Fracture parameters of nanoreinforced cement mortars: The effect of CNT functionalization

1. Introduction

Cementitious nanocomposites, such as mortars and concrete, reinforced by carbon fibers at the nanoscale are a novel topic of recent and ongoing composite materials research. The dramatic enhancement of cement based nanocomposites’ stiffness, strength, toughness and energy absorption capability, obtained from relatively small quantities of carbon nanotubes (CNTs) has been a strong motivation for further exploration.

Earlier results in the literature suggest that multiwall carbon nanotubes (MWCNTs) fundamentally modify the nanostructure of cement based materials [1], offering outstanding improvement to the fracture properties of cementitious matrices [2,3]. The resulting enhancements can vary widely depending on the geometrical and physical characteristics, and structure of MWCNTs, the density, aspect ratio, surface and functionalization, play an important role in their dispersion. Structurally, MWCNTs consist of multiple layers of graphite superimposed and rolled in on them to form a tubular shape. MWCNTs are endowed with exceptionally high aspect ratio, low bulk density and an extended specific area, which is of particular interest for adsorption and reactions taking place on the nanotubes’ surface [4,5].

The interface length and strength depend directly on the geometrical characteristics of the nanotubes, hence a higher aspect ratio is required for an effective transfer load between from the matrix to the nanotube [6]. Another important parameter affecting the contribution of MWCNTs on the properties of the cement based materials is their uniform dispersion within the matrix. At the nanoscale the van der Waals forces between MWCNTs are very high, and due to their large surface area, MWCNTs tend to entangle to each other and form bundles, making their separation a difficult procedure. Besides the surface area, characteristics such as the wall number, bulk density, aspect ratio and surface defects induced by methods of functionalization, also play a crucial role in their dispersion, hence affecting their reinforcing efficiency [7–11].

Most studies in the literature so far focused on experimentally determining the strength and the stiffness of nanoreinforced cement pastes and mortars. However, fracture mechanics characterization of cementitious nanocomposites is of great importance, mainly because of the quantitative description of the mechanical state of the deformable body containing cracks, with a view towards characterizing and measuring the resistance of materials to crack growth. There are practically few reports available in the literature on the determination of the fracture parameters such as stress intensity factor/fracture toughness, or strain energy release rate of MWCNTs reinforced cement pastes or mortars. Hu et al. [12] prepared cement paste composites with various amounts of MWCNTs or MWCNTs with a carboxyl group (MWCNTs-COOH), ranging from 0% to 0.1 wt%. The fracture toughness and energy was measured by the three-point bending method suggested by RILEM. The results showed that fracture toughness and energy of cement paste reinforced with the two types of MWCNTs, pristine MWCNTs and MWCNTs-COOH, could be improved. By adding 0.1 wt% of MWCNTs, the specimen’s fracture energy and toughness increased by 26.2% and 11.4%, respectively. The fracture energy and fracture toughness of the 0.1 wt% MWCNT-COOH composites increased by 42.9% and 19.2%, respectively. Stynoski et al. [13] studied the fracture properties of Portland cement mortars containing silica fume (SF), carbon nanotubes (CNTs) and carbon fiber (CF), using notched three-point bend specimens and following the Two Parameter Fracture Model developed by Jenq and Shah [14,15]. They observed that CNTs provided a slight improvement in fracture toughness (K_IC) of about 5–10% at 7 and 28 days of age. Their effect on critical crack tip opening displacement (CTOD_c) was more significant, achieving a 20% improvement at 28 days. Using SF and CNTs together, a significant improvement in K_IC and CTOD_c of about 35% and 56% after 28 days was observed.

