1. Introduction
The road construction sector all around the world is the largest consumer of natural aggregates [
1]. It is reported that global annual use of the aggregates constitutes over 40 billion tons, 90% of which is produced on the basis of natural resources. The rest are aggregates, made of the recycled materials (about 5%), synthetic aggregates (2%) and aggregates, extracted from the sea area (about 2%) [
2]. Despite the presence of a number of areas, rich in natural aggregate sources, certain countries are characterized by a deficit of natural aggregates. It is often the case that a fine aggregate is inapplicable in construction because of too small a grain size.
The use of industrial by-products in road construction can contribute to the discussion on reducing the consumption of natural resources and reducing the areas of landfills. Glass waste could be considered as a potential alternative secondary raw material in road construction [
3,
4,
5,
6]. Waste glass is an excellent material to be subjected to repeated recycling [
7,
8]. At the same time, due to its composition and structure, glass does not pose any hazard to the environment as a recycled material. It is estimated that annually, in Europe, 11–40 kg of glass waste is generated per capita, depending on the country. Overall production of waste is constantly growing. Glass constitutes approximately 10%–15% of municipal waste, depending on the waste management practices in the country.
According to the World Bank, annual waste production is about 2 billion tons [
9,
10,
11,
12]. The development of waste glass processing technology allows reaching up to 100% recycling, which contributes to the reduction of energy consumption for processing primary raw materials for the production of glass, i.e., sand, soda, and limestone dust [
13,
14]. There are many concerns about the industrial use of the recycled products. With this in mind, many studies are being carried out, confirming the possibility of using waste materials, including glass cullet [
15]. Expanded glass aggregate, where cullet is the key component, is proposed to be applied for the improvement of the road sub-grade. The possibility of using shredded waste glass in road engineering, as a substitute for coarse aggregate, was carried out by [
16,
17,
18,
19]. Another solution was the use of shredded glass instead of natural sand. Such studies were conducted by [
20,
21,
22,
23]. Another lightweight aggregate, which has already been used in road construction, is produced from expanded clay or fly ash, coming from coal combustion or municipal waste incineration plants. It has been well-known and successfully used for quite a long time [
24,
25,
26,
27,
28,
29]. The examples of application of lightweight aggregate fillers, presented in the publications, confirm that this solution is technically feasible in the road construction industry, and its significant advantages influence the possibility of reducing costs of road construction as compared to traditional materials [
30,
31,
32,
33,
34,
35,
36,
37,
38,
39]. Therefore, the authors have made an attempt to use GEGA as a substitute for the natural aggregate in the sub-grade and the frost protection layer of the road foundation. The main assumption, which is made here, is that construction material should meet all the requirements, related to, among others, mechanical properties, durability, and economic coefficient [
40,
41].
The authors put forward the thesis that it is possible to use GEGA of diameter 8/11.2 mm with cement grout, instead of two layers: a sub-grade and a frost-protection layer of the road. At the same time, special attention has been paid to the quality and the cost of the recycled materials. The main assumption was that the quality, cost, and durability of the new solution cannot turn out less beneficial for the interested parties than the solution, traditionally used in road construction. The main profit of the new solution implementation can be seen in lower consumption of natural resources, application of waste glass and, therefore, a reduction of CO2 emission.
On the basis of the research, carried out by [
42,
43], the authors assessed the suitability of the glass waste in the form of foam glass as an alternative to natural sand, used as a sub-grade and a frost-protection layer of the pavement road.
The authors of the article below undertook a research into the possibilities of using GEGA from the waste glass in road construction. The main goal of the research was to find a new solution that would allow reducing the consumption of natural resources and, instead, to use the material derived from glass waste recycling.
