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Browsing by Subject "Shrinkage"

Now showing 1 - 9 of 9
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    Calibration of New Concrete Shrinkage and Creep Model for ACI Committee 209
    (2022-04) Clement, Timothy
    A new semi-empirical concrete shrinkage and creep model is proposed and calibrated in which creep ages according to solidification theory. The impetus for this CRC-funded research is to propose new creep and shrinkage design guidelines for ACI Committee 209; previous guidelines were published in 1982 and are no longer up to date with modern understanding of time-dependent behavior. The new model proposes a coupling between autogenous and drying shrinkage using a volume-average pore relative humidity and treats drying creep as an additional stress-dependent shrinkage, linking together all these phenomena. The proposed expressions are designed to facilitate traditional integral-type analysis, but also uniquely support rate-type calculations that can be leveraged by analysis software. Model calibration uses the Northwestern University (NU) database of creep and shrinkage tests to determine new model parameters. Profile likelihood curves for each individual mix in the database are computed to quantify the sensitivity of the fitting parameters to the available data and to mix design inputs.
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    Cure induced stress generation and viscoelasticity in polymer coatings.
    (2010-01) O’Neal, Daniel Jeffrey
    Coatings solidified by free-radical polymerization and crosslinking (curing) reactions initiated with ultraviolet (UV) light do so quickly and at room temperature. Low viscosity monomer or oligiomer makes the use of volatile solvent unnecessary, decreasing energy use and making the process more environmentally friendly but photoinitiators can be toxic, limiting certain applications. Stress may be generated by a changing specific volume during cure, and stress-induced defects are undesirable. The goal of this research is to understand stress generation in UV irradiated coatings and to model stress generation and viscoelasticity seen during curing. Two new mathematical models were created to accomplish viscoelastic stress modeling. The first, a network model, uses a two-dimensional network of one-dimensional elements to replicate deformation in the coating. The second uses continuum momentum conservation and linear viscoelastic equations. Inertial forces can be neglected and a substitution performed, making the solution more rapid and simple with standard finite element methods. Stress generation in uniformly cured coatings depends on how quickly the specific volume and physical properties change. Reaction kinetics, volume, and stress are calculated simultaneously. Rapid initiation from high initiator concentration or UV light intensity delays volume change, generating more stress because the volume changes with a higher modulus. An optimum curing schedule would insure the actual specific volume and its equilibrium value remain the same. Inhomogeneities in the substrate or the presence of defects change the stress field. Knowing forces on the coating boundaries suggests defect locations and types. Probing the types of geometries and surface roughnesses seen in different types of coatings shows that restricted deformation increases stress concentrations and surface forces seen. Also, avenues for reducing stress via relaxation are discussed. The two-dimensional stress profiles used in these analyses are not possible to measure experimentally, making computational modeling essential. The models developed and methodology presented may be extended to other UV cured coatings or to other methods of coating solidification. Process windows of allowable final conversion-stress-energy-time states suggest what tradeoffs must be made to meet constraints.
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    Dimensional Changes of ProRoot white Mineral Trioxide Aggregate, EndoSequence Root Repair Material, and Biodentine During Setting Using Digital Image Correlation
    (2016-09) Zedler, Amber
    Introduction: Dimensional changes in the materials used for root end fillings could enhance or weaken the seal of the filling. The purpose of this study is to quantify and compare the dimensional changes of ProRoot white Mineral Trioxide Aggregate (Dentsply Tulsa Dental Specialties, Johnson City, TN), EndoSequence Root Repair Material (Brassler, Savanah, GA), and Biodentine (Septodont, Cambridge, ON, Canada) in a simulated root end model during setting of the respective materials. Methods: Dimensional changes of the experimental materials in simulated root ends were measured using digital image correlation. Results: The white MTA demonstrated shrinkage was from 0.86% to 2.98%, with a mean of 1.84% and standard deviation of 0.61%. Shrinkage of Biodentine was measured to be 0.43% to 1.88%, with a mean of 1.30% shrinkage and standard deviation of 0.42%. The ERRM expanded from 0.17% to 2.56%, with a mean of 1.23% and a standard deviation of 0.88%, with all samples demonstrating expansion. Conclusions: The water available during the setting of Mineral Trioxide Aggregate, EndoSequence Root Repair Material, and Biodentine has an effect on the dimensional changes of the material. White MTA and Biodentine have previously been shown to expand with adequate hydration, but may shrink in an environment that is drier.
