Papers by Keyword: Shrinkage

Abstract: The Paper deals with problematics of analysis of shrinkage biodegradable polymer materials PLA and PLLA in consequence with setup of parameters. Samples for analysis were prepared by technology of injection molding with parameters set according to Taguchi method. The parameters setting consist of injection speed, holding pressure, temperature and wall thickness. Dependency of parameters measured inside of the mold was compared to parameters set on molding machine. By this comparison we have received data for material processing in praxis.

Abstract: This study focuses on the analysis of plastic injection moulding process simulation using Autodesk Moldflow Insight (AMI) software in order to minimize shrinkage by optimizing the process parameters. Two types of gates which is single and dual gates have been analysed. Nessei NEX 1000 injection moulding machine and P20 mould material details are incorporated in this study on top of Acrylonitrile Butadiene Styrene (ABS) as a moulded thermoplastic material. Coolant inlet temperature, melt temperature, packing pressure and cooling time are selected as a variable parameter. Design Expert software is obtained as a medium for analysis and optimisation to minimize the shrinkage. The polynomial models are obtained using Design of Experiment (DOE) integrated with RSM Center Composite Design (CCD) method in this study. The results show that packing pressure is a main factor that contributed to shrinkage followed by coolant inlet temperature, while melt temperature and cooling time has less significant for both single and dual gates. Meanwhile, single gate shows a better result of shrinkage compared to the dual gates.

Abstract: In this study parts having Open Cell Porous Regular Interconnected Metallic Structure (OCPRIMS) of cubic unit cell and OCPRIMS with inner solid core were fabricated through developed process of Rapid Manufacturing (RM) process. Two approaches were used to fabricate inner solid core i,e one by using metallic phosphor bronze powder and other by using solid phosphor bronze rod. The specimens were fabricated in accordance with ISO 3928: 1999 and subjected to tension-tension and compressive-compressive fatigue testing. The endurance limit of OCPRIMS with solid rod cored structure was found to be on the higher side in comparison to other types of specimens. The results of Scanning Electron Microscopy (SEM) and images of fractured surfaces showed the presence of voids, shrinkage cavities and pointed edges on the designed pores. They acted as stress raisers and facilitated crack initiation and propagation in the specimens.

Abstract: The article is devoted to compare the size of the creep for a time period of cement pastes with admixture of different amounts of fly ash. Attention is given to the size of the deformations that occur in materials saturated with water and also in the dried matrices. The presented results comparing creep size for cement pastes containing fly ash 30 and 50 %. Influence of the amount of ash and the amount of water to the creep size is evaluated in the conclusion.

Abstract: Unburned clay was used for the traditional engineering works for thousands years. Its qualities have not been too appreciated in the last few decades. Because of that, its material characteristics were never researched very rigorously. In last years, we could see increasing demand for use of this building material in building practice and research. The basic problem that should be mitigated by civil engineers handling unburned clay is its considerable shrinkage during drying. The degree of shrinkage depends on the amount of mixture water and on the amount and type of clay minerals. In the unburned clay, like in the cement, clay minerals play role of the binder. The problem is that clay minerals are causing volume changes. This paper is focused on the degree of shrinkage and its dependence on the species and quantity of clay mineral.

Abstract: Generally, volume changes commences almost immediately after the cement and water come in contact during concrete mixing. Humidity of surrounding environment affects the magnitude of shrinkage. Hence, we took it into account by means of Eurocode 2. This paper proposes the evaluation of residual stresses in concrete. To describe the development of residual stresses, magnitude of free shrinkage, degree of its restrain and corresponding value of elastic modulus has to be known. All required quantities was measured apart from degree of restraint. It was tested three types of concrete mixtures. Obtained results show that the free shrinkage value can be significantly reduced by technological way such as the change of concrete composition.

Abstract: The paper deals with evaluation of experimental data collected during the Oparno arch bridge construction and with subsequent analysis of the construction process and long-term behavior with regard to rheological properties of concrete. The Oparno valley bridge is composed of two separate concrete arch structures with spans of 135 metres (this is currently the second longest span of concrete arch bridge in the Czech Republic). It was built using cantilever casting technology with temporary cable-stays and auxiliary pylons. The data recorded for this study include detailed geodetic measurement of the bridge structure during construction, along with measured strains and temperatures in the arches. Most of the data was measured during the bridge construction in 2008 and 2009. Data significant for long term behavior of structure are still being collected. Verification of different concrete material models and their suitability for design of arch bridges built by free cantilevering will be a main result of the analysis. On the basis of a detailed comparison of numerical results and measured deflections, strains and temperatures, it is possible to quantify the impact of rheological properties of the material (or their individual input parameters) on the resulting structural behavior. Unlike previous research, the examined structure is made of reinforced concrete (not prestressed) and consists of compact solid section and in the final state it is mainly in compression.

Abstract: At the beginning of this year prof. Z. P. Bažant and his team published a new numerical model for predicting creep and shrinkage in concrete structures. Model, named B4, is conceptually based on the previous version B3. While early prediction models were based mostly on classical concrete composition, the new model allows for observation the variability of modern concrete compositions, i.e. the effects of admixtures, various aggregate types and increasing concrete strength. The model also captures the effects of environment temperature, multi-decade prediction and autogenous shrinkage. This is important for concretes that are produced in Czech Republic and have higher, but not high strength (about 50 MPa). The model also allows to determine internal parameters according to experimental measurements on laboratory specimens or structural members. Therefore it is possible to refine the prediction of the behavior of structures made of this concrete in the long time periods. However, the increased number of input parameters leads to a higher complexity and it is necessary to have computational tools for practical model application. To provide the model to wider engineering community open structure computational program (in MATLAB environment) was created. Software is freely available for download on the internet. Description of the innovations of the model B4 and demonstration of its relatively simple applications using newly developed software products is a subject of this paper.

Abstract: This paper suggests an interpretation of the concrete resistance to cracking caused by shrinkage. Cement hydration leads to many chemical processes which cause volume changes and changes of the other physico-mechanical properties of concrete. It is necessary to know the development of residual stresses and corresponding capacity of concrete to establishing of the resistance to shrinkage cracking. Intersection of stress and capacity is a time point which determines that resistance. All necessary concrete properties was established experimentally. The three types of micro-concretes were monitored. On the basis of acquired results it can be stated that the optimal composition of micro-concrete was found because no shrinkage cracks should occur during its life-time.

Abstract: Effects of long-term concrete volume variations (creep, shrinkage). The value of the variation of stress in the tendons due to creep, shrinkage and relaxation at time t, the value of the variation of stress in the tendons at time t, due to the relaxation of the prestressing steel.