Thermal Stability of Latex Modified Mortars Containing CNTs
International Journal of Concrete Structures and Materials
Thermal Stability of Latex Modified Mortars Containing CNTs
Durability of concrete and cementitious materials has been of a great concern to the construction industry in the last two decades due to the deterioration of large number of concrete structures which were built in the 60 and 70 s. Among different environmental conditions, the continuous exposure to freeze-thaw and thermal fatigue cycles remains one of the most aggressive conditions to concrete structures and bridges. On the other hand, the use of nanomaterials, such as carbon nanotubes, has shown promising results in improving the properties of cementitious materials. In this paper, the role of using carbon nanotubes (CNTs) in the thermal stability and durability of latex modified mortars (LMM) is examined. CNTs contents of 0.5 wt%, 1.0%, and 1.5 wt% of latex were added prior to mixing the latex modified mortar components and the resulting LMM specimens were subjected to freeze-thaw and thermal fatigue cycles. The mechanical properties and dimensional stability of LMM specimens were then evaluated. In general, it was observed that the addition of CNTs improve the compressive strength of LMM specimens. On the contrary, CNTs have limited or no influence in the tensile strength, development of shrinkage strains, and flexural capacity of LMM specimens under same thermal conditions.
CNT; freeze and thaw; thermal fatigue; durability
Introduced almost 50 years ago, nanotechnology
represents a relatively new and very important research field with
numerous current and potential future applications in
different industrial sectors. Currently, the use of
nanotechnology extends to cover several mechanical, electrical, and
. Of the mechanical
applications, the use of nanomaterials as additives to
construction materials has been seldom introduced in industry
compared to other mechanical applications.
attributed this slow progress due to the lack of research on
the use of the nanomaterials in construction industry.
Applications of nanomaterials in construction industry
include concrete, steel, coatings and paintings, solar cells,
and structural health monitoring
(Lee et al. 2009)
concrete and cementitious materials represent the most
construction materials that are widely used in construction
industry, research on using nanomaterials in concrete
mixtures have grown rapidly in the last few years showing
promising results for enhancing their mechanical, electrical,
and thermal properties. Among all nanomaterials, carbon
nanotubes (CNTs) appear to be the strongest nano-sized
reinforcement for concrete so far.
A number of investigations have addressed the effect of
adding nanomaterials such as CNTs on the performance of
the mortar or concrete mixture. For example,
Kumar et al.
investigated the effect of adding multi-walled CNTs
on mechanical properties of cement paste. For compressive
strength, CNTs were found to affect the early stage of
concrete. 0.50% CNT content seemed to be the optimum content
for the cement mix.
Siddique and Mehta (2014)
effect of CNTs on properties of cement mortars. As CNTs
content rose, compressive strength increased with a
maximum observed strength at 1% content. Under high strain
loading rate, the compression strength, flexural strength, and
Young’s modulus were also improved with the addition of
CNTs. A low-cost pyrolysis technology that produced CNTs
and have hydrogen as the only byproduct without carbon
dioxide was developed. It was reported that making mixes
with 0.5 wt% CNTs increased the compressive strength of
the concrete by up to 30%. Compared to other common
admixtures, CNTs seemed to provide more benefits with the
addition of a smaller amount of material.
Two major challenges have been reported that typically
limits the use of nanomaterials in cementitious materials.
First is the difficulty in obtaining uniform dispersion of the
nanomaterials in cementitious matrix. To address this, some
studies suggested the use of ultra sonication as a
suitable method for obtaining fair dispersion of the
nanomaterials in the mixtures
(Tyson et al. 2011; Sobolkina et al.
