National Aviation University – Stabilized Soil – New Material for Road Construction (TPD2008031)

One of the most widespread methods is the strengthening of soils with mineral binders in the form of cement or lime, characterized by the formation of crystallization structure. This structure is formed as a result of irrigating the crystals of a new solid phase that arises from a supersaturated solution, as is the case, for example, with hydration hardening of mineral binder materials.

Soil strenghtening methods are constantly being improved in the direction of improving their structural and mechanical properties. In the process of development of various methods of soil strengthening, they were improved, and at the same time the new effective solutions to significantly improve their structural and mechanical properties were found. The following was established and confirmed by many years of observations in production conditions: when strengthening the soils with two different binders that are characterized by quite different but non-antagonistic properties and structure, they gain increased shear, frost and temperature resistance and, if necessary, can be less rigid and more deformable materials.

The most common method in the practice of road construction was the method of strengthening the soil with cement in combination with calcium chloride, caustic and sodium sulphide [2].

Especially widespread in the practice of road construction is a complex method of soils stabilization with cement in combination with organic additives [3]. This method was developed by V.M.Bezruk and his students. It helps to obtain optimal indicators of stabilized soil, which provide a long service life of the structural layer in the road structure. Bituminous emulsions and pastes, cutback bitumen, crude highly resinious oil and foamed bitumen have been used as organic additives in recent years.

In the practice of road construction, to improve the properties of soil cement, various surfactants are used; they are introduced into the mixture for plasticization and subsequent hydrophobicity of the material formed.

The effectiveness of additives depends on the type of soil, its chemical and mineralogical composition, type of additive and its quantity.

Improved design characteristics and physical and mechanical properties of stabilized soils can be achieved by introducing into the mixture of various synthetic materials in the form of strips or threads. Small scope of researches in this direction allowed to establish that introduction of reinforcement made of synthetic materials into cement soil positively influences its physical and mechanical properties and design characteristics. Cement content of the mixture is of great importance in this case. Characteristically, the compressive strength of samples and their frost resistance increase when introducing the synthetic materials.

When cement is stabilized with fine one-dimensional sands and sand mixtures, the cement consumption rate is high enough to ensure the required strength characteristics of the material, due to the increase in specific surface area of the material. Improvement of the quality of the material is possible by introducing various additives that form an additional structure in it which is common with the structure of the binder. Fuel ashes, slags and ash-and-slag mixtures, which can be simultaneously granulometric and active additives, are quite effective in this respect [4].

The best performance at stabilization of bound soil is obtained by using slaked or ground quicklime as a binder. Clayey part of soil plays a role of a kind of hydraulic additive in relation to lime. As a result, the lime, which is an air binder form, gains the properties of a hydraulic binder.

The most advanced method of soil strengthening is its stabilization, i.e. active influence on the clay-colloid part of the soil, which is the most sensitive to binders that are applied to the soil.

The stabilizer is an active substance, usually of polymeric origin, which is introduced into the soil in micro-doses and improves its building properties.

Stabilization of the soil is a set of measures aimed at providing the soil with a stable condition, constancy, preservation of unchanged properties with the help of stabilizer.

Prof. N.N. Filatov first formulated the basic principle of soil stabilization – the impact on highly dispersed part of the soil to fundamentally change the physical and mechanical properties in general. Therefore improvement of soil properties is possible only on the basis of studying the properties and composition of their finely dispersed part – absorbing complex. Correctly using the adsorption ability of the soil, actively influencing its finely dispersed part with additives of various substances, it is possible to build such road pavement that meet technical requirements.

At the same time, there are two main requirements for stabilizing the soils used for the construction of road foundations and pavements. Firstly, the soil, after the end of hardening and structural formation period, should gain such a degree of stability and irreversible viscosity between the particles that provides reliable resistance to external loads without deformation (deflection and shear) which exceeds the permissible limits. Secondly, it must withstand for a long time the stress arising from the external environment, for example, swelling during humidification and freezing, preserve the structure it has gained during long-trm humidification, temperature rise, etc. [5].

The strength of stabilized soil and its resistance to natural factors required to meet the above requirements may vary depending on the intensity of traffic and axle loads of cars and local natural conditions. This indicates the need for a differentiated approach in assessing the strength and deformation properties of stabilized soil.

