The thermal conductivity measurement by a simplified transient hot-wire technique is applied to geomaterials in order to show the relationships which can exist between effective thermal conductivity, texture, and moisture of the materials. After a validation of the used “one hot-wire” technique in water, toluene, and glass-bead assemblages, the investigations were performed (1) in glass-bead assemblages of different diameters in dried, water, and acetone-saturated states in order to observe the role of grain sizes and saturation on the effective thermal conductivity, (2) in a compacted earth brick at different moisture states, and (3) in a lime-hemp concrete during 110 days following its manufacture. The lime-hemp concrete allows the measurements during the setting, desiccation and carbonation steps. The recorded
The thermal and hydric transfers in geomaterials are fundamental phenomena which can govern firstly the comfort in buildings and residences and secondly the durability of the materials. The microclimatic simulations of the hygrometric and thermal conditions prevailing in the building interiors need the estimation of the wall material characteristics: moisture and effective thermal conductivity face to the outer climatic changes. In fact, the thermal and hydric conductivities are intrinsic properties of the materials. The heat transfers mainly depend on the frequency of grain contacts, size of pores, and air-water ratio infilling the porosity. The moisture transfers mainly depend on the connectivity associated to the meso-to-micro porosity distribution. The development and implementation of “ecomaterials” for building have to demonstrate their ecologic qualities both by their productions and building techniques and also by their thermal and hydric insulation properties. In this “ecomaterial” domain, the earth material and concretes using plant fibers as aggregates are more and more used. Their uses are governed by their mechanical and thermal properties, that is, minimum of mechanical resistance face to the construction necessities and insulation or heat storage abilities. The building techniques of earth material have been largely discussed and normalized. The material cohesion is assumed by the clay mineral matrix often added of some percents of lime or cement. The mechanical resistances are due (1) to the clay matrix cohesion which acts as cement between the sand grains and (2) to the compactness of the clay-sand assemblage which decreases the volume of microporosity. This compactness of the material influences the meso-to-micro porosity distribution. The concretes using plant fibers as aggregates are more and more used with increasing cement-to-lime replacement. These new cement- or lime-plant fiber concretes are mainly appreciated for their small effective thermal conductivities. Nevertheless, the thermal conductivities evolve with time according to the structure and saturation index evolutions of the material during the setting, desiccation, and carbonation steps.
The THW method has been progressively developed from 1931 until now [
The objective of the work is to show that a transient hot-wire (THW) technique, simplified to only one hot-wire embedded in geomaterials, allows rapid characterization on the thermal-hydric properties of the building materials which remain realistic face to their heterogeneity. The texture-moisture and thermal effective conductivity relationships are studied firstly on glass-bead assemblages of different diameters in order to demonstrate the role of grain size and saturation index. Secondly, the measurements have been performed in materials used for “ecoconstruction,” that is, sand + clay matrix compacted material (compacted earth brick) and lime-hemp concrete in order to follow the evolution of the effective thermal conductivity during their maturation. The technique is tested in such very thin divided and microporous materials taking into account the moisture evolution due to the desiccation and structure evolutions.
The THW method is a transient dynamic technique based on the measurement of the temperature rise of a linear heat source (hot wire) embedded in the tested material [
Equation (
From this ideal model and with known
For practical applications of the THW method, wire and material sample dimensions, among other ideal model hypothesis, are finite and the deviations from the ideal model have then to be evaluated. In fact, the
Taking into account (
The heat transfers were measured in assemblages of glass beads and in two geomaterials, that is, a compacted earth brick and a lime-hemp concrete brick. The data (temperature versus time) were recorded using a very pure Ni (>99.98%) wire of 0.125 mm diameter insert in the glass-bead assemblages or in the earth brick and lime-hemp brick during the manufacture (Figure
Details of the Transient Hot-Wire device. (a) Cylindrical cell used for the glass-bead assemblages. (b) Cylindrical cell filled by the beads and Keithley current generator and nanovoltemeter. (c) Schematic representation of the earth brick and lime-hemp brick instrumentation (
Successive glass-bead media have been tested. Three types of glass beads Sili S, SL, and M are used. They differ by weak differences in chemical compositions and associated densities (Table
Diameter ranges (
Mean diameter | Bead density | Porosity | |
Type M | |||
1.50–1.70 | 1.50 | 2.54 | 0.38 |
1.90–2.30 | 2.00 | 2.50 | 0.37 |
2.40–2.70 | 2.50 | 2.54 | 0.38 |
2.70–3.10 | 2.80 | 2.50 | 0.36 |
3.80–4.10 | 3.90 | 2.52 | 0.37 |
Type S | |||
0.40–0.60 | 0.45 | 2.51 | 0.40 |
0.70–1.10 | 0.85 | 2.52 | 0.39 |
1.00–1.40 | 1.10 | 2.53 | 0.39 |
1.70–2.20 | 1.80 | 2.52 | 0.39 |
2.50–3.15 | 2.50 | 2.51 | 0.38 |
2.85–3.60 | 3.00 | 2.53 | 0.39 |
3.75–4.50 | 3.75 | 2.51 | 0.37 |
Type SL | |||
0.50–0.80 | 0.71 | 2.61 | 0.40 |
0.70–1.05 | 0.85 | 2.61 | 0.39 |
0.90–1.50 | 1.10 | 2.59 | 0.39 |
