Carbon Nanofiber Reinforced Lightweight Engineered Cementitious Composites
Carbon Nanofiber Reinforced Lightweight Engineered Cementitious Composites
Table of Contents
CNFs and CNTs in Cementitious Materials
Benefits of Using CNFs Over CNT
Conclusion of Literature Review
Sampling and Curing Conditions
1.1.11 Scanning Electron Microscopy
5.0 Conclusion and Recommendations
Abstract
Engineering cementitious composites (ECC) is classified as a special type of high-performance fibre-reinforced cementitious composites (HPFRCCs), designed with micromechanical principles, comparing to 5-10% volume ratio of fibres in HPFRCCs, ECC’s fibre volume ratio is less than 2%. Under normal conditions, ECC behaves like a normal concrete, but under excessive loading, instead of fracture, the material undergoes plastic behaviour and deform like a steel plate.
While significant amount of studies has already been conducted on investigating the mechanical properties of PVA reinforced ECC, there have been limited studies conducted on lightweight ECC. In the past decades, lightweight concrete has been used in various structural and non-structural applications, offering considerable weight saving. A special type of lightweight filling, hollow glass microspheres (HGMS) has been introduced in this study, samples with different ratios of HGMS have been prepared and mechanical tests have been conducted. In the last decades, carbon nanofibers (CNFs) are quickly becoming one of the most promising nanomaterials because of its unique properties and various of studies have shown that they can be used as a Nano reinforcing fibre in cementitious materials. Thus, in this research four different types of CNFs have been introduced to enhance the mechanical properties of the lightweight ECC. In order to discern how fibers can influence the mechanical properties of lightweight ECC, various ratios of these four-different type of CNFs have been added to the lightweight ECC.
To evaluate the strength of the CNF reinforced lightweight ECC, samples were cured for periods of 7 and 28 days and then tested for mechanical properties by compression test and three-point bending. Furthermore, the morphological observations to confirm the dispersion of the lightweight filler and fibers have been captured.
Keywords: Lightweight engineered cementitious composites; Carbon nanofibers, Hollow glass microspheres;
1.0 Introduction
More recent, extraordinary sorts of carbon nanomaterials have attracted enormous attention from some concrete researchers, due to their exceptional mechanical, chemical, electrical and thermal properties, and brilliant performance in reinforcing polymer-based materials. Microfibers may postpone the nucleation and development of cracks on the concrete at the microscale, in contrast, nanomaterials will additionally defer the nucleation and development of cracks at the nanoscale. Nanomaterials includes carbon nanofibers (CNFs) and carbon nanotubes (CNTs) may end up being better choices than conventional fibres and potential candidate for the future development of high-performance and multifunctional cement-based materials and structures
Recently in construction industry, there is a strong demand for lightweight construction materials. The primary use of structural lightweight concrete is to reduce dead load of a structure, which then allows the structural designers to reduce the size of the columns, footings and other load bearing elements, besides, they also improve the thermal protection of buildings. Structural lightweight concrete provides a more efficient strength-to-weight ratio and in most cases, the marginally higher cost of the lightweight concrete is balanced by size reduction of structural elements, less reinforcing steel and reduced volume of concrete, resulting in lower overall cost. However, the application of lightweight concrete in today’s construction industry is as non-structural wall panels, bricks and architectural exterior finishing, due to the fact that comparing to normal weight concrete, lightweight concrete is more brittle and its mechanical properties are much lower. In order to utilize lightweight concrete in construction, their mechanical properties need to be improved, they need to be strong, durable and lightweight at the same time. Such properties can be obtained by the introduction of hollow glass microspheres (HGMS) in cementitious materials. HGMS, also called glass bubble, is possesses an ultra-lightweight hollow structure, adding it to cement can potentially increase the compressive strength and also reduce the density.
