Next Article in Journal
Renewable Energy: A Curse or Blessing—International Evidence
Previous Article in Journal
Integration of Indigenous and Local Knowledge in Policy and Practice of Nature-Based Solutions in China: Progress and Highlights
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Sustainability
Volume 15
Issue 14
10.3390/su151411105
sustainability-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Thermal Performance Assessment of Lightweight Aggregate Concrete by Different Test Methods

by
Maria da Glória Gomes
*,
José Alexandre Bogas
,
Sofia Real
,
António Moret Rodrigues
and
Rita Machete
CERIS, Department of Civil Engineering, Architecture and Georesources (DECivil), Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisbon, Portugal
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(14), 11105; https://doi.org/10.3390/su151411105
Submission received: 12 May 2023 / Revised: 10 July 2023 / Accepted: 12 July 2023 / Published: 17 July 2023
Browse Figures
Versions Notes

Abstract

:
Structural lightweight aggregate concrete is currently an alternative to normal-weight concrete when thermal insulation properties are required to meet the objectives of energy efficiency and sustainability. The accurate evaluation of the thermal performance is thus essential for designing structural lightweight concrete elements. This paper aims to evaluate the thermal behavior of structural lightweight aggregate concrete, assessed through different tests methods. To this end, a vast experimental campaign was carried out involving specimens produced with several types of lightweight aggregate and different water/cement ratios. The thermal performance was established by thermal conductivity, which was determined according to a modified transient pulse method and a quasi-stationary method, and specific heat capacity, which was determined through a transient pulse method and a heat transfer method. Normal-weight concrete was also tested for comparison purposes. Experimental evidence showed that lightweight aggregate concretes with lower density are associated with up to about 50% lower thermal conductivity and higher specific heat capacity than normal-weight concrete. Moreover, the study demonstrated that the expeditious transient pulse method is suitable for assessing the thermal conductivity of this type of concrete, and that both the transient pulse method and the heat transfer method are adequate to determine the specific heat capacity.

1. Introduction

Building energy consumption has become highly expensive and scarce along with the development of construction activities [1]. In fact, buildings are responsible for approximately 40% of the primary energy consumption in the USA and the EU [2]. Therefore, the minimization of heat transfer through the building envelope towards energy-neutral buildings is a major target [3]. Particularly, reinforced concrete is the most used structural material in buildings, affecting their thermal performance and energy efficiency, especially through the introduction of thermal bridging effects [4,5]. In order to improve the intrinsic thermal insulation behavior of concrete, structural lightweight aggregate concrete (LWAC) may potentially reduce the thermal conductivity by as much as about 50–70% [6,7]. Therefore, the use of LWAC as an alternative to normal-weight concrete (NWC) contributes to a decrease of either the global heat loss through the building envelope or the corrective insulation in thermal bridges required to meet increasingly demanding thermal insulation regulations [4,8,9]. These are important issues in the use stage of the building, which is generally deemed the longest stage and the largest contributor to the life cycle energy of the building. The use of LWAC in structural and non-structural building elements might provide an opportunity for more energy-efficient and environmentally sustainable buildings [3,5,10,11,12].
The energy savings and tolerable environmental impacts of LWAC are claimed to make this material a leader in energy and environmentally sustainable design [9,13].
Based on a two-dimensional heat transfer program (Therm) and an energy simulation program (EnergyPlus), Real et al. [5] analyzed the performance of LWAC in reducing the thermal bridging and improving the buildings’ energy efficiency. The authors showed that, depending on the type of aggregate, LWAC could reduce the heating energy needs by as much as 15% compared to the same NWC solution. Moreover, for a representative intermediate floor apartment, the fraction of energy allocated to thermal bridges could be reduced by up to 19%.
In general, thermal conductivity (λ), specific heat capacity (cp) and thermal diffusivity (α) are considered the fundamental thermal properties of concrete. Thermal conductivity describes the ability of a material to conduct heat, which in the case of normal concrete is quite high, requiring the installation of insulation systems to reduce heat transfer through the building parts [14]. The most relevant factor affecting this property is porosity [4,15,16,17]. In fact, the very good experimental correlation between density and thermal conductivity has been demonstrated [18,19,20,21]. As the aggregates occupy about 70% of the concrete volume, increasing the porosity of this phase is the most efficient way to reduce thermal conductivity. Real et al. [4] reported a decrease of 8.3% and 0.6% in thermal conductivity per 1% increment in LWA volume and LWA porosity, respectively.
The air encased in lightweight aggregate (LWA) pores, with a lower thermal conductivity, improves LWAC’s insulation behavior [22,23]. Real et al. [7] presented a numerical model for the LWAC thermal conductivity estimation, depending on the type and volume of coarse LWA, where the great influence of aggregate thermal properties was confirmed. Therefore, the use of natural or artificial LWA, produced either by sintering, cold bonding or autoclaving, is the most common and effective solution. In this case, expanded clay, slate or shale aggregates, as well as fly ash and some types of natural LWA, are more adequate for structural LWAC [24]. Other materials, such as wastewater treatment sludge and desulphurization device sludge [25], granite quarry dust [26] and ground granulated blast furnace slag [27], have also been used as LWA. However, properties related to the increase of porosity, such as transport properties, may negatively affect the mechanical and durability properties. Nevertheless, various studies have demonstrated that depending on the type of aggregate, LWAC may have similar to higher structural efficiency (strength per unit weight) and at least the same durability as those of NWC [24,28].
The type of binder may also affect the thermal properties. The partial cement replacement with fly ash or silica fume is reported to slightly reduce thermal conductivity by reducing concrete density and increasing the amorphous nature of the cementitious paste [17,18,29,30,31]. Other properties, such as the moisture content, pore size distribution, temperature, phase crystallinity and chemical composition, assume less relevance [4,6,23]. Regarding the effect of moisture content, a 3 to 9% variation in the thermal conductivity per each 1% change in the concrete water content is reported in the literature [4,32].
Through a comprehensive study involving LWAC with a wide range of strength (LC12/13-LC55/60) and density (D1.6–2.0), Real et al. [4] documented a 40–53% reduction of thermal conductivity when the 35% volume of coarse normal-weight aggregate (NWA) was replaced by LWA. Tajra et al. [14] found that lightweight concrete produced with cold-bonded aggregate achieved 28% and 43% lower thermal conductivity when coarse normal aggregate was replaced by 50% and 100% LWA, respectively. A more significant effect over a two- to three-fold reduction in thermal conductivity was documented by other authors [32,33].
The specific heat capacity is a measure of the energy needed to raise the temperature of a unit mass by 1 °C, providing an indication of the heat storage capacity of concrete. According to Neville [34], the specific heat capacity of concrete is affected by its density, water content and temperature, and to a lesser extent by the mineralogical composition of aggregates. The higher these properties, the higher the specific heat capacity. Approximate specific heat capacity values in dry concrete of 0.86, 1.25 and 1.35 kJ/kg°C have been indicated for NWC, LWAC with expanded shale and expanded clay LWA, respectively [16]. Values of specific heat capacity near 1 kJ/kg°C are reported by Hoff [32]. Similar values were found by Nguyen et al. [35] in LWAC (1–1.1 kJ/kg°C) and in NWC (0.77 kJ/kg°C). The specific heat capacity may be significantly affected by the water content due to the high specific heat capacity of water (4.2 kJ/kg°C) contained in the concrete pores [36,37]. Therefore, at least after production, LWAC tends to present higher specific heat capacity than NWC due to their typically higher water content and longer drying periods [28,38].
Another relevant thermal property is the thermal diffusivity (α), which measures the ability of a material to conduct heat relative to its ability to store heat. This property is numerically defined as the quotient between thermal conductivity and the product of the specific heat capacity by density. Therefore, assuming a less significant influence of the specific heat capacity and that the thermal conductivity increases exponentially with density, the thermal diffusivity increases with density. The thermal diffusivity of LWAC has been reported to be in the range of 30–50% of that of NWC [32,35].
The thermal properties results are also affected by the type of test and test conditions [4,39,40,41]. Tests may be categorized in stationary and transient methods. In stationary methods, a thermal gradient is established between two points and a constant unidirectional heat flux is measured. Although accurate, the method is laborious and time-consuming and more adequate for homogeneous materials [42]. The heat flow meter (EN 12667 [43]) and the guarded hot plate (EN 12667 [43]) are the most used stationary tests. In transient methods, the response to a heat impulse, which creates a heat flux, is measured. The most common methods are the transient plane source, transient line source and modified transient plane source. These expeditious methods provide results in a short period, but with lower precision than stationary methods [40,42]. Moreover, the transient method is more adequate for heterogeneous materials and different moisture content [44]. Van Geem et al. [39] compared the application of the hot wire test, guarded hot plate test and calibrated hot box, and confirmed the influence of the test setup on the measured thermal conductivity.
Few studies have been carried out on the thermal characterization of LWAC in stationary conditions, especially involving common concrete in an extensive range of density and strength classes. Moreover, to the best of the authors’ knowledge, no study regarding the comparison of different testing methods in assessing the thermal behavior of LWAC has been carried out yet. In this context, this present paper aims to characterize the thermal behavior of concrete produced with aggregates of distinct porosity by means of different test methods. The thermal properties are determined in terms of thermal conductivity and specific heat capacity, and results from quasi-stationary and heat transfer methods are compared with those from a transient pulse method.