. Second challenge arises from the need for creating
strong chemical bonding between the nanomaterials and
surrounding matrix. The presence of chemical coupling
agents between the CNTs and the cement hydration products
is necessary to transfer the loads between the mixture
constituents. Recently, Soliman et al. (2011) obtained fair
dispersion of CNTs in latex modified mortar specimens through
creative methods of mixing CNTs with surfactants in
polymer latex before adding the cement components. Adding
CNTs improved the properties of Latex modified mortars
(LMMs) such as compressive and tensile strengths, failure
strain, and toughness. In addition, early age compressive
strength was not affected by the amount of CNTs.
et al. (2012)
have also shown that the addition of CNTs to
Styrene butadiene rubber (SBR) latex increases the polymer
cross-linking resulting in stronger and stiffer latex
Konsta-Gdoutos et al. (2010)
aided by surfactant treatment, to disperse CNTs with
different lengths in cement paste. It was found that fracture
properties of cement were enhanced with the proper
Durability of concrete and cementitious materials has been
of a great concern to the construction industry as well as
research institutes in the last two decades. Large number of
concrete infrastructures which was built in the 60 and 70 s
has been deteriorated due to poor durability and the exposure
to aggressive environments. Several studies investigated the
deterioration of ordinary concrete due to freeze and thaw
Cai and Liu (1998)
explained the mechanism of
concrete deterioration due to the presence of freezed water
inside the concrete pores. The freezed water causes high
internal hydraulic pressure, which in turn damages the
Sun et al. (1999)
concluded that the applied stress/
strength ratio and the grade of concrete are the most
important parameters that affect the concrete durability when
exposed to freeze and thaw cycles.
Mu et al. (2002)
shown that concrete deterioration was further accelerated
when freeze–thaw cycles, loading, and chloride salt attacks
all acted simultaneously.
Limited number of studies investigated the freeze and
thaw resistance of concrete mixtures containing
(Cwirzen and Habermehl-Cwirzen 2013; Salemi et al.
2014; Wang et al. 2014; Li et al. 2015)
. It was concluded in
many studies that CNTs can retain more strength and less
cracking after being exposed to freezing and thawing tests.
While ordinary concrete suffers significantly when exposed
to freeze and thaw cycle, polymer modified concretes, such
as latex modified concrete, have been introduced to the
construction industry as a more durable class of construction
material to serve several structural applications such as
bridge deck overlays, water tanks, and swimming pools
(Ohama 1995, ACI 548.3R 2009)
. In this study, the
durability of polymer modified mortar (PMM) mixes containing
CNTs has been investigated. Different contents of CNTs
were dispersed in latex modified mortar (LMM) mixtures
and the durability of LMM mixes was evaluated through
freeze–thaw and thermal fatigue tests as discussed later.
2. Experimental Program
2.1 Mix Design
Ordinary ASTM C 150 Type I portland cement was used
in all mortar specimens along with natural sand, water, and
commercial SBR-Latex. Multi-walled carbon nanotubes
(MWCNTs) were obtained from Cheap Tubes, Inc.
According to the manufacturer’s specifications, the
MWCNTS had outer diameter of 8 nm, inner diameter of
2–5 nm, length of 10–30 lm, specific surface area of
500 m2/g, and bulk density of 0.27 g/cm3. In addition to the
control LMM specimens which had no MWCNTs, three
MWCNTs contents were added as a percentage of the weight
of latex: 0.5, 1.0, and 1.5%. Table 1 shows the mix
proportions of all LMM specimens examined in the study.
Sandto-cement, water-to-cement, and latex-cement ratios of 2.7,
0.38, and 0.15 were utilized in this study. This mix
proportions conform to range for the conventional LMM mix
proportions reported by
. The LMM
specimens with no MWCNTs were considered the control
specimens in this investigation.
