A. PADOVNIK, V. BOKAN BOSILJKOV: ADHESIVE-STRENGTH ASSESSMENT OF LIME INJECTION GROUT ...
587–593
ADHESIVE-STRENGTH ASSESSMENT OF LIME INJECTION
GROUT USING STANDARDISED AND MODIFIED TEST
METHODS
OCENA SPRIJEMNE TRDNOSTI APNENE INJEKCIJSKE MASE S
STANDARDNO IN MODIFICIRANO PRESKUSNO METODO
Andreja Padovnik, Violeta Bokan Bosiljkov
*
Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova 2, SI-1000 Ljubljana, Slovenia
Prejem rokopisa – received: 2022-04-05; sprejem za objavo – accepted for publication: 2022-09-06
doi:10.17222/mit.2022.609
The adhesive strength of non-structural lime-based grouts used to stabilise sensitive, detached, decorative plasters is an impor-
tant mechanical property. However, it is difficult to determine it due to the lack of suitable standard test methods. The existing
standard procedures are mainly aimed at testing the properties of hydraulic binders and are not suitable for injection grouts or
mortar specimens based on a lime binder. In the present study, the focus is on a comparison of the pull-off results between the
standardised method (EN 1015-12) performed on pre-drilled specimens on sandwich panels (PMS) and the modified method us-
ing sandwich discs (DSS). It was found that the modified method with sandwich discs (DSS) achieved an up to 58 % higher ad-
hesive strength than the standard method with sandwich panels. In the sandwich-panel specimens, fracture occurred in the grout
since pre-drilling reduced the cohesive strength of the grout. For the sandwich discs (DSS), fractures occurred predominantly at
the interface between the mortar and the grout.
Keywords: lime-based grout, pull-off strength, modified test method, sandwich panel, sandwich disc
Sprijemna trdnost nekonstrukcijskih injekcijskih me{anic, ki so namenjene stabilizaciji ob~utljivih dekorativnih ometov, je
klju~na lastnost za u~inkovitost utrditvenega ukrepa. Realno ovrednotenje sprijemne trdnosti je {e vedno te`avna naloga, saj za
tovrstne aplikacije ne obstajajo standardne metode. Ve~inoma so obstoje~i standardi namenjeni hidravli~nim vezivom in zato
niso primerni za injekcijske mase ali malte z apnenim vezivom. V tej {tudiji je poudarek na primerjavi rezultatov sprijemne
trdnosti med standardizirano metodo, ki vklju~uje vrtanje vzorcev na sendvi~ plo{~ah (PMS), in modificirano metodo s sendvi~
diski (DSS). Izkazalo se je, da z modificirano metodo s sendvi~ diski (DSS) dose`emo do 58 % vi{je sprijemne trdnosti kot pri
standardni metodi na sendvi~ plo{~ah. V primeru sendvi~ plo{~ se je zaradi vibracij med vrtanjem vzorcev poru{itev pojavila v
sami injekcijski masi. Ocenjujemo, da so vibracije poslab{ale kohezivne trdnosti injekcijske mase. Poru{itev na sendvi~ diskih
(DSS) je bila ve~inoma na stiku med malto in injekcijsko maso.
Klju~ne besede: apnena injekcijska masa, odtr`na sprijemna trdost, modificirana preskusna metoda, panelni sendvi~, sendvi~
disk
1 INTRODUCTION
Stabilization intervention by grouting is a common
method to restore the adhesion between delaminated
plaster layers. When the grout is injected behind the
plaster to fill voids and cracks, it becomes an irremov-
able part of the wall and results in an irreversible inter-
vention. Therefore, the physical-mechanical compatibil-
ity between the grout and the original plaster is crucial.
The requirements for the hardened state of the grout are
very often specific in relation to the properties of delami-
nated plasters: porosity, water vapour permeability and
capillary water absorption similar to the original materi-
als; mechanical strength similar to or lower than that of
the original plasters; and good adhesion properties.
1
However, there are still some issues related to a real-
istic evaluation of the mechanical properties of the
lime-based grouts. This applies in particular to the adhe-
sive strength of non-structural pure lime grouts, as there
are still no specific international laboratory standard test
methods in this area. In a limited number of previous
studies, the adhesive strength of grouts was measured
predominantly by a pull-off test described in the standard
EN 1015-12.