2. Experimental Program

In a previous study, comprehensive research work was carried out by the authors [4] regarding the thermal characterization of LWAC through a transient pulse method. This study is a continuation of that experimental work, considering some of those concrete compositions and characterizing them using different test methods. The thermal conductivity was determined according to a modified transient pulse method and a quasi-stationary method. In addition, the specific heat capacity was determined through a transient pulse method and a heat transfer method. The experimental program, including compositions, specimen production and test setup, is summarized in the next subchapters.

2.1. Materials, Compositions and Production

For the production of LWAC, four types of coarse LWA of varying characteristics were chosen: two expanded clay LWA from Portugal (Leca and Argex; the latter provided in two different granulometric fractions, designated Argex 2–4 and Argex 3–8F), sintered fly ash LWA from the UK (Lytag) and expanded slate LWA from the USA (Stalite). For the production of reference NWC, normal-weight aggregates (NWA) consisting of two crushed limestone aggregates of various granulometric fractions were used. To consider similar grading curves as those of Stalite and Leca, crushed limestone was composed of 34% and 66% fine and coarse gravel and Argex of 70% and 30% Argex 2–4 and Argex 3–8F, respectively. Moreover, for both LWAC and NWC, fine aggregates consisting of 70% and 30% coarse and fine siliceous sand were selected. The main properties and granulometric curves of these aggregates are presented in Table 1 and in Figure 1 and Figure 2, respectively. Cement Type I 42.5 R (EN 197-1 [45]) and a polycarboxylate-based superplasticizer (SP) were also used in concrete production.
In order to extend the scope of this study, concrete was produced with different water/cement ratios (w/c) and types of aggregate, comprising 15 compositions, as indicated in Table 2. The w/c relates to the effective water available for cement hydration. The concretes were produced with 350 L/m3 of coarse aggregate according to the mix design suggested by Bogas et al. [47].
Both LWAC and NWC were manufactured in a vertical shaft mixer with bottom discharge. Prior to concrete production, the LWA were immersed in water for 24 h and then surface-dried before being introduced in the mixer to guarantee an adequate control of the workability and effective water content of concrete. The aggregates were mixed with sand and 50% of the total water for 2 min and then left to rest for 1 min. Subsequently, the binder and the rest of the water were added to the mixture. When used, the SP was gradually added with 10% water after 1 min. In the case of concrete with Argex, the aggregates were introduced dry in the mixer, and the water absorption of Argex was estimated to take into account the correction of the total water mix based on the procedure suggested by Bogas et al. [48].

2.2. Specimen Production

The concretes were tested for dry density (ρd) and compressive strength (fcm), as well as their thermal properties. The dry density was determined according to EN 12390-7 [49] using two 100 mm cubic specimens per composition, which were demolded at 1 day of age and subsequently water-cured until testing age. The 28-day compressive strength was determined according to EN 12390-3 [50] using four 150 mm cubic specimens per composition, following the same conditioning of the dry density tests. The dry thermal properties were determined for two 0.3 × 0.3 × 0.08 m3 slabs per composition, which were oven-dried at 100 °C until reaching constant mass after 28 days.

2.3. Modified Transient Pulse Method

The modified transient pulse method tests were performed with an ISOMET 2114 portable hand-held heat transfer analyzer (Figure 3) from Applied Precision Enterprise (Bratislava, Slovakia). ISOMET 2114 measures thermal properties by means of a modified transient pulse method and is equipped with a surface measurement probe for solid and hard materials. A heat flux is imposed by a heat impulse on the specimen in thermal equilibrium with the adjacent environment (ASTM D5334 [51], ASTM D5930 [52]). Thermal conductivity, λT, in W/m°C; volumetric heat capacity, cρ, in J/m3K; and average testing temperature, Tmean, in °C are measured. The cρ corresponds to the product of cp and density. From the previously stated properties, thermal diffusivity, (α), in m2/s is also determined. Each of the two slabs per composition were measured in two different places and the measurement performed in two locations on the sample’s smoother face to improve the contact of the measuring probe with the surface. According to the supplier, the accuracy of the equipment is about 10% for λT within the range of 0.7–2.5 W/m°C; 5% + 0.001 W/m°C for λT within the range of 0.0015–0.7 W/m°C; and 15% + 1 × 103 J/m3°C for cp.
This modified transient method was also performed in a previous research work [3] where the thermal conductivity of the same concrete compositions tested in the present study was determined in ϕ105 × 50 mm specimens, sawn from ϕ105 × 250 mm cylinders (λT,cyl). The specimens were cured in water for 28 days and then tested under different moisture contents. The detailed description of the experimental campaign and obtained results are presented in Real et al. [3]. The average values of thermal conductivity (λT,cyl) in dry conditions are summarized in Table 2 and compared with the results of the present study in Section 3.1.

