Recently, remarkable types of carbon nanofilaments called carbon nanotubes (CNTs) have raised the interest of many concrete and cementitious composite researchers due to their significant mechanical, electrical, thermal, kinetic, and chemical properties. These nanofilaments are considered promising applicants to use in producing high-performance cement-based composite materials. In this research, the effect of CNT use on the flexural strength, strain capacity, permeability, and microstructure of concrete was investigated. Concrete batches of 0, 0.03, 0.08, 0.15, and 0.25 wt.% CNTs were prepared using a mixing method that consisted of a 30-minute solution sonication and a 60-minute batch mixing. On the 28th day, the mechanical properties were determined. The results indicated that concrete prepared using high CNT contents of 0.15 and 0.25 wt.% increased the flexural strength by more than 100% in comparison with 0% CNT concrete. Furthermore, the results showed that CNTs would increase the ductility of concrete beams by about 150%. The permeability test results showed the benefits of CNT inclusion in reducing the permeability of concrete. The permeability coefficient (kT) decreased by at least 45% when CNTs were added to concrete. A qualitative microstructural analysis illustrated the uniform dispersion of CNT filaments within the concrete hydration products in all batches.
Concrete is the most well-known material used in building construction. It comprises of water, aggregates, and cement. These ingredients are usually combined with steel reinforcement to achieve the desired mechanical properties. However, a major shortcoming in concrete is related to its brittle nature caused by its cement constituent, which is characterized by its poor resistance to crack formation, low tensile strength, and low strain capacities. In the past decade, researchers [
Most studies performed in this field investigated the effect of CNTs or CNFs usage on cement paste properties. At this stage, the reasons for investigating the effect of such materials on cement paste only is the simplicity of capturing or tracking the nanofilaments’ dispersion within the matrix compared to mortars and concrete, and the large costs of preparing concrete using the nanofilaments. Few studies investigated the effect of adding carbon nanofilament on the properties of mortar [
Up to date, the results obtained from studies that investigated the effect of long CNT weight fractions higher than 0.2% on ordinary concrete mechanical, physical, and microstructural properties were not positive. The proposed research will combine CNT reinforcement in concrete using a prolonged mixing technique to investigate their effect on the concrete mechanical and physical properties.
In this research, concrete batches containing CNTs of weight fractions ranging between 0.03 and 0.25 wt.% were prepared. Table
Composite concrete test batches.
Batch # | Batch name | Cement (kg) | Water (kg) | Fine agg. (kg) | Coarse agg. (kg) | CNTs (g) | CNTs/cement (%) | Total superplasticizer (g) | Superplasticizer used in the sonication process (g) | Remaining superplasticizer used in CNT solution and concrete mixing process (g) |
---|---|---|---|---|---|---|---|---|---|---|
1 | 0 | 19.2 | 9.6 | 44 | 48 | 0 | 0 | 240 | 0 | 240 |
2 | 0.03CNT | 19.2 | 9.6 | 44 | 48 | 5.8 | 0.03 | 240 | 23 | 217 |
3 | 0.08CNT | 19.2 | 9.6 | 44 | 48 | 154 | 0.08 | 240 | 61.4 | 178.6 |
4 | 0.15CNT | 19.2 | 9.6 | 44 | 48 | 28.8 | 0.15 | 240 | 115.2 | 124.8 |
5 | 0.25CNT | 19.2 | 9.6 | 44 | 48 | 48 | 0.25 | 240 | 192 | 48 |
The cement used in this research was a Portland cement, CEM I, Class 42.5 R, complying with EN 197-1. It was bought from Qatar National Cement Company (QNCC). The mixing water used was tap water attached to a filter. The point-of-use filter fixed on a faucet helps in removing chlorine, lead, and bacterial contaminants. The fine and coarse aggregates used in preparing the concrete samples were bought from Qatar Primary Materials Company (QPMC). The properties of the materials met the requirements of ASTM C-33, Standard Specification for Concrete Aggregates. The fine aggregates consisted of natural sand, while the coarse aggregates consisted of gabbro stones. The used CNTs were multiwalled carbon nanotubes (MWCNTs) produced using the catalytic chemical vapor deposition (CVD) process and provided by US Research Nanomaterials. The physical properties of the nanofilaments are shown in Table
CNT physical properties (
CNT type | Aspect ratio | Purity (wt.%) | Outside diameter (nm) | Inside diameter (nm) | Length ( |
SSA (m2/g) | Color | Youngs modulus (GPa) | Tensile strength (GPa) | Density (g/cm3) |
---|---|---|---|---|---|---|---|---|---|---|
Long-thin | 1,333 | >95 | 10-20 | 5-10 | 10-30 | >200 | Black | 1200 | 150 | 2.6 |
The mixing procedure was divided into two parts. The first part comprised of CNT dispersion in water, while the second part comprised of mixing the dispersed solution with cement, coarse, and fine aggregate in the concrete mixer. The dispersion was done in nine repetitions consisting of 1.1 liters each. The reasons for performing the sonication in repetitions were related to the capacity of the sonicator available and to ensure having similar sonication parameters such as energy, temperature, and amplitude in similar studies performed using smaller-scale mortar and cement paste samples. The surfactant/superplasticizer concentration used in the dispersion process was fixed at 4 : 1 of the selected CNT weight fraction. This amount was subtracted from the overall batch surfactant/superplasticizer amount that was constant in all batches. The nanofilaments were mixed with water and surfactant in the first mixing phase at the specified percentages (Table
(a) Combined CNT solution. (b) Concrete CNT mixing.
(a) Specimen compaction using a vibrator. (b) Specimen after 28 days of curing.
