Libor Topolar

Application acoustic emission method during concrete frost resistance

Frost resistance is one of the most frequent characteristics of concrete. It is a very complex subject and the test methods themselves are still in development and the international consensus on methodology is still sought, too. The determination concrete frost resistance takes several weeks, months or even more than a year. However micro-structural changes as micro-cracks have not been described sufficiently. Acoustic Emission Method as unusual Non-Destructive Methods can help to monitor structural changes during common frost resistance measuring. Note the Acoustic Emission Method detects only active “defects” into monitored structure. Thus when e.g. crack grows some acoustic waves spread from crack place, i. e. from acoustic emission source. The method does not detect geometric discontinuities and “passive” defects. Selected Non-Destructive Methods as Ultrasound, Non-Linear Ultrasonic Spectroscopy, Impact Echo etc. are used to confirm micro-structural changes. The article describes the first experiment with its imperfections, difficulties and possibilities.

Nondestructive Testing of Advanced Concrete Structure during Lifetime

The paper reports on measurements and analysis of the measurements during hardening and drying of specimens using selected acoustic nondestructive testing techniques. An integrated approach was created for better understanding of the relations between the lifetime cycle and the development of the mechanical properties of concrete. Acoustic emission, impact echo, and ultrasonic techniques were applied simultaneously to the same mixtures. These techniques and results are presented on alkali-activated slag mortars. The acoustic emission method detects transient elastic waves within the material, caused by the release of cumulated stress energy, which can be mechanical, thermal, or chemical. Hence, the cause is a phenomenon which releases elastic energy into the material, which then spreads in the form of an elastic wave. The impact echo method is based on physical laws of elastic stress wave propagation in solids generated by mechanical impulse. Ultrasonic testing is commonly used to find flaws in materials or to assess wave velocity spreading.

Determine Parameters for Double-K Model at Three-Point Bending by Application of Acoustic Emission Method

As concrete is one of the most popular building materials, it is important to know its basic properties and behaviour especially at loading. When cracks occur in concrete, released fracture energy will be proportionally transformed into the energy contained in acoustic emission. According to this physical phenomenon, acoustic emission technique provides an effective monitoring method for fracture process of concrete through generated acoustic emission. However, such monitoring is limited in qualitative evaluation of fracture process in most occasions. Quantitative interpretation of fracture process is difficult to accomplish by simply acquiring the amount of acoustic emission generated or by conducting parameter-based acoustic emission analysis. This paper investigates the use of the double-K fracture model and acoustic emission for prediction of load at the beginning of stable crack propagation in three-point bending tests of concrete specimens. Double-K model combines the concept of cohesive forces acting on the effective crack increment with a criterion based on the stress intensity factor.

Alkali Activated Binders Based Concrete Specimens: Length Change and Fracture Tests

The aim of this paper is to quantify mechanical fracture and length change parameters of the two types of concrete with alkali activated binder. The six beam specimens (75 × 75 × 295 mm) were made from each mixture. After demolding specimens were placed in air storage for 28 days. During this period length change (shrinkage) were recorded in accordance with ASTM C490 (2011). After that the three-point bending test was performed on these specimens with initial stress concentrator at the age of 28 days to obtain the mechanical fracture parameters. Records of fracture tests in form load versus deflection (F–d) diagrams were evaluated using effective crack model and work of fracture method.

Parameters of Acoustic Emission Signals Obtained During the Setting and Hardening of Concrete Mixtures with Different Water-Cement Ratio

Concrete hardening and setting processes are the most critical phases during construction work, influencing the properties of concrete structure. For this reason applying non-destructive testing in the early age of concrete lifetime can be useful. Acoustic emission method is a powerful tool for determination of lifetime concrete structures. Nevertheless, its application in civil engineering area is not easy because many building structures are inhomogeneous. This method can describe material changes during concrete lifetime. Acoustic Emission Method monitors concrete structure continuously. Changes in the whole concrete structure are recorded. The acoustic emission phenomenon is directly associated with nucleation of cracks in building materials, therefore the changes result from the volumetric expansion causing formation micro and macro cracking in structure, which we can recognize. Application of Acoustic Emission Method during concrete hardening and setting with different water-cement ratio will be the aim of this article and his influence on parameters of acoustic emission. A comprehension of microstructure-performance relationships is the key to true understanding of material behaviour.

Experimental Determination of Wheel and Guiding Forces on the Tracks

Force interaction between a railway track and a vehicle affects the safety, comfort, and last but not least, economical maintenance. Train of wagons incidence on track in both transversal and vertical direction is simplified by qualifying of force wheel (FQ), guiding (FY) and perpendicular and transverse acceleration. An experimental basis for dynamic determination of vertical, FQ, and lateral, FY, forces at the wheel using strain measures in the foot of the rail is given. Measurements of the dynamic forces during passing train are normally very costly and uneasy. These method consists of measuring strains at selected points of the rail profile is very simple and therefore interesting.