Thatch fibres grow in large quantity in the Adamawa region of Cameroon. During the long dry season, these fibres cause numerous fire incidents, which not only devastate large areas of cash crops, but also contribute to increase emissions of greenhouse gases into the atmosphere. This article aims to show how fibres could be used with compressed clay bricks to manufacture an insulating material used in building. Four fibre contents 1%, 2%, 3% and 4% made up the sample studied. The asymmetric hot plate methodology was used to determine the thermophysical properties of these composite materials. The volumetric heat capacity and the thermal effusivity of these materials were estimated. These two parametres were used to determine their apparent thermal conductivities. The results obtained show that the thermal conductivity decreases as the volume of fibres in the mixture increases. It is 0.689 W·m - 1·K- 1 for simple compressed clay bricks and 0.510 W·m -1·K - 1 for a dosage at 3% of thatch fibres. In a bit to validate the results of the pilot study of the apparent thermal conductivity, the heat mass capacity of this composite material was achieved through the use of the dehydration method. The relative difference obtained with the results of the volumetric heat capacity carried out with these two methods was good. All results showed that the use of fibres in compressed laterite brick gives a more insulating composite material that respects Civil Engineering Norms.
The tendency of the 21st century in industrialised nations is to limit the impact of human activity on the environment. It is thus within this context that construction industries had to innovate its construction practices in order to improve the use of energy in buildings as well as proposing cutting-edge technologies on materials that meet new user demands and environmental impact laws. Greenhouse gases emission, which makes up one of the main cause of environmental pollution, is constantly on the rise. These gases touch a wide range of sectors such as: agriculture, waste treatment plant, industries, transportation, and building construction etc. The global emission rate of greenhouse gas (GHG) in the Building sector is 4.1 Gt eq CO2 (gigatonnes in CO2 equivalent) in 2016 [
Our research is therefore, based on this concept of the used of laterite composite material as insulant in the interior of buildings. The sample composite material used in this study is laterite bricks mixed with thatch fibres all from the locality of Meiganga. In the Adamawa region of Cameroon, otherwise known as Watershed, 80% of houses are built with laterite brick not industrially treated nor having any geotechnical, mechanical, thermophysical or acoustic characterisation prepared initially. The bricks used do not meet civil engineering standards. A lot of research works have been carried out in the domain of characterisation of composite material made from laterite soil. Meukam et al. [
This work has as main objective the enhancement of thatch fibres as thermal insulation in buildings to reduce the quantity usually burnt during the long dry season. It was assessed when mixed with laterite bricks to get the thermophysical properties. This material can be used to curb energy consumption when used as filling for buildings.
Around the Meiganga vicinity, more than 80% of houses are built with laterite soil as opposed to the West Region of Cameroon where the laterite soil is rather dry. Although the objective is to get a composite material with insolation properties, it’s however imperative to begin with a mastery of the geotechnical properties of the sample laterite soil. Cameroon Civil Engineering standards stipulates (CS: 102 - 114) that, filling material for building such as compressed terracotta brick wall recommended for making laterite bricks should be used with plasticity index of Ip ≤ 30% [
W = 100 P h − P s P s (1)
where: P h is the mass of soil sample in its natural state and P s is the mass of the soil sample after passing through heat for 105˚C for 24 hours.
In order to determine the physico-chemical and mineralogical characteristics of this laterite, a grading analysis (according to BS 1377: 1975 [
- The diametre of particles is given by the stockes Law
D 2 = 18 η U g ( γ s − γ w ) (2)
- For the percentage of the diametre particles inferior to D, we use the equation
≤ D = γ s ( C 3 L + C 4 ) P S ( γ s − γ w ) ( 1 − ( ≥ 2.38 ) ) (3)
where: γ s = 26.5 × 10 3 N ⋅ m − 3 is the bulk specific weight of the solid particles; γ w = 10 N ⋅ m − 3 is the bulk specific weight of water; g is accelerating gravity in cm・s−2; U is the falling speed of particles in cm・s−1, η is the fluid viscosity measured in poise; C3 is the conversion coefficient from 1/1000; C4 is the reading for water only and L is the vertical reading 1/1000 beyond one.
