Mechanical and Thermal Performance Characterisation of Compressed Earth Blocks

Abstract

The present research is focused on an experimental investigation to evaluate the mechanical, durability, and thermal performance of compressed earth blocks (CEBs) produced in Portugal. CEBs were analysed in terms of electrical resistivity, ultrasonic pulse velocity, compressive strength, total water absorption, water absorption by capillarity, accelerated erosion test, and thermal transmittance evaluated in a guarded hotbox setup apparatus. Overall, the results showed that compressed earth blocks presented good mechanical and durability properties. Still, they had some issues in terms of porosity due to the particle size distribution of soil used for their production. The compressive strength value obtained was 9 MPa, which is considerably higher than the minimum requirements for compressed earth blocks. Moreover, they presented a heat transfer coefficient of 2.66 W/(m 2 ·K). This heat transfer coefficient means that this type of masonry unit cannot be used in the building envelope without an additional thermal insulation layer but shows that they are suitable to be used in partition walls. Although CEBs have promising characteristics when compared to conventional bricks, results also showed that their proprieties could even be improved if optimisation of the soil mixture is implemented.

Full text

energies
Article
Mechanical and Thermal Performance
Characterisation of Compressed Earth Blocks
Elisabete R. Teixeira 1, Gilberto Machado 1, Adilson de P. Junior 1, Christiane Guarnier 2,
Jorge Fernandes 1, Sandra M. Silva 1and Ricardo Mateus 1, *
1Department of Civil Engineering, Institute for Sustainability and Innovation in Structural
Engineering (ISISE), University of Minho, 4800-058 Guimarães, Portugal; b8416@civil.uminho.pt (E.R.T.);
a73118@alunos.uminho.pt (G.M.); cpjunior.adilson@gmail.com (A.d.P.J.); jepfernandes@me.com (J.F.);
sms@civil.uminho.pt (S.M.S.)
2Federal Center for Technological Education “Celso Suckow da Fonseca” (CEFET/RJ),
Rio de Janeiro-RJ 20271-110, Brazil; christiane.guarnier@cefet-rj.br
*Correspondence: ricardomateus@civil.uminho.pt; Tel.: +351-253-510-200
Received: 30 April 2020; Accepted: 2 June 2020; Published: 10 June 2020


Abstract:
The present research is focused on an experimental investigation to evaluate the mechanical,
durability, and thermal performance of compressed earth blocks (CEBs) produced in Portugal.
CEBs were analysed in terms of electrical resistivity, ultrasonic pulse velocity, compressive strength,
total water absorption, water absorption by capillarity, accelerated erosion test, and thermal
transmittance evaluated in a guarded hotbox setup apparatus. Overall, the results showed that
compressed earth blocks presented good mechanical and durability properties. Still, they had some
issues in terms of porosity due to the particle size distribution of soil used for their production.
The compressive strength value obtained was 9 MPa, which is considerably higher than the minimum
requirements for compressed earth blocks. Moreover, they presented a heat transfer coefficient of
2.66 W/(m
2·
K). This heat transfer coefficient means that this type of masonry unit cannot be used in
the building envelope without an additional thermal insulation layer but shows that they are suitable
to be used in partition walls. Although CEBs have promising characteristics when compared to
conventional bricks, results also showed that their proprieties could even be improved if optimisation
of the soil mixture is implemented.
Keywords:
compressed earth blocks (CEBs); compressive strength; durability; guarded hot box;
thermal transmittance
1. Introduction
Earth has been used as a building material since ancient times in several different ways around
the world [1–4]. Industrialised building systems and the dissemination of materials, like concrete [5],
have replaced earthen construction. Today, earthen construction is associated with poverty [
2
], and most
of this type of construction is located in developing countries. The continuous increase in the energy
cost of some building materials (cement and ceramic bricks) and environmental issues are promoting
the use of sustainable materials, such as the earthen materials known by their abundance and low-cost
production [5–7].
Compressed earth blocks (CEBs) are one of the most widespread earthen building techniques.
They represent a modern descendent of the moulded earth block, commonly called as the adobe
block [
7
]. The compaction of earth improves the quality and performance of the blocks [
4
] but also
promotes several environmental, social, and economic benefits [
8
,
9
]. Regarding the environmental
advantages of using earthen products, a previous study showed that in a cradle-to-gate analysis of
Energies 2020,13, 2978; doi:10.3390/en13112978 www.mdpi.com/journal/energies

Energies 2020,13, 2978 2 of 22
different walls, the use of earthen building elements could result in reducing the potential environmental
impacts by about 50% when compared to the use of conventional building elements [2].
Earthen construction is known to undergo rapid deterioration under severe weather conditions [
10
].
If not built adequately, earthen buildings have lower durability and are more vulnerable to extreme
weather conditions and rainfall than conventional buildings. This situation means higher maintenance
and repair costs during the life cycle of a building [11].
In the last few years, there has been increasing interest in overcoming the mechanical and durability
issues related to earthen blocks. Different stabilisation techniques were used to improve durability and
compressive strength [
6
,
11
–
13
]. Dynamic compaction alone or together with chemical stabilisation
using several additives has been shown to considerable improve the mechanical performance of
CEBs [
10
,
13
]. In contrast, compaction increases thermal conductivity [
14
]. The stabilisation process of
raw earth refers to any mechanical, physical, physicochemical, or combined methods that enhance
its properties [
10
]. Bahar et al. [
15
] studied the effect of several stabilisation methods on mechanical
properties. The results showed that the combination of compaction and cement stabilisation is
an effective solution for increasing the strength of earth blocks. Amoudi et al. [
16
,
17
] developed
an experimental program to study the mechanical properties of cement-stabilised earth blocks.
They verified that cement in the presence of water forms hydrated products that occupy the voids
and wrap the soil particles. That process leads to an improvement in compressive strength, water
absorption, dimensional stability, and durability. In many countries, CEBs are stabilised with cement
or lime, and there are successful examples of their use in the construction of buildings. Several
studies highlighted their lower construction cost, simpler construction processes, and the contribution
of this material to maintaining a better indoor environment quality when comparing to the use of
conventional building materials [
8
,
11
,
18
–
20
]. Besides that, some studies show that the addition of lime
to compressed earth blocks can improve their mechanical and hydrous properties [21,22].
Regarding the thermal proprieties of earthen building products, since they are massive,
they contribute to increasing the thermal inertia of the buildings. This feature can have a positive
influence on the thermal performance of buildings in certain climates. A previous study showed
that in locations with hot summers and temperate winters, such as the Mediterranean areas, earthen
construction could provide comfortable indoor temperature by passive means alone [
23
]. This property
can reduce heating and cooling energy needs and therefore contribute to lower life cycle environmental
and financial costs. Nevertheless, compared to the number of studies focusing on mechanical properties,
there are fewer studies related to the thermal properties of compressed earth blocks [
11
]. Adam and
Jones [
24
] measured the thermal conductivity of lime and cement stabilised hollow and massive earth
blocks using a guarded hot box method. The authors verified that the thermal conductivity was
higher on stabilised blocks (0.20 W/m
2·
K and 0.50 W/m
2·
K). The compressive strength used in the
compaction of the blocks, the type of soil, and the additives used can significatively influence the
thermal conductivity of a CEB building element [
24
]. For that reason, in the literature, it is possible to
find very different thermal conductivity values for earthen products. For example, according to the
Portuguese thermal regulation [
25
], the thermal conductivity to consider for adobe, rammed earth,
and compressed earth blocks is 1.10 W/(m
2·
K). At the same time, other studies show quite different
values, also depending on the considered earthen building technique—earth materials with fibres
(0.42–0.90 W/(m2·K)), adobe (0.46–0.81 W/(m2·K)), or rammed earth (0.35–0.70 W/(m2·K)) [1,26].
When designing a sustainable building, the design team must have comprehensive information
regarding the different building products they can use [
19
]. Information should include that related
to the life cycle environmental (e.g., embodied energy and global warming potential), functional
(e.g., mechanical and thermal) and economic (e.g., construction and maintenance cost) performances.
Based on this context, this research is within a series of studies that are being developed by
the same authors to develop comprehensive information about earthen construction. Past studies
include those related to analysing the contribution of this type of construction in improving the indoor
environmental quality [23] and reducing the embodied environmental impacts [2].