2
However, the specimen preparation differs
considerably among different institutions.
1,3
Azeiteiro et
al.
3
and Padovnik et al.
1
studied the pull-off strength us-
ing a specimen that simulated the detachment between
plaster layers that were later injected with grout. This ap-
proach, referred to as the "panel sandwich test," was
used to simulate detachments of 2 mm to 5 mm between
fine plaster and rough plaster. The specimen preparation
consists of first applying rough plaster and then fine
plaster with purposely fabricated detachments, to a sub-
strate, and cutting/drilling a circular specimen of a spe-
cific diameter in the detachment area. After that, gluing a
circular metal pull-head plate with the same diameter to
the specimen surface is carried out to transfer the in-
creasing tensile force to the specimen until its rupture.
In addition, it was found that there are problems in
evaluating the actual adhesion strength using the stan-
Materiali in tehnologije / Materials and technology 56 (2022) 5, 587–593 587
UDK 666.946.7:693.625-048.24 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 56(5)587(2022)
*Corresponding author's e-mail:
violeta.bokan-bosiljkov@fgg.uni-lj.si (Violeta Bokan Bosiljkov)

dard test method EN 1015-12, especially when pre-cut-
ting a circular area after the plastered panel has been
cured.
5
During cutting, the vibrations of the drilling ma-
chine might cause the weak plaster or grout layer to de-
tach or significantly reduce the cohesion of the grout
and/or plaster. Another limitation of the pull-off method,
when using standard circular pull-head plates with a di-
ameter of 50 mm, is the low adhesion strength of the
lime-based grout. As a result, the standard EN 1015-12
procedure shows a low repeatability and accuracy of the
test results because the tensile load is often below 10 %
of the capacity of the pull-off testing machine.
In the latest study, Pasian et al.
4
developed a sand-
wich system in which the grout is injected between two
plaster layers without support. The advantages of the
new proposal for sample preparation are that such a sys-
tem does not require drilling a circular area and that the
larger load-bearing surface allows for a more realistic ad-
hesion-strength evaluation as well as greater accuracy of
the test results. On the other hand, the proposed sam-
ple-preparation procedure also has a weakness. The in-
jected sandwich samples are sealed in a non-porous plas-
tic container for 28 d. Thus, CO
2
access required for the
carbonation of the lime binder is restricted. In addition,
the authors highlighted that the proposed sandwich sam-
ple allows evaluating the behaviour of the whole plas-
ter-grout-plaster system in terms of water-vapour perme-
ability and capillary-water absorption.
This study presents a new approach to evaluating the
lime-grout adhesive strength based on the preparation
and testing of a 100-mm disc-sandwich model. The grout
is injected into a deliberately prepared air pocket be-
tween the fine and rough plaster layers, and subsequent
core drilling is omitted. The obtained test results are
compared to those of the panel sandwich test, performed
by the standard pull-off method. It is important to note
that this study does not provide information about fresh
grout properties, which is generally very important in the
assessment of the test results. Since the same grout com-
positions as in earlier studies were used to prepare the
samples, fresh grout and mortar properties reported there
represent relevant data, if needed.
2 EXPERIMENTAL PART
2.1 Composition and testing of grouts and plasters
Two lime grouts were used to compare the efficiency
of the standard EN 1015-12 and a modified test method
to evaluate their adhesive strength. The grout composi-
tions are shown in Table 1. Commercially available dry
hydrated lime of class CL 90-S (EN 459-1)
6
was used as
a binder. A finely ground limestone from Slovenia
(CALCIT, Stahovica, Slovenia) and thin-walled soda-
lime-borosilicate glass microspheres (3M Glass Bubbles
K1) were used as fillers. To achieve an adequate viscos-
ity and injectability of the grout in a fresh state,
polycarboxylate ether-based superplasticiser (PCE-SP)
was used to reduce the water content of the grout (tap
water at a temperature of (20 ± 1) °C). The grout mix-
tures were prepared using a KitchenAid mixer with a
power of 300 W and a stainless-steel gate anchor blade.
First, the lime and filler were mixed. Then 70 % of the
water was added and mixed for 2 min at low revolutions
(540 min
–1
). In the last 15 s of the low-speed mixing, the
PCE-SP and 30 % of the water were added. Finally, each
grout was mixed at high revolutions (1200 min
–1
) for
3 min.
