Injection mortars for road pavement subgrade fixing

Road pavements have structural layers that perform various functions [1], [2]. A subgrade on which the other layers are located is of great importance, because the slightest deformation of the base causes a slow but irreversible destruction of the entire road structure [3], [4]. The subgrade is a layer that, together with the road surface, redistributes and reduces the pressure from vehicle tires, transferring it to the underlying base layers or directly to the subgrade soil [5,6]. The subgrade can be made of materials such as gravel, sand, crushed stone, etc., and has a special structure to ensure optimal strength and stability [7].

Cement-soil single-layer bases and road surfaces in II and III road-climatic zones (RCZ) are made of Portland cement with a class of at least 32.5 for surfaces and at least 22.5 for subgrade [8]. The lower layer of foundations in II–V RCZ is constructed using soils reinforced with cement of grade not lower than 200; and on category III roads magnesia Portland cements, slag magnesia cements and other cements of grade 300 can be used [9], [10].

European and American structural layers of road pavements of main highways have several distinctive features:

• –    the bases are erected mainly from reinforced stone materials, while reinforced soil is used exclusively in the lower layers of the subgrades [11], [12];

• –    the functions of binders are often performed not only by such traditional materials as cement and bitumen, but also by local low-active resources – granulated blast-furnace slag, slag flour, and fly ash [4], [13];

• –    the construction of road pavement subgrades from concrete class higher than B10 is irrational, therefore lean concrete or stone materials are used for them, which are reinforced with cement (up to 5 wt.%) [14], [15];

• –    the construction of the upper layers of road pavement subgrades is carried out mainly using stone materials that are reinforced with organic binders [16]. Therefore, the total thickness of layers with bitumen binder (coating + top layer of the base) reaches (and sometimes exceeds) 20 cm [17];

• -    the use of reinforced materials opens up the possibility of using local relatively low-strength stone road-building resources, which, without reinforcement, are not applicable for creating the subgrades of roads of the highest categories [8], [18].

Subgrades made of stone materials fixed with organic binders can be made using various methods such from hot asphalt concrete (black crushed stone) [6], [12]; by mixing crushed stone (gravel) with bitumen (or bitumen emulsion with Portland cement) on a road; by impregnating the crushed stone layer with bitumen or binders based on it [13], [14].

Subgrades made of materials with the use of cement are widely used in road construction, which is justified by the use of local stone materials and soils, i.e. resources that are not used without reinforcement on roads above category IV [15], [16]. For the creation of foundations using crushed stone, the use of IMs based on local natural and man-made raw materials is promising [17], [18]. Since there are no granite deposits in the Belgorod region and the cost of its transportation from other regions is constantly increasing, and there is also an increase in bitumen prices, the idea of mass installation of reinforced road pavement bases arises, ensuring a minimum reduction in the resource intensity of road pavements with the simultaneous utilization of local technogenic sands and metamorphic shales [26]– [28] . The research gap is that despite the large number of publications on the creation and study of a wide range of effective materials for strengthening the subgrades of roads, issues of increasing the efficiency of building materials on man-made resources remain insufficiently studied. In this paper, in addition to the utilization of technogenic raw materials, it is proposed to use the developed composite binders to reduce the cost of cement concrete.

The object of research is a range of injection mortars based on composite binders, crushed to S sp = 500 m²/kg, obtained using CEM I 42.5 N, partially replaced by technogenic resources with the addition of superplasticizer Polyplast PFK-NLM. The subject of research is IM fresh and physical-mechanical properties, as well as the performances of the fixed bases

The purpose of the article is the development and comprehensive study of injection mortars for road pavement bases. The tasks to achieve this goal are design of the composition of the IMs, study of their fresh and physical-mechanical properties, as well as the performances of the fixed bases