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    Effect of Creep and Coldness-Induced Shrinkage on the Performance of Laminated Timber Bridge Decks
    (1990-01) Seavey, Robert; Erickson, Robert
    The purpose of this research has been to analyze the effect of creep on stress laminated timber bridges where creep refers to the time-dependent deformational behavior of the wood. Of particular interest has been the cause of stress loss in stress laminated bridges. During the course of this research, another important factor which affects the level of rod stress in timber bridges was discovered: coldness-induced shrinkage of wood. After the construction of a stress-laminated bridge in northern Minnesota in October, 1988, a significant reduction in rod stresses was noted during a particularly cold period in December of that year. Proper rod stresses are needed for a stress-laminated bridge to perform as an orthotropic plate whereby there is a high degree of load sharing between adjacent laminae. An abrupt reduction in rod stress due to external temperatures poses an important design and service issue for timber bridges.
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    Mechanical Properties of High Strength Concrete
    (Minnesota Department of Transportation, 1998-01) Mokhtarzadeh, Alireza; French, Catherine E.
    Researchers conducted an experimental program to investigate production techniques and mechanical properties of high-strength concrete and to provide recommendations for using these concretes in manufacturing precast/prestressed bridge girders. High-strength concretes with 28-day compressive strengths in the range of 8,000 to 18,600 psi (55.2 to 128 MPa) were produced. Test variables included total amount and composition ofcementitious material, portland cement, fly ash, and silica fume; type and brand of cement; type of silica fume, dry densified and slurry; type and brand of high-range water-reducing admixture; type of aggregate; aggregate gradation; maximum aggregate size; and curing. Testing determined the effects of these variables on changes in compressive strength and modulus of elasticity over time, on splitting tensile strength, on modulus of rupture, on creep, on shrinkage, and on adsorption potential as an indirect indicator of permeability. The study also investigated the effects of test parameters such as mold size, mold material, and end condition. More than 6,300 specimens were cast from approximately 140 mixes over a period of three years.
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    Self-Compacting Concrete (SCC) for Prestressed Bridge Girders
    (Minnesota Department of Transportation, 2008-10) Erkmen, Bulent; Shield, Carol K.; French, Catherine E.
    Researchers conducted an experimental program to investigate the viability of producing self-consolidating concrete (SCC) using locally available aggregate, and the viability of its use in the production of precast prestressed concrete bridge girders for the State of Minnesota. Six precast prestressed bridge girders were cast using four SCC and two conventional concrete mixes. Variations in the mixes included cementitious materials (ASTM Type I and III cement and Class C fly ash), natural gravel and crushed stone as coarse aggregate, and several admixtures. The girders were instrumented to monitor transfer length, camber, and prestress losses. In addition, companion cylinders were cast to measure the compressive strength and modulus of elasticity, and to monitor the creep and shrinkage over time. The viability of using several test methods to evaluate SCC fresh properties was also investigated. The test results indicated that the overall performance of the SCC girders was comparable to that of the conventional concrete girders. The measured, predicted, and calculated prestress losses were generally in good agreement. The study indicated that creep and shrinkage material models developed based on companion cylinder creep and shrinkage data can be used to reasonably predict measured prestress losses of both conventional and SCC prestressed bridge girders.
