MATEC Web of Conferences 378, 02013 (2023)
SMARTINCS’23
https://doi.org/10.1051/matecconf/202337802013
Applicability of cementitious capsules in concrete production:
initial assessment on capsule robustness, mechanical and selfsealing properties of concrete
Harry Hermawan1,2, Alicia Simons1, Silke Teirlynck1, Pedro Serna2, Peter Minne1, Giovanni Anglani3, Jean-Marc
Tulliani4, Paola Antonaci3, Elke Gruyaert1*
KU Leuven, Department of Civil Engineering, Materials and Constructions, Ghent Campus, Gebroeders De Smetstraat 1, 9000 Ghent,
Belgium
2 Instituto de Ciencia y Tecnología Del Hormigón (ICITECH), Universitat Politècnica de València, Camino de Vera S/n, 46022
Valencia, Spain
3 Department of Structural, Geotechnical and Building Engineering (DISEG), Politecnico di Torino, Corso Duca degli Abruzzi 24,
10129 Torino, Italy
4 INSTM Research Unit PoliTO-LINCE Laboratory, Department of Applied Science and Technology (DISAT), Politecnico di Torino,
Corso Duca degli Abruzzi 24, 10129 Torino, Italy
1
Abstract. The use of macrocapsules in self-healing applications offers a potential benefit by carrying a
larger amount of healing agent in comparison with microcapsules. However, the application of
macrocapsules is still limited to paste and mortar levels on lab-scale. This is due to a concern that most
capsules might be broken when mixed with concrete components. In this study, cementitious tubular
capsules were used and they were considered as a partial replacement of coarse aggregates (2 vol% gravel).
The capsules have a dimension of 54 mm and 9 mm in length and outer diameter, respectively. A waterrepellent agent (WRA) was entrapped in the capsules as a proposed agent to seal the crack. Initial results
revealed high survivability of capsules during concrete mixing: 100% survival ratio when tested in a drum
mixer and 70–95% when tested in a planetary mixer. The mechanical and self-sealing properties of concrete
containing embedded capsules were evaluated. With the addition of capsules, around 8% reduction of
compressive strength was noticed, but no further effect on splitting tensile strength was detected as compared
with concrete without capsules. Ultrasonic pulse velocity (UPV) tests confirmed that the presence of
capsules also did not significantly affect the compactness of the hardened concrete. Furthermore, the
embedded capsules were able to break when a crack was introduced and it was found that 90% sealing
efficiency was achieved by capsule-based concrete as a result of the successful release of sealing agent into
the crack.
1 Introduction
In recent self-healing/self-sealing concrete technology,
some healing/sealing agents, which will be introduced
in the cementitious composite, are stored inside a vessel.
The agent is often called as healing agent if it is able to
autonomously heal the crack with (precipitated)
products ensuring a crack closure phenomenon (e.g.
bacteria, sodium silicate, polyurethane). On the other
hand, sealing agent refers to a specific agent with the
ability to seal the crack in the way that water or liquid
cannot penetrate into the cementitious matrix (e.g.
water-repellent agent). In this case, the sealing agent
seals the crack without having a crack closure
phenomenon. A lot of research is done on encapsulation
techniques for both healing and sealing agents. There
are numerous vessels such as capsules [1,2], aggregates
[3], and polylactic acid particles [4]. Especially capsules
are often developed in two distinct technologies: microencapsulation and macro-encapsulation. In the micro-
encapsulation method, the microcapsules are produced
through a series of chemical processes (e.g. in-situ
polymerization, emulsification, etc). In fact, a tiny
amount of agent is stored inside a microcapsule but the
microcapsules are normally produced in bulk. In
contrast, the macro-encapsulation method comprises
storage of agent in bigger-sized capsules. These
capsules can be made from commercial materials such
as glass [5], ceramic [6], or polymeric [7] tubes, that can
be cut into different sizes. In this case, the agent can be
stored in a bigger amount as compared with
microcapsules. However, there is sometimes an
incompatibility issue between the healing/sealing agent,
capsule material and cementitious material. Gruyaert et
al. [1] reported a premature polymerization of healing
agent (i.e., polyurethane) inside the polymeric capsules.
