Improving Impact Resistance of Polymer Concrete Using CNTs
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Polymer concrete (PC) has been favoured over Portland cement concrete when low permeability, high adhesion, and/or high durability against aggressive environments are required. In this research, a new class of PC incorporating Multi-Walled Carbon Nanotubes (MWCNTs) is introduced. Four PC mixes with different MWCNTs contents were examined. MWCNTs were carefully dispersed in epoxy resin and then mixed with the hardener and aggregate to produce PC. The impact strength of the new PC was investigated by performing low-velocity impact tests. Other mechanical properties of the new PC including compressive, flexural, and shear strengths were also characterized. Moreover, microstructural characterization using scanning electron microscope and Fourier transform infrared spectroscopy of PC incorporating MWCNTs was performed. Impact test results showed that energy absorption of PC with 1.0 wt% MWCNTs by weight of epoxy resin was significantly improved by 36 % compared with conventional PC. Microstructural analysis demonstrated evidence that MWCNTs significantly altered the chemical structure of epoxy matrix. The changes in the microstructure lead to improvements in the impact resistance of PC, which would benefit the design of various PC structural elements.
Keywordspolymer concrete carbon nanotubes impact strength microstructure cracking
Concrete polymer composites were introduced to the construction industry in three types: polymer modified concrete (PMC), polymer concrete (PC) and polymer impregnated concrete (PIC). The use of PMC, PC and PIC has remarkably increased in the last 20 years. This paper is focused on PC where a thermoset polymer (e.g. epoxy, polyester) replaces cement as the adhesive matrix for concrete based on ACI Committee 548 (2009). PC has gained wide acceptance because of its relatively high strength, high bond to concrete and steel substrates, rapid setting and ease of placement. The early use of PC in the construction field was limited to repair and rapid repair applications (Wang et al. 2003). However, PC is currently used in applications with aggressive service environments such as bridge deck overlays, industrial floors, utility rooms and manholes and for architectural precast panels (ACI Committee 548 2009; Radlińska et al. 2014). Because of its high damping characteristics, PC was also favored in applications with high dynamics such as machine foundations (Orak 2000). Many investigations were performed in the last two decades to modify mechanical characteristics of PC. Such investigations included replacing aggregate filler in PC by fly ash (Rebeiz and Craft 2002; Rebeiz et al. 2004), or by recycled plastic wastes (Bignozzi et al. 2000; Tawfik and Eskander 2006; Jo et al. 2007).
The impact resistance of concrete was typically improved using additives, specifically fibers, to the polymer matrix or by replacing aggregate with energy absorbent ones. Li and Xu (2009) improved the energy absorption capacity and deformation at failure of PC by adding basalt fibers. The dynamic modulus of elasticity and energy absorption were also improved using marble particles (Martínez-Barrera and Brostow 2010). Moreover, dispersed chopped glass and carbon fibers were used to produce fiber reinforced polymer concrete with enhanced strength, stiffness and energy absorption (Wang et al. 2013; Sett and Vipulanandan 2004). For instance, chopped carbon fibers were proved to improve the energy absorption of isophthalic polyester mortar by up to 360 % (Sett and Vipulanandan 2004). Other researchers showed that carbon fibers with as low as 0.2 % volume fraction were able to improve the strength and energy absorption of concrete (Xu et al. 2010).
Carbon nanotubes (CNTs) are one of the nano-scale inorganic particles utilized as additives in composites. CNTs are tabular materials made from concentrically rolled single or multiple graphite sheets. They are produced using the carbon arc-discharge method normally consisting of hollow carbon hexagonal networks concentrically arranged around each other and containing multiple inner chambers (Zhou and Chow 2003). CNTs were first introduced as single-walled carbon nanotubes (SWCNTs) and proved to have superior mechanical properties, but SWCNTs showed to be significantly expensive. High-purity SWCNTs are synthesized and grown using thermal chemical vapor deposition (CVD) at a growth temperature above 700 °C (Sharma and Iqbal 2004; Ting et al. 2008). Multi-walled carbon nanotubes (MWCNTs) appeared later as a cheaper alternative form of CNTs. In CVD method MWCNTs are synthesized at a relatively low temperature of 450 °C compared to 700 °C for SWCNTs (Sharma and Iqbal 2004). Although CNTs production is a challenging process, low-cost large-scale production of MWCNTs has been reported by Huang et al. with a yield as high as 10 kg/h using fluidized bed catalytic CVD technique (Huang et al. 2012).
