T. TURK et al.: CHARACTERISATION OF CONCRETE FROM THE RUPNIK MILITARY LINE
579–586
CHARACTERISATION OF CONCRETE FROM THE RUPNIK
MILITARY LINE
KARAKTERIZACIJA BETONOV RUPNIKOVE LINIJE
Tilen Turk
1
, Petra [tukovnik
1
, Marjan Marin{ek
2
, Violeta Bokan Bosiljkov
1*
1
University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova cesta 2, 1000 Ljubljana, Slovenia
2
University of Ljubljana, Faculty of Chemistry and Chemical Technology, Ve~na pot 113, 1000 Ljubljana, Slovenia
Prejem rokopisa – received: 2022-03-28; sprejem za objavo – accepted for publication: 2022-09-06
doi:10.17222/mit.2022.608
The Rupnik military line was established in about 1935 in the Kingdom of Yugoslavia as a defence system against the Kingdom
of Italy. It consists of more than 4000 reinforced concrete military bunkers positioned on the eastern part of the Rapallo border,
varying in size and purpose, and was a top-secret project at the time. Different non-destructive and non-invasive techniques were
used to characterise the selected bunkers and carefully conceived concrete samples to gain an insight into the concrete technol-
ogy used to build these historic military infrastructures. The results of the non-destructive techniques were further compared to
those of destructive techniques for a given property. It was established that very different concrete compositions were used to
build the bunkers, and an extensive dispersion of the properties was confirmed for each composition. The average compressive
strength of the Schmidt hammer for a given position is an acceptable estimate of the actual compressive strength of concrete
without destructive intervention into a bunker. It also enables an estimation of the secant modulus of elasticity using the
ModelCode2010 approach. Cylinders drilled from a bunker provided additional information about the concrete petrography, its
physical and mechanical properties and the durability of reinforced concrete.
Keywords: historic concrete, military bunker, non-destructive testing, ultrasound, Schmidt hammer, optical microscopy, image
analysis
Rupnikova linija je linija voja{kih utrdb, ki je bila v ~asu vladavine Jugoslavije zgrajena kot obrambni sistem pred vdorom
vladavine Italije, okrog leta 1935. Sestavlja jo ve~ kot 4000 armiranobetonskih voja{kih utrdb vzhodno od rapalske meje,
razli~nih velikosti in namembnosti. Zato so natan~ne mikrolokacije bunkerjev in tehnologijo gradnje poznali le redki. Za
karakterizacijo izbranih bunkerjev so bile uporabljene razli~ne neporu{ne in neinvazivne tehnike ter skrbno odvzeti betonski
valji, da bi dobili vpogled v tehnologijo, uporabljeno za gradnjo teh zgodovinskih voja{kih utrdb. Rezultate neporu{nih tehnik
smo primerjali z rezultati poru{nih tehnik za posamezno lastnost. Ugotovili smo, da so bile za gradnjo bunkerjev uporabljene
zelo razli~ne sestave betona, za vsako sestavo pa je bila potrjena velika razpr{enost lastnosti. Povpre~na tla~na trdnost, ocenjena
s Schmidtovim kladivom za posamezno pozicijo, predstavlja sprejemljivo oceno dejanske tla~ne trdnosti betona brez posega v
konstrukcijo. Omogo~a tudi oceno stabiliziranega sekantnega modula elasti~nosti, z uporabo pristopa ModelCode2010. Valji,
izvrtani iz bunkerja, so omogo~ili pridobitev dodatnih informacij o petrografiji betona, njegovih fizikalnih in mehanskih
lastnostih ter trajnosti armiranega betona.
Klju~ne beside: zgodovinski beton, voja{ki bunker, neporu{ne preiskave, ultrazvok, sklerometer, opti~na mikroskopija, analiza
slik
1 INTRODUCTION
The Rupnik military line, located in the western part
of Slovenia, is a fortification line built at the time of the
Kingdom of Yugoslavia, along the Rapallo Border be-
tween the Kingdom of Yugoslavia and the Kingdom of
Italy, established with the bilateral agreement signed on
November 12, 1920.
