Using Reclaimed Cement Concrete in Pavement Base Mixes with Foamed Bitumen Produced in Cold Recycling Technology

The paper presents the results of exploratory research on the use of reclaimed cement concrete in cold-recycled mixes with foamed bitumen. Because reclaimed cement concrete, unlike natural aggregates, is expected to have a residue of the non-hydrated cement covering the aggregate grains, which may result in a secondary cementation process after its application in a road base, this avenue was explored by tracking the time evolution of the compressive strength of the final material. The tests were performed using two mixtures, i.e., a reference mixture and a mixture containing 25% reclaimed cement concrete. The mixtures containing reclaimed cement concrete were characterized by increased uniaxial compressive strengths after each curing period (3, 4, 7, 14 and 28 days)—by 11.5 kPa on average and e.g., 498 kPa vs. 506 kPa after 28 days. The obtained differences between the mixtures were not found to be statistically significant. The small effects of the incorporation of reclaimed cement concrete were attributed to the time passed typically between the demolition and new pavement construction and to the presence of a second binding material—bitumen.


Use of Recycled Materials in Pavement's Upper Structural Layers
Recycling techniques are now widely used in road construction and they most prominently include producing top asphalt pavement courses with reclaimed asphalt pavement (RAP) and deep cold recycling techniques, enabling road bases to be produced using recycled materials in high quantities, as well as other underlying structural materials. The main purpose of using deep cold recycling technology is to maximize the use of the material obtained from the old worn-out layers of the road surface [1][2][3][4][5]. Most of the roads reconstructed with this technology are pavements with flexible or semi-rigid structures [6,7] and the processed structural layers may, in addition to asphalt layers [8], include improved subgrade [9], cement concrete [10] and unbound mixtures. Materials obtained from the above-mentioned layers come in the form of reclaimed asphalt pavement (RAP), reclaimed cement concrete (RCC) and reclaimed aggregates (RA) from unbound mixtures, which all can be used in the composition of a recycled road base layer. It should be borne in mind that the layout of the existing construction layers determines the percentage share of individual components in the mixture of the recycled base course. The most commonly used amount of reclaimed asphalt is in the range of 20-70% depending on the specific construction, climatic or operating conditions of the rehabilitated pavements, enabling the reuse (recycling) of a large amount of other used building materials. These percentages typically obtain high levels of mechanical performance and resistance to moisture. An amount of RAP equal to 20% was used in research by Niazi and Jalili [11] for assessing the effect of Portland cement and hydrated lime on the properties of a mineral-binder mixture with foamed asphalt. It was shown that the addition of 2% of Portland cement to the mixture improved its indirect tensile strength in dry conditions (ITS dry ) by approx. 70%, and after water conditioning (ITS wet ) the increase was 250%, significantly improving the water sensitivity (TSR). Iwański and Chomicz-Kowalska [2], when assessing compaction methods, used 50% RAP in the composition of the base mix and obtained ITS dry values exceeding 500 kPa and ITS wet values of over 400 kPa, regardless of the compaction method used. Iwański and Buczyński used approximately 40% to 60% reclaimed asphalt pavement [12,13] and obtained a material with extremely high stiffness (exceeding 10,000 MPa under dynamic loading) with even higher ITS values (above 1.1 MPa in dry state) when high contents of fines were introduced. The influence of the amount of reclaimed asphalt (0%, 50% and 70%) in the composition of the recycled base with foamed asphalt on the change of the complex modulus was presented by Godenzoni et al. [14] who obtained values approx. in the range of 1000-3000 MPa. These results show that even when very high rates of recycled materials are used in these types of mixtures, adequate mechanical performance can be achieved. In the design process, adequate mix gradation, fines content and amounts of hydraulic binder have to be used to provide adequate mechanical performance and moisture resistance and to mitigate over-stiffening of the material.
Regarding the cold-recycled base mixes with foamed asphalt, in the case of improper graining of the mineral mix in the existing layers, it is required to introduce a new grading aggregate. The use of a new component translates into an increase in the costs of making a recycled foundation. As a result, it is more advantageous to increase the cost of equipment and bring the existing layers to the required grain size by, e.g., grinding or introducing other types of recycled materials. This approach makes it possible to use the materials from the existing construction layers to the maximum extent in the recycled base course.
The relative proportions of the individual components (reclaimed material, grading material, hydraulic binder, asphalt binder), as well as the type of the reclaimed portion of the mix, determine the strength and deformation properties of MCAS mixtures and the future construction layer of the road surface.