• 2    Materials and Methods

A wide range of IMs has been developed from composite binders ground to S sp = 500 m²/kg, obtained based on CEM I 42.5 N (Belgorod Cement, Russia), partially replaced by technogenic resources with the addition of the superplasticizer Polyplast PFK-NLM. Waste from wet magnetic separation (WMS) of ferruginous quartzites is a technogenic fine-grained sand of dark gray color, consisting of unrolled quartz particles (about 60%), feldspars, amphiboles, carbonates, magnetite, hematite and their aggregates. Technogenic fibrous materials (TFM), which are waste from basalt production (Izovol mineral wool), were used as binder components. Polyfractional screenings of crushed shale rock with a fineness modulus of 2.7 were used as fine filler. Portland cement was replaced by waste WMS of ferruginous quartzites in the amount of 50 wt.% and 60 wt.% with the addition of 1 wt.% TFM and 0.6 wt.% SP. W/B varied in the range from 0.57 to 0.67, and the proportion of binder and filler was 1 to 8.6.

The specific surface area of bulk raw materials was studied using the PSH-11 device. The workability of concrete mixtures was determined using a standard cone based on the slump flow value. Viscosity was determined by the time it took for IMs to flow through a Marsh viscometer. The average density of the samples was calculated by dividing the mass by the volume. Compressive strength was determined using the standard method of Russian Standard GOST 310.4-81 on cubes with a 70 mm edge. Flexural strength was determined by the three-point method on prismatic samples measuring 40×40×160 mm on a SHIMADZU press. The modulus of elasticity was calculated using the formula

E . = * , ^ε 1 y

where σ 1 is the stress increment up to 30% of the breaking load;

• ε 1у is the sample deformation increment corresponding to the load level Р 1 =0.3Рр;

Р р is the breaking load;

• Р 1 is the corresponding external load increment.

The mechanical properties of the fixed base (compressive strength and deformation modulus) were evaluated on cylindrical samples.

According to Russian Standards GOST 59538-2021 [29] and GOST R 70308-2022 [30], several characteristics must be evaluated for injection mortars: water-cement ratio, workability, water separation, mixture density, compressive strength, frost resistance.

• 3    Results and Discussion

  3.1    Development of compositions and study of the properties of IMs

The developed compositions of IMs from composite binders, crushed to S sp = 500 m²/kg, are presented in Table 1.

Table 1. Composition of injection mortars

Mix ID (binder used)	Consumption, kg per 1 m3					W/B
	binder	crushed stone screening	sand	water	SP
IM1(CEM I 42.5 N)	225	1320	620	128	1.35	0.57
IM 2(CEM I 42.5 N +1%TFM)	225	1320	620	128	1.35	0.57
IM 3(CEM I 42.5 N +SP+1% TFM)	225	1320	620	128	1.35	0.57
IM 4 (CB-50 SP)	225	1320	620	140	1.35	0.62
IM 5 (CB -50+1% TFM)	225	1320	620	140	1.35	0.62
IM 6 (CB -50 SP +1% TFM)	225	1320	620	140	1.35	0.62
IM 7 (CB -60 SP)	225	1320	620	151	1.35	0.67
IM 8 (CB -60+1% TFM)	225	1320	620	151	1.35	0.67
IM 9 (CB -60 SP +1% TFM)	225	1320	620	151	1.35	0.67

The viscosity characteristics of the developed injection solutions indicate their high penetrating ability, since the flow time of the studied materials through the Marsh viscometer for the IM5 and IM6 compositions is 39 and 33 seconds at W/B=0.62 and a water consumption of 140 l/m3 (Fig. 1). Achieving equal flowability of IMs (slump flow=12-13 cm) was carried out by varying the water-binder ratio. At the same time, according to Russian Standard GOST 59538-2021, all the obtained mortars are characterized by workability grade P4 (cone slump is more than 30 cm). The onset of setting is at least 75 minutes, which allows for effective fixing of road pavement bases.

1M1 IM2 IM3 IM4 1M5 IM6 IM7 IMS 1M9

Mix

Fig. 1 - Workability of injection mortars

Low water separation was established - up to 14% after 230 minutes (Fig materials are effective for the construction of road pavement bases.

