Effect of graphite nanoplatelets and carbon nanofibers on rheology, hydration, shrinkage, mechanical properties, and microstructure of UHPC
Introduction
With appropriate combination of cement and supplementary cementitious materials, adequate sand gradation, low water-to-binder (w/b) ratio, fiber reinforcement, and high-range water reducer (HRWR), ultra-high-performance concrete (UHPC) can be produced to deliver high flowability, high mechanical properties, and excellent durability [1], [2]. By incorporating 2% or more micro steel fibers as reinforcement, UHPC can exhibit strain hardening behavior and ductile failure modes in tension and in flexure [3], [4], [5]. However, the high amount of steel fibers significantly increases the initial cost and self-weight of UHPC [1], [6], [7]. In addition, although the steel fibers can be used to bridge cracks and increase the overall tensile properties, such as the tensile capacity and fracture energy, the steel fibers are ineffective in delaying the presence of microcracks. This is possibly due to the relatively large spacing and less interlocking between the steel fibers [8], [9]. The presence of cracks makes concrete vulnerable to ingress of moisture and deleterious materials, thus resulting in accelerated deteriorations [10]. It was found that when the width of microcracks was < 50 μm, the chloride diffusion coefficient is significantly lower than that of cracks larger than 150 μm in a steel reinforced concrete composite [11]. Therefore, controlling the cracking resistance at microscale is critical for improving the durability of concrete structures.
To further enhance the cracking resistance and fracture toughness of cement-based materials, carbon nanofibers (CNFs) and graphite nanoplatelets (GNPs) have been utilized to provide microscale reinforcement [12], [13], [14], [15], [16], [17]. Typically, the CNFs have a diameter of tens of nanometers and a length of tens to a few hundreds of micrometers; the GNPs have a diameter of tens of nanometers and a thickness of a few nanometers. Gao et al. [14] observed that the compressive strength of a cementitious composite made with 0.16% CNFs increased by 40% compared with that of plain cementitious composite without any CNF. With an addition of 0.13% of GNPs, Peyvandi et al. [15] obtained a 70% increase in the flexural strength of a cement paste with a w/b of 0.2. By adding 0.2% of CNFs, Tyson et al. [16] reported an increase in flexural strength of 80% and fracture toughness of 270% compared with the cementitious composite without any CNF. Huang [17] obtained an increase of 80% in flexural strength by using 0.2% GNPs in paste with a w/b of 0.6. Mechanical property improvement of these cement-based nanocomposites could be attributed to reduction in porosity and nucleation effect [17], [18], [19], [20]. The nanoscale materials promote growth of hydration products, i.e. calcium-silicate-hydrate (C-S-H), on their surface [21], [22]. In addition, the nanoscale spacing and high specific surface areas make large-aspect-ratio nanomaterials effective in suppressing inception and propagation of microcracks [23], [24].
While the enhancement of mechanical properties by incorporating CNFs and GNPs was demonstrated in the above studies, the effect of their addition on other key properties of UHPC has not been fully investigated. The introduction of CNF or GNP may affect the compatibility between cementitious particles and chemical admixture [25], thus influencing the rheological properties of UHPC. Due to the low w/b, typically < 0.25, UHPC can exhibit large early-age autogenous shrinkage, which can cause cracking [26], [27]. Thus far, there is limited information on the effects of CNF and GNP on autogenous shrinkage, hydration kinetics, and the pore structure of UHPC. Such knowledge gap constrains the wider acceptance of nanomaterials in developing UHPC and drives the need of further research to advance the understanding.
Another constraint of using nanomaterials in UHPC is the lack of effective dispersion method. The effective and efficient use of nanomaterials depends on their dispersion condition. Given the very small size, agglomeration of nanomaterials becomes notable and can highly compromise their reinforcing performance [28], [29]. Ultra-sonification has been used to undermine agglomeration and facilitate uniform dispersion of nanomaterials in aqueous media [30]. Surfactants have been used to convert hydrophobic surface into hydrophilic surface for better dispersion of nanomaterials [31].
Based on the above review, research on CNF and GNP have been focused on two main aspects: (1) how to render nanomaterials uniformly dispersed in cementitious matrix [30], [31]; (2) whether the use of nanomaterials offers substantial improvement in performance of cementitious matrix and at what dosage of nanomaterials [17], [18]. For the case of UHPC, the first aspect is reported in Section 2.2. For the second aspect, the incorporation of nanomaterials has been found able to increase the compressive, tensile, and flexural properties of UHPC [32]. As the content of CNFs or GNPs was increased from 0 to 0.30%, the compressive, tensile and flexural properties were all significantly increased; the bond strength and post-debonding performance of the interface between steel fiber and cementitious matrix were also improved [32].
The objective of this study is to evaluate the effects of two types of GNP and one type of CNF on rheological properties, hydration kinetics, autogenous shrinkage, and pore structure of UHPC. The mechanical properties of the investigated mixtures are also discussed. The content of the nanomaterials is increased from 0 to 0.3% by mass of binder. The higher dosages of nanomaterials were not investigated due to the high capital investment and difficulty in uniformly dispersing the nanomaterials. The study also seeks to compare the dispersion methods for the nanomaterials. The outcome of this research provides insight of CNF and GNPs used as a key component of UHPC.
Section snippets
Materials and mix design
A cost-effective UHPC [2] with w/b of 0.2 and sand-to-binder volume ratio of 1.0 was used. The binder was composed of ASTM Type III Portland cement, Class C fly ash, and silica fume with volume fractions at 55%, 40%, and 5%, respectively, of the total binder. The Blaine finenesses of the cement and the fly ash are 560 and 465 m2/kg, respectively. Fine silica fume with particles smaller than 1 μm in diameter, mean diameter of 0.15 μm, the BET specific surface area of 18,200 m2/kg, and a SiO2 content
Fresh and physical properties
The effects of nanomaterials on flowability of UHPC mixtures are associated with the nanomaterials content, as shown in Fig. 1. Note that the coefficients of variation of all investigated parameters are < 5%. The HRWR demand, which allows the mixtures to achieve an initial mini-slump of 280 ± 5 mm for securing self-consolidating properties, is given as the active powder weight percentage of the cementitious materials. The HRWR demand is a key parameter to evaluate flowability of UHPC. A mixture
Conclusions
Based on the above experimental investigation, the following conclusions can be drawn:
- (1)
With the increase in nanomaterial content from 0 to 0.3%, by mass of binder, the air content was increased from 2.5% to approximately 3.0%. When the nanomaterial content was no > 0.05%, the HRWR demand decreased with the addition of the nanomaterials. However, as the nanomaterial content was > 0.05%, further addition of nanomaterials had adverse effects on the HRWR demand. Compared with CNF, the GNP-C and GNP-M
Acknowledgements
This study was funded by the Energy Consortium Research Center [grant number SMR-1406-09] and the RE-CAST University Transportation Center at Missouri S&T [grant number DTRT13-G-UTC45]. The authors would like to thank Dr. Dale P. Bentz and Dr. Pietro Lura for discussion on microstructure characterization and Zemei Wu for help with MIP measurement.
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