Energies 2020,13, 2978 3 of 22
The present research is focused on an experimental investigation to evaluate the mechanical,
durability and thermal performances of compressed earth blocks produced by a Portuguese company.
This study aims to analyse the functional quality of the abovementioned product and assess its potential
to be used in the construction of buildings.
2. Materials and Methods
The compressed earth blocks tested are a commercial product made by a manufacturer located in
the city of Serpa, district of Beja (southern Portugal), which is also a contractor that builds earthen
and conventional buildings. This contractor is one of the leading earth building systems builders in
Portugal. The share of the earthen building systems corresponds to around 12% of the total company’s
activity, and during the year 2014, the company produced 338 m
3
of rammed earth and 36 m
3
of
compressed earth blocks. Usually, rammed earth is used to build 60-cm-thick walls, and the dimensions
of the CEBs produced by this company varies. In this work, 30 cm
×
15 cm
×
7 cm compressed earth
blocks were studied, since it is the most common block produced by the company. Additionally,
this size is the most used in the Portuguese construction. The soil mixture is stabilised by using 6%
by weight (wt) of hydraulic lime and 1% wt of hydrated lime. The mix also uses water, generally
extracted on-site (groundwater) (10% by weight), which evaporates during the drying process. In the
majority of cases, earthen building elements are built from soil extracted from the construction site.
Additionally, according to the company’s data, the compressed earth blocks are made and compacted
using a mechanical tapping machine. The company provided compressed earth blocks and the soil
used for their production. They were experimentally analysed in different labs of the Department
of Civil Engineering of the University of Minho, located in the city of Guimar
ã
es, district of Braga
(northern Portugal).
2.1. Soil Characterisation
The soil was characterised in terms of particle size distribution, sand equivalent, clay content,
cohesion limits, and compaction properties. These properties evaluate the quality of soil to be used
in earthen construction. The particle size distribution was determined according to the EN 196:1966
standard [
27
]. The main goal of the sand equivalent test is to estimate the percentage of sand that exist
in a soil fraction with particles with less than 2 mm. This test was done according to EN 933-8:2002 [
28
].
The methylene blue test allows the quantification of clay content present in a soil sample through
the ionic change between the cations that exist in the soil particles and was done according to EN
933-9:2002 [
29
]. The cohesion limits of soil are fundamental for the final quality of CEBs. The main goal
of this test is assessing the liquid limit (L
L
), the plastic limit (L
P
), and the index of plasticity (I
P
) of the
soil. The cohesion limits were determined and calculated according to the EN 143:1969 standard [
30
].
The compaction properties of soil are fundamental in earthen products since there is a direct
relation between dry density and compressive strength of a product. A more compact product presents
higher strength. The main goal of the Proctor compaction test consists in analysing the optimum
water content. This water content corresponds to the water content of a soil that allows it to achieve
its dry density for specific energy of compaction. This test was done according to LNEC E 197:1966
standard [31], considering two types of compaction (light and hard) in a small mould.
Table 1and Figure 1summarise the characteristics of the soil used for CEBs production. The soil
presented a good particle size distribution and showed the four types of particles in significant
percentages (15.9% pebble, 47.2% sand, 17.6% silt, and 19.4% clay). As shown in Table 1, the soil has
a liquid limit of 29% and a plasticity index of 11%. Therefore, it can be classified as a fair to poor
clayed soil (type A6) according to the American Association of State Highway Transportation Officials
(AASHTO) system [
32
]. However, according to the CRATerre group [
33
], these figures are within the
limits of the recommended classes for soil to be used as a construction material. The methylene blue
test shows 2.28 g of methylene blue per 100 g of soil, indicating a low degree of expansion, as also
confirmed by the plasticity index, suggesting a low clay content in the studied soil [
34
]. This result

Energies 2020,13, 2978 4 of 22
is good since expansive soils are affected by humidity variations that change its consistency [
35
].
The analysed soil has a maximum dry density between 1.95 g/cm
3
and 1.99 g/cm
3
, which means that
this soil is classified as “very good” to be used as construction material [33].
Table 1. Particle size distribution and Atterberg limits of the soil used.
Property Parameter Value
Particle size distribution
Gravel (>4.75 mm) 15.90
Sand (2.00–0.06 mm) 47.20
Clay and silt (<0.06 mm) 36.95
Atterberg limits
Liquid limit Ll(%) 29.00
Plasticity limit Lp(%) 18.00
Plasticity index Ip(%) 11.00
Sand equivalent - (%) 23.49
Fine content by methylene blue test
Methylene blue value (g/100 g soil)
2.28
Modified Proctor test—Light Optimum water content (%) 12.00
Maximum dry density (g/cm3)1.95
Modified Proctor test—Heavy Optimum water content (%) 11.80
Maximum dry density (g/cm3)1.99
Energies 2020, 13, x FOR PEER REVIEW 4 of 21
consistency [36]. The analysed soil has a maximum dry density between 1.95 g/cm3 and 1.99 g/cm3,
which means that this soil is classified as “very good” to be used as construction material [34].
Figure 1. Particle size distribution of the soil, according to the results of the sedimentation test.
Table 1. Particle size distribution and Atterberg limits of the soil used.
Property Parameter Value
Particle size distribution
Gravel (> 4.75 mm) 15.90
Sand (2.00–0.06 mm) 47.20
Clay and silt (< 0.06 mm) 36.95
Atterberg limits
Liquid limit Ll (%) 29.00
Plasticity limit Lp (%) 18.00
Plasticity index Ip (%) 11.00
Sand equivalent - (%) 23.49
Fine content by methylene blue test Methylene blue value (g/100g soil) 2.28
Modified Proctor test—Light Optimum water content (%) 12.00
Maximum dry density (g/cm3) 1.95
Modified Proctor test—Heavy Optimum water content (%) 11.80
Maximum dry density (g/cm3) 1.99
2.2. Compressed Earth Blocks Characterisation
2.2.1. Electrical Resistivity
The electrical resistivity was measured using the ResipodProceq equipment, made by Proceq SA
(Schwerzenbach, Switzerland), which comprises four equidistant (38 mm) electrodes (Figure 2).
During this test, an alternate current was provided between the external electrodes and the electrical
potential difference between the internal electrodes was measured. The electrical resistivity was
measured through the Ohm’s law and computed by the equipment used. The tested samples were
the ones used for the water absorption by capillarity test after they achieved the saturation point.
Four measurements were done for each saturated compressed earth block sample.
(a)
(b)
Figure 2. Electric resistivity test: measurements were done in two samples faces—(a) width and (b)
length.
0.001 0.01 0.1 1 10 100
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Particle size
(
mm
)
Percent Passing (%)
Figure 1. Particle size distribution of the soil, according to the results of the sedimentation test.
2.2. Compressed Earth Blocks Characterisation
2.2.1. Electrical Resistivity
The electrical resistivity was measured using the ResipodProceq equipment, made by Proceq SA
(Schwerzenbach, Switzerland), which comprises four equidistant (38 mm) electrodes (Figure 2). During
this test, an alternate current was provided between the external electrodes and the electrical potential
difference between the internal electrodes was measured. The electrical resistivity was measured
through the Ohm’s law and computed by the equipment used. The tested samples were the ones used
for the water absorption by capillarity test after they achieved the saturation point. Four measurements
were done for each saturated compressed earth block sample.