Recycling of Portland Cement Concrete in Pavement Construction
Most of the waste construction materials which can be used again for producing pavement structures are typically sourced from the demolition and reconstruction of old road, railroad and bridge infrastructure. These waste materials are usually referred to as construction and demolition waste (CDW). Concrete waste is also created as a by-product during production in the fabrication plants of building materials, such as ready-mixed concrete or prefabricates [15].
The effects of the development of the construction market are an increased demand for concrete and an excessive amount of construction waste. Current efforts are focused on environmental protection as a result of a demand to lower the energy consumption of production processes. These include: use of recycling technology in many industrial sectors, enforcing rational waste management and using waste from the road construction sector. Concrete used in old pavements and other concrete structures has a high recycling potential. Thanks to the use of reclaimed concrete in construction, the need for new natural resources can be decreased. The use of recycled building materials lowers the costs of mining and transporting new rock materials [16].
Concrete aggregate may have different grain sizes and functional properties depending on the crushing method. The results of research conducted by Pedro et al. in Portugal [17] indicate, however, that the aggregate properties have a greater impact on the strength and deformation parameters of materials made of such aggregate than the crushing method itself. Sabai et al. [18] showed that the parameters of prefabricated elements based on recycled concrete aggregate allow their use in new building structures. However, it was found that prefabricated elements made of recycled concrete aggregate are characterized by lower strength compared to elements produced based on natural aggregate. This may be due to the greater water absorption of recycled aggregate compared to natural aggregates.
In the studies of Wagih et al. [19], it was shown that the content of recycled aggregate at the level of 25% does not significantly affect the properties of cement concrete. Concretes with the content of 25-50% of recycled aggregate were characterized by lower strength parameters compared to conventional concretes, but their features allowed the use of the final product in structures. At the same time, high water absorption and high abrasiveness of recycled concrete aggregate have been demonstrated.
All over the world (including in China, the USA and Norway), reclaimed concrete resulting from the crushing process is used as a road foundation in road structures [20][21][22]. Moreover, reclaimed concrete is classified as a material suitable for the construction of auxiliary foundations, main foundations and cut-off layers, as well as a filler in soundabsorbing embankments [23][24][25]. Reclaimed concrete was investigated for its use in asphalt concrete, including hot and warm mix asphalt techniques [26][27][28][29].
Reclaimed concrete material, in contrast to natural aggregate material, is distinguished by the presence of a hydrated cement matrix that remained on the top layer of the aggregate grains, becoming a component of the hardened concrete mix. Therefore, reclaimed concrete may be subject to additional cementation, which is a consequence of the release of unbound pozzolanic particles after crushing of the original material [30]. Residues of pozzolanic particles result in a reduction in the specific density of grains, a greater ability of water to enter the waste material and a reduction in the quality of aggregate contained in the concrete waste [31][32][33]. During the setting of cement concrete, not all of the cement particles are hydrated, hence the aggregate from the recycling of cement concrete undergoes secondary setting over time, which causes the physical and mechanical parameters of the base layer to change over time. The confirmation of this effect of secondary cementation in the layer of aggregate from recycled concrete was shown [34] both in laboratory tests and in the example of the value of the secondary modulus of deformation of the foundation layer on the road surface.
Based on the information presented above, an investigation was conducted to evaluate the effects of reclaimed cement concrete on the uniaxial compressive strength of deep-cold recycled mixture with foamed bitumen. The study involved evaluating the time evolution of the unconfined compressive strength of a reclaimed concrete-bearing and a reference mixture and included the use of a fine fraction of the reclaimed cement concrete material. The evaluation of uniaxial compressive strength was selected as the most basic material strength property with uniform stress and strain fields. The tests were conducted without any additional use of hydraulic binder to expose the evaluated effects. The objective was to conduct an initial assessment of the possibility and feasibility of utilizing this material, as well as to evaluate the potential presence of secondary setting in the reclaimed cement concrete material.

