2.1. Materials
The concrete studied is made of Portland cement CEM I 42.5 R (Malogoszcz, Poland) that is mixed with water to yield the
w/
c ratio equal to 0.45. The workability of the mixtures is further increased by an addition of plasticizers BASF BV 18 and BASF Glenium SKY 591. There is also fine and coarse aggregates of which type and amount differ in particular mixtures as summarized in
Table 1. The natural aggregates originate from Dwudniaki, Poland (riverbed from Dunajec river), being formed mostly by sandstone (40–50%), granite (15–20%), porfire/melafire (5–30%), and quartzite (5–25%). The recycled aggregates are made of crushed concrete pavement blocks with compressive strength of 66.8 MPa, modulus of elasticity of 41.2 GPa, and bulk density of 2290 kg·m
−3. The get the better overview, the particle size distribution curves of particular aggregates are depicted in
Figure 1. The natural coarse aggregates is replaced either partially (REC50) or fully (REC100), being compared to the reference mixture (REF) without the recycled aggregates being contained.
Recycled aggregate was made simply by crushing the concrete pavement blocks that were dismounted after exploitation period of 15 years (see
Figure 2). The only modification was the sieving to achieve the particle size range demanded.
The individual fractions after the crushing and sieving procedures are shown in
Figure 3. Accounting for 24.59%, the particles smaller than 4 mm were then rejected.
The concretes specimens were cast and stored in plastic cubic or cylindrical molds for the first 24 h. After preliminary curing, water evaporation was prevented by covering them with plastic lids for 7 days. The specimens were then stored in natural air-drying conditions at the temperature of (20 ± 5) °C and relative humidity of (50 ± 5)%.
2.2. Experimental Analysis
Experimental analyses involved a determination of selected basic physical properties as well as heat and moisture storage and transport parameters to evaluate the influence of the recycled aggregates substitution on the functional properties of the concrete.
The bulk density was determined in a standard way [
18] by averaging individual values obtained for a set of cubic samples (4 samples of 50 × 50 × 50 mm
3, 3 samples of 100 × 100 × 100 mm
3) and a set of prism samples (3 samples of 40 × 40 × 150 mm
3).
The heat transport and storage parameters represented by thermal conductivity and specific heat capacity, respectively, were obtained by means of transient heat pulse method using the ISOMET 2114 device (Applied Precision, Ltd., Bratislava, Slovakia). Contrary to the steady state methods defined in ČSN EN 12,664 [
19], the dynamic method used provides almost instant results which is its biggest advantage. Even if the accuracy is not as high as in case of the standard steady state methods, the probes calibration according to ASTM D5334-14 [
20] guarantees the correctness of the data obtained. Being equipped with a surface probe, the sample surface is heated up so that its thermal response can be recorded and evaluated. The measurements were conducted repeatedly on both, dry (nine times) and fully saturated samples (five times).
Following the principles given in ČSN EN ISO 12,572 [
21], the water vapor transport parameters were analyzed and obtained by means of both dry-cup and wet-cup method, as depicted in
Figure 4 for illustration.
Three samples of each mixture with dimensions of 100 × 100 × 30 mm
3 were placed in the cups that contained either water or silica gel, depending on the type of arrangement, and the lateral sides were sealed to enable a one-dimensional moisture flux as the most dominant transport mode. Being placed in the climatic chamber with the controlled environment (25 °C, 50% RH), the water vapor transport had been initiated and the mass change of the cup was continuously recorded. The water vapor diffusion resistance factor could then be expressed as
where
Da = 2.82 × 10
−5 m
2·s
−1 is the water vapor diffusion coefficient in air at 25 °C,
M = 0.01802 kg·mol
−1 is the molar mass of water,
t (s) is time,
S (m
2) is the cross-section area of the sample, Δ
pv (Pa) is the partial pressure difference above and under the sample, Δ
m (kg) is the mass change,
d (m) is the sample thickness,
R = 8.314 Pa·m
3·mol
−1·K
−1 is the gas constant, and
T (K) denotes the temperature.
Liquid water transport parameters were quantified using moisture diffusivity as a result of the vertical water sorption test [
22,
23]. Within the experimental procedure, the 50 × 50 × 50 mm
3 samples insulated on their lateral sides by epoxy resin were partially immersed in water to initiate the water suction due to capillary forces. Based on mass increase observed as a function of time, the apparent moisture diffusivity can be calculated as
where Δ
m (kg) is the mass difference,
S (m
2) is the sample area in contact with water,
(s
0.5) is time, and
wcap (kg·m
−3) stands for the capillary moisture content. The scheme of the experiment is shown in
Figure 5.