Araujo et al. [5] stated that the use of glass capsules
might induce alkali-silica reaction which is critical for
concrete durability. To mitigate these issues,
cementitious capsules have been recently developed and
* Corresponding author: elke.gruyaert@kuleuven.be
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution
License 4.0 (http://creativecommons.org/licenses/by/4.0/).
MATEC Web of Conferences 378, 02013 (2023)
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https://doi.org/10.1051/matecconf/202337802013
proposed using a polymer-modified cement paste [8,9].
The advantages of using the cementitious capsules are
reduced brittleness, reduced risk of alkali-silica reaction,
and higher compatibility with the surrounding matrix,
with respect to the glass capsules [10]. A mechanical
regain of the cementitious composite containing
cementitious tubular capsules (with polyurethane as
healing agent) under static and cyclic loading was also
found [11]. Nevertheless, the application of any type of
macrocapsule is still limited to the paste and mortar level
[12], and only few studies [5,13] applied the capsules
into the concrete. The assessment of the fresh and
hardened properties of the capsule-based composite is
also rarely done because the main objective of using
capsules still relies on the healing or sealing
performances. This has been identified by the authors as
a research gap. Eventually, this paper attempts to
investigate the effects of the macrocapsules in the fresh
and hardened concrete. The cementitious capsules (54
mm in length and 9 mm outer diameter) are used in this
study. The robustness of capsules toward mixing forces
is initially evaluated. The self-sealing properties of
capsule-based concrete are assessed by means of a
capillary water absorption test. An additional test is
conducted to quantify the sealing coverage area as a
result of the released sealing agent from the capsules.
gravels (see Table 1). The length, outer diameter and
inner diameter of cementitious capsules were 54, 9 and
6 mm, respectively. On the outer surface of the capsule,
a sand layer was applied in order to provide a good
bonding between the capsule and concrete matrix.
Water-repellent agent (Sikagard 705L) was selected as
a sealing agent that was stored inside each capsule with
an approximate amount of 1 mL. The detailed
composition and manufacturing process of cementitious
capsules can be found in [11,14].
Fig. 1. Cementitious capsules
2.2 Preliminary test: capsule survivability test
Before incorporating the capsules into the concrete, the
capsule survivability test was initially performed to
determine the robustness of the capsules during concrete
mixing. To realise this test, two types of mixers were
employed: a tilting drum mixer (Lescha SM 145 S) and
a planetary/rotary pan mixer (Zyklos ZZ 75 HE). In this
regard, the REF mixture was used as a preliminary test
with a target volume of 20 L. The capsules were added
in two different dosages per type of mixing, with a total
of 20, 40, or 80 pcs in each case (see Table 2). A normal
concrete mixing procedure was implemented. First, the
raw materials (i.e., cement and aggregates) were mixed
for 30 sec in the drum/planetary mixer. Second, the
mixing water and superplasticizer were added and the
mixing was continued. After 6 minutes of mixing,
capsules were added into the mixer. For the survivability
test with the drum mixer, the fresh mixture with the
capsules was continuously mixed for another 3 minutes,
while with the planetary mixer, it was only mixed for
another 2 minutes. The survivability test was performed
by taking the fresh concrete little-by-little from the
mixer onto a sieve and then shaking the sieve under
water in order to remove the fresh mortar from the
aggregates and capsules. In this way, the capsules can
be easily found and the number of intact and broken
capsules was counted manually. A quick overview of
this developed test is depicted in Figure 2. The survival
ratio was calculated as the total number of intact
capsules over the total number of originally added
capsules in percentage.
2 Materials and methods
2.1 Materials
Table 1 shows the mix designs of reference concrete
(REF) and concrete containing capsules (CAPS). CEM
III/A 42.5N, having 52% clinker and 48% blast furnace
slag, was used as a binder component. Sea sand 0/2.5
was used as fine aggregate, while two fractions of
gravels (4/8 and 8/16) were used as coarse aggregates.
The specific gravities of sand 0/2.5, gravel 4/8 and
gravel 8/16 were 2.67, 2.59 and 2.60, respectively. The
water-cement ratio (w/c) was fixed at 0.50 for all
mixtures. Polycarboxylate-ether (PCE) superplasticizer
(Fluvicon 801, supplied by CUGLA B.V.) with a 20%
solid content was used to improve the workability of the
mixes.