For the sandwich samples, rough and fine lime plas-
ters were used (Table 1). The rough plaster was prepared
from (1 : 3) lime putty : coarse sand (0/4 mm) lime mor-
tar, and the fine plaster from (1 : 2) lime putty : fine sand
(0/1 mm) lime mortar, and they were mixed with a
RILEM-CEN mortar mixer for 180 s.
The hardened properties of the lime grouts, and the
rough and fine lime mortar plasters, were evaluated after
90 d. The fresh mixtures were cast in cylindrical moulds
with a diameter and height of 50 mm and demoulded af-
ter 48 h. Curing was carried out under controlled ambi-
ent conditions (RH (60 ± 10) % and (19 ± 1) °C) until
the test day.
The compressive test was performed according to the
standard EN 1015–11.
7
The splitting tensile test followed
the ASTM C496/C496 M-1 standard.
8
The compressive
and splitting tensile strengths were determined on four
specimens per composition. The tests were performed
with a Roel Amsler HA 100 servo-hydraulic testing ma-
chine (Zwick GmbH & Co. KG, Ulm, Germany), com-
plemented by a load cell with the capacity adjusted to the
compressive (25 kN) and splitting tensile (5 kN)
strengths of the tested specimens.
Table 1: Composition of injection grout mixtures and lime plasters
Grout
A
Grout
B
Fine
plaster
Rough
plaster
Binder – hydrated lime
CL70 volume ratio
11––
Binder – slaked lime
putty
1
volume ratio
11
Limestone aggregate
0/4 mm volume ratio
–––3
Limestone aggregate
0/1 mm volume ratio
––2
Limestone filler: glass
microspheres volume ratio
3:0 2:1 - -
Water/binder mass ratio 1.86 1.76 –
2
–
2
Water/(binder, limestone
filler and glass micro-
spheres) mass ratio
0.41 0.50 – –
PCE-SP (%) 0.5 0.5 – –
1
Slaked lime putty is 2 years old and it contains about 51 % water and
49 % Ca(OH)
2
2
the only water used for the preparation of rough and fine plaster was
the water already contained in the slaked lime putty
A. PADOVNIK, V. BOKAN BOSILJKOV: ADHESIVE-STRENGTH ASSESSMENT OF LIME INJECTION GROUT ...
588 Materiali in tehnologije / Materials and technology 56 (2022) 5, 587–593

2.2 Panel sandwich test
The panel-sandwich models were prepared based on
the instructions in Ref. 1 and Ref. 3 to simulate a smaller
(2 mm) and a larger (5 mm) detachment of the fine plas-
ter from the rough plaster due to air pockets. At the age
of 1 year, the air pockets were first prewetted and then
grouted using a syringe.
2.3 Modification of the sandwich model
In this study, the preparation of the sandwich system
from Ref. 4 was modified. The sandwich-system modifi-
cation focuses on simplification of the model preparation
procedure. The newly proposed sandwich model consists
of two discs made of lime mortar, representing two plas-
ter layers.
The discs are cast using cylindrical moulds with an
inner diameter of 100 mm and a height of 20 mm. The
rough plaster discs are prepared as 20-mm-thick slabs. In
the fine plaster discs, however, a cavity of 90 mm in di-
ameter and 2 mm or 5 mm in height is made. Before
casting the fine plaster, a plastic disc with a diameter of
90 mm and a height of 2 mm or 5 mm is inserted and at-
tached to the mould base centre. In this way, a fine plas-
ter model with a cavity height of 2 mm or 5 mm and an
edge-rim thickness of about 5 mm is prepared. In addi-
tion, two small holes are cut in the rim at opposite posi-
tions to ensure efficient cavity grouting and air release
during grouting (Figure 1a). By joining the two
1-year-old discs, detachment of the fine plaster from the
rough plaster is simulated (Figure 1b).
After 1 year, the rough and the fine discs with the
cavity are assembled into a model using 40-mm-wide ad-
hesive tape (Figure 1c). Surfaces of the cavity are
pre-wetted with a syringe through one of the holes to re-
duce the absorption of the grout water and grout shrink-
age.
