A comparison of nondestructive evaluation methods for bridge deck assessment

Abstract

Concrete bridge deck deterioration is a significant problem that must be addressed to preserve highway infrastructure investments in bridges around the world. Reducing the cost of bridge deck maintenance is critical to government and private agencies responsible for maintenance of bridges. Maintenance challenges increase as many bridges begin to approach the end of their design life and traffic loads continue to increase. One means of reducing the cost of bridge deck maintenance is to accurately evaluate the condition of the structure and its constituent materials. Current methods used to evaluate deterioration of bridge decks include acoustic, electrochemical, electromagnetic, and visual inspection techniques. The purpose of this study was to assess the advantages and limitations of three available evaluation methods. The evaluation techniques included in this study were ground penetrating radar, the chain drag method, and IE. These techniques represent an important group of inspection methods currently used to evaluate in-service bridge decks. The bridge deck used in the study contains significant delaminations but exhibits virtually no outwardly visible signs of these deficiencies, so a detailed visual inspection survey of the deck was not included. Cores were taken from the bridge deck at selected locations to confirm the accuracy of the results obtained through each evaluation method. This paper presents the findings from each method and describes their respective advantages and limitations.

Introduction

The highway infrastructure in the United States and around the world faces a deterioration problem of unprecedented magnitude. Challenging environmental conditions and increased traffic volumes make bridges more susceptible than ever to deterioration. Reinforced and precast concrete bridge decks are subjected to very demanding conditions, and virtually all bridge decks will fail to serve out the useful life of the bridge [1]. Accurately evaluating the condition of a bridge deck over time and using this information to choose appropriate maintenance techniques has become critical for the efficient allocation of bridge deck maintenance resources.

Many nondestructive evaluation (NDE) methods have been developed for bridge deck assessment but few have been evaluated in a performance-based comparison. Further, most of these methods have not been integrated into a working bridge management system that makes use of the important data provided by these methods. Four basic categories of bridge deck assessment methods utilized include visual inspection, acoustic, electromagnetic, and electrochemical testing. Methods within these four basic categories have been implemented in a variety of ways, but the commonly accepted approaches have narrowed to only a few widely used methods. These include visual inspection, chain drag, IE, ground penetrating radar (GPR), infrared thermography (IR), and half-cell potential.

The primary goal of these methods is to detect corrosion-related damage in a bridge deck. For most of these methods, the delaminations in a deck (which result from volume expanding corrosion processes at the reinforcing steel) are detected or measured. However, half-cell potential methods are designed to identify locations in the deck, where corrosion activity is likely to be ongoing, which may result in or may already be areas of delaminated concrete.

Commonly used bridge deck assessment methods are briefly described here. Visual inspection involves the assessment of the bridge deck for cracks, spalls and other irregularities in the deck surface that can be detected visually [2]. Visual inspection is the most common method of deck assessment, and requires few tools and minimal training of inspectors. Subsurface or internal defects in the deck are difficult to evaluate by this method.

Common acoustic techniques include the chain drag method and IE. The chain drag method involves dragging a chain across the bridge deck surface while listening for changes in the acoustic response. This method involves only a qualitative assessment, but is rapid and inexpensive. IE is frequently used when a more thorough acoustic assessment is needed [3]. The more expensive and time consuming IE method is quantitative, since the response to an impact is detected by an acoustic transducer, recorded, and subsequently analyzed for anomalies.

Two electromagnetic methods used for bridge deck assessment are GPR [4] and IR. GPR systems record the response to a radar impulse or a series of radar impulses incident on the bridge deck surface. Careful assessment of the GPR responses can often yield information about the material properties of the deck and its condition. IR can be used for bridge deck assessment, but it is highly susceptible to environmental conditions. IR records electromagnetic waves emitted in the infrared portion of the electromagnetic frequency spectrum. These infrared wavelength emissions vary in response to environmental conditions as well as the geometry and emissivity properties of the feature under investigation. For a bridge deck application, diurnal temperature variations can lead to temperature variations in the deck that are manifested in thermal gradients on the bridge deck surface. Under favorable conditions, these gradients may be related to subsurface bridge deck features. Factors such as wind, rain, shadows and small diurnal temperature variation can present challenges for this method [5].

Finally, electrochemical methods that measure corrosion activity by half-cell potentials are common methods of bridge deck assessment [6]. These methods can be effective for locating active corrosion sites, but are time consuming and expensive to implement. Further, because half-cell measurements relate to corrosion activity, physical characteristics of the bridge deck such as delaminations are not directly assessed.

Three of the six aforementioned methods of bridge deck evaluation are described and compared in this study. Each method has advantages and limitations that can be assessed more thoroughly when they are compared to one another. Some work comparing bridge deck evaluation methods has been done in the past [7] but results from these studies have not been conclusive. The Van Buren Road Bridge (Fig. 1), which spans Quantico Creek in Virginia west of Interstate 95, was selected as the test bridge for this study.