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Cracks and cavities belong to two basic forms of damage to the concrete structure, which may reduce the load-bearing capacity and tightness of the structure and lead to failures and catastrophes in construction structures. Excessive and uncontrolled cracking of the structural element may cause both corrosion and weakening of the adhesion of the reinforcement present in it. Moreover, cracking in the structure negatively affects its aesthetics and in extreme cases may cause discomfort to people staying in such a building. Therefore, the following article provides an in-depth review of issues related to the formation and development of damage and cracking in the structure of concrete composites. It focuses on the causes of crack initiation and characterizes their basic types. An overview of the most commonly used methods for detecting and analyzing the shape of microcracks and diagnosing the trajectory of their propagation is also presented. The types of cracks occurring in concrete composites can be divided according to eight specific criteria. In reinforced concrete elements, macrocracks depend on the type of prevailing loads, whereas microcracks are correlated with their specific case. The analyses conducted show that microcracks are usually rectilinear in shape in tensioned elements; in shear elements there are wing microcracks with straight wings; and torsional stresses cause changes in wing microcrack morphology in that the tips of the wings are twisted. It should be noted that the subject matter of microcracks and cracks in concrete and structures made of this material is important in many respects as it concerns, in a holistic approach, the durability of buildings, the safety of people staying in the buildings, and costs related to possible repairs to damaged structural elements. Therefore, this problem should be further investigated in the field of evaluation of the cracking and fracture processes, both in concrete composites and reinforced concrete structures.

Disposal of the coal fly ash (CFA) generated from thermal power plants in huge quantities is one of the major concerns for the industry, as well as the natural environment. On the other hand, CFA can be used within a certain percentage range in the cement concrete mix as a replacement for cement. Nanomaterials can also be used to improve the properties of concrete. Therefore, this study investigated the effects of nanosilica (nS) on the mechanical parameters and microstructure of CFA cement concretes. This study utilized an nS content of 5%, along with three CFA contents, i.e., of 0, 15, and 25% by volume. Mechanical property tests and a thorough overview of changes in the structure of modified concrete were carried out to study the effect of the CFA content on the analyzed parameters of concrete containing nS. This study had the goal of elucidating the reinforcing mechanisms of CFA concrete by nS and providing design guidance for the practical engineering applications of CFA-nS composites. Based on the conducted studies, it was found that the combined usage of nS and CFA has synergistic and positive effects on improving mechanical parameters and microstructure in such concretes. The combined strengthening of a cement matrix by nS and CFA can fill the pores and microcracks in concrete composites and effectively improve the mechanical properties and microstructure of such materials. In this study, the optimal improvement was achieved when the concentration of additions was 5% nS and 15% CFA. The 28-day compressive strength and splitting tensile strength were increased by 37.68 and 36.21%, respectively, in comparison to control concrete. Tailored blended cements composed of nS and CFA content (up to 30% replacement level) can significantly improve the parameters of concrete composites, as well as reduce the carbon footprint of cement-based materials—constituting a step toward the production of eco-friendly concretes.

The article presents studies of plain concretes prepared based on a quaternary binder containing various percentages of selected supplementary cementitious materials (SCMs). The possibilities of nanotechnology in concrete technology were also used. An additional important environmental goal of the proposed solution was to create the possibility of reducing CO2 emissions and the carbon footprint generated during the production of ordinary Portland cement (OPC). As the main substitute for the OPC, siliceous fly ash (FA) was used. Moreover, silica fume (SF) and nanosilica (nS) were also used. During examinations, the main mechanical properties of composites, i.e., compressive strength (fcm) and splitting tensile strength (fctm), were assessed. The microstructure of these materials was also analyzed using a scanning electron microscope (SEM). In addition to the experimental research, simulations of the possible reduction of CO2 emissions to the atmosphere, as a result of the proposed solutions, were also carried out. It was found that the quaternary concrete is characterized by a well-developed structure and has high values of mechanical parameters. Furthermore, the use of green concrete based on quaternary binders enables a significant reduction in CO2 emissions. Therefore quaternary green concrete containing SCMs could be a useful alternative to plain concretes covering both the technical and environmental aspects. The present study indicates that quaternary binders can perform better than OPC as far as mechanical properties and microstructures are concerned. Therefore they can be used during the production of durable concretes used to perform structures in traditional and industrial construction.

