Recommended Practice on Strand Bond-Recommended Practice on Strand Bond Webinar Handout

Recommended Practice on Strand Bond Webinar Handout

Pdf Summary

The "2025 Recommended Practice on Strand Bond" webinar, led by Dr. Clay Naito, addresses updates to the PCI (Precast/Prestressed Concrete Institute) Recommended Practice (RP) for assessing and controlling strand/concrete bond properties in ASTM A416 prestressing strand. The RP reflects advancements in testing methods, quality control (QC), and quality assurance (QA), aligned with the newly integrated AASHTO LRFD Bridge Design Specifications (expected officially in 2027).

Key aspects include:

1. <strong>Strand Bond Fundamentals:</strong> 
   Strand bond comprises adhesion, mechanical interlock, friction, and wedge action (Hoyer effect), crucial for transfer length (precompression in pretensioned members), development length (flexural strength), and shear capacity in beams.

2. <strong>Testing Methods:</strong>  
   ASTM A1081 remains the standard surrogate cementitious material pullout test for strand surface quality, maintaining minimum bond thresholds (six-quarter running average ≥14,000 lb). However, inherent variability led to refinement of acceptance criteria.  
   Two concrete-based tests at precast plants—the Prestressed Strand Block Pullout Test and the Strand Draw-in Test—evaluate actual strand/concrete bond quality, supporting resolution when ASTM A1081 and plant QA tests conflict.

3. <strong>Quality Control and Assurance:</strong>  
   PCI Certified plants are recommended to perform regular QC testing and periodic re-verification, especially when concrete mixes or strand suppliers change. The RP encourages annual or biennial testing during qualification and ongoing plant production.

4. <strong>Design Guidance:</strong>  
   The RP provides updated expressions for transfer and development lengths, recognizing different concrete strengths. Developers emphasize using PCI’s refined calculations and hold to conventional transfer length of 60 strand diameters.  

5. <strong>AASHTO Integration:</strong>  
   The 2025 RP is incorporated into upcoming AASHTO LRFD BDS (Articles 5.9.4.3.1 and related), enabling fit-for-service evaluation and providing clarity on standard vs. high bond strand applications. Standard bond strands are recommended for most bridge pretensioning; high bond strands are reserved for special, low-redundancy cases.

6. <strong>Implementation and FAQs:</strong>  
   The RP offers comprehensive FAQs addressing strand procurement, QC/QA conflicts, testing frequency, fitness-for-service assessments, and interpretive guidance. The RP is a living document, with periodic updates reflecting industry feedback, new research, and evolving materials such as the effects of cement types on bond behavior.

In summary, the 2025 PCI RP advances the understanding, testing, and specification of strand bond in prestressed concrete, reinforcing safety and performance while enabling consistent quality control and adoption into major bridge design standards.

Keywords

2025 Recommended Practice

Strand Bond

Dr. Clay Naito

PCI Recommended Practice

ASTM A416

Precast/Prestressed Concrete

Strand Bond Testing

Quality Control

Quality Assurance

AASHTO LRFD Bridge Design