2.4. Quasi-Stationary Method

The thermal conductivity was also ascertained through a quasi-stationary method (λQS) using a climatic chamber with an internal temperature of 40 °C and an external variable ambient temperature (Figure 4). Before testing, the slabs were previously oven-dried until constant mass. Then, the slabs were placed in the chamber’s wall and properly sealed with polyurethane. The remaining wall consisted of a double-wall insulated with XPS. Five thermocouples, type T (three internal, one on the inside surface and other on the outside surface), were attached to each slab to determine the temperature evolution across the slab (Figure 5). A Hukseflux (Delft, The Netherlands) HF P01 heat flux meter was also attached to the outside surface of each slab to ascertain the heat flow (q) that traveled though the slab (Figure 6).
The flux meters and thermocouples were connected to a data acquisition system Datataker DT8. The temperature and heat flow were measured every minute. Each slab set was tested for about 6 to 7 days in order to guarantee an approximately constant heat flow.
The thermal conductivity was calculated from the temperature and heat flow results for each concrete slab. A progressive average method was employed to treat the results according to EN ISO 9869-1 [53]. Essentially, the thermal transmittance, U, at each measurement time matches the average of the transmittances obtained from the previous measurements. These were calculated from the heat flow and external and internal surface temperatures of each measurement (Equation (1)), where t is the measurement time (min), Qi is the heat flow at instant i (W/m2) and T i i and T e i are, respectively, the interior and exterior temperature at instant i (°C).
U = ∑ 0 i − t Q i ∑ 0 i − t T i i − ∑ 0 i − t T e i   ( W / m 2 ° C )
Finally, assuming a constant heat flow, the thermal conductivity coefficient (λQS) was achieved through Equation (2), where Q is the heat flow (W/m2), e is the slab’s thickness (m) and Tsi and Tse are the interior and exterior surface temperatures (°C), respectively.
λ Q S = Q × e T s i − T s e   ( W / m 2 ° C )
All thermal conductivity test results were then converted to a reference temperature of 10 °C, following the ISO/FDIS 10456 [54], since the thermal conductivity coefficient of CE-marked products is evaluated at that average temperature ITE50 [33].

2.5. Heat Transfer Method

The specific heat capacity, cp, was also determined based on a heat transfer method [55] using Holometrix Rapid-k equipment attached to a Thermo Scientific thermal bath. Before testing, the slabs were previously oven-dried. The slab specimens, with thermocouples and heat flux meters, were placed inside the equipment between two hot upper and lower plates with adjustable temperatures (Figure 7). For this purpose, the plates were set to a temperature of 40 °C in order for the samples to be heated symmetrically from both the upper and lower plates. The heat flux meters and thermocouples were connected to a data acquisition system DeltaT DL2e (Figure 8). The temperatures and heat flux were measured every minute. Each slab set was tested for about 9 h until a thermal equilibrium in all points of the slab and an approximately null heat flux was achieved, i.e., the stationary state. Only a reduced number of slabs were tested, comprising the reference NWC and LWAC with a w/c of 0.55, except that with Leca, as well as LWAC with Argex and variable w/c (0.35, 0.45 and 0.55). Concrete with Leca was only tested with a w/c of 0.45.
The specific heat capacity was then obtained indirectly through the temperatures measured on the slabs by using the differential evolution algorithm [56], which is based on natural selection mechanisms and population genetics, using mutation, crossing and selection operators to generate new agents in search of more adapted results. In the case of the problem under study, the population size corresponds to the number of temperature measurements made at each point of the slabs until a stationarity state is obtained. The population elements (specific heat capacity values) started by being randomly generated, after which they were subjected to mutation and selection operations according to the procedures of the method until the optimal solution of the specific heat capacity was found, that is, the one that numerically generates the temperature curve over time that best fits the experimental curve. In the iterative process involved, the successive elements of the population (specific heat capacity values), obtained by mutation and selection, are parameters of the dynamic heat conduction equation (Equation (3)).
ρ · c p ∂ T ∂ t = ∂ 2 T ∂ x 2
where T is the temperature (°C), ρ is the density (kg/m3) and cp is the specific heat capacity (J/kg/°C).
This equation is discretized in time and space for the central node of the slabs and, with the specific heat capacity and the temperatures of the adjacent nodes, allows obtaining the new temperature solutions that are compared with the experimental ones. The process ends when the correlation coefficient, R2, which represents the quality of the matching between the experimental and numerical curves, is close to unity.

3. Results

Common LWAC with LC12/13-LC55/60 strength classes and D1.6-D2.0 density classes (EN 206 [57]) were analyzed in this study (Table 2). Depending on the w/c and type of aggregate, the compressive strength of LWAC was up to 60% lower than that of reference NWC, whereas the dry density decreased between 17% and 35% when the NWA was replaced with LWA. As discussed in previous studies [3,18,24], the reduction of the compressive strength was greater in LWAC with lower w/c and LWA with higher porosity (Argex). Of note is the higher structural efficiency attained in LWAC with denser LWA (Stalite). On the other hand, LWAC with Argex presented the worst compromise between the compressive strength and the dry density.