More recently, Gdoutos and co-workers [2,3] provided a thorough fracture mechanics characterization through the experimental determination of the fracture parameters of nanomodified Portland cement mortars, reinforced with well-dispersed MWCNTs and CNFs. The fracture parameters of the nanoreinforced mortars were determined by following the two parameter model by Jenq and Shah [14,15]. The authors were able to demonstrate an exceptional reinforcing and toughening effect of MWCNTs and CNFs: the critical value of the stress intensity factor (namely the fracture toughness) was improved by 128.6%; the strain energy release rate by154.9%, CTOD by 39.7% and the effective crack length by10.3%.

In this study, a comprehensive analysis on the effect of the mechanical functionalization of commercially available MWCNTs in the development of Portland cement nanomodified mortars is presented. The MWCNTs’ uniform distribution in the matrix was achieved through a one-step dispersion method, involving the application of ultrasonic energy and the use of a superplasticizer (SP). For the fracture mechanics characterization, three point bending notched specimens of OPC mortars reinforced with two different types of MWCNTs, pristine and mechanical functionalized, at an amount of 0.1 wt% of cement were manufactured and tested under servohydraulic controlled conditions, using the CMOD as the feedback signal. The fracture parameters of the nanoreinforced mortars were then determined following the two parameter model by Jenq and Shah. The excellent reinforcing efficiency and toughening capability of both the pristine and functionalized MWCNTs is demonstrated by significant improvements in all the fracture properties: critical stress intensity factor (125%), strain energy release rate (131%), crack tip opening displacement (25%), effective crack length (11%) and material length (47%).

2. Materials and experimental procedure

In this study, the mortar specimens were cast using Type I ordinary Portland cement (OPC) 42.5 R and standard sand according to EN 196-1. Two different types of MWCNTs were used to reinforce the mortar matrix. Type 1000C1 is designed as “pristine” MWCNTs and was produced in a fluidized bed chemical vapor deposition reactor, employing proprietary methodologies and metal catalysts as well as ethylene as carbon source [16]. Type 1000C3 resulted by a mechanical functionalization of the pristine1000C1 MWCNTs, applied by the manufacturer. As it is depicted from the characteristic properties of MWCNTs shown in Table 1, types 1000C1 and 1000C3 have the same length, almost the same diameter, however type 1000C3 exhibits a significantly lower bulk density.

The values of an estimated fiber count, the number of MWCNTs, which are theoretically evenly distributed in a unit volume of the matrix assuming perfect dispersion, according to ACI 544.1R-96: Report on Fiber Reinforced Concrete [17], were determined and are shown in Table 2. Generally, for any given volume percentage of fibers of equal length that can ideally be assumed uniformly distributed in a material mix, the number of individual fibers per unit volume varies inversely with the square of the individual fiber diameter and can be calculated by using the following equation: Fiber count = V IF V f .(1)

Where the V_IF is the volume of an individual fiber and V_f is the volume fraction of the fiber per unit volume, which can easily calculated by the equation: (2) Where M_f is the fiber content, percent by mass and 𝜌_B is the bulk density of the MWCNTs.

From the calculation of the fiber count of MWCNTs per unit of volume matrix, it is clearly observed that it directly depends on the bulk density of carbon nanotubes: the lower bulk density leads in a higher fiber count.