MWCNTs have multiple energy and structural applications. It can be used for hydrogen storage and gas absorption (Tan et al. 2012). Moreover, MWCNTs were used to modify glass fiber reinforced polymer (GFRP) laminates to create electrically conductive networks in the matrix for structural health monitoring applications (Viets et al. 2009). MWCNTs have also been proved able to significantly improve the matrix dominated properties of GFRP (e.g. interlaminar shear strength) (Gojny et al. 2005) and to enhance the off-axis shear strength of carbon fiber reinforced polymer (CFRP) (Soliman et al. 2012a). In addition, CNTs are used to alter the impact resistance of polymer nanocomposites (Laurenzi et al. 2013) and woven fabric composite materials (Soliman et al. 2012b). It was shown that latex modified concrete with enhanced properties can be produced by incorporating CNTs in the polymer matrix during PC fabrication (Reda Taha et al. 2013). In order to enhance the bond between CNTs and polymer systems, CNTs were functionalized by chemical groups, which react with the polymer resin and hardener. Although the dispersion of CNTs in polymer matrices is a challenging process, several methods have been reported as suitable techniques for large-scale dispersion of CNTs in polymer matrices including melt blending, extrusion, latex polymer technology (Choudhary and Gupta 2011; Ma and Kim 2011). In this case, the use of nanoscale imaging techniques, such as scanning electron microscope, transmission electron microscope, and X-ray photography, can be used to verify the dispersion of CNTs in the polymer matrices (Nam and Lee 2015).
Previous research has shown the potential use of CNTs as sensors for structural health monitoring applications. In particular, since CNTs are electrically conductive material, the addition of small amount, known as the percolation threshold, may alter the electrical conductivity of non-conductive polymer-based construction materials (Zamal 2011). For instance, Swain et al. (2012) reported improvement in the electrical properties of unsaturated polyester resin by incorporating allylester functionalized MWCNTs and silane functionalized MWCNTs. Furthermore, wax coated MWCNTs enhanced the electrical conductivity and improved the mechanical properties of high-density polyethylene system (Jiang and Drzal 2011). Moreover, the fatigue damage of glass fiber reinforced polymer (GFRP) composites was monitored using CNTs (Reda Taha et al. 2014).
This research investigates the ability of COOH functionalized MWCNTs to alter epoxy-based PC microstructure and improve PC impact resistance. The epoxy resin contains reactive epoxy groups (C–O–C) at the two ends of the polymer chain. Similarly, the hardener typically contains amino groups (NH2). It is suggested that carboxyl (COOH) groups functionalized MWCNTs will react with epoxy to produce epoxy-MWCNTs nanocomposite that will improve the mechanical characteristics and specifically the impact resistance of PC.
2 Experimental Procedures
Properties of MWCNTs (Cheaptubes, Inc.).
Outer diameter (nm)
Inner diameter (nm)
MWCNTs ash (wt%)
Specific surface area (m2/g)
Bulk density (g/cm3)
Electrical conductivity (S/cm)
COOH content (wt%)
Mix proportions by weight of neat PC, kg/m3.
For the neat PC mix, the required amounts of resin and hardener were mixed together for 2–3 min using a low speed mixer, after which silica filling powder followed by the required coarse aggregate was added. Mixing continued for 2–3 min until the mixture was uniform. For the three other mixes, MWCNTs were added to the required amount of the resin, and the mix was stirred for 2 h at 110 °C using magnetic stirring. This relatively high mixing temperature was used to reduce the resin viscosity and improve the dispersion of MWCNTs. The mix was then sonicated for 2 additional hours at 65 °C. During the sonication, sound waves are generated from the transducer and radiate through the liquid causing high and low pressures releasing high amount of energy and improving the dispersion of MWCNTs. The epoxy-MWCNTs nanocomposite was left to reach room temperature and then mixed with the hardener for 2–3 min. The required aggregates were added and mixing continued for 3 additional minutes until the PC mixture was uniform. All PC specimens were cured in air at 22 °C and 50 % relative humidity for 7 days as recommended by ACI Committee 548 (2009).
2.2 Testing Methods
2.2.1 Flowability Test
The test was performed according to ASTM C1437 where a flow table, a flow cone, and a test caliper were used (ASTM C1437 2009). The cones’ smaller diameter was 70-mm, larger diameter was 100-mm, and the height was 50-mm. For testing, each mix was prepared and the cone was filled by fresh mix in two layers, and each layer was tamped 20 times. The cone was lifted in 4 s and 25 strikes were applied to the fresh specimen in 15 s. After the 25 strikes, four perpendicular readings were taken by the test caliper. The flowability percentage was considered as the sum of the previous four readings.
