Andrea J. Schokker

Dynamic buckling of composite shells

The dynamic instability of interior ring stiffened composite shells under hydrostatic pressure is investigated. A shell structure such as a submarine vessel can undergo suddenly applied overpressure or successive shocks. In the presence of imperfections, the dynamic instability so triggered leads to a reduction of the load carrying capacity of the shell from that associated with quasi-static loading. Further, the large amplitude vibrations that occur prior to reaching the dynamic limiting pressure can have a damaging effect on the material of the shell. An asymptotic procedure is used in conjunction with p-version finite elements to extract the buckling mode and the associated second-order field. A single differential equation involving cubic nonlinearity is developed to characterize the dynamic behavior of the shell structure. This is solved by the Newmark method for time step integration along with Newton-Raphson iterations. Attention is focused on the reduction of the buckling pressure of the shell under dynamic loading, as well as the shell response at various increasing load levels until the displacements become unbounded.

PERFORMANCE OF SUPPLEMENTARY CEMENTITIOUS MATERIALS IN CONCRETE RESISTIVITY AND CORROSION MONITORING EVALUATIONS

A testing regime was developed to optimize strength and durability characteristics of a wide range of high-performance concrete mixtures. The aim of the selected designs was to offer multiple solutions for creating a highly durable and effective structural material that would be implemented on Pennsylvania bridge decks, with a life expectancy of 75-100 years. A prime method for optimizing the mixtures was to implement supplemental cementitious materials, at their most advantageous levels. Fly ash, slag cement and microsilica all proved effective in creating more durable concrete design mixtures. These materials have also shown success in substantially lowering chloride ingress, thus extending the initiation phase of corrosion. An additional benefit studied in this program is the ability of these materials to extend the propagation phase of corrosion due to the high resistivity they impart to the concrete. Ternary mixtures from these materials were particularly effective, showing much higher resistivity values than the materials used separately.

Grouts for bonded post-tensioning in corrosive environments

Problems with corrosion of prestressing steel in post-tensioned structures due to inadequate grouting have become more apparent in recent years. To improve grouting practices, a series of fresh property tests, accelerated corrosion tests, simulated field tests, and long-term corrosion tests were used in this study to develop optimum grouts for post-tensioning. Grouts possessing adequate workability and bleed resistance were tested in an accelerated corrosion test to evaluate relative corrosion protection properties. This paper focuses on the accelerated corrosion test results. The most promising of these grouts were then tested for placeability under simulated field conditions. A fly ash grout and a thixotropic grout are recommended from this testing program. Fluidity, bleed, and corrosion resistance data was also acquired for a number of different combinations of pozzolans and admixtures.

Dynamic interactive buckling of ring stiffened composite shells

The dynamic instability associated with the interactive buckling of ring stiffened composite shells under hydrostatic pressure is investigated. An optimally designed shell has its static local and overall buckling pressures close to one another. The shell response is then governed by the nonlinear interaction between the modes, which makes the shell very imperfection sensitive. A shell structure, such as a submarine vessel, can undergo suddenly applied overpressure or successive shocks. In the presence of imperfections, the dynamic instability will be triggered which would lead to a reduction of the load carrying capacity of the shell from that associated with quasistatic loading. Further, the large-amplitude vibrations that occur prior to reaching the dynamic limiting pressure can precipitate some form of material failure. The dynamic interactive buckling analysis developed in this study is a combination of the amplitude modulation technique and the asymptotic procedure. The nonlinear differential equations of motion for the structure so developed are solved by the Newmark method for time step integration along with Newton-Raphson iterations. Significant reductions in the load carrying capacity of the shells are observed as a combined result of the dynamic application of the load and the modal interaction. Damping was found to be of marginal influence in enhancing the dynamic limit load. Interlaminar stresses accompanying the dynamic response are monitored, and these reach significant values prior to the onset of dynamic instability.