2. Materials and Methods
2.1. Assumptions of Pavement Structure and Adopted Materials
Two main types of pavement structure were analyzed: type 1—flexible pavement with asphalt mixture layers and base course, made of an unbound mixture, and type 2—rigid pavement with a concrete slab in a wearing course layer and base course layer, made of a mixture, bound with a hydraulic binder. Traffic categories were marked in accordance with the guidelines for the road pavement design in Poland [
44]. For flexible pavements, road structures for heavy traffic (22.00–52.00 million equivalent standard axle load (ESAL) of 100 kN/lane), for medium traffic (2.50–7.30 million of ESAL of 100 kN/lane) and for light traffic (0.09–0.50 million of ESAL of 100 kN/lane) were assumed. Additionally, there were subsurface and groundwater conditions for cohesive soils adopted, e.g., sandy clay, and high groundwater level. In the analyzed examples, the proposed new solution, which was assumed, was frost protection layer performing the function of a drainage layer at the same time. For rigid pavements, road structures for heavy traffic (42.63–101.25 million of ESAL of 100 kN/lane), for medium traffic (6.39–15.99 million of ESAL of 100 kN/lane), and for light traffic (0.15–0.75 million of ESAL of 100 kN/lane) were assumed. Additionally, subsurface and groundwater conditions, as well as conditions for the application of a drainage layer, were assumed analogously as for flexible pavements. The layout of type 1 and 2 pavement layers is schematically shown in
Figure 1.
The new solution assumes the substitution of the traditional solution with one, single layer, made of permeable lightweight GEGA concrete, constituting, at the same time, a frost protection course and improved soil sub-grade. The characteristic feature of the permeable lightweight GEGA concrete is liquid permeability. Porous lightweight GEGA concrete is freeze and thaw resistant. Permeable concretes can be made of a single- or double- fraction aggregate with a grain size of more than 4 mm. The amount of cement paste shall be used in the quantity, allowing to cover individual grains of the aggregate and to create an interfacial transition zone (ITZ) between the grains of the aggregate.
Figure 1 shows structural layers of flexible and rigid pavements, made of traditional and permeable concrete, and the ones, made of permeable concrete with solutions using GEGA for traffic load: heavy, medium, and light traffic.
Apparent, the density of condensed fine aggregate, stabilized with cement, ranges up to 1700–2000 kg/m3 whereas, in the case of using artificial aggregates with the addition of fly ash or clay, the apparent density reaches only 1400 kg/m3. If we use GEGA, the apparent density does not exceed 1000 kg/m3. The size of the spaces between the grains can be controlled to some extent by the grain size of the coarse aggregate. The amount and the size of the spaces between the grains depends on the diameter of the aggregate and on the amount of cement grout surrounding the grains of the aggregate and partially filling the pores. The greater the void content of the aggregate composition is, and the less pore filling is, the larger the porosity and the higher the water permeability are. Additionally, the amount of slurry or mortar will influence the quality of ITZ between the grains of the aggregate and cement grout. With the increase of the amount of slurry and mortar, mechanical properties of the composite will be more beneficial. The overall porosity of the composite and, therefore, permeability change with the thickness of the ITZ. The higher the amount of slurry is, the thicker the contact zone is and, at the same time, the higher compression strength is.
2.2. Materials
Portland cement CEM II/A-V 42.5 N with 20% fly ash, according to EN 197-1, was used to perform the tests. Chemical content and physical properties of the cement CEM II/A-V 42.5 N are shown in
Table 1. The tests have been carried out in the laboratory of Gdańsk University of Technology.
The main component of cement is CaO, and its content is 54%, while the content of SiO
2 silica is approximately 25%. Natural fine aggregate with a grain size from 0 to 4 mm (NATU), meeting the requirements of EN 12620: 2010, was used for the test. Distribution curves of NATU are shown in
Figure 2. The chemical composition of NATU is shown in
Table 2 and
Table 3.
Figure 3 presents microscopic images of fine aggregate grains in the vicinity of cement paste.
Artificial aggregate (GEGA) is manufactured from clear construction glass recycling and municipal waste recycling. The resources for clear glass production are quartz sand and additives, such as: sodium and calcium carbonate, flux: boron and lead oxide. Glass waste is ground in a ball mill and, next, cement is added, as well as fly ash, zeolite, metakaolin, foaming substances, and water. Then, out of the mix the granules are formed and placed in the furnace at 900 °C. Finally, a light-gray or beige porous granulated product is obtained. Granulated aggregate is sorted according to grain diameter. For this research GEGA (
Figure 4) with a grain size ranging from 8 to 11.2 mm was applied. Its chemical and physical properties are shown in
Table 2 and
Table 3.
The main component of NATU and GEGA is SiO2 silica, and its content in NATU is 97.5% and in GEGA is 63.3%.
