Hana Šimonová

Complex Evaluation of Fatigue Tests Results of Plain C30/37 and C45/55 Class Concrete Specimens

The aim of this paper is to summarize basic fatigue parameter values obtained for plain C30/37 and C45/55 class concrete specimens during dynamic tests. Selected approximation curves for relative mechanical-fracture parameter values over time − compressive cube strength, modulus of elasticity, effective fracture toughness and specific fracture energy − were used to determine the most accurate fatigue parameter values corresponding to the age of specimens at the time dynamic tests were performed. Three models are used for standard description of the S−N curve: the Weibull and Gumbel models developed by Castillo et al. as well as the power law equation. Fatigue properties were obtained from three point bending tests conducted on beam specimens (100×100×400 mm nominal size) with a central edge notch (depth 10 mm).

Component Wedge-Splitting/Bending Test of Notched Specimens with Various Crack-Tip Constraint Conditions: Experiments and Simulations

The paper presents a fracture-mechanics study conducted on a novel test configuration based on a combination of the wedge splitting and the bending test geometry. Four variants of this configuration differing in parameters of the specimen shape and boundary conditions were considered in the study. These selected variants exhibit significantly different stress state conditions at the crack tip, or, more generally, in the whole specimen ligament. The variation of the crack-tip stress constraint is of a crucial importance for the study because the expected differences in the fracture process zone extent and the amount of energy dissipation related to this zone is of particular interest here. An extensive experimental campaign was conducted on concrete specimens made of the same mixture. Processing and evaluation of the test data using the standard work of fracture technique was accompanied with numerical simulations by means of finite elements with implemented cohesive crack model to correlate the amount of energy dissipation with the simulated damaged zone extent. Variations of the fracture energy and also the stress/inelastic strain distribution with the change of the crack-tip constraint parameter and the notch length are observed.

Effect of Fibre Type in Concrete on Crack Initiation

Concrete is a traditionally used building material and its crack resistance can be improved by the addition of different types of fibres. Wedge splitting tests on specimens fabricated from five types of concrete mixture (six identical specimens from each mixture) were used to quantify the mechanical fracture parameters of these materials – a reference specimen without fibres, two others with steel fibres (Dramix, Tabix), and a final two with synthetic fibres (Enduro, Strux). Vertical load versus crack mouth opening displacement (P–CMOD) diagrams were recorded during the tests. The data points creating these diagrams were filtered first, subsequently processed and then evaluated using the double-K fracture model. Thus, values were obtained for e.g. Young’s modulus and fracture toughness. This paper is focused on quantification of the effect of the fibre type used in concrete on crack initiation, which corresponds to the beginning of stable crack growth in this sort of composite material.

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.

Evaluation of Concrete Fatigue Measurement Using Standard and Non-Linear Regression Model

The paper deals with the problem of estimating the S-N curve for the cement based composites under fatigue loading. Selected approximation curves for mechanical-fracture parameter values derived from specific fracture energy in time are used to determine the most accurate fatigue parameter values corresponding to the age of specimens when dynamic tests were performed. For standard description of S-N curve the power dependence and Weibull model developed by Castillo et al. are used to discuss the problem under study and the suitability of using models as an input data for finite element modeling

Modelling of interfacial transition zone effect on resistance to crack propagation in fine-grained cement-based composites

In this paper, the attention is paid to investigation of the importance of the interfacial transition zone (ITZ) in selected fine-grained cement-based composites for the global fracture behaviour. This is a region of cement paste around the aggregate particles which specific features could have significant impact on the final behaviour of cement composites with a crack tip nearby this interface under applied tension. The aim of this work is to show the basic interface microstructure by scanning electron microscopy (SEM) done by MIRA3 TESCAN and to analyse the behaviour of such composite by numerical modelling. Numerical studies assume two different ITZ thicknesses taken from SEM analysis. A simplified cracked geometry (consisting of three phases – matrix, ITZ, and aggregate) is modelled by means of the finite element method with a crack terminating at the matrix–ITZ interface. ITZ’s modulus of elasticity is taken from generalized self-consistent scheme. A few conclusions are discussed based on comparison of the average values of the opening stress ahead of the crack tip with their critical values. The analyses dealing with the effect of ITZ’s properties on the stress distribution should contribute to better description of toughening mechanisms in silicate-based composites.

Fracture Process in a Fine-Grained Cement-Based Composite Monitored with Nanoindentation and Acoustic Emission

The contribution is devoted to investigation of the fracture process zone (FPZ) in a fine-grained cement-based composite made from hydrated Portland cement. Particularly, experimental investigations and description of the stable crack propagation using fracture mechanics model are conducted. Three-point bending tests on small composite beams with a central edge notch were performed. The damage due to fracture was monitored by means of nanoindentation performed around the macroscopically observable crack. Acoustic emission events were recorded during the three-point bending test and correlated with load–displacement data. The beneficial effect on the fracture resistance of fine-grained mortar specimens compared to plain cement pastes was quantified.

Mechanical and Fatigue Parameters of Two Types of Alkali-Activated Concrete

The alkali-activated concrete is prepared as a new potential material for the production of concrete elements developed by ZPSV, a. s. company. Note that civil engineering structures are usually made of ordinary Portland cement (OPC) based concrete but today, the cement industry is responsible for emitting between 6% and 7% of all the CO2 emission into the atmosphere. Therefore, it is essential to seek different binders to provide environmental friendly materials. One possible alternative is the application of alkali-activated concrete. The optimal design of concrete mixture was studied in this investigation. Two types of concrete have a similar application and therefore the fatigue parameters can be compared. To this aim, specimens were prepared and tested under static (compressive cube strength) and cyclic loading (fatigue parameters − Wöhler curve). The experimentally obtained results (both mechanical and fatigue) of both types of concrete are compared and the suitability of these types of composites for its application is discussed.

The Effect of the Carbon Nanotubes on the Mechanical Fracture Properties of Alkali Activated Slag Mortars

New nanomaterials such as carbon nanotubes and nanofibres considerably improve performance of current building materials and they can contribute to new application facilities. Alkali activated slag is a material having a great potential to be used in practice. The main drawback of this material is a high level of autogenous and especially drying shrinkage, which causes a deterioration of the mechanical fracture properties. The aim of this paper is introduce the effect of carbon nanotubes admixture on the microstucture and mechanical performance of alkali activated slag mortars. The three-point bending tests of specimens with central edge notch were performed. Method of acoustic emission was used during this testing.

Effect of Cement Dosage on Selected Mechanical Fracture Parameters of Concretes

Mechanical fracture parameters obtained from three-point bending tests on concrete specimens with a central edge notch are introduced in this paper. A total of four sets of specimens were tested. The concrete used in each set differed in cement dosage, which ranged from 250 to 455 kg per m3 of fresh concrete. Three specimens in each set were tested at the age of 28 days. Increasing the dosage of cement influences the mechanical fracture properties of concretes in both positive and negative ways.