2.2 CNTs Dispersion
To incorporate MWCNTs in LMM mixtures, a
combination of mechanical and chemical dispersion techniques was
implemented. The mechanical techniques involved the
development of ultrasonic waves (e.g. ultrasonication) or
high shear mixing forces (e.g. extrusion, ball milling) needed
to disentangle the nanotubes. In this study, ultrasonication
was implemented as a relatively cheap and effective method
for the mechanical dispersion. Ultrasonic bath Model B-52,
supplied by Branson , was used to perform the
ultrasonication in this study. The ultrasonic bath has a basket
dimensions of 292 mm 9 230 mm 9 150 mm and operates
on frequency of 50–60 Hz. Approximately, 70% of the
ultrasonic bath tank capacity was filled with distilled water
to perform the ultrasonication. On the other hand, the
chemical techniques involved the formation of weak (i.e.
non covalent) or strong (i.e. covalent bond) to promote the
dispersion process. In SBR latex, the presence of surfactants
in the ingredients of the latex facilitated the dispersion
through the formation of noncovalent Van der Waal bond
between the surfactants and the CNTs. The MWCNTs were
used first dispersed in deionized water using ultrasonication
for 30 min. The resulting MWCNTs–water mixture was
added to the SBR latex and the suspension was
ultrasonicated for 2 h. The ultrasonication setup is shown in Fig. 1.
The dispersion procedures were utilized previously by
Soliman et al. (2012)
who showed a fair distribution of the
CNTs in LMM.
2.3 Casting and Curing LMM Specimens
Two mechanical mortar mixers, supplied by Hobart, Inc.
(Fig. 2), were used to prepare all the mixes. Cement and
sand were first added in the mixer and were mixed for 5 min
at a rotational speed of 261 r.p.m. This is followed by adding
the CNTs-SBR-water suspension and mixing the composite
for 10 min. The LMM specimens were then cast in 50.8 mm
wide 9 50.8 mm high 9 279.4 mm long prisms, 50.8 mm
diameter 9 101.6 mm high cylinders, and 50.8 mm 9
SBR-Latex (kg/m3) CNT (kg/m3)
50.8 mm 9 50.8 mm cubes. Specimens were then cured
following ACI 548.3R, 2009 for 28 days. The curing regime
included water curing for 2 days, followed by air curing for
5 days, and finally water curing for the remainder of
28 days. The specimens were submerged in lime water for
48 h before they were subjected to 300 cycles of two types
of tests: thermal fatigue and freeze–thaw tests. The LMM
prisms were used to examine shrinkage and flexural
response of different LMM mixes while the cylinders and
cubes were used to examine the tension and compression
strengths respectively. At least three replicas were tested at
any given time, exposure, and loading conditions.
2.4 Thermal Exposure
The thermal exposure in this investigation consisted of
freeze–thaw cycling and thermal fatigue cycling tests. The
freeze–thaw and thermal fatigue tests were proposed in this
study to simulate the outdoor temperature changes during
the winter and summer times respectively. For the two tests,
the thermal cycles were applied directly after the curing of
specimens following ASTM C-666B (2015) standard. In the
freeze–thaw test, the temperature varied between
(- 18 ± 2 C) and (4 ± 2 C) for 300 cycles. The duration
for a single cycle was selected three hours following the
ASTM standards recommendations, which suggest duration
between 2 and 5 h for each cycle during freeze–thaw testing.
Mechanical testing and shrinkage measurements were
applied at specified intervals to monitor the damage
evolution throughout the test. The test was performed by first
cooling the air around the specimens to (- 18 ± 2 C) then
thawing the specimens by pumping water at a temperature of
(4 ± 2 C). On the other hand, similar procedures were
followed in conducting the thermal fatigue test by altering
the specimen’s temperature between (16 ± 2 C) and
(40 ± 2 C).