9
After pre-wetting, the sandwich model is turned
into position with one hole facing the laboratory room
ceiling. Through this hole, the lime grout is slowly in-
jected using a syringe. The air pocket is considered filled
when the grout flows from the opposite hole, and the
grouting is stopped (Figure 1c). The adhesive tape is re-
A. PADOVNIK, V. BOKAN BOSILJKOV: ADHESIVE-STRENGTH ASSESSMENT OF LIME INJECTION GROUT ...
Materiali in tehnologije / Materials and technology 56 (2022) 5, 587–593 589
Figure 1: Test specimens: a) a cylindrical cavity disc with a flat bottom and small holes at opposite sides of the rim, b) two discs are joined to the
sandwich model, c) grout injection through the hole, d) sandwich system after grout injection

moved from the injected sandwich after 24 h (Fig-
ure 1d).
The modified sandwich model was cured under con-
trolled ambient conditions (RH (60 ± 10) % and
(19 ± 1) °C) for 90 d. The same curing regime was
adopted for the panel sandwich model.
2.4 Adhesive strength
The adhesive strength of the grout was evaluated on
the panel-sandwich models by following the standard
EN 1015-12 procedures (Figure 2a). Circular test areas
of 50 mm in diameter were cut through the grouted plas-
ter layers and 2 mm into the substrate using a core-drill-
ing machine. The metal pull-heads with a diameter of
50 mm were glued centrally on the test areas. The
Proceq pull-off tester DY-206 with an operating range of
0.3–3.1 MPa (0.6–6 kN for the 50 mm test disc/
pull-head) was used to apply the tensile load to the test
surface.
On the modified disc-sandwich model with a diame-
ter of 100 mm, the adhesive strength of the grout was
evaluated using the same testing machine (Proceq
DY-206 pull-off tester). In Pasian et al. a diameter of
95 mm was proposed for the sandwich specimens. The
operating range of the tester was thus shifted from
0.08 MPa to 0.8 MPa, i.e., to an interval where the adhe-
sive strength of the lime grout and mortar values fall. On
the test day, two circular aluminium pull-head discs (di-
ameter 100 mm and height 50 mm) are glued to the
rough and fine plaster surfaces of the sandwich model
using epoxy resin or thermoplastic hot glue. Special care
should be taken to ensure that the discs are horizontal
and parallel. Prefabricated discs for the sandwich speci-
men with the same diameter as the pull-head discs elimi-
nate the cutting of the test areas and thus possible dam-
age to the grout-plaster bond or reduced cohesion of the
grout and/or plasters. The specimen with the glued
pull-heads is placed in a specially designed metal frame,
which allows efficient clamping of the lower pull-head
and consequent execution of the pull-off test (Fig-
ure 2b). Due to the larger specimen diameter, the pro-
posed modified test method also provides higher accu-
racy of the test results compared to the pull-off test pro-
cedure in the EN 1015-12.
3 RESULTS AND DISCUSSION
The comparison of adhesive strengths measured us-
ing the panel-sandwich test (PSM) and the disc-sand-
wich system (DSS) with a 2-mm or 5-mm air pocket is
shown in Figure 3. The type of rupture is presented in
Table 2.
It can be seen that the adhesive strengths evaluated
by DSS are higher than those measured in the PSM. The
A. PADOVNIK, V. BOKAN BOSILJKOV: ADHESIVE-STRENGTH ASSESSMENT OF LIME INJECTION GROUT ...
590 Materiali in tehnologije / Materials and technology 56 (2022) 5, 587–593
Figure 2: Pull-off test: a) according to the EN 1015-12 for panel-sandwich models (PSM), b) according to the modified standard for disc-sand-
wich systems (DSS)
Figure 3: Adhesive strengths of grouts A and B injected into 2-mm or
5-mm air pocket, for the panel-sandwich models (PMS) and
disc-sandwich systems (DSS)

values for the A_PSM and B_PSM grouts injected into
the 2-mm or 5-mm air pockets ranged from
0.05–0.08 MPa, i.e., 10–58 % lower than for DSS.
When comparing the results for the 2-mm and 5-mm
air pockets, two opposite trends can be observed, de-
pending on the type of sandwich model used. For the
PSM, the adhesive strength decreases with increasing
thickness of the air pocket, while for the DSS, the
strength is higher for the thicker 5-mm air pocket. A
plausible explanation for the observed trends is the dam-
age to the grout-render layer interface and/or reduced co-
hesion of the lime grout and/or mortar related to the
core-drilling procedure in the PSM.
