No abstract

The crack resistance is the most important parameter of strength. It characterizes the ability of a material structure to resist the beginning of the movement and development of cracks at mechanical and other force impacts and is the most important parameter of the limit state of the structure. In this regard, the evaluation of parameters of the mechanics of rocks, used as fillers (dispersed phase) and the cement stone (matrix phase) for concrete, influencing factor of the actual strength of the cement composite, is not only of theoretical interest, but also of great practical importance, since the issues in regards to the problem of choosing fillers are very poorly addressed in the professional literature, especially re the new generation of concretes.

The problem of a time-dependent gain of a cement dispersed system’s strength remains the focus of both domestic and foreign researchers in terms of its practical significance. To study the above problem, we used non-additive sulfate-resistant Portland cement grade 400 made by Volsk city plant with a normal density of 24 %. 9 series of samples were made from fresh Portland cement. The kinetics of changes in the cement stone’s physical and mechanical properties was observed on series’ samples for 9.5 and 18 years. The paper imparts the values of the cement, water, superplasticizer consumption and compaction ration derived from compacted cement paste after forming test pieces, numerical values of properties in check dates as a result of natural moisture exchange with the environment and drying of test samples to a constant mass at 105°C, as well as the degree of its hydration at the age of 18 years, which was determined by calculation from non-evaporable, that is, chemically bound, water, taking into account that its amount in fully hydrated cement comprises 23% of the cement mass. Changing of the superplasticizer introduction procedure, even with a minimum consumption of S-3, led to a change in the yield value of the cement stone. The experiments targeting to reveal the long-time behavior of a superplasticized cement stone allow us to speak of a clear trend in the influence of SP and its introduction procedure on changes in long-time strength values. However, this is only a trend, since there are many other factors of influence than those discussed in this paper, which testifies to the necessity for a more detailed study of this problem due to its complexity considering the development and application of a new generation superplasticizer and composite binders to construction practice.

The currently widespread assessment of concrete quality, including reliability, by traditional mechanical characteristics is ineffective, since these state parameters do not take into account changes in the strength and deformation properties of concrete under the influence of temporary processes of microcracking during loading when diagnosing structural materials. Modern development of fracture mechanics has made it possible to establish scientific principles for evaluating materials and testing methods and to propose together force, strain, energy and acoustic criteria for crack resistance that determine the actual limit States of their structure in terms of strength and deformability and thus allow predicting the performance of the material under load. This paper presents the results of experimental evaluation of mechanical failure criteria and regularities of changes in acoustic emission parameters from the loading intensity during non-equilibrium mechanical tests for three-point bending of experimental samples with an initial incision depth of 13 mm. Analysis of the influence of the concentration and the introduction of SP-3 on regularities of changes in the coefficients of increase of controlled criteria of crack resistance in time relative to 28-day age quite clearly suggests, first, that the change of parameters of criteria of crack resistance in time has a wavelike nature, and the change in strength under axial compression, and, secondly, about, that the modified structures of cement stone samples relative to the control composition generally give significantly better indicators not only of the growth coefficients of the compared quality criteria, but also of the absolute values of the criteria for crack resistance up to 18 years of age.

It is shown that the construction of buildings and structures requires concrete with universal characteristics, primarily with compressive strength in the range of 35-200 (MPa) with high tensile strength, crack resistance and durability. Strength and elastic-plastic and other properties of concretes and other cement composites are regulated by creating their various formulations. In this regard, it is important to conduct comprehensive studies of the properties of composites in relation to their compositions. Cement composites of various compounding compositions, including fillers, plasticizers, pigments, and biocidal preparations, are considered. Attention is also paid to the technological aspects of preparing mixtures and manufacturing products, namely, the influence of activated water on the properties of concretes of mixing and hardening conditions. Indicators of strength, modulus of elasticity, and brittleness were obtained for compositions that differed in the type of plasticizer (“Melflux 1641 F”, “Melflux 5581 F”, “Fortrace–strong”, “Hidetal-P-5”, “Hidetal GP-9” gamma “A”), a biocidal preparation (Teflex Antimould, Teflex Disinfectant, Teflex for Industrial sights), a pigment (iron oxide red, iron oxide yellow, green glauconito-vogo, meerkat iron). The influence on the properties of powder – activated concretes, rheologically active (rock flour), reactively active (micro - silica), fillers, as well as quartz sand fractions 0, 16 – 0.63 and 0.63-5.0 mm. The results of research showed an increase in the strength of concrete when using plasticizer “Melflux 1641F” and activated mixing water in the technology of their preparation. To increase the strength and modulus of elasticity led to the introduction of rock flour and silica. The introduction of plasticizers led to a decrease in the elastic modulus, and the use of activated closing water does not affect this indicator. From the examined compositions, compositions with the addition of pigments are characterized by increased fragility. The brittleness index of composites is mainly reduced when using the heat-and-water treatment mode compared to normal hardening modes.