Experimental Plan
In order to evaluate the use of reclaimed cement concrete (RCC) in cold-recycled mixtures with foamed bitumen, an experimental plan was set up to investigate the unconfined compressive strength of such mixtures containing RCC material. The study involved comparing a reference mixture (REF-Mix) with a mixture containing 25% of the reclaimed cement concrete material (RCC-Mix). To formulate the investigated mixtures, a number of materials were selected (mostly recycled and reclaimed): Recycled and reclaimed concrete materials are produced from the demolition of existing concrete structures. The methods for obtaining these materials include demolition or milling of slabs, beam walls of buildings, parts of engineering structures and pavements comprising hydraulically bound layers. This reclaimed material can be used as a construction aggregate or an anthropogenic subgrade [35,36].
The investigation utilized reclaimed concrete material from used concrete road slabs. The original reclaimed cement material (RCM) was prepared by 2-mm sieve screening which produced the reclaimed cement concrete (RCC) to be used in the RCC-Mix. The grading of the sieved RCC material was made comparable to that of the natural aggregate grading improving material used in the REF-Mix. Additionally, this increased the share of the fraction responsible for the presumed formation of secondary cementation, i.e., <0.6 mm [37].
The results of the tests for determining the granulometric composition of the original reclaimed concrete and the sieved material prepared from reclaimed concrete according to [38] are presented collectively in Figure 2 and are summarized in Table 1. Table 2 presents a summary of the obtained results for the geometric and physical properties of reclaimed cement material (RCM).  Recycled and reclaimed concrete materials are produced from the demolition of existing concrete structures. The methods for obtaining these materials include demolition or milling of slabs, beam walls of buildings, parts of engineering structures and pavements comprising hydraulically bound layers. This reclaimed material can be used as a construction aggregate or an anthropogenic subgrade [35,36].
The investigation utilized reclaimed concrete material from used concrete road slabs. The original reclaimed cement material (RCM) was prepared by 2-mm sieve screening which produced the reclaimed cement concrete (RCC) to be used in the RCC-Mix. The grading of the sieved RCC material was made comparable to that of the natural aggregate grading improving material used in the REF-Mix. Additionally, this increased the share of the fraction responsible for the presumed formation of secondary cementation, i.e., <0.6 mm [37].
The results of the tests for determining the granulometric composition of the original reclaimed concrete and the sieved material prepared from reclaimed concrete according to [38] are presented collectively in Figure 2 and are summarized in Table 1. Table 2 presents a summary of the obtained results for the geometric and physical properties of reclaimed cement material (RCM). Recycled and reclaimed concrete materials are produced from the demolition of existing concrete structures. The methods for obtaining these materials include demolition or milling of slabs, beam walls of buildings, parts of engineering structures and pavements comprising hydraulically bound layers. This reclaimed material can be used as a construction aggregate or an anthropogenic subgrade [35,36].
The investigation utilized reclaimed concrete material from used concrete road slabs. The original reclaimed cement material (RCM) was prepared by 2-mm sieve screening which produced the reclaimed cement concrete (RCC) to be used in the RCC-Mix. The grading of the sieved RCC material was made comparable to that of the natural aggregate grading improving material used in the REF-Mix. Additionally, this increased the share of the fraction responsible for the presumed formation of secondary cementation, i.e., <0.6 mm [37].
The results of the tests for determining the granulometric composition of the original reclaimed concrete and the sieved material prepared from reclaimed concrete according to [38] are presented collectively in Figure 2 and are summarized in Table 1. Table 2 presents a summary of the obtained results for the geometric and physical properties of reclaimed cement material (RCM).   The physical and mechanical properties of reclaimed cement material (RCM) are similar to the physical and mechanical properties of new continuously graded aggregates produced in mineral raw materials mines. Therefore, it can be concluded that it is possible to use such a recycled material in the composition of the mineral-cement mixture with foamed bitumen (MCAS).
The chemical composition of the reclaimed cement concrete (RCC) used in the experiments is given in Table 3. EDX (energy-dispersive X-ray) spectroscopy was used to validate the composition of the RCC. The samples for analysis were prepared by covering them with a thin layer of conductive by sputtering with gold (Au). A microstructural analysis of the reclaimed material followed, performed in four distinct selected areas of the investigated sample as shown in Figure 3.    Structural analysis of reclaimed cement concrete material based on scanning electron micrographs showed the presence of cement hydration products, which were accompanied by the formation of calcium hydroxide and a stable structure of hydrated calcium silicates (CSH). The microstructure of the hydrating cement slurry is enlarged for Item 2 ( Figure 3). The analysis of the chemical composition also shows the presence of aluminum, magnesium and sodium compounds to a lesser extent at points 2 and 3. There are also visible pores, not filled with CSH gel. The chemical composition identified in the places of grain occurrence (points 1 and 4) shows the presence of mainly silicon and, to a lesser degree, calcium.