According to the average pull-off strengths (Fig-
ure 3), the difference between the PSM and DSS sand-
wich models with the 2-mm air pocket is less significant,
i.e., 20 % for grout A and 17 % for grout B. The refer-
ence strength is that of the DSS specimen. The adhesive
strength of grout A in the 2-mm air pocket reached the
value of 0.08 MPa and 0.10 MPa for the PSM and the
DSS model, respectively. The failure occurred within the
grout (100 %) in the PSM. In the DSS, it occurred pre-
dominantly along the grout to the rough-plaster interface
(90 %). Therefore, the average tensile strength of grout
A seems to be higher than 0.10 MPa, since, in the DSS
model, the adhesive strength between the grout and the
rough plaster was the weakest sandwich layer. Thus, the
lower tensile strength of grout A measured for the PSM
model can be related to the reduced grout cohesion due
to core-drilling damage. However, its value is at the same
time well below the operating range of the pull-off tester.
Grout B injected into the 2-mm air pocket of the PSM
and DSS models showed lower average pull-off strengths
of 0.05 MPa and 0.06 MPa, respectively. For the PSM
model, the rupture occurred predominantly in the grout
(70 %) and along the interface between the grout and the
rough plaster (30 %), while for the DSS model, the fail-
ure was in the interface (100 %). Thus, the cohesion of
grout B can be estimated as higher than 0.06 MPa, and
reduced cohesion or interface damage is responsible for
the lower pull-off strength of the PSM. These results
show that the interface between the lime plaster and
grouts A or B is the weakest layer in the disc-sandwich
specimens (DSS) with a 2-mm air pocket. The tensile
rupture of the grouted layer in the panel-sandwich speci-
mens (PSM) is caused by a reduced cohesion of the
grout due to core drilling. The same trend was observed
in Faria et al., where the authors showed that cutting
specimens by core drilling harms the adhesive strength
of the grout.
For the 5-mm air pocket, the differences between the
average pull-off strengths of the panel and the disc-sand-
wich models are higher than for the 2-mm air pocket. A
difference of 58 % was obtained for grout A. On the
other hand, the pull-off test was not performed for grout
B and the PSM model, because the specimens failed dur-
ing the drilling process due to a failure within the grout
and the grout to fine plaster interface. The rupture of the
A_PSM specimens occurred predominantly in the grout
(90 %) and partly through the interface between the
grout and the fine plaster layer (10 %), resulting in a
pull-off strength of 0.05 MPa. In the case of the A_DSS
specimens, a considerably higher average pull-off
strength was measured (0.12 MPa), and the mixed-frac-
ture mode was partly along the grout (40 %), partly at
the interface between the grout and the rough plaster (50
%) and in the rough plaster itself (10 %). These results
indicate approximately the same values of cohesive
strengths in the grout and rough plaster and the adhesive
strength between the grout and the rough plaster. For the
B_DSS specimens with the 5-mm air pocket, the average
pull-off strength reached 0.09 MPa, with the fracture in
the fine plaster layer (20 %), through the grout (60 %),
and through the interface between the grout and the
rough plaster (20 %). The results indicate that grout B’s
cohesive strength of 0.09 MPa is probably the weakest
link in the B_DSS specimens.
A relatively large, injected area in the disc-sandwich
system also allows the effectiveness of the grouting pro-
cess to be evaluated by the presence of voids not occu-
pied by the grout and by the presence of drying cracks
formed in the grout. These features were visible on the
specimens after the pull-off test. It was observed that the
grout did not completely fill the air pocket due to the for-
mation of air bubbles during the injection process. De-
spite their good working properties, partial filling of the
air pockets was observed for both grouts A and B.