This paper presents the experimental results and deep discussion on the simultaneous effect of fly ash (FA) and synthetic nano-SiO 2 on the main strength parameters and fracture toughness expressed by critical stress intensity factor, K Ic S of a new concrete based on ternary blended cements (TC). Four series of concretes were made, one of which was the control concrete and the remaining three were TC. The effect of constant content of 5% nano-SiO 2 and various FA contents such, i.e.: 0, 15 and 25% wt. as partial replacement of cement were studied. During the studies the DIC technique was applied to determine the deformation of the concrete beams in the propagation area of the modelled initial crack. Based on the studies it was found that the addition of 5% nano-SiO 2 without FA increases the strength and fracture toughness of concrete by approx. 20%. On the other hand supplementing the composition of the binder with 5% nano-SiO 2 in combination with the 15% FA additive causes an increase in all mechanical parameters by another approx. 20%. However, an increase in the FA content in the concrete mix by another 10% causes a smaller increase in the all analysed factors, i.e. by approx. 10% compared to composite with the addition of nano-SiO 2 modifier only. In addition, based on the studies using DIC technique it was observed that in concrete including only nano-SiO 2 the crack paths were almost perfectly rectilinear in shape, with a significant width of opening. However, in concrete containing 5% nano-SiO 2 + 15% FA the crack paths were curvilinear with characteristic additional microcracks in the vicinity of the main crack, whereas in concrete with the addition of 5% nano-SiO 2 + 25% FA the crack paths were very strongly curved and had pronounced branching and numerous additional microcracks in the vicinity of the main crack. From an application point of view, concretes involving FA and nano-SiO 2 can be used in the execution of specific types of concrete and reinforced concrete structures that require a material with reduced brittleness and at the same time increased fracture toughness.

The following article introduces, in a thorough manner, how the chemical pozzolanic reaction takes place in cement composites containing the fly ash (FA) additive. In the research part, however, the development of phases in the structure of the cement paste in the initial period of its curing and after 28 days from its preparation was traced. For this purpose, a Scanning Electron Microscope (SEM) was used. In order to accurately highlight all the characteristic stages of the formation of the structure of the composite containing FA, an analysis of the cement matrix was carried out between 0.5 and 28 days of their curing. Microstructural studies were complemented by tests of pozzolanic activity of FAs used. In order to conduct a full analysis of this feature, experiments were carried out using two types of research methods, i.e., chemical and physical. On the basis on the conducted studies it was found that: in cement composites with the addition of FA, in the period until the third day of curing, the development of the material structure is mainly the result of the hydration reaction, and between the seventh and fourteenth day after sample preparation, the first signs of the pozzolanic reaction on FA grains are visible; however, in the period between 14 and 28 days, there is a clear homogenization of the structure of the cement composite with the addition of FA, resulting from the change of disordered phases into compact and homogeneous forms and filling in the composite of porous places with pozzolanic reaction products. The use of cement composites based on materials whose application makes it possible to reduce GHG emissions to the atmosphere, reduce energy consumption, and reduce industrial waste landfills leads towards the development of ecological and sustainable building engineering.

This article is devoted to the study of the combined effect of siliceous fly ash (FA), silica fume (SF), and nanosilica (nS) on the cement matrix morphology and size of microcracks occurring in the Interfacial Transition Zone (ITZ) between the coarse aggregate and the cement paste of concrete composites based on ordinary Portland cement (OPC). The manuscript contains analyses of width of microcracks (Wc) occurring in the ITZ area of concretes based on quaternary blended cements and changes in ITZ morphology in the concretes in question. Experiments were planned for four types of concrete. Three of them were composites based on quaternary blended cements (QBC), while the fourth was reference concrete (REF). Based on the observations of the matrices of individual composites, it was found that the REF concrete was characterized by the most heterogeneous structure. However, substitution of part of the cement binder with active pozzolanic additives resulted in a more compact and homogenous structure of the cement matrix in each of the QBC series concretes. Moreover, when analyzing the average Wc values, it should be stated that the modification of the basic structure of the cement matrix present in the REF concrete resulted in a significant reduction of the analyzed parameter in all concretes of the QBC series. For QBC-1, QBC-2, and QBC-3, the Wc values were 0.70 μm, 0.59 μm, and 0.79 μm, respectively, indicating a decrease of 38%, almost 48%, and 30%, respectively, compared with the working condition of concrete without additives. On the basis of the above results, it can therefore be concluded that the proposed modification of the binder composition in the analyzed materials clearly leads to homogenization of the composite structure and limitation of initial internal damages in concrete.

The paper presented herein investigates the effects of using supplementary cementitious materials (SCMs) in quaternary mixtures on the compressive strength and splitting tensile strength of plain concrete. In addition, environmental benefits resulting from the proposed solutions were analysed. A total of four concrete mixtures were designed, having a constant water/binder ratio of 0.4 and total binder content of 352 kg/m3. The control mixture only contained ordinary Portland cement (OPC) as binder, whereas others incorporated quaternary mixtures of: OPC, fly ash (FA), silica fume (SF), and nanosilica (nS). Based on the obtained test results, it was found that concretes made on quaternary binders containing nanoadditives have very favorable mechanical parameters. The quaternary concrete containing: 80% OPC, 5% FA, 10% SF, and 5% nS have shown the best results in terms of good compressive strength and splitting tensile strength, whereas the worst mechanical parameters were characterized by concrete with more content of FA additive in the concrete mix, i.e., 15%. Moreover, the results of compressive strength and splitting tensile strength are qualitatively convergent. Furthermore, reducing the amount of OPC in the composition of the concrete mix in quaternary concretes causes environmental benefits associated with the reduction of: raw materials that are required for burning clinker, electricity, and heat energy in the production of cement.

