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Experimental studies of the deformation properties of concrete based on sulfate-resistant slag Portland cement as a material for hard coatings of agricultural airfields

The paper presents the results of experimental studies of the deformation properties of concretes based on sulfate-resistant slag Portland cement intended for hard coatings of agricultural tracks and airfields. The effect of concrete hardening conditions on the value of its relative shrinkage deformation has been studied. The role of surfactants (plasticizers) as an additional ingredient to concrete is also investigated. Based on these experimental tests, the regularities of the development of plastic deformations formed in concrete in time at a constant load are revealed. The results of the two series of experiments have made it possible to demonstrate the influence of concrete hardening conditions on the values of creep deformation that are also described and analyzed. This article analyzes the kinetics of concrete shrinkage deformations without additives, comparing the results with those for concrete with the addition of a lignosulfonate superplasticizer and also taking into account various modes of heat and humidity treatment of concrete. A comparison of the deformation properties in concrete with sulfate-resistant Portland slag cement with the deformation properties of concrete based on ordinary Portland cement is carried out. The conclusion is made about suitability of concrete with Portland slag cement for use in construction of hard coating for agricultural tracks and airfield runways.


Introduction
To improve the efficiency and reduce the cost of rigid concrete coatings of agricultural tracks and airfields, it becomes necessary to develop new solutions in the field of materials science. In particular, the reduction in the cost of cement, which is part of concrete, is promising direction. The corrosion resistance of the coating is also of great importance. In this case, to determination of the coating deformation it seems important, since this is associated with the safety of aviation and automobile transport.

Formulation of the Problem
Since slag Portland cement is a new material in the construction of pavements of agricultural tracks and airfields, determination of the deformation modulus, and to study the creep and shrinkage of concrete based on this material is the scientific and practical interest.
Studies of the deformation properties of concrete are devoted to the work of many scientists [1-10]. A A Gaikovich and R V Ovchinnikov substantiated the possibility of obtaining concretes of different strengths with given properties based on a cement-ash binder. This made it possible to achieve a reduction in the cost of production by 8.5%, which is equivalent to saving almost 100 kg of cement per 1 m3 of concrete mixture [1][2][3].
The influence of the mineral-chemical additive of the boiler slag on the hardening of cement is considered in the works of Korovkin M O, Eroshkina N A , Zamchalin M N. It has been proven that replacement of the 8-15% of cement by crushed slag reduces the mobility of the mixture, but with an increase its consumption to 20%, workability remains the same as for compositions without additives [4]. To eliminate the negative effect of slowing down the rate of cement hardening with the introduction of slag, it is proposed to introduce a hardening accelerator into the complex additive The works of Belov V V, Subbotin S L, Kulyaev P V are devoted to the study of the deformative properties of concretes with a finely dispersed limestone component. The use of this additive made it possible to reduce the creep and shrinkage of concrete in comparison with concretes without additives [7].
The possibility of using in construction of fine-grained slag concrete on the basis of sifting out from of crushing cast slag crushed stone is substantiated in the works of such scientists as B A Bondarev, N N Chernousov, R N Chernousov, V A Sturov. Due to the introduction into the composition of the cinder-concrete mixture the steel fiber, which has high tensile strength and increased modulus of elasticity, it has become possible to abandon the rod reinforcement in some structures [8].
Studies [9] have shown that the use of high-strength concretes can increase the service life of the structures of cement-concrete road surfaces or achieve significant material savings due to the thinner slab thickness. The use of complex plasticizing additives made it possible to obtain road concretes of C35 / 40 and higher classes [9-10].
The work [11] is devoted to the study of the strength properties and frost resistance of concretes based on slag Portland cement. It substantiates the possibility and feasibility of using sulfate-resistant slag Portland cement for the construction of rigid coatings of agricultural tracks and airfields. At the same time, it is important to determine the deflection of the coating from the load and volumetric deformations of concrete. Therefore, it became necessary to study the deformation characteristics of such concrete.
The aim of the work is to experimentally determine the deformation modulus of concrete on sulfate-resistant slag Portland cement under different hardening conditions; revealing the influence of concrete hardening conditions on the value of its relative shrinkage and creep deformation; study of the effect of additives of surfactants -plasticizers in the form of lignosulfonate on the deformation characteristics of concrete.

