Carbon Nano Fibre Reinforcements In Concrete

Graphite nanomaterials offer distinct advantages over microscale reinforcing fibers in terms of engineering properties and geometric attributes. Through dispersion and effective interfacial interactions of proper functionalization of carbon nanofibers are the prerequisites for their effective use in high-performance cementitious matrices. Furthermore, the use of nanoand micro-scale reinforcements together provides reinforcing effects at different scales, thus rendering balanced gains in engineering properties of the matrix. However, their uses in coarser high-performance matrices have not been evaluated thoroughly. The results show improvements in all flexural attributes, impact and abrasion resistance of concrete with addition of 0.16% of oxidized and polyacrylic acid physisorbed carbon nanofibers, over the corresponding properties of plain matrix. The results also pointed to synergetic effects of hybrid reinforcements on improving the various engineering properties of DSP concrete matrix, especially with low modulus polypropylene microfibers. © 2019 Tim Pengembang Jurnal UPI Article History: Received 16 Oct 2016 Revised 20 Aug 2018 Accepted 31 Jan 2019 Available online 09 Apr 2019 ____________________

Graphite nanomaterials offer distinct advantages over microscale reinforcing fibers in terms of engineering properties and geometric attributes. Through dispersion and effective interfacial interactions of proper functionalization of carbon nanofibers are the prerequisites for their effective use in high-performance cementitious matrices. Furthermore, the use of nano-and micro-scale reinforcements together provides reinforcing effects at different scales, thus rendering balanced gains in engineering properties of the matrix. However, their uses in coarser high-performance matrices have not been evaluated thoroughly. The results show improvements in all flexural attributes, impact and abrasion resistance of concrete with addition of 0.16% of oxidized and polyacrylic acid physisorbed carbon nanofibers, over the corresponding properties of plain matrix. The results also pointed to synergetic effects of hybrid reinforcements on improving the various engineering properties of DSP concrete matrix, especially with low modulus polypropylene microfibers.

INTRODUCTION
Advanced technological aspects of cement based materials have recently focused on developing high-performance cementitious composites, which exhibit high compressive strengths. Such composites, however, exhibit also extremely brittle failure, low tensile capacity and appear sensitive to early age microcracking as a result of volumetric changes due to high autogenous shrinkage stresses. These characteristics of cement based materials are serious shortcomings that impose constrains in structural design and affect the long term durability of structures. To overcome the aforementioned disadvantages from the reinforcement of cementitious materials, it is typically provided at the millimeter and/or the micro scale using macrofibers and microfibers.
Cementitious matrices, however, exhibit flaws at the nanoscale, where traditional reinforcement is not effective. Graphite Nanomaterials, including carbon nanofibers (CNFs), present several distinct advantages as a reinforcing material for high strength/performance cementitious composites as compared to more traditional fibers. First, they exhibit significant greater strength and stiffness than conventional fibers, which should improve overall mechanical behavior. Second, their higher aspect ratio is expected to effectively arrest the nanocracks and demand significantly higher energy for crack propagation. Third, CNFs are uniformly dispersed. Then, due to their nanoscale diameter, fiber spacing is reduced. This has opened a new field for nanosized reinforcements that should theoretically hinder the formation and later propagation of microcracks at the very beginning. Few attempts have been made to add different graphite nanomaterials as reinforcement in cementitious matrices.
Most of the works involving use of graphite nanomaterials in cementitious matrices have used very fine matrices to evaluate the efficiency of their reinforcement and reported modest gains in some of the engineering properties (Makar and Beaudoin, 2004;Makar and Chan, 2009;Li et al., 2005;Cwirzen et al., 2009;Cwirzen et al., 2008;Metaxa et al., 2009;Metaxa et al., 2010). A comprehensive approach was taken to evaluate suitably functionalized carbon nanofibers in high-performance cementitious materials of higher complexity, especially concrete. The results of functionalized and nonfunctionalized graphite nanomaterials in high performance DSP paste and mortar matrices have been mentioned elsewhere (Alkhateb et al., 2013).
Based on previous studies Asmara et al., 2018;, the focus of this research paper is on the use of high-performance (DSP) concrete as the matrix in which the reinforcement efficiency of suitably functionalized carbon nanofibers and/ or different microfibers is evaluated. The larger aggregate size and content in concrete, when compared with mortar and paste, adds to complexity of behavior and failure modes by introducing an interfacial zone, generating interactions between aggregates and propagating microcracks, and the need to disperse nanomaterials in the space between aggregates.
Evaluation of the reinforcement efficiency of carbon nanofibers in a coarser cementitious matrix, high-performance concrete, was undertaken to see the effect of increased particle size on the interaction of nano reinforcements with the relatively coarser matrix. This has also highlighted the filler effect and dimensional stability brought about by use coarser aggregates in a well graded matrix. Furthermore, micro-scale fibers along with their hybrid combination with carbon nanofibers were evaluated to achieve desired balance of performance and cost efficiency.
Cementitious materials are essentially particulate composites, which are rarely used without aggregates (particulates). Given the micro-to millimeter-scale dimensions of the sand and gravel particles used in DSP concrete, hybrid reinforcements which complement the reinforcing and dimensional stabilizing actions of particulates with multiscale reinforcement mechanisms could produce particularly positive effects. In the past, the concept of using hybrid (micro-and millimeter-scale) reinforcement has been explored by the concrete industry for achieving balanced gains in material properties (O'Connell et al., 2001;Banthia and Sappakittipakorn, 2007;Moranville-Regourd, 2002;Lawler et al., 2005).