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2.0 The research on the integration of
3.0 Literature Review
1.3. Introduction
1.4. Historical Background of ECC
The improvement of fibre reinforced concrete has experienced various stages. Romauldi and his collaborators conducted experiments in 1960’s and have established the viability of short steel fibres in decreasing the brittleness of normal concrete. This advancement has proceeded with extension to various types of fibres, for example: glass, carbon, synthetics and natural fibres. In 1980’s, research focus has shifted to creating a type of fibre reinforced concrete that possess high tensile property, due to the fact that in fibre reinforced concrete, the toughness of the material is increased, but ductility remains the same. To improve the ductility in concrete, Krenchel and Stang proved that high tensile ductility can be achieved by continuous aligned fibres, which can be hundreds of times stronger than normal concrete. Additionally, investigations of the performance of discontinuous fibres at high dosage (4-20%) in concrete were conducted by Allen, which demonstrate a higher tensile strength than normal concrete but less ductile than continuous aligned fibre reinforced concrete (Li 2007).
The materials described above can be classified as High-Performance Fibre Reinforced Cementitious Composites (HPFRCC). Although HPFRCC materials can improve the ductility of normal concrete, they have for most part been limited to academic research or particular applications so far. This is due to extra request in mechanical tasks, especially in on-site construction feasibility and economical plausibility. These two requirements are hard to meet when either continuous aligned fibres or high fibre content are utilized as a part of the composites (Li 2007).
Engineered Cementitious Composite (ECC) is special type of HPFRCC which was originally developed by Victor Li at the University of Michigan with a ductility of 3-5% and tensile strength ranging from 4-6Mpa. The design approaches for ECC was to maximize the tensile ductility by developing firmly dispersed various micro cracks while limiting the fibre content to 2% or less by volume (Li 2007). As far as material constituents, ECC use comparative ingredients as fibre reinforced concrete: cement, water, sand, fibre and some chemical additives. ECC doesn’t contain any coarse aggregates as they have a tendency to negatively influence its extraordinary ductile behaviour. Due to the limited amount of fibres in ECC, the mixing procedure of ECC is similar to the procedure utilized in mixing normal concrete. Additionally, by intentionally constraining the amount of fibres, various investigations have proven that ECC can be used in particular structure applications (Li & Kanda 1998).
1.5. Why add PVA in ECC
Over these years, researchers have been trying to optimise the performance of ECC by investigating the behaviour of ECC with different types of fibres. Sathishkumar and his team conducted experiments to study the behaviour of ECC with Polyvinyl Alcohol fibres (PVA), Poly Propylene fibres, Polyester fibres and Polyethylene fibres by adding various proportions: 1%, 1.5%, 2%, 2.5% and then compare their strength. It was found that, the specimen that is reinforced with PVA fibres possess the best performance, followed by Polyethylene, Polyester and Polypropylene. Likewise, when the outcomes are analysed for percentage addition of fibres, obviously the performance increased when the fibre content is increased, however, the workability and strength of ECC has decreased when the fibre content increased from 2% to 2.5%. Consequently, the performance of the mixture that reinforced with 2% PVA has the best performance, it possesses the highest compressive strength, spilt tensile and flexural strength (Sathishkumar et al. 2016). This finding is consistent with findings of Victor Li. According to Wang and Li, PVA fibre is considered as a standout amongst the most appropriate polymeric fibres to be utilized as the reinforcement for ECC. PVA was selected from a bunch of high performance fibres due to its hydrophilic nature, the bound of PVA and cementitious matrix is very strong so that the fibres are adept to break rather than being pulled out (Wang & Li 2017).
1.6. CNFs and CNTs in Cementitious Materials
Experimental tests on CNFs have demonstrated them to have a Young’s Modulus of around 400GPa, and a tensile strength of 7GPa. On the other hand, the average Young’s Modulus and tensile strength for CNTs is around 1TPa and 60GPa respectively. Therefore, CNTs are rapidly becoming a standout amongst all the other carbon nanomaterials on account of their novel mechanical properties. Contrasted with steel, the elasticity of CNTs are 5 times higher and elastic strain capacities are 60 times better, what’s more, the specific gravity of CNTs are 1/6 of steel. Even though CNFs and CNTs’ mechanical properties are extraordinary, adding them into cement do not ensure a significant change in mechanical properties, since the properties of nanocomposites are greatly influenced by two factors. The first one is the dispersion of nanomaterials within cementitious matrix and the second one is the bound strength between the cementitious matrix and carbon nanomaterials’ surface. Carbon nanomaterials are strongly bound together and difficult to be separated due to high van der Waals forces. In any case, one can anticipate that CNTs will be influenced more by van der Waals force than CNFs in light of their bigger surface-area-to-volume ratio. This stronger attraction results in CNTs to be more prone to agglomeration than CNFs. Thus, CNTs need to be treated with surfactants prior to be added to cement and mix with an ultrasonic mixer. The energy in the shock wave is exceptionally high, essentially quickens chemical reactions and breaks the clusters and agglomerations of particles. Previous researches have successfully dispersed both CNFs and CNTs within aqueous solutions (Tyson et al. 2011 ). Another research conducted by Metaxa and his team members have proven that utilizing a surfactant and ultrasonic processing can achieve good dispersion of CNFs (Metaxa, Konsta-Gdoutos & Shah 2013).