Reclaimed Asphalt Pavement (RAP)
A major component comprising the investigated mixtures was reclaimed asphalt obtained from the milling of existing asphalt courses (wearing and binding layer), which contained 5.6% of bituminous binder as determined in the extraction test in accordance with EN 12697-1:2012. The evaluation of the particle size distribution of reclaimed asphalt and recovered aggregate (after solvent extraction of the binder) determined in accordance with [38] are shown in Figure 4 and are summarized in Table 4. Structural analysis of reclaimed cement concrete material based on scanning electron micrographs showed the presence of cement hydration products, which were accompanied by the formation of calcium hydroxide and a stable structure of hydrated calcium silicates (CSH). The microstructure of the hydrating cement slurry is enlarged for Item 2 ( Figure 3). The analysis of the chemical composition also shows the presence of aluminum, magnesium and sodium compounds to a lesser extent at points 2 and 3. There are also visible pores, not filled with CSH gel. The chemical composition identified in the places of grain occurrence (points 1 and 4) shows the presence of mainly silicon and, to a lesser degree, calcium.

Reclaimed Asphalt Pavement (RAP)
A major component comprising the investigated mixtures was reclaimed asphalt obtained from the milling of existing asphalt courses (wearing and binding layer), which contained 5.6% of bituminous binder as determined in the extraction test in accordance with EN 12697-1:2012. The evaluation of the particle size distribution of reclaimed asphalt and recovered aggregate (after solvent extraction of the binder) determined in accordance with [38] are shown in Figure 4 and are summarized in Table 4.    Knowing the maximum particle size of reclaimed asphalt mixture (U) and the aggregate grain size after extraction of the binder, the tested reclaimed asphalt, in accordance with the 13108-8: 2016 standard, was designated as 16 RA 0/8, i.e., reclaimed asphalt material with aggregate size of 8 mm and asphalt particles of a maximum size of 16 mm.
Basic tests were carried out based on the EN 13108-8: 2016 standard for the asphalt extracted from the RAP. The results of the analysis are presented in Table 5. Based on the basic tests of asphalt recovered from reclaimed asphalt, it can be concluded that the binder in the RAP, in terms of penetration, can be classified as 50/70 paving grade bitumen. Based on the results of penetration and softening point determinations, the value of the penetration index was determined (PN-EN 12591: 2009). The calculated penetration index was set at IP = −0.1. The obtained result of the average elastic recovery excludes the possibility of classifying the recovered asphalt as a modified binder.
The chemical composition of the RAP used in the experiments is given in Table 6 and its microscopic image is depicted in Figure 5. EDX spectroscopy was used to validate the composition of the RAP. Scanning microscope studies revealed the complex structural nature of the material of RAP samples. There are several elements of internal structure with a different chemical composition, defined for example at points 1-3 ( Figure 5). Asphalt mastic appears as smooth, amorphous coatings, seen at point 2, with a chemical composition indicating both the presence of asphalt (hydrocarbon compounds) and fine aggregate fractions mainly derived from sedimentary rocks. The analysis revealed the presence of, among others, elements of carbon, silicon, aluminum and calcium. The components of coarse aggregate grains are shown in Items 1 and 3, where the predominant share of calcium compounds was found. There are also visible spaces that are not filled with binding material. Scanning microscope studies revealed the complex structural nature of the material of RAP samples. There are several elements of internal structure with a different chemical composition, defined for example at points 1-3 ( Figure 5). Asphalt mastic appears as smooth, amorphous coatings, seen at point 2, with a chemical composition indicating both the presence of asphalt (hydrocarbon compounds) and fine aggregate fractions mainly derived from sedimentary rocks. The analysis revealed the presence of, among others, elements of carbon, silicon, aluminum and calcium. The components of coarse aggregate grains are shown in Items 1 and 3, where the predominant share of calcium compounds was found. There are also visible spaces that are not filled with binding material.