Materials
Oxidized (CNF-OX) and polyacrylic acid physisorbed (CNF-PAA) carbon nanofibers, carbon microfibers (CMF) and polypropylene microfibers (PP) at different volume fractions were used in Densified with Small Particles (DSP) concrete matrix, as seen in Figure 1.
These carbon nanofibers have an outer diameters in the 60-150 nm range and lengths ranging from 30 to 100 μm, 50-60 m 2 /g specific surface area (SSA), ~1.95 g/cm 3 true density, and >95% purity. These pristine and oxidized nanofibers were purchased from Pyrograf Products, Inc. and PAA was physisorbed on to pristine nanofibers using a procedure described in the subsequent section. Carbon microfibers (CMF) with 6mm length and 6-μm diameter were obtained from Toho Tenax America, Inc., US. Polypropylene microfibers (PP) with 19 mm in length and 39 μm in diameter were obtained from New Nycon, Inc., US. Poly(acrylic acid) (PAA, average Mw of about 100,000, 35 wt.% of H2O) was purchased from Sigma-Aldrich, US. Deionized (DI) water was used for all solution preparations and were purchased from PT Rumah Publikasi Indonesia, Indonesia.

Dispersion of Nanotubes in Water
The nanomaterial dispersion procedure comprised the following steps: (1) Add the required amount of CNF-OX and CNF-PAA (carbon nanofibers and poly acrylic acid 1:1 by weight) to the mixing water of cementitious materials in order to achieve the targeted nanofiber volume fraction.
(3) Sonicate the mixture using a probe as follows: (i) Sonicate for ten minutes at different amplitudes (30%, 45%, 65% and 75%) with 1-minute breaks between different amplitudes; (ii) Pulse (1 minute on and 30 seconds off) for 10 minutes at 85% amplitude; (iii) Turn off the sonic probe for 2 minutes, and repeat the pulsing cycle two more times; and (iv) Repeat the whole sonic probing cycle one more time.
(4) For the microfiber and hybrid reinforcement systems, the microfibers were added to the mixing water without using the above dispersion procedure. Microfibers were added to the mixing water half an hour before mixing it with other ingredients of the DSP concrete in mixer.