Revisions to Accelerated Corrosion Test Method for Post-Tensioning Grout

A minimum degree of corrosion protection should be conferred to an embedded prestressing strand by post-tensioning grouts, along with placement-related requirements. The accelerated corrosion test (ACT), an anodic polarization test procedure, is currently used to measure corrosion protection in the United States. Approval of a specific grout currently requires two conditions: a 1,000 hour average ACT time-to-corrosion minimum result; and a 0.45 water-cement ratio standard mixture (only water and portland cement) less than or equal to an ACT time-to-corrosion result tested under the same conditions. Different combinations of the two approval conditions are used in various specifications. Criticism has resulted since the ACT test procedure does not currently require that grout resistance variations be accounted for during selection of applied voltage. The addition of admixtures and pozzolans can significantly vary grout resistance, perhaps affecting test results adversely. The authors report on research on different ACT test post-tensioning grout mixtures with admixture level variation and resistivity effects inherent to electrochemical systems compensation option. Study results show that resistivity variation significantly affected test type variation, so when testing equipment does not allow IR compensation, potentially lower quality grouts may occur.

Non-contact ultrasonic imaging for post-tensioned bridges to investigate corrosion and void status

Corrosion of the nations transportation infrastructure is a widespread and costly problem. Recent corrosion problems in post-tensioned bridge structures have increased the need for a reliable method for determining grout voids and level of corrosion in post-tensioned tendons. Corrosion monitoring techniques such as half-cell potential and corrosion rate measurements face problems when used in this type of structure and standard NDE (nondestructive evaluation) methods such as impact-echo have also encountered problems. This study begins the evaluation of a method called C-Scan ultrasonic imaging to evaluate grouted post-tensioned tendons. While this paper focuses on post-tensioning applications, the C-Scan technique may be valuable for investigation of any type of reinforced concrete structure.

ULTRASONIC C-SCAN IMAGING: PRELIMINARY EVALUATION FOR CORROSION AND VOID DETECTION IN POSTTENSIONED TENDONS

Corrosion of the nation’s transportation infrastructure is a widespread and costly problem. The most prevalent durability issue in reinforced concrete structures is chloride-induced corrosion of the reinforcing steel. A reliable method of determining grout voids and corrosion levels in posttensioned bridge structures is needed. Traditional techniques of corrosion monitoring (e.g., half-cell potential and corrosion rate measurement) are problematic when used in this type of structure, as are standard nondestructive evaluation (NDE) methods, such as impact echo. C-scan imaging, an ultrasonic technique used primarily in the composites industry for detecting delamination, is examined as a method of evaluating grouted posttensioned tendons. This method exhibits many promising qualities: it can be used for internal or external tendons and on metal or plastic ducts; access to only one side of a specimen is required; strong imaging allows easy interpretation of results; the technique poses no risk to users or the environment; and the method has strong potential for development as a handheld field tool. The C-scan technique may be valuable for the investigation of not only posttensioning applications but other types of reinforced concrete structures as well. Results of preliminary investigations on lab specimens indicate that the C-scan technique holds promise. The ultimate goal of the research is to provide a user-friendly, robust system for the NDE of posttensioned tendons for voids, corrosion, and wire breaks. Recommendations for optimal acquisition and processing techniques as well as for the future development of the equipment as a field tool are proposed.

Linear Polarization Resistance Tests on Corrosion Protection Degree of Post-Tensioning Grouts

This article discusses the current industry practice for acceptance testing of post-tensioning grouts. The practice includes an accelerated corrosion test (ACT), as recommended in Post-Tensioning Institute (PTI) M55.1-03, “Specification for Grouting of Post-Tensioned Structures.” The current ACT method typically takes 1 to 2 months to complete and is seen as a downside. However, the linear polarization resistance (LPR) technique requires only a few hours to complete. The research presented in this paper indicates a strong correlation between the time to corrosion, as measured by the ACT method, and the system polarization resistance, as measured by the LPR technique. The authors recommend the use of the LPR method as a prescreen for very high-quality grouts so that lengthy ACT testing is not necessary.

Bridge deck patching material evaluation

Deck patching is a common practice used when bridge decks have local deterioration but do not yet warrant full replacement. Rapid setting patch materials provide a quick and economical way to patch deteriorated areas without significant lane closure time. While these materials set quickly and often achieve high early strength, their long-term performance in corrosive environments and under heavy traffic loads is sometimes poor. In addition, little guidance is provided in terms of standardized testing for these materials that addresses the key properties needed for long-term service life. This paper details a testing program that evaluates a number of deck patch materials and the development of a testing protocol for material evaluation.