Table 1. Mix designs
Material
CEM III/A 42.5N
Sand 0/2.5
Gravel 4/8
Gravel 8/16
Effective water
Superplasticizer
Effective w/c
Capsules
Unit
kg/m3
kg/m3
kg/m3
kg/m3
kg/m3
kg/m3
vol% vs. gravel
REF
325
740
701
378
163
0.89
0.50
-
CAPS
325
740
678
365
163
0.89
0.50
2
The REF mixture was made without the addition of
capsules. In case of CAPS mixture, the cementitious
capsules (see Figure 1) were added into the concrete mix
with a dosage of 2% by the volume of coarse aggregates.
The capsules were treated as a partial replacement of
Fig. 2. Capsule survivability test
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crack that will be generated in the middle span of the
prism, it was decided to manually place the capsules
instead of adopting a randomly placement. In order to
keep the same condition, the REF mixture was cast in
the same way as the CAPS mixture but no capsules were
added. One day after casting, the specimens were
demoulded. The cube and cylindrical specimens were
stored in a water tank at a temperature of 20±2 °C, while
the prismatic specimens were stored in a curing chamber
at 20±2 °C and 60% RH.
2.3 Modified mixing and casting procedure for
capsule-based mixture
A concrete volume of 35 L was aimed both for REF and
CAPS mixtures. Especially for the CAPS mixture, a new
mixing process was developed. Initially, all dry
materials (i.e. cement, sand, gravel) were put into the
drum mixer and were mixed for 30 s. Mixing water was
then added into the mixer and after 2 minutes of mixing,
the mixer was stopped and the superplasticizer was
added. Next, the mixing process was continued. After 6
minutes of mixing, a big portion of the fresh mix (~20
L) was poured into a bucket and around 15 L of fresh
mix remained in the mixer. Fifty-two capsules
(corresponding to 2 vol% of gravel in a 15 L fresh
mixture) were added into the mixer and the leftover mix
(~15 L) was mixed again for 90 s. Hence, the fresh
mixture containing capsules was tested by means of
slump and air content tests. Fresh concrete from the
bucket was then prepared to be combined with capsules
and cast into moulds. Three cube moulds (150 mm in
side), three cylindrical moulds (Ø100×200 mm) and
three prismatic moulds (100×100×400 mm3) were
prepared to cast the capsule-based concrete. It was
initially calculated that 2% of capsules by the volume of
gravel corresponds with 14 capsules per cube specimen
and 6 capsules per cylindrical specimen. A small
amount of fresh mix was taken from the bucket into a
bowl and depending on the volume of the designated
specimens, the desired correct number of capsules was
added into the bowl. Next, the fresh mix and capsules in
the bowl were manually mixed by a scoop until a
homogeneous mixture was obtained which was then
poured into the mould. The fresh mix with capsules was
compacted by a vibrating table. In this way, a random
distribution of capsules was attained. This method is
proposed to have the target amount of capsules per
specimen. A preliminary test also showed that the
capsules were completely intact after concrete mixing in
the drum mixer, thus this method can be used to simulate
the application of capsules in the concrete and to
understand the effects of capsules toward mechanical
performance of hardened concrete.
For the prismatic specimens, the capsules were
manually placed into the moulds by the following
procedure:
o the first layer of fresh mix was poured into the mould
with a height of approximately 40 mm,
o on top of the first layer, three capsules were placed
in the middle span of the mould,
o the second layer of fresh mix was poured into the
mould until reaching a height of 60 mm from the
bottom of the mould,
o three capsules were placed again in the middle span
of the mould and finally, the fresh mix was poured
until it fully covered the entire mould.
The cross section of the prism can be found in Figure 3.
