3.1 Materials

3.1.1 Cement

The cement used was Portland cement type CPV-ARI. The cement was selected because it has no pozzolanic ash and contain higher amount of clinker in its composition. It eliminates another variable in the study, since pozzolanic materials may interact with CNTs and interfere in results (^{Chaipanich et al., 2010)}.

The physical and chemical composition of the cement are evidenced in Table 2. The specific cement average weight is 3.12 g/cm³ in accordance with the NM NBR 23/2001 (^{NBR NM 23, 2001}).

Table 2 Physical properties and chemical composition of cement CP V - ARI 

* Na₂Oe = Na₂O + 0.658 K₂O

3.1.3 Carbon nanotubes and Admixture

The CNTs used in this study were produced from Nanocyl SA Company, located in Belgium. The CNTs cost 120 euros per kilo. These are multi-walled CNTs synthesized by the chemical vapor deposition method or also called CVD - chemical vapor deposition. Commercially the product is specified by the name of NC 7000. Figure 1 shows an image of this material with the use of scanning electron microscopy (SEM).

Figure 1 SEM Image of carbon nanotubes multi-walled produced by Nanocyl SA. 

Its properties and composition are showed in Tables 5 and ⁶.

Table 5 Physical properties of NTCs 

Property	Unit.	Value
Average diameter	Nanometer	9.5
Average Length	Mícron	1.5
Surface Area	m2/g	250-300
Average Density	g/l	60

Table 6 NTCs composition  

Components	% (weight)
Synthesized graphite (NTC)	90%
Cobalt Oxide	< 1%
Metal Oxide	10%

From the scope of the experiment, it was used a polycarboxylate as superplasticizer, used to set the desired consistency.

The admixture used in the experiment is a carboxylic ether polymer modified, with concentration average of 49%. It is in accordance to ^{ASTM C 494 (2011)} (Type A and F), ^{ASTM 1017 (1998)} and ^{NBR 11768 (2011)}.

Polycarboxylate macromolecules are used as dispersants in cementitious compositions as high efficiency by reducing the viscosity and minimizing the amount of water used for processing (^{Mehta; Miller, 2008}).

Some properties of admixture is showed in Table 7.

Table 7 Superplasticizer informations 

Suggested dosage (by cement mass)	pH (ABNT 10908)	Specific gravity (ABNT 10908)
0.3% to 2%	5.5 + 1.0	1.10 + 0.02 g/cm3

3.2 Specimens

The concrete without CNTs was called REF (without adding CNTs) and was used to compare with the concrete with the addition of CNTs. Thus, it was adopted a simply conventional concrete mix as a reference and then reproduced series adding the CNTs. The mix proportions established are appointed in Table 8. It should be noted that the CNT content relative to the cement mass was maintained at 0.3% for all composites. This settlement was made based on the studies of ^{Melo et al. (2011)} that indicated 0.30 % (in cement mass) as the optimum content.

Table 8 Concrete mix proportions 

*in reference to cement mass.

The nomenclature used in Table 8 refers to the REF as the mark without the carbon nanotubes; the CD, such as the insertion mark with the additive and carbon nanotubes (with dispersion in water and additive ultrasound); and SD, the mark containing only the NTC added powder (without the dispersion in the additive and without application of ultrasonic energy - mixed into the cement powder using only with mixer attached to a drill).

The water/cement ratio used in this study was set at 0.55. During this work, it was noted that the addition of CNTs to the concrete caused reduction of fluidity to the SD serie, which damaged its feasibility and prevented the use of smaller ratio values water/cement ratio. On the other hand, it is considered the limit value as specified by the ^{NBR 6118 (2007)} for the purpose of application of concrete in marine environments.

Cement content is 352 kg/m³ and is in accordance to the suggested consumption established by ^{NBR 12655 (2006)}, which in the case of concrete placed in class III environment indicate the lower limit of 320 kg of cement per cubic meter of concrete.

From the scope of tests, they were made in the mixer 3 mixtures, one for each mix proportion, and to prepare the solution to be used for CD feature of preparation, there was the addition of CNTs water with superplasticizer and was followed with sonification. The wave application time was 1 hour. This time was predetermined based on work done by ^{Marcondes (2012)} due to a visual analysis indicated that after 60 minutes the samples did not appoint any changes to the tonality and turbidity.

Furthermore, as of 40 minutes sonication the samples are not presented (visually) CNTs decantation elapsed time of 24 hours in the solution rest, as can be seen in Figure 2.

Figure 2 Visual analysis of samples with 40 minutes of sonification elapsed in 24h rest (^{MARCONDES, 2012}). 

The ultrasonic equipment rendered low frequency, model 0-14 C / I manufacturer Thornton INPEC Electronics SA with a nominal frequency of 40 kHz and 100W of power. The dispersed solution was used within 30 minutes after sonication.

