Eco-Innovative Concrete for Infrastructure Obtained with Alternative Aggregates and a Supplementary Cementitious Material (SCM)

Abstract

Concrete is a heterogeneous material, one of the most widely used materials on the planet, and a major consumer of natural resources. Its carbon emissions are largely due to the extensive use of cement in its composition, which contributes to 7% of global CO₂ emissions. Extraction and processing of aggregates is another source of CO₂ emissions. Many countries have succeeded in moving from a linear economy to a circular economy by partially or fully replacing non-renewable natural materials with alternatives from waste recycling. One such alternative consists of partially replacing cement with supplementary cementitious materials (SCMs) in concrete mixes. Thus, this work is based on the experimental investigation of the fresh and hardened properties of road concrete in which crushed river aggregates were replaced with recycled waste aggregates of uncontaminated concrete. At the same time, partial replacement of cement with a SCM material in the form of glass powder improved the durability characteristics of this sustainable concrete. The microstructure and compositional features of the selected optimum mix have also been investigated using polarized light optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction by the Powder method (PXRD) for the qualitative analysis of crystalline constitutive materials.

1. Introduction

Transitioning from “Linear Economy” [1] towards the waste management concept of European Directive 2008/98/EC [2] and the Industrial Emissions Directive, known as European Directive 2010/75/EU, in favor of “Circular Economy” [3,4] is an absolute necessity. The new paradigm is based on reducing the consumption of natural resources through the use of recycled waste from various industries and industrial by-products, which might become alternatives to the raw materials used in the construction industry. A direct, long term result will be the creation of more ecological construction products. The uncontrolled consumption of natural resources may have led to the climate and economic changes we are already facing, which are the result of the still-existing linear economies in certain areas of the world [4]. The increasingly large volume of waste in landfills, the lack of storage space, and the stringent reduction of non-renewable natural resources are strong motivations for researchers to mobilize and support recyclers in finding innovative ways to use recycled waste as raw materials in new concrete mixes, which represents one of the most eco-friendly options [5,6,7,8,9,10].

Conventional concrete is the most widely used artificial material in the construction industry and worldwide, second only in total use to water. Its production involves the depletion of the natural resources and increased use of cement, the production of which requires significant amount of electrical energy and limestone, leading to significant environmental impacts and CO₂ generation in addition the high amount of released emissions [11,12,13,14,15,16,17].

Recycled wastes used in the production of concrete, such as glass, plastic, concrete, construction waste, rubber, and ceramics, may facilitate the transition from a linear economy to a circular one [4]. Extensive research has been conducted on concrete mixtures using glass powder as partial replacement for cement in conventional concrete [18]. It has proven to be an innovative method of supporting the recycling companies’ in reducing of waste in landfills [19] and their transformation into final recyclers by putting into production the new eco-friendly concrete for various construction elements (paving curbs, hollow blocks, platforms, sidewalks) [20,21,22,23]. The recycling of concrete from demolitions or that obtained by crushing prefabs is equally welcomed. If the volume allows, concrete obtained from tests of samples subjected to destructive or non-destructive laboratory determinations, with or without a known track, can be used [24].

Numerous studies have been conducted worldwide with respect to the use of glass powder or recycled concrete aggregates individually. However, studies of the mixture containing both in a single composite are relatively limited in number.

Glass powder resulting from recycling (WGP—waste glass powder), used as a substitute for cement in proportions of 10–25% in concrete/mortar mixes, has been highlighted in recent studies [25,26,27,28,29,30]. It has also been used in geopolymer concrete in proportions of 20–80% [31,32,33]. Furthermore, studies have been conducted on the substitution of up to 60% of materials and the monitoring of strength development up to 90 days [34,35] focusing on the efficiency of pozzolanic activity of cementitious materials [32,36,37,38,39]. The properties of mortars with cathode ray tube (CRT) glass waste (Pb-containing) added in the amounts of 30%, 40%, and 50% by weight percent as replacement for river sand were also investigated. The experimental results indicated that all the samples containing glass waste achieved higher compressive strength compared to the control mortar. After 14, 28, and 42 days of maintaining the mortar in water, no evidence of Pb was detected in the solutions [40].

All these studies can encourage recyclers to create their own SMEs and to produce and implement such eco-friendly concretes in the production of various precast elements, like paving or curbs, Lego-type hollow blocks, sidewalks, bike lanes, industrial platforms, parking lots, etc., depending on the specific requirements and regulations of each region.

The purpose of this study is to evaluate the possibility of integrating waste materials such as WGP (waste glass powder) and RCA (recycled concrete aggregates) into concrete and quantify their influence on mechanical and durability properties, particularly in terms of abrasion resistance.

As a result, the findings of this study confirm a favorable contribution to the mechanical characteristics and abrasion resistance of road concrete when using WGP waste materials. The current use of WGP-RCA in concrete composition may redirect the construction industry towards a circular and sustainable economy in line with the main directions and requirements of EU.

2. Materials and Methods

2.1. Materials

An in-process flow diagram of substituting cement with WGP and coarse aggregates with PCA in the concrete mix is presented in Figure 1.

2.1.1. Aggregates

The aggregates used for the control road concrete and for the eco-friendly road concrete are presented synthetically in

Table 1. Types of aggregates used in the design of road concrete.

Size of Aggregate [mm]	Type of Aggregate
0/4	Natural river aggregate (NRA) for all mixes (gravel)
4/8	Crushed river aggregates (CRA) (crushed gravel)
4/8	Recycled concrete aggregates (RCA)
8/16 mm, 16/25	Crushed aggregates/chippings (CAC) for all mixes

.

In Figure 2, images of the aggregates used in the design for the control/reference concrete (Figure 2a) and the recycled coarse aggregate (4/8 mm) RCA that replaces the natural aggregate 4/8 mm (Figure 2b) are presented.

In the present study, concrete mix design was carried out through careful analysis of the characteristics of the used aggregates, especially for the recycled aggregates [41], followed by the adjustment of appropriate proportions in such a way that the designed mixes comply with the requirements of the current standards in Romania for the production of road concrete [42,43].

Special attention was paid to the total aggregate volume, as it represents approximately 60%–80% of the concrete composition. Recycled concrete aggregate exhibits inconsistency in quality due to its various sources often lacking a known history. In this study, RCA (recycled concrete aggregate) comes from a local laboratory that consistently verifies the quality of the designed/implemented materials, ensuring that they have not been contaminated in storage. The RCA were obtained by crushing C25/30-concrete-class cubic specimens [41].

The design of concrete mixes took into consideration the widest possible applicability of alternative aggregates to replace traditional, natural aggregates. The popularity of the new concrete is increasing as it is used as a secondary raw material in construction products. Through the sustainable design of the resulting concrete, a smaller carbon footprint is achieved by using RCAs. They are approximately 10% lighter compared to traditional mineral aggregates due to their porosity and the presence of cement paste. However, high porosity in aggregates might result in higher water absorption, lower mechanical strengths, and reduced resistance to repeated freeze–thaw cycles. Therefore, strict quality control is essential for recycled aggregates compared to traditional, natural aggregates. Some studies indicate methods of hydrophobization or increasing the density of aggregates through various techniques to close the pores and, as a consequence, to reduce water absorption [44,45,46,47].

In the present study, the criteria imposed on aggregates (according to SR EN 933-1:2012) [48] for single-layer road wearing courses have been chosen. The criteria are identical to the requirements for the types of aggregates used for double-layer road-wearing courses, including the wearing layer, according to NE 014:2002 [42].

In the following, the determinations performed on aggregates in the laboratory will be presented.

The Granularity of the Aggregates

The granularity was determined in accordance with the SR EN 933-1:2012 specifications [48]. The test consists of separating the material into several sizes with decreasing dimensions based on grain size by using a series of sieves. The masses of particles retained on the different sieves are reported relative to the initial mass of the material. The cumulative percentages of passage through each sieve are presented in numerical form (

Table 2. Passage of 0/4 mm natural river aggregate for all the mixes (NRA).