1
The Yugoslav military leadership
considered the Kingdom of Italy to be the main threat to
the freedom of the Kingdom of Yugoslavia. Therefore,
they decided to build two fortification systems: a low-
land fortification system and a ridgetop fortification sys-
tem, positioned to the eastern side of the Rapallo Border,
following the French building model used in the Maginot
line.
2
In the lowland fortification systems, primarily ma-
chine-gun nests, light-artillery bunkers and obstacle sys-
tems were built. The ridgetop positions were used to pro-
tect the lowland system in the case of heavy enemy at-
tacks, so many changes to the landscape (forestation and
deforestation) were also made to ensure the blending of
the bunkers with nature. Reinforced concrete bunkers
were positioned strategically to ensure proper border
protection.
3
The military bunkers are built of reinforced concrete.
The concrete walls are often rendered with a unique
cementitious mixture to protect the concrete from envi-
ronmental factors, such as rain, snow, freezing/thawing,
etc., and to blend the bunkers with nature (Figure 1 left).
Inside the bunkers, there is often evidence of poor com-
paction, indicating that the fresh concrete had poor fill-
ing capacity (Figure 1 right). When these defects were
extensive, they were repaired with cement render (exter-
nal surfaces) or plaster (internal surfaces).
Determining the mechanical properties of heritage
materials to design compatible repair or strengthening
materials is often difficult because only a limited number
Materiali in tehnologije / Materials and technology 56 (2022) 5, 579–586 579
UDK 623.126:691.328(497.1) ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 56(5)579(2022)
*Corresponding author's e-mail:
Violeta.Bokan-Bosiljkov@fgg.uni-lj.si (Violeta Bokan Bosiljkov)

of samples can be taken from historical structures. Thus,
various non-destructive testing methods are used for the
in situ characterisation of particular properties. Concrete
condition and building technology can be determined in
part by the ultrasonic waves test method, which mea-
sures the transmission times of longitudinal and shear
waves. Dynamic E-modulus, Poisson’s ratio, and shear
G-modulus can be straightforwardly calculated when the
ultrasound transmission times are known.
4
On the other
hand, compressive strength can be non-destructively esti-
mated using a Schmidt hammer.
Non-destructive techniques are extremely useful tools
for the in-situ characterisation of concrete and its quality
control, which is reflected in the available standard test
methods in this area.
5,6
However, historic reinforced con-
crete structures very often show a lack of homogeneity
and a high concentration of defects compared to modern
concrete structures. This work, therefore, focuses on the
characterisation of 80-year-old concrete samples taken
from two bunkers of the Rupnik military line, using ul-
trasonic wave and Schmidt hammer test methods. The
results of the non-destructive testing are compared with
those of the conventional destructive testing performed
on concrete cylinders taken from the bunkers.
The measured mechanical properties are further sub-
stantiated by a petrographic analysis of concrete speci-
mens, in which aggregate type, microstructure changes
and cement type were determined.
2 EXPERIMENTAL PART
2.1 Sampling
Due to the requirements of the competent authority
for minimal interventions into the reinforced concrete el-
ements of the bunkers, only three concrete cores per
bunker were drilled in the first phase of the study. The
concrete cores were carefully taken from a selected low-
land bunker (group A1) and a ridgetop bunker (group
C1). Before drilling, the two bunkers were scanned with
the Hilti Ferroscan PS 300 device to determine the loca-
tion of the steel reinforcement and thus avoid drilling
through the reinforcement, if possible. On selected drill-
ing positions, the rebound index was determined first by
using the Schmidt rebound hammer (Proceq) and follow-
ing the standard procedure.
6
The cores were taken from
inside the bunkers, according to the EN 12504-1 stan-
dard.
7
The Hilti diamond coring machine with a core
drill bit of 100 mm was used to obtain concrete cores
with a diameter of 94 mm and length of 200–300 mm.
The drilled holes were immediately filled by repair mix-
ture compatible with the original concrete.