The thermal cycles were applied to LMM specimens in the
environmental chamber facility available at Ohio Northern
University. The temperature varying processes were
controlled by a DirectLOGIC D4-450 PLC (programmable logic
controller). The controller enabled setting the upper and
lower temperature limits, the water fill time, the delay before
draining time, and the drain time before putting the PLC in
automatic or manual control mode. User input and process
output was displayed on a C-more EA7-T8C 8-inch
touchscreen. The touch screen displayed the current temperatures
of the two thermocouples used, along with indicators for the
current operation in progress. Current water fill, freezer run,
and water drain times along with the current cycle and last
cycle times and cycle count were also displayed on the touch
The effect of thermal exposure on the mechanical and
dimensional stability of different LMM specimens was
evaluated through mechanical testing and shrinkage
measurements. In order to determine the effect of shrinkage on
the LMM specimens, the length of the LMM was measured
after every 30 cycles (i.e. almost every 4 days) using a
digital comparator. The change in length was then used to
compute the shrinkage strains induced due to the application
of freeze–thaw and thermal fatigue cycles. The prisms were
later tested in flexure according to ASTM C348 (2014)
standards using three point bending test after the completion
of the 300 cycles, which corresponds to concrete age of
66 days (Fig. 3). In the flexural test, the load was applied at
the mid-span of each specimen with a supported length of
228.6 mm. The load and mid-span deflection was recorded
and the load–deflection curves for different LMM specimens
were presented and discussed. In addition, compressive and
tensile strengths were determined for cubes and cylinders
after 28 days, after the exposure to 150 thermal cycles, and
after the exposure to 300 thermal cycles, which correspond
to concrete age of 47 days and 66 days respectively. The
strength of each specimen was determined and the average
of the three replicas was calculated and presented. The
compression test was performed in accordance with ASTM
C39 (2016), whereas the splitting tensile test was performed
in accordance with ASTM C496 (2004). The same standard
testing machine was used to conduct both of the
compression and tensile tests.
3. Results and Discussions
3.1 Effect of CNTs Content on Compressive and Tensile Strengths
Figures 4 and 5 presents the compressive strength of
LMM samples due to the freeze–thaw and thermal fatigue
cycles, respectively. In addition, statistical analysis using
student t test is conducted and presented in Tables 2 and 3 to
study the statistical significance of the compressive strength
test results due to changing CNTs content and number of
thermal cycles, respectively. For freeze–thaw testing, after
150 cycles, the LMM control samples experienced a drastic
drop in compressive strength; however, it gained part of the
lost strength (about 50%) after 300 cycles. The student t-test
shows the drop and the regain in the compressive strength is
statistically significant (Table 3). Similarly, LMM specimens
with different CNTs contents exhibited similar drop in
strength after the first 150 cycles. However, they regained
almost all the strength after 300 cycles. The student t-test
also shows that the drop and regain in compressive strength
for all CNTs contents is statistically significant. The drop in
strength in the first 150 cycles could be attributed to the early
development of microcracks due to the exposure to freeze–
thaw cycles, which in turn reduced the compressive strength.
As the LMM specimens were subjected to more freeze–thaw
cycles, no further development of microcracks occurred. In
addition, the low permeability of LMM helped reduce the
water loss from the mixtures and therefore contributed to the
late curing of cement and latex films. The late curing
resulted in a regain in the compressive strength
. The role of CNTs is evident in increasing the regain
in compressive strength as shown in Fig. 4. The mechanism
of CNTs in improving the compressive strength is due to
their reinforcing effect on the cured SBR latex, which yields
an integrated network of latex membrane that limits water
loss from the LMM mixtures. The reduced water loss
encourages late curing of cement and contributes
significantly on the regain in compressive strength. This
mechanism is in line with the observation reported by
et al. (2012)
in which the cured SBR latex films reinforced
by CNTs showed superior mechanical response when
compared to cured plain SBR latex films
(Soliman et al. 2012)
For thermal fatigue testing (Fig. 5), no regain was
observed in compressive strength after 300 cycles, instead a
drastic drop was observed. For the control specimen, the
drop in the compressive strength reached of 70%. Similar
drop in compressive strength of 60% was observed with
LMM specimen with high CNTs content of 1.5%. The
difference between the effect of freeze–thaw and thermal
fatigue tests on compressive strength is attributed to the
difference in the temperature exposure range. In freeze–thaw
cycles, the temperature was altered between - 18 to 4 C,
which helped maintain water in the mixture for late curing.