Reclaimed Aggregate (RA) Natural reclaimed aggregate produced by removing an existing road pavement base course was used in this study. This type of material is defined by the EN 13242 standard as an aggregate resulting from the processing of inorganic or mineral material previously used in construction [39].
Natural reclaimed aggregates are favored for use in the construction of new infrastructure as it decreases the environmental impact of such endeavors. Utilization of this type of material is particularly well suited to the construction of roads, pavements, bicycle paths, squares and car parks, as well as for levelling roads that do not have bituminous pavements [40]. In many cases, incorporation of reclaimed aggregates in such projects requires additional crushing, sieving and sorting before they can be re-used for the construction of a new road or reconstruction of the existing ones. However, when cold recycling is considered, reclaimed aggregates can often be used in place, without any additional processing.
Natural reclaimed aggregate utilized in the present study was obtained from an existing unbound road base. The material, which was designated as a continuously graded Reclaimed Aggregate (RA) Natural reclaimed aggregate produced by removing an existing road pavement base course was used in this study. This type of material is defined by the EN 13242 standard as an aggregate resulting from the processing of inorganic or mineral material previously used in construction [39].
Natural reclaimed aggregates are favored for use in the construction of new infrastructure as it decreases the environmental impact of such endeavors. Utilization of this type of material is particularly well suited to the construction of roads, pavements, bicycle paths, squares and car parks, as well as for levelling roads that do not have bituminous pavements [40]. In many cases, incorporation of reclaimed aggregates in such projects requires additional crushing, sieving and sorting before they can be re-used for the construction of a new road or reconstruction of the existing ones. However, when cold recycling is considered, reclaimed aggregates can often be used in place, without any additional processing.
Natural reclaimed aggregate utilized in the present study was obtained from an existing unbound road base. The material, which was designated as a continuously graded 0/22 mm size aggregate, was incorporated in both the reference REF-Mix and the recycled RCC-Mix.
The results of the tests for determining the granulometric composition of the reclaimed aggregate according to [38] are shown in Figure 6 and are summarized in Table 7. Table 8 presents a summary of the geometric and physical characteristics. The results of the tests for determining the granulometric composition of the reclaimed aggregate according to [38] are shown in Figure 6 and are summarized in Table  7. Table 8 presents a summary of the geometric and physical characteristics.   The reference REF-Mix mixture incorporated a 0/4 sized fraction of virgin aggregates in its original design in order to improve its grading. A dolomite aggregate was selected for this purpose because of its wide availability in the region. The utilized dolomite material is typically used in asphalt concrete mixtures and cement concrete formulations and therefore meets all the typical requirements for these kinds of usage.
The results of the tests for determining the grain size composition of aggregate (VA) according to [38] are shown in Figure 7 and summarized in Table 9. Table 10 presents a summary of the geometric and physical characteristics. The reference REF-Mix mixture incorporated a 0/4 sized fraction of virgin aggregates in its original design in order to improve its grading. A dolomite aggregate was selected for this purpose because of its wide availability in the region. The utilized dolomite material is typically used in asphalt concrete mixtures and cement concrete formulations and therefore meets all the typical requirements for these kinds of usage.
The results of the tests for determining the grain size composition of aggregate (VA) according to [38] are shown in Figure 7 and summarized in Table 9. Table 10 presents a summary of the geometric and physical characteristics.

