Creep and shrinkage behavior of ultra high-performance concrete under compressive loading with varying curing regimes

This Ultra High Performance Concrete research involves observing early-age creep and shrinkage under a compressive load throughout multiple thermal curing regimes. The goal was to mimic the conditions that would be expected of a precast/prestressing plant in the United States, where UHPC beams would be produced quickly to maximize a manufacturing plant’s output. The practice of steam curing green concrete to accelerate compressive strengths for early release of the prestressing tendons was utilized (140°F [60°C], 95% RH, 14 hrs), in addition to the full thermal treatment (195°F [90°C], 95% RH, 48 hrs) while the specimens were under compressive loading. Past experimental studies on creep and shrinkage characteristics of UHPC have only looked at applying a creep load after the thermal treatment had been administered to the specimens, or on ambient cured specimens. However, this research looked at mimicking current U.S. precast/prestressed plant procedures, and thus characterized the creep and shrinkage characteristics of UHPC as it is thermally treated under a compressive load. Michigan Tech has three moveable creep frames to accommodate two loading criteria per frame of 0.2f’ci and 0.6f’ci. Specimens were loaded in the creep frames and moved into a custom built curing chamber at different times, mimicking a precast plant producing several beams throughout the week and applying a thermal cure to all of the beams over the weekend. This thesis presents the effects of creep strain due to the varying curing regimes. An ambient cure regime was used as a baseline for the comparison against the varying thermal curing regimes. In all cases of thermally cured specimens, the compressive creep and shrinkage strains are accelerated to a maximum strain value, and remain consistent after the administration of the thermal cure. An average creep coefficient for specimens subjected to a thermal cure was found to be 1.12 and 0.78 for the high and low load levels, respectively. Precast/pressed plants can expect that simultaneously thermally curing UHPC elements that are produced throughout the week does not impact the post-cure creep coefficient.

Biomechanics of meniscal horn attachments

Menisci are anchored to the tibia by means of ligament-like structures called meniscal attachments. Failure material properties of bovine meniscal attachments were obtained. There were no significant differences in the structural properties or ultimate stress between the meniscal attachments (p>0.05). Furthermore, Glycosaminoglycan (GAG) fraction and crimping frequency was obtained for each attachment using histology and differential interference contrast (DIC) respectively. Results showed that the anterior attachment’s insertion had the greatest GAG fraction when compared to the posterior attachment’s insertion. Crimp frequency of the collagen fibrils was homogeneous along the length. Moreover, Scanning Electron Microscopy (SEM) technique was used to reveal the morphology of collagen in human meniscal attachments. Its midsubstance was composed of collagen fascicles running parallel to the longitudinal axis, with a few fibrils running obliquely, and others transversely. There were no differences between attachments for crimping angle or length. Since ligamentous-type tissues are comprised mainly of water, the fluid pressure within meniscal horn attachments was measured using a Fiber Optic Microsensor (FOM). Four cadaveric human joints were subjected to 2BW compressive load (ramp) at 0-, 15-, and 30-degrees of flexion for a minute and then the load was hold for 20 minutes (equilibrium). There were significant differences between 0- and 15- (p1– c5) were obtained. Significant differences were found on the straightened collagen fibers coefficient (c5) between MP and LA attachments (p