John S Lawler

Measuring three-dimensional damage in concrete under compression

To study the relationship between surface and internal cracking, 2 high-resolution, non-destructive evaluation techniques were used to measure crack growth caused by compressive loading in the fracture process of cement-based materials. Digital image correlation (DIC) was used to characterize the 2-D, surface fracture pattern, while 3-D, internal behavior was measured with x-ray microtomography (XMT). Rectangular mortar specimens containing both sand and graphite aggregates were examined. These techniques gave complementary information about crack geometry and development: DIC was more effective at determining crack width and location of small cracks, while XMT depicted the shape of larger cracks more successfully and showed the influence of internal features on the fracture process. The effect of aggregate shape and strength and the subsequent influence of increased crack distribution on ductility are discussed.

Fracture processes of hybrid fiber-reinforced mortar

Subregion Scanning Computer Vision (SSCV), a digital image based method for measuring surface deformation is used to examine the role of the fibers in the fracture process of mortars reinforced with hybrid blends of microfiber (less than 22 μm in diameter) and macrofiber (500 μm in diameter). Closely-spaced microfibers interact with cracks at the microstructural level and hamper the widening of coalesced microcracks, thus encouraging the growth of multiple cracks. The microfibers improved pre-peak mechanical performance and strength by delaying the formation of a through-specimen macrocrack. Macrofibers were most effective at bridging macrocracks and imparting ductility to the composite due to their geometry and greater length. Compared to mortar reinforced with a single fiber type, an increase in strength and toughness was seen with a blend of steel macrofibers and either steel or PVA microfibers. Finally, based on the crack topography observed, the reduction in water permeability of cracked mortar achieved with hybrid fiber-reinforcement, measured directly in a parallel study, was governed by multiple crack development.RésuméLa visualisation assistée par ordinateur (en anglais Subregion Scanning Computer Vision (SSCV), méthode basée sur les images numériques pour mesurer les déformations de surface, est utilisée ici pour examiner les processus de fissuration de mortiers de ciment renforcés à l’aide de mélanges hybrides de fibres: microfibres (diamètre inférieur à 22μm) et macrofibres (diamètre supérieur à 500μm). Les microfibres faiblement espacées interagissent avec les microfissures et rendent difficile l’élargissement de celles-ci et leur éventuelle union en fissures plus importantes. Ces microfibres ont amélioré les performances mécaniques et la résistance du mortier en deçà de la contrainte de rupture en retardant l’apparition de macrofissures traversant l’ensemble de l’échantillon.Les macrofibres ont été les plus efficaces pour «coudre» les macrofissures tout en conférant une ductilité accrue au composite en raison de leur géométrie et de leur longueur plus importantes. Par comparaison avec des mortiers renforcés avec un seul type de fibre, ceux renforcés avec un mélange de macrofibres en acier et microfibres en alcool polyvinylique (PVA) ont révélé une résistance et une robustesse accrues.Le renforcement avec un mélange hybride de fibres a permis aussi d’observer une diminution de la perméabilité du mortier fissuré. Cette diminution de perméabilité est due au développement d’une fissuration multiple par opposition a des microfissures qui se transforment en macrofissures.

Condition Survey of Older West Virginia Bridge Decks Constructed with Epoxy-Coated Reinforcing Bars

Chloride-induced corrosion of steel reinforcing bars is a major cause of deterioration in concrete bridge decks in northern climates. Corrosion-resistant epoxy-coated reinforcing (ECR) bars were introduced in the 1970s. In 1993, the West Virginia Department of Transportation (DOT) performed surveys of bridge decks treated with ECR and uncoated bars built in the mid-1970s. While deterioration was observed on uncoated bar decks, deterioration of decks built with ECR was found to be limited. The current investigation consisted of a questionnaire and review of the condition reports of the decks examined by the West Virginia DOT in 1993, followed by field condition surveys of six decks built with ECR to determine how the ECR was performing. The six bridge decks inspected were in good to excellent condition, with the exception of two spans of Bridge 2930, which were reinforced with uncoated black bars. The decks with ECR exhibited less than 0.15% corrosion-induced deterioration. Any deterioration observed on ECR decks was concentrated at cracks or construction joints. All actively corroding ECR bars had an average coating thickness less than 7 mil (0.007 in.), the current minimum specified thickness. Decks reinforced with uncoated black bars included in the 1993 study had an initial service life of 18 to 21 years, and an overlay was applied to all of these decks to address corrosion-related damage. The decks constructed with ECR are now 33 to 35 years old, and none have required rehabilitation to address corrosion-related deterioration.

Acceptance Tests for Surface Characteristics of Steel Strands in Prestressed Concrete

This report provides practical tests to identify and measure residues (e.g., rust, lubricants used in manufacturing processes, or corrosion inhibitors) on the surface of steel prestressing strands and to establish thresholds for residue types found to affect the strength of the strands bond to concrete. Key products presented here are four test methods suitable for use in a quality assurance program for the manufacture of steel prestressing strand. The report will be of particular interest to bridge engineers in state highway agencies and industry and to suppliers of steel prestressing strand.

Extending the Life of Historic Concrete Arch Bridges: State of the Art

Abstract Open-spandrel, concrete arch bridges were a common bridge design in the United States during the early 1900s. Many of these bridges are now urban landmarks and listed historic structures that local jurisdictions wish to rehabilitate for years to come, including widening of the deck to more safely accommodate pedestrians and bicyclists. However, decades of exposure in harsh climates have led to advanced deterioration and reduced load ratings for most extant examples. Furthermore, the original concrete is non-air-entrained, potentially lower-strength, and often chloride-contaminated and deeply carbonated, all of which make the concrete difficult to repair and vulnerable to future deterioration. Further complicating rehabilitation, the height, and arch-reliant behavior of these bridges make construction access, staging, and maintenance of traffic challenges. Drawing upon the authors’ experience with several bridges of this type, this paper discusses best practices and special considerations for investigating and rehabilitating historic concrete arch bridges to extend their life.