The results from Equations (2) and (3) enabled us to represent the texture diagram presented in
Distribution of particle size grading analysis of laterite soil as illustrated in
In analysing the diagram in
W L = W 1.419 − 0.3 log N (4)
W P = 100 M t h − M t s M t s − M t (5)
where: Mt tare mass; Mth is tare mass + wet sample; Mts is tare mass + dry sample
Placticity index IP of laterite came from the equation:
I p = W L − W p (6)
We can then conclude that though the laterite texture curve does not fall within the grading range, its plastic index meets BTC conception standards that stipulates that Atterberg’s limitation must be between 10 < Ip < 20. The laterite was not adulterated with in anyway.
Ref | colour | Aspect | % gravel Φ > 2 mm | % of sand 2 > Φ > 0.02 mm | % of limons 0.02 > Φ > 0.02 mm | % of clay Φ < 0.02 mm |
---|---|---|---|---|---|---|
Late | Red | Clay-Sandy | 6.67 | 34.34 | 23.02 | 38.85 |
Late: Lateritic soil.
Material | WL (%) | WP (%) | IP (%) |
---|---|---|---|
Lateritic | 56 | 38.3 | 17.7 |
Cameroon Standards | 25 - 50 | 20 - 35 | 2 - 30 |
Several researchers have worked on fibres as natural insulators that help improve thermal performance of a composite material. Petkova et al. [
The process of extracting dry fibres uses as follow:
- The fibres were picked wet (
- Chopped at regular length and dried in the open (
- After the crushed stem mixed with lateritic soil (
- They were later dried in an oven at 45˚C (
- After that they were packed in plastic bags so as to keep their water content close to zero.
Measuring the apparent density of dry fibres (
The dry fibres obtained are then mixed with the laterite at different proportions to get different samples of composite material.
The experimental setup used in making samples were made of three cuboid-shaped metallic mould of (width 1, length L and thickness h): 4 cm × 8 cm × 2 cm (French standards Crater standard NFP-31-212 [
Ref | MS (g) | Vf (ml) | ρapp(f) (kg・m−3) |
---|---|---|---|
Fibres | 9 | 170 | 52.91 |
Ms: dry mass of fibres; Vf: volume of fibre; ρapp(f): Apparent density of dry fibre.
Samples for: | Flexural 16 cm × 4 cm × 4 cm | Compressive 4 cm × 4 cm × 4 cm | Thermophysical 10 cm × 10 cm × 3 cm | |||||||
---|---|---|---|---|---|---|---|---|---|---|
Ref | Fbre (%) | mL (g) | mF (g) | V (l) | mL (g) | mF (g) | V (l) | mL (g) | mF (g) | V (l) |
E0 | 0 | 400 | - | 60 | 150 | - | 22.5 | 400 | - | 60 |
E1 | 1 | 396 | 4 | 59.4 | 148.5 | 1.5 | 22.3 | 396 | 4 | 59.4 |
E2 | 2 | 392 | 8 | 58.8 | 147 | 3 | 22 | 392 | 8 | 58.8 |
E3 | 3 | 388 | 12 | 58.2 | 15.5 | 5.5 | 21.8 | 388 | 12 | 58.2 |
mL (g): mass of laterite: mF (g): mass of fibres:; V (l): volume of water.
Mechanical characterisation is of prime importance in Civil Engineering. Depending on the nature of the material to be used, either as a load-bearing structure or as a filling of the building, this characterisation helps to verify whether the material meets Civil Engineering standards. In our case, filling materials were used in the interior of the building. We were therefore interested in two mechanical characteristics: Flexural and the Compressive strength.