In terms of how CNFs and CNTs could enhance the mechanical properties of cementitious composite, investigation have been conducted by Tyson and his team. In their study, CNFs and multiwalled carbon nanotubes (MWCNTs) were added to cement paste at 0.1 wt% and 0.2 wt% (by weight of cement). To achieve good dispersion, they first dispersed the carbon nanomaterials in an aqueous solution and then treated them with a commercially available surfactant, followed by using an ultrasonic mixer. They studied the ultimate strength, ultimate strain capacity, elastic modulus and fracture toughness of all mixes. Tyson’s finding provides evidence that, comparing to plain mortar the addition of CNFs and CNTs increased the peak displacement up to 150%, which is essential for construction applications in which higher strain capacity and high ductility to failure is required. This finding also suggests that for the sample reinforced with 0.1% CNFs, its overall performances could match the plan mortar in practically every classification, whereas the flexural strength, Young’s Modulus and fracture toughness decreased in the early ages, 7 and 14 days. Be that as it may, at 28 days, these properties increased beyond the plan mortar. SEM images supported the fact that CNFs and CNTs were not normally distributed in the samples. The deferred improvements in strength, ductility and toughness were likely a direct result of a shift in the bounding between the carbon nanomaterials and the cement matrix. What’s more, CNFs’ performances are superior than CNTs due to their higher aspect ratio (Tyson et al. 2011 ).
The above finding is consistent with the study by Gdoutos and his team, through their experimental study, they established that CNFs and MTCNTs significantly increased the critical stress intensity factor and critical strain energy release rate, as well as reducing the critical tip opening displacement. Their research also indicated that comparing to MWCNTs, CNFs enhance the reinforcing and toughening effect greater. As mentioned above, this is due to the higher aspect ratio of CNFs, their external surface comprises of narrowly formed graphite planes inclined with respect to the longitudinal fibre axis, and create a stronger interfacial bounding between the fibre and the matrix (Gdoutos, Konsta-Gdoutos & Danogilis 2016).
1.7. Benefits of Using CNFs Over CNT
It appears from aforementioned investigations that the performance of cementitious materials reinforced with CNFs exhibit better performance than CNTs, other than that, there are more reasons to utilize CNFs instead of CNTs to reinforce cementitious materials. First of all, comparing to CNTs it is easier to disperse CNFs. This is because of the van der Waals forces between CNTs are much stronger, this force causes the CNTs to form ropes or reassemble after being dispersed, thus chemical dispersants and ultrasonic techniques need to be utilized to help and maintain dispersion. In contrast, CNFs are less influenced by van der Waals force and tend to maintain dispersed for longer period of time. Secondly, the cost of CNFs and CNTs fluctuate upon the manufacture. CNFs are accessible in substantial volumes (up to 31.75 tons per year) and price range from $200 per kilogram to $1000 per kilogram. On the other hand, the price of CNTs differs broadly, and are extremely reliant on the quality and purity of CNTs, the price varies from as low as $200 per kilogram to as much as $1500 per kilogram or even more. And keep in mind that, this is estimated cost for the raw material, which still need to be processed. Considering the final product properties are commonly identical or better for CNFs reinforced composites comparing to CNTs reinforced composites, CNFs usually have a lower general effect on the cost of delivering the nanocomposites (PyrografProducts 2011a).