Energies 2020,13, 2978 5 of 22
Energies 2020, 13, x FOR PEER REVIEW 4 of 21
consistency [36]. The analysed soil has a maximum dry density between 1.95 g/cm3 and 1.99 g/cm3,
which means that this soil is classified as “very good” to be used as construction material [34].
Figure 1. Particle size distribution of the soil, according to the results of the sedimentation test.
Table 1. Particle size distribution and Atterberg limits of the soil used.
Property Parameter Value
Particle size distribution
Gravel (> 4.75 mm) 15.90
Sand (2.00–0.06 mm) 47.20
Clay and silt (< 0.06 mm) 36.95
Atterberg limits
Liquid limit Ll (%) 29.00
Plasticity limit Lp (%) 18.00
Plasticity index Ip (%) 11.00
Sand equivalent - (%) 23.49
Fine content by methylene blue test Methylene blue value (g/100g soil) 2.28
Modified Proctor test—Light Optimum water content (%) 12.00
Maximum dry density (g/cm3) 1.95
Modified Proctor test—Heavy Optimum water content (%) 11.80
Maximum dry density (g/cm3) 1.99
2.2. Compressed Earth Blocks Characterisation
2.2.1. Electrical Resistivity
The electrical resistivity was measured using the ResipodProceq equipment, made by Proceq SA
(Schwerzenbach, Switzerland), which comprises four equidistant (38 mm) electrodes (Figure 2).
During this test, an alternate current was provided between the external electrodes and the electrical
potential difference between the internal electrodes was measured. The electrical resistivity was
measured through the Ohm’s law and computed by the equipment used. The tested samples were
the ones used for the water absorption by capillarity test after they achieved the saturation point.
Four measurements were done for each saturated compressed earth block sample.
(a)
(b)
Figure 2. Electric resistivity test: measurements were done in two samples faces—(a) width and (b)
length.
0.001 0.01 0.1 1 10 100
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90
95
100
Particle size
(
mm
)
Percent Passing (%)
Figure 2.
Electric resistivity test: measurements were done in two samples faces—(
a
) width and
(b) length.
2.2.2. Ultrasonic Pulse Velocity
The Ultrasonic Pulse Velocity (UPV) test evaluates some materials properties, such as elasticity
modulus, homogeneity, mechanical resistances, and cracking. It is also possible to calculate the
propagation velocity [
36
]. The UPV tests consists of measuring the time that a given sound pulse takes
to pass through a known section of a specimen. This is based on the wave propagation theory, where
a sound pulse propagates faster in a dense material and slowly in a porous material. It is therefore
possible to calculate the propagation velocity [
36
], and this test allows the indirect determination of
the intrinsic characteristics of a given sample [
36
]. There is almost nothing in the literature about the
use of this test in compressed earth blocks. Nevertheless, there are some studies that have already
been performed on rammed earth [
37
,
38
] that disclose that there is a relation between the UPV and
the compressive strength of earthen products. The UPV measurement was developed according to
EN 12504-4:2007 [
39
] in two directions (direct and indirect—see Figure 3). The measure of UPV in
the direct position was obtained with the transmitter and receiver transducers positioned on two
opposite sides. The indirect measurements were done by placing the transmitter on one face and the
receiver on a perpendicular side. An appropriate coupling gel was applied between the transducers
and the sample to prevent the existence of voids in the contact area. Three independent readings were
registered for each sample. Equation (1) is used to calculate the UPV, which is the ratio between the
distance (L) between the transductors (emission and receptor) and the propagation time (t).
UPV (m/s)=L
t(1)
Energies 2020, 13, x FOR PEER REVIEW 5 of 21
2.2.2. Ultrasonic Pulse Velocity
The Ultrasonic Pulse Velocity (UPV) test evaluates some materials properties, such as elasticity
modulus, homogeneity, mechanical resistances, and cracking. It is also possible to calculate the
propagation velocity [37]. The UPV tests consists of measuring the time that a given sound pulse
takes to pass through a known section of a specimen. This is based on the wave propagation theory,
where a sound pulse propagates faster in a dense material and slowly in a porous material. It is
therefore possible to calculate the propagation velocity [37], and this test allows the indirect
determination of the intrinsic characteristics of a given sample [37]. There is almost nothing in the
literature about the use of this test in compressed earth blocks. Nevertheless, there are some studies
that have already been performed on rammed earth [38,39] that disclose that there is a relation
between the UPV and the compressive strength of earthen products. The UPV measurement was
developed according to EN 12504-4:2007 [40] in two directions (direct and indirect—see Figure 3).
The measure of UPV in the direct position was obtained with the transmitter and receiver transducers
positioned on two opposite sides. The indirect measurements were done by placing the transmitter
on one face and the receiver on a perpendicular side. An appropriate coupling gel was applied
between the transducers and the sample to prevent the existence of voids in the contact area. Three
independent readings were registered for each sample. Equation (1) is used to calculate the UPV,
which is the ratio between the distance (L) between the transductors (emission and receptor) and the
propagation time (t).
 (m/s) = 
 (1)
(a)
(b)
Figure 3. Ultrasonic Pulse Velocity (UPV) test—(a) direct method and (b) indirect method.
2.2.3. Compressive Strength
The compressive strength test used a hydraulic press machine with a capacity of 3000 kN (Figure
4), coupled with a hydraulic control system, according to NP EN 772-1 [41]. For the test, two
transducers were used, one that belongs to the press and another external to measure the vertical
displacement (LVSTs). The test used displacement control, with a regular load velocity of 0.5 kN/s.
The experiment consisted of applying an increasing compressive load until the load achieved 40% to
50% of the failure value after registering the maximum load peak. Six samples were tested to assess
the compressive strength of CEBs.
Figure 3. Ultrasonic Pulse Velocity (UPV) test—(a) direct method and (b) indirect method.
2.2.3. Compressive Strength
The compressive strength test used a hydraulic press machine with a capacity of 3000 kN (Figure 4),
coupled with a hydraulic control system, according to NP EN 772-1 [
40
]. For the test, two transducers
were used, one that belongs to the press and another external to measure the vertical displacement

Energies 2020,13, 2978 6 of 22
(LVSTs). The test used displacement control, with a regular load velocity of 0.5 kN/s. The experiment
consisted of applying an increasing compressive load until the load achieved 40% to 50% of the failure
value after registering the maximum load peak. Six samples were tested to assess the compressive
strength of CEBs.
Energies 2020, 13, x FOR PEER REVIEW 6 of 21
Figure 4. Compressive strength test.
2.2.4. Total Water Absorption
The assessment of the total water absorption of a block is essential since it can be used for routine
quality checks, classification according to required durability and structural use, and to estimate the
volume of voids [4,42]. Usually, the less water a block absorbs and retains, the better its structural
performance and durability. Reducing the total water absorption capacity of a block has often been
considered as one way of improving its quality [42].
The total water absorption test consists of immersing a block in the water until no further
increase in apparent mass is observed. This experiment followed the LNEC E 394:1993 standard [43].
It is considered that there was no increase in the apparent mass when two consecutive measurements
did not differ by more than 0.1% by mass. The test was carried out at atmospheric pressure in which
three samples were immersed in water for 1, 2, 3, 4, and 5 hours. After each period, the surface of the
specimen was wiped with a cloth to remove any adsorbed water. Then the samples were weighed.
Initially, the test was done for 24 h, as recommended by the standard and observed in other studies
[4,42]. However, after 24 h immersion in water, the CEBs disintegrated, and it was not possible to
measure its wet weight. The percentage of water absorbed (A) was calculated using Equation (2). Wh
is the weight of the specimen after each period of immersion, and Ws is the dry block weight.

(%) = (−
)

 (2)
2.2.5. Water Absorption by Capillarity
This test consists of quantifying the amount of water absorbed by capillarity in the compressed
earth block. The experiment was performed in three entire blocks following the LNEC E 393:1993
standard [44]. This is a Portuguese standard for analysing the water absorption in concrete, and it
was used since the procedure is similar to the international standards specific for earthen products
[11,41,45]. Before being immersed, each specimen dried for 14 days in an oven at a controlled
temperature of 60 ± 5 °C. In the following step, each sample was weighed with a precision of 0.1 g,
and then its lower face was immersed in a 5 mm water bath. Samples were left in the bath for 10, 20,
30, 45, 60, and 90 min, and 2, 3, 4, 6, 24, and 72 h, to identify the water saturation point (Figure 5). The
water absorption by capillarity coefficient, Cb, was calculated for 10 min, according to
UNE 41410:2008 [45] and using Equation (3).
=100 × (−
)

√
 (3)
where
Cb is the water absorption by capillarity coefficient (g/cm2·min0.5);
M1 is the weight of the block after immersion in water (g);
M0 is the weight of the block before immersion in water (g);
S is the immersed area (cm2);
t is the immersion time (min).
Figure 4. Compressive strength test.
2.2.4. Total Water Absorption
The assessment of the total water absorption of a block is essential since it can be used for routine
quality checks, classification according to required durability and structural use, and to estimate the
volume of voids [
4
,
41
]. Usually, the less water a block absorbs and retains, the better its structural
performance and durability. Reducing the total water absorption capacity of a block has often been
considered as one way of improving its quality [41].
The total water absorption test consists of immersing a block in the water until no further increase
in apparent mass is observed. This experiment followed the LNEC E 394:1993 standard [
42
]. It is
considered that there was no increase in the apparent mass when two consecutive measurements did
not differ by more than 0.1% by mass. The test was carried out at atmospheric pressure in which three
samples were immersed in water for 1, 2, 3, 4, and 5 h. After each period, the surface of the specimen
was wiped with a cloth to remove any adsorbed water. Then the samples were weighed. Initially,
the test was done for 24 h, as recommended by the standard and observed in other studies [
4
,
41
].
However, after 24 h immersion in water, the CEBs disintegrated, and it was not possible to measure its
wet weight. The percentage of water absorbed (A) was calculated using Equation (2). W
h
is the weight
of the specimen after each period of immersion, and Wsis the dry block weight.
A(%)=(Wh−Ws)
Ws(2)
2.2.5. Water Absorption by Capillarity
This test consists of quantifying the amount of water absorbed by capillarity in the compressed
earth block. The experiment was performed in three entire blocks following the LNEC E 393:1993
standard [
43
]. This is a Portuguese standard for analysing the water absorption in concrete, and it was
used since the procedure is similar to the international standards specific for earthen products [
10
,
40
,
44
].
Before being immersed, each specimen dried for 14 days in an oven at a controlled temperature of
60 ±5◦C
. In the following step, each sample was weighed with a precision of 0.1 g, and then its lower
face was immersed in a 5 mm water bath. Samples were left in the bath for 10, 20, 30, 45, 60, and 90 min,
and 2, 3, 4, 6, 24, and 72 h, to identify the water saturation point (Figure 5). The water absorption
by capillarity coefficient, C
b
, was calculated for 10 min, according to UNE 41410:2008 [
44
] and using
Equation (3).
Cb=100 ×(M1−M0)
S√t(3)
where

