Petr Daněk

Comprehensive Testing Techniques for the Measurement of Shrinkage and Structural Changes of Fine-Grained Cement-Based Composites during Ageing

The paper deals with an experimental analysis focusing on the utilization of a specific measurement technique for determining the development of shrinkage and for monitoring structural changes in fine-grained cement-based composites during their ageing. Advanced measurement equipment and procedure allowing simultaneous measurement of length changes, mass losses, acoustic responses, and temperature development were designed and verified by the experimental investigation. The main scope of the experiments performed was focused on finding the relationships between the characteristics being investigated while maintaining a uniform test setup. For the purpose of the experimental measurement, three fine-grained cement composite mixtures were designed and manufactured. The mixtures differed in the water-to-cement ratio (w/c) and in the amount of plasticizer. The measurement outputs are presented in the form of diagrams showing the relations between the studied parameters, such as relative length changes, mass losses, temperature progress, and acoustic emission (AE) activity during solidification of the composites. The measurement results showed close relations between the examined characteristics. The progress of relative length changes together with the progress of mass losses and temperature development is reflected in the AE activity. The advanced measurement procedure and technique provided valuable information about the behaviour of cement-based composites during early setting and long-term hardening.

The Effect of Concrete Quality on the Acoustic Emission Parameters during Three-Point Bending Fracture Test

Acoustic emission is an experimental tool well suited for monitoring fracture processes. The paper presents experiment focused on analysing acoustic emission signals captured during three-point bending fracture test of specimens of concrete. Quantitative acoustic emission techniques were used to measure micro fracture properties. For three different concrete mixtures typical acoustic emission patterns were identified in the acoustic emission records to further describe the under-the-stress behaviour and failure development. If we have a better understanding of the relationships between micro structural events and macroscopic behaviour we can better formulate predictive models for large-scale structural performance and reliability. An understanding of microstructureperformance relationships is the key to true understanding of material behaviours. Three-point bending fracture tests were conducted on these specimens and load versus crack mouth opening displacement (Load-CMOD) diagrams were recorded during the testing.

Experimental Analysis on Shrinkage and Swelling in Ordinary Concrete

The paper deals with the experimental determination of shrinkage development during concrete ageing. Three concrete mixtures were made. They differed in the amount of cement in the fresh mixture, 300, 350, and 400 kg/m3. In order to determine the influence of plasticiser on the progress of volume changes, another three concrete mixtures were prepared with plasticiser in the amount of 0.25% by cement mass. Measurements were performed with the goal of observing the influence of cement and plasticiser content on the overall development of volume changes in the concrete. Changes in length and mass losses of the concrete during ageing were measured simultaneously. The continuous measurement of concrete mass losses caused by drying of the specimen’s surface proved useful during the interpretation of results obtained from the concrete shrinkage measurement. During the first 24 hours of ageing, all the concrete mixtures exhibited swelling. Its magnitude and progress were influenced by cement, water, and plasticiser content. However, a loss of mass caused by water evaporation from the surface of the specimens was also recorded in this stage. The measured progress of shrinkage corresponded well to the progress of mass loss.

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.

Thermal Analysis of Concrete from Panels Subjected to Fire Experiments

This paper presents the results of the differential thermal analysis of cement matrix samples taken from concrete panels with nominal dimensions of 2300 × 1300 × 150 mm after one-sided exposure to high temperatures. The panels were subjected to maximum nominal temperature loads of 550, 600, 800 and 1000 °C. Concrete was also taken from a reference panel (without temperature loading) and investigated. Five samples with a nominal thickness of 20 mm were taken for thermal analysis. They were cut from the central part of the panels using a diamond blade saw. The thermal analysis covered the effects of temperature load on the concrete to a depth of approximately 100 mm from the heated surface of the panel.

Possibilities of Experimental Determinations of the Modulus of Elasticity in the early Stage of Ageing of Cement Composites

This paper deals with the possibilities of experimental determination of the dynamic and static modulus of elasticity of fine-grained cement composites in the early stage of setting and hardening - up to 72 hours. Several cement pastes and cement mortars were produced for the purpose of this experiment. The measurement of the modulus of elasticity on the manufactured cement-based composites was carried out in the first 24 hours, each time only by the ultrasonic pulse velocity test using the innovative Vikasonic instrument. In the following 48 hours, the resonance method and the static load test were employed. The results of the pilot measurement and particularly the assessment of the possibilities of determination of the moduli of elasticity are presented in this paper.

Influence of a Shrinkage-Reducing Admixture on the Damage to the Internal Structure of Alkali-Activated Composites during Testing of the Modulus of Elasticity

This article deals with an experimental determination of the static modulus of elasticity in compression on fine-grained composites based on alkali-activated slag. This experiment included an alkali-activated composite without a shrinkage-reducing admixture and the same composite with a shrinkage-reducing admixture. The test specimens were subjected to testing of the dynamic modulus of elasticity using the ultrasonic pulse velocity test and the resonance method as well as of the static modulus of elasticity in compression. The static modulus of elasticity test was accompanied by the measurement of the acoustic activity of the material using the acoustic emission method, whose advantages is the possibility to detect early formation and propagation of cracks in the internal structure of the material. The output of the described experiment is a detailed evaluation of the differences in the behaviour of the tested alkali-activated composites based on the observed values of the modulus of elasticity and the recorded acoustic activity of the material during loading.

Experimental Determination of how the Static Modulus of Elasticity is Influenced by the Value of the Upper Loading Stress

There is a large number of factors that have a strong influence on the elastic modulus of concrete. These are technological influences as well as issues connected with the testing methods for its determination, one of which is the choice of the upper loading stress of cyclic loading during testing the static modulus of elasticity of concrete in compression. The upper loading stress should be equal to 1/3 of the specimen’s compressive strength, however, its choice is often made incorrectly. This paper describes an experiment focused on discovering the impact that deviating from the 1/3 upper loading stress has on the measured value of the static modulus of elasticity.

Detailed Determination of Mechanical Fracture Parameters of Concrete after Fire Experiments

The aim of this paper is to describe the procedure of determining the mechanical fracture parameters of selected concrete specimens taken from panels after the fire experiments. The records (in form load vs displacement diagrams) of three-point bending fracture tests of these specimens with initial stress concentrators was first advanced corrected and subsequently evaluated using the Effective Crack Model and the work-of-fracture method. The increasing temperatures during the fire experiments ranging between 550 to 1000 °C led to a decrease of modulus of elasticity and fracture toughness values and to the increase of fracture energy value. The 2D laser profile scanner was used to estimate the degree of complexity of fracture surfaces; its statistical dependence on the mechanical fracture parameters proved to be moderate – the absolute value of the correlation coefficient was about 0.5°[–].

Influence of Coarse Aggregate Grain Size on Frost Resistance of Concrete

This paper deals with the influence of the coarse aggregate grain size on the frost resistance of concrete. For the purpose of the experiment, three types of concrete were produced, varying only in the maximum grain size of the used natural aggregate. The evaluation of the frost resistance of concrete is based on non-destructive (NDT) dynamic and static testing methods. The results of the experiment indicate influences of the aggregate used in the production of the concrete on the results of its frost resistance.