3.1. Thermal Properties from the Modified Transient Pulse Method

The thermal conductivity measured in slab specimens through the modified transient pulse method varied between 0.93 and 2.1 W/m°C, increasing with the concrete density (Table 2). Indeed, the dry density was the property that best related to thermal conductivity (Figure 9) with an R2 over 0.9. An exponential increase in the thermal conductivity with the dry density was confirmed, as also reported by other authors [4,16,20,58,59] (Figure 9).
The LWAC thermal conductivity was, on average, 28% to 52% lower than that of NWC based on the type of aggregate. Only for LWAC with dense Stalite was the reduction lower than 40%. The level of reduction was slightly lower than that found in a previous study, which varied between 40 and 53% [4]. Nevertheless, LWAC with LWA of over 1000 kg/m3 was able to achieve 10–30% lower thermal conductivity per unit strength (λT,slab/fcm) than NWC.
For the same composition, the thermal conductivity was lower in concrete with more porous LWA since the concrete porosity was greatly increased. According to Real et al. [3], considering the same compositions tested in this study, an average reduction of 0.6% was observed in the thermal conductivity per each increment of 1% in the LWA porosity. In the present study and based on the values of porosity indicated in Table 1, the variation of thermal conductivity was about 0.6–0.8% per each 1% variation in LWA porosity. The thermal conductivity was also affected by the paste characteristics, having decreased with the increase of w/c for a given type of aggregate, which increased the concrete porosity. However, as found by Real et al. [4], these trends are not only attributed to the variation of density but also the characteristics of the constituent phases since, for the same density, the thermal conductivity varied as much as 25% (Figure 9).
A good agreement was obtained between the results of this study and those reported in Real et al. [3], considering the same compositions but measured from small cylindrical specimens (Figure 10). On average, the measurements from slab specimens were about 10% higher than those from cylindrical specimens. This should be related to the fact that the modified transient pulse method measures the thermal conductivity of the concrete’s surface, which is different in cylindrical and in slab specimens. In concrete slabs, the probe was applied over a molded surface, protected by a thin layer of paste and sand (wall-effect), whereas in the cylindrical specimens, the thermal conductivity was measured over a cut surface with exposed LWA, and hence, with higher porosity. This justifies the slightly higher thermal conductivity obtained in slab specimens. In addition, the results obtained from cylindrical specimens tended to be even higher than those reported in other studies for a given concrete density (Figure 9). This may be related to the fact that the other authors [4,16,20,58,59] in Figure 9 adopted stationary methods involving the full specimen’s thickness.
The specific heat capacity, cp,T, varied between 767 and 1191 J/kg°C depending on the type of aggregate. On average, the specific heat capacity of LWAC was 16% to 49% higher than that of NWC. These values were within the range of those reported in the literature (Section 1). As shown in Figure 11, the increasing trend of cp,T with the reduction of concrete density was confirmed, as documented by other authors [16,60], showing a high R2 of 0.94. In fact, concretes with high density are associated with high thermal conductivity, requiring less energy to raise their temperature. However, if we take into account the volumetric heat capacity, cρ = cp. ρd, differences between NWC (cρ,T of 1755–1856 × 103 J/m3°C) and LWAC (cρ,T of 1663–1890 × 103 J/m3°C) were not significant, showing that NWA and LWA solid constituents present similar thermal properties. Thus, the volumetric heat capacity is similar in LWAC and NWC.
The influence of the type of aggregate is clearly demonstrated in Figure 12, where, on average, the cp,T decreased about 0.5–0.7% per 1% increment in aggregate porosity. Nevertheless, for a given density, slight variations of cp,T were observed, which suggests that other less relevant factors may also affect this property. Regarding the influence of the w/c, a clear dependence of cp,T on this parameter was not confirmed (Table 2).
The thermal diffusivity, αT,slab, calculated from the quotient between λT,slab and the product of density and cpT,slab, varied between 0.54 and 1.17 × 10−6 m2/s, with a clear influence from the type of aggregate (Figure 13). These results are in line with the tendency reported in the literature [32,35]. On average, the thermal diffusivity was 31–49% lower in LWAC than in NWC. Thus, the reduction of concrete density promoted by LWA led to a lower λ, higher cp and lower α. Therefore, LWAC showed a higher thermal insulation capacity than NWC, either under stationary (lower λ, higher cp) or non-stationary conditions (lower λ and α).

3.2. Thermal Conductivity from the Quasi-Stationary Method

In general, as expected, a higher heat flux was measured in concretes with higher dry density and thermal conductivity, namely, in NWC and LWAC with denser LWA (Stalite) (Figure 14). The difference in heat flux between NWC and LWAC was higher than that between different types of LWAC, which is in line with the difference of thermal conductivity of about 45%, on average, between NWC and LWAC and up to 18% among LWAC. As stated in Section 2.4, this test was carried out over 6–7 days, a period that proved to be sufficient to obtain a quasi-stationary regime, as confirmed by the approximately constant heat flux values of Figure 14 before the descending heat flux limb that occurs after the climatic chamber had been switched off. In fact, since the exterior temperature varied slightly during testing (laboratory air temperature, Section 2.4), the heat flux and temperature across the specimens were only approximately constant (Figure 14).
Overall, the variation rate of heat flux was higher in NWC than in LWAC (initial slope of the heat flux curves in Figure 14). This is explained by the lower thermal diffusivity of LWAC (41% on average, Section 3.1) than that of NWC. Additionally, the period from the beginning of each test until equilibrium was reached was relatively independent from the type of concrete, potentially due to the reduced difference between the volumetric heat capacity (cρ) of NWC and LWAC of just about 5%.
As described in Section 2.4, the temperatures across the slabs’ thickness were monitored (Figure 15). As would be expected, the temperatures decreased across the slabs’ thickness towards the exterior surface, following a linear trend that confirmed the quasi-stationary nature of this test. However, due to the variation in exterior temperature over the test period, the mean surface temperature deviated slightly from the linear trend. This difference was more relevant in NWC due to its greater thermal inertia, reacting more slowly to temperature variations.
The thermal conductivity calculated from the results of the quasi-stationary method, λQS, varied between 0.88 and 1.55 W/m°C for LWAC and between 2.16 and 2.37 W/m°C for NWC (Table 2). The same trends discussed in Section 3.1 were confirmed regarding the influence of density, type of aggregate and mortar composition (Figure 16, Table 2). In this case, on average, the thermal conductivity of LWAC was 34% to 59% lower than that of NWC, displaying a greater reduction than found in the modified transient pulse tests (Section 3.1). This is explained by the higher values of the thermal conductivity determined for NWC, contrary to those of LWAC, which were more similar between the modified transient pulse and quasi-stationary methods. This difference is confirmed in Figure 16 and also in Figure 17, where both methods are directly compared using the same specimens.
In fact, the λ from the expeditious modified transient pulse method tended to be lower than that of the quasi-stationary method for λ above 1.6 W/m°C (Figure 17). The new values estimated for NWC are more in line with those suggested in FIP [16] (Figure 9). As discussed in Section 2.4, the accuracy of the modified transient pulse method tends to decrease for higher values of measured λT. On the other hand, in the quasi-stationary tests, NWC was more affected by the aforementioned phenomenon of surface temperature rise. In addition, contrary to the modified transient pulse method, the slabs used in the quasi-stationary method were not fully dry since, during this long-term test, the equilibrium humidity was altered. This will increase the concrete λ, especially in NWC, where the thermal characteristics of the cementitious matrix assume more relevance than in LWAC. In LWAC, the water content of LWA is not significantly altered during testing due to their coarser porosity than the surrounding paste [61]. Nevertheless, the λ determined from both methods was very similar in LWAC (Figure 17), which shows that the expeditious modified transient pulse method is adequate for assessing the thermal conductivity of this type of concrete.
Finally, comparing the results obtained in both methods with those from the literature, the modified transient pulse method led to thermal conductivity results closer to those reported by other authors, in particular, in NWC (Figure 9). However, as mentioned, the quasi-stationary method was the one that best followed the thermal conductivity and dry density relationship reported in the literature.

