Methodology For Determination of Long‐Term Properties and Crack Development Research in Extra Fine Aggregate Cement Composites
2015
Andīna Sprince

Defending
10.04.2015. 14:15, Rīgas Tehniskās universitātes Būvniecības inženierzinātņu fakultātē, Āzenes ielā 16, sēžu zālē

Supervisor
Leonīds Pakrastiņš, Aleksandrs Korjakins

Reviewers
Robertas Balevicius, Bradley Justin Pease, Ainārs Paeglītis

The last couple of decades have seen an increasing growth in the development of new, innovative and different structural cement composite materials, from reduced amount of cement and smaller dimensions of the aggregate, to disperse reinforcement in the form of fibers, chemical additives and a reduced water-cement ratio, creating cement composite matrixes with improved physical and mechanical properties. Although the influence on these properties has been conscious, there are obstacles that prevent a more widespread application of these new cement composites in construction. One of the main obstacles is the insufficient information about the long-term properties and the durability parameters — creep, shrinkage, and cracking — of these improved cement composites, although they are essential in ensuring safe and long exploitation of structures. The main problem is further complicated by the fact that the existing standards for determination of these properties are deficient and ambiguous. In Europe, the mandatory standards include standard EN 12390 “Testing hardened concrete”, which does not provide information on how to determine such long-term and durability-related properties as creep, shrinkage, research of crack development. Another major obstacle is the delay in the introduction of design standards. The calculation methods that have been tested in practice and are used for the traditional, normal-strength concretes are not applicable to the new cement composites. The values given in the empirical schemes are approximate and, for the most part, can be used mainly for predicting the long-term properties of conventional structures, because in each individual project the actual cement compositions, the raw materials used, the environmental conditions, the geometry and age of the specimens can differ from the ones taken into consideration in the models. Unless most of the parameters coincide, it is not permissible to use the developed models, as that would lead to major inaccuracies in the calculations. The theoretical part of the work provides a description of the mechanical properties of cement composites, with focus on the research of long-term and durability-related phenomena. It summarizes different methodologies that can be used for the determination of long-term and durability-related properties and shows various mathematical and empirical models for establishing the long-term properties. Based on the performed literature analysis, a new methodology has been established for determining the long-term properties and crack development research in extra fine aggregate cement composites (with the largest aggregate dimension ≤ 5 mm). The methodology has been approbated experimentally, and the achieved results have been analyzed. Based on the literature analysis, the developed methodology, and the analysis of the experiment data, the conclusion of the thesis presents recommendations for practical application of these results, namely, for improvement of standards and design rules. The developed methodology is of essential importance for the determination of the characteristic properties of the modern and different normal, high, and ultra-high-strength cement composites, which are necessary for the design of safe, durable structures. These properties are also vital for further development of more rational building materials, which would allow reduction of the overall consumption of the material and the weight of the structure, to build higher structures, to use structural elements with more effective cross sections, as well as to design more graceful thin-walled structures by implementing new, more complex forms. The improved properties of the cement composite will make it the most viable choice from the financial point of view, and the only possible material in the field of specialized construction. The newly developed methodology for the long-term properties and crack development research in EFACC describes the composition of the cement composites, the procedures for preparing the specimens and performing the experimental tests, as well as for the processing of the obtained data and the equipment selection. Within the scope of the methodology development, a modified cement composite testing stand for the determination of uniaxial tensile creep strains, and a cement composite testing stand for the determination of uniaxial compression creep strains was successfully adopted. A new uniaxial tensile testing stand specifically suited for the research of cracks and small-sized specimens was also developed. The new methodology can be used for the determination of the mechanical properties of materials in laboratory conditions, and the obtained data can be used for the design of safe buildings and the improvement of the existing standards. The methodology has been approbated for various normal, high, and ultra-high-strength cement composites (15 mixtures) at various ages (including early), with various humidity conditions (humid and air-dry), with specimens of various geometric shapes and dimensions (cubes, cylinders, prisms, plate-type); in total. Various properties of the materials were determined via experiments — compressive strength, tensile strength, modulus of elasticity, uniaxial creep and uniaxial drying shrinkage strains, creep coefficient, specific creep. Crack initiation, development, propagation, characteristics and the size of the crack mouth openings displacement were also determined, and the strain intensity factor, the strain field at the end of the crack and the toughness of the failure were established. Based on the statistical processing of the experiment data obtained during the approbation phase, it can be stated that the new methodology for the determination of EFACC long-term properties and crack development research is stable and can be generally used for the determining the characteristics of various cement composites. Recommendations for the practical application of the obtained results have also been developed with the aim of improving the standards and design rules. The thesis is presented on 172 pages and contains 92 figures, 8 tables, a list of references with 158 items, and 2 appendices.


Keywords
METHODOLOGY; LONG‐TERM; CREEP; SHRINKAGE; CRACK; EXTRA FINE AGGREGATE CEMENT COMPOSITES

Sprince, Andīna. Methodology For Determination of Long‐Term Properties and Crack Development Research in Extra Fine Aggregate Cement Composites. PhD Thesis. Rīga: [RTU], 2015. 172 p.

Publication language
Latvian (lv)
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