Influence of dispersed reinforcement on frost resistance of high-strength basalt concrete

. This work is devoted to the study of the inﬂuence of various types of dispersed reinforcement of basalt ﬁber on increasing the frost resistance of high-strength modiﬁed basalt concrete. The article presents the results of experimental studies of one composition of coarse aggregate concrete modiﬁed with the admixture "Embelit 8-100" and three compositions of basalt ﬁber concrete. 3 types were considered as dispersed reinforcement: mono-reinforcement with basalt ﬁber of 24 mm length – soft ﬁber, mono-reinforcement with basalt micro-reinforcement of 25-40 mm length – stiﬀ ﬁber, poly-reinforcement with a mixture of soft and stiﬀ ﬁber in a ratio of 1:1. According to the test results, experimental values of physical and mechanical characteristics of coarse aggregate high-strength concrete and basalt ﬁber concrete with three types of dispersed reinforcement (at the age of 7, 14, 28 days) were determined: cube strength, concrete compressive strength class, elastic modulus, frost resistance grade concrete. The analysis of the eﬀect of diﬀerent types of dispersed reinforcement on increasing the strength and frost resistance of coarse aggregate basalt concrete modiﬁed with the admixture "Embelit 8-100" was also carried out. The conclusion is made about the prospects of using mono-reinforcement in the form of soft basalt ﬁber to increase the frost resistance of high-strength basalt concrete.


Introduction
High-strength concrete is one of the young and promising materials in the construction industry.It is used in the construction of bridges, roads, skyscrapers, subways and other critical buildings and structures due to its high characteristics (excellent machinability, good mechanical properties and durability compared to conventional concrete).However, some problems limit the use of high-strength concrete, especially in extreme conditions of the far north.Alternating freezing and thawing is the main cause of concrete destruction in cold climatic regions.The expansion of freezing water in the micropores of concrete stone leads to the formation of internal microcracks, which gradually expand as the freeze-thaw cycles increase, which seriously impairs the durability, mechanical properties and abrasion resistance of concrete [1][2][3].In view of this, special requirements for durability and frost resistance are imposed on objects that are constantly exposed to low temperatures and operated both in the air and in contact with aggressive liquids.Now many studies are aimed at the active introduction of high-strength concrete in construction [4,5], standardization of the method of assessing the frost resistance of concrete [6], optimization of the ratio of raw materials in the production of high-strength concrete and prediction of the frost resistance of concrete [7][8][9].In addition, many studies are being conducted to improve the durability and mechanical properties of concrete by including various types of fibers, mineral additives and industrial waste in their composition [10][11][12][13][14][15].The frost resistance of concrete depends on various parameters: water-to-cementitious materials ratio [16], the porosity and degree of moisture saturation of concrete, the curing time of concrete [17], the amount of freezing water, the freezing rate and the type of filler [16].Now the use of basalt in the construction industry is rapidly developing as a reinforcing material with high resistance to aggressive environments and strength (compared with fiberglass) [18][19][20][21].This work is devoted to the study of the influence of various types of dispersed reinforcement from basalt fiber on increasing the frost resistance of high-strength modified basalt concrete due to the improvement of new materials with high strength characteristics and the emergence of the need for construction in extreme climatic conditions.

Materials and Methods of Research
This work is devoted to experimental studies of high-strength concretes containing the complex organic-mineral admixture "Embelit 8-100" and basalt concretes with different types of reinforcement with soft basalt fiber (Fig. 1, a) and stiff basalt fiber -basalt micro-reinforcement (Fig. 1, b).Experimental studies were conducted in the laboratory "Building Materials and Structures" of the Department of Civil Engineering of RUDN University.
The complex organic-mineral admixture "Embelit 8-100" with expanding components produced by LLC "Enterprise Master Concrete" was used in order to improve the physical and mechanical characteristics of concrete.The admixture "Embelit 8-100" is a powdered multicomponent product in which the content of the plasticizer C-3 is 8%, the content of the expanding component in the mineral part of the modifier is 90-100%.
Wet chopped basalt fiber BS16-24-76 manufactured by LLC "Alliance-Construction Technologies" with a fiber length of 24 mm was used as soft fiber, the diameter of monofilaments is 13-17 microns, the type of oiling is water-compatible (it is necessary for basalt fiber to easily disintegrate into monofilaments in an aqueous environment and to be easily distributed in the concrete matrix).
Basalt micro-reinforcement MKA 1.5-4 was used as a stiff fiber.The length of the fiber is 15-40 mm.The study was carried out with the following concrete composition: binder -Portland cement type I = 700 kg/m 3 ; coarse aggregate -crushed stone = 524 kg/m 3 ; concrete admixture "Embelit 8-100" = 56 kg/m 3 ; river sand with a grain size of 2.7 = 830 kg/m 3 as a fine aggregate; water = 283 l/m 3 .
The percentage of basalt fiber reinforcement was assumed to be equal to 2% of the cement mass for basalt concrete compositions.
The studies were carried out on 4 set of samples (Table 1).The test samples were made in the form of cubes with dimensions of 100×100×100 mm.Compressive strength tests were carried out during the curing period for 7, 14, 28 days.The number of samples for determining frost resistance for each set of samples was assumed to be 3, for determining compressive strength for the set C1.00 -12 samples, for all other sets -18 samples each in accordance with the experimental plan (Table 1).The following equipment was used for experimental studies: hydraulic press "Matest CPC040PN167", freezer CM 5/100-80, volumetric deformation measuring device "CONCRETE-FROST", ultrasound device "Pulsar-1".
The frost resistance of the samples was determined by the accelerated dilatometry with a single freezing of water-saturated samples-cubes with dimensions of 100×100×100 mm, using the volumetric deformation measuring device "CONCRETE-FROST" (Fig. 2) in according to GOST 10060.3-95.
The results of study for all four sets of tested samples to determine compressive strength are presented in Table 2 and Figures 3 and 4. The results of study to determine frost resistance are shown in Table 3 and Figure 5.