3.3. Specific Heat Capacity from the Heat Transfer Method

As discussed in Section 2.5, to calculate the specific heat capacity through the heat transfer method, each slab was heated in a Rapid-k equipment until reaching thermal equilibrium, which was achieved when the surface heat flux tended to zero, and the interior temperature was about 38–40 °C (Figure 18).
In the modified transient pulse method measured in cylindrical specimens (Section 3.1), the specific heat capacity decreased with concrete density (Figure 11). The same trend was observed in heat transfer tests, although with greater dispersion (Figure 19). As mentioned, the higher the aggregate porosity, the higher the cp (Table 2). As per the modified transient pulse method, a clear trend between cp and w/c was found. In general, both methods led to similar values of cp for a reasonable R2 of 0.74. The exception occurred in LWAC with Stalite and a w/c of 0.55, which was the most pronounced outlier (Figure 20). In this case, an abnormally high value of cp, out from the relation between this property and density (Figure 19), was obtained through the heat transfer method, possibly affected by a less effective drying during pre-conditioning. Excluding this point, the R2 would be as high as 0.90. In this case, the maximum difference between cpT and cpHT was 6.7%, without a prevailing trend.

4. Conclusions

The focus of this study was to characterize the thermal properties of LWAC manufactured with different compositions and types of aggregate through different thermal test methods.
The thermal properties essentially depend on the porosity of the concrete. The thermal conductivity and specific heat capacity tended to increase and decrease with the density of concrete, respectively. The thermal conductivity and specific heat capacity of LWAC were about 28–52% and 16–49% of those of NWC. However, the volumetric heat capacity was similar in LWAC and NWC.
Although the thermal conductivities calculated from the quasi-stationary method were slightly higher than those of the modified transient pulse method, they followed the same trends regarding the influence of density, type of aggregate and mortar composition. In this case, on average, the thermal conductivity of LWAC was 34% to 59% lower than that of NWC, displaying a greater reduction than that found in the modified transient pulse tests. This can be explained by the reduction of the accuracy of the modified transient pulse method for higher thermal conductivities. The specific heat capacity tended to decrease with concrete density, similarly to the modified transient pulse method, although with greater dispersion.
Overall, the results demonstrate the improved thermal performance of lightweight concrete, leading to a more energy-efficient solution when compared with normal-weight concrete. In addition, the test methods used in this study provided consistent results and proved to be capable of adequately characterizing the thermal properties of LWAC. Moreover, it is expected that these experimental methods for determining the thermophysical properties of LWAC can also be employed for other construction materials. Given the simplicity of the modified transient pulse method, this would be the most practical choice to determine the thermal properties of building materials.