Cementitious Matrices
Dense cement-based matrices with a smooth particle size gradation covering nano-to micro-scale range promise to effectively mobilize the tremendous mechanical qualities of graphite nanomaterials within cementitious nanocomposites. One category of cementbased matrices meeting these requirements is referred to as Densified with small particles (DSP). DSP cement-based materials comprise micro-scale cement and nano-scale silica fume particles, dispersed and densified with a superplasticizer, which is shown in Figure 2 (Guerrini, 2000).
Using this basic concept and introducing other ingredients (e.g., high-quality aggregates and discrete reinforcement), it is possible to obtain highly desired combinations of mechanical performance and durability suiting demanding fields of application (Guerrini, 2000). Based on a comprehensive review of the literature on DSP (Aı̈tcin, 2000;Badanoiu et al., 2003;and Moranville, 1999), the cementitious concrete matrix introduced in Table 1 was selected for evaluation of the merits of graphite nanomaterials and/or microfibers in cement-based materials.   The materials selected for use in cementbased matrices included Type I Portland cement (Lafarge-North America), undensified silica fume (Norchem, Inc.) with the average particle size of 200 nm, the specific surface area of 15 m 2 /g, and minimum 7-day pozzolanic activity index of 105%, silica sands (Fairmount Minerals) with average sizes of about 39 and 350 μm, comprising >99.5% of silica, granite gravel ranging in sizes of from 1 mm to less than 9 mm (average particle size of 3.55 mm). ADVA ® Cast 575 is a polycarboxylate-based Type F ASTM C 494 superplasticizer.
Cementitious materials (with and without carbon nanotubes and/or microfibers dispersed in the mixing water via sonication) were prepared using following the ASTM C 192 and C 305 procedures. The specimens were moist-cured inside molds after casting (ASTM C 192) over a 24-hour period, and were then demolded and subjected to 48 hours of steam curing at 70 o C. The samples were subsequently conditioned at 50% of relative humidity for seven days prior to testing. At least two batches were casted with at least four specimens for each test condition in each batch for all reinforcement conditions and engineering properties.

Experimental Methods
The test procedures employed to determine the engineering properties of cement-based materials are described in this section. Compression tests (ASTM C 109) were performed on 50 mm of cube specimens. Flexure tests (ASTM C 1185) were performed on 12.5 x 50 x 150 mm specimens by center-point loading on a span of 125 mm using a deflection-controlled mechanical test system, with load and deflection data collected using a data acquisition system. Impact tests (ASTM D 7136) were performed on 12 x 150 x 150 mm specimens. Abrasion tests (ASTM C 944) were conducted on the surface of cylindrical specimens with 100 mm in diameter (and 50 mm in height). Scanning electron microscopy (SEM) was also employed to gain further insight into the structure and failure mechanisms of cementbased nanocomposites. Experimental results were evaluated using the analysis of variance (ANOVA) and pair-wise comparison techniques. Response surface analysis (RSA) was used to identify reinforcement volume fraction, which provides optimum reinforcement and gains in various engineering properties of the cementitious matrix.

RESULTS AND DISCUSSION
Acid-oxidized (CNF-OX) and Poly-acrylic acid physisorbed (CNF-PAA) carbon nanofibers were used as reinforcement at 0.16 vol.%; polypropylene (PP) and carbon microfibers (CMF) were used at 0.24 vol.%. Combinations of polypropylene and carbon microfibers with PAA physisorbed carbon nanofibers were also used as hybrid reinforcement for concrete, with PP/ CMF