Comparing the results in Figure 3 with the pull-of
strengths reported in comparable studies
3,4
shows that the
A. PADOVNIK, V. BOKAN BOSILJKOV: ADHESIVE-STRENGTH ASSESSMENT OF LIME INJECTION GROUT ...
Materiali in tehnologije / Materials and technology 56 (2022) 5, 587–593 591
Table 2: Location of failure after the pull-off test
Air pocket height/
specimen
2mm 5mm
A_PSM 100 % within the grout
90 % within the grout
10 % along the grout–fine plaster interface
B_PSM
70 % within the grout
30 % along the grout–rough plaster interface
90 % within the grout
10 % along the grout–fine plaster interface
A_DSS
90 % along the grout–rough plaster interface
10 % within the rough plaster
40 % within the grout
50 % along the grout–rough plaster interface
10 % within the rough plaster
B_DSS 100 % along the grout–rough plaster interface
20 % along the grout–fine plaster interface
60 % within the grout
20 % along the grout–rough plaster interface

values obtained in this study are higher than those in the
referred studies. In Ref. 4 the 150-d pull-of strengths of
the lime-grout-injected disc sandwiches were
0.032–0.041 MPa when injected into an 8-mm air
pocket. These results are similar to the pull-off strengths
of specimens A_PSM and B_PSM in the present study.
On the other hand, specimens A_DSS and B_DSS that
are more comparable to those in Ref. 4. provided much
higher adhesive strengths for comparable grout composi-
tions. Moreover, the maximum pull-off strength in Ref. 3
after 28 d and 60 d was 0.015 MPa. The sandwich model
used is similar to the PSM system in our study, but the
lime-based grouts contained 10–30 % of metakaolin.
The values of different studies cannot be directly
compared with each other, as parameters, such as grout
and plaster composition, air-pocket shape and thickness,
and curing time and conditions, can influence the test re-
sults.
4
Table 3: Average compressive strength and splitting tensile strength at
90 d
Specimen
Average compressive
strength (MPa)
Average splitting ten-
sile strength (MPa)
A 3.5 ± 0.3 0.16 ± 0.04
B 2.6 ± 0.2 0.18 ± 0.04
Fine plaster 2.2 ± 0.1 0.18 ± 0.01
Rough plaster 1.3 ± 0.1 0.13 ± 0.01
The average mechanical strengths of grouts A and B
after 90 d are shown in Table 3, along with the corre-
sponding standard deviation. The compressive strengths
of grouts A and B were 3.5 MPa, and 2.6 MPa, respec-
tively. The splitting tensile strengths were 0.16 MPa and
0.18 MPa for grouts A and B, respectively, which is
close to the same strength of the lime mortars for the fine
and rough plasters (Table 3). The reported results differ
from those in Ref. 1 where the same compositions were
used; the measured values are, as a rule, higher. The mix-
ing procedure was modified for this study and may be re-
sponsible for the increased strength properties.
A comparison of the splitting tensile-strength values
in Table 3 with the DSS pull-off strengths in Figure 3
shows that the weakest link in the A_DSS samples is
most likely the interface between the grout and the rough
plaster layer, and the rough mortar itself, since the
A_DSS average pull-off strength is the same as the
rough plaster average splitting tensile strength for the
5-mm air pocket. The grout governs the failure mode and
the pull-off strengths for the B_DSS samples with a
5-mm air pocket. Due to its much lower pull-off strength
compared to the splitting tensile strength of the grout, the
presence of entrapped air pores in the lower-density
grout B is a plausible explanation for the measured val-
ues. In the 2-mm air-pocket samples, the adhesive
strength between the grout and the rough plaster seems
to control the pull-off strength, which is considerably
lower than the grout-splitting tensile strength of the indi-
vidual grout. It seems that efficient grouting is more
complex for the 2-mm air pocket than for the 5-mm one,
and the reduced pull-off strength may be due to the in-
complete filling of the detachment model. Lower-density
grout B is less efficient in air-pocket filling than grout A.
4 CONCLUSIONS
The study presents a new approach to the evaluation
of lime grout’s adhesive strength, based on the 100-mm
disc-sandwich system (DSS) and pull-off tester, and
compares the results of this modified adhesive-strength
test with those of the standard EN 1015-12 adhe-
sive-strength test (PSM). The PSM test involves cutting
the samples by core drilling. In both methods, the adhe-
sive strength of the lime grout is evaluated by injecting
the grout into the air pocket between two plaster layers.
The DSS sample simulates two plaster layers with
detachment in terms of the composition of each layer
and the detachment size between two layers, which can
be easily adjusted to the actual situation on site. More-
over, the DSS model with a diameter of 100 mm makes it
possible to evaluate the filling of the space between two
plaster layers and the formation of drying cracks after
the pull-off test.
In addition, the shape of the upper disc with a
5-mm-thick wall of mortar around the cavity makes it
more difficult for CO
2
to access the injected grout in the
disc-sandwich model, which reflects the situation on site
more closely than the solution in
4
.