Unlike a random distribution of the capsules in cube and
cylindrical specimens, the prismatic specimens were not
aimed to assess the mechanical performance of the
concrete. It was targeted to assess the ability of capsules
to break during cracking and at the same time to evaluate
the sealing properties. As the point of interest here is a
Fig. 3. Schematic design of the capsule-based prism
2.4 Test methods
The fresh properties of concrete were assessed by means
of slump, air content and fresh density tests (EN 123502,6,7), while the hardened properties of concrete were
evaluated by means of compression and tensile splitting
tests at 28 days (EN 12390-3,6). An ultrasonic pulse
velocity (UPV) test based on EN 12504-4 was also
conducted on the hardened cube specimens to evaluate
the compactness of the specimens with and without the
presence of capsules. During the UPV test, the
transmitter and receiver were always arranged in the
same way per each specimen, either REF or CAPS, i.e.
the UPV probes were placed at the center of two
opposite faces of cube, and the wave propagation
direction was perpendicular to the casting direction. The
prisms were subjected to the three-point bending test to
induce a crack at the age of 14 days. During the threepoint bending test, the loading was manually controlled
by the user with a relatively slow loading rate and it was
directly unloaded as soon as a crack occurred. Since the
rebars were embedded 20 mm from the top of the
prisms, the specimens did not experience a sudden
failure and after unloading, the crack width slightly
reduced due to the rebar relaxation. Then, the cracked
prisms were stored in an oven at 40°C for 10 days until
constant weight was achieved. Constant weight was
considered to be achieved when the change in mass over
a period of 24 hours was less than 0.2% [15]. Next, the
sides and the bottom of the specimens were partially
covered with epoxy resin (Episol Designtop SF) as
shown in Figure 3. The prisms were later returned to the
oven for 3 days and the capillary water absorption test
was finally conducted. A small area around the crack (20
mm in width) was left uncovered which was assigned as
the contact area for this test (see Figure 3). The
specimens were immersed in water with the water level
3 mm above the bottom surface of the prism. The weight
of each specimen was recorded after 10, 20, 30, 60, 90,
120, 180, 240, 360, 480, and 1440 minutes. During the
capillary water absorption test, it was ensured that the
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water level remained constant. As a note, the capillary
water absorption test was performed on 3 uncracked
reference prisms (REF_UNCR), 3 cracked reference
prisms (REF_CR), and 3 cracked capsule-based prisms
(CAPS_CR). In addition, the water droplet test was later
executed on the split cracked specimens. After
performing the capillary water absorption test, all prisms
were deliberately cracked until failure, resulting into
two parts of prisms per specimen. One part of the
specimen was taken and the cross section was tested by
the water droplet test. The droplet test was performed by
releasing water droplets from the pipette to the crack
surface (entire area of the cross section), and the water
droplets were remained for approximately one minute.
Next, the crack surface was captured by the camera and
the area where the water droplets were not absorbed by
the matrix indicates a sealed area and the wet area where
the water droplets were absorbed by the matrix indicates
an unsealed area.
aggregates). The fresh densities of both mixtures were
identical regardless the addition of capsules.
Table 3. Fresh properties (note: single measurement for slump
and air content tests)
Fresh properties
Unit
REF
CAPS
Slump
mm
130
100
%
3.0
4.0
kg/m3
2328 ± 14
2329 ± 6
Air content
Fresh density
Figure 4 illustrates the mechanical performance of
REF and CAPS concretes and the evaluation of UPV
results. The REF concrete had a compressive strength of
42.9 MPa and the introduction of 2% capsules caused an
8% strength reduction. Statistical analysis by a t-test
revealed that the difference of compressive strengths
between REF and CAPS concretes was statistically
significant (p-value = 0.002 < 0.05 as significance
level). Interestingly, the tensile splitting strength of both
mixtures had the same value of 4.1 MPa. Additionally,
the UPV of hardened concretes with and without
capsules had identical values at roughly 4810 m/s. It can
be concluded that the addition of capsules mainly affects
the compressive strength and no notable effects are
found on the tensile splitting strength and the matrix
compactness. It may be attributed to the effect of the
random capsule distribution inside the concrete. One
possible reason for strength reduction was due to the
packing disturbance and the capsules were regarded as
‘weak’ spots in the matrix.
3 Results and discussions
3.1 Capsules robustness
The results from the capsule survivability test were
summarized in Table 2. It was shown that a 100%
survival ratio of capsules was achieved during mixing
the capsules in a drum mixer, while in case of using a
planetary mixer, the survival ratio was attained in the
range of 70–95%. A lower survival ratio in a planetary
mixer occurred due to the high shear mixing force which
eventually damaged a few capsules. Nevertheless, the
capsules were considered robust enough to resist the
mixing forces. As a proof-of-concept, in order to
guarantee no single capsule was broken during the
mixing process, the drum mixer was further opted for
the final REF and CAPS mixtures.