Aggregate	Passes, in %, through the Size Sieve (mm):
	0.125	0.250	0.500	1	2	4	8	16	31.5
0/4 mm	4.23	15.18	38.30	64.70	86.30	99.43	100	100	100

,

Table 3. Passage of 4/8 mm, 8/16 mm crushed river aggregates (CRA).

Aggregate	Passes, in %, through the Size Sieve (mm):
	0.125	0.250	0.500	1	2	4	8	16	31.5
4/8 mm	0.19	0.22	0.24	0.27	1.33	27.50	96.90	100	100
8/16 mm	0.05	0.06	0.06	0.07	0.07	0.09	1.76	94.62	100

,

Table 4. Passage of 4/8 mm, recycled concrete aggregates (RCA).

Aggregate	Passes, in %, through the Size Sieve (mm):
	0.125	0.250	0.500	1	2	4	8	16	31.5
4/8 mm	0.02	0.02	0.03	0.03	0.04	0.12	79.90	100	100

and

Table 5. Passage of 8/16 mm, 16/25 mm crushed aggregates/chippings (CAC).

Aggregate	Passes, in %, through the Size Sieve (mm):
	0.125	0.250	0.500	1	2	4	8	16	25
8/16 mm	1.00	0.11	0.11	0.11	0.11	0.11	7.27	95.51	100
16/25 mm	0.08	0.09	0.09	0.09	0.10	0.10	0.10	5.02	100

). The obtained results are the average of 3 determinations.

Real Density and Water Absorption Coefficient of RCA

The determination was carried out according to SR EN 1097-6: 2013 [49].

The real density was calculated based on the mass-to-volume ratio. The mass was determined by weighing the saturated test specimen, with dry surface, and again after drying in an oven. The volume was calculated based on the mass of the displaced water, determined by weighing using the pycnometer method (for aggregates of size 0/4 and 4/8) or by reducing the mass using the wire loop method (for aggregates of size 8/16 and 16/25).

The following parameters have been calculated and are presented in

Table 6. Real volumetric mass and coefficient of water absorption of the aggregates.

Symbol Aggreg. Sorts	Characteristics of Aggregates
(mm)	ρa (Mg/m3)	ρrd (Mg/m3)	ρssd (Mg/m3)	WA24 (%)
NRA_0/4	2.70	2.57	2.63	3.00
CRA_4/8	2.68	2.59	2.62	2.40
RCA_4/8	2.70	2.32	2.46	6.00
CAC_8/16	2.65	2.56	2.61	1.40
CAC_16/25	2.67	2.59	2.62	1.20

: absolute volume mass (ρ_a), real volume mass determined via oven drying (ρ_rd), real volume mass on the saturated dry surface (ρ_ssd), and water absorption coefficient (WA24) (expressed as a percentage of the dry mass after 24 h of immersion in water), according to the equations:

Absolute volumetric mass:

$${\mathrm{\rho }}_{\mathrm{a}}={\mathrm{\rho }}_{\mathrm{w}}\frac{{\mathrm{M}}_4}{{\mathrm{M}}_4-\left({\mathrm{M}}_2-{\mathrm{M}}_3\right)} \mathrm{M}\mathrm{g}/{\mathrm{m}}^3$$

(1)

Actual density determined after drying in oven:

$${\mathrm{\rho }}_{\mathrm{r}\mathrm{d}}=\frac{{\mathrm{M}}_4}{{\mathrm{M}}_1-\left({\mathrm{M}}_2-{\mathrm{M}}_3\right)} \mathrm{M}\mathrm{g}/{\mathrm{m}}^3$$

(2)

Actual density on the saturated dry surface:

$${\mathrm{\rho }}_{\mathrm{s}\mathrm{s}\mathrm{d}}=\frac{{\mathrm{M}}_1}{{\mathrm{M}}_1-\left({\mathrm{M}}_2-{\mathrm{M}}_3\right)} \mathrm{M}\mathrm{g}/{\mathrm{m}}^3$$

(3)

The water absorption coefficient (expressed as a percentage of the dry mass) after 24 h of immersion (WA24) was calculated according to the following equation:

$$\mathrm{W}\mathrm{A}24=\frac{100\times \left({\mathrm{M}}_1-{\mathrm{M}}_4\right)}{{\mathrm{M}}_4}\%$$

(4)

where:

• ρ_w—volumetric mass of water at the test temperature (0.973 at T = 24 °C), Mg/m³;
• M₁—mass in air of saturated and superficially dried aggregates, g;
• M₂—mass of the pycnometer containing the sample of saturated aggregates, g;
• M₃—pycnometer mass filled with water only, g;
• M₄—mass in air of the test sample dried in the oven, g.

Resistance to Fragmentation and Abrasion

The determination of resistance to fragmentation of coarse aggregates was performed in accordance with SR EN1097-2:2020 [50] specification. The Los Angeles coefficient (LA) was calculated based on the following equation:

$$\mathrm{LA}=\frac{5000-\mathrm{m}}{50}$$

(5)

where m—the mass of the material retained on the 1.6 mm sieve, (g).

The obtained results are presented in

Table 7. Los Angeles coefficient (LA) for the aggregates.

Symbol of Aggregate	Sorts of Aggregates	LAmed (%)	Traffic Class
RCA	4/8 mm	30.9	Reduced
CRA	4/8 mm	31.0	Reduced
CAC	8/16 mm	16.0	Intensive
CAC	16/25 mm	15.0	Intensive

High value means less resistance to crushing.

:

Abrasion Testing of Coarse Aggregate (MicroDeval Coefficient) for (RCA)

The abrasion testing was performed according to SR EN 1097-1:2011 [51]. The Micro-Deval coefficient (MDE) of the test in the presence of water is calculated according to the following equation:

$$\mathrm{MDE}=\frac{500-\mathrm{m}}{5}$$

(6)

where m—mass of the retained aggregates on the 1.6 mm sieve. The results, considered as an average of 2 determinations, are presented in

Table 8. Micro-Deval (MDE) coefficient in presence of water for (RCA).

Symbol of Aggregate	Sorts of Aggregates	MDEmed (%)	Traffic Class
RCA	4/8 mm	20.8	Medium
CRA	4/8 mm	10.1	Intensive
CAC	8/16, 16/25 mm	14.0	Intensive

.

Flattening Coefficient of RCA

The flattening coefficient of RCA was determined following the guidelines of SR EN 933-3:2012 [52]. The overall flattening coefficient was calculated as the total mass of particles passing through the grate with slots, expressed as a percentage of the total dry mass of the particles tested.

The overall flattening coefficient (A) was calculated based on the following equation:

$$\mathrm{A}=\frac{{\mathrm{M}}_2}{{\mathrm{M}}_1}\times 100$$

(7)

where:

• M₁—sum of the aggregate masses of the elements di/Di, (g);
• M₂—sum of the masses of the granules passed through the slotted grate corresponding to the opening Di/2, (g).

The obtained results are summarized in

Table 9. Overall flattening coefficient (A) for RCA 4/8 mm.

Sorts/Elementary Aggregates di/Di		The Nominal Opening of the Grill Slots, mm	Ai	M1	M2	A
4/8 mm	8/10	5	0	600	76	13
	6.3/8	4	5
	5/6.3	3.15	29
	4/5	2.5	26

:

2.1.2. Cement

A CEM I 42.5R rapid-hardening cement was used to cast the road concrete considered in this research. The cement properties are presented in

Table 10. Cement characteristics for CEM I 42, 5R.

Characteristics CEM I 42, 5R		Value	According to
Composition	Clincher Portland (%)	95–100	SR EN 197-1 [53]
	Minor component (%)	0–5	SR EN 197-1 [53]
Chemical Characteristics	Sulphate content (in the form of SO, %)	≤4	SR EN 196-2 [54]
	Chloride content (%)	≤0.1	SR EN 196-2 [54]
	Loss of calcination (%)	≤5	SR EN 196-2 [54]
	Insoluble residue (%)	≤5	SR EN 196-2 [54]
Physicomechanical Characteristics	Setting time (min.)	≥60	SR EN 196-3 [55]
	Stability (mm)	≤10	SR EN 196-3 [55]
	Compressive strength at 2 days (MPa)	≥20	SR EN 196-1 [56]
	Compressive strength at 28 days (MPa)	≥42.5 ≤62.5	SR EN 196-1 [56]

.

