Craig M. Newtson

A Gauss–Newton full-waveform inversion for material profile reconstruction in viscoelastic semi-infinite solid media

An inversion framework employing a Gauss–Newton method is developed to reconstruct material profiles in heterogeneous, viscoelastic, semi-infinite domains. In particular, a full-waveform inversion approach is investigated to image the elastic and attenuating parameters of a layered media. To account for the viscoelasticity of the medium, a Generalized Maxwell Body with one spring and two Maxwell elements in parallel (GMB2) is adopted in the forward and inverse wave propagation problems. Perfectly-matched-layers were introduced as wave absorbing buffers to simulate the semi-infinite extent of the domain. Using transient wave equations endowed with the GMB2 constitutive relation and the PML, a partial-differential-equations-constrained optimization scheme was implemented that lead to classic KKT (Karush–Kuhn–Tucker) conditions including time-dependent state, adjoint, and time-invariant control problems. An optimal solution of the viscoelastic parameters was obtained using a reduced-space approach based on a line search algorithm where the search direction was computed by the Gauss–Newton method. Considerable improvements on the accuracy and convergence rate of solutions were made by the developed Gauss–Newton inversion procedure compared to previous research using the Fletcher–Reeves method.

Case Studies Using Ultrahigh-Performance Concrete for Prestressed Girder Bridge Design

Ultrahigh-performance concrete (UHPC) develops very high compressive strengths and exhibits improved tensile strength and durability properties that make UHPC a promising material for bridge applications. Through case studies on typical prestressed concrete girder bridges (simple and continuous), the potential impact of implementing UHPC in New Mexico was investigated. Two existing bridges with high-performance concrete girders were redesigned using UHPC with a compressive strength of 155.1 MPa (22,500 psi) and a modulus of rupture of 8.0 MPa (1,160 psi). The redesign used a modified load factor design procedure for the Service III flexure limit state and a modified load and resistance factor design procedure for the ultimate shear limit state that considered the compressive strength, modulus of rupture, and modulus of elasticity of UHPC. Additionally, 15- and 18-mm-diameter (0.6- and 0.7-in.) prestressing strands were investigated. The use of UHPC and 18-mm-diameter (0.7-in.) prestressing strands reduced the required volume of girder concrete by up to 40%. Additionally, the contribution of the steel fibers in the UHPC significantly reduced the required shear reinforcement.

Load Rating a Prestressed Concrete Double T-Beam Bridge Without Plans by Field Testing

Bridges with no design plans are an issue in New Mexico because of the many that exist throughout the state. Conventional load rating techniques cannot be used because these bridges have limited or no design documentation. This lack of information has created uncertainties regarding the load-carrying capacity of these structures. Only a few states have formal procedures on how these particular bridges should be load rated. A project was conducted for the New Mexico Department of Transportation to develop a procedure for load rating bridges without plans, including prestressed concrete bridges. In accordance with the AASHTO Manual for Bridge Evaluation, a prestressed concrete double T-beam bridge was evaluated with advanced analyses and experimental methods (including load testing and nondestructive material evaluation techniques). A four-step load rating procedure was implemented that included estimating the prestressing steel by Magnel diagrams, verifying the estimate with a rebar scanner, testing the bridge at both diagnostic and proof loads based on strain measurements, and using the proof test results to rate the bridge. Rating factors and posting loads were determined for AASHTO and New Mexico legal loads. Because of the poor condition of the shear keys (some of which were broken), it is shown that the load distribution between beams was adversely affected and the bridge should be load posted.

Feasibility Analysis of Using UHPC in Prestressed Bridge Girders

Ultra high performance concrete (UHPC) is an emerging material which develops very high compressive strengths and exhibits improved tensile strength and durability properties. Recent construction of three UHPC bridges in the United States has sparked interest in the use of UHPC for prestressed bridge girders in several states. UHPC offers many advantages including longer spans, improved durability, and smaller structural members, making it an appealing material to use in prestressed bridge construction. This paper presents a case study conducted to investigate the feasibility of implementing UHPC in bridge design in New Mexico. Using a modified LFD procedure, bridge designs were developed considering the compressive strength, modulus of rupture, and modulus of elasticity of UHPC as well as design requirements for flexure and shear, girder cross-section geometry, and prestressing strand area. Comparisons with as-built bridge configurations show that using UHPC with compressive strengths between 100 MPa and 150 MPa may potentially lead to a reduction in required girder lines and reduce or eliminate mild steel shear reinforcement. Compared to regular concrete, a cubic yard of commercially available UHPC can cost as much as 10 times more, making designers and precasters hesitant to adopt this material. An economic analysis, assuming that local materials can be used to produce UHPC, indicates that the initial high cost can be offset by careful selection of materials, reduced construction time and maintenance, and the increased service life of the structure. Costs were evaluated based on local material prices, design parameters of the bridge, modified production practices, and a projected maintenance schedule. This study aids in the development of design procedures and recommendations for implementing UHPC as a prestressed concrete girder material.