F (N) is flexural samples load (measured by CONTROLS.CAT machine No. T1004, Mat No. 3558), D = 50 mm distance from cylindrical support, 2 L = 80 mm length of block and h = 20 mm the average thickness of block. The flexural strength of MPa calculated using the equation:
R f = 3 F D 2 L h (7)
If F is the compressive break-down load (observed in reading the machine CONTROLS No. of rings 4117, Fmax = 30 kN), we then obtain RC constraint starting from the relation (h2 is the section):
R C = F h 2 (8)
The testing tool used to show the thermophysical characterisation is the asymmetric hot plane method [
Material with specifications of (10 cm × 10 cm × 3) is placed on a heat sensor between two polystyrene blocks of (10 cm × 10 cm × 5 cm). A generator is used to heat the resistor. The increase in temperature at the centre of the heat resistor was as a result of type K thermocouple that registers the test temperature on the hot surface of the material. The experimental temperatures Texp(t) were registered using the acquisition module TC 08-USB Picolog. The dropping of experimental and simulated temperature obtained after modelling the testing instrument helped us to estimate E and ρCP. It is therefore important to note that the temperature above the polystyrene blocks remains at its initial state, to arrive at this; we placed two aluminium blocks of 10 cm × 10 cm × 4 cm above and below the polystyrene blocks.
Given that all the blocks used for the pilot test were thick, we could therefore model heat transfer to other blocks using the heat equation.
∂ 2 T i ∂ x 2 + ∂ 2 T i ∂ y 2 + ∂ 2 T i ∂ z 2 = 1 a i ∂ T i ∂ t where i = s and h (9)
where T s ( x , y , z , t ) modelises the temperature thermogram on the heated face of the composite material while T h ( x , y , z , t ) does same on the heat sensor. By assuming that the heat transfer remains unidirectional for a time t, solving this 1D equation can be done using the quadrupole formalism developed by Maillet et al. [
d d x ( θ ϕ ) = M ( θ ϕ ) (10)
where: M is matrix or matrix product.
We can use this formalism according to the characterisation method used in evaluating thermophysical parameters E, ρCP and λ. In this study, the asymmetrical hot plate method enables us to estimate by using the complete 1D model, the parametres E and ρCP yielding to λ.
Considering the sample and polystyrene insulating block as a semi-infinite medium and hoping that the heating sensor is a thin device (
( θ s Φ h s ) = ( 1 0 ρ h c h e h S p 1 ) ( 1 S R h s 0 1 ) ( θ 1 E p θ 1 ) (11)
( θ s Φ h p o ) = ( A p o B p o C p o D p o ) ( θ 2 E p o p θ 2 ) (12)
where: E and Epo are respectively the thermal effusivity of samples and the polystyrene blocks while P is the Laplace parametre; S is the surface area of the sample which is supposed to be equal to Sh, area of heating element and Spo, area of the polystyrene blocks.
Combining Equations (11) and (12) yields to:
θ s ( z , p ) = 1 ρ h c h e h p + ( 1 + R h m ρ h c h e h S p ) E p 1 + R h m E S p + E p o p 1 + R h p o E p o S p Φ ( 0 , p ) (13)
For a considerable period of time (p®0), the analytic inversion of the above equation, using Laplace table gives:
Δ T a s y m ( 0 , t → ∞ ) = δ t + φ S ( E 2 R h s + E p o 2 R h p o ( E + E p o ) 2 − ρ h c h e h S ( E + E p o ) 2 ) (14)
The pre-estimated thermal effusivity may be calculated with the simplified 1D model from numerical calculation of the slope δ(t) of the linear part of the curve T ( t ) = f ( t ) when Texp(t) and Tsinf(t) are overlapped.
E = E p r e s = 2 φ δ π − E p o (15)
The pre-estimated value of volumetric heat capacity given by the equation (15), may be approximated from the slope η(t) of the linear part of the curve T ( t ) = f ( t ) when Texp(t) and Tsinf(t) are overlapped.
