This study examines the verification of the mechanical properties of ordinary concrete with Portland cement (OPC) and high-performance concretes (HPCs) after exposure to the effects of varying temperatures, in addition to durability tests as part of the experimental testing. Particular emphasis is placed on the comparison between OPC and HPC. Before describing the behavior of concrete under the influence of heat and the difference between ordinary and high-performance concrete, the microstructures of both hardened mixtures were monitored, and the chemical compounds of the used Portland cement and silica fume were analyzed using a scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR). For the experiments, compressive strength and flexural strength tests were performed before and after stressing at elevated temperatures. Furthermore, the experiments included the following: a study of concrete color changes under the influence of elevated temperatures, the use of the ultrasonic pulse method, the determination of resistance to water and chemical de-icing agents, and an examination of the frost resistance of concrete. This study considers the differences between the two concretes—OPC and HPC—especially in terms of their mechanical properties, as well as their durability and resistance to chemicals. This research contributes to the understanding of the properties of the two types of concrete; subsequently, it can help in the selection of concrete for specific applications.

The presented experimental program focuses on the design of high-performance dry concrete mixtures, which could find application in advanced manufacturing technologies, for example, additive solutions. The combination of high-performance concrete (HPC) with advanced or additive technologies provides new possibilities for constructing architecturally attractive buildings with high material requirements. The purpose of this study was to develop a dry mixture made from high-performance concrete that could be distributed directly in advanced or additive technologies of solutions in pre-prepared condition with all input materials (except for water) in order to reduce both financial and labor costs. This research specifically aimed to improve the basic strength characteristics—including mechanical (assessed using compressive strength, tensile splitting strength, and flexural strength tests) and durability properties (assessed using tests of resistance to frost, water, and defrosting chemicals)—of hardened mixtures, with partial insight into the rheology of fresh mixtures (consistency as assessed using the slump-flow test). Additionally, the load-bearing capacity of the selected mixtures in the form of specimens with concrete reinforcement was tested using a three-point bending test. A reference mixture with two liquid plasticizers—the first based on polycarboxylate and polyphosphonate and the second based on polyether carboxylate—was modified using a powdered plasticizer based on the polymerization product Glycol to create a dry mixture; the reference mixture was compared with the developed mixtures with respect to the above-mentioned properties. In general, the results show that the replacement of the aforementioned liquid plasticizers by a powdered plasticizer based on the polymerization product Glycol in the given mixtures is effective up to 5% (of the cement content) with regard to the mechanical and durability properties. The presented work provides an overview of the compared characteristics, which will serve as a basis for future research into the development of additive manufacturing technologies in the conditions of the Czech Republic while respecting the principles of sustainable construction.

Arguably the most important element in the sustainability of concrete development is the discovery of an optimal sustainable binder and substitution for the increasingly depleted reserves of natural aggregates. Considerable interest has been shown in alkali-activated materials, which possess good characteristics and could be considered environmentally friendly because of their use of secondary materials in production. The aim of this study was the determination of the mechanical properties of three different mixtures based on the same locally accessible raw materials. The reference mixture contained Portland cement, the second mix contained a finely ground granulated blast furnace slag instead of cement, and the third mixture contained a portion of light artificial aggregate. The experiments focused on the testing and mutual comparison of the processability of the fresh mixture and mechanical characteristics (like compressive and flexural strength, as well as resistance to high temperatures and surface layer tear strength tests). Reinforced concrete beams without shear reinforcement and with three levels of reinforcement were also tested with a three-point bend test. The results show that, overall, the mechanical properties of all the tested mixtures were similar, but each had its own disadvantages. For example, the blast furnace slag-based mixture had a more vulnerable surface layer or a debatable loss of bulk density in the light aggregate mix at the expense of the mechanical properties. One of the main results of the research is that it was possible to technologically produce beams from the alkali-activated concrete (AAC) mixture. Then, the performed beam experiments verified the mechanism of damage, collapse, and load capacity. The obtained results are essential because they present the use of AAC not only in laboratory conditions but also for building elements. In beams without shear reinforcement, the typical tensile cracks caused by bending and shear cracks appeared under loading, where their character was affected depending on the degree of beam reinforcement and loading.

This study presents a comparison of the mechanical properties of selected high-performance concrete mixtures, some of which contained a proportion of recycled concrete aggregate (15% or 30%) as a substitute for natural aggregate. A reference mixture without recycled concrete aggregate was used for comparison. Initially, the properties of concrete containing both the natural and recycled aggregate types were characterized. This was followed by a series of mechanical tests investigating the compressive strength, flexural strength, and chemical resistance (including resistance to de-icing agents and sulfuric acid). The structural performance of reinforced concrete (RC) beams produced from the mixtures was assessed, and surface morphology was evaluated using a digital microscope. The results confirmed that the use of recycled aggregate had a measurable yet limited effect on the properties of hardened concrete. While the compressive strength tended to decrease slightly with an increasing degree of replacement, the flexural strength remained stable in all the mixtures. The tested mixtures demonstrated adequate resistance to de-icing agents and sulfuric acid. Interestingly, specimens subjected to a frost-resistance test showed improved flexural strength, potentially due to ongoing hydration or microcrack healing. In addition, the RC beams with partial aggregate replacement achieved a higher load-bearing capacity compared to the reference beams. The optical surface evaluation method proved to be a valuable tool, complementary to conventional strength testing. This research enhances the current understanding of recycled aggregate concrete and supports its potential for structural applications.

The paper deals with an alternative possibility to evaluate the degradation of concrete composites by means of surface changes of surfaces exposed to the relevant degradation agents using digital microscopy. A uniform, rapid and efficient determination of the nature and level of degradation for the various methods dealing with the durability and service life of portland cement-based systems is crucial for eliminating the time-economic costs in the case of diagnostic, verification or type tests. The experimental section specifically addresses the issue for specimens subjected to the test of the resistance to water and de-icing agents and acid attacks.

The paper deals with the area of the use of recycled materials with regard to the potential of sustainable development of the construction industry. The need for recycling and new solutions in materials engineering is constantly growing. Potential sources of recycled raw materials also include rubber recyclate, where two mixtures with different proportions of recycled rubber are compared within the proposed experimental program. The researched mixtures reflect the need for local resources, which also reflects the needs of the circular economy. The experimental program is focused on typical mechanical properties, which are supplemented by specialized tests. Among the results of the experimental program is that with significant proportions of rubber granulate, not only the mechanical properties are affected, but also the durability is significantly affected with regard to the action of the external environment.