Author Contributions

Conceptualization, M.d.G.G. and J.A.B.; methodology, M.d.G.G., J.A.B. and A.M.R.; investigation, M.d.G.G., J.A.B., S.R., A.M.R. and R.M.; writing—original draft preparation, M.d.G.G., J.A.B. and R.M.; writing—review and editing, M.d.G.G., J.A.B., S.R. and A.M.R.; supervision, M.d.G.G. and J.A.B. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to acknowledge the Portuguese Foundation for Science and Technology (FCT) for funding the research project PTDC/ECM-COM1734/2012, the research unit CERIS (UIDB/04625/2020) and the Ph.D. scholarships SFRH/BD/100397/2014 (S. Real) and 2020.09705.BD (R. Machete).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors also acknowledge the companies Saint-Gobain Weber Portugal, Argex, Stalite, Lytag, SECIL and BASF for material supply.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Dong, B.; Cao, C.; Lee, S. Applying support vector machines to predict building energy consumption in tropical region. Energy Build. 2005, 37, 545–553. [Google Scholar] [CrossRef]
  2. Cao, X.; Dai, X.; Liu, J. Building energy-consumption status worldwide and the state-of-the-art technologies for zero-energy buildings during the past decade. Energy Build. 2016, 128, 198–213. [Google Scholar] [CrossRef]
  3. Liu, M.Y.J.; Alengaram, U.J.; Jumaat, M.Z.; Mo, K.H. Evaluation of thermal conductivity, mechanical and transport properties of lightweight aggregate foamed geopolymer concrete. Energy Build. 2014, 72, 238–245. [Google Scholar] [CrossRef]
  4. Real, S.; Bogas, J.; Gomes, M.; Ferrer, B. Thermal conductivity of structural lightweight aggregate concrete. Mag. Concr. Res. 2015, 68, 798–808. [Google Scholar] [CrossRef]
  5. Real, S.; Gomes, M.G.; Moret Rodrigues, A.; Bogas, J.A. Contribution of structural lightweight aggregate concrete to the reduction of thermal bridging effect in buildings. Constr. Build. Mater. 2016, 121, 460–470. [Google Scholar] [CrossRef]
  6. Holm, T.; Bremner, T. State-of-the-Art Report on High-Strength, High Durability Structural Low-Density Concrete for Applications in Severe Marine Environments; ERDC/SL TR-00-3; US Army Corps of Engineers, Structural Laboratory: Washington, DC, USA, 2000. [Google Scholar]
  7. Real, S.; Maia, C.; Bogas, J.; Gomes, M. Thermal conductivity modelling of structural lightweight aggregate concrete. Mag. Concr. Res. 2021, 73, 798–809. [Google Scholar] [CrossRef]
  8. Kim, H.K.; Jeon, J.H.; Lee, H.K. Workability, and mechanical, acoustic and thermal properties of lightweight aggregate concrete with a high volume of entrained air. Constr. Build. Mater. 2012, 29, 193–200. [Google Scholar] [CrossRef]
  9. Mousa, A.; Mahgoub, M.; Hussein, M. Lightweight concrete in America: Presence and challenges. Sustain. Prod. Consum. 2018, 15, 131–144. [Google Scholar] [CrossRef]
  10. Tajra, F.; Elrahman, M.; Stephan, D. The production and properties of cold-bonded aggregate and its applications in concrete: A review. Constr. Build. Mater. 2019, 225, 29–43. [Google Scholar] [CrossRef]
  11. Kumar, P.; Pasla, D.; Jothi Saravanan, T. Self-compacting lightweight aggregate concrete and its properties: A review. Constr. Build. Mater. 2023, 375, 130861. [Google Scholar] [CrossRef]
  12. Vives, I.; Varona, F.; Tenza-Abril, A.; Pereiro-Barceló, J. A parametric study to assess lightweight aggregate concrete for future sustainable construction of reinforced concrete beams. Sustainability 2021, 13, 13893. [Google Scholar] [CrossRef]
  13. Ries, J.; Speck, J.; Harmon, K. Lightweight aggregate optimizes the sustainability of concrete, through weight reduction, internal curing, extended service life, and lower carbon footprint. In Proceedings of the Concrete Sustainability Conference, National Ready Mixed Concrete Association, Tempe, AZ, USA, 13–15 April 2010. [Google Scholar]
  14. Tajra, F.; Elrahman, M.; Lehmann, C.; Stephan, D. Properties of lightweight concrete made with core-shell structured lightweight aggregate. Constr. Build. Mater. 2019, 205, 39–51. [Google Scholar] [CrossRef]
  15. Bessenouci, M.Z.; Bibi Triki, N.E.; Khelladi, S.; Draoui, B.; Abene, A. The apparent thermal conductivity of pozzolana concrete. Phys. Procedia 2011, 21, 59–66. [Google Scholar] [CrossRef] [Green Version]
  16. FIP. FIP Manual of Lightweight Aggregate Concrete, 2nd ed.; FIP/Surrey University Press: Glasgow, Scotland, 1983. [Google Scholar]
  17. Chi, J.; Huang, R.; Yang, C.C.; Chang, J.J. Effect of aggregate properties on the strength and stiffness of lightweight concrete. Cem. Concr. Compos. 2003, 25, 197–205. [Google Scholar] [CrossRef]
  18. ACI 213R; Guide for Structural Lightweight-Aggregate Concrete. American Concrete Institute (ACI): Farmington Hills, MI, USA, 2003.
  19. Uysal, H.; Demirboǧa, R.; Sahin, R.; Gül, R. The effects of different cement dosages, slumps and pumice aggregate ratios on the freezing and thawing of concrete. Comput. Concr. 2004, 34, 845–848. [Google Scholar] [CrossRef]
  20. Alengaram, U.J.; Al Muhit, B.A.; Jumaat, M.Z.; Jing, M.L.Y. A comparison of the thermal conductivity of oil palm shell foamed concrete with conventional materials. Mater. Des. 2013, 51, 522–529. [Google Scholar] [CrossRef]
  21. Gomes, M.; Rodrigues, A.; Bogas, J.; Freitas, A. Thermophysical properties under different hygroscopic conditions of an innovative composite concrete pre-walls system. Constr. Build. Mater. 2021, 307, 124938. [Google Scholar] [CrossRef]
  22. Demirboǧa, R.; Gül, R. The effects of expanded perlite aggregate, silica fume and fly ash on the thermal conductivity of lightweight concrete. Cem. Concr. Res. 2003, 33, 723–727. [Google Scholar] [CrossRef]
  23. EuroLightCon R2. LWAC material properties—State-of-the-art. In European Union—Brite EuRam III; BE96-3942/R2; Delft University of Technology: Delft, The Netherlands, 1998. [Google Scholar]
  24. Bogas, J.; Gomes, A. Compressive behavior and failure modes of structural lightweight aggregate concrete—Characterization and strength prediction. Mater. Des. 2013, 46, 832–841. [Google Scholar] [CrossRef]
  25. Ferone, C.; Colangelo, F.; Messina, F.; Lucolano, F.; Liguori, B.; Cioffi, R. Coal combustion wastes reuse in low energy artificial aggregates manufacturing. Materials 2013, 6, 5000–5015. [Google Scholar] [CrossRef] [Green Version]
  26. Thomas, J.; Harilal, B. Properties of cold bonded quarry dust coarse aggregates and its use in concrete. Cem. Concr. Compos. 2015, 62, 67–75. [Google Scholar] [CrossRef]
  27. Hwang, C.-L.; Tran, V.-A. A study of the properties of foamed lightweight aggregate for self-consolidating concrete. Constr. Build. Mater. 2015, 87, 78–85. [Google Scholar] [CrossRef]
  28. Bogas, J.A.; Real, S. A review on the carbonation and chloride penetration resistance of structural lightweight aggregate concrete. Materials 2019, 12, 3456. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  29. Oktay, H.; Yumrutaş, R.; Akpolat, A. Mechanical and thermophysical properties of lightweight aggregate concretes. Constr. Build. Mater. 2015, 96, 217–225. [Google Scholar] [CrossRef]
  30. Fu, X.; Chung, D.D.L. Effects of silica fume, latex, methylcellulose, and carbon fibers on the thermal conductivity and specific heat of cement paste. Cem. Concr. Res. 1997, 27, 1799–1804. [Google Scholar] [CrossRef]
  31. Xu, Y.; Chung, D. Effect of sand addition on the specific heat and thermal conductivity of cement. Cem. Concr. Res. 2000, 30, 59–61. [Google Scholar] [CrossRef]
