George Morcous

Mechanical and Bond Properties of 18-mm- (0.7-in.-) Diameter Prestressing Strands

For several years, 18-mm- (0.7 in.-) diameter strands have been successfully used in cable bridges and for mining applications. The use of these large diameter strands in pretensioned concrete girders could allow approximately 35% increase in the prestressing force compared to the same number of 15-mm- (0.6 in.-) diameter strands and 92% increase compared to 13-mm- (0.5 in.-) diameter strands. Consequently, this process will allow for longer spans, shallower structural depth, and/or wider girder spacing in bridge construction. For the same prestressing force, the use of 18-mm- (0.7 in.-) diameter strands results in fewer strands to jack and release, fewer chucks, and greater flexural capacity due to lowering the center of gravity of the strands. Despite the advantages of using large diameter strands in pretensioned concrete girders, the lack of data on their mechanical and bond properties hinder their wide use in bridge construction. In this paper, the mechanical and bond properties of 18-mm- (0.7 in.-) diameter strands are evaluated. One hundred and two strand specimens were obtained from different strand producers and production cycles to evaluate the ultimate strength, yield strength, modulus of elasticity, and elongation at two different laboratories. Test results indicated that all strands adequately met the requirements of the ASTM standard A416-06, with the exception of the minimum yield strength requirements (90% of the specified ultimate strength). The power formula for stress-strain relation- ship was used to provide an accurate predictor of the behavior of strands. Also, 58 strand specimens were tested for their bond in mortar and concrete using the North America Strand Producers (NASP) test method. Test results demonstrated that the bond of 18-mm- (0.7 in.-) diameter strands is proportional to the concrete strength. A formula for predicting the NASP pull-out test value as a function of concrete strength was also developed. In addition, NASP test results for clean and rusted strands were measured and compared at different slip values. DOI: 10.1061/(ASCE)MT.1943-5533.0000424.

Structural performance of precast/prestressed bridge double-tee girders made of high-strength concrete, welded wire reinforcement, and 18-mm-diameter strands

This paper presents the development of high-strength precast prestressed double-Tee girders for bridge construction. These girders use high-strength concrete (103 MPa), Grade 550 welded wire reinforcement, and 18-mm-diameter Grade 1860 prestressing strands at 51- by 51-mm spacing. The double-Tee section was used to simplify girder production and erection and to maximize span-to-depth ratio, which improves the construction economy and speed. To evaluate the efficiency of the developed girders, two full-scale 15.24-m long, 1.21-m wide, and 0.5-m deep single-Tee girders were fabricated by a precast producer and tested at the University of Nebraska structural laboratory. Transfer length measurements, development length testing, flexure capacity testing, and vertical and horizontal shear testing were conducted for each specimen. Test results have shown that the proposed high-strength bridge double-Tee girder can be designed using current LRFD bridge design specifications. Preliminary design charts for different girder sizes were presented to demonstrate the efficiency of these girders for short- and medium-span bridges.

Efficient Prestressed Concrete-Steel Composite Girder for Medium-Span Bridges. II: Finite-Element Analysis and Experimental Investigation

AbstractIn this paper, finite-element analysis (FEA) of the prestressed concrete-steel composite (PCSC) girder is performed to investigate strain and stress distributions in the girder sections and determine the influence of stud distribution on stresses in the concrete bottom flange. Approaches of FEA are discussed for the material and element models of steel, concrete, and strands, and element models of the bond between the concrete and strand and the shear studs, loading and boundary conditions, and convergence issues. A PCSC girder specimen is fabricated and instrumented in the structural laboratory to validate the proposed fabrication and design procedures. FEA and service design using the age-adjusted elasticity modulus method (AEMM) are both validated using the strain profiles at different sections and values of concrete surface strains and camber/deflection. Test results indicate that the cracking moment, ultimate moment, and ultimate shear of the PCSC girder can be well predicted using the AEMM a...

Concrete-Filled Steel Tubular Tied Arch Bridge System: Application to Columbus Viaduct

The tied arch bridge system provides a unique solution to the several challenges associated with the construction of railroad overpasses and water crossings, such as restricted vertical clearance, undesirable or impractical arrangement for intermediate piers, and extremely limited traffic control during construction. The paper presents the design and construction challenges pertinent to a novel concrete-filled steel tubular tied arch system that was first introduced in the Ravenna viaduct (53 m) and applied later to the Columbus viaduct (79 m). The main structural components of the Columbus viaduct are described in detail and the advantages of the system are summarized. The detailed analysis of the system at different construction stages and design checks of main components and connections under various loading conditions are discussed. Experimental investigations conducted on concrete-filled steel tubular arch and tie specimens to validate their theoretical capacities are demonstrated. The three-dimensional nonlinear finite element model developed to analyze the tie-to- arch connection and evaluate the lateral stability of arches is presented. Finally, the main construction procedures and challenges of the three tied arches of the Columbus viaduct are highlighted. DOI: 10.1061/(ASCE)BE.1943-5592.0000205.

Pareto Analysis for Multicriteria Optimization of Bridge Preservation Decisions

A new approach is presented for solving infrastructure maintenance optimization problems involving multiple, conflicting, and incommensurable criteria. Pareto analysis is adapted from the theory of social welfare economics to identify the solutions that mutually achieve the best compromise among competing criteria. The proposed approach is applied to the domain of bridge maintenance management to assist decision makers in optimizing bridge preservation decisions. Two criteria are considered in this study: the minimization of life-cycle costs and the maximization of bridge network condition. Feasibility and capability of the proposed approach are demonstrated by using an application example of concrete bridge decks. The uncertainty in deck deterioration is considered by using stochastic Markov chain models developed from the condition data of concrete bridge decks in Nebraska. Further research is needed to expand the application of the proposed approach to other infrastructure facilities, and to consider other decision criteria, such as network reliability, functionality, and user cost.