The road concrete classes to be designed with this type of cement are: BcR3.5, BcR4, BcR4.5 and BcR5. The quantity of cement in the road concrete mix for class BcR4 is set by the normative NE 014:2002 to a quantity of 330 kg/m³ [42]. The concrete shrinkage is influenced by the mineralogical nature of Portland cement—its specific surface area and cement ratio.

2.1.3. WGP—Waste Glass Powder

Recycled glass waste, a pozzolanic material, in the form of powder [57] was obtained through grinding in a ball mill. When incorporated into cement, mortar, or concrete compositions, pozzolanic materials have a major role in reducing carbon dioxide (CO₂) emissions and are referred to in the literature as “Supplementary Cementitious Materials (SCMs)” or “additional constituents with cementitious (hydration or pozzolanic) characteristics”. Depending on their size, glass particles transition from being inert minerals to becoming reactive materials during cement hydration in the concrete mixture, especially when ground into a fine powder with reduced particle sizes [58,59,60].

For particle sizes smaller than 0.250 mm [57] alkali–silica reactions (ASR) are cancelled [61,62]. Moreover, this particle size improves the properties of cement-based materials, such as mortar and concrete (strength and wear resistance), due to the pozzolanic reaction which leads to the development of secondary reaction products during and after the hydration process of cement.

Pozzolanicity is the ability of a natural or artificial material to react with Ca(OH)₂ in the presence of water. Pozzolan reaction rate depends on the intrinsic characteristics of pozzolan itself, such as specific surface area, chemical composition, and content of the active phase [63]. According to ASTM C618 prescriptions [64], the following condition should be fulfilled for a material to be considered a pozzolan: SiO₂ + Al₂O₃ + Fe₂O₃ ≥ (50–70)%.

SEM (scanning electron microscopy) investigations were previously conducted on the plain cement matrix and cement mixed with waste glass powder [57].

Scanning electronic microscopy was performed using SEM Tescan VEGA II LSH to investigate the microstructure and the raw material. The test was carried out using secondary as well as backscattered electron detectors.

The mortar mixes were tested at the age of 28 days during a previous stage of the research showed that when substituting 10% of the cement content with glass waste in the form of powder (CEM—10% WGP1), the compressive strength was slightly higher compared to the conventional control mix. This confirmed the effective pozzolanic activity of the glass powder at the 10% substitution ratio.

The SEM images highlighted that the microstructure showed a complete consumption of fine glass particles in the mixture with 10% WGP, compared to the one with 20% WGP [57], due to the pozzolanic reaction during cement hydration favoring the evolution of compressive strengths [65]. In both cases, the fracture surfaces of the mortar specimens indicated a compact microstructure [66].

The chemical compositions of the considered WGP and CEM I 42.5R cement were determined using X-ray fluorescence (XRF-Qualitax, Italy) at the School of Materials Engineering, University of Malaysia Perlis (UniMAP), Perlis, Malaysia [67] and are presented in

Table 11. XRF Analysis result—chemical composition (%) of the cement CEM I 42.5R and WGP.

Oxides	SiO2	K2O	Fe2SiO3	CaO	Al2O3	MgO	Na2O	Oder
CEM I 42.5R	14.30	1.08	3.70	71.46	2.90	0.86	5.70	-
WGP	77.70	1.01	0.44	13.6	0.06	0.01	5.27	1.92

.

In

Table 12. ≤0.125 mm waste glass powder (WGP).

WGP	Passing (in %) through the Sieve (Size in mm)
	0.63	0.125	0.250	0.500
≤0.125 mm	43.80	100.00	100	100

, the fineness of the glass powder is observed. Due to its small particle size, it reacts very effectively within the concrete matrix during cement hydration process, similar to other supplementary cementitious materials (SCMs) [68,69,70]. The XRD pattern showed the amorphous feature of the glass waste and the presence of C2S, C3S, C3A, C4AF, and gypsum as the main mineralogical phases in the CEM I 42.5 R cement [31].

C₃S (tricalcium silicate) is the most important component, showing optimal characteristics for road cements from all perspectives, and it is recommended to be equal to or greater than 55% (

Table 13. Favorable behavior of the mineralogical component C₃S of cement [70,71].

Properties	Mechanical Strength	Shrinkage	Abrasion Resistance	Freeze–Thaw Resistance	Modulus of Elasticity	Hydration Rate
C3S	Very high	Low	Good	Very good	Very high	Moderate

) [71].

2.1.4. Water and Additives

The water–cement ratio (w/c) is imposed by the current norms [42,43] for the designed concrete class BcR4 as 0.45 for the content of aggregates with continuous gradation. The water fulfills the requirements set in SR EN 1008:2003 [72].

Two types of additives were used in the composition of the road concrete mixtures. The first one was a superplasticizer for high-performance concrete (MasterGlenium 115 BASF), and the second one was an air-entraining admixture (MICROAir 107-2 BASF), which is imposed by the current standard [43] to provide the concrete with better freeze–thaw resistance. The additives are compatible with Lafarge CEM I 42.5R cement and meet the requirements of the standard SR EN 934-2 + A1:2012 [73]. The necessary amount of the additive was calculated as a percentage of the cement mass.

2.2. Methods

2.2.1. Design and Preparation of BcR4 Class Road Concrete Mix

Three road concrete mixes were designed in Variant I, referred to as Var. I. Mix 1—the control mix BcR-NA with natural aggregates (river aggregates, river crushed aggregates, and crusher aggregates/chippings). The second mix design (BcR-RCA) differs from the control mix by replacing the river crushed aggregate with alternative aggregates obtained from recycled concrete waste (RCA) in a sorted form in accordance with the normative for road concrete NE 014:2002 [42] and the harmonized standards SR EN 12620 + A1:2008 [74] and SR EN 12043:2013 [75]. The third mix (BcR-RCA-WGP) includes the same types of aggregates as BcR-RCA but with the addition of substituting 10% of the cement, by mass, with glass waste in the form of powder (WGP).

The gradations of aggregates for the concrete mixes for road pavements was chosen in accordance with NE 014:2002 [42], as shown in

Table 14. Sorts of aggregates used in the layers of road wearing course in accordance with NE 014:2002 [42].

Pavements Realized	Nature of Aggregates	Sorts of Aggregates	Gradation of Total Aggregates
Single layer	Natural Sand	0/4
Two layers	Crushed Gravel	4/8	0/25
	Chipping	8/16
Wearing course	Chipping	16/25

. Thus, the chosen variant coincides with both methods of laying road concrete either in a single layer or in two layers for the wearing layer.

The natural sand of 0/4 mm fraction was used in each mix to support good compaction and workability of the concrete due to the rounded shape of the particles (all other aggregates in the concrete mix were obtained by crushing). The proportions of the aggregates were the same for all mixes in Variant I: 30% for 0/4 mm natural river sand (NRA); 16% for 4/8 mm crushed gravel (CRA); 24% for 8/16 mm crushed aggregate (CAC); 30% for 16/25 mm crushed aggregate (CAC). These percentages have been calculated to ensure that the total gradation curve falls within the “Favorable Zone”, as presented in Table 14.

In a similar manner, the water/cement ratio of 0.45—in accordance with NE 014:2002 [42] and SR EN 206-1:2021 [43]—was maintained for all mixes to observe any changes in workability/consistency of the concrete based on the variations in the mixture.

The percentages of the aggregates have been calculated in such a way to ensure that the total gradation curve falls within the “Favorable Zone”, which is delimited by the limits of granulometry curves of the total aggregate for road concretes made with continuous gradation aggregates of 0/25 mm. The representation of the total curve can be found in Figure 3.

In

Table 15. The total gradation curves for the variant with natural aggregates and recycled aggregates.