2.3. Preparation of Mix and Samples
Research of the properties was carried out for two variants of the material, marked NATU and GEGA. In the first variant, NATU, the samples were representative for the material, used in the cement-stabilized sub-grade and for a frost protection road layer in Poland. A mixture of NATU, stabilized with cement, contained 20% fly ash. Properties of the components are presented in
Table 1,
Table 2 and
Table 3. NATU mixture, bound with a hydraulic binder, was prepared with CEM II/A-V 42.5 N cement, 129 kg; NATU, 1742 kg; and water, 107 kg. The composition was designed on the basis of the tests, aiming to determine the maximum density and optimal moisture content of the material. The components of the mixture were mixed in a mechanical mixer. First, NATU was dry-mixed with cement for 2 min, then water was added and mixed for another 3 min. The maximum density of NATU with cement and water of max = 1.84 g/cm
3 was determined and the optimum water content constituted up to 7.97%. The relationship of bulk density and moisture is shown in
Figure 5.
The mixture was poured into special cylindrical forms. Samples were prepared in special cylindrical forms of dimensions of Ø 100 mm; h = 115 mm (
Figure 6a,b). The samples were compacted in three layers, 25 strokes per layer, by means of the Proctor method according to standard EN 13286-2, using a Ø 50 mm compactor of a weight of 2.5 kg, falling freely from a height of 305 mm. In total, 18 cylindrical samples of the dimensions of Ø 100 mm and h = 115 mm were made, for compressive strength testing at 7, 28, and 56 days. There were six samples tested on each date of the test. Additionally, six cubic samples of dimensions of 10 × 10 × 10 cm
3 were made for frost resistance testing.
Samples, having been prepared, were stored for 24 h in a mold, at a temperature of 20 ± 2 °C, followed by subsequent storage in a chamber, in the humidity of 95%–100% and temperature of 20 ± 2 °C and protected against drying. Three days before the test, the samples were submerged in water of the temperature of 20 ± 2 °C. One hour before compressive strength testing, the samples were taken out of the water and their surface was dried. Volume density, compressive strength, and frost resistance coefficient were tested.
The material with GEGA was proposed as a new solution. The mixture of the following composition was designed: CEM II/A-V 42.5 N, 205 kg; NATU, 90 kg; GEGA, 150 kg; water, 125 kg. The components of the mixture were mixed in a mechanical mixer. First, the slurry, composed of cement and water, was mixed for 2 min. Then, NATU was added and the mortar was mixed for next 1 min. GEGA was added to the mortar and mixed for 2 min.
The mixture was laid to special cylindrical molds of Ø 100 mm; h = 115 mm. The mix was compacted 25 times in two layers. The compaction method in accordance with the EN 206 standard is suggested by the authors. During concrete compacting, a hand-rammer of Ø 50 mm and weight of 1 kg was used. In order to ensure proper merger of the components, it is important to properly compact permeable ready mix with GEGA. It should be noted that too much compaction energy will reduce the porosity of the concrete, which, in turn, will reduce its permeability. Additionally, compacting shall be carried out in a way that prevents damage of the grains of the lightweight GEGA aggregate. For the compressive strength testing at 7, 28, and 56 days, 18 cylindrical samples of Ø 100 mm and h = 115 mm were made as shown in
Figure 7a,b. There were six samples, tested on each assigned date of the tests. Additionally, 6 cubic samples of dimensions of 10 × 10 × 10 cm
3 were made for frost resistance testing. Test samples were stored for 24 h in molds at a temperature of 20 ± 2 °C, followed by subsequent storage in a chamber at a humidity of 95%–100% and temperature of 20 ± 2 °C and protected against drying. One hour before the resistance test, the samples were taken out of the chamber and left to dry in the air at a temperature of 20 ± 2 °C. Volume density, compressive strength, and frost resistance coefficient were tested.
2.4. Test Methods
2.4.1. Volume Density Test and Porosity Test
Volume density was determined for three samples, made of NATU and for three samples of the material, made of GEGA:
where
ρ0 is the volume density of the materials,
m0 is the weight of the cylindrical sample, and
V is the volume of cylindrical sample.
Volume density tests were carried out on cylindrical samples, intended for compressive strength testing. The volume of the sample was determined by measuring the sample’s dimensions.