The flexural strength testing of concrete CNT samples was conducted according to ASTM C78/C78M−16, which is the standard test method for flexural strength of concrete using a simple beam with a third-point loading [ If the fracture initiates in the tension surface within the middle third of the span length, the flexural strength was calculated follows:
If the fracture occurs in the tension surface outside of the middle third of the span length by less than 5% of the span length, the flexural strength was calculated as follows:
If the fracture occurs on the tension surface outside of the middle third of the span length by more than 5% of the span length, the results of the test were discarded.
The load-deflection curves were determined using an LVDT that was tightly attached to a magnetic holder (Figure
(a) LVDT fixing using a magnetic holder. (b) LVDT vertical alignment at midspan.
The permeability test is a nondestructive method used to evaluate the durability of concrete. The proposed method complies with the European standard, SN, EN 206-1 [
(a) Setting the permeability tester. (b) Measurements of the permeability coefficient on the concrete sample surface.
Microstructural analysis of the broken samples was performed using a scanning electron microscope (SEM), model Nova NanoSEM. A secondary electron mode of imaging was used to capture the images with two types of scales. The first scale was a large scale between 0.5 and 1 mm used to examine the void percent, whereas the second scale was a small scale between 1 and 3 micrometers used to investigate the dispersion of the CNTs at the nanolevel. The voltage used was about 5 kV, whereas the working distance varied from 4.5 to 8.5 mm. The preparation procedure consisted of first drying the samples using a vacuum chamber, followed by coating them with a gold-palladium to dissipate any excess charges. The thickness of the gold sputtering coating was 10 nanometers every 40 seconds. After that, the samples were mounted rigidly on a specimen holder using a conductive adhesive and the scanning process was then commenced. The microstructure analysis of the SEM images was done using only qualitative techniques to understand the effect of the tested parameters on the strengths obtained for the strongest and weakest specimens. A total of fifteen images were taken for every batch. It was shown, via the analysis, that samples taken from every selected batch were monolithic.
Figure
Samples’ flexural strength results.
Figures
Load-deflection curves for (a) plain concrete (control) samples and (b) 0.03 wt.% CNT concrete samples.
Load-deflection curves for (a) 0.08 wt.% CNT concrete samples and (b) 0.15 wt.% CNT concrete samples.
Load-deflection curves for (a) 0.25 wt.% CNT concrete samples and (b) strongest sample in each batch.
However, to confirm this hypothesis, a microstructural analysis should be performed to investigate CNT dispersion within the concrete constituents. The maximum attained load and deflection for the 0.03 wt.% CNT batch specimens (Figure
Wang et al. [
Figure
Permeability test results.
Microstructural analysis of the fractured surfaces showed a few observations related to the behavior of the nanofilaments within the concrete hydration products. In terms of dispersion, it was observed that the quality of CNT dispersion in concrete products is good in the batches of low CNT content of 0.03 and 0.08%. However, few agglomerations were seen in the batches with higher CNT content of 0.15 and 0.25%. At small scales images of 1-3
(a) 0.03% content batch. (b) Nanoscale voids (scale: 1-3
(a) 0.15% CNT batch. (b) 0.25% CNT batch (scale: 1-3
(a) 0.03% CNT batch. (b) 0.08% CNT batch (scale: 0.5-1 mm).
(a) 0.15% CNT batch. (b) 0.25% CNT batch (scale: 0.5-1 mm).
To correlate strength results with SEM analysis, it was shown that the effect of higher CNT content on the cohesiveness of the samples may be the main reason for the incremental increase in the flexural strength of the samples containing CNTs compared to plain concrete. Higher CNT contents mean denser composites that occupied nanovoids, and hence, this will result in higher flexural strengths and strain capacities. To correlate with permeability measurements, it was not possible to understand the decrease and increase of the CNT samples’ permeability via using microstructural analysis. Even though the samples of higher CNT contents of 0.15 and 0.25% were denser, the permeability values of those samples prepared using lower CNT contents of 0.03 and 0.08% were lower.
This study illustrated a few conclusions related to the effect of CNTs’ addition on the flexural strength, ductility, and permeability of concrete. The results indicated that high CNT contents of 0.15 and 0.25 wt.% CNTs would increase the flexural strength of concrete by more than 100%. Furthermore, the results also showed that CNTs would increase the ductility of concrete by about 150%. The permeability test results showed the benefits of CNT addition in reducing the permeability of concrete. The permeability coefficient (kT) decreased by at least 45% when CNTs were added to concrete. The relationship between concrete’s mechanical and physical property improvements and the CNT weight fraction was primarily explained. The addition of CNTs to concrete resulted in a denser composite with higher flexural strengths and strain capacity and lower permeability when compared to plain concrete. The findings of this study could be considered one of the few studies that incorporated CNT addition to concrete to produce composite members of enhanced performance.
The limitations observed during this work include the following:
Dispersion of CNTs within the water solution was a time-consuming process due to the need to sonicate limited quantities of a maximum of 1 liter in the repetition The reduced amount of water in the concrete batches compared to cement paste batches resulted in the nonuniform dispersion of CNTs
The elemental compositions of concrete hydration phases must be investigated using energy-dispersive X-ray analysis to understand the effect of CNTs on these phases There is also a need to further investigate the erratic behavior and nonuniform results obtained during the load-deflection-based tests
The data used to support the findings of this study are available from the corresponding author upon request.
The statements made herein are solely the responsibility of the authors.
The authors declare that they have no conflicts of interest.
We express our sincere appreciation and gratitude to the Qatar National Research Fund (QNRF) for funding this research work via NPRP 4-1142-2-440 and PDRA 3-0402-17003. Furthermore, we acknowledge Qatar National Library’s (QNL) support in making this research publishable.