2.2 Preparation of samples
The two groups of concrete samples (A1 and C1) are
cylinders with a diameter of 94 mm and a height of
92–95 mm. The samples were obtained by cutting the
concrete cores taken from the bunkers. If a steel bar was
present in the core, this part of the core was cut off be-
fore sample preparation. The cut-off discs were used to
make thin sections and to determine the aggre-
gate-to-binder ratio. The cylinders were stored under lab-
oratory conditions at a humidity of about 50 % until test-
ing. Prior to testing, the ends of the samples were ground
to achieve the tolerances of flatness and perpendicularity,
according to EN 12390-1.
8
2.3 Estimation of aggregate-to-binder ratio
The aggregate-to-binder-to-pore ratio was estimated
by image analysis of the cross-sectional area of the cylin-
ders. The total area of the aggregate grains and the total
area of the pores in the cross-section were divided by the
cross-sectional area (stereological theory). A Hirox
KH-3000 VD scanning optical microscope was used for
the surface imaging.
2.4 Optical microscopy
Polarised transmission optical microscopy was per-
formed on thin sections of the samples. For mineralogi-
T. TURK et al.: CHARACTERISATION OF CONCRETE FROM THE RUPNIK MILITARY LINE
580 Materiali in tehnologije / Materials and technology 56 (2022) 5, 579–586
Figure 1: Ridgetop bunker perfectly blended with nature (left) and poor concrete compaction due to low workability (right)

cal characterisation of the aggregate the samples were
coloured with the alizarine red colourant. Microscope
images were taken at 50× magnification in PTT mode
with a Zeiss LSM-700 microscope (Co-Namaste).
2.5 Dynamic elastic parameters and density
For measuring the transmission time of longitudinal
ultrasonic waves (P-waves) and ultrasonic shear waves
(S-waves), the Proceq Pundit PL-200 device was used,
with an operating frequency range of 20 kHz to 500 kHz.
With the ultrasonic method, the dynamic elastic modu-
lus, dynamic Poisson’s ratio and dynamic shear modulus
were determined using 250-MHz Olympus shear-wave
transducers. Modern concrete cast in structural elements
is a homogeneous, isotropic elastic material; therefore,
the equations for calculating the dynamic elastic parame-
ters of such materials can be used (Equation (1) to (3)).
9
The equations are based on the transmission velocity of
ultrasonic P- and S-waves and the bulk density of the
material:
"
d
ps
ps
=
−
−
VV
VV
22
22
2
2( )
(1)
EV V
d p
dd
d
s d
=
+−
−
=+
22
11 2
1
212  ""
"
"
() ()
()
() (2)
G
E
V
d
d
d
p
=
+
=
22
2
"
 (3)
where"
d
stands for the dynamic Poisson’s ratio, V
P
is
the transmission velocity of ultrasonic P-waves, V
S
is
the transmission velocity of ultrasonic S-waves, E
d
is
the dynamic elastic modulus, G
d
the dynamic shear
modulus and is the concrete density.
The concrete density was determined for each sample
as the quotient of its mass and volume.
2.6 Compressive strength and static modulus of elasticity
The static modulus of elasticity (E-modulus) was es-
timated using the modified EN 12390-13 standard proce-
dure.
10
Modifications used are the length-to-diameter ra-
tio of the specimen, which is about 1, instead of 2 to 4,
and the estimation of concrete compressive strength,
based on our own expertise, in order to determine the up-
per stress (1/3 of the compressive strength) to which the
load was increased during the test. These two modifica-
tions were necessary due to the limited number of speci-
mens that could be taken from the concrete cores. The
strain measuring instruments were two deformeters with
a gauge length of 50 mm.
Method A of the EN 12390-13 standard was applied
in order to determine the initial (E
sI-in
) and stabilised
(E
sIII-dec
) secant (static) E-modulus, with one modification
– stabilised secant E-modulus was determined in the
third unloading cycle (denotation III-dec), instead of the
third loading cycle, according to the COST TU1404 re-
searchers’ proposal.
4
The E-moduli were calculated with
an upper-stress value of 16 MPa and a lower-stress value
of 6 MPa, and the corresponding deformations.
The compressive strength was determined only after
the static E-modulus test had been completed, using the
EN 12390-3 standard procedure.
