Browsing by Subject "Durability"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Chemo-mechanical Modeling of Stress Corrosion Cracking of High Density Polyethylene in Bleach Solutions
    (2020-10) Tripathi, Anu
    High density polyethylene (HDPE) is increasingly being used in infrastructure applications with a design service lifetime of several decades. In many cases, the member is exposed to a corrosive environment, such as in pipes carrying potable water, where the dissolved bleach selectively attacks the loosely packed amorphous phase of the polymer. The failure mode of HDPE transitions from a ductile to a brittle mode as the corrosion level increases. This leads to subcritical crack propagation that deteriorates the load capacity and long-term behavior of HDPE structure exposed to a chlorinated environment. In this study, we develop a coupled chemo-mechanical model to simulate stress corrosion cracking (SCC) of HDPE members in a bleach solution. The mechanical response of the polymer is described by a constitutive model to considers the individual deformation and damage mechanisms of the amorphous and crystalline phases. The model accounts for the intermolecular deformation and homogeneous void growth in the crystalline and amorphous phases, along with entangled network resistance and craze damage in the amorphous phase. The embrittlement due to corrosion is captured by relating the amorphous phase parameters to the polymer molecular weight which decreases with corrosion level. The proposed model is calibrated using uniaxial tensile tests at different deformation rates, crystallinities, and corrosion levels. The model is used to simulate the double-edge notched (DEN) tension specimens at different corrosion levels. The constitutive model can capture the rate-dependent elasto-viscoplastic behavior of HDPE under the unexposed condition as well as the brittle failure behavior after exposure to a highly corrosive environment. The decrease in the molecular weight of HDPE due to exposure to bleach environment is captured by a reduced-order corrosion kinetics model. The selective diffusion and chemical reaction of bleach into the amorphous phase lead to polymer chain scission that reduces the molecular weight. The corrosion kinetics model describes this diffusion-chemical reaction of bleach and expresses the extent of chain scission as a function of the bleach concentration. The proposed material constitutive model and the diffusion-reaction model are combined in a single finite element (FE) code to investigate the SCC behavior of double edge notched HDPE specimens. The simulation yields the stress-life curves which qualitatively match the measured stress-life data of polymer pipes. The stress-life curve is shown to exhibit different regimes corresponding to distinct failure mechanisms, as indicated by the stress and strain distributions in the specimen. The simulations also provide the fracture kinetics under different environments, which can be used to predict the service life of an HDPE specimen with any geometry and applied load.
  • Thumbnail Image
    Item
    Hygrothermal Performance of Residential Cantilevered Floors
    (2014-05) Stone, Richard Byron
    This field investigation was designed to compare three insulation strategies commonly used in residential cantilevered floors. The first objective is to determine floor surface temperatures above insulated cantilever cavities, with respect to occupant thermal comfort. The second objective is to determine moisture behavior within insulated cantilever cavities, with respect to durability. The experimental set-up was installed in the cantilevered floor of an existing Minnesota home. The first six months of investigation, from mid-summer into early winter, provided data for analysis and discussion presented in this thesis. Investigation results support the view that cantilevered floor cavities open to adjacent conditioned space have warmer floor surfaces above them. Condensation and wetting in some cantilever cavities during colder weather suggests that durability risks are increased in cavities where there is air movement through the thermal insulation. Further investigation is expected to provide a more comprehensive representation of the annual hygrothermal cycle.
  • Thumbnail Image
    Item
    Impact of Low Asphalt Binder for Coarse HMA Mixes
    (Minnesota Department of Transportation, 2017-06) Dave, Eshan V.; DeCarlo, Christopher; Hoplin, Chelsea M.; Helmer, Benjamin; Dailey, Jay; Williams, R. Christopher
    Asphalt mixtures are commonly specified using volumetric controls in combination with aggregate gradation limits, like most transportation agencies, MnDOT also uses this approach. Since 2010 onward, several asphalt paving projects for MnDOT have been constructed using coarser asphalt mixtures that are manufactured with lower total asphalt binder contents. Due to the severe cold climate conditions in Minnesota, there are concerns of premature cracking and inferior durability in asphalt mixtures with lower asphalt binder contents. This research project evaluated 13 low asphalt binder content mixes from 10 actual field projects to determine whether there is potential for poor cracking performance and high permeability. Assessment of field performance indicated an average of 7.75 years of life until 100% transverse cracking level is reached. The pavement structure played a significant factor in controlling the cracking rates. Thin overlays showed almost ten times inferior transverse cracking performance as compared to asphalt wearing courses on full-depth reclamation. Asphalt mixture volumetric factors did not show a statistically significant effect on cracking rates; however, the asphalt binder grade did show a strong effect. Eight out of the 13 coarse asphalt mixtures evaluated in this study have higher permeability than the typical dense graded asphalt mixtures. Performance evaluations using lab measured properties predicted poor thermal cracking performances. No discernable trends were observed between measured or predicted cracking performance and mix volumetric measures. Use of performance tests based on specifications for design and acceptance purposes is reinforced through this study.