Table 2. Capsule survivability (results based on a single test)
Concrete mixer
Concrete volume (L)
Number of added
capsules (pcs)
Number of
Intact
capsules after
capsules
mixing –
Broken
manual
counting (pcs) capsules
Survival ratio (%)
Drum
mixer
20
20
Planetary
mixer
20
20
40
80
20
40
40
80
14
38
0
0
6
2
100
100
70
95
Fig. 4. Hardened properties of the REF and CAPS concretes
(notes: n = 3 for compression tests, n = 3 for tensile splitting
tests and n = 3 for UPV tests)
3.3 Self-sealing properties
The 14-days-old prisms were cracked by means of the
three-point bending test. In case of the REF concrete, the
average crack width of three cracked prisms was 135 ±
10 µm. The CAPS concretes were also cracked, but the
average crack width (184 ± 17 µm) was slightly higher
than for the REF. When the CAPS prisms were cracked,
a release of sealing agent was observed as shown in
Figure 5. This confirms that the capsules were
successfully ruptured during the cracking stage. In this
condition, the released water-repellent agent was seen to
be rapidly absorbed by the concrete matrix in the crack
3.2 Fresh and hardened properties
The fresh properties of REF and CAPS concretes were
summarized in Table 3. The slump of REF concrete was
130 mm, while CAPS concrete had a lower slump of 100
mm. Nevertheless, both slump results were categorized
in the S3 slump class. The introduction of capsules
increased the air content of the fresh mixture from 3%
(REF) to 4% (CAPS). This may be due to the fact that
the presence of capsules that disturb the packing of the
concrete materials (especially on the packing of
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zone. In order to evaluate the sealing ability, the
capillary water absorption was performed on uncracked
and cracked specimens. Figure 6 shows the relationship
between the water uptake and the water immersion time
during the test. Based on the REF concretes, it was clear
that the presence of a crack caused a significant increase
of the water uptake with increasing immersion time with
respect to the uncracked specimen. With the cracked
CAPS prisms, the progress of water uptake over time
was considerably low and it was less dramatic than
cracked REF prisms. Nevertheless, the water uptake of
cracked CAPS prisms was slightly higher than the
uncracked REF prisms. It proves that the released
sealing agent seals the concrete matrix by preventing the
penetration of water via the crack. According to Figure
6, the sorption coefficients (S) were recorded at 1.18,
6.98 and 1.78 kg/m2h0.5 for uncracked REF, cracked
REF and cracked CAPS concretes, respectively. The
sealing efficiency (SE) of the CAPS concretes can be
calculated by Equation (1) and it was found that 90% SE
was achieved by the CAPS concrete. A previous study
by Anglani et al. [14] also showcased a 92% sealing
efficiency with the use of the cementitious capsules
filled with WRA in the mortar matrix and although the
definition for the SE was slightly different, the results of
Anglani support the finding in this study.
𝑆𝑆𝑆𝑆 =
𝑆𝑆𝑅𝑅𝑅𝑅𝑅𝑅_𝐶𝐶𝐶𝐶 −𝑆𝑆𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶_𝐶𝐶𝐶𝐶
𝑆𝑆𝑅𝑅𝑅𝑅𝐹𝐹_𝐶𝐶𝐶𝐶−𝑆𝑆𝑅𝑅𝑅𝑅𝑅𝑅_𝑈𝑈𝑈𝑈𝑈𝑈𝑈𝑈
× 100%
presence of the water repellent agent in that area. The
AutoCAD software was used to measure the total area
of the concrete surface and the total unsealed area. In
this way, the sealed area can be quantified. It was found
that based on three repetitions, the total sealing coverage
area for the CAPS concretes was in the range of 80–
88%.