A Gauss–Newton full-waveform inversion in PML-truncated domains using scalar probing waves

Abstract This study considers the characterization of subsurface shear wave velocity profiles in semi-infinite media using scalar waves. Using surficial responses caused by probing waves, a reconstruction of the material profile is sought using a Gauss–Newton full-waveform inversion method in a two-dimensional domain truncated by perfectly matched layer (PML) wave-absorbing boundaries. The PML is introduced to limit the semi-infinite extent of the half-space and to prevent reflections from the truncated boundaries. A hybrid unsplit-field PML is formulated in the inversion framework to enable more efficient wave simulations than with a fully mixed PML. The full-waveform inversion method is based on a constrained optimization framework that is implemented using Karush–Kuhn–Tucker (KKT) optimality conditions to minimize the objective functional augmented by PML-endowed wave equations via Lagrange multipliers. The KKT conditions consist of state, adjoint, and control problems, and are solved iteratively to update the shear wave velocity profile of the PML-truncated domain. Numerical examples show that the developed Gauss–Newton inversion method is accurate enough and more efficient than another inversion method. The algorithms performance is demonstrated by the numerical examples including the case of noisy measurement responses and the case of reduced number of sources and receivers.

Behavior Comparison of Prestressed Channel Girders from High-Performance and Ultrahigh-Performance Concrete

In response to the demand for sustainable and improved bridge design practices, the development of emerging materials like ultrahigh-performance concrete (UHPC) is at the forefront of structural innovation. UHPC offers significant advantages to bridge superstructure design as it provides advanced mechanical and durability properties, including high compressive strength and increased tensile capacity. With the introduction of high-strength steel fibers into mixture proportions, postcracking tensile and flexural tensile capacities are increased. These increased tensile capacities provide greater ductility and reduce or possibly eliminate the need for mild steel reinforcement. The present research investigated the behavioral response of full-scale prestressed bridge girders subjected to four-point flexural loading. Two channel-shaped girders were designed to provide equal design moment capacities to facilitate comparative analyses of performance. The first of these girders was designed with high-performance concrete [HPC; 9.5 kips per square inch (ksi; 66 MPa)] and mild steel reinforcement typical of that used in New Mexico bridge designs. The second girder used nonproprietary UHPC [20 ksi (138 MPa)] mixture proportions consisting primarily of local materials, local mixing procedures, and a local curing regimen, all of which were developed at New Mexico State University, Las Cruces. Digital image correlation was used to capture deformations throughout the testing. This information creates a full field of displacements that captures tensile and compressive strain behaviors in the pure moment region. This investigation demonstrated the advantages and improved performance of UHPC and the contribution of steel fiber reinforcement to postcracking strength and flexural capacity.

Simplified Procedure to Obtain LRFD Preliminary Design Charts for Simple-Span Prestressed Concrete Bridge Girders

Abstract The latest bridge design manual from the Precast/Prestressed Concrete Institute (PCI) provides preliminary design charts for selecting the girder size and prestressing strands for a given span length and beam spacing but only for fc′ = 55 MPa and 0.6-in. (15-mm) diameter strands. This single concrete strength and strand size may limit the use of the charts, particularly in states considering high- and ultrahigh-performance concrete. Accordingly, this paper presents a simplified procedure to develop preliminary design charts for prestressed concrete bulb-tee girders based on the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) bridge design specifications considering service load stress limits and flexural strength. The procedure is demonstrated for a BT-72 section and new LRFD charts are generated to compare the effects of higher concrete strength and larger strand size on bridge girder design. The benefits and limitations of the...

Materials Specification Needs for Future Development of Ultra High Performance Concrete

Over the next several years, new mixtures for ultra high performance concrete (UHPC) products are expected to move from prepackaged mixtures toward the use of locally available materials to make UHPC economically viable for future projects. Because a wider variety of materials will be used in UHPC, it will be necessary to develop or modify material specifications to ensure high quality UHPC mixtures are produced that provide strength and durability properties consistent with designers’ expectations. New or modified specifications will be needed for chemical requirements on the cement and supplementary cementitious materials considered for use in UHPC as well as to guide producers in the selection of aggregates for use in UHPC. This paper discusses these material issues along with issues related to admixture and fiber selection that are important to the future development of UHPC. Suggestions for development or modification of specifications that will ensure selection of high quality materials and potential performance based specifications that may evolve in the future are also provided.

Freezing and Thawing Durability of Ultra High Strength Concrete

Resistance to freezing and thawing of two UHSC (ultra high strength concrete) mixtures was evaluated in accordance with ASTM C 666 Procedure A. The two mixtures (plain and fiber reinforced) were developed using materials local to southern New Mexico, USA. Three different curing regimens were investigated for the mixture with fibers and one curing regimen was studied for the mixture without fibers. All curing regimens included 24 h of ambient curing followed by four days of wet curing at 50 o C, and then two days dry curing at 200 o C. At an age of seven days, one batch of fiber reinforced specimens was air cured at ambient conditions for the following six days and then placed in a water bath at 4.4 o C for 24 h prior to initiating freezing and thawing cycles. The second batch was air cured from day seven to day 12, and then wet cured for one day at 23 o C prior to being placed in the 4.4 o C water bath. The final batch was wet cured at 23 o C from the seventh day to an age of 13 days and then placed in the 4.4 o C water bath. The mixture with no fibers was air cured from the seventh day to an age of 12 days and then wet cured for one day at 23 o C prior to being placed in the 4.4 o C water bath. Higher moisture levels during curing produced greater initial dynamic elastic modulus values and durability factors at the end of the freezing and thawing tests, with the greatest durability factor being 87.5. Steel fibers were observed to improve both compressive strength and durability factor for UHSC.