Thomas E. Cousins

TRANSFER LENGTH OF EPOXY-COATED PRESTRESSING STRAND

To provide corossion protection for 7-wire prestressing strand in prestressed concrete used in adverse environments, a manufacturer has developed an epoxy-coated strand. The effect of the epoxy coatings and grit on transfer length for 3 different sizes of prestressing strand was investigated. The transfer length of 3 differenct densities of grit were also studied. Fifty-three specimens were cast to determine the transfer length of epoxy-coated and bare (uncoated) prestressing strand. The transfer length was determined by measuring concrete strains before and after release. Also studied were the effects of time on transfer length of epoxy-coated strand - referred to as transfer-length creep - the relationship between end slip and transfer length of prestressing strand, and the effects of elevated temperatures on the transfer length of epoxy-coated strand. The experimental and analytical results from this research were compared to the results in the literature, provisions in the ACI Building Code (ACI 318-83), and previously proposed bond provisions.

Conformable tire patch loading for FRP composite bridge deck

Fiber-reinforced polymer (FRP) composites are increasingly being used in bridge deck applications. However, there are currently only fledgling standards to design and characterize FRP deck systems. One area that should be addressed is the loading method for the FRP deck. It has been observed that the type of loading patch greatly influences the failure mode of a cellular FRP deck. The contact pressure distribution of a real truck loading is nonuniform with more concentration near the center of the contact area as a result of the conformable contact mechanics. Conversely, the conventional rectangular steel patch on a FRP deck act like a rigid flat punch and produces stress concentration near the edges. A proposed simulated tire patch has been examined for loading a cellular FRP deck with the load distribution characterized by a pressure sensitive film sensor and three-dimensional contact analysis using ANSYS. A loading profile is proposed as a design tool for analyzing FRP deck systems for strength and dur...

Fatigue of Diaphragm-Girder Connections

Distortion-induced fatigue cracking has occurred at hundreds of diaphragm-girder connections in multigirder steel bridges in Birmingham, Alabama, in recent years. Repairs have been ineffective in some cases. The goal of the reported research was an improved strategy for repair and maintenance of the bridges so that the potential for future cracking is minimized. The investigation included field measurements of distortion-induced stresses at connections, field measurements of the effects of removing diaphragms from two in-service bridges, structural evaluations and finite-element method analyses of typical bridge designs, and laboratory testing of bolted connections. The results indicate that interior diaphragms can be removed from many existing bridges without significant negative effects. A new bolted diaphragmgirder connection was designed, installed in the field, and tested. Tests confirmed that the new design performed better than the original design. Recommendations for maintaining the Birmingham bridges include removing unneeded diaphragms and repairing old connections with the new design at needed diaphragms.

Experiments on a Hybrid-Composite Beam for Bridge Applications

This paper details a study of the structural behavior of hybrid-composite beams (HCBs), which consist of a fiber-reinforced polymer (FRP) shell with a tied concrete arch. The HCB offers advantages in life-cycle costs through reduced transportation weight and increased corrosion resistance. Through a better understanding of system behavior, the proportion of load in each component can be determined, and each component can be designed for the appropriate forces. A long-term outcome of this research will be a general, structural analytical framework, which can be used by transportation departments to design HCBs as rapidly constructible bridge components. This study focused on the identification of the load paths and load sharing between the arch and FRP shell in an HCB and on the test of an HCB with a composite bridge deck. Tests were performed through the application of point loads on simple span beams (before the bridge deck was placed) and with a three-beam, skewed composite bridge system, which resulted in strain data for the arch and FRP shell. The test results showed that strain behavior was linear elastic at service loads, and the FRP shell had a linear strain profile. Curvature from strain data was used to find internal bending forces, and the proportion of load within the arch was found. A stress integration method was used to confirm the internal force contributions. The arch carried about 80% of the total load for the noncomposite case without a bridge deck. When composite with a bridge deck, the arch made a minimal contribution to the HCB stiffness and strength, because most of the arch was below the neutral axis and cracked under the maximum live load expected for the bridge. For this composite case, the FRP shell and prestressing strands resisted about 80% of the applied load, while the bridge deck carried the remaining 20% to the end diaphragms and bearings.

Shear strength of a lightweight self-consolidating concrete bridge girder

Lightweight self-consolidating concrete (LWSCC) is advantageous in the bridge industry because members made with this material have a significantly lower self-weight, and in its fresh state, LWSCC has a low viscosity which eliminates the need for vibration during fabrication. A composite section was fabricated with a single precast bulb-tee LWSCC beam and a lightweight concrete cast-in-place deck. A simply supported test configuration was constructed with two point loads to quantify the web-shear strength of the girder. The experimental shear strength is compared to four analytical models from different AASHTO specifications. Based on the results of this limited study, the theoretical predictions for the web-shear strength of this girder were all conservative when compared to the experimentally measured failure strength. With these results in mind, further research is recommended on the use of LWSCC girders in the bridge industry to better understand the material properties, structural properties, and cos...

