Shield, Carol K.

The behavior of concrete integral abutment bridges was investigated through a field experiment and a numerical parametric study. The field investigation focused on Bridge #55555 in Rochester, Minnesota, which was monitored from November 1996 to February 2004. Over 150 instruments were installed during construction of the bridge to measure abutment horizontal movement, abutment rotation, abutment pile strains, earth pressure, pier pile strains, prestressed girder strains, concrete deck strains, thermal gradients, and weather. The collected data were used to understand the behavior of Bridge #55555 due to the effects of temperature, creep and shrinkage. Two live load tests were conducted in 1997 and 1999, to examine the behavior of the bridge under live load. The overall performance of the integral abutment bridge was good. Bridge shortening was observed from the readings of different sensors. A steadily increasing tendency of average pile curvatures was observed from the measured data. Possible reasons were investigated through a time-dependent numerical analysis. A 3D finite element model of the test bridge was developed which took into account soil-structure interaction. The model was calibrated using data collected from the truck tests and the data from the seasonal and daily temperature variations. A parametric study was conducted to extend the results of the test bridge to other integral abutment bridges with different design variables including pile foundation type, bridge span and length, and orientation and length of wingwalls. Several design recommendations are made regarding the temperature range, use of predrilled holes around the piles, pile analysis method, and the applications of simplified design approaches for concrete integral abutment bridges.

This report investigates a method of repairing fatigued steel bridge girders using carbon fiber reinforced polymer (CFRP) strips. This type of repair would be used to prevent the propagation of cracks which could lead to failure of the bridge girders. The main advantage of using CFRP is it is lightweight and durable, resulting in ease of handling and maintenance. Therefore, it would not require the closing of traffic on the bridge during rehabilitation. Effective bond length was determined by a series of experimental tests with actual materials, as well as through the use of analytical equations. Finally, tests were conducted on full-scale cracked girders; the application of the CFRP strips to the steel girders resulted in significant strain reduction, except in the case of small cracks where it was difficult to clearly identify the benefits.

Vertical cracks near the midspan of large-sized prestressed concrete bridge girders may develop during the curing process and can extend through the depth of the girder. The cracking is attributed to restrained shrinkage and thermal effects prior to release of the prestressing strands. Eighteen full-scale Minnesota Department of Transportation Type 28M prestressed concrete beams were tested to investigate the effects of the cracks on the performance of the beams. Thirteen beams tested in this study incorporated manmade pre-release cracks. All of the beams were tested under static loading to investigate the effects of pre-release cracks on concrete strains, flexural crack initiation and re-opening loads, overall beam stiffness, and ultimate flexural capacity. Three of the beams were subjected to cyclic testing to evaluate possible effects of the pre-release cracks on the strand stress ranges and fatigue life of the beams. Unlike the field observations, the pre-release cracks in the test beams did not close completely under the beam's weight and pre-stressing force. The pre-release cracks were found to cause changes in beam strains around the crack locations. The overall stiffnesses of the beams were also affected by the reduction in the moment of inertia of the pre-release crack section. Following pre-release crack closure, the beams recover the stiffness comparable to that of the uncracked beams. No significant effect of pre-release cracks was observed on the behavior of the beams near the ultimate capacity. Results from the cyclic testing of three beams indicated that a beam that develops pre-release cracks is more likely to experience fatigue problems and tend to cause a reduction in the beam's fatigue life. Guidelines are proposed for the assessment of girders that develop pre-release cracks during production.