Total Gradation Curves for the Concrete Mixes	Passing % through Sieve with Size (mm)
	0.125	0.250	0.5	1	2	4	8	16	25
Concrete mixture with natural aggregates [38]	1.29	4.64	11.59	19.51	26.06	32.86	47.58	70.43	100
Concrete mixture with recycled aggregates	0.81	4.02	11.24	19.17	25.92	29.99	44.56	70.43	100
Lower limit	1.5	2	5	8	15	20	35	62	100
Upper limit	7	8	17.5	27	34	42	60	83	100

the total gradation curves are presented for the control mix and the other two mixes design, where the modification involves replacing the 4/8 mm crushed river aggregate (CRA) with recycled concrete aggregate (RCA). This replacement was carried out as a first phase of the study. The replacement is relevant because the two types of aggregates have very similar Los Angeles abrasion resistance values (31.0% and 30.9%, respectively) and have been obtained through crushing processes.

Design parameters according to standards and cement quantities (kg/m³) for preparing concretes are presented in

Table 16. Design parameters for road concrete BcR, imposed by NE 014:2002 [42] norm.

Design Parameters BcR4	Min. Cement Ratio CEM I 42,5	(w/c)	Consistency Class S1 (mm)	Air Void Content (%)	Freeze-Throw Circles	fc 28 Days MPa	fct,fl 28 Days MPa
NE 014 [42] and SR EN 206-1: 2021 [43]	330 kg/m3	max. 0.45	10–40	3.5 ± 0.5	100	min. 35 max. 50	min. 4 max. 5

.

In

Table 17. The composition of the road concretes in Variant I (kg/m³).

Mix Components	BcR-NA-1	BcR-RCA-1	BcR-RCA-WGP10
Water/Cement ratio	0.45	0.45	0.45
Cement I 42.5R	330	330	297
DSP(WGP) < 0.125 mm—10%	-	-	33
NRA—0/4 mm	569	569	569
CRA—4/8 mm	303	-	-
RCA—4/8 mm	-	303	303
CAC—8/16 mm	455	455	455
CAC—16/25 mm	569	569	569
Admixture 1 (Master Glenium 115)—1.80%	5.94	5.94	5.94
Admixture 2 (MICROAir 107-2)—0.25%	0.285	0.285	0.285

, the compositions of the designed/calculated and realized mixes are presented, where it can be remarked that the difference in the cement amount occurs in the concrete made with glass waste (in the form of powder with particle size < 0.125 mm), where it substitutes 10% of the cement content compared to the control mix.

Aiming to use of as much recycled material in the road concrete mix as possible and since the total aggregate curve allows entry into the favorable zone, new percentages for aggregates have been adapted, as shown in Figure 4. Additionally, 20% of the cement mass was substituted with WGP in a second mix design variant (Var. II), presented in

Table 18. The composition of the road concretes in Variant II (kg/m³).

Mix Components	BcR-NA-2	BcR-RCA-2	BcR-RCA-WGP20
Water/Cement ratio	0.55	0.55	0.55
Cement I 42.5R	330	330	264
DSP(WGP) < 0.125 mm—20%	-	-	66
NRA—0/4 mm	607	569	569
CRA—4/8 mm	379	-	-
RCA—4/8 mm	-	379	379
CAC—8/16 mm	341	341	341
CAC—16/25 mm	569	569	569
Admixture 1 (Master Glenium 115)—2.30%	7.59	7.59	7.59
Admixture 2 (MICROAir 107-2)—0.25%	0.285	0.285	0.285

.

In Variant II, the first mixtures have been prepared using natural river aggregates (NRA) and crushed aggregates / chipping (CAC), along with recycled concrete aggregates (RCA) with a size of 4/8 mm. In the third mixture, 20% of the cement was substituted with WGP to enhance wear resistance while maintaining the water-to-cement ratio (W/C) and admixture dosage. However, after preparing the mixtures, a significant reduction in workability was observed, as expected, due to the 6% water absorption of RCA, which necessitated an increase in the superplasticizer admixture (Admixture 1) from 1.8% to 2.3% relative to the cement quantity as well as maintenance of the W/C ratio of 0.55. Consequently, a consistency class of S1 (10–40 mm) was achieved in accordance with NE 014:2002 [42].

Several studies have highlighted that crushed concrete aggregates in the size range of 4 to 8 mm exhibit the highest amount of adhered mortar, which implies that aggregate size has a significant effect on water absorption and concrete strength [76,77]. The determined water absorption value for RCA (sort 4/8 mm) was 6.0%, while for NRA it was 3%.

2.2.2. Determinations of Fresh Properties of BcR Concrete

In the study the following determinations have been conducted:

• Concrete temperature (°C), in accordance with SR EN 206-1:2021 [43];
• Consistency using the slump test (mm), in accordance with SR EN 12350-2:2019 [78];
• Density (kg/m³), in accordance with SR EN 12350-6:2019 [79];
• Air content (%), in accordance with SR EN 12350-7:2019 [80].

The temperature was measured using a specialized thermometer for concrete determinations, at various points on the poured material. The consistency, which indicates workability, was determined using the slump cone method. Three determinations were performed for each mix. The density of fresh concrete mixes was determined using a vessel of known volume, which was weighed before and after filling. The difference in masses compared to volume, provided the density value.

The air content was determined using equipment with a manometer. The vessel was filled in three layers and the material was vibrated until no more air bubbles escaped. The vessel was then covered with a lid equipped with valves, an air release valve, and a manometer indicating the percentage of entrained air.

2.2.3. Mechanical Properties of BcR Concrete

The following determinations were conducted for BcR concrete:

• Flexural strength (f_ct,fl) in accordance with SR EN 12390-5:2019 [81];
• Compressive strength (f_cm) in accordance with SR EN 12390-3:2009 [82];
• Splitting tensile strength (f_ct,sp) in accordance with SR EN 12390-6:2010 [83];
• Hardened concrete density (ρ_a) in accordance with SR EN 12390-7:2019 [84].

For determining the flexural strength, a minimum of three prismatic specimens measuring 150 mm × 150 mm × 600 mm were considered for each BcR concrete mix. The results from this test are used as a reference for establishing the class of BcR concrete compared to conventional concretes, where the compressive strength on cylinders or cubes is relevant. The hardened specimens were stored in water at a controlled temperature of 20 ± 2 °C immediately after demolding until the reference age of 28 days and then tested using the four-point loading test, Figure 5a, in a universal testing machine. The loading rate was set at 0.04–0.06 MPa/s, as per SR EN 12390-5:2019 [81].

The flexural strength was determined using the equation:

$${\mathrm{f}}_{\mathrm{ct},\mathrm{fl}}=\frac{\mathrm{F}\times \mathrm{l}}{{\mathrm{d}}_1\times {\mathrm{d}}_2^2}$$

(8)

where:

• f_ct,fl is the tensile strength in MPa/s (N/mm²∙s);
• F is the maximum load in N;
• l the span between the supports in mm;
• d₁ and d₂ are the cross-sectional dimensions of the specimen in mm (as shown in Figure 6b).

The flexural strength was expressed rounded to the nearest 0.1 MPa (N/mm²).

For determining the compressive strength (Figure 5b), in addition to the series of cubes taken with a side length of 150 mm, determinations also on the prism fragments have been conducted using non-deformable metal plates for the testing surface.

The loading rate should remain constant in the range of 0.6 ± 0.2 MPa/s (N/mm².s). After applying the initial load, which should not exceed approximately 30% of the ultimate load, the load is applied without shock to the specimen, and it is continuously increased at the chosen constant rate ± 10% until the specimen can no longer withstand a higher load.

The compressive strength was determined by the equation:

$${\mathrm{f}}_{\mathrm{c}}=\frac{\mathrm{P}}{{\mathrm{A}}_{\mathrm{c}}}$$

(9)

where f_c is the compressive strength in MPa (N/mm²), P is maximum load at yield in N, and A_c is the cross-section of the specimen perpendicular to the load direction in mm².