Fig. 7. Sealing coverage area based on the water droplet test
As a proof-of-concept, a qualitative assessment
was made to split the cube and cylindrical specimens,
which had been previously tested by compression and
tensile splitting tests, respectively, in order to count the
amount of broken capsules in a certain crack plane. The
specimens were placed in a tensile splitting setup, and
they were tested until failure, as shown in Figures 8 and
9. The number of broken capsules was counted after
performing the splitting tests on three cylindrical
specimens and three cube specimens containing 2%
capsules. Few capsules in a certain plane of specimens
broke during the splitting test. As shown in Figure 8,
(1)
Fig. 5. Successful rupture of embedded capsules by releasing
the sealing agent through the crack
Fig. 8. Splitting the cube specimens to count the amount of
broken capsules in a certain crack plane (note: the darkest
concrete surface is the area covered by WRA)
Fig. 6. Capillary water absorption results (note: n = 3 for
capillary water absorption tests on each specimen series)
Fig. 9. Splitting the cylindrical specimens to count the amount
of broken capsules in a certain crack plane (note: the darkest
concrete surface is the area covered by WRA)
An additional test was performed to assess the
sealing coverage area in the crack zone by water droplet
test. Figure 7 clearly shows a 100% unsealed area for a
REF specimen and a distinction between sealed and
unsealed areas for a CAPS specimen. The sealed area
represents a hydrophobic coating confirming the
based on three repetitions, 2-5 out of 14 embedded
capsules were present and broken in the crack plane of
the cube specimens. On the other hand, as shown in
Figure 9, based on three repetitions, 2-3 out of 6
embedded capsules were present and broken in the crack
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plane of cylindrical specimens. Despite the fact that not
all of the embedded capsules were broken, the fracture
surface covered by the sealing agent appears to be quite
large with respect to the total fracture surface. This is
due to the fact that a single macrocapsule can carry a
considerable amount of sealing agent.
These tendencies occur due to a random distribution
of the capsules inside concrete, thus the location of the
capsules is not concentrated in one specific location. For
the CAPS prisms, the capsules were deliberately placed
in the middle span of the specimens, thus when they
were split (see Figure 7), all embedded capsules were
broken. This observation implies that the
distribution/placement of the capsules plays a key role
on the successful capsule breakage when a crack
penetrates into the concrete. Pros and cons related to the
capsule placement are discussed below:
• In the case of a random distribution of capsules in the
concrete, it is not guaranteed that all capsules can be
broken when a single crack is introduced. It strongly
depends whether the capsules are present in the crack
plane. However, it can also be beneficial if the cracks
occur in different locations. It will open a high
possibility of ‘in situ’ repair where the capsules in
random locations are broken and the release of
sealing agent may occur in many places.
• In the case of a specific placement of capsules in the
concrete (like CAPS prisms), all capsules are mostlikely at the ‘right’ place when a single crack is
introduced. However, if cracks occur in any other
location in which the capsules are not present, the ‘in
situ’ repair will not be achieved. This concept of a
specific capsules placement might be useful in case
the cracks are predicted to occur in a specific
location, thus the specific capsules placement can be
strived to allow a local repair.
4. Based on the capillary water absorption test, the
water uptake of the cracked CAPS specimens was
almost as low as the water uptake of the uncracked
REF specimens. This occurred due to the released
water-repellent agent from the capsules that sealed
the crack. A 90% sealing efficiency was achieved for
the capsule-based concrete with the sealing coverage
area of 80–88%.
4 Conclusions
[8]
This project has received funding from the
European Union’s Horizon 2020 research
and innovation programme under the
Marie Skłodowska-Curie grant agreement
No 860006.
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This study aims to investigate the introduction of
macrocapsules in the concrete production by evaluating
the capsules robustness, mechanical and self-sealing
properties of hardened concrete. Cementitious capsules
with a length of 54 mm and an outer diameter of 9 mm
were used and the water-repellent agent was stored
inside the capsules. The dosage of capsules was fixed at
2% by the volume of coarse aggregates. Based on this
study, the key findings are summarized as follows:
1. The cementitious capsules were found to be robust
to resist the mixing forces with 100% survival ratio
when tested in a drum mixer and 70–95% when
tested in a planetary mixer.
2. The addition of capsules slightly reduced the slump
value but resulted in a higher air content.
3. A reduction of compressive strength by 8% was
found when the capsules were introduced at 2% by
the volume of coarse aggregates which potentially
occurs due to the disturbance of the packing and the
presence of capsules could be ‘weak’ spots in the
concrete matrix. Furthermore, the presence of
capsules did not alter the compactness of concrete
matrix and the tensile splitting strength of concrete.
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[10]
[11]
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