Keywords

1 Introduction

Novel nano- and micro-cementitious materials result a topic of growing interest in scientific literature (Shah et al. 2009; Ubertini et al. 2016). Cement-matrix materials result particularly suitable for a micro-structural modification, due to the presence of components and pores at different scales. The fillers can confer to the materials multifunctional properties, as new or enhanced capabilities (D’Alessandro et al. 2016a; 2016b; Laflamme et al. 2015; Yang and Che 2015). In particular, phase change materials (PCMs) possess high heats of fusion and, during the melting or solidifying transitions at a fixed temperature, are capable of storing or releasing energy (Sharma et al. 2009). Their use in building materials permits to optimize thermal-energy efficiency and construction sustainability of constructions (Kalnæs and Jelle 2015; Navarro et al. 2016; D’Alessandro et al. 2016a; 2016b). An optimal phase change material for concrete needs to exhibit appropriate mechanical performances, a proper phase change temperature and a great melting enthalpy. Also, the choice of such materials usually is also led considering other physical, technical and economic aspects, (Cabeza 2015; Fernandes et al. 2014). Research investigations present in literature mainly concern the capability of PCMs to improve the thermal performance of concrete, assessing whether the mechanical properties were severely flawed (Zhang et al. 2004; Lecompte et al. 2015). In order to ensure such acceptable properties, most of these studies focus on the integration of microencapsulated PCMs in cementitious admixtures, thus avoiding leakage problems which could negatively affect the compression resistance of the samples (Konuklu et al. 2015).

The present paper is aimed at investigating the mechanical and thermal performance of concretes with PCMs for structural applications in comparison to ordinary concrete. The paper is organized as follows. Firstly, a literature overview on the use of PCMs as multifunctional additives in concrete is presented in Sect. 1. Materials and samples developed within this work are presented in Sect. 2. The experimental methodology is described in detail in Sect. 3, while test results are presented in Sect. 4 and discussed in Sect. 5. Finally, the paper presents the main findings and conclusions.

2 Materials and Experimental Methods

2.1 Phase Change Materials

The PCMs utilized in the experimentation were Microtek Microencapsulated PCM. They appeared as a white powder (Fig. 1a) with a mean particle size between 17 and 20 µm. The core material inside the microcapsules, consisted of a inert polymer, was a paraffin-wax with heat absorption capabilities and a melting point at 18 °C. The PCM content was about 85–90% with respect to the total mass.

Fig. 1.
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(a) Appearance of MicroPCM; (b) Microscope enlargement of concrete with 5% of PCM

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2.2 Concrete Preparation Procedures

Table 1 shows the components of the normal reference concrete and the concretes prepared with the addition of 1, 3 and 5% of PCMs with respect to the whole mass of the composite. The water/cement ratio was 0.45 for all the admixtures, except for 5% PCM-concrete, with a w/c ratio of 0.5, necessary to obtain similar workability. The cement was type 42.5, pozzolanic. The aggregates were constituted by sand and gravel with nominal dimensions from 0 to 4 mm and from 4 to 8 mm, respectively.

Table 1. Mix design of normal reference concrete and concretes with PCMs.
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The sample preparation process consisted in the preliminary mix of cement, sand, gravel and PCMs (Fig. 2a), and the subsequent addition of water and plasticizer (Fig. 2b). Then, the dough was mixed and carefully poured into oiled moulds, in order to avoid the separation of the fillers (Fig. 2c). The samples were unmolded for curing after some days (Fig. 2d). After mixing, microPCM capsules appeared intact, since no oil was observed in the composites.

Fig. 2.
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Preparation process of concretes with PCMs

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The samples were cubes with sides of 100 mm, for mechanical tests, and prisms with squared bases of 190 × 190 mm2 and height of 50 mm, for thermal experiments. Figure 3 represents the structure of the normal and the PCM-concrete, with dispersed microcapsules. An enlarged image of a fragment of hardened concrete with PCMs can be observed in Fig. 1b. The microscope used for the elargements was type Bresser, Biolux NV, with magnifications up to 1280X.

Fig. 3.
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Schematic representation of the structure of Normal Concrete and Concrete with microPCM

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2.3 Experimental Setup

Mechanical Tests

After curing, the sample were measured and weight and then compression tests were carried out, using an Advantest machine, model Controls 50-C7600 (Fig. 4a) with a servo-hydraulic control unit model 50-C9842 (Fig. 4b), instrumented with three LVDT placed at 120° (Fig. 4c). The compressive loads were applied under displacement control, up to the collapse. Both resistance and ductility after peak have been investigated. The test speed was 2 μm/s, in agreement with the EN 12390-3 standard.