1. 2.  Research of the roads state

The authors from the National Aviation University (NAU), the Department of Interior Design, have conducted extensive studies to determine the impact of such stabilizers on the most common soils in Ukraine (heavy dusty loams, light dusty loams, loamy sands and other): Rodbond ЕН – 1 (manufacture of the USA) RRP-235 Special – Reynold and Rod Packer (manufacture of Germany and Canada) Rodbon SPP (manufacture of RSA and Russia); stabilizers GRB-1, GRB-2, GRB-3, GRB-4, GRB-5 (manufacture of the Institute of high-molecular binders of the National Academy of Sciences (NAS) of Ukraine); stabilizer SG (manufacture of the NAS of Ukraine); stabilizer Soiltac (manufacture of company Soilworks of the USA) hydrophobic liquids ГКЖ-II Б, ГКЖ-12, КЖ-94 (manufacture of Zaporizhia joint-stock company “Kremniipolymer”) soil stabilizer Perma-Zyme 11x (manufacture of the USA); liquid glass; calcium chloride (CaCl2), Open Company “Dnipro Association – К” enzyme stabilizer Roadzyme. Stabilizers are recommended for use in the form of diluted aqueous solution.

The results of studies have shown that the best indicators of strength and water resistance can be obtained by using a complex stabilizer consisting of polymeric substance, binders and aggregate). Reinforcing additives in the form of nanofibers are used as aggregate. The combination of different substances in a single composition allows to obtain a stabilized soil with increased strength, moisture and frost resistance properties.

Also, the use of integrated soil stabilization can solve the following problems:

–      Expansion of the types of soils that are suitable for stabilization and longer use in transport facilities (acidic, humus, salted, overwetted soils);

–      prolongation of the construction season due to the possibility of processing the overwetted soils and works fulfillment at adverse temperatures;

–      increased deformability;

–      expansion of application areas at the arrangement of road foundations and pavements and at manufacturing the road design objects on the roads of different categories in all road and climatic zones of Ukraine.

On the basis of the performed studies of NAU jointly with the Institute of Chemistry and Oil Chemistry of the National Academy of Sciences of Ukraine, the design of complex stabilizers on the basis of polymeric high- molecular substance with addition of inorganic binder, basalt aggregate for such types of soils were developed: КGC -1 – for loamy soils (dusty loams). КGС -2 – for sandy soils (sandy loamy sands). The developed compositions of stabilizers differ by the content of polymer and aggregate.

When using loamy soils in КGС -1, the content of high molecular weight polymer ranges from 10- 30% by weight and of basalt aggregate – 25-45% by weight. To stabilize sandy and loamy sandy soils in КGС -2, the content of high molecular polymer is 20-50% by weight, and of basalt nano-aggregate – 20-30% by weight.

As inorganic binder, it is possible to use Portland cement of PC -300- PC -400 grade or quicklime.

The purpose of the work was to solve the assigned task:

–      designing of soil compositions with complex stabilizers КGС -1 and КGС -2;

–      determination of the optimal cost of these complex stabilizers for soil strengthening;

–      determination of physical and mechanical properties of stabilized soil by additives КGС -1 and КGС -2;

–      determination of durability of design objects made of stabilized soil.

Materials for study. For solving these tasks, the samples of different soils were taken for determination of the effect of mineralogical composition, humus substances and acidity of the environment on the properties of stabilized soil. Soils are represented by loamy sand (sample №1) and heavy dusty loam (sample №2) with different specific surface.

The study consisted in comparing the physical and mechanical properties of soils with the addition of stabilizers КGС -1 and КGС -2 in the amount of 5, 10, 14 % by weight of soil. From these mixtures at optimal humidity test cylinder samples and test beam samples were formed. The samples were put into special sealed chamber (in humid conditions) – over a vessel with water for 90 days and tested to determine these characteristics on the 90th day in a water-saturated state [6].

Methodology and results of the study. Characteristics of strengthened soil were determined according to standards [7].

Compressive strength was determined on test cylinder samples using a hydraulic press with a piston free move speed of 3 mm / min.

Tensile strength at bending was determined on test beam samples using the testing device MII -100 at a temperature of +20°C and a strain rate of 3 mm/min.

Freeze resistance tests were performed on three samples of each composition after curing for 90 days. After complete water saturation, the samples were alternately frozen for four hours and thawed.

The number of freeze-thaw cycles was 5, 10, 15. The freezing temperature was -18°C.

The modulus of elasticity of the samples was determined using a pendulum test device at a temperature of +20°C and the duration of the loading 0.1 s.

Durability was determined by the calculation method according by the degradation model which determines the relationship between the reliability of the element and its lifetime.

1. 3.  Results of study

Determination of physical and physical and mechanical properties of selected soil samples are in the tables 1-2.

Table 1. Physical and mechanical properties of soils.

As can be seen, addition of a complex stabilizer improves the physical and mechanical properties of soils of different types. Increasing of density, decreasing of humidity, as a result – the stabilized soil strengthens. The optimal number of stabilizers ranges from 5% to 14% by weight of soil.

The dependences of compressive and tensile strength at bending of stabilized soils on the number of used КGС -1 and КGС -2 are shown in Figure 1-2.

