17

Experimental investigations to study the effect of

carbon nanotubes reinforced in cement-based matrix
composite beams
A M Hunashyal1, G V Sundeep1, S S Quadri1, and N R Banapurmath2*
1
Department of Civil Engineering, B.V. Bhoomaraddi College of Engineering and Technology, Hubli, India
2
Department of Mechanical Engineering, B.V. Bhoomaraddi College of Engineering and Technology, Hubli, India

The manuscript was received on 4 March 2011 and was accepted after revision for publication on 11 July 2011.

DOI: 10.1177/1740349911418570

Abstract: The present paper investigates the behaviour of cement beams reinforced with mul-
ti-walled carbon nanotubes (MWCNTs). The addition of MWCNTs was varied at 0.25, 0.5, 0.65,
0.75, 0.85, and 1 per cent by weight of cement. Dispersion of MWCNTs was carried out using
ultrasonic energy. Composite beams were tested under flexure in order to evaluate their
mechanical properties such as flexural strength and load–deflection criteria. These results
were then compared with those from plain cement control beams. The present work also
investigated the optimum percentage of MWCNTs that gives the best results in terms of both
enhanced properties and economy. Scanning electron microscopy was conducted to examine
the bond between the MWCNTs and the cement matrix.

Keywords: multi-walled carbon nanotubes, cement composites, single-point loading, ultimate

load, scanning electron microscopy

1 INTRODUCTION used to control cracking in fibre-reinforced

concrete [2].
Failure in many construction materials at the micro Nanotechnology is one of the most active research
scale is typically the result of tiny microcracks. Plain areas, with both novel science and useful applica-
concrete is a brittle material and although it is not tions that have gradually established its importance
normally designed to resist direct tension, knowl- over the past two decades [3, 4]. It enables scientists
edge of the tensile strength of concrete is of value in to work at the molecular level atom by atom to
estimating the load under which cracking will develop new materials with fundamentally new phys-
develop [1]. Advanced technological aspects have ical and chemical properties [5]. Carbon nanotubes
increased the demand for higher-performing (CNTs) are the subject of one of the most important
cement-based materials with improved tensile areas of research [6]. CNTs were discovered in 1991
strength and toughness for use in civil engineering by Iijima in Japan [7], and were found to possess a
projects. Cementitious materials are typically char- Young’s modulus of 1 TPa, a yield stress of 100–
acterized as quasi-brittle materials with low tensile 300 GPa [2], and a tensile strength of 63 GPa [6, 8].
strength and low strain capacity, and hence their CNTs may be classified as either single-walled
use may affect the long-term durability of struc- (SWCNTs) or multi-walled (MWCNTs). Nanoscience
tures. These days, discrete short fibres are widely has a central role to play in producing innovative
concrete materials for the 21st century. The incor-
poration of MWCNTs in cement increases the stiff-
*Corresponding author: Department of Mechanical Engineering, ness of the calcium silicate hydroxide (C-S-H) gel,
B.V. Bhoomaraddi College of Engineering and Technology, resulting in a stronger material [9]. While the
Vidyanagar, Hubli 580031, Karnataka, India. exploration for various applications of nanotechnol-
email: nr_banapurmath@rediffmail.com ogy to develop innovative construction materials

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18 A M Hunashyal, G V Sundeep, S S Quadri, and N R Banapurmath