There are many wooden bridges in the United States. Their decks are often built of timber beams nailed together and covered with asphalt. The asphalt plays a mechanical role, and it provides environmental protection for the wood deck. The asphalt layer deteriorates and requires replacement. That leads to a faster deterioration of the deck, increased maintenance, and shorter bridge life. The flexibility of the deck is a probable cause of the fast deterioration of the asphalt. Low temperatures lead to deformations of the deck and may lead to cracks, which are propagated by mechanical and environmental effects. This project investigates stiffening the bridge deck by connecting a beam perpendicularly to the deck planks with metal bolts to reduce deformations of the deck. The additional beam is called a Transverse Stiffener Beam (or TSB). It can be incorporated as a part of new bridges or be attached to existing bridges. The investigations show the TSB significantly reduces deformations of the deck in most cases. The study indicates the positive effects of the TSB's should be expected in other applications. The magnitude of the effects can be analyzed with the computer program developed during this project.

This project involved the construction of two long-span, high-strength composite prestressed bridge girders to investigate their structural behavior and the adequacy of American Association of State Highway and Transportation Officials (AASHTO) 1993 provisions for their design. The scope of the research included examining prestress losses, transfer length, cyclic load response, and ultimate flexural strength. The research revealed that prestress losses could not be determined solely from strain gage instrumentation. Foil strain gages attached to the strand cannot measure losses caused by relaxation and drift over time. Vibrating wire strain gages embedded in the concrete cannot account for losses in the prestressing strand before the concrete hardens. Researchers used vibrating wire gage data to measure the prestress losses incurred since the time of strand release. To backcalculate the losses that occur before release, researchers used total prestress losses determined from flexural cracking and crack reopening loads. The measured prestress losses were found to be much higher than those predicted by analytical methods. Prestress losses predicted by AASHTO not only ignore concrete stress before release but also overestimate the high-strength concrete modulus, leading to lower initial losses, and overpredict the creep and shrinkage, leading to higher long-term losses.

This report summarizes an experimental program that investigated the development length and variability in bond of glass-fiber-reinforced-polymer (GFRP) reinforcement in concrete.

This research project investigated the effects of pre-release cracks on girder camber, flexural cracking capacity, and steel stress ranges. The research included a parametric study investigating stress ranges in the prestressing strands in uncracked, cracked and partially cracked girder sections to determine if steel fatigue was a concern. An analytical study also was performed, which modeled several pre-release cracks, including models of two experimental girders that developed pre-release cracks, to determine the effect of various cracks on girder stress and camber. The study concluded that steel fatigue in the prestressing strand is a concern in girders that become cracked in service. A loss of compressive stress is believed to occur in the bottom fiber of the girder because of pre-release cracks, which may result in the section cracking at lower applied load. Finite element modeling determined the loss of compressive stress in bottom fiber of girders with pre-release cracks. Analytical models also showed that pre-release cracks remained local to the crack location, that non-linear stress distributions occurred during the process of crack closure, and that the magnitude of the pre-release crack effects depended on the number of cracks, the crack width, and the crack depth.

In this project, researchers investigated methods for mitigating corrosion in reinforced concrete structures on the substructure of a bridge in Minneapolis, Minnesota. They treated several corrosion-damaged columns and pier caps with electrochemical chloride extraction (ECE). Then selected ECE-treated and untreated structures were wrapped with fiber reinforced polymer (FRP) wraps or sealed with concrete sealers to prevent future chloride ingression. They installed embeddable corrosion monitoring instrumentation in the field structures to evaluate the effectiveness of ECE treatment. Although the ECE process reduced average chloride levels in the treated structures by approximately 50%, several locations still had chloride concentrations in excess of the established corrosion threshold following ECE treatment. Resistivity probe failures that occurred at some of these locations indicated corrosion within the treated structures still could occur, despite re-passivation of the reinforcing steel following ECE treatment. Continued monitoring of the installed instrumentation is required to evaluate the long-term effectiveness of ECE treatment and concrete wrapping/sealing as a corrosion mitigation technique. In laboratory testing of the three FRP wrap types, the Mbrace CFRP and GFRP reported higher peeling loads and lower diffusion rates than the AMOCO CRFP, and thus were considered more effective sealant systems. However, concrete sealers are recommended to prevent future chloride ion ingress, instead of FRP wraps, because the use of sealers does not prevent visual inspection of the concrete for corrosion damage.