Benjamin A. Graybeal

Visual Inspection of Highway Bridges

The Federal Highway Administration recently conducted an investigation to assess the reliability of visual inspection of highway bridges as implemented by state departments of transportation. The study focused on routine and in-depth inspection, the two most prevalent forms of visual inspection of bridges. Because visual inspection is a highly subjective nondestructive evaluation technique, the results of these bridge inspections can be highly variable and are dependent on many factors. This study included the completion of 10 bridge inspection tasks by 49 practicing state bridge inspectors. With regard to routine inspections, this study showed that condition ratings are assigned with significant variability. Specifically, approximately 68% of condition ratings are within 1 rating point of the average. The results of the in-depth superstructure inspections indicated that these inspections are not likely to detect and identify the specific types of defects for which this inspection procedure is sometimes prescribed. Specifically, widespread defects such as those normally noted during a routine inspection are likely to be noted; however, local deficiencies, such as crack indications, are unlikely to be detected. The results of the deck delamination survey indicate that the accuracy of this type of inspection is relatively poor, with few inspection teams providing results that could be considered to accurately portray the condition of the deck.

Highway Bridge Inspection: State-of-the-Practice Survey

The congressionally mandated National Bridge Inspection Program requires states to inspect periodically all highway bridges on public roads, among other activities; visual inspection (VI) is the primary tool used to perform these inspections. A survey was conducted to help determine current policies and practices that may affect the accuracy and reliability of VI. The survey had three main objectives: to compile a “state of the practice” for bridge inspection, particularly as it pertains to VI; to gather information about bridge inspection management to study how inspection management may influence the reliability of inspections; and to gather data about the current use of nondestructive evaluation technologies and to identify current and future research needs. State departments of transportation, local departments of transportation, and select bridge inspection contractors participated in the survey. Conclusions drawn from this study indicate that the use of nondestructive evaluation has increased since 1993 and that the use of American Society of Nondestructive Testing Level III–certified personnel is increasing. VI was cited as the most frequently used nondestructive evaluation technique; however, vision testing for inspectors is almost nonexistent. Typically, professional engineers were not on site for inspections. State departments of transportation indicated a large demand for future research into the nondestructive evaluation of prestressed concrete superstructures and concrete decks.

Ultrasonic testing of reactive powder concrete

Concrete is a critical material for the construction of infrastructure facilities throughout the world. Traditional concretes consist of cement paste and aggregates ranging in size from 6 to 25 mm that form a heterogeneous material with substantial compressive strength and a very low tensile strength. Steel reinforcement is used to provide tensile strength for reinforced concrete structures and as a composite the material is useful for structural applications. A new material known as reactive powder concrete (RPC) is becoming available. It differs significantly from traditional concrete; RPC has no large aggregates, and contains small steel fibers that provide additional strength and, in some cases, can replace traditional steel reinforcement. Due to its high density and lack of aggregates, ultrasonic inspections at frequencies 10 to 20 times that of traditional concrete inspections are possible. This paper reports on the initial findings of research conducted to determine the applicability of ultrasonic testing techniques for the condition assessment of RPC. Pulse velocities for shear and longitudinal waves and ultrasonic measurement of the modulus of elasticity for RPC are reported. Ultrasonic crack detection for RPC also is investigated.

Modeling Structural Performance of Ultrahigh Performance Concrete I-Girders

Ultrahigh performance concrete (UHPC) is an advanced cementitious composite material that has been developed in recent decades. When compared with more conventional cement-based concrete materials, UHPC tends to exhibit superior properties such as increased durability, strength, and long-term stability. This computational investigation focused on modeling the structural behaviors of UHPC components including prestressed UHPC AASHTO Type II girders. The concrete damaged plasticity model was tailored to model UHPC within a commercially available finite-element analysis package. This manuscript focuses on modeling three UHPC I-girders tested under flexural or shear loading configurations. The concrete damaged plasticity model was demonstrated to replicate both linear and nonlinear structural responses of I-girders reasonably well. A set of UHPC constitutive properties were developed that facilitate the model replication of the local and global responses observed in the series of physical tests.

RELIABILITY AND ACCURACY OF ROUTINE INSPECTION OF HIGHWAY BRIDGES

Routine inspection is the most common type of highway bridge inspection completed to satisfy the requirements of the National Bridge Inspection Standards. Routine inspections of highway bridges are typically completed using only visual inspection (VI) and rely heavily on subjective assessments made by bridge inspectors. Given this practice and the fact that VI may have limitations that affect its reliability, the Federal Highway Administration completed a comprehensive study to examine the reliability of VI as it is currently practiced in the United States. The accuracy and reliability of condition ratings generated through routine inspections of six in-service and decommissioned highway bridges were studied. These timed inspections were completed in normal summer weather conditions under direct observation. To ensure that results from this study would be applicable to normal bridge inspections, state department of transportation bridge inspectors made up the study sample, which included 49 inspectors from 25 states, representing a diverse cross section of the bridge inspector population. Results of the study indicate that routine inspections are completed with significant variability. Specifically, 95 percent of primary bridge element condition ratings will vary within two rating points of the average, and only 68 percent of these ratings will vary within one rating point. Additionally, the National Bridge Inspection Standards definitions of condition rating may not be sufficiently refined to allow for reliable routine inspection results.