. 2). Accordingly, these

---1M1

---IM2

---1M3

---1M4

---1M5

1M6

---IM7

---IM8

---IM9

0    --------------------------------------------

15   30   45    60    75    90   120   230

Time, min

Fig. 2 - Water separation of injection mortars

The change in the average density of IM is given in Fig. 3.

Fig. 3 - Changing injection mortars density

An increase in the average density of hardened samples is noted with the use of composite binders, and especially in the presence of a mineral modifier, which indicates an increase in the packing density of crystalline new growths. The use of fine-ground composite binders leads to an increase in the density of cement paste, which can be useful for improving the mechanical properties and strength of the material. In addition, an increase in the content of the high-density phase CSH (I) will contribute to a decrease in the volume of gel submicroporosity.

It was found that the effect of increasing the compressive strength of IMs increases with the use of a composite binder (Fig. 4). The maximum effect is noted for early strength values, especially at the age of 2 days. In particular, at the age of 2 days, the compressive strength of IMs based on CB-50 using the superplasticizer and the mineral modifier increased by 56% compared to the composition without additives.

1M1     1M2 IM3 1M4 IM5 1M6     1M7     1M8    1M9

Mix

Fig. 4 - Changing in compressive strength of injection mortars

The flexural strength also increased by 75% at the same age (Fig. 5). This indicates that the composite binder (including Portland cement, waste WMC ferruginous quartzites, TFM and SP) contributes to a significant increase in the early strength of the IMs.

Fig. 5 - Changing in flexural strength of injection mortars

In addition, the ratio of the strength properties on the second day to the same indicators on the 28th is also high (Figure 6). For compressive strength, this ratio is 0.41 (compared to 0.33 for the control compositions), and for flexural strength - 0.47 (compared to 0.36 for compositions without additives). This confirms the stability and preservation of high strength of IMs with the addition of wet magnetic separation waste, technogenic fibrous materials and carbon black throughout the entire period of operation. The high early strength of the developed IMs allows its effective use for urgent construction and comprehensive repair of road pavements, where rapid creation and restoration of structural strength is required.

Mix

Fig. 6 - Ratio of 2-days to 28-days strength for compression and bending of injection mortars

By the age of seven days, the rate of growth of the compressive strength of the studied IMs based on CB stabilizes to some extent, but still exceeds the values of the control composition by up to 42% (see Fig. 4). At the same time, the flexural strength exceeds the value for the control composition by up to 50% (see Fig. 5). It is interesting to note that the ratio of the tensile strength in bending and compression at the age of 7 days (0.12) exceeds the similar characteristic of the control composition based o Portland cement (Fig. 7). This occurs despite the replacement of Portland cement clinker by Fediuk, R.; Berdnikov, A.; Shilonosov, A.; Panarin, I.; Fediuk, G.

Injection mortars for road pavement subgrade fixing;

more than 50 wt. % with waste WMS of ferruginous quartzites, TFM and SP. The high value of this ratio indicates the development of crack resistance of the material. For a rationally developed composition (IM6), the ratio of flexural and compressive strength also increases with age: on the second day, it is 0.13, on the seventh - 0.11, and on the 28th - 0.12. All these values are not lower, and often exceed the values of the control compositions.

Fig. 7 - Ratio of compressive strength to flexural strength at 2, 7 and 28 days of injection mortars

It was found that the materials based on CB have a more intensive strength gain compared to the control samples made of pure Portland cement. This is explained by the positive effect of the polycarboxylate chemical modifier and active fillers, which help to reduce water demand and accelerate the hydration of alite and tricalcium aluminate, as well as increase heat generation. Superplasticizers help to reduce the water demand of the material, which leads to an improvement in its density and compactness. This contributes to more efficient hydration of clinker minerals and the formation of a stronger structure. In addition, superplasticizers can also affect the rheological properties of the material, ensuring its more uniform distribution and filling of voids. Polymineral components, such as WMS waste from ferruginous quartzites and TFM, actively participate in hydration reactions and the pozzolanic reaction with calcium hydroxide. This contributes to more intensive hydration and the formation of additional strong hydration products.