Energies 2020,13, 2978 7 of 22
Cbis the water absorption by capillarity coefficient (g/cm2·min0.5);
M1is the weight of the block after immersion in water (g);
M0is the weight of the block before immersion in water (g);
Sis the immersed area (cm2);
tis the immersion time (min).
Energies 2020, 13, x FOR PEER REVIEW 7 of 21
(a)
(b)
(c)
(d)
Figure 5. Water absorption by capillarity test: (a) after 10 min of water immersion; (b) after 30 min of
water immersion; (c) after 45 min of water immersion; and (d) after 60 min of water immersion.
2.2.6. Accelerated Erosion
This test analyses the degradation process of a specimen caused by water falling on it. This
experiment verifies the surface resistance to erosion, thus evaluating the durability of the analysed
blocks. The test was performed according to NZS 4298:1998 [40], and a rain simulator was used
(Figure 6). The climate parameters used were the ones for Penhas Douradas region, Guarda district
since it is the Portuguese region with the highest precipitation values (1715 mm). In the experiment,
a direct rainfall exposure index (worst scenario) was considered, which means that a flow rate of
14.26 l/min was used in the rainfall simulation. The outlet pressure in the water nozzle was 45 kPa,
respecting the conditions recommended for erosion tests in the international standards.
(a)
(b)
(c)
(d)
Figure 6. Accelerated erosion test: (a) setup; (b) sample preparation; (c) and (d) sample test.
Figure 5.
Water absorption by capillarity test: (
a
) after 10 min of water immersion; (
b
) after 30 min of
water immersion; (c) after 45 min of water immersion; and (d) after 60 min of water immersion.
2.2.6. Accelerated Erosion
This test analyses the degradation process of a specimen caused by water falling on it.
This experiment verifies the surface resistance to erosion, thus evaluating the durability of the
analysed blocks. The test was performed according to NZS 4298:1998 [
39
], and a rain simulator was
used (Figure 6). The climate parameters used were the ones for Penhas Douradas region, Guarda
district since it is the Portuguese region with the highest precipitation values (1715 mm). In the
experiment, a direct rainfall exposure index (worst scenario) was considered, which means that a flow
rate of 14.26 L/min was used in the rainfall simulation. The outlet pressure in the water nozzle was
45 kPa, respecting the conditions recommended for erosion tests in the international standards.

Energies 2020,13, 2978 8 of 22
Energies 2020, 13, x FOR PEER REVIEW 7 of 21
(a)
(b)
(c)
(d)
Figure 5. Water absorption by capillarity test: (a) after 10 min of water immersion; (b) after 30 min of
water immersion; (c) after 45 min of water immersion; and (d) after 60 min of water immersion.
2.2.6. Accelerated Erosion
This test analyses the degradation process of a specimen caused by water falling on it. This
experiment verifies the surface resistance to erosion, thus evaluating the durability of the analysed
blocks. The test was performed according to NZS 4298:1998 [40], and a rain simulator was used
(Figure 6). The climate parameters used were the ones for Penhas Douradas region, Guarda district
since it is the Portuguese region with the highest precipitation values (1715 mm). In the experiment,
a direct rainfall exposure index (worst scenario) was considered, which means that a flow rate of
14.26 l/min was used in the rainfall simulation. The outlet pressure in the water nozzle was 45 kPa,
respecting the conditions recommended for erosion tests in the international standards.
(a)
(b)
(c)
(d)
Figure 6. Accelerated erosion test: (a) setup; (b) sample preparation; (c) and (d) sample test.
Figure 6. Accelerated erosion test: (a) setup; (b) sample preparation; (c) and (d) sample test.
2.2.7. Thermal Transmittance
The characterisation of the thermal properties of the CEBs is based on the analysis of the thermal
transmittance (U-value) of the product. The thermal transmittance was measured using a guarded hot
box set up apparatus, built for this study in the Department of Civil Engineering of the University
of Minho, according to ASTM C1363-11:2011 [
45
] (Figure 7a). The hot box consists of two five-sided
chambers (dimensions: 2.0 m
×
1.4 m
×
1.6 m), the cold and the hot one. The envelope is well
insulated, made of extruded polystyrene (20 cm; U =0.21 W/(m
2·
K)), to reduce the heat flux through
the envelope and minimise heat losses by conduction. The specimen is placed in the mounting ring
placed between the two chambers (Figure 7b). The setup is placed in an indoor environment with a
controlled temperature below to the ones in the measurement chambers.
The thermal transmittance of the sample is obtained by measuring the heat flux rate needed to
maintain the hot chamber at a steady temperature (in this study 35
±
5
◦
C). Two ventilation devices
were placed in the back wall of the cold chamber (Figure 7a), which allow the cold air to enter into the
chamber and the hot air to exit. The ventilation is necessary to maintain uniform heat flux conditions
through the specimen. In the hot chamber, there is a heating system, controlled by a temperature
controller, that controls the defined temperature. The temperature in the chambers was measured
by four thermocouples (two in each chamber—one in the middle of the chamber, and the other near
the sample). A heat flux sensor was installed in the centre of the sample (Figure 7c). Preliminary
calibration measurements were carried out successfully to evaluate the heat losses and the heat transfer
through a wall with known thermal transmittance.
In this study, the heat flux method was used to determine the U-value. There is a heat flux through
a material when there is a temperature difference between two sides. Heat flows from the warmer side to
the colder side. It is possible to calculate the U-value of a specimen using the standardised methodology
of ISO 9869-1:2014 [
46
] by assessing the heat flux together with the temperatures in both chambers.
In this experiment, the greenTEG gSKIN
®
U-Value Kit (KIT-2615C) was used to automatically quantify
the temperatures, the heat flux through the material and the U-value. The U-value is obtained from the
average values of the heat flux through a small CEBs wall sample (composed of three blocks) and the
temperature difference,
∆
T, between the chambers, using Equation (4). In this experiment, the heat
flux was assessed in two points of the CEBs wall.

Energies 2020,13, 2978 9 of 22
Energies 2020, 13, x FOR PEER REVIEW 8 of 21
2.2.7. Thermal Transmittance
The characterisation of the thermal properties of the CEBs is based on the analysis of the thermal
transmittance (U-value) of the product. The thermal transmittance was measured using a guarded
hot box set up apparatus, built for this study in the Department of Civil Engineering of the University
of Minho, according to ASTM C1363-11:2011 [46] (Figure 7a). The hot box consists of two five-sided
chambers (dimensions: 2.0 m × 1.4 m × 1.6 m), the cold and the hot one. The envelope is well insulated,
made of extruded polystyrene (20 cm; U = 0.21 W(m2·K)), to reduce the heat flux through the envelope
and minimise heat losses by conduction. The specimen is placed in the mounting ring placed between
the two chambers (Figure 7b). The setup is placed in an indoor environment with a controlled
temperature below to the ones in the measurement chambers.
The thermal transmittance of the sample is obtained by measuring the heat flux rate needed to
maintain the hot chamber at a steady temperature (in this study 35 ± 5 °C). Two ventilation devices
were placed in the back wall of the cold chamber (Figure 7a), which allow the cold air to enter into
the chamber and the hot air to exit. The ventilation is necessary to maintain uniform heat flux
conditions through the specimen. In the hot chamber, there is a heating system, controlled by a
temperature controller, that controls the defined temperature. The temperature in the chambers was
measured by four thermocouples (two in each chamber—one in the middle of the chamber, and the
other near the sample). A heat flux sensor was installed in the centre of the sample (Figure 7c).
Preliminary calibration measurements were carried out successfully to evaluate the heat losses and
the heat transfer through a wall with known thermal transmittance.
In this study, the heat flux method was used to determine the U-value. There is a heat flux
through a material when there is a temperature difference between two sides. Heat flows from the
warmer side to the colder side. It is possible to calculate the U-value of a specimen using the
standardised methodology of ISO 9869-1:2014 [47] by assessing the heat flux together with the
temperatures in both chambers. In this experiment, the greenTEG gSKIN® U-Value Kit (KIT-2615C)
was used to automatically quantify the temperatures, the heat flux through the material and the U-
value. The U-value is obtained from the average values of the heat flux through a small CEBs wall
sample (composed of three blocks) and the temperature difference, ΔT, between the chambers, using
Equation 4. In this experiment, the heat flux was assessed in two points of the CEBs wall.
− = ∑


∑∆


(/(·
), (4)
where
n is the total number of data points;
φ is the heat flux in (W/m2);
ΔT is the temperature (°C) difference between the two sides of the specimen.
Energies 2020, 13, x FOR PEER REVIEW 9 of 21
(a)
(b)
(c)
Figure 7. Hot box apparatus. Legend: (a) hotbox closed; (b) cold chamber, mounting ring, and hot
chamber; (c) longitudinal plan view of the hotbox apparatus and position of the measurement
equipment.
Figure 7.
Hot box apparatus. Legend: (
a
) hotbox closed; (
b
) cold chamber, mounting ring, and hot
chamber; (
c
) longitudinal plan view of the hotbox apparatus and position of the measurement equipment.