1.60–2.20 | 1.80 | 2.61 | 0.39 |
2.50–3.20 | 2.70 | 2.60 | 0.38 |
The compacted earth brick is made by mechanical compression of earth plus sand mixture added of 5% lime. In weight percentage, the composition of the compacted brick is 62% earth, 28% sand, 5% lime, and 5% water. The dimensions of the bricks are 29.5 cm length, 14 cm width, and 9.5 cm height (Figure
The lime-hemp concrete is composed of mixture of 25 kg water, 5.08 kg (60 L) hemp (chenevotte), and 35 kg lime. The weight formulae of the lime-hemp concrete evolve from 38.5% of water, 7.8% of chenevotte, and 53.8% of lime for the fresh concrete. The percentage in weight of lime increases to 76% for mature concrete. The lime is a Batichanvre Saint Astier (NHL5). The dimensions of the earth and lime-hemp bricks are 29.5 cm length, 14 cm width, and 9.5 cm height (Figure
The heat transfer measurements are represented in
(a) Superimposition of the
(a) Superimposition of the
This one hot-wire probe was firstly validated by measurements in water and toluene (22°C). Ten successive measurements give water and toluene thermal conductivities of 0.602 W/mK and 0.1313 W/mK, respectively, in good agreement with bibliographic data [
The short time
The precision of the measurements is influenced by many sets. The precisions of the generator (10−6 A) and of the nanovoltmeter (10−8 V) minimize a cumulate error on the
For the glass-bead materials, all the
Example of the evolution of the slopes (
Representation of the
For the successive tests performed on the different materials, the
The two obtained
Thermal conductivity (
Medium | |
---|---|
air | 0.025 |
acetone | 0.16 |
water | 0.60 |
glass | 1.05 |
Evolution of the
Medium | Maxwell | Ktupiczka | ||
---|---|---|---|---|
Air saturated | 0.1–0.04 | 0.20–0.40 | 0.20 | 0.18 |
Acetone saturated | 0.35 | 0.64 | 0.63 | 0.48 |
Water saturated | 0.85 | 0.85 | 0.98 | 0.87 |
The measurements for the different saturation states were made on the 2.8–3.4 mm diameter glass-bead assemblages. Three types of the curve characteristic of the dried bead assemblage with the two successive a very flat curve two slope curve for the acetone saturation, a straight line for the water saturation.
Evolution the
Evolution of weight, gravimetric water content (
The
The thermal conductivities were measured during the 110 days of the brick desiccation. The manufacturing of the earth brick by mechanical compression induced a quite saturated material initially characterized by a saturation index (
For this material, only one brick and, consequently, only one texture was tested. The sand grains are coated by the earth clay material. In fact, the clay-to-sand skeleton and the small initial porosity of the compacted material have limited the shrinkage phenomenon. Thus, the porosity may be counted as constant during the desiccation and mainly assumed by the clay matrix microporosity. The scanning electronic microscope confirms this microstructure of the compacted brick. The clay matrix is very compact. All the porosity is constituted of microporosity disseminated between the clay particles and almost micropores have sizes lower than 20
SEM microphotography of the earth brick microstructure. The sand grain is isolated inside the compacted clay matrix. The clay matrix appears very compact and homogeneous. The microcracks are consequence of the sampling of the milimetric block from the brick.
Evolution of the
Nevertheless, the
Evolution of
Evolution of
The evolutions of
Two days were necessary to allow the removal of the test piece from the mould. The measurements by the THW method began 2 days after the manufacturing of the lime-hemp concrete. The weight of the test piece was measured from 2 to 110 days. The relative lost of weight was 42.10%. It includes the lost of water by desiccation, the hydration, and the carbonation of the lime. The curve of weight versus time shows a “hyperbolic” shape characterized by (Figure a very high weight lost (170 g/day equivalent to 4.47%/day) during the 8 first days, a weight decrease following a exponential law from 8 to 55 days; that is, a low decrease from 55 to 110 days (2 g/day equivalent to 0.05%/day).
Weight lost of the lime-hemp test piece with time.
The successive recorded
Evolutions of the
The obtained one-slope
The 0 to 20 days; 20 to 110 days;
Evolution of the thermal conductivity (
The final
The investigations, using only one hot wire, in spite of its simplicity and basic aspect face to the recent developments, were tested on granular and geomaterials. Despite the measurement precisions lower than those obtained on pure phases by recent investigations (3% against 1% for Assael et al. results; [ the diameter/length ratio of wire has to be very small to allow the simplification as infinite linear wire, according to the theory. Nevertheless, it has to be appropriated to the manufacture of the test pieces that is; very low end effect error and sufficient resistance of the embedded wire face to the eventual compression of the material in moulds, a very accurate measurement of the wire length and of the associated currents and voltages.
In order to avoid all the artifacts of material surface/wire contact, the hot wires were embedded into the materials during their manufactures. The wire insertion is particularly easy for the glass-bead assemblages and during the earth brick or lime-hemp concrete compaction, and, therefore, for different other types of manufactured geomaterials. The experiments on glass beads show the interest in recording the
Tested on the lime-hemp concrete from its manufacture to advance desiccation + setting steps (110 days), the method confirms its validity for geomaterials. The
These preliminary results suggest the possibility of investigation on the relationship which prevails between the microstructure and saturation state of geomaterials and their effective thermal conductivity. The technique may be extended to