2.2.2 Impact Test
2.2.3 Compressive Strength Test
The compressive strength test was performed in a standard compression machine on three 50.8-mm cubes made from each mix. The cubes were tested at 7 days of age with loading rate of 6 kN/min according to ASTM C579 (2012).
2.2.4 Flexural Strength Test
In order to perform the flexure test, polymer concrete prisms were fabricated from each mix. The prisms dimensions were 25.4-mm × 25.4-mm × 116-mm. The prisms were tested in a three-point bending test setup at 7 days of age with a loading rate of 0.3 kN/min according to ASTM C78 (2002). The distance between the supports was 96-mm. The mid span deflection was recorded using a linear variable differential transducer (LVDT). The flexure load and displacement were performed using a servo hydraulic system and recorded using a data acquisition system with a sampling rate of 10 Hz for all experiments.
2.2.5 Shear Strength Test
The shear test was performed following the guidelines of ASTM D4475 in what is known as a short beam setup (ASTM D4475 2008). To enforce shear failure in that test, the distance between supports was limited to 32-mm and the distance between points of loading was set at 24-mm such that the distance between each support and the adjacent point of loading “shear span” was limited to 4-mm as shown in Fig. 2d. The prisms were tested in a four-point bending at 7 days of age. The shear test was performed using servo hydraulic system and recorded data acquisition system with sampling rate of 10 Hz for all experiments.
2.3 Microstructural Characterization
The microstructural characterization tests of epoxy-MWCNTs nanocomposite were performed on specimens of neat epoxy and epoxy with 1.0 wt% MWCNTs to explain the improvement observed in the macroscale mechanical response of PC incorporating MWCNTs. The microstructural characterization tests included scanning electron microscope (SEM), microscopic images, and Fourier transform infrared spectroscopy (FTIR) tests. In both the SEM and FTIR, no filler powder or aggregate was used in the microstructural characterization tests while in the microscopic images all polymer concrete components are examined.
The main goal of SEM investigation is to give an insight on dispersion of CNTs in polymer matrix and the mechanical mechanism of CNTs in polymer concrete composites. The microstructural features of a fractured surface of epoxy reinforced with 1.0 wt% MWCNTs were investigated under SEM with a guaranteed resolution of 0.5–1.7 nm at 30–1 kV, respectively. In addition, 100× magnification microscopic images are presented for the PC with neat epoxy and epoxy incorporating 1.0 % MWCNTs to assess the quality of mixing polymer concrete components (e.g. epoxy, aggregates, and powder).
In FTIR, samples of neat epoxy and epoxy incorporating MWCNTs were analyzed with biconical reflectance Micro-Fourier Transform Infrared Spectroscopy (Micro-FTIR) apparatus. The FTIR has a continuum microscope with a Globar source, XT-KBr beam splitter and a MCT-A detector over a 100 × 100 µm area with a 4-cm−1 resolution. Spectra were background corrected using a reflective gold slide and converted to absorbance using the Kramers–Kronig equation as per standard FTIR analysis method (Roessler 1965).
3 Results and Discussion
3.1 Impact and Mechanical Properties of PC
Effect of MWCNTs content on maximum absorbed impact energy in J.
Mean ± SD
26.8 ± 4.8
34.4 ± 2.2
36.4 ± 0.3
31.9 ± 4.3
Ductility index (DI) of PC with varying contents of MWCNTs in %.
Initiation energy (Ei) (J)
Propagation energy (Ep) (J)
Ductility Index (DI) (%)
Compressive strength of PC with varying contents of MWCNTs in MPa.
Mean ± SD
28.9 ± 2.1
31.8 ± 0.3
28.5 ± 0.5
28.1 ± 1.1
Flexural strength of PC with varying MWCNTs content in MPa.
Mean ± SD
9.2 ± 0.5
13.0 ± 0.6
11.6 ± 0.8
9.0 ± 0.5
Flexural failure strain of PC with varying MWCNTs content in (%).
Mean ± SD
13.1 ± 0.7
13.0 ± 0.6
14.7 ± 1.1
21.6 ± 1.8
Flexural toughness of PC with varying MWCNTs content in N mm/mm3.
Mean ± SD
0.81 ± 0.09
1.14 ± 0.13
1.10 ± 0.15
1.24 ± 0.14
Shear strength of PC with varying MWCNTs content in MPa.