Energies 2020,13, 2978 10 of 22
U−value =Pn
j=1ϕj
Pn
j=1∆TjW/(m2·K), (4)
where
nis the total number of data points;
ϕis the heat flux in (W/m2);
∆Tis the temperature (◦C) difference between the two sides of the specimen.
3. Results
In this section, the mechanical, durability, and thermal characterisation of the CEBs will be
described and discussed.
3.1. Electrical Resistivity
Electrical resistivity was measured to analyse the porosity of CEBs, and the results are presented in
Figure 8. CEBs have electrical conductivity mainly because ions can propagate in their body. Electrical
resistivity is directly dependent on CEBs permeability. In water-saturated CEBs with higher porosity,
the propagation of ions is easier, and therefore, there is a lower electrical resistivity [
47
]. From Figure 8,
it is possible to conclude that the measurements done in the direction of the bigger dimension (length)
of the sample showed similar results for all samples. However, in the measurements carried out in
the other direction (width), sample 2 presented higher values than samples 1 and 3. This result can
be an indication that sample 2 was denser, with fewer pores or with pores with smaller dimensions
(meaning reduced permeability and conductibility). These outputs disclose some disparity between
the porosity of tested CEB samples.
Energies 2020, 13, x FOR PEER REVIEW 10 of 21
samples 1 and 3. This result can be an indication that sample 2 was denser, with fewer pores or with
pores with smaller dimensions (meaning reduced permeability and conductibility). These outputs
disclose some disparity between the porosity of tested CEB samples.
Figure 8. Results of the electrical resistivity tests of the three specimens carried out in the two
directions.
3.2. Ultrasonic Pulse Velocity
Figure 9 presents the results for the UPV for each sample, using the same samples used in the
electrical resistivity test. Five measurements were carried out for every sample (for each direction,
direct and indirect) and results presented in Figure 9 are the average of the results obtained for each
sample.
A sonic impulse propagates with lower velocity in a porous body and with higher velocity in a
denser one. Therefore, according to the analysis of the results, it is possible to conclude that sample
2 presented slightly lower UPV, which can be an indication of a higher number of voids. These results
are similar to the electrical resistivity test results. The sample performance differences could be due
to the incorrect homogenisation of the soil mixture and/or cracking.
Figure 9. Ultrasonic Pulse Velocity measurements.
3.3. Compressive Strength
The compressive strength test is considered a reference test for CEBs since it is regarded as an
essential indicator of masonry strength. Figure 10 presents the results obtained for the compressive
strength of six samples, as well as the average value obtained for this parameter. The results showed
a variation on compressive strength between 7.8 MPa and 11.0 MPa, being the average 9.0 ± 1.3 MPa.
0
0.5
1
1.5
2
2.5
3
3.5
123
Electri cal Resist ivity [ kΏm.cm]
Test sample
Len
g
th Width
0
200
400
600
800
1000
1200
1400
1600
180
0
123
Ultrasonic Pulse Velocity [m/s]
Test sample
Direct Indirect
Figure 8.
Results of the electrical resistivity tests of the three specimens carried out in the two directions.
3.2. Ultrasonic Pulse Velocity
Figure 9presents the results for the UPV for each sample, using the same samples used in the
electrical resistivity test. Five measurements were carried out for every sample (for each direction, direct
and indirect) and results presented in Figure 9are the average of the results obtained for each sample.

Energies 2020,13, 2978 11 of 22
Energies 2020, 13, x FOR PEER REVIEW 10 of 21
samples 1 and 3. This result can be an indication that sample 2 was denser, with fewer pores or with
pores with smaller dimensions (meaning reduced permeability and conductibility). These outputs
disclose some disparity between the porosity of tested CEB samples.
Figure 8. Results of the electrical resistivity tests of the three specimens carried out in the two
directions.
3.2. Ultrasonic Pulse Velocity
Figure 9 presents the results for the UPV for each sample, using the same samples used in the
electrical resistivity test. Five measurements were carried out for every sample (for each direction,
direct and indirect) and results presented in Figure 9 are the average of the results obtained for each
sample.
A sonic impulse propagates with lower velocity in a porous body and with higher velocity in a
denser one. Therefore, according to the analysis of the results, it is possible to conclude that sample
2 presented slightly lower UPV, which can be an indication of a higher number of voids. These results
are similar to the electrical resistivity test results. The sample performance differences could be due
to the incorrect homogenisation of the soil mixture and/or cracking.
Figure 9. Ultrasonic Pulse Velocity measurements.
3.3. Compressive Strength
The compressive strength test is considered a reference test for CEBs since it is regarded as an
essential indicator of masonry strength. Figure 10 presents the results obtained for the compressive
strength of six samples, as well as the average value obtained for this parameter. The results showed
a variation on compressive strength between 7.8 MPa and 11.0 MPa, being the average 9.0 ± 1.3 MPa.
0
0.5
1
1.5
2
2.5
3
3.5
123
Electri cal Resist ivity [ kΏm.cm]
Test sample
Len
g
th Width
0
200
400
600
800
1000
1200
1400
1600
180
0
123
Ultrasonic Pulse Velocity [m/s]
Test sample
Direct Indirect
Figure 9. Ultrasonic Pulse Velocity measurements.
A sonic impulse propagates with lower velocity in a porous body and with higher velocity in a
denser one. Therefore, according to the analysis of the results, it is possible to conclude that sample 2
presented slightly lower UPV, which can be an indication of a higher number of voids. These results
are similar to the electrical resistivity test results. The sample performance differences could be due to
the incorrect homogenisation of the soil mixture and/or cracking.
3.3. Compressive Strength
The compressive strength test is considered a reference test for CEBs since it is regarded as an
essential indicator of masonry strength. Figure 10 presents the results obtained for the compressive
strength of six samples, as well as the average value obtained for this parameter. The results showed a
variation on compressive strength between 7.8 MPa and 11.0 MPa, being the average 9.0
±
1.3 MPa.
These values are very good ones since it is known that the minimum compressive strength requirements
for CEBs, varying between 1.0 MPa and 2.8 MPa [
7
,
21
]. These higher values can be related to the
compaction process used since compacting the soil using a press improves the quality of the material.
The higher density obtained by compaction significantly increases the compressive strength of the
blocks [
48
]. Another reason for the higher compressive strength is the presence of lime in the mixture.
Lime allows the development of calcium silicate hydrate (CSH) together with the formation of minor
amounts of calcite, which causes increased strength [21].
Energies 2020, 13, x FOR PEER REVIEW 11 of 21
These values are very good ones since it is known that the minimum compressive strength
requirements for CEBs, varying between 1.0 MPa and 2.8 MPa [7,22]. These higher values can be
related to the compaction process used since compacting the soil using a press improves the quality
of the material. The higher density obtained by compaction significantly increases the compressive
strength of the blocks [49]. Another reason for the higher compressive strength is the presence of lime
in the mixture. Lime allows the development of calcium silicate hydrate (CSH) together with the
formation of minor amounts of calcite, which causes increased strength [22].
Figure 10. Compressive strength results.
3.4. Total Water Absorption
Analysing Figure 11, the maximum values obtained for the total water absorption for each
sample were 8.7%, 11.3%, and 10.0% for samples 1, 2, and 3, respectively. It is possible to conclude
that the total water absorption varied between 8.7 and 11.3%, being these values favourable when
compared with clay bricks (0%–30%), concrete blocks (4%–25%), or calcium silicate bricks (6%–16%)
[4]. Although this result seems good, the fast absorption and desegregation of blocks can influence
the durability negatively. Total water absorption is influenced by the granulometry of the soil and
compaction pressure. These two aspects have a meaningful impact in the density, mechanical
strength, compressibility, permeability, and porosity of CEBs [4,49].
Figure 11. Total water absorption results. The results highlighted by the orange box represents the
maximum values obtained in the total water absorption test.
3.5. Water Absorption by Capillarity
0
2
4
6
8
10
12
123456
Compressive strength [MPa]
Test sample
9.0 ± 1.3 MPa
0
2
4
6
8
10
12
12345
Total water absorption [%]
Time [h]
Sample 1 Sample 2 Sample 3
Figure 10. Compressive strength results.

