Browsing by Author "Dr. Michael L. Leming, Committee Member"

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Analysis of Vertical Load Distribution in Shoring and Slab Systems of Multistory Concrete Structures under Construction.
(2010-04-30) McGurl, Michael Patrick; Dr. James M. Nau, Committee Member; Dr. Michael L. Leming, Committee Member; Dr. David W. Johnston, Committee Chair
Analysis of the construction load distribution during construction of multistory concrete buildings is critical for both construction safety and economy. Recent reduction of load factors used in design of the permanent structure has resulted in less strength availability in the construction stage and need for tools to better support shoring and reshoring analysis. Using the traditional simplified method to analyze loads in a concrete structure under construction, spreadsheet models have been developed which distribute load inputs chosen by the engineer. The models determine the vertical loads resisted by the following shoring systems: one set of shores with either two or three sets of reshores; two sets of shores with either one or two sets of reshores; or three sets of mechanical drophead shores. Analysis can be performed using either unfactored or factored loads and using either actual uniform loads or loads expressed as a proportion of slab dead load, D. Slab liftoff was then analyzed to prevent improper loading conditions during the construction cycle of multistory concrete buildings. The results of the models were validated by comparison with previously published shoring analysis. Lastly, a sensitivity analysis is performed to compare the loads supported by individual slabs and sets of shores in each model during the placement of upper floors.
Development of APA Design Criteria for Surface Mixtures
(2006-09-12) Kadayam, Vijay V; Dr. John R Stone, Committee Member; Dr. Michael L. Leming, Committee Member; Dr. N. Paul Khosla, Committee Chair
Several test methods are in practice to assess the rutting potential of a mixture. The commonly used procedures are Diametral tests, Uniaxial test, Triaxial tests, Shear tests, Empirical tests, and Simulative tests. Of all these test methods, simulative test methods are relatively easier to use and ready for immediate adoption. The APA test is the most widely used simulative test. It is imperative that the rut depth criteria for the APA test should be developed for its employment. In a recent study conducted at NCSU, it was concluded that the APA could clearly detect poorly performing mixtures. With the limited availability of data, a reasonable correlation was observed between the APA tests and Repeated Shear at Constant Height (RSCH) tests. A comprehensive research study was conducted on all surface mixtures of NCDOT. The rut depths measured from the APA test were compared with the corresponding shear strains of the RSCH test. In addition, the shear and APA tests were fine-tuned by addressing issues related to air voids, test temperatures and aggregate sources. Statistical analysis was performed on the test results as measured by the APA and the RSCH tests. This was used to characterize rut resistance of mixtures used in this study. The correlations estimated using the data obtained from the APA and Shear tests were used to develop rut depth criteria for the APA test. The developed rut depth criteria for the APA test could be adopted for immediate use in practice.
Drophead Formwork System Implications in Flat Plate Concrete Floor Construction
(2005-08-11) Lewis, David Dylan; Dr. Michael L. Leming, Committee Member; Dr. David W. Johnston, Committee Chair; Dr. Mervyn J. Kowalsky, Committee Member
This study analyzed aspects of shoring and reshoring of multi-story concrete construction using retractable dropheads for the temporary support of the flat plate floor slabs. A comparison was made between this method and the traditional method using removed shores. The objective was to determine the ability of the slabs to support construction loads at the stage of very early age removal of the forming panels. The strength evaluations were conducted considering punching shear, flexural strength, and beam shear. The results were presented as a series of charts showing the structural capacities of a slab for various slab depths and concrete compressive strengths attained. An analysis is also presented of the distribution of the construction loads throughout a structure using both the traditional method and the method with the retractable dropheads. An additional analysis was also done considering the effects of having various percentages of slab activation due to partial rather than full release of the shores.
Evaluation of the Effects of Compaction Methods on the Predicted Performance of Superpave Mixtures
(2004-11-14) Sadasivam, Suriyanarayanan; Dr. M. Shamimur Rahman, Committee Member; Dr. Pierre A. Gremaud, Committee Member; Dr. N. Paul Khosla, Committee Chair; Dr. Michael L. Leming, Committee Member
Several compaction methods are used to fabricate specimens for performance testing in the laboratory. As the physical properties of the specimens depend on the method of compaction used for fabrication, the compaction methods adopted in the laboratory should simulate the properties of the pavement in the field. The effects of different compaction methods on the performance of mixtures were investigated in this study. Laboratory compaction methods such as Superpave Gyratory Compaction (SGC) and Rolling Wheel Compaction (RWC) were compared with field compaction. Four field mixtures were selected. Various performance evaluation tests were conducted on the field cores and specimens fabricated using SGC and RWC. The mixtures were evaluated using the Shear tester, the Asphalt Pavement Analyzer (APA) and the NCSU Wheel Tracking Device. The results indicate that the laboratory compacted mixtures tend to be superior in their performance than the field cores. The laboratory compacted mixtures have higher stiffness values and lower shear strain values than the field cores. The RWC seems to simulate field compaction better than the SGC. A good correlation was found between the results of the Repeated Shear tests at Constant Height tests, the APA tests and the NCSU WTD rut tests. The mixtures which failed to satisfy the RSCH test criteria had rut depths greater than 0.5 inch, as measured by the APA and NCSU WTD. The mixtures that passed the RSCH tests had rut depths less than 0.5 inch. The APA test and the NCSU WTD test can be used as a simulator to examine the rutting susceptibility of a mixture. The compactability of the mixtures is studied using the SGC, IPC Servopac Compactor and GLPA. The compaction parameters that relate with the rutting behavior of mixtures were compared with the RSCH shear strains at different air void contents. The correlations showed different trends with two sets of parameters with one set measuring the compactability of the mixtures and another set measuring the shear behavior of the mixtures. The compaction parameters measured based on the theory of rate of densification could not satisfactorily predict the rutting behavior of the mixtures.
An Investigation of Critical Success Factors for Robotic Masonry
(2006-04-17) Rihani, Rami Awni; Dr. Leonhard E. Bernold, Committee Chair; Dr. William L. Bingham, Committee Member; Dr. Michael L. Leming, Committee Member; Dr. William J. Rasdorf, Committee Member
Automating masonry wall construction faces several challenging mechanical control issues reaching from the dehacking and delivery of bricks, the application of mortar, to actual placement of bricks to create straight and even elements. The first objective of this research work was to study the complex operations done by hand in order to understand what is necessary to meet standards, what affects the brick-mortar interfaces, and what leads to required bond strengths. The Critical Success Factor method was used to identify the most relevant problem areas that needed to be addressed and solved. Using this approach, the following six factors were defined: 1) design automation, 2) dehacking, 3) brick quality control, 4) brick placement quality, 5) mortar application quality, and 6) brick-mortar bond strength. Consequently, each factor was studied followed by the design and execution of necessary experiments. The research showed that isolating these factors was useful in identifying a focused list of topics that affect them. The issue of brick-mortar interaction and its importance in achieving bond strength was especially important in that the skill of a mason had to be replicated with a mechanical approach. Experimental tests showed that the bond strength was affected by the profile of the mortar joint applied and the depth of mortar penetration in the brick holes. The final step of this study included a comparison between robotically and manually placed bricks. Utilizing a standardized Bond Wrench Test apparatus, it was found that the consistency of bond strength values of ten prisms placed robotically matched that of ten prisms laid manually. Mortar application proved to be the most difficult problem. Consistently smooth pulsation-less mortar joints were not achieved even after many redesigns of the progressive cavity pump setup. Conceptual pressure models demonstrated that the resultant pressure at the end of the pumping apparatus, after friction losses and pressure drops, was critical in producing acceptable mortar joints. A modified pump model was recommended for future research. The experiments highlighted the factors affecting mortar pumping using this kind of pumping mechanism, such factors included: 1) the speed of the pump motor drive, 2) the size of the rotor-stator opening, and 3) the length of the rotor-stator assembly. Measuring the pressure inside the pump lay outside the scope of this research but should be the focal point in future work. Overall, the work showed difficulty in achieving established quality standards in masonry construction using robotics. On the other hand, the project solved some unique problems by integrating electronics, computing, and mechanical concepts in innovating ways. For example, the problem of dehacking work led to the design of the pneumatic adaptive brick gripper. For the design of the brick quality control work cell, data collected from a photoelectric sensor was manipulated in order to substitute the human sensory skill of detecting different colors. For applying mortar bed joints, a nozzle plate was redesigned, with the same cross-section as that of a manually placed mortar joint, in order to pump mortar where it is most effective.
Use of Lime as Anti-Strip Additive for Mitigating Moisture Susceptibility of Asphalt Mixes Containing Baghouse Fines.
(2005-11-10) Shidhore, Aniruddha Vilas; Dr. N. Paul Khosla, Committee Member; Dr. Akhtarhusein A. Tayebali, Committee Chair; Dr. Michael L. Leming, Committee Member
Recent NCDOT research suggests that baghouse fines with gradation similar to the natural and manufactured fines passing #200 sieve seems to have beneficial effect on stiffness and rutting characteristics of the asphalt mix. However, these studies conclude that mixes containing baghouse fines were highly moisture susceptible, and recommended that baghouse fines be metered into the mix to create a uniform percentage throughout the mix. This study assesses the effectiveness of hydrated lime as an anti-strip additive in mixes containing excess baghouse fines. Comparison of test results of the mixes containing hydrated lime versus the mixes containing organic anti-stripping additive (LOF 6500) was also done. Two different types of baghouse fines, one from Boone, NC and one from Enka, NC, were used in HMA mixtures in the amounts of 1.5, 5.5 and 6.5-percent. Modifications were made to the available JMF and specimens were prepared in the laboratory and several different tests were performed. Wet sieve analysis was first done to check the gradation of materials. Using this gradation and the available JMF, aggregate proportioning was done to satisfy NCDOT mix design criteria. Moisture susceptibility of mixes was determined by performing TSR tests on mixes with different proportions of BHF, and with or without lime. TSR testing showed that moisture susceptibility was dependant on both the concentration of baghouse fines and anti-strip additive. Presence of hydrated lime in mixes increased the resistance to moisture damage. Specimens were also tested using the SST machine. Samples were compacted and sawed and one half of the specimens were moisture conditioned. The FSCH and RSCH tests were then performed on the samples to determine the material properties as well as the rutting resistance and fatigue life. In general, the test results indicate that addition of lime enhances the mix performance - |G*| values are higher, rut depths are lower, and fatigue resistance is higher. Based on the results of this study, it may be concluded that, addition of 1-percent hydrated lime to asphalt mixtures with up to 5-percent additional BHF (total of 6.5-percent) enhances the mix performance. Addition of lime also helped in the mitigation of moisture susceptibility of asphalt mixes. It is thus recommended that NCDOT should consider addition of 1-percent hydrated lime (by weight of dry aggregates) to mixes which are expected to have excess BHF content.