Materials and methods
Samples in the form of a prism with dimensions of 10 × 10 × 30 cm were made for the experimental determination of the modulus of deformation of concrete on sulfate-resistant slag Portland cement binder [1, 12, 13]. They were tested at the age of 28 days.
For testing, samples of grades CK 5, CK 3 and CK 9 were selected, which after thermal and moisture treatment were in normal hardening conditions and had the composition shown in table 1 [13]. Determination of the concrete deformation modulus for these samples was carried out by loading them with a load equal to 0.3 -0.5 of the prismatic strength (fcd) and measuring the corresponding deformations. For this, metal corners were glued to the samples on four sides before testing in order to install dial indicators, figure 1. The deformation modulus was determined by dividing the vertical stress in the sample by the corresponding averaged relative deformation.
Samples in the form of a prism with dimensions of 10×10×30 cm were made to study the shrinkage deformations of concrete on a slag Portland cement binder. They began to be tested at the age of one day. As in the determination of the deformation modulus, metal corners were glued to the samples before the study to establish dial indicators, figure 1.
The samples were regularly monitored and the readings of indicators were periodically taken. It is made it possible to determine the relative deformations of concrete shrinkage at various points in time. The maximum duration of the experiments was 250 days. For the study samples of grades CK 1, CK 3, CK 5, CK 9, CK 11, CK 13 and CK 17 [11] were selected, the hardening they took place under normal conditions at a humidity of 90 -95% [11].
In total, several series of experiments were carried out, which made it possible to reveal the influence of concrete hardening conditions on the value of its relative shrinkage deformation. In addition, the role of surfactant additives -plasticizers introduced into concrete was investigated.
As you know, concrete creep is the phenomenon of a gradual increase of deformation under a constant load. Creep is not only a negative phenomenon, since it indicates the relaxation of stresses in time [14].
Typically, in a creep study, concrete specimens are loaded with a load that is about 30% of the breaking value. Taking into account the importance of this characteristic, concrete on slag Portland cement was subjected to tests, which made it possible to reveal how its plastic deformations develop over time under constant load.
Samples were made of concrete on slag Portland cement with dimensions of 10×10×30 cm to carry out these tests, which began to be tested at the age of 7 days. As in the previous cases, corners were glued to the samples to establish dial indicators, figure 1.
Samples of grades CK 3 and CK 9 were selected (table 1 [11]) to test concrete for creep. They hardened after thermal and moisture treatment under normal conditions. After a constant load equal to 30% of the destructive load was applied to the samples, they were regularly monitored and readings of indicators were periodically taken.
Two series of experiments were carried out, which made it possible to reveal the influence of concrete hardening conditions on the values of creep deformation. The maximum duration of the experiments was 220 days.

Research results
The results of determining the modulus of concrete deformation are given in table 1. As can be seen from table 1, the values of the modulus of deformation of concrete on slag Portland cement coincide practically with the data given in the literature for heavy concrete on Portland cement [15,16,17].
The study results of the concrete shrinkage deformations are presented in figures 2 and 3. As can be seen from the graph of the kinetics of shrinkage deformations of concrete without additives, figure 2, the curves are characterized by reaching a plateau parallel to the abscissa axis by 40-50 days. Subsequently, shrinkage deformations begin to increase from the moment of time 150 days, and second part of the curve has a damping character by 250 days. The noted regularities also take place for the graphs of the kinetics of concrete shrinkage deformations of with additives, figure 3, but they are less pronounced.
It is known that concrete shrinkage deformations arise from a reduction in the volume of the cement-water system during cement hydration [18][19][20][21]. The values of the concrete relative deformation of shrinkage on ordinary Portland cement by 180 days at a water-cement ratio of 0.4 are 4-8•10 -4 , depending on the ratio between the aggregate and the cement [22].
The data obtained as a result of experimental studies indicate that, in general, the relative shrinkage deformations of concretes on a slag Portland cement binder, both with and without additives, are in close to the values of the concrete relative shrinkage deformations on ordinary Portland cement in order of magnitude.
The study results of concrete on sulfate-resistant slag Portland cement on creep are presented in the form of a graph, figure 4. In accordance with the data available in the literature [14][15][16][17][18][19][20][21][22], the value of the creep relative deformation of concrete on Portland cement when using gravel aggregate is about 6 • 10 -4 0,5 years after the start of loading.
From the graph shown in Figure 4, it can be seen that the greatest value of the creep characteristic for concretes based on slag Portland cement is 0,9 -1,1. It corresponds to the value of the relative creep deformation, equal to (2,7 -3,2) • 10 -4 also half a year after the start of loading. It follows from this that the relative creep deformations for concretes both on slag Portland cement and ordinary Portland cement are of the same order.

Conclusions
Experimental studies of the deformation properties of concretes on sulfate-resistant slag Portland cement have shown that it has: 1) shrinkage deformations of the same order as that of concrete on Portland cement; 2) the same order of magnitude of creep deformation as for concretes on Portland cement. This magnitude has a damping character at loads of 30% of the destructive value.
The following conclusions can be drawn based on the analysis of the kinetics of the development of concrete shrinkage deformations on a slag Portland cement binder: 1) the nature of the curves for all, without exception, samples hardened under different conditions with and without additives of a superplasticizer in the form of lignosulfonate is the same; 2) for concretes without additives, the kinetic curves of the development of shrinkage deformations have virtually the same shape, and the quantitative differences between them are relatively small; 3 The creep characteristic for concretes based on slag Portland cement is close in order of magnitude to the corresponding characteristic of concrete on ordinary Portland cement.
It was found that elastic deformations and shrinkage and creep deformations are among the structure-sensitive characteristics. This can explain the influence of the steaming mode on their values.
Thus, the studies carried out have shown that concrete based on slag Portland cement binder under use plasticizing additives, in all basic deformation characteristics is at the level of concrete on Portland cement binder and can be used for the construction of prefabricated coverings for agricultural airfields.