11
The compressive
strength determined on a cylinder with a diameter-to-
length ratio of about one is considered as the cube com-
pressive strength. The 100/100 mm cylinder compressive
strength is about7%lowerthan the 150 mm cube com-
pressive strength and can be used to determine a con-
crete’s compressive strength class.
T. TURK et al.: CHARACTERISATION OF CONCRETE FROM THE RUPNIK MILITARY LINE
Materiali in tehnologije / Materials and technology 56 (2022) 5, 579–586 581
Figure 2: Image of steel reinforcement scanned with Hilti PS-1000 for the lowland (left) and the ridgetop (right) bunker

3 RESULTS
3.1 Visual appearance of steel reinforcement and con-
crete cores
Figure 2 presents images of steel reinforcement in
the lowland and the ridgetop bunker. From the images
we can conclude that the steel mesh sheets were used to
reinforce the concrete bunkers and that at least two
sheets were used in the inner part of the concrete wall
where they are positioned at a depth between 0 mm and
80 mm. The smaller lowland bunker was reinforced more
heavily than the ridgetop bunker, since the clear distance
between the steel bars is between 100 mm and 150 mm
for the former and about 200 mm for the latter.
Based on the steel-reinforcement images, the optimal
positions of the Hilti coring assembly were determined
(Figure 3 left). However, for the lowland bunker the ac-
quired cores contained parts of steel bar (Figure 3
right), which can be attributed to the small opening of
the steel mesh and the overlap of the two layers. These
steel-bar segments revealed the good condition of the
steel reinforcement in the bunker, even after 80 years.
There is no sign of steel corrosion, despite the presence
of voids between the concrete and the surface of the steel
bars. These voids are the consequence of inefficient con-
crete compaction during construction works.
The visual appearance of the concrete cores (Figure
3 right) shows an efficient skeleton of aggregate grains
that predominantly occupy the concrete volume, and a
just large enough content of hydrated cement paste to
bind the grains into the concrete composite.
3.2 Aggregate shape and aggregate-to-binder ratio
The aggregate-to-binder-to-pore ratio for samples
A1, taken from the lowland bunker, was estimated to be
around 78 : 20 : 2, while for samples C1, taken from the
ridgetop bunker, it was 47 : 51 : 2. The aggregate grains
are predominantly angular, similar to today’s crushed
natural aggregate grains. However, much higher volumes
of elongated particles were observed in the concrete
samples (Figure 3 right), compared to today’s concrete
compositions. It is not yet clear how the angular grains
for concrete production were obtained. According to the
oral information given by a local resident, the stone
grains separated from the rocks of the nearby mountains
T. TURK et al.: CHARACTERISATION OF CONCRETE FROM THE RUPNIK MILITARY LINE
582 Materiali in tehnologije / Materials and technology 56 (2022) 5, 579–586
Figure 4: Petrography of samples A1 and C1 under polarised transmission optical microscope
Figure 3: Coring of the concrete samples (left) and concrete cores taken from the lowland bunker (right)

(due to erosion) were transported into the valleys by tor-
rents and were deliberately used as a source of aggre-
gate. Engineering solutions that enable the grain deposi-
tion were applied to collect the aggregate for the
concrete. In addition, there is information that the army
has also opened some quarries to produce aggregate for
the concrete.
3
3.3 Optical microscopy
Petrographic analyses of the thin sections showed
that concrete composition A1 was prepared using dolo-
mite aggregate (Figure 4 left), and concrete composition
C1 with limestone aggregate (Figure 4 right). Binder
used for the two concrete compositions was most proba-
bly a mixture of Portland cement clinker and blast fur-
nace slag (Figure 4). Moreover, the presence of
dedolomitization and secondary calcite formation is evi-
dent from the thin section A1, and the microcracks are
filled with secondary products in both compositions.
3.4 Concrete density, compressive strength and elastic
properties
The main properties of hardened concrete specimens
A1 and C1 are given in Table 1. As reference values, the
same properties of modern concrete M1 are provided.