continues, it is already clear that the science of the MWCNTs are desirable to know the actual beha-
very small is making big changes, with numerous viour in a cement matrix and the difficulties
economic benefits for the construction industry [10]. encountered in the dispersion of MWCNTs.
By using CNTs, a new material which exhibits Experimental work [2] has shown that the addition
enhanced tensile strength and Young’s modulus and of 0.25 wt% of MWCNTs to plain cement beams
improved early-age strain capacity can be created results in a remarkable enhancement in both flexur-
[11]. The various contributors to the research work al strength and toughness. In comparison with plain
on the utilization of CNTs in cement-based matrices cement beams, the CNT-reinforced beams showed a
are presented in Table 1 [12–16]. 47 per cent increase in load-carrying capacity and an
Hydrated cement is porous with a pore size distri- average increase in toughness of 25 per cent. Metaxa
bution that ranges from nanometres to millimetres. et al. [17] tested CNT/cement composites under
The dimensional range of solids and pores in a flexure and found that, with proper dispersion, the
hydrated cement paste has been reported in the litera- fracture properties of cement matrices can be sub-
ture [1]. The pore size classification of concrete stantially increased by adding a very low amount of
includes micropores of radius less than 1.25 nm, CNTs (i.e. 0.025 to 0.10 per cent of MWCNTs by
mesopores of radius 1.20–25 nm, and macropores of weight of cement). Scanning electron microscopy
radius 30–5000 nm. The pores are an important com- (SEM) has shown that the addition of a small
ponent of the microstructure of a cement-based com- amount of CNTs enables the control of matrix
posite, and have a great influence on the strength and cracks at the nano scale, and nano-indentation tests
durability properties of the material. A number of have suggested that CNTs can modify and reinforce
research groups have investigated the influence of the cement paste matrix at the nano scale. The fore-
CNTs on the pore structure of ordinary Portland going results suggest that CNTs have potential to be
cement (OPC)/CNT composites. The findings of the used as reinforcement for concrete; however, there
studies suggest that the combination of hydration is still a significant need for further research studies
product nucleation and pore bridging may result in a in this area to completely understand the structural
densification of the OPC/CNT composite and a reduc- behaviour of CNT-reinforced cement [18, 19].
tion in the overall porosity and pore continuity of the Fundamentally, hydrated cement paste is a nano-
material. However, two problems with the addition of material. Knowledge at the nano-scale level of the
CNTs and carbon fibres to any material are the composition and characteristics of materials will
clumping together of the tubes/fibres and the lack of promote the exploration of new applications and
cohesion between them and the bulk matrix material. new products that can be used to repair or to
A uniform dispersion must be achieved to get the improve the properties of construction materials
desired results [8]. The cost of adding CNTs to con- [18]. The compression strength of the cement paste
crete may be prohibitive at the moment but work is increased with the addition of CNTs that decreased
being done to reduce their price, and at such time the the number of mesopores [20]. CNTs have the
potential to enhance the strength, effectively hinder
cost/benefit ratio offered by their addition to cementi-
crack propagation in cement composites, and act as
tious materials may become more acceptable.
nucleating agents [4]. Reinforcing concrete with
Most of the published papers on the use of
CNTs will produce tougher concretes by interrupt-
MWCNTs in a cement matrix have focused on pre-
ing crack formation as soon as it is initiated.
liminary work involving very low additions varying
The interactions of CNTs in a cement matrix that
from 0.08 to 0.25 per cent by weight of cement,
hinder crack propagation have been reported in the
keeping in mind the cost and dispersion issue of
literature [5].
MWCNTs. Studies using a higher percentage of
The present work utilized MWCNTs, having dia-
meters of 10–30 nm, densities of 1000–2000 kg/m3,
and an aspect ratio of 1000:1, to produce reinforced
Table 1 Increment in mechanical properties with cement beams. The addition of MWCNTs was varied
incorporation of MWCNTs for cement-based from 0.25 to 1 per cent by weight of cement, and dis-
material persion of MWCNTs was carried out using ultraso-
Researchers Flexural strength increment (%) nic energy. The mechanical properties of the
composite beams were tested and the results com-
Li et al. [12] 25
Cwirzen et al. [13] 20 pared with those from plain cement control beams.
Konsta-Gdoutos et al. [14] 25 The optimum percentage of MWCNTs that gives the
Konsta-Gdoutos et al. [15] 40 best results in terms of both enhanced properties
Hunashyal et al. [16] 43.75 (0.75% MWCNTs)
and economy was investigated, and SEM was

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 Experimental investigations to study the effect of reinfoced carbon nanotubes 19

conducted to examine the bond between the satisfactory dispersion [21]. A w/c ratio of 0.4 was
MWCNTs and the cement matrix found to be appropriate in terms of mixing and
workability of the paste after adding the CNTs. After
this process, the paste was carefully placed into pre-
2 EXPERIMENTAL PROGRAMME pared moulds and compacted using a tamping rod.
The cast specimens, of dimensions shown in
The properties of the MWCNTs used in the present Table 3, were demoulded after 24 h, cured for 28
study are given in Table 2; the MWCNTs were of days, and tested on day 28 day. Serving as the con-
industrial grade with a purity of greater than 95 per trol, plain cement beams without any reinforcement
cent. Uniform dispersion of MWCNTs against their were also prepared. The designation of the test spe-
agglomeration due to van der Waals’ bonding is the cimens used is shown in Table 4.
first step in the processing of nanocomposites. Experimental tests were conducted to investigate
Dispersion is a critical issue while mixing CNTs in the effect of the uniformly dispersed and randomly
either water or organic solvents. Ethanol was used oriented MWCNTs in the cement paste as per the
for pre-dispersion of MWCNTs in cement using a Bureau of Indian Standards [22–24]. For experimen-
sonication method, as suggested by Makar and tal accuracy, ASTM C348 was followed to determine
Beaudoin [6]. Care was taken to ensure evaporation the average values of flexural strength (ASTM C 348
of the ethanol, and later, water was added without determines the flexural strength of hydraulic-cement
taking into account the amount of ethanol used mortars). Three-point bending tests as shown in
when calculating the water-to-cement (w/c) ratio of Fig. 1 were carried out. SEM was also conducted on
the paste. Moreover, the method of sonication, all samples with varying percentages of MWCNTs.
duration of sonication, and method of casting the
specimens were maintained uniformly throughout.
Different amounts of MWCNTs were added to 3 RESULTS AND DISCUSSION
cement as shown in Table 3 and the whole mixture
was kept in an ultrasonicator for 90 min to achieve The objective of the present study was to find the
optimum percentage of CNT dosage and the effect
of CNTs on the ultimate load-carrying capacity of
Table 2 Properties of the MWCNTs used in the pres- the MWCNT-reinforced cement composite under
ent work, supplied by Intelligent Private three-point loading. Figure 2 shows the load–deflec-
Limited tion curves for composites with different percen-
tages of MWCNTs in the cement matrix.
Diameter 10–30 nm
Length 1–2 mm The ultimate load was calculated for composites
Purity .95% containing 0, 0.25, 0.5, 0.75, and 1 per cent of
Surface area 350 m2/g MWCNTs by weight of cement, as shown in Fig. 3.
Bulk density 0.05–0.17 g/cm3
The variation of flexural strength was studied
keeping PC beams as reference; the flexural strength
followed an increasing trend up to 0.75 wt% of
Table 3 Specimen characteristics MWCNTs as shown in Table 5.
Size 20 mm 320 mm 380 mm Specimen A3 with 0.75 wt% MWCNTs showed a
Water/cement ratio 0.4 maximum ultimate load of 500 N, while the refer-
Amount of CNTs 0.25, 0.5, 0.65, 0.75, 0.85, 1 per cent by
weight of cement ence beam gave a value of 270 N. Since the compo-
site, at greater MWCNT contents, has a tendency to