Material porosity was marked as
and defined as a ratio of the pores volume in the cylindrical sample to the total volume V of the sample. Porosity is the ratio of the volume of pass-through pores in a unit of the volume of a sample:
Volume porosity is defined as:
where
is the material porosity,
V is the total volume,
Vp is the volume of pores, and
Vs is the volume of the solid material.
A porosity test was carried out on three cylindrical samples. The sample volume was determined by the measurement of its dimensions. The volume of open pores was determined by measuring water volume which permeated inside the sample. Water, filling in the pores, was measured by capillary forces, without any external pressure. The pore volume Vp was obtained by determining the mass of water that penetrated the pores of the sample. It has been assumed that 1 dm3 of water is equal to 1 kg.
2.4.2. Compressive Strength Tests
The compressive strength test was carried out in the Advantest 9, Controls, Liscate MI, Italy) in a uniaxial state of stress in accordance with the procedure, defined in EN 13286-41. For the compressive strength testing at 7, 28, and 56 days, 18 cylindrical samples of Ø 100 mm and h = 115 were made. On each assigned date six cylindrical samples were tested. Compressive strength of the material was determined on the basis of the arithmetic mean of the results for all six. It was assumed that none of the results can deviate from the mean more than 10%. Standard deviation was determined.
2.4.3. Frost Resistance Coefficient Tests
Samples for frost resistance coefficient test were stored for 28 days in a chamber with the humidity of 95%–100%, and temperature of 20 ± 2 °C. Then, they were submerged in water for 24 h, and placed in a frost chamber. Then they underwent 14 frost and thaw cycles. One cycle procedure involves freezing the samples for 8 h in the temperature of −23 ± 2 °C and thawing them for 16 h in the temperature of 18 ± 2 °C. Frost resistance coefficient was determined for three samples in accordance with Equation (5). The average compression strength
RcZ−O was adopted for the calculations. The following frost resistance coefficient was adopted:
for
Mmin = 0.7; where
is the average compressive strength of the samples, subjected to freeze-thaw cycles, and
Rc is the average compressive strength of reference samples (cared in air and water).
4. Analysis and Application of the Traditional Pavement Structure and with Use of a GEGA Permeable Layer
Taking into account favorable properties of permeable concrete, the analysis of the possibilities to apply this material for a road course in typical solutions for pavement structure was carried out in accordance with Polish requirements [
45,
46].
The procedure for the calculation of the required thickness of the lightweight GEGA permeable concrete course, as an alternative to a typical frost protection course and improved soil sub-grade, was carried out in accordance with the guidelines, contained in [
47,
48,
49]. For each of the analyzed solutions the values of elastic deflections “w” on the surface of the improved sub-grade course and on the surface of the sub-base courses of the pavement structure were determined. Calculations were carried out using BISAR 3.0 software (Bitumen Stress Analysis in Roads, Shell, Gdansk, Poland). The required
Eequivalent substitute modulus on the surface of the analyzed layers was calculated by means of using the Boussinesq equation (Equation (6)) from the theory of elastic half-space, which is the following:
where
Eequivalent is the substitute modulus, determined on the surface of sub-base course layer of pavement structure and on the surface of improved subgrade, MPa;
q is the contact pressure of the wheel
q = 0.65 kPa;
D is the substitute diameter of the wheel tract,
D = 0.313 m;
ν is Poisson’s ratio,
ν = 0.3; and
w is the deflection on the surface of road profile, m.
Table 4,
Table 5 and
Table 6 summarize the proposal of application of lightweight concrete sub-grade course containing GEGA material instead of traditional frost-resistant layer and improved sub-grade course. Calculations of the required thickness of the GEGA concrete road pavement layer for flexible and rigid pavements were carried out, depending on the traffic category, i.e., the traffic load. The results of the calculations, presented in
Table 4,
Table 5 and
Table 6, refer to three selected representative road traffic categories, according to [
44], representing heavy, medium, and light traffic.
The computational analysis showed that it is possible to replace the traditional solution of the frost protection course and the improved soil sub-grade course with a single course of lightweight concrete containing GEGA material. The proposed alternative solution which may be used in both flexible and rigid pavement structures, features the required load-bearing capacity and durability parameters.