River gravel and sand with predominantly carbonate
grains and a grain density of about 2750 kg/m
3
occupy
75 % of the M1 concrete volume. From the results in Ta-
ble 1 it is evident that concrete specimens A1 have a
lower density, compared to the concrete specimens C1,
despite the higher density of the dolomite aggregate (val-
ues from 2810–2840 kg/m
3
), compared to the limestone
aggregate (values of 2660–2760 kg/m
3
).
12
The character-
istic compressive strength of concrete A1 is about
25 MPa (average value – 2·standard deviation), and for
concrete C1 it is 30 MPa. The repeatability of the com-
pressive-strength results for concrete compositions A1
and C1 is low, with standard deviations of 11 MPa and
15 MPa, respectively. For the modern concrete composi-
tions, the standard deviation of compressive strength is
3–6 MPa, which is confirmed by the M1 test results.
Estimation of the concrete’s compressive strength us-
ing the Schmidt hammer average value gave realistic re-
sults for the cylinder’s compressive strength (0.8 cube
compressive strength) for all samples but A1-1, where
the estimated compressive strength was extremely low.
The modulus of elasticity is also higher for composi-
tion C1, although its aggregate volume is much lower
(about 50 %) than for concrete A1 (about 80 %). This is
true for the dynamic (Young’s) and static moduli of elas-
ticity.
However, there is a large difference between the ini-
tial and stabilised secant E-moduli, which is not consis-
tent with modern concrete properties, where the differ-
ences are small. Figure 5 illustrates typical stress-strain
diagrams used to determine the static E-modulus of sam-
T. TURK et al.: CHARACTERISATION OF CONCRETE FROM THE RUPNIK MILITARY LINE
Materiali in tehnologije / Materials and technology 56 (2022) 5, 579–586 583
Table 1: Properties of hardened concrete
 /(kg/m
3
) f
c
/MPa E
d
/GPa "
d
G
d
/GPa E
sI-in
/GPa E
sIII-dec
/GPa f
c-Schmidt
/MPa
A1-1 2284 46 42 0.32 16 23 32 22 ± 7.5
A1-2 2381 58 54 0.27 21 55 53 46 ± 13.5
A1-3 2260 37 40 0.30 15 26 32 32.5 ± 13.3
A1aver 2310 47 46 0.30 18 35 39 34
STD 60 11 8 0.03 3 18 12 12
C1-1 2483 76 55 0.33 21 38 43 56.5 ± 15.3
C1-2 2400 46 45 0.35 17 25 32 47.5 ± 27.8
C1-3 2472 59 47 0.30 18 60 63 51 ± 17.8
C1aver 2450 60 49 0.33 19 41 46 52
STD 50 15 5 0.03 2 18 16 5
M1-1 2426 58 51 0.27 20 37 37 –
M1-2 2435 60 54 0.26 21 39 40 –
M1-3 2448 63 58 0.25 23 40 41 –
M1aver 2440 60 54 0.26 21 39 39 –
STD 10 3 4 0.01 2 2 2 –
 – concrete density, f
c
– compressive strength, E
d
– dynamic elastic modulus,"
d
– dynamic Poisson’s ratio, G
d
– dynamic shear modulus,
E
sI-in
– initial and E
sIII-dec
– stabilised secant (static) E modulus, f
c-Schmidt
– compressive strength estimation with Schmidt hammer
Figure 5: Compressive stress-strain diagrams for the determination of
static E-modulus

ples M1-1 (f
c
=5 8M P a ,E
sI-in
=3 7G P a ,E
sIII-dec
=
37 GPa), A1-2 (f
c
= 58 MPa) and C1-1 (E
sI-in
=38GPa,
E
sIII-dec
= 43 GPa). Samples M1-1 and C1-1 have almost
the same E
sI-in
, with the compressive strength of C1-1
about 38 % higher. For the historic sample C1-1, rela-
tively large residual deformations were measured after
the last loading/unloading cycle. Sample A1-2, on the
other hand, has approximately the same compressive
strength as M1-1, but its average static E-modulus and
residual deformations are considerably higher.