The effective dispersion of MWCNTs is one of the major parameters that strongly influence the mechanical and fracture properties of mortar nanocomposites. These nanoscale fibers have strong tendency to agglomerate due to presence of van der Waals forces, originating from their polarizable extended 𝜋-electron systems in their surface [18]. The dispersion of carbon nanotubes in a general is a critical process that depends on the physical and geometrical characteristics such as aspect ratio, bulk density, surface area and volume fraction. Different covalent and non covalent methods have been tried so far to achieve homogeneous dispersion of carbon nanotubes in water such as using solvents [19], surfactants [20–23], functionalization with acids [24] and other techniques [25–27]. The best physical technique used for the dispersion of carbon nanotubes is the ultrasonication. In this study, a simple covalent dispersion method [28–30] was followed to achieve homogeneous dispersion of MWCNTs in the mortar mixture. In a typical procedure the MWCNT/suspensions are prepared by adding MWCNTs to an aqueous surfactant solution, at a surfactant to MWCNT weight ratio of 4.0. The application of ultrasonic energy to the suspensions is achieved by a 500 W cup-horn high intensity ultrasonic processor with a standard probe of a diameter of 19 mm and a temperature controller. The sonicator is operated at amplitude of 57% so as to deliver constant energy rate of 1900–2100 J/min, at cycles of 20 seconds in order to prevent overheating of the suspensions. The MWCNT/suspensions were then added into the OPC and sand at a water to cement ratio w∕c = 0.485, and sand to cement ratio s∕c = 2.75. Mixing of the materials was performed according to the procedure outlined by ASTM 305, using a standard robust mixer capable of operating from 140 ± 5 revolutions per minute (r/min) to 285 ± 10 r/min. After mixing, the mixtures were cast in 20 × 20 × 80 mm oiled molds. Following demolding, the samples were cured in lime saturated water, until testing. A 6 mm notch was introduced into the prismatic 20 × 20 × 80 mm specimens using a water cooled band saw machine. The length of the notch was calculated based on the RILEM standard, which requires a notch to depth ratio of 1/3.

Load-CMOD curves from neat mortar and mortar nanocomposites reinforced with the two different types of MWCNTs, pristine (1000C1) and functionalized (1000C3), at an amount of 0.1 wt% of cement, obtained from linear elastic fracture mechanics tests are shown in Fig. 1. The load-CMOD curves of the neat mortar and the 0.1 wt% 1000C1 and 1000C3 MWCNT reinforced mortars show the same pattern, consisting of a linear elastic stage before crack initiation, nonlinear stage of stable crack propagation preceding unstable failure and unstable extension stage after the peak load. Nevertheless, it is observed that the peak load values of the load-CMOD curves of both the pristine and functionalized MWCNT reinforced mortars are higher compared to the neat mortar; therefore, as shown in Table 3 the ultimate fracture load is also higher. Especially for the functionalized 1000C3 mortars, it is observed that they exhibit the highest value of fracture load. It should be noted here that the fiber count of individual 1000C3 MWCNTs in the mortar matrix is approximately 3 times higher from that of the 1000C1 MWCNTs (Table 2). Table 3 also provides the Young‘s modulus values of the 1000C1 and 1000C3 MWCNT nanocomposites, both of which are substantially higher compared to the neat mortar. It is of interest to note here that the values of Young‘s modulus measured using the two parameter fracture model are in perfect agreement with the values of Young’s modulus obtained from 3 point bending tests on 40 × 40 × 160 mm prismatic beams, uniaxial compression tests on the half prisms 40 × 40 × 80 mm, and linear elastic fracture mechanics tests on notched 20 × 20 × 80 mm beams [31,32].

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Fig. 1.

Load-CMOD curves for a 28d neat mortar and mortars reinforced with 1000C1 and 1000C3 MWCNTs at an amount of 0.1 wt% of cement.