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    Self-compacting concrete for prestressed bridge girders
    (2008-10) Erkmen, Bulent
    Researchers conducted an experimental program to investigate the viability of producing self-consolidating concrete (SCC) using locally available aggregate, and the viability of its use in the production of precast prestressed concrete bridge girders for the State of Minnesota. Six precast prestressed bridge girders were cast using four SCC and two conventional concrete mixes. Variations in the mixes included cementitious materials (ASTM Type I and III cement and Class C fly ash), natural gravel and crushed stone as coarse aggregate, and several admixtures. The girders were instrumented to monitor transfer length, camber, and prestress losses. In addition, companion cylinders were cast to measure the compressive strength and modulus of elasticity, and to monitor the creep and shrinkage over time. The viability of using several test methods to evaluate SCC fresh properties was also investigated. The test results indicated that the overall performance of the SCC girders was comparable to that of the conventional concrete girders. The measured, predicted, and calculated prestress losses were generally in good agreement. The study indicated that creep and shrinkage material models developed based on companion cylinder creep and shrinkage data can be used to reasonably predict measured prestress losses of both conventional and SCC prestressed bridge girders.
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    Self-Compacting Concrete for Prestressed Bridge Girders
    (2008-10) Erkmen, Bulent
    Self-consolidating concrete (SCC), which is different from conventional concrete especially in its fresh state, is a highly workable concrete that flows through congested reinforcement under its own weight alone, filling the formwork without segregation of its constituent materials with a void-free structure, and can be placed without any vibration. Self-consolidating concrete was first developed in Japan in the early 1980s, and the main issues that promoted the development of SCC were the shortage of skilled labor and the emergence of heavily reinforced structures that made it difficult to sufficiently consolidate the concrete which is crucial for its durability. Although some raw materials and chemical admixtures may increase the initial cost, its use is on the rise worldwide for precast concrete construction mainly due to its ease of placement over conventional concrete. Some benefits of using SCC for precast concrete applications are easily quantified such as faster construction, reduced noise level, and improved surface finish which eliminates the need for patching. Other less tangible benefits include worker safety improvements and extended life of the precasting forms. Although SCC has been developed and successfully used for numerous precast and cast-in-place applications worldwide, and both fresh and hardened properties of SCC have been investigated, concerns have remained regarding mix proportioning, acceptance criteria of SCC in its plastic state, and long term behavior (e.g., creep and shrinkage) of SCC precast/pretensioned elements in service. Limited literature is available to evaluate the hardened and long-term behavior of SCC members, particularly creep, shrinkage, and elastic modulus. Furthermore, there is a wide variation in the findings regarding the long-term behavior of SCC. Due to these reasons, many state departments of transportation, including the Minnesota Department of Transportation (Mn/DOT), have been hesitant to allow SCC for precast bridge girder applications. This study was initiated with the intent to investigate the viability of using SCC developed at local precast plants with locally available materials for the construction of precast prestressed SCC girders in the State of Minnesota. The primary objective of the research was to determine both short-term and long-term properties of SCC bridge girders, evaluate the applicability and accuracy of available test procedures, design equations, and material models for SCC bridge girders. The research was divided into several phases. In the first phase, SCC trial mixes were developed using locally available materials from two local precast concrete plants (Plant-A and Plan-B). The developed trial SCC mixes were studied to identify the main parameters that affect the performance of SCC in its fresh state (e.g., flowability and segregation resistance) such as cement, high-range water reducing admixture dosage, and fresh concrete temperature. It was found that variations in cement from the same supplier with no difference in the cement mill report can significantly affect the flowability of SCC, and recommendations were included for the effect of concrete temperature and admixture dosage on fresh concrete properties. In addition, a testing program was undertaken to evaluate the static and dynamic one-dimensional free flow and flow through reinforcing obstacle segregation resistances of SCC and passing ability of coarse aggregate through reinforcing obstacles. Correlations between different test results were investigated to minimize the required number of test methods to adequately evaluate SCC mixtures. The next phase involved casting four SCC and two conventional concrete precast prestressed bridge girders using locally available materials from Plant-A and Plant-B (three girders per plant). The