Flexural Performance
The flexural attributes test results for the high-performance concrete reinforced with different volume fractions of carbon nanofibers and/or microfibers are summarized in Table 2. Significant gains in all flexural performance characteristics of concrete are observed with carbon nanofiber reinforcement used alone or in combination with microfibers. Polymer wrapping of nanofibers improves their reinforcement efficiency in concrete. The most desired balance of properties for a single reinforcement system was realized with 0.16 vol.% of PAA-physisorbed carbon nanofibers (CNF-PAA). The corresponding improvements in flexural strength, energy absorption capacity and maximum deflection versus plain concrete were 20.9, 134, and 120%, respectively. The experimental data generated in the past pointed at the positive effects of hybrid (nano-and micro-scale) reinforcement systems in high-performance cementitious materials (Peyvandi et al., 2013). The test data presented here suggest that micro-scale fibers of lower modulus and lower cost (when compared with high-modulus carbon microfiber) could effectively complement the reinforcing effects of polymer-wrapped carbon nanofibers in high-performance (DSP) concrete. These lower-modulus microfibers, when used as hybrid reinforcement together with both treated and untreated carbon microfibers, produced balanced gains in the flexural performance attributes of the highperformance (DSP) concrete by interacting with and arresting the cracks developing in matrix at different scales. The hybrid reinforcement of low-modulus polymer (polypropylene) microfibers with nanofibers produced particularly pronounced gains in the mechanical properties of DSP concrete. The best balance of flexural attributes was realized with the hybrid reinforcing material comprising PAA-physisorbed carbon nanofibers (0.16 vol.%) and polypropylene microfiber (0.24 vol.%) (which were produced to have 35.8, 644, and 371%) rise in the flexural strength, maximum deflection, and energy absorption capacity of the highperformance concrete.
The hybrid reinforcement systems also overcame the adverse effects of microfibers on flexural strength, as reported earlier. The synergistic reinforcing action of nano-and micro-scale reinforcement was further enhanced by polymer wrapping of carbon nanofibers. The effectiveness of hybrid reinforcement was also evident for highmodulus carbon microfibers when used with different PAA physisorbed carbon nanofibers. This hybrid reinforcement resulted in improvement of all engineering properties versus plain matrix and also versus a similar hybrid reinforced materials incorporating untreated carbon nanofibers.
The experimental results were subjected to statistical analysis of variance (ANOVA). Due to the large number of observations, ANOVA only gives the general trends in test results. The ANOVA outcomes indicate that there are statistically significant (at 0.05 significance level) improvements in all flexural attributes of high-performance concrete due to the addition of nano-scale and hybrid reinforcement systems. Pair-wise comparisons were carried out in order to assess the statistical significance of the effects associated with the addition of nanoand/or micro-scale reinforcement systems. These results indicate that nano-scale reinforcement systems at 0.16 vol.% produced statistically significant improvements in all flexural attributes of the plain matrix at 0.05 of significance level when compared with plain concrete. Both microfibers produced statistically insignificant drops in flexural strength (0.297 and 0.481 of significance levels for carbon and polypropylene microfibers, respectively). Pair-wise analysis also pointed at statistically insignificant gains in energy absorption capacity (0.502 of significance level) and maximum deflection (0.402 of significance level) realized with 0.24 vol.% of carbon microfiber. The use of low-modulus polypropylene microfiber as well as highmodulus carbon microfiber in conjunction with different surface functionalized carbon nanofibers reversed any negative trends in the effects of individual reinforcement, producing balanced gains in the engineering properties of high-performance DSP concrete at significance levels varying form 0.000 to 0.047. The lower values of significance level were only for maximum deflection when carbon microfibers were used in conjunction with carbon nanofibers.

Compressive Strength
The compressive strength test results (mean values and standard errors) for highperformance concretes with nano-and/or micro-scale reinforcement systems are presented in Table 3. As was the case with DSP paste and mortar (Alkhateb et al., 2013), PAA physisorbed carbon nanofibers as well as both microfibers produced relatively small and statistically insignificant (at 0.05 of significance level) effects on the compressive strength of high-performance concrete.
Outcomes of pair-wise comparisons also indicate the effects of nano-and microscale reinforcement systems on compressive strength are not statistically significant (at 0.05 of significance level). The use of hybrid reinforcement systems comprising carbon or polypropylene microfiber and carbon nanotubes restored the compressive strengths of DSP concrete by inducing interactions with matrix and its cracks at different scales. The hybrid reinforcement system comprising functionalized carbon nanofibers and micro-scale polypropylene fiber produced a small rise in the compressive strength of high-performance concrete.

Impact Resistance
The impact test data are summarized in Table 4. All nano-and/or micro-scale reinforcement systems produced improvements in the impact resistance of high performance concrete. The maximum rise in impact resistance realized with a single reinforcement (69%) was for 0.16 vol.% of CNTF-PAA. In the case of micro-scale fibers, polypropylene produced a higher gain (60%) in the impact resistance of high-performance DSP concrete.
All hybrid nano-and micro-scale reinforcement systems produced further rise in the mechanical properties of highperformance concrete when compared with individual (nano-or micro-scale) reinforcement used alone. A significant (0.000 significance level) increase in impact resistance was observed for the hybrid reinforcement comprising polypropylene microfiber with CNF-PAA, which confirms the effectiveness of hybrid reinforcement. The maximum increase in impact resistance (115%) was brought about by hybrid use of 0.16 vol.% of CNFPAA and 0.24 vol.% of PP microfiber. Outcomes of ANOVA and pair-wise comparisons point at the statistical significant (0.000 to 0.012 significance levels) of nano-and/ or microscale reinforcement effects on the impact resistance of high-performance concrete. When compared with high performance cement paste and mortar (with nano-scale reinforcement), concrete (with nano-scale reinforcement) provides higher levels of impact resistance; this observation points at the well-known contributions of aggregates (in concrete) to the toughness of cementitious matrices.