Foamed Bitumen
The bituminous binder used in the investigated mixtures was a 50/70 paving grade bitumen as in [41][42][43] foamed prior to mixing with the mineral material using the WLB10S (Wirtgen, Windhagen, Germany) laboratory foamer. The 50/70 asphalt binder was selected based on the findings of other researchers on the effects of bitumen type on the properties of foamed bitumen mixtures [3]. The properties of the utilized binder are shown in Table 11, while a graphical representation of the determination of foaming water content is presented in Figure 8. The physical parameters of the asphalt foam were determined as shown in Figure 8, by introducing a requirement for one of the analyzed features. In this case, an optimal foaming water content FWC = 3.0% (foaming water content) was determined at the values of ER = 13.3 (maximum expansion ratio) and HL = 15.4 s (bitumen foam half-life) as read from the graph.

The Mix Design of Recycled Mixtures
The mix design of the recycled mixtures includes the determination of the particle size distribution of the mixture and the contents of binding materials, which typically include foamed asphalt binder and Portland cement, which is required to obtain the necessary moisture and frost damage resistance of cold-recycled mixtures [2,13,44]. However, given the aim of this study, incorporation of fresh Portland cement or any other kind of hydraulic binder would conceal the potential effects of secondary setting in the reclaimed cement concrete material. Therefore, no additional hydraulic binder was used in this study. The mineral composition of the investigated mixtures is shown in Table 12, while their designed particle size distribution is shown in Figure 9. The materials used in the study conformed to the requirements provided in the respective guidelines [36].  The physical parameters of the asphalt foam were determined as shown in Figure 8, by introducing a requirement for one of the analyzed features. In this case, an optimal foaming water content FWC = 3.0% (foaming water content) was determined at the values of ER = 13.3 (maximum expansion ratio) and HL = 15.4 s (bitumen foam half-life) as read from the graph.

The Mix Design of Recycled Mixtures
The mix design of the recycled mixtures includes the determination of the particle size distribution of the mixture and the contents of binding materials, which typically include foamed asphalt binder and Portland cement, which is required to obtain the necessary moisture and frost damage resistance of cold-recycled mixtures [2,13,44]. However, given the aim of this study, incorporation of fresh Portland cement or any other kind of hydraulic binder would conceal the potential effects of secondary setting in the reclaimed cement concrete material. Therefore, no additional hydraulic binder was used in this study. The mineral composition of the investigated mixtures is shown in Table 12, while their designed particle size distribution is shown in Figure 9. The materials used in the study conformed to the requirements provided in the respective guidelines [36].  Figure 9 shows the requirements regarding particle size distribution regarding coldrecycled mixtures. The designed investigated mixtures conformed to the grading area and both foamed mixtures, REF-Mix and RCC-Mix, had the same particle size distributions. Both mixtures were designed with the same total asphalt binder content of 5.5%. The asphalt binder comprised in the RAP amounted to 2% of the final mixture and the added foamed bitumen amounted to 3%. The amount of the asphalt binder was in accordance with the relevant recommendations (not exceeding 6%) [44,45].

Optimum Moisture Content (OMC)
The optimum moisture content (OMC) was determined in accordance with the PN EN 13286-2:2010 standard using the Proctor compaction test (large cylinder, method B). The obtained relationship between the moisture content of the mix and its dry density is presented in Figure 10. This relationship permitted establishing the moisture content at the maximum dry density of the mineral mixture which amounted to 6.1%. The mixing moisture content of the recycled mixtures was set at 75% of the OMC.   Both mixtures were designed with the same total asphalt binder content of 5.5%. The asphalt binder comprised in the RAP amounted to 2% of the final mixture and the added foamed bitumen amounted to 3%. The amount of the asphalt binder was in accordance with the relevant recommendations (not exceeding 6%) [44,45].

