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In this study, the effect of carbon black and graphite filler on the crack initiation and fracture parameters of fly ash geopolymer mortar is investigated. The carbon black was added in the amount of 0.5 and 1.0% and graphite powder in the amount of 5 and 10% relative to the fly ash mass. The reference mixture without any filler was also prepared. The fracture characteristics were determined based on the results of the three-point bending test of prismatic specimens provided with an initial central edge notch. The fracture experiments were conducted at the age of 48 days. The vertical force ( F ), the displacement measured in the middle of the span length (d), and the crack mouth opening displacement ( CMOD ) were continuously recorded during the test. The records of fracture tests were subsequently evaluated using the effective crack model, work-of-fracture method, and double- K fracture model. The addition of both fine fillers led to a decrease in monitored mechanical fracture parameters in comparison with reference mortar.

This paper investigates the combination of commonly used blast furnace slag (BFS) and quite non-traditional cement kiln by-pass dust (BD) for the preparation of alkali-activated binder. More specifically, the possibilities of alkali-activated slag (AAS) shrinkage compensation and cracking mitigation using BD were studied. For this purpose, mixtures with ratios of BFS/BD ranging from 100/0 to 60/40 were prepared and cured in different environment, namely autogenous conditions, water curing, air curing and their combinations. The results showed that curing regime is a determinative parameter for performance of the mixtures: BD containing specimens showed a certain expansion during the first days of water curing leading to AAS shrinkage compensation, while in absence of water curing increasing dose of BD increased drying shrinkage significantly.

The aim of the presented work was evaluation of an effect of various conditions on the performance of two developed concretes based on alkali-activated slag and cement kiln by-pass dust (BD). BD was used as a partial replacement of natural aggregates while slag as an aluminosilicate precursor activated by a combination of waterglass and sodium hydroxide solution (resulting silicate modulus of 0.5). The concretes differed only in an activator dose which was 4 and 6% of Na2O with respect to the slag weight. The prepared specimens were sealed-cured for the first 28 days and then their resistance to freeze-thaw cycles and aggressive solutions (ammonium nitrate, acetic acid and sulphates) was tested. Evolution of dynamic modulus showed that both concretes resisted well to freeze-thaw cycles as well as to most solutions, where their dynamic modulus tended to increase in most cases or did not change significantly. Only the concrete with 4% Na2O showed poor resistance to acetic acid solution as the specimens completely disintegrated until 50 days.

The main objective of the paper is to employ a multi-parameter fracture mechanics concept to describe crack propagation through a specimen loaded in mixed-mode. This concept in particular was used because it has been shown that application of the generalized fracture mechanics concept can play a key role for materials with specific fracture behaviour, i.e. when fracture processes occur not only in the very vicinity of the crack tip, but also at larger distances from it. Two mixed-mode (I+II) geometries for the investigation of crack behaviour are presented here. The Williams series expansion is used for crack-tip stress field approximation. Then, considering the higher-order terms of the Williams expansion with regard to maximum tangential stress criterion can provide better estimates of the crack deflection angle. The coefficients of the Williams expansion were determined by means of the over-deterministic method for the purposes of this work. This analysis was performed for each cracked configuration, which is very time-consuming and makes the analysis very extensive. The crack propagation angle obtained by means of the generalized fracture criterion is discussed in detail. It was found that single-parameter fracture mechanics is sufficient when applied close to the crack tip and when mode I of loading prevails, while multi-parameter fracture mechanics can be recommended at larger distances from the crack tip and for configurations where mode II becomes dominant.

The paper deals with the experimental determination of the shrinkage process of the alkali-activated slag mortars (AASM). Two mortars which differed especially in the type of used aggregate were mixed for the experiment. The slag-aggregate ratio was 1:3 for both mortars. The waste sludge, obtained from the water glass production, was used as an activator. The desired workability of the fresh mixtures was reached by addition of water. Because of the high water demand of waste sludge, the water-slag coefficient (w/s) was relatively high - 0.98 for the mortar with the natural sand and 0.73 for the mortar with the standard sand. All test specimens were removed from the moulds at the age of 24 hours and left to dry freely in the laboratory conditions with a temperature of 21 ± 2°C and relative humidity of 50 ± 10% during the whole time of measurement. The results show that AASMs which contain waste sludge exhibit rapid shrinkage with a high magnitude independent on the type of aggregate. During the first 40 hours of ageing, both mortars reached approx. 50% of the final value of shrinkage. The high shrinkage was strengthened by a high w/s coefficient needed to maintain the workability of the fresh mixtures.