  32. Hoff, G. High strength lightweight aggregate concrete for artic applications—Part 1–3. In Structural Lightweight Aggregate Concrete Performance; ACI SP-136; Holm, T.A., Vaysburd, A.M., Eds.; ACI: Farmington Hills, MI, USA, 1992. [Google Scholar]
  33. ITE 50. Thermal Transmittance Coefficients of Building Envelope Elements; Laboratório de Engenharia Civil (LNEC): Lisbon, Portugal, 2006. (In Portuguese) [Google Scholar]
  34. Neville, A. Properties of Concrete, 4th ed.; Standards Updated to 2002; Longman: Harlow, UK, 2004. [Google Scholar]
  35. Nguyen, L.H.; Beaucour, A.L.; Ortola, S.; Noumowé, A. Influence of the volume fraction and the nature of fine lightweight aggregates on the thermal and mechanical properties of structural concrete. Constr. Build. Mater. 2014, 51, 121–132. [Google Scholar] [CrossRef]
  36. Malešev, M.; Radonjanin, V.; Lukić, I.; Bulatović, V. The effect of aggregate, type and quantity of cement on modulus of elasticity of lightweight aggregate concrete. Arab. J. Sci. Eng. 2014, 39, 705–711. [Google Scholar] [CrossRef]
  37. Weigler, H.; Karl, S. Structural lightweight aggregate concrete with reduced density- lightweight aggregate foamed concrete. Int. J. Cem. Compos. Light. Concr. 1980, 2, 101–104. [Google Scholar] [CrossRef]
  38. Bogas, J.A.; Carriço, A.; Pontes, J. Influence of cracking on the capillary absorption and carbonation of structural lightweight aggregate concrete. Cem. Concr. Compos. 2019, 104, 103382. [Google Scholar] [CrossRef]
  39. Van Geem, M.; Fiorato, A.; Musser, D. Calibrated hot box tests of thermal performance of concrete walls. In Proceedings of the ASHRAE/DOE Conference on Thermal Performance of the Exterior Envelopes of Buildings II, Las Vegas, NV, USA, 6–9 December 1982; ASHRAE: Atlanta, GA, USA, 1982. [Google Scholar]
  40. Asadi, I.; Shafigh, P.; Hassan, Z.; Mahyuddin, N. Thermal conductivity of concrete—A review. J. Build. Eng. 2018, 20, 81–93. [Google Scholar] [CrossRef]
  41. Gomes, M.; Flores-Colen, I.; Silva, F.; Pedroso, M. Thermal conductivity measurement of thermal insulating mortars with EPS and silica aerogel by steady-state and transient methods. Constr. Build. Mater. 2018, 172, 696–705. [Google Scholar] [CrossRef]
  42. Bristow, K. Thermal Conductivity, Methods of Soil Analysis: Part 4: Physical Methods; Wiley Online Library: Hoboken, NJ, USA, 2002. [Google Scholar]
  43. EN 12667; Thermal Performance of Building Materials and Products-Determination of Thermal Resistance by Means of Guarded Hot Plate and Heat Flow Meter Methods-Products of High and Medium Thermal Resistance. European Committee for Standardization (CEN): Brussels, Belgium, 2001.
  44. Gandage, A.; Rao, V.; Sivakumar, M.; Vasan, A.; Venu, M.; Yaswanth, A. Effect of perlite on thermal conductivity of self-compacting concrete. Procedia Soc. Behav. Sci. 2013, 104, 188–197. [Google Scholar] [CrossRef] [Green Version]
  45. EN 197-1; Cement-Part 1: Composition, Specifications and Conformity Criteria for Common Cements. European Committee for Standardization (CEN): Brussels, Belgium, 2011.
  46. EN 12620; Aggregates for Concrete. European Committee for Standardization (CEN): Brussels, Belgium, 2013.
  47. Bogas, J.; Gomes, A. A simple mix design method for structural lightweight aggregate concrete. Mater. Struct. 2013, 46, 1919–1932. [Google Scholar] [CrossRef]
  48. Bogas, J.; Gomes, A.; Gomes, M. Estimation of water absorbed by expanding clay aggregates during structural lightweight concrete production. Mater. Struct. 2012, 45, 1565–1576. [Google Scholar] [CrossRef]
  49. EN 12390-3; Testing Hardened Concrete. Compressive Strength of Test Specimens. European Committee for Standardization (CEN): Brussels, Belgium, 2019.
  50. EN 12390-7; Testing Hardened Concrete—Part 7: Density of Hardened Concrete. European Committee for Standardization (CEN): Brussels, Belgium, 2009.
  51. ASTM D5334; Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure. American Society for Testing & Materials (ASTM): West Conshohocken, PL, USA, 2014.
  52. ASTM D5930; Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique. American Society for Testing & Materials (ASTM): West Conshohocken, PL, USA, 2009.
  53. ISO 9869-1; Thermal Insulation. Building Elements. In-Situ Measurement of Thermal Resistance and Thermal Transmittance. Part 1: Heat Flow Meter Method. International Organization for Standardization (ISO): Geneva, Switzerland, 2014.
  54. ISO/FDIS 10456; Building Materials and Products—Hygrothermal Properties—Tabulated Design Values and Procedures for Determining Declared and Design Thermal Values. International Standards Organization (ISO): Geneva, Switzerland, 2007.
  55. Cheng, R.; Pomianowski, M.; Wang, X.; Heiselberg, P.; Zhang, Y. A new method to determine thermophysical properties of PCM-concrete brick. Appl. Energy 2013, 112, 988–998. [Google Scholar] [CrossRef]
  56. Prince, K.; Storn, R.; Lampinen, J. Differential Evolution: A Practical Approach to Global Optimization; Springer: Berlin/Heidelberg, Germany, 2005. [Google Scholar]
  57. EN 206; Concrete. Specification, Performance, Production and Conformity. European Committee for Standardization (CEN): Brussels, Belgium, 2013.
  58. Pan, Z.; Hiromi, F.; Wee, T. Preparation of High-Performance Foamed Concrete from Cement, Sand and Mineral Admixtures. J. Wuhan Univ. Technol. Mater. Sci. Ed. 2007, 22, 295–298. [Google Scholar] [CrossRef]
  59. Lo-Shu, K.; Man-Qing, S.; Xing-Sheng, S.; Yun-Xiu, L. Research on several physicomechanical properties of lightweight aggregate concrete. Int. J. Light. Concr. 1980, 2, 185–191. [Google Scholar] [CrossRef]
  60. Van Geem, M.; Fiorato, A. Thermal Properties of Masonry Materials for Passive Solar Design—A State-of-the-Art Review; Construction Technology Laboratories, Portland Cement Association: Skokie, IL, USA, 1983. [Google Scholar]
  61. Bogas, J.Á.; Nogueira, R.; Almeida, N.G. Influence of mineral additions and different compositional parameters on the shrinkage of structural expanded clay lightweight concrete. Mater. Des. 2014, 56, 1039–1048. [Google Scholar] [CrossRef]
Sustainability 15 11105 g001 550
Figure 1. Granulometric curves of the lightweight aggregates.
Figure 1. Granulometric curves of the lightweight aggregates.
Sustainability 15 11105 g001
Sustainability 15 11105 g002 550
Figure 2. Granulometric curves of the normal-weight aggregates.
Figure 2. Granulometric curves of the normal-weight aggregates.
Sustainability 15 11105 g002
Sustainability 15 11105 g003 550
Figure 3. Measurement device (ISOMET 2114).
Figure 3. Measurement device (ISOMET 2114).
Sustainability 15 11105 g003
Sustainability 15 11105 g004 550
Figure 4. Climatic chamber with slabs in place.
Figure 4. Climatic chamber with slabs in place.
Sustainability 15 11105 g004
Sustainability 15 11105 g005 550
Figure 5. Slab scheme with placement of thermocouples for quasi-stationary method.
Figure 5. Slab scheme with placement of thermocouples for quasi-stationary method.
Sustainability 15 11105 g005
Sustainability 15 11105 g006 550
Figure 6. Slab with flux meter and thermocouples in the climatic chamber.
Figure 6. Slab with flux meter and thermocouples in the climatic chamber.
Sustainability 15 11105 g006
Sustainability 15 11105 g007 550
Figure 7. Slab scheme with placement of thermocouples and heat flux meters for heat transfer method.
Figure 7. Slab scheme with placement of thermocouples and heat flux meters for heat transfer method.