Table 4 Details of the test specimens

Sample no. Specimen reference Specimen details
Constituents Percentage of MWCNTs by weight of cement

1 PC Plain cement Nil

2 A1 Plain cement + MWCNTs 0.25
3 A2 Plain cement + MWCNTs 0.50
4 A3 Plain cement + MWCNTs 0.65
5 A4 Plain cement + MWCNTs 0.75
6 A5 Plain cement + MWCNTs 0.85
7 A6 Plain cement + MWCNTs 1

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Table 5 Test results for the MWCNT-reinforced

cement composites
Sample no. Specimen Ultimate Maximum
reference load (N) deflection (mm)

1 PC 270 0.36
2 A1 280 0.44
3 A2 340 0.46
4 A2* 360 0.45
5 A3 500 0.49
6 A3* 490 0.48
7 A4 140 0.29

Fig. 1 Placing of the specimen for testing Variation of load carrying capacity for different proportions
subjected to single point loading
0.6
0.5 0.49
0.5
0.6 0.34 0.36
Load, kN
0.4
PC 0.27 0.28
0.3
0.5 A1 0.14
0.2
A2
0.4 0.1
A3
0
Load kN

A4
0.3 PC A1 A2 A2* A3 A3* A4
Proportions
0.2
Fig. 4 Variation of the ultimate load of the specimens
in single-point loading
0.1

0
0 0.1 0.2 0.3 0.4 0.5 reinforcements in composite materials. In addition
Deflection in mm to their strength and elastic constant, CNTs have
extremely high aspect ratios, with values typically
Fig. 2 Load–deflection curves for the specimens in higher than 1000:1 and reaching as high as
single-point loading
2 500 000:1 [1]. The size and aspect ratio of CNTs
mean that they can be distributed on a much finer
scale than commonly used reinforcing fibres. As a
Variation of deflection for different proportions
for single point loading result, cracks are interrupted much more quickly
0.6 during propagation in a CNT-reinforced matrix, pro-
0.44 0.46 0.45 0.49 0.48
0.5 ducing much lower crack widths at the point of first
Deflection, mm

0.36
0.4 contact between the moving crack front and the
0.29
0.3 reinforcement. As a result of these properties, CNT
0.2 reinforcements are expected to produce signifi-
0.1 cantly stronger and tougher composites than tradi-
0 tional reinforcing materials.
PC A1 A2 A2* A3 A3* A4
Proportion Figure 5 shows a SEM micrograph of the CNT/
cement composite with 0.25 vol% CNTs, from which
Fig. 3 Variation of deflection for different specimens it follows that CNTs act as bridges across pores and
cracks. They are tightly wrapped by C-S-H. This indi-
cates that high bonding strength between the CNTs
and cement matrix is achieved. This is consistent
undergo large deflections as shown in Fig. 4, the with other results published in the literature [25, 26].
CNTs provide additional toughness to the compo-
site. By this means the energy-absorbing capacity of
the composite generally increases, i.e. a greater 4 CONCLUSIONS
amount of load is carried since the CNTs resist the
crack propagation. Experimental observations revealed that the
The mechanical properties of CNTs have drawn MWCNT-reinforced cement composite beams
intense interest owing to their potential for use as showed increased strength compared with plain

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 Experimental investigations to study the effect of reinfoced carbon nanotubes 21

FUNDING
This research received no specific grant from any
funding agency in the public, commercial, or not for
profit sectors.

Ó Authors 2011

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