Browsing by Subject "Shrinkage"

Now showing 1 - 3 of 3

Binder Jetting of 316L process simulation tools evaluation
(2022) Caballero, K.; Medrano, V.A.; Arrieta, E.; Sandoval, H.; Wicker, R. B.; Medina, F.
Binder Jetting has become one of the most popular Additive Manufacturing technologies over the years due to its low cost and fast production times, nevertheless this technology has a steep learning curve due to the shrinkage induced to parts during sintering. Since shrinkage is not uniform along the part, it’s hard to efficiently determine what areas will be distorted hence this needs to be taken into consideration when designing a new part and many iterations need to be printed until dimensional accuracy is achieved, as a result production time and cost significantly increase. New Binder Jetting simulation tools are being developed and tested; this software will help the technology be more robust and user-friendly for the industry. The software computes a sintering simulation and can provide displacement results making support positioning more efficient, in addition, newer versions of the software can export a compensated model which will be able to be sintered without supports. To evaluate the simulation software, a dimensional test artifact model was designed and printed, then compared with the software predicted model simulation results. The simulation software was used in an initial evaluation of the test artifact geometry to identify areas of concern in the model and document them so efficiency when predicting material behavior during the sintering process can be evaluated. In addition, an evaluation of the effects of different sintering process parameters on the physical and mechanical properties of the material will be analyzed considering the inert sintering atmosphere of the process. Finally, printing parameters of the machine such as layer thickness, binder saturation, and recoat speed among others will also be evaluated.
Investigation of vertical shear strength in unreinforced interfaces between concrete cast at different times
(2022-05) Small, Luke James; Bayrak, Oguzhan, 1969-
This thesis presents an experimental study of vertical interface shear strength between layers of concrete cast at different times. Current design provisions allow only for load to be transferred across reinforced interfaces. However, previous research has suggested that unreinforced interfaces can be manipulated to sustain significant load transfer. The first part of this project was to identify key factors affecting interface shear strength through a review of previous research. The key factors identified were interface roughness, differential stiffness, and differential shrinkage. To quantify the effects of these factors, an experimental program consisting of twenty-seven specimens was conducted in two phases. Phase one consisted on specimens with a precast shell and a cast-in-place core, while phase two consisted of a cast-in-place shell with a precast core. The variables tested were smooth interfaces compared to rough, sandblasted interfaces, conventional concrete compared to shrinkage reducing concrete, and circular compared to square interfaces. Interface slip data was also recorded to better characterize the behavior of the interfaces under loading. Interface strengths observed ranged from 41 psi to 236 psi. It was found that roughened, circular interfaces tended to be the strongest and circular smooth interfaces with shrinkage reducing concrete were the weakest. Results indicated that circular interfaces were preferable compared to square interfaces. Further, the use of shrinkage reducing admixture greatly reduced interface shear strength, which was an unexpected result. Finally, it was concluded that interface roughening, even in a modest amount, can provide significant strength gains independent of concrete strength or stiffness. This research also suggests a modified version of the AASHTO interface shear transfer equation to allow for future discussion on the use of unreinforced interfaces in design applications.
Model of strain-related prestress losses in pretensioned simply supported bridge girders
(2014-05) Gallardo Méndez, José Manuel; Bayrak, Oguzhan, 1969-
Prestressed concrete construction relies on the application of compressive stresses to concrete elements. The prestressing force is typically applied through the tensioning of strands that react against the concrete and induce compression in the concrete. Loss of prestress is the decrease of this pre-applied stress. The conservative estimation of the prestress losses is imperative to prevent undesired cracking of the prestressed element under service loads. A large fraction of the prestress losses is a consequence of concrete deformations. This fraction of the losses can be identified as strain-related losses, and these occur due to instantaneous elastic shortening, and time-dependent creep and shrinkage. Creep and shrinkage of concrete depend on many factors that are extremely variable within concrete structures. The time-dependent behavior of concrete is not well-understood, but recent findings in the topics of concrete creep and shrinkage provide a better understanding of the underlying mechanisms affecting the nature of these two phenomena. However, current design practices and prestress loss estimation methods do not reflect the state-of-the-art knowledge regarding creep and shrinkage. The main objective of this dissertation was the study and estimation of strain-related prestress losses in simply supported pretensioned bridge girders. Simply supported pretensioned girders are widely designed, produced and frequently used in bridge construction. Due to this common use, pretensioned concrete bridge girders has become fairly standardized elements, which results in a reduced variability in the behavior of pretensioned bridge girders, as compare to that of less standardized concrete structures. Hence, a simplified method was calibrated to estimate prestress losses within pretensioned girders to an adequate level of accuracy. To achieve an acceptable accuracy experimental data from the monitoring of pretensioned simply supported girders was used for the calibration of the method. The accuracy of this simplified method is comparable to that achievable using more elaborate methods developed for generic concrete structures.