Fly ash (FA) is a waste product generated in a thermal power plant in huge quantitie has been posing problems of disposal. Therefore, the purpose of this study is to make a meaningful utilization of FA as a binder in concrete composites, while the research involved low volume fly ash concrete (LVFAC). The cements used in the studies were ordinary Portland cement (OPC), (FA-00) and two blended OPC containing FA in the amount of: 20% (FA-20) and 30% (FA-30). This research confronts the following concrete properties: water absorptions by immersion ( wa ) and compressive strength ( f cm ). The methodology of the wa tests was to reflect the conditions prevailing in the case of reinforced concrete structures operating below the water table. Both evaluated properties were measured at 28 days. In addition, the microstructure of all materials was also assessed. The results show that, the best f cm , i.e. 48.96 MPa can be obtained in FA-20 concrete. However, in the case of LVFAC containing 30% FA the compressive strength is decreases to 45.10 MPa. On the other hand the average values of wa was 4.6% for FA-30 and 5.3% for FA-20, respectively. Based on the microstructural studies it was found that this behavior is attributed to reduced pore diameters and densification of cement matrix microstructure in FA-30 concrete. On the other hand concrete of series FA-20 showing a loose and porous microstructure, and wa in this composite increased accordingly. Based on the conducted studies it can be stated that, the LVFAC containing 30% FA may increase durability of reinforced concrete structures subjected to immersion conditions. Furthermore, from an application point of view, the obtained research results may be helpful in understanding the impact of the FA additive on the level of wa in concretes with this waste.

This work presents a discussion of the basic properties of broken mineral limestone aggregates with the specification of the properties affecting the fracture toughness of concretes made with these aggregates. To determine the influence of the grain-size distribution of coarse aggregates for each concrete series, two types of aggregate grain were used, with maximum grain sizes of 8 mm (series of concrete L1) and 16 mm (series of concrete L2). Fracture-toughness tests were carried out using mode I fractures in accordance with the RILEM Draft recommendations, TC-89 FMT. During the experiments the critical stress-intensity factor (KIcS) and crack-tip-opening displacements (CTODc) were determined. The main mechanical parameters, i.e., the compressive strength (fcm) and splitting tensile strength (fctm), were also assessed. Based on the obtained results, it was found that the grain-size distribution of the limestone aggregate influenced the concrete’s mechanical and fracture-mechanics parameters. The obtained results showed that the series-L2 concrete had higher strength and fracture-mechanics parameters, i.e.,: fcm—45.06 MPa, fctm—3.03 MPa, KIcS—1.22 MN/m3/2, and CTODc —12.87 m10−6. However, the concrete with a maximum grain size of 8 mm (series of concrete L1) presented lower values for all the analyzed parameters, i.e.,: fcm—39.17 MPa, fctm—2.57 MPa, KIcS—0.99 MN/m3/2, and CTODc —10.02 m10−6. The main reason for the lower fracture toughness of the concretes with smaller grain sizes was the weakness of the ITZ in this composite compared to the ITZ in the concrete with a maximum grain size of 16 mm. The obtained test results can help designers, concrete producers, and contractors working with concrete structures to ensure the more conscious composition of concrete mixes with limestone aggregates, as well as to produce precise forecasts for the operational properties of concrete composites containing fillers obtained from carbonate rocks.

The paper presents results of tests on the effect of the addition of fly ash (FA) in the amounts of 0%, 20%, and 30% by weight of cement on the interfacial microcracks in concrete composites subjected to dynamic loads. The analyses were carried out based on the results of the microstructural tests using a scanning electron microscope (SEM). The average width of the microcracks (Wc) in the interfacial transition zone (ITZ) of coarse aggregate with cement matrix was evaluated. During the studies beneficial effect of the addition of FA on reduction of the size of Wc in the ITZ of concrete composites under dynamic loading were observed. Based on obtained test results, it was found that using the 20% FA additive causes favorable changes in the microstructure of mature concrete. In this composite, the average value of Wc was lower by more than 40% compared to the result obtained for the reference concrete. In contrast, concrete containing 30% FA additive had greater microcracks in the ITZ area by over 60% compared to the material without additive. In all analyzed composites, an increase in the Wc value by almost 70% to over 110% in the case of occurrence of dynamic loads was also observed. This was the most evident in the case of concrete with a higher content of FA.