accommodate progressive loading and concrete strength gain.

Key Specifications

• Transfer Strength (Clause 2.74): Concrete must attain a minimum strength before prestress transfer, typically f'c ≥ 15–20 MPa (as per IS 1343 or project specs).
• Development Bond Stress (Clause 2.21): Ensures adequate anchorage of tendons; depends on concrete strength and tendon type.

Typical Procedure

1. Stage 1: Initial stressing at early concrete strength (e.g., 15 MPa).
2. Stage 2: Subsequent stressing after further strength gain or additional concrete placement.

Important Considerations

• Avoid excessive tendon slip or concrete cracking.
• Calculate losses (elastic shortening, creep, shrinkage) after each stage.

Simplified Formula for Prestress Loss in Multistage Stressing

[ \text{Loss}{total} = \sum{i=1}^{n} \text{Loss}_i ]

Where each stage loss includes:

• Elastic shortening
• Creep
• Shrinkage
• Relaxation of steel

Typical Table: Minimum Concrete Strength Before Transfer

flowchart TD
    A[Cast Concrete] --> B[Wait for Strength (15 MPa)]
    B --> C[Stage 1: Initial Prestressing]
    C --> D[Wait for Further Strength (25-30 MPa)]
    D --> E[Stage 2: Final Prestressing]
    E --> F[Complete Structural Element]

Summary: Multistage stressing requires careful monitoring of concrete strength and staged application of prestress to optimize structural performance and minimize losses.

IS 6461 Part 11: Transfer of Prestress - Key Points

Definitions:

• Transfer Strength (Clause 2.74): Minimum concrete compressive strength before prestress is transferred.
• Transfer (Clause 2.71): Act of shifting prestress from jacks/bed to concrete.
• Transfer Bond (Clause 2.72): Bond stress at tendon-concrete interface during transfer.

Key Specifications:

Transfer Stress Calculation:

[ f_{pt} = \frac{P_t}{A_p} ]

• (P_t) = Prestressing force at transfer
• (A_p) = Area of prestressing steel

Important Notes:

• Ensure concrete reaches transfer strength before releasing prestress.
• Transfer bond depends on concrete maturity and tendon surface condition.
• Follow IS 1343 for detailed prestress loss and transfer procedures.

flowchart LR
    A[Prestressing Jack] --> B[Prestressing Tendon]
    B --> C[Concrete Member]
    C --> D[Transfer of Stress]
    D --> E[Bond Stress at Interface]

This diagram shows the prestress transfer path from jack to concrete via tendon and bond.

IS 6461 Part 11: Sheathing & Ducts Key Points

Definitions:

• Sheathing/Sheeting (Clauses 2.58 & 2.59): Material forming the contact face of formwork, also called lagging.
• Sheath (Clause 2.57): Enclosure around post-tensioned tendons to prevent bonding with concrete.
• Duct (Clause 2.22): Hole or pipe in concrete for post-tensioning tendons or utilities.

Key Specifications:

• Sheathing Material: Should be smooth, durable, and resistant to concrete adhesion to ensure easy stripping.
• Ducts for Post-Tensioning:
  • Must be rigid, corrosion-resistant (usually galvanized steel or plastic).
  • Diameter sized to allow tendon passage plus grout cover.
  • Must maintain shape during concrete placement to avoid tendon damage.

Typical Formula for Duct Size:

[ D_{duct} = D_{tendon} + 2 \times C_{grout} ]

• (D_{duct}): Internal diameter of duct
• (D_{tendon}): Diameter of tendon
• (C_{grout}): Minimum grout cover (typically 5-10 mm)

Table: Typical Duct Sizes (Example)

flowchart LR
    Sheathing["Sheathing (Lagging)"]
    Duct["Duct (Post-Tension Tendon Enclosure)"]
    Sheath["Sheath (Tendon Enclosure)"]

    Sheathing -->|Contact face of form| Concrete
    Duct -->|Houses tendons| Concrete
    Sheath -->|Prevents bonding| Tendon

Summary: Use smooth, durable sheathing for formwork. Design ducts with adequate diameter for tendon passage and grout cover, ensuring corrosion resistance and shape stability.

Stress Relaxation in IS 6461 Part 11

• Definition (Clause 2.65):
  Stress relaxation is the reduction in stress over time when a prestressing steel is held at constant strain (constant length).

• Key Concept:
  When prestressing steel is tensioned and anchored, over time, the stress reduces due to internal molecular rearrangements even if the strain remains constant.

Typical Formula for Stress Relaxation Loss

[ \Delta f_{sr} = f_{pi} \times R(t, f_{pi}) ]

• ( f_{pi} ) = Initial prestress in steel
• ( R(t, f_{pi}) ) = Relaxation factor dependent on time ( t ) and initial stress level

Typical Relaxation Values (Indicative)

Values vary with steel grade and temperature.

Related Losses to Consider (IS 6461 Part 11)

• Shrinkage Loss (Clause 2.60): Stress loss due to concrete shrinkage.
• Curvature Friction (Clause 2.17): Friction loss due to cable bends.
• Development Bond Stress (Clause 2.21): Stress transfer via bond.

Summary Diagram

flowchart LR
    A[Initial Prestress] --> B[Time under Constant Strain]
    B --> C[Stress Relaxation Loss]
    B --> D[Other Losses: Shrinkage, Friction, Bond]
    C & D --> E[Total Prestress Loss]

Note: For design, IS 1343 and manufacturer data provide detailed relaxation curves and factors. Always check steel grade and temperature effects.

IS 6461 (Part 11) primarily covers Prestressed Concrete terminology and aspects, but does not explicitly provide detailed formulas or tables for "Special Components and Techniques."

Key Relevant Points from IS 6461 Part 11:

• Wobble Friction (Clause 2.82): Friction caused by unintended variation of prestressing steel duct profile.
• Focus on prestressing steel, ducts, and anchorage systems.
• Glossary and definitions related to prestressed concrete components and processes.

For Special Components and Techniques in Prestressed Concrete (General Engineering Knowledge):

• Losses in Prestress:

  • Immediate losses: elastic shortening, anchorage slip.
  • Time-dependent losses: creep, shrinkage, relaxation.

• Development Length (ld): [ l_d = \frac{\sigma_{sd} \times \phi}{4 \times \tau_{bd}} ] where:

  • (\sigma_{sd}) = design stress in steel,
  • (\phi) = diameter of prestressing steel,
  • (\tau_{bd}) = bond stress.

• Friction Losses: [ \Delta P = P_0 \times (e^{-\mu \alpha} \times e^{-k x}) ] where:

  • (P_0) = initial prestressing force,
  • (\mu) = coefficient of friction,
  • (\alpha) = total angle of curvature,
  • (k) = wobble coefficient,
  • (x) = length along the tendon.

Summary Table: Prestressing Losses

If you need detailed design formulas or tables, refer to IS 1343 (Prestressed Concrete - Code of Practice) which complements IS 6461 Part 11.