Pavel Padevět

Micromechanical Analysis of Cement Paste with Carbon Nanotubes

Carbon nanotubes (CNT) are an attractive reinforcement material for several composites, due to their inherently high strength and high modulus of elasticity. There are controversial results for cement paste with admixed CNT up to 500 µm in length. Some results show an increase in flexural or compressive strength, while others showing a decrease in the values. Our experiments produced results that showed a small increase in fracture energy and tensile strength. Micromechanical simulations on a CNT-reinforced cement paste 50×50 µm proved that CNT clustering is the crucial factor for an increasein fracture energy and for an improvement in tensile strength.

Utilization of Recycled Fine-Ground Concrete from Railway Sleepers for Production of Cement-Based Binder

The environmental awareness and potential cost reduction have promoted the recycling of materials in civil engineering. This paper is dealing with the recycling and secondary use of old concrete railway sleepers for reconstruction of old railway lines. In particular, it is focused on the investigation of material properties of a binder prepared from finely crushed old concrete sleepers. This material could be used for strengthening of subsoil and embankments supporting the railway structures. The study shows that the compressive and bending strength of the investigated material is sufficient for this purpose and that the strength can be further increased by suitable curing conditions. These findings could contribute to the utilization of old concrete sleepers during railway reconstructions and safe money spent for the purchase of new materials and disposal of old sleepers.

Influence of Aggregate Stiffness on Fracture-Mechanical Properties of Lime-Based Mortars

Addition of relatively stiff aggregates into lime-based mortars is responsible for an increase of the effective mortar stiffness and stress concentrations around aggregates during mechanical loading. To observe the damage development during the three-point bending and splitting tests a 2D plane-stress nonlinear finite element analysis utilizing isotropic damage model was carried out and the results were validated against experimentally obtained data. The study revealed that the finite element model is able to capture the trends observed during the experimental investigation. The results of the numerical modeling and experimental investigation show the advantages of the use of relatively compliant crushed brick aggregates in ancient structures.

Microstructure of Unburned Clay and its Shrinkage during Drying

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.

The Influence of High Temperatures and Time on the Material and Mechanical Properties of Cement Binder with Fly Ash

The paper deals with the use of fly ash in cement and concrete. Fly ash, primarily waste material, can be used as a binder because of pozzolanic properties. This fact positively influences the economic and ecological part of the construction. At present, also in the context of higher fire protection requirements fly ash, it is mentioned as a possible material for the protection of structures from fire. The article presents and discusses the results of destructive tests of tensile strength and compressive strength and non-destructive verification of dynamic modulus of elasticity of cement pastes with fly ash after high temperature load. The test samples were almost six years old.

Comparison of creep of the cement pastes included fly ash

The paper is devoted to comparison of creep of cement pastes containing fly ash admixture. The size of creep in time depends on the amount of components of the cement paste. Attention is paid to the content of classical fly ash in cement paste and its impact on the size of creep. The moisture of cement pastes is distinguished because it significantly affects the rheological properties of the material.

The Influence of the Chosen Type of Form Lubricator in the Form for Cement Paste with Fly Ash on the Fracture Process Zone

This paper is focused on the cracks growth in mode II of specimens made of cement paste with fly ash. Three prescriptions were made for testing in mode II, during the making air bubbles arose at the bottom of the specimen that were lubricated with oil. These little caverns can have an influence on the growth of the cracks in the fracture process zone. On the other side, specimens that were lubricated with a wax, no caverns were observed. There are complex mechanisms of crack growth in the fracture process zone, cavern or inertial elements have a fundamental effect on the success of the real tests in mode II. Specimens lubricated with wax have a 50 % higher successful running of the test than those, whose forms were lubricated with the oil.

WATER ABSORPTION CAPACITY COEFFICIENT AND MASS MOISTURE OF RAMMED EARTH MATERIAL

This article presents the development of mass moisture of rammed earth material and determines the water absorption capacity coefficient for rammed earth with illite-kaoline clay. Specimens of two prescriptions were rammed in the moulds. They were settled in the box with soft foam that was moistened. The level of moistening was kept constant. The specimens were regularly measured and weighted. Two measuerements were carried out – one of montmorillonite clay during 13 days and one of illite-kaoline clay during three hours. The development of moisture increase was captured and the water absorption capacity coefficient for illite-kaoline clay was determined and compared to coefficients of common building materials.

DRYING CHARACTERISTIC OF RAMMED EARTH WITH ILLITIC-KAOLINITIC CLAY CONTENT

Clay is a traditional construction material which has got to background with introduction of modern materials to building practice. There is no proper material available for clay constructions design due to lack of proper examination of its mechanical properties. This paper focuses on drying rate of rammed earth. Drying is a primary way in which clay gets its strength. It is thus essential to know a duration needed to a construction to get dry. After pass of this time the maximum strength is attained and it is possible to load a construction. Properties of unburned clay are dependent on clay mix composition. In this contribution two sets of testing bodies with different composition are presented. As a bonding agent an illitic-kaolinite clay was used. It was presupposed that amount of used clay in clay mixture has major influence on the speed of drying and a final equilibrium moisture content. This presumption was disproved.

Changes of the Properties of Cement Paste with Fly Ash Exposed to the High Temperature

This paper describes influence of exposure to high temperatures on material properties of cement paste with addition of fly ash. The properties of cement pastes are significant to the assumption behaviour of concrete and concrete structures. In the cement paste containing fly ash, the effect of high temperature up to 600 ° C causes the changes of content in physically bound water and the change in the material structure. The results of research indicate changes that are reflected in the material properties of the cement paste as compressive strength, tensile strength in bending.