Energies 2020,13, 2978 12 of 22
3.4. Total Water Absorption
Analysing Figure 11, the maximum values obtained for the total water absorption for each sample
were 8.7%, 11.3%, and 10.0% for samples 1, 2, and 3, respectively. It is possible to conclude that the total
water absorption varied between 8.7 and 11.3%, being these values favourable when compared with
clay bricks (0–30%), concrete blocks (4–25%), or calcium silicate bricks (6–16%) [
4
]. Although this result
seems good, the fast absorption and desegregation of blocks can influence the durability negatively.
Total water absorption is influenced by the granulometry of the soil and compaction pressure. These two
aspects have a meaningful impact in the density, mechanical strength, compressibility, permeability,
and porosity of CEBs [4,48].
Energies 2020, 13, x FOR PEER REVIEW 11 of 21
These values are very good ones since it is known that the minimum compressive strength
requirements for CEBs, varying between 1.0 MPa and 2.8 MPa [7,22]. These higher values can be
related to the compaction process used since compacting the soil using a press improves the quality
of the material. The higher density obtained by compaction significantly increases the compressive
strength of the blocks [49]. Another reason for the higher compressive strength is the presence of lime
in the mixture. Lime allows the development of calcium silicate hydrate (CSH) together with the
formation of minor amounts of calcite, which causes increased strength [22].
Figure 10. Compressive strength results.
3.4. Total Water Absorption
Analysing Figure 11, the maximum values obtained for the total water absorption for each
sample were 8.7%, 11.3%, and 10.0% for samples 1, 2, and 3, respectively. It is possible to conclude
that the total water absorption varied between 8.7 and 11.3%, being these values favourable when
compared with clay bricks (0%–30%), concrete blocks (4%–25%), or calcium silicate bricks (6%–16%)
[4]. Although this result seems good, the fast absorption and desegregation of blocks can influence
the durability negatively. Total water absorption is influenced by the granulometry of the soil and
compaction pressure. These two aspects have a meaningful impact in the density, mechanical
strength, compressibility, permeability, and porosity of CEBs [4,49].
Figure 11. Total water absorption results. The results highlighted by the orange box represents the
maximum values obtained in the total water absorption test.
3.5. Water Absorption by Capillarity
0
2
4
6
8
10
12
123456
Compressive strength [MPa]
Test sample
9.0 ± 1.3 MPa
0
2
4
6
8
10
12
12345
Total water absorption [%]
Time [h]
Sample 1 Sample 2 Sample 3
Figure 11.
Total water absorption results. The results highlighted by the orange box represents the
maximum values obtained in the total water absorption test.
3.5. Water Absorption by Capillarity
CEBs used in a structure may undergo alternating phenomena of absorption and release of water,
mainly because of the capillarity effect [
49
]. The curves shown in Figure 12 illustrate the variation of
water absorption by capillarity until the total saturation of each sample is reached, as a function of the
square root of test time. Figure 13 shows the variation of the water absorption coefficient (average
of the three samples tested) of compressed earth blocks as a function of the square root of test time.
This coefficient was determined for all test periods. However, the literature mentions that the value at
the end of 10 min (represented with an orange rectangle in Figure 13) is representative of the behaviour
of masonry exposed to a violent storm [
21
]. At the end of this period, the water absorption coefficient
value was 34.6, and therefore, the blocks are classified as having high capillarity [
21
]. This result can be
related to the fact that the soils used to manufacture the CEBs have a high percentage of sand (Table 1).
In this study, the high presence of bigger particles (in terms of size) in CEBs seems to be an issue related
with the quality of the particle size distribution of the soil, used for the CEBs production, than the
quality of the soil itself, which did not cause good reaction with lime and resulted in CEBs with high
porosity [10].

Energies 2020,13, 2978 13 of 22
Energies 2020, 13, x FOR PEER REVIEW 12 of 21
CEBs used in a structure may undergo alternating phenomena of absorption and release of
water, mainly because of the capillarity effect [50]. The curves shown in Figure 12 illustrate the
variation of water absorption by capillarity until the total saturation of each sample is reached, as a
function of the square root of test time. Figure 13 shows the variation of the water absorption
coefficient (average of the three samples tested) of compressed earth blocks as a function of the square
root of test time. This coefficient was determined for all test periods. However, the literature mentions
that the value at the end of 10 min (represented with an orange rectangle in Figure 13) is
representative of the behaviour of masonry exposed to a violent storm [22]. At the end of this period,
the water absorption coefficient value was 34.6, and therefore, the blocks are classified as having high
capillarity [22]. This result can be related to the fact that the soils used to manufacture the CEBs have
a high percentage of sand (Table 1). In this study, the high presence of bigger particles (in terms of
size) in CEBs seems to be an issue related with the quality of the particle size distribution of the soil,
used for the CEBs production, than the quality of the soil itself, which did not cause good reaction
with lime and resulted in CEBs with high porosity [11].
Figure 12. Total water absorption results.
Figure 13. Variation of the average water absorption during the test period.
3.6. Accelerated Erosion
The degradation analysis of CEBs was done, and the results are presented in Figures 14–16. A
total of seven blocks were analysed in this test. Initially, three blocks were tested for one hour. Since
blocks did not present any type of erosion, this test was repeated in four additional blocks, and the
results were the same. Based on these results and according to NP EN 12504-4 [40], CEBs were
classified with an erosion index of 1 (erosion depth between 0–20 mm/h), which means that they had
very good results in terms of durability.
0
1
2
3
4
5
6
7
8
9
10
0 10203040506070
Water ab sorption by capillarity
[g/cm
2
]
Time (min
0.5
)
Average Sample 1 Sample 2 Sample 3
0
5
10
15
20
25
30
35
40
0 3 4 5 7 8 9 111315193866
Average water absorption coefficient
[g/cm
2
.min
0.5
]
Time (min
0.5
)
Figure 12. Total water absorption results.
Energies 2020, 13, x FOR PEER REVIEW 12 of 21
CEBs used in a structure may undergo alternating phenomena of absorption and release of
water, mainly because of the capillarity effect [50]. The curves shown in Figure 12 illustrate the
variation of water absorption by capillarity until the total saturation of each sample is reached, as a
function of the square root of test time. Figure 13 shows the variation of the water absorption
coefficient (average of the three samples tested) of compressed earth blocks as a function of the square
root of test time. This coefficient was determined for all test periods. However, the literature mentions
that the value at the end of 10 min (represented with an orange rectangle in Figure 13) is
representative of the behaviour of masonry exposed to a violent storm [22]. At the end of this period,
the water absorption coefficient value was 34.6, and therefore, the blocks are classified as having high
capillarity [22]. This result can be related to the fact that the soils used to manufacture the CEBs have
a high percentage of sand (Table 1). In this study, the high presence of bigger particles (in terms of
size) in CEBs seems to be an issue related with the quality of the particle size distribution of the soil,
used for the CEBs production, than the quality of the soil itself, which did not cause good reaction
with lime and resulted in CEBs with high porosity [11].
Figure 12. Total water absorption results.
Figure 13. Variation of the average water absorption during the test period.
3.6. Accelerated Erosion
The degradation analysis of CEBs was done, and the results are presented in Figures 14–16. A
total of seven blocks were analysed in this test. Initially, three blocks were tested for one hour. Since
blocks did not present any type of erosion, this test was repeated in four additional blocks, and the
results were the same. Based on these results and according to NP EN 12504-4 [40], CEBs were
classified with an erosion index of 1 (erosion depth between 0–20 mm/h), which means that they had
very good results in terms of durability.
0
1
2
3
4
5
6
7
8
9
10
0 10203040506070
Water ab sorption by capillarity
[g/cm
2
]
Time (min
0.5
)
Average Sample 1 Sample 2 Sample 3
0
5
10
15
20
25
30
35
40
0 3 4 5 7 8 9 111315193866
Average water absorption coefficient
[g/cm
2
.min
0.5
]
Time (min
0.5
)
Figure 13. Variation of the average water absorption during the test period.
3.6. Accelerated Erosion
The degradation analysis of CEBs was done, and the results are presented in Figures 14–16. A total
of seven blocks were analysed in this test. Initially, three blocks were tested for one hour. Since blocks
did not present any type of erosion, this test was repeated in four additional blocks, and the results
were the same. Based on these results and according to NP EN 12504-4 [
39
], CEBs were classified
with an erosion index of 1 (erosion depth between 0–20 mm/h), which means that they had very good
results in terms of durability.
To understand how these blocks behave if exposed for more time to the erosion test and if there is
any relation with the other physical properties mentioned before, the analysis was extended for one
additional hour.