Fig. 4.
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Experimental setup for mechanical tests: (a) testing machine; (b) central control unit; (c) cubic concrete sample instrumented with LVDTs

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Thermal Tests

The thermal investigations of the prismatic samples were carried out by means of Transient Plane Source (TPS) method, using a Hot Disk 2500 system, in accord with the ISO 22007-2 standard and an ATT DM340SR climatic chamber equipped with 12 PT100 thermocouples. In the environmental chamber the samples were insulated by XPS panels: just the upper side was exposed to the controlled environment of the chamber (Fig. 5a). Each sample was instrumented with four PT100: one on bottom surface, one on the upper surface and two probes on the lateral sides. The thermal program consisted of five subsequent segments, 8 h long, with linear varying temperature between 26 °C and 10 °C and a fixed RH value of 50%. The thermal tests in the climatic chamber were aimed at investigating the melting temperature of the PCMs within the concretes and the phase change enthalpy. The Hot Disk tests were carried out in a single sided configuration, installing a superinsulating material on the other side of the probe. These tests were aimed at assessing the thermal conductivity and the thermal diffusivity of the concretes, and to derive the specific heat.

Fig. 5.
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(a) Prismatic samples in the climatic chamber; (b) setup of the hot-disk

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3 Mechanical Tests

The mass density of concretes with PCMs decreased with the increase of the filler content.

In particular, 5% PCM-concretes demonstrated a density reduction of about 11% with respect to normal concrete (Fig. 6a). So, they resulted suitable for the realization of lightweight concretes. Figure 6b represents the fracture pattern of a specimen of 5%PCM-concrete after compressive test: it exhibits a bi-pyramidal fracture, indicating a good behavior of the material.

Fig. 6.
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(a) Variation of density in concrete samples with different amounts of PCM; (b) sample of 5% PCM-concrete after compressive test

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The concrete samples were compared in terms of average compressive strength, Rm, and of characteristic compressive strength, Rck, in order to evaluate their feasibility as structural materials. Rck represents the strength value which the 5% of the tested samples don’t achieve. According to literature, it is analytical defined as:

$$ R_{ck} = R_{m} - k \cdot s $$
(1)

where s is the standard deviation, and k depends on the number of tested samples. It was assumed equal to 3.40 and 2.75 for PCM-concretes (5 samples) and normal concrete (8 samples), respectively. Also, the coefficient of variation is particular relevant for structural applications because low values of this characteristic result in higher structural reliability:

$$ CV = {s \mathord{\left/ {\vphantom {s {R_{m} }}} \right. \kern-0pt} {R_{m} }} $$
(2)

All samples with and without PCMs exhibit the typical non-linear stress-strain curve, with decaying branch after peak.

Figure 7 shows the results of all the tested samples without PCM and with microPCM. As expected, the compressive strength of the concretes decreased with the increase of the content of PCMs because of the lower strength of PCM capsules with respect to aggregates.

Fig. 7.
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(a) Average compressive strength with standard deviation, and (b) characteristic compressive strength of the tested concretes with different amounts of PCMs

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The characteristic compressive strength shows instead a better performance of concretes with low amounts of PCMs. Microcapsules can provide higher specific area for nucleation sites in cement hydration, and could have a beneficial thermal effect in the composition of the hydration products.

Table 2 reports the average compressive strength resulted from the axial compression tests. The table also shows the values of the coefficient of variation (CV) of the composites, their characteristic compressive strength and their average density. The CV of normal concrete tested in the experimentation is equal to 0.09, within the typical range of ordinary concretes. The CV values of PCM-concretes resulted smaller or equal than 0.09 and demonstrated the good reliability capabilities of the composites for structural applications. Moreover, 1% PCM-concrete possessed a coefficient of variation of 0.02, considerably less than the value of normal concrete. The low dispersion exhibited by resistances of 1% PCM-concrete resulted in an increased structural class from C25/30 of normal concrete to C28/35. Moreover, all concretes with PCMs demonstrated structural properties compatible with the use as structural concretes. Figures 8 represents the single stress-strain curves resulted from the compressive tests at displacement control, for all the normal concrete samples and samples with PCMs. The graphs show clearly the behavior of the materials with increased strain: the peak value represents the maximum compressive resistance, while the post peak branches are representative of the ductility properties. Each type of concrete exhibits a similar behavior even if a decrease of the maximum resistance can be observed. PCMs seem not to affect the ductile capabilities of the concretes. Figure 9 shows the average values of the stress-strain curves for each typology of concrete. The values were obtained by calculating the mean stress for fixed strains. The diagrams clearly show that the elastic modulus is greater for the concrete samples with higher compressive strength.