The effect of dispersed reinforcement on the change in the strength and deformation characteristics of concrete
The following patterns can be seen by analysing the results of tests to determine the compressive strength: the presence of dispersed reinforcement changes the fracture pattern of samples.The mechanisms of destruction of samples are shown in Fig. 4.
The brittle nature of destruction is inherent for set of samples C1.00 without fiber.Angular cracks formed, which opened and destroyed the concrete sample as the load increased during the test.
The destruction is brittle for the poly-reinforced set of samples C2.00.However, vertical cracks parallel to each other are observed.It indicates the occurrence of plastic flow deformations in the fiber-concrete sample due to the inclusion of basalt fibers (both soft fiber and rigid micro-reinforcement) in addition to the appearance of angular cracks as in the control sample.
The load curve and the dynamics of crack development the set of samples C3.00 with mono-reinforcement of rigid fiber clearly indicates the brittle fracture pattern.The fracture pattern is similar to the fracture pattern the set of samples C1.00.However, there is a decrease in the angle of opening of angular cracks, which indicates an increase in the strength characteristics of fiber concrete.
The destruction of fiber concrete samples with soft fiber of the C4.00 set occurs without obvious signs of destruction, which indicates a viscous non-dangerous fracture pattern.
Thus, the most advantageous option of dispersed reinforcement from the point of view of the work of concrete in the structure is the option of mono-reinforcement with soft basalt fiber.The fracture pattern of destruction has changed from brittle to viscous only for set of samples C4.00.
It can be noted that the presence of dispersed reinforcement led to a decrease in compressive strength at the design age of 28 days analyzing the test results presented in Table 2: for samples of the C2.00 set -by 3.5%, the C3.00 set -by 1.75%, the C4.00 setby 15%.

The effects of dispersed reinforcement on the frost resistance of concrete
Different types of dispersed reinforcement considered in this study affect the frost resistance of concrete to varying degrees as can be seen from Table 3 and Table 5.The addition of fiber into the composition of concrete increases the porosity of concrete and leads to an increase in the amount of absorbed water during water saturation of samples.The increase in the mass of water-saturated samples differs for different sets: fibro concrete samples of the C 4.00 set have the smallest increase in mass compared to other fibro concrete samples of the C2.00 and C3.00 sets.
According to the indicator of frost resistance -Value of the coefficient of frost resistance (Table 3) fiber concrete samples showed changes compared to control samples without fiber of the C1.00 set: -samples of the C2.00 set: decrease in the value of the indicator by 17% (indicates an increase in frost resistance); -samples of the C3.00 set: increase in the value of the indicator by 24.17% (indicates a decrease in frost resistance); -samples of the C4.00 set: decrease in the value of the indicator by 43.78% (indicates an increase in frost resistance).
These experimental results describe the operation of soft basalt fiber BS16-12-76.Fibers in the body of concrete exhibit a hydrating effect, reduce the amount of water in the pores of fiber concrete, resulting in reduced volumetric deformations inside the body of the test sample, which leads to an increase in the number of freezing and thawing cycles.
The most advantageous type of dispersed reinforcement to increase the frost resistance of concrete was the type using mono-reinforcement with soft basalt fiber BS16-24-76 set C4.00.The most unfavorable type was the type of mono-reinforcement with rigid fiberbasalt micro-reinforcement MKA 1.5-4 mm set C 3.00.The intermediate position is occupied by the type of poly-reinforcement with soft fiber BS16-24-76 (50%) and rigid fiber micro-reinforcement MKA 1.5-4 mm (50%) of the C2.00 set.

Conclusions
Based on the conducted studies of samples with different types of dispersed reinforcement with basalt fiber, the following conclusions can be drawn: 1.The most optimal type of dispersed reinforcement for high-strength coarse-grained concrete using the admixture "Embelit 8-100" is a type of mono-reinforcement with soft basalt fiber BS16-24-76, as a percentage of 2% by weight of cement.The samples of this set are characterized by a non-dangerous viscous fracture pattern, and there is also a significant increase in the frost resistance of these basalt concrete samples compared to control samples without fiber.The addition of dispersed reinforcement in the form of soft fiber into the concrete composition made it possible to increase the frost resistance grade of concrete from F200 to F300; 2. The presence of dispersed reinforcement leads to a decrease in compressive strength.Depending on the type of dispersed reinforcement, the compressive strength decreases to varying degrees compared to samples without fiber: for samples mono-reinforced with micro-reinforcement (stiff fiber) by 1.75%, mono-reinforced with soft fiber -by 15%, poly-reinforced with micro-reinforcement and soft fiber -by 3.5%; 3. Dispersed mono-reinforcement of modified coarse-grained concrete with stiff fiber in the form of micro-reinforcement MKA 1.5-4 mm, as well as poly-reinforcement with soft fiber BS16-24-76 (50%) and stiff fiber micro-reinforcement MKA 1.5-4 mm (50%)

Table 1 .
Sets of test concrete samples.

Table 2 .
Compression strength test results.

Table 3 .
Frost resistance test results.