1.8. Lightweight ECC
There are only limited number of researches on lightweight ECC. In Wang and Li’s research, they successfully achieved lightweight ECC by adding different types of lightweight fillers: glass micro-bubbles, polymeric micro-bubbles, expanded perlite and air bubbles into conventional PVA-ECC. The main findings can be summarised as follow: Firstly, all mixtures exhibit strain hardening behaviour and multiple cracks can be seen on the sample. And it is better to add small size light weight aggregate, e.g. diameter smaller than
100μm, since the smaller the diameter, the minimal the negative influence on the compressive strength and tensile strength, additionally, they can help to maintain the workability of PVA-ECC. Secondly, fillers with prescribed sizes are supported over air voids. Since the size distribution of air voids is hard to control and manage. Besides, a closed shell structure is more beneficial as it guarantees the partition of voids and does not absorb water. Among all these four lightweight fillers, glass micro-bubbles were found to be the best lightweight fillers due to its small diameter and closed shell structure. It contributed in reducing the overall density while maintaining the unique mechanical property of ECC: tensile strain capacity above 3% and compressive strength above 40 MPa (Wang & Li 2003).
1.9. Hollow Glass Microsphere
In recent year, Hollow glass microsphere (HGMS) made by different materials and sizes have been created. HGMS demonstrates an alternate physical behaviour contrasted with traditional solid materials since the macroscopic behaviour is decided by the cell wall material and cellular structure. Because of the high porosity, the material can withstand high compressive force and the unique structure enable it to absorb energy at low and constant stress level. HGMS is normally used as a lightweight filler in oil and mud drilling industry. It has the ability to increase the compressive strength and to reduce the density of the mixture. Additionally, the structure of HGMS is spherical with 2-120
μm, which is an ideal lightweight aggregate to be used to produce lightweight ECC. However, as far we know, short studies have been conducted on the application of HGMS in concrete.
Perfilov and his team introduced HGMS into cement in order to solve the problem of increasing ecological safety, effectiveness and quality of mortars and grouting mortars, and studied HGMS’ reaction with the cement matrix as well as the role of HGMS in lightweight cement. Authors reported that HGMS can be uniformly distributed in the cement system, which was determined by the microstructure analysis that the voids between HGMS is filled with the cement matrix. Similarly, HGMS helps to dense the intergranular space and actively interacts with the products of cement hydration (Perfilov, Oreshkin & Semenov 2016).
1.10. Conclusion of Literature Review
The key findings from the aforementioned literature reviews are summarised as follow: ECC is a relatively new material that can potentially be applied in construction industry in the future, PVA is considered as one of the most suitable polymeric fibres for ECC, commonly referred as PVA-ECC and the fibre content should be constrained to 2% or less. CNFs can enhance the mechanical behaviour of cementitious composite, but to successfully achieve that, good dispersion of CNFs within the cementitious material must be satisfied. ECC can be lightweight and durable at the same time by adding small size and closed shell lightweight aggregates and HGMS is a desirable candidate to achieve that goal.
1.11. Gaps in Literature Review
According to current literatures, CNFs can enhance the performance of cementitious composites but very little attention has been focused on improving the mechanical properties of ECC. Additionally, not much literatures are investigating lightweight ECC, the research conducted by Wang and Li was 15 years ago and since then limited studies were concentrating making ECC lightweight and durable at the same time. Therefore, the research on lightweight ECC is at a relatively novel stage. The current research and gap provides a strong rational to investigate the mechanical property of CNF reinforced lightweight ECC.
4.0 Experimental Programs
1.12. Introduction
The main objective of the thesis is to study the mechanical behaviour of CNF reinforced lightweight PVA-ECC and comparing the results with CNF reinforced PVA-ECC. The mechanical parameters studied in this research include compressive strength and flexural strength. Additionally, density is another factor to be evaluated.
1.13. Material
1.1.1 Cements
The cementitious material used in this study was General Purpose Cement, it fully complies with the requirements for type GP cement in Australian Standard AS3972- General purpose and blended cements. This type of cement is suitable for the manufacture of fibre cement products; thus, it is considered as an appropriate cement to be used in this research. The chemical, physical and mechanical properties of the General Purpose Cement is listed in (table).