Modeling Structural Performance of Second-Generation Ultrahigh-Performance Concrete Pi-Girders

AbstractThe concrete-damaged plasticity (CDP) model with proposed material properties replicated the observed deflection and strain responses of three experimentally tested I-girders and was determined to be consistent for different spans under both flexural and shear tests. In this study, the CDP model was further tested in modeling the behaviors of a prestressed second-generation ultrahigh-performance concrete (UHPC) pi-girder. The computational aspects include discussion of the various parameters that influenced the accuracy of the model and investigation of the scenarios regarding the limits that are useful for further optimization of the girder. The CDP model was reconfirmed to be consistent and reliable in replicating the observed structural response of both the UHPC pi-girder and a modified structural configuration referred to as the “UHPC pi-girder-with-joint.” The finite-element analysis modeling techniques developed herein are expected to be valuable in the future development of additional UHPC ...

Fatigue Response in Bridge Deck Connection Composed of Field-Cast Ultra-High-Performance Concrete

The use of prefabricated concrete bridge deck components can offer many advantages over conventional cast-in-place construction techniques. However, completing the overall bridge system requires the installation of connecting elements. The state of the practice for these connecting elements has been deficient in resilience, durability, and ease of construction. An ongoing research effort at FHWA, in conjunction with partners from the New York State and Iowa Departments of Transportation, is focused on engaging the advantageous properties of ultra-high-performance concrete (UHPC) to develop a new type of detail applicable to deck-level connections between prefabricated modular bridge components. This physical testing program investigated the structural performance of a field-cast UHPC connection under repeated truck wheel loading. The connection—150 mm (5.9 in.) thick and 152 mm (6 in.) wide—was designed as a noncontact lap splice with straight 16M (No. 5) steel reinforcement. After more than 11 million structural-loading cycles at progressively increasing load levels, individual bars within the connection began to fail in metal fatigue. The stress range in the reinforcement at initial fatigue failure was conservatively estimated at 197 MPa (28.6 ksi). No debonding or slippage of reinforcement was observed. The performance of this connection detail demonstrated the types of details that can be developed and deployed with field-cast UHPC.

Full-Scale Testing of Shear Key Details for Precast Concrete Box-Beam Bridges

AbstractBox-beam bridge shear key connection designs are investigated by conducting full-scale structural tests. The authors’ evaluation included connection details, which are considered current good practice using high-strength nonshrink grout in combination with transverse posttensioning and novel details using ultra-high-performance concrete (UHPC). The factors that affect shear key performance, including simulated traffic loading, thermal loads, level of transverse posttensioning, and different connection designs, are presented in this paper. A simplified model for calculating the transverse shear is proposed, and the possible transverse tensile force caused by eccentrically placed loads is discussed. The performances of different connection details are presented and compared. The findings of the research can be used to assist bridge owners in the use of this economical bridge type and can provide innovative solutions that advance state-of-the-practice in bridge construction.

Dimensional Stability of Grout-Type Materials Used as Connections between Prefabricated Concrete Elements

Prefabricated bridge elements and systems (PBES) construction relies on field-cast, grout-type materials to complete the connections between precast concrete elements. This PBES construction facilitates and accelerates bridge construction (ABC), increases safety, and minimizes the inconveniences to the traveling public while delivering a superior product. Although prefabricated concrete components are produced in a controlled environment, field-cast grouts have at times shown serviceability issues mainly associated with dimensional stability. This paper assesses the dimensional stability (primarily shrinkage) of a total of seven prebagged grouts currently used in the construction industry. Their shrinkage performance is compared to that of an ultrahigh-performance concrete. The feasibility of the test methods used for evaluating the dimensional stability of nonshrink grouts is also discussed. Although many grouts are referred to as nonshrink materials, the results show shrinkage, especially in drying conditions. The use of the internal curing technology as an emerging solution for mitigating shrinkage in grout-type materials is also discussed. The results obtained in two of the cement-based grouts, including internal curing, show a reduction of both autogenous and drying shrinkage. Based on the results obtained, recommendations are given to end-users to provide guidance in selecting an appropriate grout-type material.

RELIABILITY AND ACCURACY OF IN-DEPTH INSPECTION OF HIGHWAY BRIDGES

In-depth inspection is one of the two most common types of bridge inspection. In contrast with routine inspection, in-depth inspection is a close-up, hands-on inspection that generally covers only a portion of a bridge. The Federal Highway Administration’s Nondestructive Evaluation Validation Center recently completed a study of the reliability of visual inspection of highway bridges. Part of this study focused on the accuracy and reliability of in-depth inspections as completed by state bridge inspectors from across the United States. Three in-depth inspection tasks were completed, two focusing on the inspection of steel superstructures and one focusing on the inspection of concrete bridge decks. Visual inspection techniques were used, with the possible use of simple inspection tools. Results show that many in-depth inspections completed by state bridge inspectors do not accurately represent the condition of the structure. In-depth inspections of the steel superstructures revealed that certain deficiencies for which this type of inspection is frequently prescribed were rarely detected. Specifically, only a small minority of state bridge inspectors located the presence of any of the existing crack indications. The delamination survey of a concrete bridge deck indicated that the accuracy of this type of inspection is relatively poor. Very few state bridge inspection teams provided results that could be considered to portray the condition of the deck accurately.