Energies 2020,13, 2978 14 of 22
Energies 2020, 13, x FOR PEER REVIEW 13 of 21
To understand how these blocks behave if exposed for more time to the erosion test and if there
is any relation with the other physical properties mentioned before, the analysis was extended for
one additional hour.
After two hours of water exposition, CEBs presented different behaviour. Sample 1 did not show
significant damage in the majority of faces exposed to water but presented a loss in one part of the
block where there was already a small defect before the test (Figure 14). Sample 2 showed similar
behaviour to sample 1. Samples 3 (Figure 15), 4m and 5 presented some damage in the two smaller
faces, even though the face in contact with the water was the one with a larger area (highlighted with
a red cross in Figure 14). Samples 6 (Figure 16) and 7 suffered significant damage, and at the end of
two hours, they were almost destroyed. It is then essential to analyse the reasoning behind the
different deterioration levels of CEBs since they were manufactured using the same soil, mixture, and
compaction process. The most probable explanation for the differences is the inadequate particle size
distribution in the soil used. According to the literature in the field of earthen blocks, the soil should
only present particles below 5 mm of diameter [49]. Nevertheless, it was possible to see particles with
higher dimensions in the most damaged samples (Figure 16d,f,h)). These big size particles affected
the homogeneity of the mixture, which negatively influenced the porosity and the porous structure
of the CEBs. The lack of uniformity in the structure of the blocks worsens their behaviour to water.
This problem also explains the results achieved in the total water absorption and water absorption
by capillarity tests. The presented porosity and durability issues can be minimised if proper soil
preparation and/or selection is considered [19].
(a)
(b)
(c)
(d)
Energies 2020, 13, x FOR PEER REVIEW 14 of 21
(e)
(f)
(g)
(h)
Figure 14. Accelerated erosion test. (a), (c), (e), and (g)—four sides of the exposed face of sample 1
before the test; (b), (d), (f), and (h)—four sides of the exposed face of sample 1 after two hours of
testing.
(a)
(b)
(c)
(d)
Figure 14.
Accelerated erosion test. (
a
,
c
,
e
,
g
)—four sides of the exposed face of sample 1 before the test;
(b,d,f,h)—four sides of the exposed face of sample 1 after two hours of testing.

Energies 2020,13, 2978 15 of 22
Energies 2020, 13, x FOR PEER REVIEW 14 of 21
(e)
(f)
(g)
(h)
Figure 14. Accelerated erosion test. (a), (c), (e), and (g)—four sides of the exposed face of sample 1
before the test; (b), (d), (f), and (h)—four sides of the exposed face of sample 1 after two hours of
testing.
(a)
(b)
(c)
(d)
Energies 2020, 13, x FOR PEER REVIEW 15 of 21
(e)
(f)
(g)
(h)
Figure 15. Accelerated erosion test. (a), (c), (e), and (g)—four sides of the exposed face of sample 3
before the test; (b), (d), (f), and (h)—four sides of exposure face of sample 3 after two hours testing.
(a)
(b)
Figure 15.
Accelerated erosion test. (
a
,
c
,
e
,
g
)—four sides of the exposed face of sample 3 before the test;
(b,d,f,h)—four sides of exposure face of sample 3 after two hours testing.

Energies 2020,13, 2978 16 of 22
Energies 2020, 13, x FOR PEER REVIEW 15 of 21
(e)
(f)
(g)
(h)
Figure 15. Accelerated erosion test. (a), (c), (e), and (g)—four sides of the exposed face of sample 3
before the test; (b), (d), (f), and (h)—four sides of exposure face of sample 3 after two hours testing.
(a)
(b)
Energies 2020, 13, x FOR PEER REVIEW 16 of 21
(c) (d)
(e)
(f)
(g)
(h)
Figure 16. Accelerated erosion test. (a), (c), (e), and (g)—four sides of the exposed face of sample 6
before the test; (b), (d), (f), and (h)—four sides of the exposed face of sample 6 after testing.
3.7. Thermal Transmittance
Regarding the thermal transmittance, Figure 17 shows the measurement results for the XPS wall
(Figure 17a) and the compressed earth blocks small wall (Figure 17b,c). From the analysis of Figure
17, it is possible to verify that the values measured for temperature and the heat flux were very stable
during the test period, in both cases. The temperature in the hot chamber practically did not change
since the heat input to the box was controlled so that the temperature established was maintained (35
°C). The heating system turns on when the temperature drops below 35 °C and turns off when the
temperature rises to 40 °C. The heat flux is presented as negative values due to heat flux sensor
placement in the sample.
(a)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-30
-20
-10
0
10
20
30
40
50
U-value [W/( m
2
K)]
Temperature [ºC] /
Heat Flux [W/m
2
]
Temperature, h ot chamber [ºC] Temperature, cold chamber [ºC]
Heat Flux [W /m2] U-Value [W/(m2/K)]
Figure 16.
Accelerated erosion test. (
a
,
c
,
e
,
g
)—four sides of the exposed face of sample 6 before the test;
(b,d,f,h)—four sides of the exposed face of sample 6 after testing.

Energies 2020,13, 2978 17 of 22
After two hours of water exposition, CEBs presented different behaviour. Sample 1 did not show
significant damage in the majority of faces exposed to water but presented a loss in one part of the block
where there was already a small defect before the test (Figure 14). Sample 2 showed similar behaviour
to sample 1. Samples 3 (Figure 15), 4 and 5 presented some damage in the two smaller faces, even
though the face in contact with the water was the one with a larger area (highlighted with a red cross in
Figure 14). Samples 6 (Figure 16) and 7 suffered significant damage, and at the end of two hours, they
were almost destroyed. It is then essential to analyse the reasoning behind the different deterioration
levels of CEBs since they were manufactured using the same soil, mixture, and compaction process.
The most probable explanation for the differences is the inadequate particle size distribution in the soil
used. According to the literature in the field of earthen blocks, the soil should only present particles
below 5 mm of diameter [
48
]. Nevertheless, it was possible to see particles with higher dimensions
in the most damaged samples (Figure 16d,f,h). These big size particles affected the homogeneity of
the mixture, which negatively influenced the porosity and the porous structure of the CEBs. The
lack of uniformity in the structure of the blocks worsens their behaviour to water. This problem also
explains the results achieved in the total water absorption and water absorption by capillarity tests. The
presented porosity and durability issues can be minimised if proper soil preparation and/or selection is
considered [18].
3.7. Thermal Transmittance
Regarding the thermal transmittance, Figure 17 shows the measurement results for the XPS wall
(Figure 17a) and the compressed earth blocks small wall (Figure 17b,c). From the analysis of Figure 17,
it is possible to verify that the values measured for temperature and the heat flux were very stable
during the test period, in both cases. The temperature in the hot chamber practically did not change
since the heat input to the box was controlled so that the temperature established was maintained
(35
◦
C). The heating system turns on when the temperature drops below 35
◦
C and turns offwhen
the temperature rises to 40
◦
C. The heat flux is presented as negative values due to heat flux sensor
placement in the sample.
The results of the preliminary calibration measurements carried out, using a reference sample with
known thermal transmittance (XPS 20 cm), showed an agreement with the technical data provided by
the manufacturer (U-value of 0.21 W/(m
2·
K) and thermal conductivity of 0.60 W/(m
·
K). The measured
thermal transmittance of the 15 cm CEBs wall was of 2.65
±
0.16 W/(m
2·
K) (thermal resistance of
0.21 (m2·K)/W on average).
The results indicate that the thermal conductivity of the CEBs studied is significantly lower than
the reference values (1.1 W/(m
·
K)) listed by the Portuguese National Laboratory of Civil Engineering
(LNEC) [
25
]. Considering the thickness of the CEBs wall analysed (15 cm), the thermal transmittance
of the CEB wall would be approximately 3.26 W/(m
2·
K) (thermal resistance of 0.31 (m
2·
K)/W)) (for
external walls). The thermal transmittance measured for the CEB wall sample was lower than the value
obtained from technical data of LNEC. This result can be explained due to a higher porosity of these
blocks, which increased their thermal resistance. This result is in accordance with the other analysis
made in those blocks; as was seen in the accelerated erosion test, the CEBs presented a different soil
composition, showing in some samples soil particles with a size larger than 5 mm, which leads to a
higher thermal conductivity (Figure 16). Moreover, the results could be better if the granulometry of
the soil was optimised, since in the experiments found soil particles larger than 5 mm and some small
rocks that increase thermal transmittance.