Figure 1. Dependence of compressive strength of soils when using different amount of stabilizers.

Determination of limit compressive strength of soil samples of heavy dusty loam and sandy loam showed that heavy dusty loam with addition of 5% of КGС -1 stabilizer has compressive strength of

0.96 MPa, which is 70% higher than that of sandy loam with the same amount of КGС -2.

As the amount of stabilizer increases, the strength of the samples increases reaching 2.9 MPa at 9% КGС -1 for heavy dusty loam and 4.32 at 13% КGС-1. Sandy loam was characterized by a 12% increase in strength. This indicates that the maximum strength can be obtained by strengthening the soils with a high specific surface area of particles, namely soils with clayey particles.

Figure 2. Dependence of tensile strength at bending of soils when using different amount of stabilizers.

Determination of limit tensile strength at bending showed that in the heavy dusty loam at 5% content of КГС-1 the strength is 0.22 MPa, in sandy loam – 0.19, which is 26 % higher. After increasing of the stabilizer amount, the strength increases and reaches 0.52 MPa and 0.4 MPa, respectively. That is, there is the same dependence as at determination of the compressive strength.

Figure 3. Dependence of freeze resistance coefficient on the number of cycles of “freezing – thawing” of soils with the use of different amount of stabilizers.

The study of freeze resistance showed that stabilized soils of heavy dusty loam and sandy loam are freeze resistant materials and can be used for the manufacture of road design objects.

The strength of stabilized soil has quite high properties similar to those of the lean concrete, so this material is suitable for the arrangement of pavement layers of roads of III-IV categories. This becomes especially relevant in the areas where there is a lack of stone materials.

For final solving of the problem of designing of road pavements with layers of stabilized soil, the modulus of elasticity was determined.

The results of determination of the modulus of elasticity of stabilized soils with different content of stabilizers КGС-1 and КGС-2 are presented in Table 2.

Table 2. The value of the elasticity modulus of stabilized soil.

An important area of use of complex stabilized soil is the manufacture of road design objects.

Durability of road design objects made of complex stabilized soil was determined by the calculation method based on the theory of reliability. It is known [8] that durability is the ability of the object to keep operable condition to the limit state with the established maintenance system.

The quantitative parameter of durability is the service life before overhaul.

The durability of design objects was determined at the stage of operation, taking into account the standard service life of the road before overhaul.

Durability is related to the equation of reliability.

¥

T = ò P(t)dt

0

where Т – average service life, year;

(1)

Р(t) – reliability function (probability of trouble-free operation)

The reliability of design objects made of complex stabilized soil was determined as the probability that the limit state (destruction) will not be reached by a certain service life:

where

Р(t) = P(S £ Sd )

Sd – reserve of operation capacity of an object;

(2)

Sd = R – Q

Sd  = R – Q

(3)

where R – generalized resistance of the object to the current destructive material; Q – generalized action of destructive factors (natural and climatic, load).

The process of destruction of design objects can be described by a model based on the theory of random Kharkiv processes.

The operable period of one or another design object is divided into 5 states depending on the surface damage in %.

The reliability of the design objects in each of the states is shown in table 3.

Table 3. Condition of design objects during operation period.

When n = 5, the end time of operation which must occur after the end of the normal service life before the overhaul of the road is determined.

Graphical interpretation of the reliability of operation of design objects is shown in Figure 4.

Figure 4. Curves of change of design object states produced of the stabilized soil.

Thus, it is determined that the non-operable (critical) state of the design objects occurs after 10 years of operation in natural conditions.

1. 4.  Examples in design

Today, the development of environmental design and the creation of small architectural forms occurs in the context of changing values of society and the priorities of technologies that ensure sustainable development with energy saving. The figures show examples of objects and objects of environmental design made of fortified soils.

Figure 5. Enclosing columns from the strengthened soil.

Figure 6. Decorative container for flowers from fortified soil.

Figure 7. Flowerports from the strengthened soil.

Figure 8. Flowerports from the strengthened soil.

Figure 9. Supports under a bench from the strengthened soil.

The main methods used for the manufacture of most MAF are vibrocasting using a vibrating table (Figure 8) and vibropressing. During vibrocasting, ready-made molds are used for future products (Figure 9). These methods are studied by students-designers of NAU during industrial practice and are applied at development of subjects of design and small architectural forms (Figure 10-18).

Figure 9. Vibrating table for the manufacture of small architectural forms by vibrocasting.

Figure   11.       Bench   with   flower    garden. Development of students. NAU N. Pilipenko.

Figure 10. Wooden forms for small architectural forms made of stabilized soil.

Figure 12. Wheelchair. Development of students. NAU A. Fabiyanska.