On the other hand, the temperature in the thermal-fatigue test
was altered between 18 and 40 C which promoted water
loss from the mixture and prevented the late curing and the
regain in compressive strength. It can also be noted that
LMM specimens with the moderate and low CNTs contents
(i.e. 0.5 and 1.0%) were more stable under thermal fatigue
cycles as they demonstrated statistically insignificant or no
drop in compressive strength after 300 cycles. It can
therefore be deduced from the compressive strength testing that
the addition of low-to-moderate CNTs contents to LMM
mixtures yields more thermally stable and more durable
mortar specimens. The inefficiency of adding high contents
of CNTs might be attributed to the poor dispersion and/or
increased surface area of the CNTs, which may slow down
the cement hydration and the latex curing. The adverse effect
of high content of CNTs (i.e. 1.5%) on LMM subjected to
thermal fatigue cycles is also evident from the high
variability of the compressive strength data, represented by the
high error bars for 1.5% CNTs case, as shown in Fig. 5,
relative to other CNTs contents.
Figures 6 and 7 show the tensile strength results of LMM
specimens subjected to freeze–thaw and thermal fatigue
tests, respectively. Unlike compressive strength, tensile
strength of LMM specimens showed much higher thermal
stability with almost no loss in tensile strength due to 150 or
300 cycles of freeze–thaw or thermal fatigue cycles. This
might be attributed to the difference in the failure
mechanisms of LMM between compression and tension. The
compression failure is typically governed by the formation
of multiple cracks which increase in numbers and intensity
till complete failure. In this case, the overall condition of the
treated LMM specimens contributes in the compressive
strength of the specimens. On the other hand, the tensile
fracture is governed by single critical crack in which the
local condition around the crack surface affects the tensile
strength and therefore, experience less effect due to thermal
treatment. Of interest in Fig. 7 is the fair improvement in
tensile strength after 300 cycles of thermal fatigue with the
use of 1.5% CNTs. The improvement in the tensile strength
could be attributed to the bridging effect of CNTs on the
crack surface. However, such improvement in tensile
strength is less than the regain observed in the compressive
strength observed in Fig. 4 given its lower magnitude and
relatively high variability.
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3.2 Effect of CNTs Content on Shrinkage
The development of shrinkage/expansion strains due to
freeze–thaw and thermal fatigue cycles are examined in
Figs. 8 and 9 respectively. Average normalized change in
length were calculated every 30 cycle from the change in the
specimen length in order to reduce uncertainties due to
precision in measurements, change in humidity, and
materials’ microstructure variability. Figure 8 shows that for
freeze–thaw cycles, all specimens observed average
normalized change in length within ± 0.6, except for the
control specimen which observed a maximum change in length
of 1.3 at the 300th cycle only. In addition, the LMM
specimen with 1.5% observed a constant and continuous
decrease in length from the 30th cycle to the 200th cycle.
Such decrease may be attributed to the fact that
incorporating high contents of CNTs reduces the mixture workability,
which in turn would introduce relatively high void content.
The high voids content will enable high shrinkage and
expansion under freeze–thaw cycles. The role of high
contents of CNTs in reducing the workability and increasing the
voids content of cementitious materials has been reported by
Van Tonder and Mafokoane (2014
). Figure 8 also shows that
the moderate contents of CNTs (e.g. 0.5 and 1.0%) are
slightly more thermally stable under freeze–thaw cycles than
the control LMM specimens and LMM specimens with high
CNTs content specimens. Lower shrinkage/expansion strains
would reduce the development of shrinkage microcracks and
yields LMM with better tensile and compressive strength as
discussed in some cases in the previous section.