Optimum Moisture Content (OMC)
The optimum moisture content (OMC) was determined in accordance with the PN EN 13286-2:2010 standard using the Proctor compaction test (large cylinder, method B). The obtained relationship between the moisture content of the mix and its dry density is presented in Figure 10. This relationship permitted establishing the moisture content at the maximum dry density of the mineral mixture which amounted to 6.1%. The mixing moisture content of the recycled mixtures was set at 75% of the OMC.  Figure 9 shows the requirements regarding particle size distribution regarding coldrecycled mixtures. The designed investigated mixtures conformed to the grading area and both foamed mixtures, REF-Mix and RCC-Mix, had the same particle size distributions. Both mixtures were designed with the same total asphalt binder content of 5.5%. The asphalt binder comprised in the RAP amounted to 2% of the final mixture and the added foamed bitumen amounted to 3%. The amount of the asphalt binder was in accordance with the relevant recommendations (not exceeding 6%) [44,45].

Optimum Moisture Content (OMC)
The optimum moisture content (OMC) was determined in accordance with the PN EN 13286-2:2010 standard using the Proctor compaction test (large cylinder, method B). The obtained relationship between the moisture content of the mix and its dry density is presented in Figure 10. This relationship permitted establishing the moisture content at the maximum dry density of the mineral mixture which amounted to 6.1%. The mixing moisture content of the recycled mixtures was set at 75% of the OMC.

Experimental Methodology
The experimental tests involved evaluating the unconfined compressive strength (UCS) of the compacted specimens produced from the recycled mixtures at predetermined time intervals of 3, 4, 7, 14 and 28 days of ageing. After compaction, the samples were conditioned for 48 h in the molds and subsequently demolded ( Figure 11) and kept in shade on a tabletop. The samples were compacted using the Proctor compactor at 4.6% (75% OMC) according to EN 13286-50 [46]. The evaluation of the unconfined compressive strength was performed to the EN 13286-41 [47] standard at 25 • C.

Experimental Methodology
The experimental tests involved evaluating the unconfined compressive strength (UCS) of the compacted specimens produced from the recycled mixtures at predetermined time intervals of 3, 4, 7, 14 and 28 days of ageing. After compaction, the samples were conditioned for 48 h in the molds and subsequently demolded ( Figure 11) and kept in shade on a tabletop. The samples were compacted using the Proctor compactor at 4.6% (75% OMC) according to EN 13286-50 [46]. The evaluation of the unconfined compressive strength was performed to the EN 13286-41 [47] standard at 25 °C.  The unconfined compressive strength (UCS) was determined ( Figure 12) by measuring the ultimate load to failure of a specimen subjected to a constant loading rate of 142 kPa/s (153 kN/min) [6]. The value of this parameter was determined based on the following relationship [6]: where: UCS-unconfined compressive strength (kPa), P-maximum compressive force (kN), d-specimen diameter (cm).  The unconfined compressive strength (UCS) was determined ( Figure 12) by measuring the ultimate load to failure of a specimen subjected to a constant loading rate of 142 kPa/s (153 kN/min) [6]. The value of this parameter was determined based on the following relationship [6]: where: UCS-unconfined compressive strength (kPa), P-maximum compressive force (kN), d-specimen diameter (cm).

Results and Discussions
The results of the determination of unconfined compressive strength of the investigated recycled mixtures are presented in Figures 13 and 14 in graphical form, while the calculated means, standard deviations, coefficients of variability and relative changes in the measured values are shown in Table 13.

