Ivana Havlíková

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).

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.

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.

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.

Evaluation of Three-Point Bending Fracture Tests of Selected Concrete: Crack Initiation and Acoustic Emission Parameters

In this paper, authors concentrate attention on crack initiation and acoustic emission (AE) parameters obtained from records of three-point bending fracture tests on eight sets of concrete specimens with initial stress concentrator at the age of 28 days. Resistance to stable and unstable crack propagation was quantified via evaluation of load vs crack mouth opening displacement diagrams using Double-K fracture model. The AE technique was used to monitor damage process taking place during testing in specimens.

Evaluation of Wedge Splitting Fracture Tests Using the Double-K Model: Analysis of the Effect of Hemp Fibre Dosage and Length in Concrete Specimens

This paper deals with the results obtained from the employment of a selected fracture model to evaluate wedge splitting fracture tests carried out on hemp fibre concrete specimens. The research work was focused mainly on the effect of the dosage and length of hemp fibres on the initiation part of crack propagation in concrete specimens, and on critical crack opening displacement. Concrete mixtures with different volumetric dosages (0.5, 1.0 and 2.0 %) and fibre lengths (10, 20 and 40 mm) were prepared, and six identical specimens were cast from each mixture. Specimens were also cast from a reference mixture, which was without fibres. The specimens were provided with an initial notch and tested using the wedge splitting test method. Load versus crack mouth opening displacement diagrams were recorded during testing and (after data filtering and appropriate modifications) subsequently evaluated using the Double-K fracture model. This model allows the evaluation of two material parameters – the initiation fracture toughness, which defines the onset of stable crack propagation, and the unstable fracture toughness, which defines the onset of unstable cracking or failure. Finally, the critical crack opening displacement was determined with the assumption of the bilinear function of softening in tension.

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.

Components of Fracture Response of Steel Fibre Reinforced Concrete Specimens

Records of fracture tests on steel fibre reinforced concrete notched specimens in a three-point bending configuration are evaluated in detail and selected results are discussed in the paper. The values of fracture parameters are determined using work of fracture method and double-K fracture model. Primarily, the role of plain concrete as a matrix in steel fibre reinforced concrete specimens is studied with regard to the recorded fracture response.

Evaluation of Fracture Tests of Concrete Specimens via Advanced Tool for Experimental Data Processing

Cement-based composites are traditionally used building materials. Concrete is the basic representative of this type of materials which exhibit the so called quasi-brittle response. Quantification of mechanical fracture parameters is performed using fracture tests on specimens with a stress concentrator. Load versus crack mouth opening displacement (P–CMOD) diagrams are recorded during these tests. In order to correctly evaluate these diagrams, an advanced own developed software tool was used for the data filtering and appropriate modifications. In this paper, the programmed Java utility is generally introduced and its utilization demonstrated on the set of recorded P–CMOD diagrams, which are further evaluated using Double-K fracture model.