An in depth fracture mechanics characterization of mortars using the TPFM typically requires determination of the critical values of the effective crack length (a_C), stress intensity factor (K_IC), strain energy release rate (G_IC), crack tip opening displacement (CTOD_C), and material length (Q). This way, failure by crack propagation can be described when stress intensity factor, strain energy release rate of crack mouth and crack tip opening displacement reach their critical values. The above mentioned quantities are functions of applied loads, the material’s characteristics, size and dimensions of the specimen, while their critical values are true material properties. The effective crack length, a_c, is equal to the length of the initial notch plus the length of the fracture process zone at the peak load. Results of the effective crack length, a_c, for the 3, 7 and 28d plain, 1000C1 and 1000C3 0.1 wt% MWCNT reinforced mortars are plotted in Fig. 2. An increase of 27% was observed for the 28d 1000C1 0.1 wt% MWCNT nanoreinforced mortar over the plain one. An even higher increase (29%) was observed for the functionalized MWCNT mix at the same age of hydration. Fig. 3 presents the values of the fracture toughness K_IC, for the neat mortar and mortars reinforced with the pristine and functionalized MWCNTs, at an amount of 0.1 wt% of cement, up to 28 days of hydration. The effect of the MWCNTs on the mortar’s resistance to cracking was significant as K_IC increases with time for both nanoreinforced mixes. This very low dosage of both MWCNTs resulted in increases in the 28d fracture toughness of 86% for the type 1000C1 and 128% for the type 1000C3. The improved toughness and mechanical performance of nanocomposites is the effect of the crack arresting mechanism at the nanoscale, which leads to a more stable coalescing process of cracks at the nano- and micro scale. Geometrical characteristics and the mechanical functionalization are the keys of a high quality dispersion of MWCNTs, hence for the successful enhancement of the fracture toughness and stiffness of the mortar matrix.

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Fig. 2.

Effective crack length of 3, 7 and 28d neat mortar and mortars reinforced with 0.1 wt% 1000C1 and 1000C3 MWCNTs.

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Fig. 3.

Critical stress intensity factor K _IC of neat mortar and mortars reinforced with pristine (1000C1), and functionalized (1000C3) MWCNTs versus time for up to 28 days.

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Table 3

Effect of pristine and functionalized MWCNTs on the fracture load, and compressive and tensile Young’s modulus of nanoreinforced cement mortars

	MWCNT count	Day	Fracture load (N)	E(GPa)∗ TPFM	E (GPa)∗ LEFM	E (GPa)∗ 3-point bending [32]	E (GPa)∗∗ compression [32]
		3	136.6	9.3 ± 0.92	9.9 ± 0.95	9.2 ± 0.96	9.5 ± 0.97
Mortar	–	7	170.8	12.6 ± 0.85	12.0 ± 0.81	10.8 ± 0.87	11.5 ± 0.83
		28	220.0	14.3 ± 0.68	14.6 ± 0.72	14.1 ± 0.71	14.0 ± 0.70
		3	197.9	17.2 ± 0.51	16.7 ± 0.41	16.0 ± 0.44	15.4 ± 0.49
M + 1000C1	3.6 × 1011	7	227.0	18.9 ± 0.17	18.4 ± 0.10	18.1 ± 0.13	18.3 ± 0.15
		28	362.3	27.5 ± 0.10	27.2 ± 0.12	26.8 ± 0.09	28.0 ± 0.12
		3	295.6	16.7 ± 0.44	17.3 ± 0.45	17.8 ± 0.43	17.4 ± 0.52
M + 1000C3	10.9 × 1011	7	337.1	23.2 ± 0.16	24.1 ± 0.14	24.0 ± 0.11	23.6 ± 0.13
		28	398.5	31.5 ± 0.10	31.4 ± 0.11	32.0 ± 0.12	31.3 ± 0.11

^∗Average value of three specimens. ^∗∗Average value of six specimens.

The use of energy principles, such as the strain energy release rate G_IC, is absolutely needed to describe the stable crack propagation and the critical fracture of quasi brittle materials such as cement mortars. The strain energy release rate development is presented in Fig. 4. Similarly to the fracture toughness and stiffness results, both nanoreinforced mixes exhibit impressive increases in G_IC, at all three ages of hydration, 3, 7 and 28d.

Interestingly, and despite the fact that the two types of MWCNTs have almost the same aspect ratio, the 28d G_IC of the 1000C3 MWCNT mortar is ≈24% higher than the 1000C1 one and 131% higher over the plain mortar. By comparison, it is obvious that the mechanical functionalization of the pristine nanotubes resulted in a modified nanomaterial (1000C3) that can greatly attain a good adhesion with the mortar matrix, manifested in extraordinary increases in fracture toughness and energy.