Results and Discussions
The results of the determination of unconfined compressive strength of the investigated recycled mixtures are presented in Figures 13 and 14 in graphical form, while the calculated means, standard deviations, coefficients of variability and relative changes in the measured values are shown in Table 13.      The obtained results of UCS testing have shown that both mixtures exhibit similar strength characteristics throughout the curing period. The reference REF-Mix, which did not contain the reclaimed cement concrete aggregates, had a slightly smaller mean value of compressive strength during the whole 28-day period. This difference, which again was not very significant and amounted to 3-9 kPa, could be attributed to the effects of the reclaimed cement concrete in the RCC-Mix. The rate at which the compressive strength of the REF-Mix and RCC-Mix mixtures changed during the curing period decreased over time in a similar manner. This may lead to a conclusion that the processes having the greatest influence on the curing process of both mixtures were similar in nature, i.e., they were not governed primarily by the formation of hydraulic bonds. Nonlinear logarithmic curves were fitted to the data and shown in Figure 14, as these functions are often used to approximate the time evolution of mechanical properties in both cement-bound materials and cold-recycled mixtures [49][50][51]. The results shown in Figure 14 reinforce the abovementioned observations. The intercept term for the RCC-Mix was slightly larger than for the REF-Mix, indicating higher initial strength of the mixture with recycled cement concrete. The coefficients of the log functions fitted for both mixtures were similar, indicating a similar increase in strength over time.
To estimate in a more rigorous manner the effects of the RCC material on the unconfined compressive strength of the RCC-Mix mixture, a statistical analysis of the obtained results was performed. The results of the initial ANOVA analysis are presented in Table 14 which summarizes the evaluation of the following factors and their effects on the unconfined compressive strength: sample age (3, 4, 7, 14 days). The performed analysis of variance reveals that, in the evaluated groups of results, only the age of the investigated samples had a statistically significant effect (α > 0.05) on the mean value of their unconfined compressive strength. It is worth noting, however, that the effect of mixture type (REF-Mix, RCC-Mix) returned a probability value close to the assumed significance level (p-value = 0.066). The significance of the interaction between the mixture type and sample age can, on the other hand, be rejected with high confidence (p-value = 0.908). To analyze these initial results in further detail, post hoc Duncan tests were performed to simultaneously compare multiple groups and isolate homogenous groups, between which significant differences could not be proven. The results of this analysis are presented in Table 15.  (Table 16). The results of these tests have shown that the differences between the unconfined compressive strengths of these mixtures were too small to be proven statistically significant.

Conclusions
The present study considered using reclaimed cement concrete in cold mixtures with foamed bitumen and has shown a satisfactory performance of the RCC-Mix containing recycled cement material.
The preliminary tests performed on the constituent materials have shown their compliance with the adequate requirements, permitting their utilization in recycled foamed mixtures. In particular, the scanning microscope revealed the complex microstructure of the reclaimed materials-the reclaimed cement concrete and reclaimed asphalt pavement.
The evaluation of the mechanical performance of the produced cold-recycled mixtures with foamed bitumen has shown that the mixtures have increased their unconfined compressive strength in the course of the 28-day curing period. It was also found that the mixture containing the reclaimed cement concrete material experienced a slightly higher increase in strength, although the observed differences were not statistically significant. This observation may be explained by two mechanisms. Firstly, the fine non-hydrated particles easily react with moisture present in air and from other sources. This, together with the typical delay between the demolition and the producing of the new structure, lessens the effects of secondary setting, unless the reclaimed cement concrete is significantly processed (milled, crushed) during the new construction process. Secondly, the presence of bituminous bonding may have reduced the significance of the secondary cement setting in the RCC-Mix. This effect may be beneficial when incorporating RCC material into recycled road bases as it reduces the risk of over-stiffening of the road base, and therefore mitigates the possible formation of cracks in the pavement structure.
Future work in this area should include the evaluation of mixtures with higher reclaimed cement content and the evaluation of cracking and fatigue resistance of such mixtures; longer curing times should also be considered. ment setting in the RCC-Mix. This effect may be beneficial when incorporating RCC material into recycled road bases as it reduces the risk of over-stiffening of the road base, and therefore mitigates the possible formation of cracks in the pavement structure.
Future work in this area should include the evaluation of mixtures with higher reclaimed cement content and the evaluation of cracking and fatigue resistance of such mixtures; longer curing times should also be considered.