Increasing the density of the material on composite binders has a positive effect on its performances and durability. Reducing the capillary porosity and permeability of the material for liquids and gases leads to the following advantages:

• 1.    Improved water resistance. Reducing the capillary porosity and permeability of the material reduces the possibility of water and moisture penetration into its structure. This improves water resistance and protection against moisture exchange, which is especially important for materials used in the construction and repair of roads, i.e. objects exposed to moisture.

• 2.    Improved resistance to chemical attack. Reducing the permeability of the material also means that it will be less susceptible to chemical attack by various aggressive environments, such as acids, alkalis and other chemically active substances that can be spilled on the road surface. This increases the material's resistance to corrosion and degradation.

• 3.    Improved mechanical strength. Compaction of the material's microstructure also helps to increase its mechanical strength. Reducing the porosity and increasing the density of the material improves its ability to resist external loads and deformations, which leads to an improvement in its strength properties.

• 4.    Improved durability. All the above factors - improved water resistance, resistance to chemical attack and increased mechanical strength in combination allow to count on the high durability of materials and structures made from them.

Studies of the characteristics have shown that the physical and mechanical properties of IMs made on CB-50 (WMC) in all cases exceed the characteristics of samples of a similar composition made on other binders and thus allows a significant reduction in the consumption of the clinker component.

All developed injection mortars show a frost resistance grade of F 2 100.

The developed highly penetrating mortars meet the requirements of Russian Standard GOST 59538-2021 (Table 2).

Table 2. Compliance of the developed injection mortars with Russian Standard GOST 59538-2021

Characteristic	IMs	Russian Standard GOST 59538-2021
Specific surface area of cement, m2/kg	450	300-500
Compressive strength class	B5-B10	B5- B20
Workability grade	P4 (slump > 30 m)	P1-P4
Water separation	≤8%	2-8%, time of complete water separation more than 30 min (stable)
	≤16%	8-16%, time of complete water separation 20-30 min (conditionally stable)
Waterproof grade	W6-W16	W4-W12
Frost resistance grade	F 2 100	F15-200

Due to the high traffic intensity on the road surface, the study of deformation characteristics is of great importance. One of the main characteristics of concrete deformation is the modulus of elasticity, for the determination of which prisms were tested at the age of 28 days in accordance with the requirements of Russian Standard GOST 24452. The longitudinal deformations of the prisms were measured using dial indicators with a division value of 0.01 mm, on the basis of which the modulus of elasticity was calculated (Fig. 8).

Fig. 8 - Deformative properties of injection mortars

An increase in the deformation characteristics of the mixture hardened on CB-50SP+1%TFM by 112% was established in comparison with the mixture on CEM I 42.5 N cement. Studies of the deformation properties of the mixtures allow us to conclude that it is possible to obtain concrete for road bases based on the developed IMs and crushed stone from metamorphic shale that meet the regulatory documentation for this type of construction [11], [14], [27].

Thus, the possibility of obtaining concrete for road pavement bases by using composite binders and filler from crystalline shale has been proven. It has been established that the strength and deformation properties of concrete using CB-50 based on WMS waste are higher than those of the control concrete on ordinary Portland cement, which can be explained by the high characteristics of the composite binder itself (high dispersion, low water demand, and high activity), due to which the state of the interfacial transition zone at the interface between "cement paste and filler" is improved, as well as the composition and structure of new formations in this zone.

Analysis of the obtained data showed that the use of WMS waste of ferruginous quartzites as a fine filler is effective in obtaining highly penetrating mixtures and allows obtaining a wide range of IMs compositions for the construction of reinforced bases of highways. It is also advisable to introduce superplasticizer additives, the use of which makes it possible to simultaneously obtain two effects such an increase in the workability of the concrete mixture and an increase in the strength of concrete.

3.2 Strengthening of road pavement bases with developed mixtures

Based on the selected compositions of mixtures with high penetrating ability, experiments were conducted to strengthen the crushed stone base layer. Samples were formed by pouring a crushed stone framework in 15x15x15 cm forms, consisting of shale crushed stone of fraction 40-70 mm (Table 3). The approximate consumption of IMs per 1 m2 of base is 100 kg.