Performance Evaluation of FRP Composite Deck Considering for Local Deformation Effects

We examine here the replacement of a deteriorated concrete deck in the historic Hawthorne Street Bridge in Covington, Va. with a lightweight fiber-reinforced polymer (FRP) deck system (adhesively bonded pultruded tube and plate assembly) to increase the load rating of the bridge. To explore construction feasibility, serviceability, and durability of the proposed deck system, a two-bay section (9.45 by 6.7 m ) of the bridge has been constructed and tested under different probable loading scenarios. Experimental results show that the response of the deck is linear elastic with no evidence of deterioration at service load level (HS-20). From global behavior of the bridge superstructure (experimental data and finite- element analysis), degree of composite action, and load distribution factors are determined. The lowest failure load ( 93.6 kips or 418.1 kN ) is about 4.5 times the design load ( 21.3 kips or 94 kN ), including dynamic allowance at HS-20. The failure mode is consistent in all loading conditions ...

DEVELOPMENT LENGTH OF EPOXY-COATED PRESTRESSING STRAND

To provide corrosion protection in adverse environments, a 7-wire prestressing strand is coated with a grit-impregnated epoxy. The effect of the epoxy coating and grit on development length for 3 different sizes of prestressing strand was investigated. The development lengths of 3 different densities of grit were studied as well. Also, some test specimens were subjected to a fatigue loading to investigate its effect on bond between pretensioned strand and concrete. Experimental and analytical results from this study were compared to the results in the literature and provisions in the ACI Building Code (ACI 318-83).

Inspecting the lightweight precast concrete panels in the Woodrow Wilson bridge deck of 1982

This paper presents the results of a detailed inspection of the deck panels of the Woodrow Wilson Bridge installed in 1982. The original cast-in-place concrete deck, constructed in 1962, was replaced with full-depth lightweight precast concrete deck panels that enabled rapid construction with minimal traffic disruption. The inspection of the Woodrow Wilson deck provides valuable information about the performance of the precast concrete panels, joints, and connections after 20 years of very harsh traffic loads and environmental stressors. The deck panels performed well overall, with the only serious problems at expansion and contraction joints. All of these joints exhibited cracking and rusting. The most prevalent type of cracking appeared to be due to restrained shrinkage between the new polymer concrete, the older precast panels, and the rigid steel joints. This location is more vulnerable to cracking and leaking because there is no prestress across the joint. The multilayered corrosion protection methods used for the transverse and longitudinal post-tensioning tendons were very successful.

Effect of Vertical Casting Position on Transfer and Development Length

This investigation includes the effect of vertical casting position on transfer and development lengths of prestressing strand. Current code provisions account for the top-bar effect in the calculation of development length for standard reinforcing bars, but fail to recognize the phenomenon when calculating transfer and development lengths of prestressing strand. Historically, this phenomenon has been dependent on the amount of concrete cast below a bar or strand. Recent research shows the amount of concrete cast above a strand to be more influential. This paper presents the results of an experimental investigation on the influence of vertical casting position of prestressing strand on transfer and development lengths. The study includes data from 20 T-beam test specimens and four sets of top-strand blocks. The results from 119 transfer zones and 39 flexural tests were evaluated and compared to vertical casting position along with current code provisions.

Pedestrian Bridge Collapse and Failure Analysis in Giles County, Virginia

A pedestrian suspension bridge over Walker Creek near Route 749 in Giles County, Virginia, partially collapsed in October of 2008, causing 10 people to fall into the creek below. The failure occurred when one of the anchors holding a suspension cable fractured. This paper presents the investigation of the failure, including a structural analysis of the bridge to determine cable forces, material property testing of the fractured anchor, laboratory testing of a different anchor from the bridge, and a simple strength evaluation of the anchor under combined axial force and moment. Based on the results of the study, it was concluded that the anchor should not have failed carrying the loads on the bridge on the day of failure. The anchor failed in brittle fracture because of a local defect in the material at the perimeter of a corrosion pit on the surface of the anchor hook. The structural analysis and laboratory testing indicated that the material around the defect was at yield before fracture initiation. The material flaws significantly influenced the bridge failure after yielding and inelastic deformation of the anchor material at the inside of the hook radius occurred.