( ρ C p ) p r e s = 1 e s ( φ η − φ p o c p o e p o − ρ h c h e h ) (16)
These pre-estimated values (Equations (15) and (16)) are then used as initial values in the estimation code of E and ρCp for complete model.
With the Complete model, the quadrupole method no longer uses the sample and insulating block as in semi-infinite medium. Consequently, matrix M representing these two blocks is shown in the Equation (17):
( A s B s C s D s ) = ( c h ( ρ C p E p e ) s h ( ρ C p E p e ) λ ρ C p E p λ ρ C p E p s h ( ρ C p E p e ) c h ( ρ C p E p e ) ) and ( A p o B p o C p o D p o ) = ( c h ( p a p o e p o ) s h ( p a p o e p o ) λ p o p a p o λ p o p a p o s h ( p a p o e p o ) c h ( p a p o e p o ) ) (17)
The solving of the matrix system equation is given by Equations (18) and (19):
( θ c Φ h s ) = ( 1 0 ρ h c h e h S p 1 ) ( 1 S R h s 0 1 ) ( A s B s C s D s ) ( 1 S R s p o 0 1 ) ( A p o B p o C p o D p o ) ( 0 Φ m ) = ( A B C D ) ( 0 Φ 1 ) (18)
( θ c Φ h p o ) = ( 1 S R h p o 0 1 ) ( A p o B p o C p o D p o ) ( 0 Φ 2 ) (19)
The total heat flux density in the Laplace field is calculated from relation:
Φ 0 = Φ 1 + Φ 2 (20)
Combining relations (18); (19) and (20), the system leads to:
θ c ( z , p ) = B p o B s D s B p o + D p o B s Φ ( 0 , p ) (21)
The theoretical curve T a s y m ( t ) = L − 1 ( θ c ( z , p ) ) was obtained by numerical inversion of the Equation (21) using the De Hoog algorithm [
Σ = ∑ i = 1 n [ Δ T e x p ( t i ) − T a s y m ( t i ) ] 2 (22)
The calculated values of apparent thermal conductivity of composite material can also be inferred from the Equation (23):
λ e s t = E 2 ρ C p (23)
Establishing thermal capacity of composite material with different fibre proportions that was achieved had as objective to confirm the results of the pilot study carried out with Equation (23). This test achievement was made known through its water content, colometry sizing which helps to validate the results. To establish the water content, several methods do exist among which the dehydration method (DM) uses in this research. Three research instruments (
This method was inspired by the manner in which objects transfer heat. The Cp was measured in cold water as well as normal temperature. The thermal heat emitted by the object is as follows:
Q = m C m Δ θ (24)
In cases where heat loss is neglected within the exchange system, we can therefore conclude that the energy produced by the object is exactly equivalent to that absorbed by water:
Q h w + Q c w + Q c a = 0 (25)
where Qhw equals energy emitted by the object, Qcw energy absorbed by water and Qca energy absorbed by calorimeter.
For calorimeter sizing, the following equation was used:
m 2 C e ( θ e q − θ i ) + C ( θ e q − θ i ) + m 2 C e ( θ e q − θ c ) = 0 (26)
If the mass of hot water is equal to that of cold water, we’ll then say that:
C = m 2 C e ( 2 θ e q − θ i − θ c ) θ i − θ e q (27)
To determine the Cp sample; we have:
m 1 C e ( θ e q − θ i ) + C ( θ e q − θ i ) + m 2 C p ( θ e q − θ w o ) = 0 (28)
Leading to:
C p = m 1 ( C e + C ) ( θ e q − θ i ) m 2 ( θ w o − θ e q ) (29)
where: m1 mass of hot water, m2 mass of cold water, θc hot water temperature, θwo Cold water temperature, θeq balance point, Calorie capacity of calorimeter
To validate the research method used, we tested the value of the thermal capacity of the calorimeter used. The specification sheet provided shows CP = 30 J・K−1. (
We notice a correlation between the average value of the thermal capacity of calorimeter calculated with dehydration method and that shown on the researcher’s specification sheet. The relative difference obtained between the two values is less than 3%. This result enables us to validate the results of the thermal capacities of the composite materials studied.
