Service Life Design Guidance for UHPC Link Slabs

Background:
Design for service life rather than just for strength against potential overload and fatigue failure is becoming a more common consideration for bridges. One aspect of design, and often bridge retrofit, with potential for a large impact is minimizing the number of transverse deck joints. Bridge deterioration can often be traced to poor performance of these deck joints due to failure of the joint seal allowing chloride laden water onto bridge girder ends, bearings, and substructure elements. Using link slabs over the piers allows for eliminating some interior joints and moving expansion joints to the end of the bridge while still maintaining typical bridge behavior. Link slabs allow the simply supported behavior expected for many bridges, yet still transmit deformations and forces to expansion joints and reduce potential penetrations in the bridge deck. Advanced materials, such as ultra-high performance concrete (UHPC) can simplify link slab details and substantially improve their durability.

UHPC link slabs are specifically relevant to accelerating bridge retrofit in that the short required debonded lengths can significantly reduce the required amount of demolition and the overall time required for the project. Debonded lengths for UHPC link slabs can be as small as 16 in. compared to several feet for conventional construction. While the concrete in the immediate area of the joint may be deteriorated and can be removed quickly, concrete further from the joint will often be sound and take substantial time and labor to remove. The hairline distributed cracks that form in a UHPC link slab limit pathways for water to penetrate to the bridge girders and substructure, and UHPC itself is inherently more durable than conventional concrete due to its very low permeability.

UHPC link slabs have been used successfully in the field by several state DOTs, several research studies have been carried out focused on link slabs, and appropriate design guidelines are available in the AASHTO LRFD Guide Specifications for Accelerated Bridge Construction (2018). However, more information is needed for quantifying the service life benefits of using UHPC link slabs compared to conventional construction.

Objective:
The main objectives of this project are to develop user friendly tools that will allow use of developed information specific to UHPC link slabs within the framework of SHRP2 R19A for service life design of bridges and to provide educational materials to help practitioners understand how to use those tools.

Scope:
This project focuses on quantification of service life design considerations for UHPC link slabs including economic analysis relative to other construction alternatives. The following tasks will be performed to achieve the project objective:

  • Task 1 –Review of Link Slab Research and Practice
    • The proposed project will include a detailed examination of the literature available on UHPC link slabs to synthesize design guidance, including consideration of service life (e.g. Azizinamini et al., 2013), and how that guidance fits with published guide specifications.
  • Task 2 – Identification of Service Life Design Considerations
    • Bridge configurations appropriate for use of UHPC link slabs for new construction, for repair/retrofit, and with an emphasis on accelerated construction/retrofit will be identified.
    • Failure mechanisms identified from previous research will be considered from both the structural and durability standpoint, and performance will be examined relative to repeated traffic loading causing cracking and fatigue and the resulting freeze-thaw durability and corrosion resistance.
  • Task 3 – Cost Analysis
    • An analysis of the cost and comparison to other possible methods for both construction and retrofit will be conducted including the economic effects of a potential extension of service life.
  • Task 4 – Link Slab Durability Specimen Construction and Preparation
    • Two full-scale link slab segments will be constructed, using non-proprietary UHPC.
    • One link slab will be subjected to 3 million service level load cycles to induce a level of damage to the specimen similar to that expected in the field.
  • Task 5 – Link Slab Durability Testing
    • Sections will be cut from each link slab specimen.
    • Sections cut from the link slab segments will be exposed to accelerated corrosion testing to examine the effect of the two different materials and the resulting interface on system durability.
  • Task 6 – Education Module Development
    • A series of voice annotated PowerPoint presentations and short videos will be developed that will be useful for training design professionals on design of UHPC link slabs including consideration of service life and durability.
  • Task 7 – Assembling Reports and “Guide for Design of UHPC Link Slabs”
    • A “Guide for Design of UHPC Link Slabs” will be developed incorporating the results of the research within the framework developed by SHRP2 R19A for service life design of bridges.

Research Team:

Principal Investigator:  Dr. Royce Floyd
Co-Principal Investigator: Dr. Jeffery Volz and Dr. Musharraf Zaman
Research Assistant:  Stephen Roswurm and Jacob Choate