The obtained result was rounded to 0.1 N/mm². The loading rate was 0.5 MPa/s according to SR EN 12390-3:2009 [82].

The determination of splitting tensile strength was carried out on cubes with sides of 150 mm and on prism fragments (Figure 5c). The specimen was centrally positioned, and the direction of applied force was perpendicular to the direction of concrete casting. The prismatic specimens were subjected to a compressive force applied over a narrow region along their length.

Loading strips made of hard wood with density > 900 kg/m³ and dimensions of width: (10 ± 1) mm, thickness: (4 ± 1) mm—with the length greater than the length of the contact line of the tested specimen—were used to transfer the applied load from the testing machine to the specimen. These strips were used only once. The loading strips were positioned along the upper and lower parts of the specimen loading plane. The loading rate was 0.04 MPa/s according to SR EN 12390-6:2010 [81,83].

The splitting tensile strength was determined using the formula:

$${\mathrm{f}}_{\mathrm{c}\mathrm{t},\mathrm{s}\mathrm{p}}=\frac{\mathrm{F}\times \mathrm{l}}{{\mathrm{d}}_1\times {\mathrm{d}}_2^2}$$

(10)

where:

• f_ct,sp—tensile splitting strength in MPa;
• F—maximum load in N;
• l—length of the contact line of the specimen in mm;
• d₁ and d₂—size of the cross-section in mm.

The obtained value was rounded to the nearest 0.05 MPa.

All mechanical property tests were performed using the Advantest 9 digital press with 2000 kN capacity, provided by the CONTROLS company.

The hardened concrete density was determined by weighing and dividing the mass of each specimen by the volume of the cube with a side length of 150 mm.

2.2.4. Durability Properties of BcR Concrete

The following determinations were conducted for BcR concrete:

• Loss of strength after 100 freeze–thaw cycles in accordance with SR 3518:2009 [85];
• Abrasion resistance in accordance with SR EN 1338:2004 [86], SR EN 1339:2004 [87], SR EN 1340:2004 [88];
• Carbonation depth determination according to SR CR 12793:2002 [89].

The loss of strength due to freeze–thaw cycles was determined on concrete specimens kept for 7 days in water at a temperature of T_water = 20 ± 2 °C and for 21 days in air at T_air = 20 ± 2 °C and relative a humidity of URA = 65 ± 5%.

Specimens aged at a minimum of 28 days were placed in a water bath at a temperature of 20 ± 5 °C for saturation 4 days before the start of the test. Water was poured into the bath up to 1/4 of the height of the specimens; after 24 h, it was added up to 1/2 of the height, and after another 24 h, up to 3/4 of the height. Three days after introducing them into the bath, the water level should be at least 20 mm above the height of the specimens, and this level should be maintained for 24 h. Afterward, the specimens are considered saturated.

Specimens intended for freeze–thaw cycles were placed in the thermostat cabinet. Control specimens were continuously kept in water.

Saturated specimens were introduced into the refrigeration chamber at −17 ± 2 °C and kept for 4 h for the freezing cycle. The thawing cycle was carried out at a temperature of 20 ± 5 °C and humidity of 95% for 4 h. The specimens must be arranged so that they are completely surrounded by air. The distance between specimens and between specimens and the walls of the installations must be at least 20 mm. The support on which the specimens are placed must be constructed in such a way that the contact surface with the base of the specimens is minimal.

After subjecting the specimens to 100 freeze–thaw cycles, the loss of compressive strength was determined by subjecting them to a compressive test. The number of specimens tested was the same as the number of control specimens. The test was stopped after reaching the number of 100 cycles or if the loss of compressive strength exceeded 25% compared to the control specimens of the same age.

The loss of compressive strength (ŋ) was determined by the relationship:

$$\mathsf{\eta }=\frac{\mathrm{Rm}-\mathrm{Ri}}{\mathrm{Rm}}\times 100$$

(11)

where:

• Rm—Arithmetic mean value of the compressive strengths of the control specimens, in N/mm².
• Ri—Arithmetic mean of the compressive strength values of the freeze-thaw specimens, in N/mm² or MPa.

For the determination of the Böhme abrasion resistance expressed as volume loss relative to 5000 mm², cubes with a side length of 71.0 ± 1.5 mm were placed on the abrasive disk of the Böhme apparatus (Figure 6), on the testing track where a standard abrasive was spread. The disk was rotated, and the specimens were subjected to an abrasive load of 29 ± 3 N for a specified number of cycles. The abrasive wear on the specimen leads to volume loss. The contact face and the opposite face of the specimen must be parallel to each other and flat. Before the test, the density of the specimen, ρ_R, was determined by measuring to the nearest millimeter and by weighing to the nearest 0.1 g. The standard abrasive used should be fused alumina (artificial corundum) designed to produce an abrasion of 1.10 mm to 1.3 mm when specimens are tested.

Before the abrasion test and after every four cycles, the specimen was weighed with an accuracy of 0.1 g. Additionally, 20 g of standard abrasive was placed on the testing track.

The specimen was tested for 16 cycles, each consisting of 22 rotations. After each cycle, the disk and the contact face were cleaned, the specimen was progressively rotated by 90°, and a new abrasive was placed on the testing track.

The abrasion was calculated after 16 cycles as the average of the lost volume ΔV, using the equation:

$$\mathsf{\Delta }\mathrm{V}=\frac{\mathsf{\Delta }\mathrm{m}}{{\mathsf{\rho }}_{\mathrm{R}}}[\mathrm{m}{\mathrm{m}}^3]$$

(12)

where:

• $\mathsf{\Delta }\mathrm{V}$ is the volume loss after 16 cycle, in mm³;
• $\mathsf{\Delta }\mathrm{m}$ is the loss of mass after 16 cycles in g;
• ρ_R is the density of the specimen in g/mm³.

The abrasion was expressed by the volume loss of the specimen after it was subjected to wear.

The depth of the carbonation layer in hardened concrete was determined using the phenolphthalein method in accordance with SR CR 12793: 2002 [89].

The carbonation of the concrete specimens was assessed in freshly exposed sections through the reaction with pH indicator substances, such as phenolphthalein, which turns the concrete red purple at a pH value of approximately 9. This measurement can be carried out at various ages (days).

Carbonation was determined at an age of 8 years (2920 days). The specimens were kept in a laboratory room at a temperature of 20 ± 3 °C throughout this period. Before the test, they were placed in water for 28 days, then taken out. Excess water was removed, and then the specimens were split open. A 1% solution of phenolphthalein in 70% ethanol was sprayed onto the fresh concrete section. The depth of carbonation is represented by the distance dk (measured in mm) from the outer surface of the concrete to the edge of the red-purple-colored region. Both the average depth, dk average, and the maximum depth, dkmax, were measured.

2.2.5. Microstructural Determinations

Optical microscopy and the X-ray powder diffraction method (PXRD) for the qualitative analysis of crystalline constitutive materials were used to assess the microstructure of the BcR concrete.

Polarized light optical microscopy was used in order to investigate the concrete/mortars samples. A thin section was prepared from all samples of investigated concrete and mortars, and a Nikon Optiphot T2—Pol was used for optical studies (texture and composition at crossed and parallel pollars, respectively) as well as for taking photos.

X-ray diffraction (XRD) was performed on the concrete/mortars using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.541874 Å), a 0.01 mm Fe filter, and a LynxEye one-dimensional detector at the Department of Geology, Babeș-Bolyai University (Cluj-Napoca, Romania). The determinations were performed on samples aged for 2920 days, as in the case of carbonation determination.

3. Results

3.1. Characteristics of Fresh State for Road Pavement Concrete

It has to be noted that the designed mixes in Variant II required an increase in the amount of admixture and the water-to-cementitious-materials ratio (w/c) due to the increased proportions of natural river aggregates (NRA) of 0/4 mm by 2% and recycled concrete aggregates (RCA) of 4/8 mm by 4% compared to the mixes in Variant I, as shown in Figure 4. The increase in water content leads to a reduction in the values of mechanical properties.

The results obtained from the determinations on the fresh concrete are presented in

Table 19. Fresh BcR composite properties.