1.1.2 Fly Ash
Fly ash is one of the slag created in combustion, and contains the fine particles that ascent with the flue gases. Ash that has been left at the base of the boiler is called bottom ash. Why used in construction industry, fly ash normally refers to ash produced during the burning of coal. Coral fly ash particles are generally collected by electrostatic precipitators or other molecule filtration hardware before the flue gases come to the chimneys of coal-fired power plants. The type of fly ash used in this research is Cement Australia’s Fly Ash, which is a Fine Grade ash that fully complies with the requirements of Australian Standard AS3582.1- Supplementary cementitious material for use with General Purpose and blended cement. The chemical and physical properties of this type of fly ash is listed in (table)
1.1.3 Silica Fume
Silica fume is a one of the by-product of the silicon and ferrosilicon alloy. A standout amongst the most valuable uses for silica fume is in concrete. When silica fume is added to Portland cement and water, the reaction starts which result in improvement in compressive, flexural and bond strength, as well as a much denser mix, especially in areas that would have contain many small air voids(TheSilicaFumeAssociation 2014). Densified Silica Fume produced at SIMCOA’s silicon plant was used in this research, it complies the requirements of Australian Standard AS3582.3- Supplementary cementitious materials for use with Portland and blended cement. Its chemical and physical properties can be found in (table).
1.1.4 Silica Sand
Silica sand also known as Quartz sands are normally used in building construction and road buildings. As mentioned in the literature review, ECC incorporate fine aggregates instead of coarse aggregates to maintain adequate stiffness and volume stability. Silica sand is the most common fine aggregate used in ECC. Another benefit of using silica sand is to optimum gradation of particles to produce good workability. Silica sand from Hanson was adopted in this experiment, its chemical constituents and particle size distribution are listed in table
1.1.5 PVA Fibres
Polyvinyl Alcohol (PVA) fibre is selected as the fibre reinforcement for ECC. According to literature review, the bond between fibre and cement matrix is very strong and it can provide durability to cementitious composites. All mixture design corporate PVA fibres at a ratio of 1.75 vol%. The fibres mechanical and geometric properties are summarized in Table 1.
Table 1Properties of PVA fibres
| Fibre | Diameter(mm) | Length(mm) | Density (g/cm3) | Tensile Strength (MPa) | Young’s Modulus (GPa) |
| PVA | 0.039 | 12 | 1.3 | 1600 | 41 |
1.1.6 Carbon Nanofiber
In this experiment, four types of extremely fine, highly graphic and inexpensive carbon nanofibers with the following commercial names: PR-19XT-LHT, PR-24-XT-LHT, PR-24-XT-PS, PR-19-XT-PS, were used. Hereafter, these carbon nanofibers will be designated as t, 19LHT, 24 LHT, 24PS, 19PS, respectively (Table 2).
24LHT and 19LHT: these types of fibre have been heat treated at 1500oC. Carbon distributed on the fibre surface as chemical vapour can be transformed into a short range ordered structure. Therefore, the conductivity of these types of fibre are enhanced. (AZoNetwork 2012)
24PS and 19PS: polyaromatic hydrocarbons have been removed from the surface of these fibres by pyrolytically stripping the as-produced fibres(PyrografProducts 2011b).
Table 2 Properties of carbon nanofibers
1.1.7 HGMS
The main feature of HGMS includes lightweight (0.2-0.6 g/cm3) with an average diameter of 2-120
μm, low thermal conductivity, high compressive strength and smooth mobility. The type of HGMS selected in this research is called H40, some of its properties is listed in (table)
Table 3 Properties of HGMS
| Type | Density | Test pressure | Particle size (
μm,by volume) |
||
| (g/cm3) | (Mpa) | D10 | D50 | D90 | |
| H40 | 0.4 | 28 | 25 | 50 | 95 |
1.1.8 Superplasticiser
To improve the workability and rheology of the ECC while maintaining a low water to cement ratio, superplasticiser (MasterEase 3000 manufactured by BASF) have been added. Other than that, when adding superplasticizer to fresh concrete which helps in dispersing constituents uniformly throughout the mix, this is accomplished by their deflocculation activity on cement particles by which water ensnared is discharged and is available for workability.