Water vapor storage parameters were represented by the hygroscopic moisture content defining a moisture content threshold after which the moisture is no longer stored in a water vapor- but only in a liquid form. According to common agreement [
24] such a threshold corresponds to the equilibrium moisture content at 97% RH. Following this principle, four samples of each mixture were placed in a desiccator with supersaturated solution of K
2SO
4 that maintains the humidity of air above the solution at this level (see
Figure 6). After the mass equilibrium of the samples had been reached, the moisture content was calculated gravimetrically.
The compressive strength and splitting tensile strength after 28 days and compressive strength after 90 days, were determined following the standard procedures for concrete defined in ČSN EN 12390-3 standard [
25].
2.3. Hygrothermal Performance
Hygrothermal performance of the mixtures was compared by means of computational prediction of heat and moisture distribution over a reference year after exposure to dynamic weather conditions. A simplified construction segment (wall) was assumed for this purpose, being made of the concrete mixtures studied and provided with thermal insulation and finishes as shown in
Figure 7.
The heat and moisture transport was computed using an advanced mathematical model of a diffusion type [
26] that is able to precisely distinguish between particular phases of water that participate on the moisture transport at given conditions. The set of balance equation was solved numerically using the finite element method. As the input parameters, the material properties of concrete mixtures obtained experimentally were used, the other parameters of the thermal insulation and the plaster were taken from studies published previously [
27,
28]. The computational simulation resulted in the determination of hourly values of moisture content and temperature in every node of the computational mesh that together form the temperature and moisture distribution fields and can be further post-processed to get an overview on the hygrothermal performance. The boundary conditions on the exterior side were represented by the test reference year for Ostrava, Czech Republic [
29], containing long-term average hourly values of selected weather parameters (temperature, relative humidity, rainfalls, wind velocity, wind direction, direct solar radiation, diffuse solar radiation). The interior boundary conditions were set to 21 °C and 55% of relative humidity according to the thermal standard [
30].
2.4. Environmental Impact Assessment
The LCA (life cycle assessment) methodology was used to perform the environmental analysis of concrete mixtures studied, which considers each manufacturing step and quantifies all the benefits following the ČSN EN ISO 19,011 [
31] and ČSN EN ISO 14,044 [
32] standards. All the results presented within this subtask are related to the functional unit which is 1 m
3.
The analysis considers two scenarios which are combined to accommodate all the mixtures studied [
33]. The natural resources scenario contemplates quarrying of granite/limestone, loading, two-stage crushing, loading, and transportation between particular stages. According to Borghi et al. [
34], transportation distances for the natural aggregates were assumed to be 30 km. The recycling scenario contemplates the concrete pavement blocks at the end-of-life which must be collected, crushed, and sieved and transported to a concrete plant where it can be used as the filler. Within this scenario, the material needs to be transported about 60 km together including its collection and consequent delivery to the concrete plant [
34].
The life cycle impact assessment performed was based on the IMPACT 2002+ methodology, which is very frequently used among researchers and environmentalists (see [
35,
36]). Following midpoint indicators were used to compare the particular mixtures studied: carcinogens (CA), non-carcinogens (NCA), respiratory organics (RO) and inorganics (RI), aquatic (AE) and terrestrial ecotoxicity (TE), terrestrial acidification/nitrification (TA/N), aquatic acidification (AC) and eutrophication (AEU), land occupation (LO), mineral extraction (ME), non-renewable energy (NRE), ionizing radiation (RI), ozone layer depletion (OLD), and global warming (GW). To depict the environmental burden of materials studied, the endpoint level categories including Human Health, Ecosystem Quality, Climate change, and Resources consumption were determined. All the data gathered to perform the analysis were obtained using the Simapro 8.5 software and Ecoinvent database 3.5.
Since the original material replacement may result in changes in the functional performance, the combined environmental/functional assessment derived from Pedreno-Rojas et al. [
37] was employed to access more reliable comparison of studied materials. The overall environmental/functional efficiency,
Ec, expresses the environmental costs per unit compressive strength (3), being represented by the weighted endpoint environmental single score:
where
E (mPt) is the normalized environmental single score, and
Rc (MPa) the compressive strength after 28 and 90 days of curing, respectively.