Measurements were carried out on five different samples E0, E1, E2, E3 and E4.
Test N0 | Mce (g) | Mhe (g) | Tce (˚C) | The (˚C) | Teq (˚C) | Cp (J・K−1) | (Cp)mean (J・K−1) | D (%) |
---|---|---|---|---|---|---|---|---|
1 | 200 | 200 | 22 | 81 | 52 | 28.827 | 29.17 | 2.76 |
2 | 200 | 200 | 22 | 81 | 52 | 28.827 | ||
3 | 200 | 200 | 24 | 81 | 53 | 29.85 |
Mce: Mass of cold water; Mhe: Mass of hot water; Tce: cold water temperature; T: hot water temperature; D: deviation.
Ref | Cp(DM) (Jkg−1) |
---|---|
E0 | 870.46 |
E1 | 869.57 |
E2 | 867.18 |
E3 | 861.73 |
E4 | 858.43 |
Three were performed on each sample as well as the average value of each of the samples. For the mechanical tests,
We notice that the apparent density ρapp and the porosity ε were subsequently obtained from the Equation (30):
ρ a p p = M s V a p p (30)
ε = ρ a p p s ρ e M h − M s M s (31)
where: Vapp is the apparent density of the composite material, Mh wet density of composite material, Ms is dry density of composite material and ρe is the mass density of water.
It should be noted that control protocol on building site recommended by MIPROMALO (Mission de Promotion des Matériaux Locaux) [
Ref | ρapp (kg・m−3) | RC (MPa) | RF (MPa) | ε | Abs |
---|---|---|---|---|---|
E0 | 1775.86 | 4.61 | 1.42 | 9.04 | 16.18 |
E1 | 1770.61 | 5.13 | 2.06 | 9.71 | 17.19 |
E2 | 1751.91 | 6.44 | 2.34 | 10.73 | 17.73 |
E3 | 1745.95 | 7.60 | 2.43 | 11.41 | 18.75 |
E4 | 1741.66 | 7.92 | 2.57 | 13.01 | 19.21 |
As concerns thermophysical characterisation, we initially verified if the new model could react to thermophysical properties when calculated with transient hot plate method. Since these thermophysical parametre are not yet known, we noticed a reduced sensitivity of the thermophysical parametres β ∂ T ∂ β by using the result obtained with the simplified model (Equations (15) and (16)) Values Epres and (ρCp)pres were calculated within a period of time [t1, t2] as much as the thermogram Tasym(t) and Tsinf(t) superimposed (
The reduced sensitivities of the temperature of parameter E, ρCp and SRchs were calculated as presented in
- We noticed that the reduced sensitivities of T to the heating sensor becomes constant after 10 s, this shows that the inertia of the heating sensor could be neglected only 10 s after having measured T. This shows that, although the effect of the inertia becomes important as the proportion of fibres in the material increases, the developed model cannot provide a reliable estimate of the thermal contact resistance at the interface of the heat sensor/material.