Poisson’s ratio"
d
of concrete samples A1 and C1 is
high (0.3 ± 0.03 and 0.33 ± 0.03, respectively) when
compared to the modern concrete values, which are
around 0.25 for mature concrete. The higher"
d
of sam-
ples C1 is due to the higher hydrated cement paste con-
tent. The Poisson’s ratio of the hydrated cement paste
can be higher by about 0.05, compared to the concrete in
which the same cement paste glues the aggregate grains
together.
9
The static E-modulus is often estimated using differ-
ent regression equations, from compressive strength or
dynamic E-modulus. By using the approach given in
ModelCode 2010
13
(Equation (4)), we first calculated the
static E-moduli from the compressive strengths of the
samples A1, C1 and M1. Next, we estimated the static
E-modulus from the dynamic E-modulus, by using Equa-
tion (5), for the three concrete compositions. The esti-
mated static E-moduli and parameters of Equations (4)
and (5) are given in Table 2.
EE
f
se s tM C c E
c
−−
=⋅
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 0
13
10
 /
(4)
Ea E
b
se s tE d
d
−−
=⋅ (5)
E
s-est-MC
stands for static E-modulus estimated by
ModelCode2010, and E
c0
· E
presents the effect of the ag-
gregate type on the E-modulus. E
s-est-Ed
stands for the
static E-modulus estimated from the dynamic modulus
E
d
, calculated from the ultrasonic test results.
Table 2: Parameters of regression equations and estimated static
E-modulus
E
c0 E
/
MPa
E
s-est-MC/
GPa
a b
E
s-est-Ed
/
GPa
E
sIII-dec
/
GPa
A1-1
22900
35
0.895 0.964
33 32
A1-2 38 42 42
A1-3 33 31 32
C1-1
22900
42
0.356 1.202
44 43
C1-2 35 35 32
C1-3 38 36 40
M1-1
23300
39
0.569 1.062
37 37
M1-2 39 39 40
M1-3 40 42 41
From the results in Table 2, it is clear that for compo-
sitions A1 and M1, the E-modulus estimation originating
from the measured E
d
(Equation (5)) is closer to E
sIII-dec
than the estimation based on the concrete compressive
strength (Equation (4)). For concrete C1, the two equa-
tions result in approximately the same E-modulus.
4 DISCUSSION
The compressive strength of concrete A1 is, on aver-
age, lower than the compressive strength of concrete C1.
This is mainly due to the lower water-to-cement ratio
(W/C ratio) of composition C1. The significantly larger
amount of hydraulic binder in this composition (paste
content of about 50 %) enables a large reduction of the
W/C ratio to achieve a preferred plastic consistency of
the fresh concrete. The estimation of the concrete’s com-
pressive strength using the Schmidt hammer resulted in
realistic values close to the cylinder compressive strength
for all but sample A1-1, where the estimated compres-
sive strength was very low. However, a possible deterio-
ration of the concrete in the protective cover above the
steel reinforcement can result in severely underestimated
concrete compressive strength. Besides, we still do not
know how the ACR reactions change the concrete’s hard-
ness.
The dry consistency of concrete A1 and the associ-
ated difficult concrete consolidation during its casting
into the bunker most likely resulted in a higher propor-
tion of voids in concrete A1 (Figure 3 right) compared
to concrete C1. An increased volume of voids leads to a
lower concrete density and thus a lower compressive
strength. Taking into account the data on the density of
dolomite aggregates in Slovenia (2810–2840 kg/m
3
) and
the volume of aggregate grains of about 78 %, we would
expect the density of concrete A1 to be around
2450 kg/m
3
for optimally consolidated concrete (2 % of
entrapped pores). This means that the share of voids in
the samples of concrete A1 is at least 5–10 %, with con-
solidation voids ranging from3%to8%.W ecanalso
conclude that there is an excellent linear correlation be-
tween the compressive strength of samples A1 and their
density (R
2
= 0.93).
On the other hand, samples of concrete C1 have den-
sities of 2400–2480 kg/m
3
. For concrete with limestone
aggregate (densities of 2660–2760 kg/m
3
), this is possi-
ble only with a high volume of hydraulic binder and ex-
cellent concrete consolidation. We estimate that the pro-
portion of consolidation voids in samples C1 does not
exceed 2 %.