Reliability Analysis of NU Girders Designed Using AASHTO LRFD

The University of Nebraskas I-girders, known as NU girders, are precast prestressed concrete girders that have unique characteristics. NU girders have a wide top flange (48.2 in) to provide a better platform for workers and shorter span for deck; and a wide and thick bottom flange (38.4 in) to accommodate a large number of prestressing strands. The depth of NU girders varies from 20.3 in. to 63.6 in. to allow the construction of bridges. that spans up to 210 ft with a high economic competency. These unique characteristics significantly affect the flexure and shear resistance of NU girders, and consequently, influence their reliability when the uncertainty of loads, material properties, and fabrication parameters are considered. This paper presents the reliability analysis of NU girders designed using the strength I limit state of the AASHTO LRFD bridge design specifications. Resistance models are developed for several types of NU girders (NU1600, NU1800, and NU2000) and considering different combinations of girder span (120 to 200 ft) and girder spacing (8 to 12 ft). The variability of flexure and shear resistance is calculated based on the uncertainty of: i) material properties, such as cast-in-place concrete strength, precast concrete initial and final strength, reinforcing steel yield strength, and prestressing strands ultimate strength; and ii) fabrication parameters, such as the dimensions of concrete sections, area of reinforcing steel, and area of prestressing strands. Load models are developed to account for the uncertainty in dead load, wearing surface load, live load, and impact load. The calculated reliability indices are examined for consistency and compared against the target reliability index for various combinations of girder type, span, and spacing.

Precast/Prestressed Concrete Sandwich Panels for Thermally Efficient Floor/Roof Applications

AbstractPrecast concrete floor systems represent a major component of the cost and weight of precast concrete buildings. Hollow-core (HC) planking is considered the most common precast concrete floor system for residential and commercial buildings because of their economy, light weight, structural capacity, and ease of production and erection. However, the high thermal conductivity of HC planks hinders their use in radiant heated floor and roof applications where thermal insulation is needed. This paper presents the development of a precast/prestressed concrete sandwich floor panel that consists of an internal wythe of insulation and two external wythes of concrete similar to those used in sandwich wall panels. The main difference between the sandwich floor and wall panels is the design of shear connectors between concrete wythes to achieve full composite action under ultimate loads while simultaneously having an adequate creep resistance under sustained loads and acceptable deflection under live loads. T...

Accuracy of ground-penetrating radar for concrete pavement thickness measurement

Core extraction is the most common method for measuring concrete layer thickness in pavement construction. Although this method provides a very accurate thickness measurement, it is destructive, time-consuming, and does not provide adequate representation of the concrete layer thickness variability. Ground-penetrating radar (GPR) is a nondestructive evaluation technique that has been successfully used in several transportation applications, such as subsurface exploration and condition assessment. The main objective of this research is to investigate the accuracy and cost-effectiveness of using GPR in thickness measurement of concrete pavement for quality assurance purposes. A high-resolution 1.6-MHz ground-coupled antenna was used to perform grid scans and measure concrete thickness for several laboratory and field experiments. Results indicated that the use of metal objects underneath the concrete layer to improve bottom surface reflectivity was necessary for a reliable thickness measurement. Also, the use of calibration cores to determine the actual dielectric properties of the concrete was essential for accurate thickness calculation. An average accuracy of 98.5% was achieved when steel plates were used underneath the concrete layer and two cores were extracted for calibration. The effect of concrete age on GPR thickness measurement accuracy was also investigated.

Efficient Prestressed Concrete-Steel Composite Girder for Medium-Span Bridges. I: System Description and Design

A new prestressed concrete-steel composite (PCSC) girder system is developed to provide a viable alternative for steel and prestressed concrete I-girders in bridges. The PCSC girder is composed of a lightweight W-shaped steel section with shear studs on its top and bottom flanges to achieve composite action with the pretensioned concrete bottom flange and the cast-in-place concrete deck. The PCSC girder is lightweight, economical, durable, and easy to fabricate. The proposed fabrication procedure is similar to those of prestressed concrete girders and does not need specialized equipment, materials, and forms. A service design procedure is proposed using the age-adjusted elasticity modulus method to evaluate the time-dependent stresses and strains in the PCSC girder caused by creep and shrinkage effects of concrete and relaxation of strands. The strength design method is proposed for the design of PCSC girders at prestress release. A design procedure is proposed to assist engineers to accomplish economic design and production of PCSC girders, and design examples are presented to illustrate the design procedure.

Deterioration Models for Life-Cycle Cost Analysis of Bridge Decks in Nebraska

Deterioration models are vital to life-cycle cost assessments of bridges. Maintenance and user costs depend on bridge conditions, which vary across the period of analysis. The quality of LCC-based decisions depends on the accuracy and efficiency of the deterioration models used to predict the remaining service life of highway bridges. In Nebraska, the national average deterioration rates are used to predict the condition of bridge components. These rates do not represent the actual performance of bridges in Nebraska, however. The Nebraska Department of Roads therefore sponsored a research project to develop deterioration models specific to Nebraskas bridges, which used condition data collected from 1998 to 2010. In this paper, deterministic and stochastic deterioration models for bridge decks in Nebraska are presented. These models reflect the impact of parameters that govern deterioration, which include average daily traffic, average daily truck traffic, wearing surface type, highway district, and deck protection. The deterioration models developed indicated that the deck deterioration rate in Nebraska was lower than the national average and that it depended on traffic volume and highway district. Also, the service life of decks with epoxy-coated reinforcement was longer than that of decks with black reinforcement.