- Sensitivities of T to ρCp and to E are decorrelated, but T is sensible to E only between 0 and 500 s and sensible to ρCp practically after 300 s. The link observed between ρCp and Ri is as a result of strong thermal initial observed 100 s after having started registering the temperature. This could be fully seen on the residual graph that shows the plotting in
Contrary to the simplified model which does not reduce the sum of quadratic differences (Equation (22)) between the model and the experimental temperature, the completed model instead reduces inversely. We therefore observed a convergence between pre-estimated and experimental results as seen in
In a bid to validate and confirm the experimental results with the asymmetric hot plate method, we compared the estimated value of (ρCp)est with that of ρapp × (Cp)DM where the value of (Cp)DM and papp are respectively shown in
λ e x p = E 2 ρ a p p ∗ ( C p ) D M (32)
All the results obtained are shown in
When comparing results from both methods which aim at determining the volumetric heat capacity, we can clearly see the mean value of apparent thermal conductivity obtained by the asymmetric hot plate method on one hand and that obtained by the Equation (32), perfectly match (
Ref | Esimpl J・m−2・˚C−1・s−1/2 | Ecompl J・m−2・˚C−1・s−1/2 | (ρCp)est J・m−3・K−1 | ρapp × (Cp)DM J・m−3・K−1 | λest W・m−1・K−1 | λexp W・m−1・K−1 | DE % | DρC % | Dλ % |
---|---|---|---|---|---|---|---|---|---|
E0 | 1051.93 | 1046.74 | 1.592 106 | 1.545 106 | 0.689 | 0.710 | 0.39 | 2.91 | 2.91 |
E1 | 995.81 | 994.90 | 1.546 106 | 1.539 106 | 0.639 | 0.642 | 0.39 | 0.45 | 0.45 |
E2 | 912.74 | 913.95 | 1.518 106 | 1.519 106 | 0.550 | 0.549 | 0.09 | 0.05 | 0.05 |
E3 | 878.34 | 876.99 | 1.505 106 | 1.512 106 | 0.510 | 0.511 | 0.13 | 0.05 | 0.05 |
E4 | 853.34 | 851.80 | 1.511 106 | 1.495 106 | 0.483 | 0.485 | 0.18 | 1.05 | 0.41 |
Simpl: simplified model; compl: completed model; est: estimated; exp: experimental; D: standard deviation.
This research work sets out to investigate the energetic and mechanical reinforcement of sun-dried bricks made with laterite and different proportions of thatch fibres from the Meiganga locality of the Adamawa region in Cameroon. Mechanical characterisation studies have shown that with up to 4% fibre incorporated, the composite materials produced have a very good compressive and flexural strength that meets civil engineering standards. The asymmetric hot plate method used to calculate the thermophysical parametres of these different materials was validated with another method, i.e. the dehydration method, used for determining the heat capacity. With the use of apparent thermal conductivity study, we came to the conclusion that with fibres used, a better thermal insulating composite material was the bi-product. These materials can then contribute to ensuring a better thermal comfort of the interior of a building. The extremely hot climate of this region gives credence to the treatment and use of plant fibres to make sun-dried bricks. These could be used to reduce energy consumption thus, limiting the emission of greenhouse gas (especially CO2) when air-conditioners are used.
The authors declare no conflicts of interest regarding the publication of this paper.
Nitcheu, M., Meukam, P., Damfeu, J.C. and Njomo, D. (2018) Thermomechanical Characterisation of Compressed Clay Bricks Reinforced by Thatch Fibres for the Optimal Use in Building. Materials Sciences and Applications, 9, 913-935. https://doi.org/10.4236/msa.2018.912066
T. Temperature (˚C)
E. Thermal effusivity (J・m−2・C−1・s−1⁄2)
λ. Thermal conductivity (W・m−1・K−1)
h. Convective heat loss coefficient (W・m−2・˚C−1)
R. Thermal contact resistance (˚C−1・W−1)
Cp. Heat capacity (J・kg−1・K−1)
ρ. Density (kg・m−3)
e. Thickness (m)
θ. Laplace transform of temperature
P. Laplace parameter
Φ. Laplace transform of heat flux
φ0. Heat flux density (W・m−2)
ρCp. Volumetric heat capacity (J・m−3・K−1)
P0. Insulating blocks
s. Sample
comp. Completed model
simpl. Simplified model
h. Heation element
exp. Experimental
Pre. Pre-estimated
DM. Dehydration Method
Sinf. Semi-infinite medium
c. Center