The higher modulus of elasticity of concrete C1 is
also due to the higher strength of hydrated cement paste
(HCP) in this composition. The significant difference be-
tween the initial and stabilized secant E-modulus is most
likely due to the inhomogeneous structure of concrete C1
due to the possible segregation and the influence of the
HCP creep during the test. The relatively even more sig-
nificant difference between the initial and stabilized se-
cant E-modulus in concrete A1 is, in addition to the
inhomogeneous composition, a consequence of a greater
volume of voids due to the difficult compaction of the
T. TURK et al.: CHARACTERISATION OF CONCRETE FROM THE RUPNIK MILITARY LINE
584 Materiali in tehnologije / Materials and technology 56 (2022) 5, 579–586

fresh concrete. Also, in concrete A1, concrete creep was
detected during the test, resulting in more significant re-
sidual deformations than in modern concrete.
However, the inhomogeneity of the A1 concrete
structure might also be the result of ACR reactions,
which is confirmed by the analysed thin sections (Figure
2 left). The course of the ACR reactions in concrete with
dolomite aggregate and their effects on the concrete
properties have been described earlier.
14
The difference in dynamic E-moduli between compo-
sitions A1 and C1 is significantly smaller than the differ-
ence in static E-moduli. For concrete A1, the linear cor-
relation between the concrete’s density and its E
d
is
perfect (R
2
= 1), mainly due to the high proportion of do-
lomite aggregate grains in the concrete, which form a
skeleton along which ultrasonic waves propagate faster
than through HCP. In concrete C1, the volume of aggre-
gate grains is much lower, and the grains are not in con-
tact, so the value of E
d
is significantly influenced by
HCP.
A comparison of the properties of historic concretes
A1 and C1 with the properties of modern concrete (M1)
shows that concrete C1 has, on average, the same com-
pressive strength as well as shear and initial static
E-modulus. The difference is in the E
d
and stabilized
static E-modulus, which is 5 GPa and 7 GPa higher for
concrete C1. This can be attributed to the large volume
of HCP in concrete C1 and the resulting more extensive
creep, which consolidates the binder structure and thus
increases the concrete’s stiffness. On the other hand,
concrete A1 showed, on average, the same stabilized
E-modulus at a significantly lower compressive strength
and E
d
compared to concrete M1.
The Poisson’s ratios show significantly higher values
for concretes A1 and C1 compared to the concrete M1.
While the higher"
d
for concrete C1 is expected due to
the large HCP volume, for concrete A1,"
d
= 0.3 is an un-
expected result. The increased"
d
of concrete A1 might
be due to ACR reactions.
The estimation of the stabilised secant E-modulus us-
ing the ModelCode 2010 for the bunkers of the Rupnik
line requires at least average compressive-strength val-
ues. We showed, on drilled cylinders, that the parameter
E
c0
· E
which represents the effect of the aggregate type,
is the same for the dolomite and limestone aggregate
used when building the bunkers. The cores enable a more
detailed study of the concrete’s properties and provide
data about the elastic properties E
d
, G
d
and"
d
. The com-
pressive strength f
c
and the initial and stabilised secant
E-moduli can also be determined if the concrete core
length is at least equal to its diameter.
5 CONCLUSIONS
The use of non-invasive and non-destructive testing
for the characterisation of historical reinforced concrete
structures provides a good general approximation and al-
lows at least a partial understanding of construction tech-
nology.
It was established that very different concrete compo-
sitions were used to build the bunkers, and the extensive
scatter of properties was confirmed for each composition
due to inhomogeneity and entrapped air voids of the cast
concrete. The average compressive strength of the
Schmidt hammer for a given position is an acceptable es-
timation of the actual concrete’s compressive strength
without a destructive intervention into the bunker. The
parameter E
c0
· E
of the ModelCode 2010 equation, deter-
mined from the test results, seems to have the same value
for all the concrete compositions of the Rupnik line.