Fresh Property	UM	Performance Level	Mix Design
			BcR-NA		BcR-RCA		BcR-RCA-WGP
			Var. I	Var. II	Var. I	Var. II	10%	20%
Temperature (T)	°C	5–30	23	22	22	21	23	22
Consistency (S)	mm	10–40	35	40	35	37	27	31
Apparent Density (ρ)	kg/m3	2400	2374	2370	2364	2352	2358	2347
Entrained Air for Aggreg. dmax-25 mm	%	3.5–4.5 (±0.5)	4.0	4.2	3.7	3.9	3.8	4.2

.

3.2. Hardened BcR Composite Properties

The results obtained from mechanical tests on cubic and prismatic concrete specimens for the two mix variants are presented in

Table 20. Hardened BcR composite properties.

Hard Property	UM	Performance Level	Mix Design
			BcR-NA		BcR-RCA		BcR-RCA-WGP
			Var. I	Var. II	Var. I	Var. II	10%	20%
Flexural strength (fct,fl)	MPa	4.0–5.0	6.7	5.4	5.6	5.5	5.4	4.3
Compressive strength (fc)	MPa	35–45	84.2	69.2	83.1	69.4	80	62.0
Splitting strength (fct,sp)	MPa	-	4.5	3.7	4.4	3.7	4.5	3.5
Density (ρa)	kg/m3	2400 ± 40	2430	2417	2425	2410	2420	2406.6
Loss of strength (η)	%	≤25	14.11	16.8	14.60	17.6	16.0	20.2
Volume loss due to abrasion (η)	ΔV/5000 mm2	ΔV ≤ 18,000 mm3	11,301	9371	11,500	9494	11,220	9436
Depth of carbonation (dk)	mm		-	0.5	-	0.5	0.2	0.5

.

In Figure 7, it can be observed that in all specimens aged for 2920 days, a slight and non-uniform carbonation outline is present, reaching a maximum depth of 0.5 mm. However, the color intensity remains pronounced after 1 h of spraying. This phenomenon, occurring at a testing age of 28 days under testing/holding conditions according to the standard, would not have existed. It can be hypothesized that the diffusion of carbon dioxide did not take place within the cement matrix, a phenomenon hindered by the compactness of the cementitious stone in the concrete [90,91].

3.3. Microstructural Determinations

3.3.1. Optical Microscopy Using Polarized Light

Taking into account the data presented in Table 19 and Table 20 and the main contribution of this research work (e.g., the use of waste glass powder as a substitute for cement and recycled concrete aggregates), it can be concluded that BcR-NA-1 and BcR-RCA-WGP10 mixes exhibited similar values. These results are encouraging from a sustainability perspective for road concrete. Modifying the percentages of aggregates from the entire aggregate volume in order to accommodate more RCA, Var. II in Figure 4, resulted in decreased values of all mixes compared to their counterparts in Var. I.

Therefore, the prime candidates for microstructural analyses were BcR-NA-1 and BcR-RCA-WGP10 mixes.

Hence, two samples from each of the above-mentioned concrete mixes were randomly chosen for microstructural analysis. From Figure 8 and Figure 9, it can be observed that both types of samples have porphyroclastic textures defined by the presence of large fragments of aggregates embedded into the microcrystalline groundmass (matrix of the concrete). In the matrix there are pores, spherical in shape, with diameters up to 0.2 mm (Figure 8 and Figure 9). The aggregates consisted of fragments (clasts) of minerals (crystalloclasts), rocks (lithoclasts), and concrete (concreteclasts). The crystalloclasts originated from river sand or resulted from the mechanical crushing of the rock (aggregates) extracted from the quarry.

In the case of BcR-NA-1, the aggregates consist of fragments of minerals and rocks (Figure 10 and Figure 11). The minerals identified into the sample BcR-NA1 are represented by quartz, muscovite, biotite (sometime chloritized), pyroxene, plagioclase, feldspars, etc. The fragments of rocks are represented mainly by dacite and subordinated by quartzite and crystalline schists. The matrix is very fine crystallized (Figure 12) and consists of hydrated calcium and aluminum silicates, calcite, and portlandite. The local brown color of the matrix indicates the presence of iron hydroxides formed on the brownmillerite from the cement. Frequently, the newly formed minerals resulted from hydration processes developed as a rim surrounding the aggregate fragments (Figure 13).

In the case of BcR-RCA-WGP10 concrete, the aggregates consisted of fragments of minerals, rocks (Figure 14), and recycled concrete (Figure 15). The fragments of minerals consisted of quartz, muscovite (Figure 16 and Figure 17), pyroxene, and feldspars and originated in river sand or resulted from the mechanical crushing of the rock (aggregates) extracted from the quarry. The lithoclasts consisted of dacite (quarry-crushed aggregates), quartzite, and crystalline schists, as shown in Figure 16 and Figure 17. Recycled concrete aggregates (RCA) are also present in the sample of BcR-RCA-WGP10 mix. Under the microscope, it was well visible that the matrix of RCA was well crystalized compared to the matrix of BcR-RCA-WGP10 sample (Figure 14). BcR-RA-WGP10 matrix was predominantly isotropic (black in color at crossed pollars), indicating the presence of glass powder. Small crystals of portlandite were also visible in the matrix.

3.3.2. X-ray Diffraction Using the Powder Method (PXRD) for Qualitative Analysis

Samples from both mixes, BcR-NA-1, and BcR-RCA-WGP10, were investigated using X-ray diffraction. The X-ray spectra obtained on the whole sample powder were dominated by the presence of the minerals forming the aggregates, especially that of quartz. In order to investigate the newly formed mineral phases of the matrix the elimination of the aggregates was crucial. Such a separation of the matrix and aggregates is almost impossible as long as some fragments of minerals are microscopic in size. The X-ray investigation performed on the sample from the BcR-NA-1 mix indicated the presence of mineral phases originated from the aggregates (quartz, muscovite, albite, clinochlore, and orthoclase) as well as newly formed minerals, such as portlandite (Ca (OH)₂) and gypsum (CaSO₄·2H₂O). Calcite (CaCO₃) was also present as the result of carbonation of concrete (Figure 18).

For a better highlighting of portlandite as well as to eliminate the strong signal of quartz, a limited X-ray spectrum between 3.8 and 20 degrees 2θ was collected (Figure 19). Besides portlandite, muscovite, clinochlore, and tourmaline (var. Schol), some typical lines for calcium silicate hydrate (CSH) and aluminum silicate hydrate (ASH) were easily visible.

The X-ray investigation performed on the sample from the BcR-RCA-WGP10 mix indicated the presence of mineral phases originated from the aggregates (quartz, muscovite, plagioclase feldspars, clinochlore, and orthoclase) as well as newly formed minerals as portlandite and gypsum (Figure 20). Calcite was also present as the result of carbonation of concrete. The shape of the spectrum indicates a high degree of matrix crystallization, suggesting the pozzolanic reaction of the glass powder (WGP).

The X-ray spectrum collected between 3.8 and 20 degrees 2θ for the sample BcR-RCA-WPG10 shows a typical line for muscovite, clinochlore, gypsum, schorl, and portlandite as well as for calcium silicate hydrate (CSH) and aluminum silicate hydrate (ASH) (Figure 21).

4. Discussion

4.1. Performance of the BcR Composites Fresh Properties

Figure 22 shows the fresh properties of the investigated mixtures. As is already known in the scientific literature, both WG particles and quarry aggregates have a detrimental effect on the workability of the material when both natural aggregates and high-water-absorption recycled aggregates (RCA) are used [7,58,92,93].

From Figure 22a, it can be observed that the densities of Var. I mixtures were higher than those of Var. II, which is natural due to the percentage composition of the coarse quarry aggregate (8/16 mm) being 6% higher in Var. I than in Var. II, as shown in Figure 4.

The values of the slump were higher for the Var. II mixes, as seen in Figure 22b, where both the w/c ratios increased from 0.45 to 0.55, and there was an additional superplasticizer input from 1.8% to 2.3%. As previously mentioned, these adjustments were necessary due to the 2% increase in the NRA volume coupled with a 4% increase in RCA volume.