Thermal fatigue test results showed more stable behavior
for All LMM specimens under thermal cycles than that
observed for the freeze–thaw cycles as shown in Fig. 9. A
maximum average normalized expansion and shrinkage
strains of ? 0.6 and - 0.6 were observed in a single
measurement for LMM specimens with 1.5% CNTs and
1.0% respectively. In addition, similar to the freeze–thaw
cycles, the LMM specimens with 1.5% CNTs observed
constant and continuous shrinkage between the 90th cycle
and the 240th cycle, which can be attributed to the decreased
workability and increased void contents. Overall, CNTs are
found to have little or no influence on the thermal stability of
LMM subjected to either freeze–thaw or thermal-fatigue
cycles. It is well known that LMC and LMM mixture have
good thermal stability
(ACI 548.3R 2009)
and therefore the
role of CNTs in improving the thermal stability shall not be
evident. However, previous research work has reported that
CNTs might improve the thermal stability of ordinary
Portland concrete due to its negative coefficient of expansion
3.3 Effect of CNT Content on Flexural Load–
Figures 10 and 11 depict the load–deflection curves of
LMM samples tested after the completion of all the freeze–
thaw and thermal fatigue cycles. For freeze–thaw test, an
average load capacity of 2500 N was observed for the
control specimens as shown in Fig. 10a. By adding 0.5%
CNTs to LMM specimens, the load capacity improved by
15% (Fig. 10b) whereas the load capacity reduced by 10%
with the addition of 1.0 or 1.5% CNTs (Fig. 10c and d). The
change in load capacity between the control specimens and
other groups are found within the acceptable single
laboratory coefficient of variation precision 1 s % of 14.4%.
Furthermore, statistical analysis using student t-test with 95%
confidence level reveals no significant statistical difference
in the load capacity between the control specimens and other
CNTs reinforced specimens. Therefore, no effect of the
adding CNTs on the load capacity of flexural specimens
subjected to freeze–thaw cycles can be deduced from
Thermal fatigue test results in Fig. 11 observed higher
flexure capacity for all LMM specimens as compared to
freeze–thaw cycles. For instance, the control specimens
observed average flexural capacities of 2500 and 4000 N
due to the application of 300 cycles of freeze–thaw and
thermal fatigue respectively. Such reduction in the flexural
capacity reflects the significant effect of freeze–thaw cycles
on the durability of LMM specimens. Moreover, similar to
the freeze–thaw testing, the addition of CNTs does not seem
to have any effect on the flexural capacity of thermal fatigue
treated specimens. This observation is in agreement with
most of the observed trend for the splitting tension strength
results of LMM specimens with various CNTs contents as
reported earlier in Figs. 6 and 7.
In this study, the effect of adding various contents of CNTs
on the thermal stability of SBR LMM was investigated.
Freeze–thaw and thermal fatigue cycles were subjected to
different LMM specimens to assess their shrinkage,
compressive, tensile and flexural performance. In general,
noticeable effect in compressive strength was observed with
the addition of 0.5 to 1.5% CNTs to LMM. However, limited
or no effect was observed on tensile strength, shrinkage
strains, and flexural behavior due to the addition of same
amounts of CNTs. Considerable loss in the compressive
strength was associated with the application of 150 cycles of
freeze–thaw, which could be attributed to the development
of microcracks. This was followed be relative regain in
compressive strength in the second 150 freeze–thaw cycles.
The role of adding CNTs to LMM specimens is evident by
regaining almost 100% of the loss in compressive strength
while on the control specimens; only 50% of the loss in
compressive strength was regained. For thermal fatigue test,
the addition of low-to-moderate CNTs contents maintained
the compressive strength unaffected by the thermal cycles
whereas a drastic drop of 70% in compressive strength was
observed in the control specimen. Unlike compressive
strength, the freeze–thaw or thermal fatigue cycles did not
influence the tensile strength of most of LMM specimens. Of
great interest is the addition of 1.5% CNTs which improved
the tensile strength of LMM specimens after the exposure to
300 cycle of thermal fatigue by 52% relative to no exposure
to thermal-fatigue cycles. This trend was not observed in the
control specimens or the LMM specimens with low and
moderate CNTs contents. Moreover, unlike other LMM
specimens, LMM specimens with 1.5% CNTs exhibited little
but constant and continuous shrinkage due to freeze–thaw
and thermal fatigue cycles. The shrinkage of such high
content of CNTs could be attributed to the reduced
workability of the LMM mixture and the corresponding increase in
The authors acknowledge Nathan Craft, Nathan Miller, Mike
Kimberlin, and Scot Cottle at the Ohio Northern University
for their help and support for the project.
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