1.14. Mix Design
Since there is no definite mixing proportion nor mixing procedure for the mix design of ECC, the mix proportion used in this research was designed based on a large number of literature review. The mix proportion is listed below in table. In total, 34 batches of cement paste were prepared to test after 7 days and 28 days, the mix designs are listed in table 4. These included two references plain ECC samples reinforced with PVA only, sixteen batches of CNF reinforced ECC with each type of CNFs at 0.2 wt% and 0.4 wt%, and sixteen batches of CNF reinforced lightweight ECC. To achieve lightweight, 40% H40 were introduced to replace 40% fly ash by volume. These batches were labelled to indicate the components and their concentrations in table.
Table 4 Mixture proportions
| Cement | FA | SF | W | Sand |
| 1.0 | 1.33 | 0.11 | 0.65 | 0.89 |
Table 5 Mix Design of the Test Specimens
| Sample | PVA | HGMS | CNF | ||
| Amount | Type | Amount | Type | Amount | |
| PVA_ECC | 1.75% | – | – | – | – |
| 19LHT_0.2% | 1.75% | – | – | 19LHT | 0.20% |
| 24LHT_0.2% | 1.75% | – | – | 24LHT | 0.20% |
| 19PS_0.2% | 1.75% | – | – | 19PS | 0.20% |
| 24PS_0.2% | 1.75% | – | – | 24PS | 0.20% |
| 19LHT_0.2%_H40 | 1.75% | H40 | 40% | 19LHT | 0.20% |
| 24LHT_0.2%_H40 | 1.75% | H40 | 40% | 24LHT | 0.20% |
| 19PS_0.2%_H40 | 1.75% | H40 | 40% | 19PS | 0.20% |
| 24PS_0.2%_H40 | 1.75% | H40 | 40% | 24PS | 0.20% |
| 19LHT_0.4% | 1.75% | – | – | 19LHT | 0.40% |
| 24LHT_0.4% | 1.75% | – | – | 24LHT | 0.40% |
| 19PS_0.4% | 1.75% | – | – | 19PS | 0.40% |
| 24PS_0.4% | 1.75% | – | – | 24PS | 0.40% |
| 19LHT_0.4%_H40 | 1.75% | H40 | 40% | 19LHT | 0.40% |
| 24LHT_0.4%_H40 | 1.75% | H40 | 40% | 24LHT | 0.40% |
| 19PS_0.4%_H40 | 1.75% | H40 | 40% | 19PS | 0.40% |
| 24PS_0.4%_H40 | 1.75% | H40 | 40% | 24PS | 0.40% |
1.15. Mixing Procedure
All mixtures were prepared in Hobart mixture figure. First of all, cement, fly ash, silica fume and silica sand were mixed for 2 minutes at low speed, then gradually add PVA fibres into the dry mixture, this procedure usually takes about 10 minutes at low speed. Then it is time to add HGMS and CNF, since HGMS and CNF are very light weight, the mixer is required to stop thus HGMS and CNF can be added to the pan. It is important that during the transportation, there is no spillage of those two materials. Next, the mixer can be turned on and all the dry materials will be mixed for another 2 minutes. Now, water and superplasticizer can be added in together and then the mortar was mixed for 2 minutes at high speed and 3 minutes at medium speed until the mixture is homogeneous.
1.16. Fresh Property of ECC
The appropriately blended ECC mixture ought to have a creamy texture and smooth surface. To guarantee good self-consolidation behaviour, ECC’s fresh property need to be checked before casting. A concrete slump cone test can be used to performed this check. The slump test measured the consistency of mortar, and its results are correlated with the yield stress. The slump cone used in the research has a top diameter of 100mm, a bottom diameter of 140mm and a height of 105mm. the cone was placed in the centre of a square steel plate and filled with ECC mortar. Then the cone was lifted up to aloe the mortar to flow. Once the flowing stopped, the spread of the mortar was recorded as the average diameter of the measured along two diagonals. According to Li, for self-compacting ECC, a characteristic deformability factor
Γshould have a minimum value of 2.75. Exorbitantly
Γvalue may show improper mix proportions and may possibly cause agglomeration in the mix (Li 2007).