Sustainability 15 11105 g007
Sustainability 15 11105 g008 550
Figure 8. Measurement device (Rapid-k and thermal bath) and data acquisition system DeltaT DL2e.
Figure 8. Measurement device (Rapid-k and thermal bath) and data acquisition system DeltaT DL2e.
Sustainability 15 11105 g008
Sustainability 15 11105 g009 550
Figure 9. Thermal conductivity (λ) versus dry density (ρd) of the present study and other authors [4,16,20,58,59].
Figure 9. Thermal conductivity (λ) versus dry density (ρd) of the present study and other authors [4,16,20,58,59].
Sustainability 15 11105 g009
Sustainability 15 11105 g010 550
Figure 10. Thermal conductivity measured through the modified transient pulse method in cylindrical specimens (λT,cyl) [3] and slab specimens (λT,slab).
Figure 10. Thermal conductivity measured through the modified transient pulse method in cylindrical specimens (λT,cyl) [3] and slab specimens (λT,slab).
Sustainability 15 11105 g010
Sustainability 15 11105 g011 550
Figure 11. Specific heat capacity (cpT,slab) versus dry density (ρd).
Figure 11. Specific heat capacity (cpT,slab) versus dry density (ρd).
Sustainability 15 11105 g011
Sustainability 15 11105 g012 550
Figure 12. Specific heat capacity (cpT,slab) versus total LWA porosity.
Figure 12. Specific heat capacity (cpT,slab) versus total LWA porosity.
Sustainability 15 11105 g012
Sustainability 15 11105 g013 550
Figure 13. Thermal diffusivity (αT,slab) versus dry density (ρd).
Figure 13. Thermal diffusivity (αT,slab) versus dry density (ρd).
Sustainability 15 11105 g013
Sustainability 15 11105 g014 550
Figure 14. Heat flux of LWAC/NWC with a w/c of 0.35.
Figure 14. Heat flux of LWAC/NWC with a w/c of 0.35.
Sustainability 15 11105 g014
Sustainability 15 11105 g015 550
Figure 15. Temperature distribution across the slabs’ thickness for concrete with a w/c of 0.35, 0.45 and 0.55, together with the ambient temperature (×).
Figure 15. Temperature distribution across the slabs’ thickness for concrete with a w/c of 0.35, 0.45 and 0.55, together with the ambient temperature (×).
Sustainability 15 11105 g015
Sustainability 15 11105 g016 550
Figure 16. Thermal conductivity from the quasi-stationary method (λQS), the modified transient pulse method in slab specimens (λT,slab) and in cylindrical specimens (λT,cyl) versus dry density (ρd).
Figure 16. Thermal conductivity from the quasi-stationary method (λQS), the modified transient pulse method in slab specimens (λT,slab) and in cylindrical specimens (λT,cyl) versus dry density (ρd).
Sustainability 15 11105 g016
Sustainability 15 11105 g017 550
Figure 17. Thermal conductivity of slab specimens measured through the modified transient pulse method (λT,slab) and the quasi-stationary method (λQS,slab).
Figure 17. Thermal conductivity of slab specimens measured through the modified transient pulse method (λT,slab) and the quasi-stationary method (λQS,slab).
Sustainability 15 11105 g017
Sustainability 15 11105 g018 550
Figure 18. Heat transfer method. Temperature distribution across the slab thickness and heat flux in each slab surface for concrete with Argex and a w/c of 0.35.
Figure 18. Heat transfer method. Temperature distribution across the slab thickness and heat flux in each slab surface for concrete with Argex and a w/c of 0.35.
Sustainability 15 11105 g018
Sustainability 15 11105 g019 550
Figure 19. Specific heat capacity measured through the heat transfer method (cpHT) versus dry density (ρd).
Figure 19. Specific heat capacity measured through the heat transfer method (cpHT) versus dry density (ρd).
Sustainability 15 11105 g019
Sustainability 15 11105 g020 550
Figure 20. Specific heat capacity of specimens measured through the modified transient pulse method (cpT,slab) and the heat transfer method (cpHT).
Figure 20. Specific heat capacity of specimens measured through the modified transient pulse method (cpT,slab) and the heat transfer method (cpHT).
Sustainability 15 11105 g020
Table
Table 1. Aggregate properties.
Table 1. Aggregate properties.
PropertyLightweight AggregatesNormal-Weight Aggregates
StaliteLytagLecaArgex
3–8F		Argex
2–4		Coarse SandFine SandCoarse GravelFine Gravel
Loose bulk density (kg/m3)7607506243303771708156913461309
Dry density (kg/m3)1483133810765976692617260526832646
Absorption at 24 h (%)3.617.915.819.321.40.30.20.40.7
Granulometric fraction (di/Di) a8/164/11.24/11.24/11.24/80/40/14/11.20/8
Total porosity (%)43.14758.976.173.1----
a di/Di correspond to the minimum/maximum aggregate size within the base series of EN 12620 [46].
Table
Table 2. Concrete compositions, dry density (ρd), average compressive strength (fcm), average thermal conductivity, specific heat capacity and thermal diffusivity determined in cylindrical specimens [3] using a modified transient pulse method (λT,cyl) and in slab specimens based on modified transient pulse (λT,slab; cpT,slab; αT,slab), quasi-stationary (λQS) and heat transfer (cpHT) methods.
Table 2. Concrete compositions, dry density (ρd), average compressive strength (fcm), average thermal conductivity, specific heat capacity and thermal diffusivity determined in cylindrical specimens [3] using a modified transient pulse method (λT,cyl) and in slab specimens based on modified transient pulse (λT,slab; cpT,slab; αT,slab), quasi-stationary (λQS) and heat transfer (cpHT) methods.
Type of Aggregate (350 L/m3)w/cMCEM I (kg/m3)Vsand (L/m3)ρd (kg/m3)fcm,28d (kg/m3)Modified Transient Pulse MethodQuasi-Stationary MethodHeat Transfer Method
λT,cyl
(W/m°C) [3]		CVλT,cyl (%)λT,slab (W/m°C)CVλT,slab (%)cpT,slab (J/kg°C)CVcpT,slab (%)αT,slab 
(×10−6 m2/s)		λQS (W/m°C)CVλQS (%)cpHT (J/kg°C)
NWA0.35450314234076.32.00-2.10376741.172.377-
0.45400310223057.71.9841.98078711.132.226-
0.55350315223047.81.8611.96183231.062.165790
Stalite0.35450314189066.81.3631.523100040.801.551-
0.45400310184049.91.2131.364100600.731.300-
0.55350315180041.50.9921.19196250.691.2001131
Leca0.35450314176043.31.1651.213103410.671.276-
0.45400310170037.61.0641.160107900.631.1451123
0.55350315164032.60.9461.102101410.661.042-
Lytag0.35450314184047.81.2051.32498620.731.35--
0.45400310177041.21.1411.152100020.651.25--
0.55350315174037.30.9331.02598040.60--941
Argex0.35450314155028.51.10111.13-1141-0.641.02-1220
0.45400310147026.10.9431.013116820.590.95-1235
0.55350315144022.50.8720.933119100.540.88-1135
Note: CV refers to the coefficients of variation, w/c to water/cement ratios, MCEM I to the mass of Cement Type I 42.5 R and Vsand to the volume of sand per m3 of concrete.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Gomes, M.d.G.; Bogas, J.A.; Real, S.; Moret Rodrigues, A.; Machete, R. Thermal Performance Assessment of Lightweight Aggregate Concrete by Different Test Methods. Sustainability 2023, 15, 11105. https://doi.org/10.3390/su151411105

AMA Style

Gomes MdG, Bogas JA, Real S, Moret Rodrigues A, Machete R. Thermal Performance Assessment of Lightweight Aggregate Concrete by Different Test Methods. Sustainability. 2023; 15(14):11105. https://doi.org/10.3390/su151411105

Chicago/Turabian Style

Gomes, Maria da Glória, José Alexandre Bogas, Sofia Real, António Moret Rodrigues, and Rita Machete. 2023. "Thermal Performance Assessment of Lightweight Aggregate Concrete by Different Test Methods" Sustainability 15, no. 14: 11105. https://doi.org/10.3390/su151411105

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop