Field Testing of an Ultra-High Performance Concrete Overlay

CHAPTER 1. INTRODUCTION

INTRODUCTION

Maintenance and rehabilitation of highway bridge decks is a continual challenge for bridge owners and transportation agencies throughout the United States. Transportation agencies must extend the service lives of existing bridge decks with limited funds and limited time needed for replacement or major rehabilitation. For context, figure 1 shows a pie chart depicting the bridge deck condition rating from U.S. bridges currently in service; the data shown is based on 2016 National Bridge Inventory (NBI) data.

Source: Data from Krauss, Lawler, and Steiner (2009).

Figure 1. Pie Chart. Concrete bridge deck condition rating based on 2016 NBI data.

The data only include highway bridges with reinforced concrete bridge decks, either cast-in-place or precast. Table 1 lists the NBI codes, condition rating, and the associated description for each rating. Approximately 38 percent of these bridges exhibit some level of deck deterioration. This is not to say that this entire group requires remedial action. However, deterioration is progressive and irreversible. Thus, bridges with decks in the satisfactory group will tend to migrate over time to the fair group, and so on. There is currently a need for resilient and durable repair and rehabilitation solutions for these aging reinforced concrete bridge decks.

Table 1. FHWA Condition ratings—1995.

Bridge deck repairs usually have one or a combination of the following objectives:

• Seal existing cracks and areas of spalled concrete.
  
  
• Water proof.
  
  
• Provide additional protection to corroded or damaged reinforcement.
  
  
• Provide a smooth riding surface for increased user comfort.
  
  
• Replace deteriorated cover concrete.
  
  
• Add material for structural strengthening.

Traditionally, conventional cement- or asphalt-based overlays have been economical and constructible options to achieve these objectives. However, these options also suffer from their own degradation mechanisms. Many transportation agencies fully recognize limited service life extension as noted in the report by Krauss, Lawler, and Steiner (2009).

Figure 2 depicts the anticipated service life of bridge deck overlays as reported by 43 transportation agencies. (Krauss, Lawler, and Steiner 2009) Most agencies feel that overlays will last between 5 and 30 years. However, this data does not reflect actual service life extensions, which may be shorter or longer than anticipated. Nevertheless, there is a need for durable, long-lasting bridge deck overlay solutions to extend the service life of existing bridge structures.

Source: Data from Krauss, Lawler, and Steiner (2009).

Figure 2. Pie Chart. Anticipated service life of bridge deck overlays as reported by 43 transportation agencies.

Ultra-high performance concrete is emerging as an innovative solution for a variety of bridge construction and rehabilitation applications, including 100 percent UHPC structural elements, structural patching and repair bridge element, jackets for columns and driven piles, and field-cast connections between prefabricated bridge elements. One emerging application of UHPC in the highway bridge sector is thin, bonded overlays for bridge deck rehabilitation. As an overlay material, UHPC can provide both structural strengthening and protection from chloride penetration and water ingress. This is achieved using a 1-in (25-mm) to 2-in (51-mm) thick layer of UHPC, which minimizes both the required material volume and additional dead load on the bridge structure. Prior to placing the UHPC overlay, a thin layer of poor and/or deteriorated concrete is removed from the existing concrete deck (if needed), and the deck surface is roughened to facilitate good bond between UHPC and existing concrete. Good bond between the UHPC overlay and the existing concrete deck is required to develop composite action between the two materials. The concept and use of UHPC overlays has been researched in Europe and has been deployed on more than 20 bridges. (Brühwiler and Denarié 2013)

UHPC BACKGROUND

Advances in concrete technology, such as high-strength steel micro-fiber reinforcement, superplasticizers, gradation optimization, and supplementary cementitious materials, began to be packaged together into a new generation of cementitious composite materials in the 1970s and 1980s. In the 1990s, this new class of materials was brought to market and has become known as ultra-high performance concrete (UHPC). Preblended, prepackaged, proprietary formulations of UHPC became commercially available in the United States in the early 2000s, and academic researchers funded by Federal or State transportation agencies have developed nonproprietary mixes. (Wille and Boisvert-Cotulio 2013; El-Tawil et al. 2016)

UHPC offers a number of advantages over conventional concretes and other cementitious materials, including enhanced material and durability properties, which have gained the attention of the highway bridge design community. Since 2005, over 140 highway bridges have been constructed using UHPC in the United States and Canada combined. Those deployments used UHPC in a variety of bridge construction and rehabilitation applications, including prefabricated structural bridge elements made entirely of UHPC, retrofit and repair of bridge decks, girders, and substructures, and field-cast connections between prefabricated bridge elements (PBEs). This last application is currently the most popular within the United States and Canada and has proved to be a common entry point for many owners new to this technology.

PROBLEM STATEMENT AND OBJECTIVE

The first U.S. deployment of UHPC as a bridge deck overlay was completed in May 2016 on a reinforced concrete slab bridge located in Brandon, IA, in Buchanan County. This bridge is the focal point of this report and is referred to as the “Laporte Road bridge.” A few months after installing the UHPC overlay, a field inspection of the Laporte Road bridge identified some locations along the deck where delamination might have occurred. These locations were identified by sounding the deck using a chain drag. However, it was not known whether delamination, if actually present, occurred at the interface between the UHPC overlay and substrate concrete, within the existing concrete deck, or within the UHPC overlay itself. It was noted, prior to installing the overlay, that there were some regions on the deck where the existing concrete appeared distressed and where corrosion of steel had caused delamination. Thus, there was a need to assess the bond between the UHPC overlay and substrate concrete and determine whether locations of potential delamination were a result of poor bond between the UHPC overlay and the concrete deck or a result of pre-existing issues.

These delamination mechanisms are illustrated in figure 3, which depicts a concrete bridge deck with a UHPC overlay. Delamination due to debonding between the UHPC overlay and the existing deck concrete might be caused by poor surface preparation of substrate concrete, poor consolidation of UHPC, excessive tensile stress normal to the material interface, or excessive shear stress parallel to the material interface. Delamination within the existing concrete deck would likely be caused by corrosion-induced spalling of cover concrete or freeze-thaw damage and could have preceded the UHPC overlay. Delamination within the UHPC overlay layer is possible, but highly unlikely.

Figure 3. Illustration. Existing concrete bridge deck with a UHPC overlay and potential locations of delamination.

APPROACH

In November 2016, researchers from the Federal Highway Administration’s (FHWA) Turner-Fairbank Highway Research Center (TFHRC) conducted a field study on the Laporte Road bridge to evaluate the bond between the UHPC overlay and the substrate concrete bridge deck. The approach included synthesis of photographic evidence, a field inspection of the bridge deck surface using a chain drag, physical testing of the UHPC-concrete interface bond according to American Society of Testing and Materials (ASTM) C1583—the direct tension bond pull-off test—and visual inspection of the UHPC-concrete interface using scanning electron microscopy.

REPORT OUTLINE

This report is divided into five chapters. Chapter 1 provides an introduction to the problem and the approach taken by the research team. Chapter 2 provides an overview of UHPC-class materials and their properties, and discusses some previous research on UHPC overlays and previous field deployments. Chapter 3 describes the Laporte Road Bridge, overlay installation, and some key observations prior to installation of the overlay. Chapter 4 describes the methodology used to develop a test plan and the details of the bond test method. Lastly, chapters 5 and 6 provide results and conclusions, respectively.