Therefore, the secant stabilised E-modulus estimate us-
ing the average compressive strength of the Schmidt
hammer and the ModelCode2010 equation is possible
for the bunkers. More exact values of the secant stabi-
lised E-modulus can be obtained from the dynamic
E-modulus, but the correlation parameters seem to de-
pend on the concrete’s composition. Further studies are
needed in the area.
The Poisson’s coefficients of the historic concrete
compositions are higher than the modern reference con-
crete. The chemical reactions associated with the disso-
lution, migration and precipitation of new products, such
as those observed in the ACR reaction, might be respon-
sible for the increased"
d
in the 80-year-old concrete.
Acknowledgment
The authors acknowledge the financial support from
the Slovenian Research Agency through Tilen Turk’s
PhD project and research programmes P2-0185 and
P1-0175.
6 REFERENCES
1
K. Ajlec, B. Balkovec, B. Repe, NE^AKOV ZBORNIK Procesi,
teme in dogodki iz 19. in 20. stoletja, 2018
2
J. E. Kaufmann, H. W. Kaufmann, The Forts and Fortifications of
Europe 1815-1945: The Central States: Germany, Austria-Hungry
and Czechoslovakia, Pen&Sword Military 2014.
3
A. Jankovi~ Poto~nik, V.Toni~, M. Perpar, Fortifying Europe’s Soft
Underbelly: The Rupnik Line, the Vallo Alpino and Other Fortifica-
tions of the Ljubljana Gap, CreateSpace Independent Publishing
Platform 2012
4
TU 1404 COST ACTION; Towards the Next Generation of Stan-
dards for Service Life of Cement-Based Materials and Structures.
RRT+ Main phase of the Extended Round Robin Testing programme
for TU1404. Testing Protocols, 2016, 28–30
5
EN 12504-2:2021 – Testing concrete in structures Non-destructive
testing. Determination of rebound number, International Organiza-
tion for Standardization, Brussels.
6
EN 12504-4:2021 – Testing concrete in structures Determination of
ultrasonic pulse velocity, International Organization for Standardiza-
tion, Brussels
7
EN 12504-1:2019 – Testing concrete in structures – Part 1: Cored
specimens – Taking, examining and testing in compression, Interna-
tional Organization for Standardization, Brussels
T. TURK et al.: CHARACTERISATION OF CONCRETE FROM THE RUPNIK MILITARY LINE
Materiali in tehnologije / Materials and technology 56 (2022) 5, 579–586 585

8
EN 12390-1 – Testing hardened concrete Shape, dimensions and
other requirements for specimens and moulds, International Organi-
zation for Standardization, Brussels.
9
M. Klun , V. Bosiljkov, V. B. Bosiljkov, The relation between con-
crete, mortar and paste scale early age properties, Materials, 6 (2021)
14, doi:10.3390/ma14061569
10
EN 12390-13:2013 – Testing hardened concrete – Part 13: Determi-
nation of secant modulus of elasticity in compression, International
Organization for Standardization, Brussels
11
EN 12390-3:2019 – Testing hardened concrete Compressive strength
of test specimens, International Organization for Standardization,
Brussels
12
F. Bohar, M. Bebar, J. Prosen, Dolo~itev kriterijev za postopka mikro
Deval (SIST EN 1097-1) in zmrzlinska obstojnost zrn (SIST EN
1367-1 in SIST EN 1367-2) za naravne, drobljene in reciklirane
agregate za nevezane in vezane (stabilizirane) plasti v vozi{~ni
konstrukciji. Rezultati laboratorijskih preiskav, Ljubljana-Polje
(2013).
13
Fédération internationale du be´ton. Model code 2010: final draft, In-
ternational Federation for Structural Concrete (fib) (2012)
14
T. Katayama, How to identify carbonate rock reactions in concrete,
Materials Characterization, 53 (2004), 85–104, doi: doi:10.1016/
j.matchar.2004.07.002
T. TURK et al.: CHARACTERISATION OF CONCRETE FROM THE RUPNIK MILITARY LINE
586 Materiali in tehnologije / Materials and technology 56 (2022) 5, 579–586