Improved balance between compressive strength and thermal conductivity of insulating and structural lightweight concretes for low rise construction
Introduction
Energy use for space heating and cooling in the residential sector accounts for over 10% of the total primary energy consumption in the USA, Europe and Chile [1], [2], [3], [4], [5]. Concrete is the material, which is the mostly used in the built environment, as it excels in terms of mechanical properties and constructability. Nevertheless, concrete houses present a market share of 25% in Chile [6] and in less than 2% in the USA [7]. The relatively low thermal resistance of concrete compared to timber and masonry construction contributes to the moderate use of the cast-in-place concrete technology in the construction sector.
Building codes provide constraints regarding the thermal resistance of construction envelope walls in Chile [8] and worldwide [9], [10], [11]. Such levels of restriction are difficult to reach using conventional concrete construction alone. For instance, the thermal conductivity of conventional concrete ranges 1.4 to 2.3 W/mK [12], [13], [14], while the maximum admissible thermal conductivity of envelope walls is set to 0.42 W/mK by the Chilean Building Regulation for concrete houses with 15 cm width walls in Santiago. Therefore, the addition of insulation materials and interior or exterior finishing systems is compulsory in concrete-based constructions in order to reach the thermal transmittance coefficients (U-values) set by regulations. This situation leads to increase direct costs (labor, material) and reduce productivity, thus provides certain limitations to the implementation of concrete-based elements compared to other materials used for wall construction structures such as masonry, steel-stud walls and timber walls. In order to save energy consumption in concrete buildings, there is a demand for structural cement-based materials with low thermal conductivity and sufficient compressive strength characteristics.
Literature review shows two main strategies to reduce the thermal conductivity of concrete: (i) the replacement of normal weight aggregates (NWA) by lightweight aggregates (LWA) [15], [16], [17] and (ii) the replacement of Ordinary Portland Cement (OPC) by supplementary cementing materials (SCM) [18], [19]. Although these strategies increase the insulation capacity of concrete, they generally compromise its mechanical properties [20], [21], [22].
LWA present a cellular pore structure and a lower particle density than NWA [22].
Several studies have shown that the use of LWA allow to reduce the thermal conductivity of concrete [15], [17], [23], [24], [25], [26], [27]. This is due to the LWA air-dry density ranging from 60 to over 1000 kg/m3, depending of its total porosity [20]. However, the lighter is the aggregate particle, the lower is the LWA intrinsic strength [22], [28], [29]. For that reason, not all LWA are suitable for making structural strength concrete. Expanded clay is among the LWA that allows the production of structural lightweight aggregate concrete (LWAC) with low thermal conductivity at oven-dry state, ranging from 0.35 to 0.43 W/mK when using 100% OPC as cementitious material [15], [30]. With the incorporation of hollow cenospheres as LWA, Wu et al. [31] developed LWAC with thermal conductivity as low as 0.28 W/mK and high compressive strength so that they can be used for structural applications.
As for SCMs, fly ash (FA), which is a by-product of coal power plants, has been used as a partial replacement of OPC in order to develop concretes with excellent mechanical properties, low permeability, and superior durability. Besides, the use of FA as a SCM aims to make sustainable concretes [32]. This is particularly relevant considering that the OPC production industry is responsible of 7% of the global CO2 emissions [33]. Previous studies have shown that the thermal conductivity of cement paste is reduced when using SCMs [18], [34]. Among SCMs, FA is one of the most effective material to reduce the thermal conductivity of concrete [18], [35]. The effect of FA on NWCs in terms of mechanical properties and durability have been widely studied for FA ratios up to 80% [32], [33], [36], [37], [38], [39], [40], [41], [42], [43]. Literature shows that the substitution of OPC by FA to replacement levels of up to 70% allows the reduction of the thermal conductivity in binder [18], [35], [44], [45], but also reduces the strength gain rate [40], [46], [47].
In spite of the investigation studying the effects of LWA and FA on concrete’s thermal properties, there is a lack of research analyzing the consequences of using both the aggregates and SCMs simultaneously. Demirboǧa and Gül [45] showed that the combination of fine LWA (expanded perlite and pumice) with FA replacement level of 10% to 30% decreased the dry thermal conductivity of concrete to 0.15 W/mK, but also decreased the 28-day compressive strength to 3 MPa [48], and therefore, the resulting mixture was not suitable for structural concrete walls. In order to save energy consumption in concrete houses, there is a demand for structural cement-based materials with low thermal conductivity and sufficient compressive strength characteristics. To the authors’ knowledge, there are no previous studies focusing on the development of low thermal conductivity concrete using both FA and LWA for structural applications in housing.
Section snippets
Research significance
This study aims to understand and assess the influence of three constituents on the thermal and mechanical properties of concrete simultaneously in order to develop an insulating and structural concrete for external bearing walls in residential buildings. The constituents are coarse expanded clay (CEC) aggregate, fine expanded clay (FEC) aggregate, and FA. The measured mechanical and thermal properties are compressive strength, elastic modulus, thermal conductivity, thermal diffusivity and
Experimental approach
Current mixture design methods aim to produce either insulating or structural lightweight concrete [49], [50], [51], that is, they allow for mixture proportioning of insulating non-structural concrete or a low-density (not necessarily insulating) structural concrete. Thus, the mixture proportioning methods are focus in either thermal conductivity without considering mechanical strength or in density and strength without considering thermal conductivity directly.
The experimental approach taken
Results and discussion
The results of mechanical and thermal properties of the studied concrete samples at equilibrium moisture conditions are presented in Table 5; where is the equilibrium density, stands for the oven-dry density, fc is the compressive strength, E refers to the elastic modulus, represents the thermal conductivity at equilibrium density, is the thermal diffusivity at equilibrium density, and the specific heat at equilibrium density. Multiple linear regression analyses are
Conclusions
This study reports on mechanical and thermal properties of LWA concretes combining CEC, FEC and FA at different mixing levels to investigate their impact on developing insulating and structural lightweight concretes. Lightweight concretes obtained in the study exhibit compressive strength and thermal conductivity characteristics ranging from 9.3 MPa to 28.8 MPa and from 0.43 W/mK to 1.04 W/mK respectively. The impact of each of the constituent on the concrete compressive strength and thermal
CRediT authorship contribution statement
Jose C. Remesar: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Writing - original draft, Writing - review & editing. Francois Simon: Formal analysis, Writing - original draft. Sergio Vera: Formal analysis, Supervision, Writing - original draft, Writing - review & editing. Mauricio Lopez: Conceptualization, Formal analysis, Resources, Supervision, Writing - original draft, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the funding provided by CONICYT/FONDEF D10I1086 and ID17I10215, CONICYT-PCHA/MagisterNacional/2013 and CEDEUS, CONICYT/FONDAP 15110020. The authors also recognize Melón Hormigones, DICTUC S.A., and Mauricio Guerra for their contributions to this research project.
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