girders were Mn/DOT 36M I-girders with a span length of 38 ft, and design concrete compressive strengths of 7.5 ksi at release and 9.0 ksi at 28 days. The girders were designed incorporating 36 straight strands in the bottom flange, and four strands in the top flange to avoid the need to drape strands (total of 40 strands). This large amount of prestressed strand was used to create a situation with congested reinforcement to challenge the SCC flow. In addition, the large amount of prestress maximized the allowable compressive stresses at release in the bottom concrete fiber to maximize the concrete creep. The section represented one of the most severe cases for the application of SCC. In addition to the girders, companion cylinders were cast to monitor compressive strength, modulus of elasticity, creep, and shrinkage over time. The girders were instrumented and stored in an outdoor storage site for a period of approximately 2 years to monitor both short-term and long-term performance, which included transfer length, camber, and prestress losses. Both short-term (e.g., elastic shortening) and long-term performance of the girders (e.g., prestress losses) were measured and compared to AASHTO (2004 and 2007), PCI Design Handbook 6th Edition (2004), and PCI General Method (PCI, 1975) predictions. The results indicated that the predicted total long-term prestress losses calculated with AASHTO 2004, PCI Design Handbook 6th Edition (2004), and PCI General Method (PCI, 1975) using measured material properties obtained from conventional cylinders were conservative for both SCC and conventional concrete girders (Note that the SCC conventional cylinders were fabricated with a slightly modified process; rather than rodding the cylinders after each lift, the sides of the mold were tapped with a rubber mallet). The predicted long-term losses at the end of the monitoring periods (i.e., approximately 600 days and 450 days for Plant-A and Plant-B, respectively) were larger than measured losses by 2 to 5% for AASHTO 2004 Lump Sum Method, 12 to 15% for AASHTO 2004 Refined Method, 4 to 7% for PCI General Method, and 8 to 11% for PCI Design Handbook Method for all girders. However, the long-term prestress losses computed with AASHTO-2007 (Approximate Estimate of Time-Dependent Losses) were either not conservative or very close to the measured losses for both the SCC and conventional concrete girders at the end of the monitoring periods. The magnitude of the difference between the measured and predicted losses was comparable for both the conventional and SCC girders. Finally the girders were tested in three-point bending to determine the cracking and crack re-opening loads at the University of Minnesota Structures Laboratory. The experimentally measured crack re-opening loads were used to indirectly calculate the remaining effective prestressing forces and total prestress losses. Also, a semi-destructive test method was used to experimentally measure the remaining tendon forces to verify the field measured losses. The measured girder prestress losses were compared to those determined from a fiber-based finite element analysis incorporating time-dependent creep and shrinkage models based on companion cylinder data. The measured, predicted, and calculated prestress losses were generally in good agreement. The study indicated that creep and shrinkage material models developed based on measured companion cylinder creep and shrinkage data can be used to reasonably predict measured field prestress losses of both conventional and SCC prestressed bridge girders.
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    Validation of Prestressed Concrete I-Beam Deflection and Camber Estimates
    (Minnesota Department of Transportation, 2012-06) O'Neill, Cullen; French, Catherine
    The camber at the time of bridge erection of prestressed concrete bridge girders predicted by the Minnesota Department of Transportation (MnDOT) was observed to often overestimate the measured cambers of girders erected at bridge sites in Minnesota, which, in some cases, was causing significant problems related to the formation of the bridge deck profile, the composite behavior of the girders and bridge deck, delays in construction and increased costs. Extensive historical data was collected from two precasting plants and MN counties and it was found that, on average, the measured cambers at release and erection were only 74% and 83.5%, respectively, of the design values. Through data collection, analysis, and material testing, it was found that the primary causes of the low camber at release were concrete release strengths that exceeded the design values, the use of an equation for concrete elastic modulus that greatly under-predicted the measured values, and thermal prestress losses not accounted for in design. Fourteen girders were instrumented and their camber measured and the program PBEAM was used to evaluate the influence of various time-dependent effects (i.e., solar radiation, relative humidity, concrete creep and shrinkage, length of cure and bunking/storage conditions) on long-term camber. Once investigated, these effects were included in long-term camber predictions that were used to create sets of both time-dependent and singlevalue camber multipliers. The use of these multipliers, along with modifications made to the elastic release camber calculations, greatly reduced the observed discrepancy between measured and design release and erection cambers.