Energies 2020,13, 2978 18 of 22
Energies 2020, 13, x FOR PEER REVIEW 16 of 21
(c) (d)
(e)
(f)
(g)
(h)
Figure 16. Accelerated erosion test. (a), (c), (e), and (g)—four sides of the exposed face of sample 6
before the test; (b), (d), (f), and (h)—four sides of the exposed face of sample 6 after testing.
3.7. Thermal Transmittance
Regarding the thermal transmittance, Figure 17 shows the measurement results for the XPS wall
(Figure 17a) and the compressed earth blocks small wall (Figure 17b,c). From the analysis of Figure
17, it is possible to verify that the values measured for temperature and the heat flux were very stable
during the test period, in both cases. The temperature in the hot chamber practically did not change
since the heat input to the box was controlled so that the temperature established was maintained (35
°C). The heating system turns on when the temperature drops below 35 °C and turns off when the
temperature rises to 40 °C. The heat flux is presented as negative values due to heat flux sensor
placement in the sample.
(a)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
-30
-20
-10
0
10
20
30
40
50
U-value [W/(m
2
K)]
Temperature [ºC] /
Heat Flux [W/m
2
]
Temperature, h ot chamber [ºC] Temperature, cold chamber [ºC]
Heat Flux [W /m2] U-Value [W/(m2/K)]
Energies 2020, 13, x FOR PEER REVIEW 17 of 21
(b)
(c)
Figure 17. Measurements of the heat flux, hot and cold superficial temperatures and U-value in the
XPS wall (a) and in the small CEBs wall (b, c).
The results of the preliminary calibration measurements carried out, using a reference sample
with known thermal transmittance (XPS 20 cm), showed an agreement with the technical data
provided by the manufacturer (U-value of 0.21 W/(m2·K) and thermal conductivity of 0.60 W/(m·K).
The measured thermal transmittance of the 15 cm CEBs wall was of 2.65 ± 0.16 W/(m2·K) (thermal
resistance of 0.21 (m2·K)/W on average).
The results indicate that the thermal conductivity of the CEBs studied is significantly lower than
the reference values (1.1 W/(m·K)) listed by the Portuguese National Laboratory of Civil Engineering
(LNEC) [26]. Considering the thickness of the CEBs wall analysed (15 cm), the thermal transmittance
of the CEB wall would be approximately 3.26 W/(m2·K) (thermal resistance of 0.31 (m2·K)/W)) (for
external walls). The thermal transmittance measured for the CEB wall sample was lower than the
value obtained from technical data of LNEC. This result can be explained due to a higher porosity of
these blocks, which increased their thermal resistance. This result is in accordance with the other
analysis made in those blocks; as was seen in the accelerated erosion test, the CEBs presented a
different soil composition, showing in some samples soil particles with a size larger than 5 mm, which
leads to a higher thermal conductivity (Figure 16). Moreover, the results could be better if the
granulometry of the soil was optimised, since in the experiments found soil particles larger than 5
mm and some small rocks that increase thermal transmittance.
4. Discussion
The compressed earth blocks characterisation is summarised in Table 2. Overall, the results are
consistent and show that these blocks presented good mechanical and durability properties and
better thermal performance than the reference values listed in the technical data for Portugal, but
they also present porosity issues. The non-destructive (electrical resistivity and ultrasonic pulse
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
-60
-40
-20
0
20
40
60
U-value [W/(m
2
K)]
Temperature [ºC] /
Heat Flux [W/m
2
]
Temperatur e, hot chamber [ºC] Temperature, cold chamber [ºC]
Heat Flux [W/m2 ] U-Value [W/(m2/K)]
0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
-60
-40
-20
0
20
40
60
U-value [W/(m
2
K)]
Temperature [º C] /
Heat Flux [W/m
2
]
Temperature, hot chamber [ºC] Temperature, cold chamber [ºC]
Heat Flux [W/m2 ] U-Value [W/(m2/K)]
Figure 17.
Measurements of the heat flux, hot and cold superficial temperatures and U-value in the
XPS wall (a) and in the small CEBs wall (b,c).
4. Discussion
The compressed earth blocks characterisation is summarised in Table 2. Overall, the results are
consistent and show that these blocks presented good mechanical and durability properties and better
thermal performance than the reference values listed in the technical data for Portugal, but they also
present porosity issues. The non-destructive (electrical resistivity and ultrasonic pulse velocity) and
the destructive (water absorption) tests for porosity analysis showed similar results. CEBs samples
presented heterogeneity on their mixture composition, which led to the production of blocks with

Energies 2020,13, 2978 19 of 22
different porosities. The high values obtained in the water absorption test highlight that this is the most
problematic characteristic of the CEBs. Contrary, to the other results, the compressive strength analysis
and the accelerated erosion test presented significant results. The compressive strength obtained was
approximately three times higher than the minimum requirement for CEBs. Moreover, these CEBs
were classified as erosion index of 1, which means that they have an erosion depth between 0–20 mm/h,
are very resistant, and have good durability properties.
Table 2. Thermophysical proprieties of the analysed compressed earth blocks.
Property Average Standard
Deviation
Coefficient of
Variance (%)
Electrical resistivity (kΩm·cm) Length 1.07 0.07 6.63
Width 2.54 0.30 11.81
Ultrasonic pulse velocity (m/s) Direct 908.34 51.97 5.72
Indirect 1647.21 12.32 0.75
Compressive strength (MPa) 9.01 1.25 13.90
Total water absorption (%) 9.98 1.29 12.96
Water absorption by capillarity
(g/cm2·min0.5)34.62 5.13 14.82
Heat Transfer Coefficient (W/m2·K) 2.65 0.15 5.59
Thermal resistance (m2·K/W) 0.21 - -
Thermal conductivity(W/m·K) 0.60 - -
One of the essential features of these building elements is thermal resistance. In this study, thermal
transmittance was analysed, corresponding to a U-value of 2.66 W/(m
2·
K). The results seem to be
in accordance with the results obtained for the quality and durability parameters, showing that the
results could be better if the granulometry of the soil was optimised, since in the experiments found
soil particles larger than 5 mm and some small rocks that increase thermal transmittance. Therefore,
the optimisation of the soil particle size distribution in the mixture before CEB production is necessary
to increase thermal performance while maintaining high mechanical resistance.
5. Conclusions
The results presented in this study show a strong relationship between soil and mixture preparation
and compaction with CEB properties. During the investigation, it was possible to observe that the
use of soil with particles with higher dimensions than the ones recommended by the international
standards for CEB production had a significant effect in some of CEBs properties, such as: porosity,
water absorption, durability and thermal performance. However, even though these blocks did not
have the proper production, they did not present mechanical resistance issues. In general, the analysed
CEBs are adequate to be used for construction of partition walls. Moreover, it was seen in the literature
that these blocks presented several benefits in terms of environmental performance. Taking this into
account, optimising the soil particle size distribution in the mixture before CEB production could be a
solution to minimise these issues (mainly in thermal performance) while maintaining high mechanical
resistance. The optimisation of the distribution will lead to a production of elements with lower
porosity, and it is known that porosity has a direct relation to mechanical resistance and durability.
However, by reducing porosity, it is expected that thermal transmittance increases. The study of CEB
optimisation and the characterisation of the optimised CEB properties will be studied in the future in
order to assess the real contribution of this optimisation in CEB mechanical, durability, and thermal
performance. In short, optimisation of compressed earth blocks is necessary to improve their functional
quality and increase their potential for use in the construction of buildings.

Energies 2020,13, 2978 20 of 22
Author Contributions:
Conceptualization, E.R.T., J.F., and R.M.; methodology, E.R.T., S.M.S., and R.M.; validation,
S.M.S. and R.M.; formal analysis, E.R.T. and J.F.; investigation, E.R.T., G.M., A.d.P.J., and C.G.; resources, R.M.;
writing—original draft preparation, E.R.T.; writing—review and editing, E.R.T., A.d.P.J., C.G., J.F., S.M.S., and R.M.;
supervision, S.M.S. and R.M.; project administration, R.M.; funding acquisition, R.M. All authors have read and
agreed to the published version of the manuscript.
Funding:
The authors would like to acknowledge the support granted by the FEDER funds through
the Competitively and Internationalization Operational Programme (POCI) and by national funds through
FCT (the Foundation for Science and Technology) within the scope of the project with the reference
POCI-01-0145-FEDER-029328, and of the Ph.D. grant with the reference PD/BD/113641/2015, which were
fundamental for the development of this study.
Acknowledgments:
The authors would like to acknowledge the support granted by DANOSA “Derivados asf
á
lticos
normalizados, S.A.” industry for the hotbox construction by providing all the necessary insulation material.
Conflicts of Interest: The authors declare no conflict of interest.
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