Table 2. Compressive strengths, CV and densities of concrete composites normal and with PCMs
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Fig. 8.
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Stress-strain curves of the complete uniaxial compression tests on concrete without and with PCM up to a strain of 10 µε.

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Fig. 9.
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Average values of stress-strain curves of the complete uniaxial compression tests on concretes without and with PCM up to a strain of 10 µε.

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4 Thermo-Physical Tests

The thermo-physical tests were aimed at investigating the phase change materials in dynamic experimental conditions in order to assess the thermal benefits of the addition of PCMs in concrete.

The results coming from the thermal cycles show that all the samples followed the imposed temperature history. The differences between the temperature applied and measured on the concrete samples are due to the high thermal inertia of the cementitious material. Moreover, the samples with PCMs exhibit a peculiar behavior in the temperature range between 17 and 12°C, in the descending ramp, and in the range between 15 and 18 °C in the increasing ramp (Fig. 10). The deflections resulted in the thermal profile of concrete with 5% of PCMs, with respect to the reference trend of the normal concrete sample, are associated to the phase change taking place in the fillers. Indeed, the paraffin inside the microcapsules has a nominal melting point of 18 °C. The first deflection referred to the solidification of the PCM, while the second one to the melting transition. During these phenomena, the PCM-concretes decrease their temperature at a lower level, since thermal energy is stored in the latent storage material, necessary to break or form molecular bonds in the PCMs. The effect of the presence of PCMs is visible up to about 2 h. Figure 11 represents the thermal behavior of concretes with 1% and 5% of PCMs, compared to the thermal behavior of the reference normal concrete, as recorded within the environmental simulation chamber. Phase change phenomena are clearly visible during cycles, closed to the transition phase temperature of the fillers. As shown in the diagrams, the concrete with the highest content of PCMs exhibited the greater deviation, resulting in a higher amount of energy absorbed and released by the concretes in phase change transition. Such behavior is highlighted also by the presence of a larger area included in the trends. Otherwise, the concretes with 1% of PCMs exhibit a thermal behavior almost linear, very similar to that of normal concrete.

Fig. 10.
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Normal and 5% PCM-concretes’ thermal profiles monitored by the PT100 probe at the bottom of the samples

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Fig. 11.
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Thermal profile of 1% and 5% PCM-concrete, as measured within the environmental simulation chamber

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Figure 12 depicts the thermal conductivity and the diffusivity measured in the concrete samples with different amounts of PCMs, in comparison with the normal ones. The samples presented a slightly variation of both thermal conductivity and diffusivity in concretes with PCMs, with respect to normal concrete. In particular, the composite materials showed a reduction of the thermal conductivity, probably due to the small and closed pores created by the addition of the PCM microcapsules. Also, the capability to propagate the thermal wave in transient conditions resulted increased. The specific heat of the different concretes with and without PCMs was also tested, as key parameter influencing the thermal inertia of buildings and typically characterizing both sensible and latent thermal energy storage components. The presence of higher percentages of fillers resulted in a reduction of the specific heat superior to 20%. Indeed, microencapsulated materials seem to produce a noticeable effect on the thermal inertia of concretes.

Fig. 12.
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Thermal conductivity, diffusivity and specific heat resulted by means of hot disk tests on normal concretes and with different contents of PCMs

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5 Conclusions

This research was aimed at investigating the multifunctional performance of PCM-concretes for building applications.

Such concretes were made by adding increasing amounts of microencapsulated PCMs, from 1% to 5% with respect to the total mass of the material. The fillers had an internal core made of paraffin with melting temperature at 18 °C. Both mechanical and thermo-physical tests were carried out, in order to investigate the multifunctional properties of such materials and their applicability to structural civil engineering. The results of mechanical test demonstrated that PCM-concretes are compatible with structural applications as lightweight concretes. With respect to normal concretes, a reduction up to 11% in weight was observed. Moreover, small additions of PCMs resulted in an increase of the characteristic resistance. At the same time, the prototyped concrete manifested thermal capabilities suitable for building energy efficiency applications. Thermo-physical tests demonstrated that the phase transition is visible in PCM-concretes with higher amounts of fillers, resulting in a mitigation of the temperature fluctuations in the matrices.