3.3 Tests procedures

3.3.2 Tensile strength by diametrical compression

The tensile strength by diametrical compression test was carried out according to the ^{NBR 7222 (2011)}. This test is focused on evaluating the tensile strength in the analyzed concrete by applying a diametrical compression load to the specimen. Similarly to the previous test, it was prepared six cylindrical specimens Ø 10 x 20 cm for each mix proportion.

The Equation 1 was used to calculate the tensile strength by diametrical compression.

1

Where: P = maximum applied load, kN; d = specimen diameter, mm; L = specimen height, mm.

3.3.3 Immersion water absorption

The immersion absorption test was performed based on ^{NBR 9778 (2005)} and was performed according to the following steps:

1. Drying of specimens until they reach constant mass (the dry out temperature was 60 °C to avoid very high temperatures, which may cause microcracks in the specimen and influence the results);

2. After the dry out in an oven, it was rendered the immersion of the sample in water at 23 ± 2 °C for a total of 72 h. The samples were kept immersed up to 1/3 of its volume during the first 4 h, 2/3 in the subsequent 4 h, remaining completely immersed in 64 h;

3. It was determined the mass of the test specimens at 24 h, 48 h and 72 h of immersion. The determinations were made after dry up the sample surface with a cloth.

The water absorption by immersion was defined by Equation 2.

2

Where: M_sat = saturated mass of specimen; M_s = mass of oven dried specimen

3.3.4 Water absorption by pipette method

This test also renowned as pipette test or ^{RILEM pipette test (Test Method II.4: Water absorption test tube, 2006)}. It was proposed by German researcher Karsten and approved by RILEM (Reunion Internationale des Laboratoires d'essais et de Recherches sur les Materiaux et les Constructions) (^{RILEM, 2006}) for the control of water infiltration in walls.

Each specimen was cut into 5 cm thick slices. These slices were placed in an oven at a temperature of 60 °C during four days. Then, the pipettes were fixed on the circumference of the specimens with silicone as shown in Figure 3. We used four slices of each series formulated for the measurements.

Figure 3 a) Pippete Positions; b) Pipette details; c) Pipette in vitro. 

With fixed pipette, they were filled with water until the zero degree (highest) and the water inlet on the substrates was monitored by the decrease in the water level on the graduated scale (Figure 3). The readings were taken at 15 minute intervals to complete 2.5 hours of testing and, after that, we proceeded with readings 24h, 48h and 72h.

In order to consider the loss of water by evaporation, a pipette was fixed on a glass plate and it was used considering the variation of water level, in this case, as the water evaporated. These were subsequently evaporating discounted values of all the tested series concrete.

4.1 Influence on consistency

The consistency results of fresh concrete mixtures obtained by the slump test upon the determined mix proportions for this experiment are showed in Figure 4.

Figure 4 Slump test results (cm). 

The test results evidenced that the form of dispersion and the presence of CNTs in the concrete have a strong influence on consistency. As it can be seen in Figure 4, the presence of CNTs Without dispersion increased the consistency of the concrete (reduced its slump), while the series with the presence of CNTs in ultrasound dispersed form presented similar results compared to the reference series. In comparison to the reference series, the powder mixture without dispersion caused the reduction of slump results from 20 cm to 7 cm.

These results highlights the difficulty in performing feasible concrete with CNTs, therefore indicating to be possible only with the use of chemical admixtures to increase the plasticity of the concrete, due to the high amount of water required for the concrete with CNTs. On the other hand, it was appointed that the prior dispersion by sonification had considerably reduced the locking effect of the fluid mass submitted by the addition of powdered CNTs.

Experience shows that the slump results reduction on the addition of powdered CNTs to be due to the large specific surface of the material (250-300 m₂/g) about 650 times higher than CP-V ARI as used in this study. The high slope consumption surface evidenced a splitting towards the entire area of ​​the particles as per CNTs addition. The improvement in the slump results with the ultrasound usage for scattering technique (CD series) led to the better distribution of the CNTs in an aqueous medium and admixture incorporation as presented on the surface of CNTs. This means, the dispersion of ultrasound as evidenced to ensure a preview area and efficient water splitting to the carbon nanotubes particles.

4.2 Compressive strength

The results of the compressive strength test at 28 days of cure may be observed in Figure 5.

Figure 5 Compressive strength (MPa) for reference series and the two forms of NTCs addition. 

The data had evidenced in both cases when CNTs were added, to estate significant improvements in compressive strength effects. To acknowledge these results it was applied the analysis of variance (ANOVA) and it appeared the averages to be significantly different, 95% of certainty. The ANOVA data can be seen in Table 9 and the difference between the averages is confirmed by the fact of F to be higher than F_critical.

Table 9: Analysis of variance (ANOVA) between compressive strength values 

The mix proportion with higher elevation of the compressive strength was the CD series, where there was 37% of increase in compressive strength compared to the reference series. The series without dispersion (SD series) showed 19% of compressive strength increase compared to the reference series.