Abrasion Resistance
The abrasion test results produced for high-performance concrete with different volume fractions of graphite nanomaterials and/ or microfiber reinforcement are summarized in Table 5. Table 5, CNF-PAA at 0.16 vol.% produced the greatest improvement (reaching 40.0%) in the abrasion resistance of high-performance DSP concrete. All nanoand/or microscale reinforcement systems produced marked gains in the abrasion resistance of this high-performance concrete matrix, which were statistically significant at 0.05 of significance level. Outcomes of pairwise comparisons confirmed that each of the reinforcement conditions considered here produced statistically significant improvements in the abrasion resistance of high-performance concrete (with the value of significance levels ranging form 0.006 to 0.000).

Response Surface Analysis
The test data on high-performance concrete with nano-and/or micro-scale reinforcement systems was subjected to response surface analysis. The objective of this analysis was to identify optimum reinforcement conditions which maximize the benefits to specific engineering properties of high-performance concrete. This section presents optimization of concrete nanocomposites for enhancing specific engineering properties, and also for simultaneous enhancement of several engineering properties. Response surface analysis (RSA) of the test data was conducted considering the volume fractions of nano-and micro-scale reinforcement systems as input variables. Flexural strength, energy absorption capacity, maximum deflection and impact resistance were used as response variables. The RSA process started with evaluating the effects of input variables on response variables, and was followed with desirability analyses considering means of response variables, using both canonical and ridge analyses.
RSA conducted for each of the flexural strength, energy absorption capacity, maximum deflection and impact resistance test data indicated that an optimum reinforcement system comprising 0 All stationary points are saddle points which do not give a maximum but a general direction to follow in which targeted properties can be maximized.
The ridge analysis outcomes can be used to adjust the reinforcement condition for achieving further gains in engineering properties, shown in Table 6. The analyses for flexural attributes and impact resistance show that the current hybrid reinforcement is the appropriate combination to maximize these properties of the matrix.
Desirability analysis of test data was also conducted in order to identify reinforcement conditions which yield a desired balance of all the engineering properties considered here. Desirability analyses were conducted using mean values obtained through both canonical and ridge analyses. These outputs indicate that maximum values of all response variables can be achieved with 0.11; 0.12 vol.% CNF-PAA and 0.26; 0.24 vol.% PP microfiber. These values are closer to what is being used as hybrid reinforcement in this high-performance concrete matrix.

Scanning Electron Microscope Evaluation
Failed surfaces of flexure and compression test specimens were evaluated under a high-precision scanning electron microscope (JOEL 7500F). All samples were coated with Osmium (Using Osmium Coater Neoc-AN, Meiwa Shoji) prior to SEM observations. Figure 3 shows SEM images of DSP concrete samples.
The density of the DSP matrix is apparent in Figures 3a and 3b. Figure 3a also highlights the positive effective of coarser particles in the matrix like sand and gravel. Figures 3c to 3e points at the uniform dispersion of nanotubes and their bridging/pullout actions across a fine (nano-scale) crack within the cementitious matrix. Figures 3f and 3g present evidence of micro-fiber pullout from the matrix. Scanning electron microscope (SEM) observations of cementitious concrete matrices reinforced with hybrid reinforcement (carbon nanofibres and microfibres) also indicated that the presence of nanofibers in the matrix around micro-fibres enhanced the interaction of matrix with micro-fibres (Figure 3g), thus benefiting the gains in matrix performance with hybrid reinforcement. This strong interaction of matrix with micro-fibres was not observed in samples reinforced with only micro-fibre reinforcement. These observations provide some insight into the synergistic action of nano-and micro-scale reinforcement systems in cementitious matrices. All these observations support the results of various engineering properties mentioned in earlier.