The paper presents the results of a pilot study focused on the shrinkage process and fracture parameters of two fine-grained materials prepared from ground granulated blast furnace slag and silica sand. Two different activators were used - liquid sodium silicate and sodium hydroxide. The components ratio and the activator dose were the same for both materials and were as follows: activator dose of 6% Na 2 O by the slag weight, 1% of lignosulfonate plasticizer, and the ratio of slag:water:sand of 1:0.45:3. All specimens matured at room temperature. For each material, four curing regimes were designed with respect to the potential application on-site (especially different upper surface treatment and demoulding time). The shrinkage measurement lasted more than 2.5 years, after which fracture tests were performed on the same sets of specimens. Although only a slight nuance was in designed curing conditions, the results showed different sensitivity of investigated materials and monitored parameters to the particular curing regimes.

A mixed-mode geometry has been chosen to investigate a crack propagation using the multi-parameter fracture mechanics concept. The so-called Williams series expansion is used for the crack-tip stress field approximation. It has been shown that application of the generalized fracture mechanics concept can be crucial for materials with specific fracture behaviour, such as elastic-plastic or quasi-brittle one, when fracture occurs not only in the very vicinity of the crack tip, but also in a more distant surrounding. Then, considering the higher-order terms of the Williams expansion in fracture criteria (describing the crack stability and/or crack propagation direction) can bring more precise results. The coefficients of the Williams expansion must be calculated numerically (for instance by means of the over-deterministic method in this work) for each cracked configuration, which is very time-consuming, and the analysis is very extensive even for a few basic cracked specimen configurations. On the other hand, a suitable choice of the geometrical configuration of the cracked disc enables performing experiments only on the specimens that could prove the theory about the importance of using the higher-order terms.

This paper aims to evaluate the effect of replacement of aggregate by recycled one on fracture response of selected composites. These are hemp fibre reinforced cement-based composites. Four different mixtures were prepared: in reference one, the only natural aggregate was used whereas in the other three mixtures, the natural aggregate was replaced by recycled one in the amount of 10, 25 and 50 %, respectively. All mixtures contained hemp fibres with a length of 10 mm in the dosage of 1 % by volume. In order to determine the selected mechanical fracture characteristics of the investigated composites, three-point bending fracture tests were carried out on prismatic specimens with nominal dimensions 40 × 40 × 160 mm 3 provided with an initial notch. During the whole course of the testing, the loading process was governed by a constant displacement increment of 0.02 mm/min. The vertical displacement (midspan deflection) was measured using the inductive sensor mounted on a special measurement frame placed on the specimens. The fracture tests were terminated when the midspan deflection reached 0.5 mm. The mechanical fracture parameters were obtained through a direct evaluation of the fracture test data via the effective crack model and the work-of-fracture method. The paper shows that composites with a higher dosage of recycled aggregate (25 and 50 wt%) achieve similar values of mechanical fracture properties, so this replacement can be recommended.

The aim of this work is to determine the physical, mechanical and fracture characteristics of concrete specimens with alkali activated binder cured at different conditions. The concrete mixture with alkali activated ground granulated blast furnace slag was selected for the purpose of the experiment. The binder was activated by the liquid sodium silicate with the silicate modulus of 2.0. Natural sand and gravel were used as a fine and coarse aggregate, respectively. The maximum aggregate size was 16 mm. The binder : aggregate : water ratio was 1 : 3 : 0.46. During the specimens ageing, the dynamic modulus of elasticity was determined using a resonance method. The fracture characteristics were determined based on the results of the three-point bending test of specimens provided with an initial central edge notch. The fracture experiments were conducted at the age of 3 and 38 days. The loading force (F), the displacement measured in the middle of the span length (d), and the crack mouth opening displacement (CMOD) were continuously recorded during the test. The records of fracture tests were subsequently evaluated using the effective crack model, work-of-fracture method and double-K fracture model. The fracture tests were also accompanied by the monitoring of acoustic responses using the acoustic emission method.

The paper reports a study of the effect of carbon fibres on the mechanical fracture parameters of alkali-activated slag mortars. The carbon fibres were added in the amount of 1, 2 and 3%, respectively with respect to the mass of the slag. The mechanical fracture parameters were determined using evaluation of fracture tests carried out on 40 × 40 × 160 mm beam specimens with an initial central edge notch. The monitored parameters were compressive strength, modulus of elasticity, effective fracture toughness and specific fracture energy. The specimen response during fracture tests was also monitored by means of acoustic emission. It was shown that as the addition of carbon fibres increased the value of compressive strength and modulus of elasticity of alkali-activated slag dropped to 50% in case of the highest amount of fibres. The effective fracture toughness is not significantly influenced by addition of carbon fibres. On the other hand, the fracture energy value gradually increases with addition of carbon fibres, up to more than twofold increase in case of the highest amount of fibres.