The entrained air content followed the same trend as the slump of the concrete. Although it led to a decrease in mechanical strength values, it may favor a reduction in loss of strength during repeated freeze–thaw cycles (Table 19 and Table 20).

4.2. Performance of the BcR Composites’ Hard Properties

In Figure 23, the graphical representations of the most relevant values of road composite properties (f_c,fl, f_c, η, and ΔV) are shown. Additionally, the increasing or decreasing trends of these properties are illustrated with their corresponding equations along with the limit value imposed by the NE 014 standard [38], highlighted by the green line.

For road concrete, the flexural strength (f_ct,fl) classifies the concrete into BcR strength classes, which are presented in

Table 21. BcR Classification Classes according to NE 014:2002 [42].

Hard Property	UM	Performance Level	Mix Design
			BcR-NA		BcR-RCA		BcR-RCA-WGP
			Var. I	Var. II	Var. I	Var. II	10%	20%
Flexural Strength (fct,fl)	MPa	4.0–5.0	6.7	5.4	5.6	5.5	5.4	4.3
Compressive Strength (fc)	MPa	35–45	84.2	69.2	83.1	69.4	80.0	62.0
Loss of Strength (η)	%	25	14.11	16.8	14.60	17.6	16.0	20.2
Achieved Strength Class	BcR	BcR4–BcR5	BcR 5	BcR 5	BcR 5	BcR 5	BcR 5	BcR 4

for the composites considered in this study.

The design of road concrete mixes meets the requirements of both national NE 012-1:2022 [92,94] and European fib Bulletin 42 [93] standards, as shown in

Table 22. Mathematical relation—comparison of European norms and national norms [20].

Source	Mathematical Relation (Cylinders with H/ Φ- 300/150 or Cube with l = 150 mm)
fib Bulletin 42 [95]	(a) fcm = fck + Δf, Δf = 8 MPa
NE 012-1: 2022 [94]	(b) fcm = fck + (6–12) MPa

. For both standards, the acceptable value of the mean compressive strength (f_cm) is obtained by adding to the characteristic strength, f_ck, a value Δf ranging from 6 to 12 units (MPa). Additionally, it is known that the flexural tensile strength (f_ct,fl) has values in the range of (1/10–1/20) of f_cm, and the values from this study also meet this criterion.

The reported values of the properties of the control composite for each of the variants (Var. I and II), the alternatives with RCA, and the ones with both RCA and WGP—as well as the ratio of properties between the control composites, those with RCA, and those with WGP from the two variants—are expressed by the coefficient of variation (Cv1). In terms of the classification criteria (f_ct,fl > 5.0 (4.0) MPa; f_c > 45 (35) MPa, loss of strength (η) after 100 freeze–thaw cycles < 25%, and the value of volume loss (ΔV) < 18,000 mm³/5000 mm² for classification in the best wear resistance class), they are expressed by the coefficient of variation (Cv2) and presented in

Table 23. Coefficients of variation of the properties of interest of road concrete composites.

Mix	fct,fl	Cv1	Cv2	fc	Cv1	Cv2	η	Cv1	Cv2	ΔV	Cv1	Cv2
Var. I	MPa			MPa			%			/5000 mm2
BcR-NA-1	6.7		0.75	84.2		0.65	14.11		1.77	11,301		1.59
BcR-RCA-1	5.6	1.20	0.89	83.1	1.01	0.66	14.60	0.97	1.71	11,500	0.98	1.57
BcR-RCA-WGP10	5.4	1.24	0.93	80.0	1.05	0.69	16.00	0.88	1.56	11,220	1.01	1.60
Var. II
BcR-NA-2	5.4		0.93	69.2		0.79	16.80		1.49	9371		1.92
BcR-RCA-2	5.5	0.98	0.91	69.4	0.997	0.79	17.60	0.95	1.42	9494	0.98	1.90
BcR-RCA-WGP20	4.3	1.26	0.93	62.0	1.12	0.89	20.20	0.83	1.24	9436	0.99	1.91
Var. I & II
BcR-NA	6.7			84.2			14.11			11,301
	5.4	1.24		69.2	1.22		16.80	0.84		9371	1.21
BcR-RCA	5.6			83.1			14.60			11,500
	5.5	1.02		69.4	1.20		17.60	0.83		9494	1.21
BcR-RCA-WGP	5.4			80.0			16.00			11,220
	4.3	1.26		62.0	1.29		20.20	0.79		9436	1.19

.

The value of f_ct,fl strength for Var. I obtained for BcR-NA-1 was higher than that of BcR-RCA-1 by 16.4% and higher than that of BcR-RCA-WGP10 by 19.4%. Considering the minimum required value of 5 MPa for classification in BcR5 Class, the value of BcR-NA-1 exceeded the limit by 34%, BcR-RCA-1 by 12%, and BcR-RCA-WGP10 by 8%.

The value of f_ct,fl strength for Var. II obtained for BcR-NA-2 was lower than that of BcR-RCA-2 by 1.9% and compared to that of BcR-RCA-WGP20 was higher by 20.4%. Considering the minimum required value of 5 MPa for classification in BcR5 Class, the value of BcR-NA-2 exceeded the limit by 8% and BcR-RCA-2 did so by 10%, whereas BcR-RCA-WGP20 fell into the BcR4 class.

The comparative values of f_ct,fl strength between Var. I and Var. II obtained for BcR-NA were higher by 19.4% for BCR-NA-1. At the same time, the value obtained for BcR-RCA-1 was only 1.79% higher than the values obtained for BcR-RCA-2. The BcR-RCA-WGP mixes exhibited similar trends to the values of f_ct,fl, as BcR-NA mixes, with an increase of 20.37% in the value obtained for BcR-RCA-WGP10 mix.

The values of the flexural tensile strength at the reference age of 28 days of all concrete mixes, except BcR-RCA-WGP20, recorded values greater than >5.0 MPa, corresponding to the requirements for the very heavy traffic class BcR 5.0 [42], the same traffic class as the reference concrete. The increase in the ratio of WGP led to a decrease in the value of the flexural tensile strength, which can be attributed to the higher absorption and porosity compared to natural sand and, consequently, an increased water demand to maintain the desired workability [94].

Apart from the BcR-RCA-WGP20 (BcR4), all other mixes fit into the BcR5 class. The values of f_c strength for Var. I obtained for BcR-NA-1 were higher than the that obtained for the BcR-RCA-1 by 1.3% and compared to that of BcR-RCA-WGP10 by 5.0%. Considering the minimum required value of 45 MPa for classification in BcR5 class, the value of BcR-NA-1 exceeded the limit by 87.11%, that of BcR-RCA-1 did so by 84.67%, and that of BcR-RCA-WGP10 did so by 77.78%.

The value of f_c strength for Var. II obtained for BcR-NA-2 was lower than that of BcR-RCA-2 by 0.3%, and compared to that of BcR-RCA-WGP20, it was 10.4% higher. Considering the minimum required value of 45 MPa for classification in BcR5 class, the value of BcR-NA-2 exceeded the limit by 53.78%, that of BcR-RCA-2 did so by 54.22% and that of BcR-RCA-WGP20 did so by 37.78%.

The comparative value of f_c strength between Var. I and Var. II lead to the conclusion that all mixes belonging to Var. I exhibited consistently higher values of the compressive strength than mixes belonging to Var. II. The differences ranged from 16.5% to 22.5%.

The values of f_ct,sp strength associated with Var. I compared to all the f_c,sp values associated with composites in Var. II were 17.77% higher for BcR-NA-1, 15.90% higher for BcR-RCA-1, and 22.22% higher for those containing WGP, as seen in Table 20.

The loss of strength (η) due to 100 repeated freeze–thaw cycles of the composites in Variant I was lower than that of those in Var. II, due to the presence of entrained air and the increased w/c ratio for Var. II mixes. The lower η was, the higher the resistance of the composites to freeze–thaw cycles.