Mean ± SD
5.0 ± 0.2
4.7 ± 0.0
4.3 ± 0.1
4.5 ± 0.1
Observing the behavior of PC with MWCNTs under compression and flexure stresses, it is obvious that addition of MWCNTs altered its mechanical performance. The results showed that MWCNTs did not have a significant effect on the compressive strength of PC. However, MWCNTs significantly improved the flexural strength, stiffness and failure strain of PC. The significance of MWCNTs on flexural strength might be attributed to the ability of the MWCNTs to produce a new epoxy-MWCNTs nanocomposite with improved mechanical properties; specially the flexural strength and the failure strain. These observations are supported by other researchers (Ganguli et al. 2005) who showed functionalized MWCNTs to act as microfibers, each individual fiber being 10-nm diameter and 10–30 μm long. These microfibers were able to bridge micro and submicron cracks in the polymer matrix and improve mechanical properties. Moreover, the functionalization of MWCNTs allows the MWCNTs to chemically bond to the epoxy matrix and thus alter the strength, failure strains and stiffness of the PC incorporating MWCNTs as reported elsewhere (Zhu et al. 2003; Zhu et al. 2004).
It can also be observed that by increasing the MWCNTs content, there is an increase followed by a decrease in the mechanical properties. This trend can be clearly observed in Tables 3 and 6, and the inset of Fig. 6 for the impact energy, flexure strength, and Young’s modulus, respectively. This trend is a result of two counteracting factors. First, with relatively small amounts of MWCNTs (e.g. 0.5 wt% and below), there is insufficient amount of CNTs that can introduce significant effect on the mechanical properties of polymer concretes. As the amount of CNTs increases, the improvements increase. On the other hand, the inefficiency of 1.5 wt% MWCNTs content can be attributed to the reduction of PC flowability at high content of MWCNTs. While the effect of MWCNTs on PC flowability was limited in the case of 0.5 and 1.0 wt% the addition of 1.5 wt% MWCNTs reduced the flowability significantly. This reduction in PC flowability might result in entrapping air that reduces the mechanical properties of PC with 1.5 wt% MWCNTs.
3.2 Cracking Pattern of PC Under Impact
3.3 Microstructural Features of PC
The above microstructural analysis shows that the improvement of mechanical properties of PC with MWCNTs can be attributed to the chemical reactivity of the COOH-functionalized MWCNTs with the polymer matrix producing a new epoxy-MWCNTs nanocomposite with improved mechanical characteristics specifically strength, ductility and energy absorption. In the meantime, it is obvious that the relatively high aspect ratio of MWCNTs results in enabling them to work as fibers reinforcing PC and limiting crack propagation and enabling energy dissipation. Further research is warranted to understand the fracture mechanics of PC incorporating MWCNTs. It is important to understand how the change in the epoxy matrix due to incorporating MWCNTs would affect the adhesion between epoxy and the aggregate particles compared with neat PC. Future research should enable engineering new impact resistant PCs needed from many applications.
PC incorporating MWCNTs has a higher impact resistance than neat PC. In particular, incorporating 1.0 wt% MWCNTs in PC per weight of the epoxy resin during PC fabrication resulted in significantly increased impact energy absorption of PC by 36 % compared with neat PC. Moreover, PC incorporating MWCNTs was more capable of dissipating the impact energy compared with neat PC. The increase in energy dissipation for PC incorporating MWCNTs resulted in a significant reduction in impact striker velocity after penetration and an increase of cracking density compared with neat PC. This improvement in the impact energy absorption and dissipation are in line with the improvement in the flexural strength of PC and might be explained by the change in the microstructure of the epoxy matrix. The change in the microstructure stems from the ability of the COOH functionalized MWCNTs to bridge the polymer matrix microcracks with its (10–30 μm) length and very high aspect ratio (about 1000) to continue carrying load. Microstructural investigations proved the good dispersion of MWCNTs in PC matrix and provided evidence that a chemical reaction of COOH functionalized MWCNTs with epoxy matrix takes place creating a new epoxy-MWCNTs nanocomposite with improved mechanical properties specifically ductility and energy absorption. On the other hand, the addition of 1.5 wt% MWCNTs limited the improvements on the impact and flexure of PC. Such relatively large amounts of MWCNTs reduce the flowability of the epoxy resin resulting in more entrapped air within the PC material. The presence of entrapped air adversely affects the mechanical properties of PC. In general, the changes in the microstructure of PC using 1.0 wt% MWCNTs can benefit the design of new class of PC structural elements with enhanced impact resistance and ductility for infrastructure applications.
This work has been supported by STC.UNM fund for investigating fatigue and impact strength of PC using MWCNTs. Support to the third author by STDF-CSE 5213 Polymer Nanocomposite Centre, Egyptian Petroleum Research Institute (EPRI) is much appreciated. The donation of polymer concrete materials by Transpo Industries is greatly appreciated.
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