Increased test area (100 mm in diameter) compared
to the standard method (EN 1015-12, 50 mm in diame-
ter) allows for higher measurement accuracy and shifts
the working operating range of the pull-off tester to ad-
hesive strengths typical for the grouts and mortars with a
predominantly hydrated lime binder.
The core-drilling elimination in the modified DSS re-
sulted in increased pull-off strength values for all the
tested combinations compared to the PSM test. The dif-
ference was 20 % (grout A) and 17 % (grout B) for the
2-mm air pocket and 58 % for the 5-mm air pocket.
These results confirm that the core-drilling procedure is
a detrimental influence when testing low-strength
lime-based materials.
The tests of the DSS samples also revealed that the
2-mm air pocket represents a more challenging environ-
ment for the grout injection than the 5-mm one. The fail-
ure mode was predominantly through the interface be-
tween the grout and the rough plaster, which indicates
poorer adhesive strength of the grout, most probably due
to the presence of entrapped air.
Finally, it can be concluded that the modified sand-
wich model proposed in this study is a better solution
than the EN 1015-12 test method when the lime-based
grout’s adhesive strength is evaluated. However, a more
extensive testing campaign with different grout and mor-
tar compositions is needed for the future.
A. PADOVNIK, V. BOKAN BOSILJKOV: ADHESIVE-STRENGTH ASSESSMENT OF LIME INJECTION GROUT ...
592 Materiali in tehnologije / Materials and technology 56 (2022) 5, 587–593

Acknowledgment
This work was financially supported by the Slovenian
Research Agency, through Programme Group P2-0185.
5 REFERENCES
1
A. Padovnik, F. Piqué, A. Jornet, V. Bokan-Bosiljkov, Injection
Grouts for the Re-Attachment of Architectural Surfaces with Historic
Value – Measures to Improve the Properties of Hydrated Lime
Grouts in Slovenia, Int. J. Archit. Herit., 10 (2016), 993–1007,
doi:10.1080/15583058.2016.1177747
2
EN 1015-12, Methods of Test for Mortar for Masonry – Part 12: De-
termination of Adhesive Strength of Hardened Rendering and Plas-
tering Mortars on Substrates, CEN, Brussels, Belgium, (2001)
3
L. C. Azeiteiro, A. Velosa, H. Paiva, P. Q. Mantas, V. M. Ferreira, R.
Veiga, Development of grouts for consolidation of old renders, Con-
struct. Build. Mater., 50 (2014), 352–360, doi:10.1016/j.conbuildmat.
2013.09.006
4
C. Pasian, F. Piqué, A. Jornet, S. Cather, A ‘Sandwich’ Specimen
Preparation and Testing Procedure for the Evaluation of Non-Struc-
tural Injection Grouts for the Re-Adhesion of Historic Plasters, Int. J.
Archit. Heritage, 1 (2019), 455–466, doi: 10.1080/15583058.
2019.1626513
5
P. Faria, J. Lima, J. Nabais, V. Silva, Assessment of adhesive strength
of an earth plaster on different substrates through different methods,
Proc. of the 5th Historic Mortars Conference, Pamplona, 2019,
51–64
6
EN 459-1, Building Lime – Part 1: Definitions, Specifications and
Conformity Criteria, CEN, Brussels, Belgium, (2010)
7
EN 1015-11, Methods of Test for Mortar for Masonry – Part 11: De-
termination of Flexural and Compressive Strength of Hardened Mor-
tar, CEN, Brussels, Belgium, (1999)
8
ASTM C496/C496M-1, Standard Test Method for Splitting Tensile
Strength of Cylindrical Concrete Specimens, ASTM International,
Harrisburg, PA, USA, (2004)
9
B. Biçer-ªimºir, R. Leslie H., Evaluation of Lime-Based Hydraulic
Injection Grouts for the Conservation of Architectural Surfaces: A
manual of Laboratory and Field Test Methods, The Getty Conserva-
tion Institute, Los Angeles 2013
A. PADOVNIK, V. BOKAN BOSILJKOV: ADHESIVE-STRENGTH ASSESSMENT OF LIME INJECTION GROUT ...
Materiali in tehnologije / Materials and technology 56 (2022) 5, 587–593 593