The value of η for Var. I obtained for BcR-NA-1 was lower than that of BcR-RCA-1 by 3.47% and than that of BcR-RCA-WGP10 by 13.39%. Considering the minimum required value of 25% for classification in BcR5 class, the value of BcR-NA-1 is lower than the limit by 43.56%, that of BcR-RCA-1 is lower by 41.60%, and the value of BcR-RCA-WGP10 is lower by 36.00%.

The value of η for Var. II obtained for BcR-NA-2 was lower than that of BcR-RCA-2 by 4.76%, and compared to that of BcR-RCA-WGP20, it was lower by 20.24%. Considering the minimum required value of 25% for classification in BcR5 class, the value of BcR-NA-2 was lower than the limit by 32.80%, that of BcR-RCA-2 was lower by 29.60%, and the value of BcR-RCA-WGP20 was lower by 19.20%.

For the comparative value of η between Var. I and Var. II, it can be observed that all values obtained for Var. I mixes were consistently lower than those obtained for Var. II mixes. The difference ranged from 19.06% up to 26.25%.

The results obtained for the mechanical properties indicate that, in general, they decreased as natural aggregate was replaced, cement was substituted with WGP, and the w/c ratio increased. However, all of them highlight better behavior than the limits imposed for the classification into road concrete classes.

The volume loss (ΔV) of the composites in Var. I was greater than that of Var. II due to the increased water content in the designed mixes, but it was also influenced by the strength of the aggregate in the cement matrix and the content of WGP. Composites containing glass powder perform the best in terms of wear, especially the BcR-RCA-WGP20 mix, thus confirming the benefic influence of WGP in the composite and fulfilling the research purpose. Abrasion behavior was also attributed to the characteristics of the constituent aggregates, which form the mineral skeleton incorporated by the cement matrix.

The lower the volume loss, the more resistant the sample was to wear. The greater the amount of FGS (fine glass powder) in the concrete composition, the more resistant the sample subjected to Böhme wear due to the dense composition and the hardness of WGP. All obtained values classify the composites in the best performance class according to the criteria in SR EN 1338:2004 [86], SR EN 1339:2004 [87], and SR EN 1340:2004 [88], specifically in Class 4—Mark I, which requires a volume loss < 18,000 mm³/5000 mm². Regarding the cement replacement variants with 10% and 20% WGP, all mechanical characteristics are higher in the 10% variant except for the volume loss in the wear test (Table 20).

The value of ΔV for Var. I obtained for BcR-NA-1 was lower than that of BcR-RCA-1 by 1.76%, and compared to that of BcR-RCA-WGP10, it was higher by 2.43%. Considering the minimum required value of 18,000 mm³ for classification in Class 4—Mark I, all mixes from Var. I fulfill the requirement by more than 36%.

The value of ΔV for Var. II obtained for BcR-NA-2 was lower than that of BcR-RCA-2 by 1.31%, and compared to that of BcR-RCA-WGP20, it was higher by 0.69%. Considering the minimum required value of 18,000 mm³ for classification in Class 4—Mark I, all mixes showed consistently lower values than the upper limit by at least 47%.

The comparative value of ΔV between Var. I and Var. II show that the mixes in Var. II were more resistant to wear. The best wear behavior among all composites was that of BcR-RCA-WGP20.

4.3. Performance of the BcR by PXRD Test

Microscopic studies have highlighted the fact that, from a matrix structure perspective, it was finely crystallized, supporting a compact composite with high mechanical strengths both in the control mix and especially for the one with 10% WGP. Microcrystalline mineral phases were visible, represented by portlandite, calcite, hydrated calcium silicates, and hydrated calcium aluminates. X-ray diffraction indicated a high degree of crystallization in the concrete matrix containing 10% WGP, suggesting a pozzolanic reaction of WGP. Additionally, X-ray diffraction has revealed the presence of calcite, portlandite and gypsum, as well as the presence of calcium silicate hydrate and aluminum silicate hydrate. Thus, the effectiveness of WGP content in BcR composites was confirmed due to its efficient pozzolanic activity and hardness, contributing to durability characteristics [95].

A continuation of this study is planned, involving the assessment of long-term characteristics of the new road concrete with RCA and WGP.

5. Conclusions

This paper focuses on the development of road concrete that aims to use recycled materials from concrete and glass at the end of their life cycle. These materials are incorporated into the cement matrix alongside natural aggregates, with WGP partially substituting Portland cement. The intention is to create a new, sustainable composite that has not been produced yet. Microscopic determinations confirm the pozzolanic efficiency of WGP in concrete through its complete consumption. Additionally, the presence of CSH contributes to the enhancement of mechanical characteristics.

This new composite offers several advantages in terms of reducing existing and potential volumes of mineral waste both economically and visually, reducing pollution, and minimizing the consumption of natural resources and energy (the production of RCA consumes less electrical energy than obtaining natural crushed aggregates, which are rapidly depleted in urban areas undergoing rapid development). Moreover, the potential for practical application is ensured due to meeting durability requirements. Typically, companies incur high fees for the removal, transportation, and disposal of this type of waste at landfills as inert solid waste, reasonable reuse of these waste being of high interest.

This study confirms the appropriate use of RCA and its integration into road concrete. The characteristics of this type of aggregate need more frequent testing compared to natural crushed river aggregate, and they must have values close to those they replace. The fact that RCA contains a quantity of mortar signals the need for water in the mix (effective water) to ensure it is sufficient for a prescribed consistency, considering the increased absorption of RCA. The high absorption of RCA is not detrimental because it can promote the hydration of cement particles that remained un-hydrated for a longer period, contributing to increased mechanical strength even at more advanced ages. This justifies the high characteristic values for composites with RCA content. Another confirmation of the study is the role of WGP in the composites. WGP significantly contributes to the favorable behavior of composites, particularly in terms of wear resistance, which is a key objective of this study. Consequently, these compositions can be used for roads with heavy traffic.

Based on the obtained results, the following conclusion can be drawn:

• The apparent density of the concrete mix decreases with the substitution of natural river aggregates (4/8 mm) with recycled concrete aggregates and by substituting cement by waste glass powder.
• High quality recycled concrete aggregates have little influence on the fresh properties of concrete, specifically on the slump values. However, the addition of glass powder leads to a decrease in the slump value due to its higher specific surface area and increased friction force between the particles.
• Increasing the volume of recycled concrete aggregates and cement replacement percentage by waste glass powder results in decreased values of the slump and significant decrease in workability. In order to counteract this drawback, a higher water/binder ratio is needed coupled with a higher dosage of superplasticizer. This leads to lower values for apparent density but increased air content.
• Substituting cement with waste glass powder and river aggregates by recycled aggregates results in a decrease in the value of flexural strength. On the other hand, the same substitution has little effect in terms of values of the compressive strength. Increasing the volume of 4/8 mm aggregates, from the total aggregate volume leads to a sharp decrease in the value of the compressive strength as well. Substituting 4/8 mm river aggregates with recycled concrete aggregates and cement with waste glass powder (20% by mass of cement) results in further decrease in mechanical properties.
• Volume loss in Böhme abrasion and loss of strength after 100 freeze–thaw cycles are significantly lower than the upper limit imposed by existing regulations for all investigated concrete mixes.

Even though the mechanical characteristic values experienced slight decreases for composites with alternative mixtures, all of them fall within the BcR 4 and mostly BcR 5 classes of road concrete, which is the highest road concrete class currently existing in the Romanian standard.

Based on the obtained results in this study, the best concrete mix in terms of mechanical and durability properties and answering to the call for sustainability, is BcR-RCA-WGP10 mix.

6. Patents

Eco-innovative road concrete based on cement, glass powder, and aggregates from recycled concrete waste for applications in the field of constructions “BcR-G”:

RO137345A0 • 30 March 2023 • UNIV TEHNICA DIN CLUJ NAPOCA [RO], earliest priority: 29 September 2022 • earliest publication: 30 March 2023. Inventors: Corbu Ofelia Cornelia [RO]; Puskas Attila [RO].