Table 3. Characteristics of shale crushed stone for a fixed base

Name of indicators	Value
Total residues on the sieve by weight, % d	97.9
Same 0,5(d+D)	67.3
Same D	6.8
Same 1.25 D	0
Content of lamellar and needle-shaped grains, %	45.2
Content of clay and dust particles, %	0.5
Content of clay in the form of lumps, %	0
Content of grains of weak rocks, %	0
Crushed stone grade by strength	800
Frost resistance, cycles	200
Bulk density	1.3
Class of crushed stone by radionuclide content	1

The average density of the strengthening base shows minimum values at the level of 2278 kg/m3 for the control composition with compaction of the material with the introduction of superplasticizer, WMS waste and mineral modifier (Figure 9). At the same time, the maximum increase in the average density is noted for compositions using all these components (WMS waste - 50 wt. %, mineral modifier - 1.5 wt. %, and superplasticizer - 0.6 wt. %).

Fig. 9 - Average density of strengthening bases

A similar trend is observed for the entire complex of physical and mechanical properties of the reinforced base. Compressive strength, prismatic strength and elastic modulus of the reinforced base show minimum values at the level of 15.3 MPa, 12.0 MPa and 9.9 GPa, respectively, for the control composition with an increase in these values with the introduction of superplasticizer, WMS waste and mineral modifier (Figure 10). At the same time, the maximum increase in all physical and mechanical properties is noted for compositions using all these components (WMS waste - 50 wt. %, mineral modifier - 1.5 wt. %, superplasticizer - 0.6 wt. %).

■ Compressive strength, MPa

Mix

Fig. 10 - Physico-mechanical properties of strengthening bases

These results are confirmed by the study of water absorption by weight, where the maximum value is noted for the control additive-free composition (4.7 wt.%) with a natural decrease upon introduction of a rational content of both superplasticizer and mineral modifier (Fig .11). At the same time, the lowest water absorption was noted for the composition on the composite binder CB-50SP (4.0 wt.%) and CB-

50SP + 1% TFM (4.1 wt.%). Slightly higher values were noted for all compositions with 60% replacement of Portland cement with WMS waste (4.2 wt.%); the equality of values is explained by dilution of the binder, leveling the effectiveness of other small additives (superplasticizer and technogenic fibrous material). Low water absorption means that the material has a low value of open capillary porosity. As a result, low water absorption leads to increased frost resistance (Fig. 12).

Fig. 12 - Frost resistance of strengthening bases

• 4 Conclusions

Based on the test results, it can be concluded that all strength values meet the requirements for the bases of category II highways:

• 1.    Tests of samples of reinforced crushed stone bases for frost resistance showed that samples of all selected compositions on a composite binder withstood 100 cycles of alternating freezing and thawing without any external changes (for control compositions on non-additive cement 75 cycles) and are characterized by low water absorption rates. The developed IMs compositions make it possible to obtain

• 2.    Reduction in material intensity and cost of road construction is possible due to the use of local raw materials and, first, man-made raw materials.

• 3.    This predetermines the need for the widespread use of concretes using local raw materials and industrial waste for the construction of both foundations and road surfaces.

• 4.    Thus, the use of mixtures with high penetrating ability based on composite binders and technogenic sands for the construction of reinforced foundations will not only eliminate expensive crushed stone, thereby reducing the material intensity of road surfaces, but also significantly improve the environmental situation, thanks to the recycling of waste, hundreds of millions of tons of which have accumulated in dumps and tailings of mining and mining and processing plants.

concrete of class B5 - B10 when strengthening crushed stone bases, and they can be used in the construction of category II highway bases.

• 5 Fundings

  The work was carried out within the framework of the state assignment of the Institute of Chemistry, Far Eastern Branch of the Russian Academy of Sciences, topic No. FWFN (0205)-2022-0002.