Analogous results are observed for the geometry independent fracture parameter, critical crack tip opening displacement, CTOD_C. As shown in Fig. 5 the 3, 7 and 28d of the MWCNT reinforced mortars are higher than the plain ones. More specifically, the 28d critical crack tip opening displacement of the 1000C3 MWCNT mortar is ≈53% higher than the 1000C1 one and 253.6% higher over the plain mortar. It should be reminded herein that the type 1000C3 of MWCNTs resulted by a manufacturer’s mechanical functionalization of type 1000C1 pristine MWCNTs. The two types have almost the same length and diameter; however type 1000C3 MWCNTs have remarkably lower bulk density. On this basis, the bulk density plays an important role in the reinforcing efficiency: the lower the bulk density the higher the count of individual MWCNTs that are theoretically uniformly distributed in the mortar matrix. For the case of type 1000C3 the MWCNT count is approximately three times the 1000C1 count (Table 2) following the same dispersion conditions and sonication procedure for the same amount.

The material length Q is proportional to the size of the fracture process zone, and can be used to characterize the brittleness of the material. Results of the 3, 7 and 28d calculated material length values for the MWCNTs mixes are shown in Fig. 6. Taking into account that the toughness values of the 1000C1 and 1000C3 mortar specimens are more than 100% higher over the plain ones, the brittleness indices of both the 0.1 wt% 1000C1 and 1000C3 MWCNT reinforced mortars are higher; hence both mixes exhibit a less brittle behavior. Furthermore, it is also observed that the brittleness index of the 28d 1000C3 MWCNT mix is higher than the 1000C1 one, mainly due to the higher count of individual carbon nanotubes in the matrix.

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Fig. 4.

Strain energy release rate G _IC of neat mortar and mortars reinforced with pristine (1000C1), and functionalized (1000C3) MWCNTs at an amount of 0.1 wt% of cement, up to 28 days of hydration.

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Fig. 5.

Critical crack tip opening displacement CTOD _C of 3, 7 and 28d neat mortar and mortars reinforced with 0.1 wt% of cement pristine and functionalized MWCNTs.

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Fig. 6.

Material length Q of neat mortar and mortars reinforced with 0.1 wt% of cement pristine (1000C1) and functionalized (1000C3) MWCNTs at 3, 7 and 28 days of hydration.

4. Conclusions

This work focuses on the effect of MWCNT functionalization on the fracture mechanics parameters of Portland cement nanoreinforced mortars. The fracture properties, namely effective crack length a_C; critical stress intensity factor/fracture toughness, K_IC; modulus of Elasticity E; critical strain energy release rate, G_IC; critical crack tip opening displacement, CTOD_C; and material length, Q, of 3, 7, and 28 days Portland cement mortars, reinforced with pristine and mechanically functionalized well dispersed 0.1 wt% of cement MWCNTs were experimentally determined. The values of toughness and fracture energy are 86--126% higher over the neat mortar. Furthermore, the material length values Q, as calculated using the TPFM, of both the pristine and functionalized MWCNT reinforced mortars are higher than those of the neat one; therefore both nanoreinforced mixes exhibit a less brittle behavior. The highest improvement in the fracture properties was seen in the mixes that contained mechanically functionalized MWCNTs with low bulk density. The impressive increase of 125% in stress intensity factor/fracture toughness, 124% in modulus of elasticity and 131% in strain energy release rate that the functionalized MWCNT nanomodified mortars presented can be attributed to the beneficial effects of the nanotubes’ good adhesion with the mortar matrix and the strengthening of the mortar/carbon nanotube interface.

Footnotes

Acknowledgements

The authors would like to acknowledge the financial support of the Research Funding Program “Study of the mechanical and electromechanical behavior of nanoreinforced concrete (200/877)”, funded by the Academy of Athens. The help provided by Mrs M. Falara, a Ph.D. student of the Laboratory of Applied Mechanics of DUTh, is greatly appreciated.

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