The results show that the dispersion with ultrasound is extremely important to maximize the benefits brought about by the addition of CNTs in Portland cement concrete.

4.3 Tensile strength by diametrical compression

Figure 6 shows the results of tensile strength by diametrical compression from the concrete series screened in this study.

Figure 6 Tensile strength by diametrical compression for reference series and the two forms of NTCs addition. 

These results indicate increase in tensile strength between 18 % and 24% with the NTCs addition to the concrete, as compared to the reference series. Although the SD series had resulted in a similar increase of tensile strength compared to the series with dispersion, the SD series presented the largest standard deviation among samples. This is an indication of which the addition of NTCs powder in the mixer resulted in less uniformity of distribution of NTCs in the concrete mass creating considerable variability in tensile concrete strength tests.

To obtain greater reliability in the comparison between the series of concrete studied, the ANOVA tool was used, of which indicated 95% reliability appointing significant differences between the results. This can be better detailed in Table 10, with F higher than F_critical.

Table 10: Analysis of variance (ANOVA) between the results of tensile strength by diametrical compression for 95% significance 

4.4 Immersion water absorption

Figure 7 evidences that samples with the NTCs there have been improvements in the immersion absorption propertie. This may be related to a better distribution of the pores, resulting in reduced macro porosity. On the other hand, to prove this it is necessary other tests, as the mercury porosimetry test to be able to better quantify the pore size.

Figure 7 Immersion absorption (%) X time (h) for reference series and the two forms of NTCs addition. 

It is easy to see the trend of reduced absorption caused by addition of NTCs in concrete as reviewed in this research. However, the reduction level not to be as high as expected, presenting a decrease of 4.4% for the SD series and 3.9% for CD series.

4.5 Water absorption by pipette method

The absorption by the pipette method was performed to add more knowledge of water absorption of concrete produced with the addition of NTCs. Their results are shown in Figure 8. Note that the reference series and the SD series showed the worst results demonstrating once again the importance of employment dispersion with ultrasound, the CNTs in the additive and water before mix the concrete.

Figure 8 Absorption by pipette method.  

The test was conducted on the cut face and it is important to emphasize the fact that the presence of coarse aggregate in the concrete may affect the water absorption results in this method. However, the trial also showed the importance of the dispersion in the variation of water penetration results and showed reduction trend in series with CNTs pre-dispersed with ultrasound technique.

4.6 Discussion.

^{Chaipanich et al. (2010)} showed that the addition of carbon nanotubes increased strength of cement with fly ash. In such research, the levels of 0.5% and 1% NTCs relative to the cement mass was analyzed, and the second with 20% fly ash showed a 10% increase in compressive strength. Comparing study data hereby presented with data ^{Chaipanich et al. (2010)}, it appears that the first case was obtained higher level of compressive strength (increase of 19 % to 37%) and this is probably due to the use of higher levels of CNT in the work of ^{Chaipanich et al. (2010)}. Due to its high specific surface, the CNT tends to make the cementitious composite less fluid and this may have been a causative point of difference between the results of two research work. However, it is not possible to compare the effect of CNTs on the fluidity in both works because the study of ^{Chaipanich et al. (2010)} was carried out on cement paste and consistency was not measured.

^{Melo (2009)} showed an increased compressive strength and tensile strength in Portland cement products with the addition of CNTs. ^{Melo (2009)} and ^{Nochaiya; Chaipanich (2011)} in their respective studies proved a reduction of the average size of the pores caused by the incorporation of NTC in Portland cement pastes, which can increase the durability of concrete structures, since smaller diameters tend to hinder the movement of aggressive agents within the concrete. According ^{Melo (2009)}, the content of 0.3% nanotubes showed the highest strength lifting capacity. According to ^{Makar et al. (2005)}, an increase in the durability of concretes and mortars with addition of CNTs should the adhesion bridges that form in the mass microstructure (Figure 9). They control cracks generated in the cementitious matrix and hence promote greater strength and decrease in porosity. ^{Li et al. (2004)} also report a connecting bridge between the hydrated cement and nanomaterials on one of the reasons of increased strength mortars had the incorporation of carbon nanotubes. Other explanations cited in the work of ^{Li et al. (2004)} and ^{Nochaiya; Chaipanich (2011)} imply the fact that the nanoparticles fill the voids of the cement paste and increases the mechanical strength of the cementitious composite.

Figure 9 Micro fissure in the cement paste with addition of CNTs acting as adhesion bridges (^{Makar et al., 2005}). 

Despite being used in various fields, carbon nanotubes also have a high cost, which can be an obstacle to the use of this material in cement composites. Experience shows that with the increasing demand and the ability to synthesize CNTs in the manufacture of clinker, which is being studied at UFMG as reported by ^{Ludving et al. (2011)} and ^{Melo et al. (2011)}, the material will become more accessible. Thus, although the cost of the material is a negative aspect at present, the tendency is that this drawback be overcome over time.