Γ=D1-DODO (Equation 3.5)
D1 is the average diameter measured along two diagonals, whereas DO represents the bottom diameter of the slump cone.
1.17. Sampling and Curing Conditions
Once the ECC was mixed, the specimens were cast. Two types of specimens were used for compressive and flexural testing. The specimens cast for compressive testing were cast into cube moulds that had the dimensions of
50×50×50 mmin accordance with ASTM C109 and the specimen dimension casted for flexural tests were
40×40×160 mmin accordance with ASTM C109. The moulds used can be seen in (figure xx), for each mix design, 4 samples were prepared for each test. In accordance with Australian Standard AS1012.8- Methods of testing concrete, all specimens were prepared without compaction or vibration. After casting, the samples were cured in moulds for 24 hours at 25
±2oC and 95% relative humidity. After 24 hours, the sample were demolded and then put back in the curing room for 7 days or 28 days until the day of testing in accordance with AS1012.8.
1.18. Testing of ECC
Specimens were tested to evaluate their mechanical properties through compression tests and three-point bending tests. The specimens were tested at the age of 7 days and 28 days.
1.1.9 Compression test
The testing machines used for compression tests are BALDWIN and INSTRON 5982 with loading capacity of 600KN and 100KN respectively. The speed of 0.5KN/mm was applied for each sample (figure).
1.1.10 Flexural test
For the flexural testing, three-point bending tests was performed. Support distance of the loading was 100mm. Testing machine INTRON 5982 was loaded at 0.5 KN/mm for each test. Detailed discussion on test results would follow next (figure).
1.1.11 Scanning Electron Microscopy
The morphology of the distribution of CNFs in ECC and the morphology of the CNF/HGMS nanocomposites’ surface were investigated using Scanning Electron Microscopy (SEM). For the SEM, small size specimens of
5mm × 5mm × 1mmwere prepared for each mix (Figure). To eliminate charging effect, the sample surface of the nanocomposites, prior to their observation, were coated with gold (Au). The SEM was performed using TESCAN VEGA3, operating at 20KV using secondary electron imaging. This mode provides high-resolution imaging of the surface morphology at medium to high magnification.
5.0 RESULTS AND DISCUSSION
1.19. Workability
According to equation xx, while using the small slump cone, the desirable spread of the mixtures should be larger than 525 mm. As shown is figure, in this research, the workability of all the mix design passed the requirement mentioned in section xx, with
Γ>2.75, which meant that the concrete mixtures were highly flowable and self-compacting. It was observed that the amount of superplasticizer added for each design mix various. For example, the amount of superplasticizer added for 19LHT_0.2%_H40 is much lower than 19LHT_0.2%,
1.20. Density
Density is an important physical property for lightweight concrete. The densities of all specimens were measured at both 7 days and 28 days are given in table. The density of CNF reinforced lightweight ECC at 7 days range from 1740 kg/m3 to 1823kg/m3, the 28 days density range from 1753 kg/m3 to 1819 kg/m3, which is 27%-24% less than that of normal concrete with a typical density of 2400kg/m3. According to ACI committee 213, structural lightweight concrete’s average density at 28 days should not exceed 1850kg/m3 (Caldarone & Burg 2004). All the ECC mixtures with HGMS exhibit density lower than 1850kg/m3 after 28 days curing time in the curing room. Hence, the CNF reinforced lightweight ECC satisfies the requirement for lightweight concrete. In terms of density decreasing from PVA-ECC, the density decreased 6%-9% due to the fact that ECC doesn’t contain any coarse aggregates, the density of ECC itself is much lower than normal concrete. According to table, when adding 0.2% CNFs to lightweight ECC, the density has decreased at 28 days comparing to 7 days for 24LHT_0.2%_H40 and 19PS_0.2%_H40, whereas for 0.4% CNFs reinforced lightweight ECC, except for 19LHT_0.4%_H40, the average density for others has increased
| Sample | Density | |
| 7 Days | 28Days | |
| PVA_ECC | 2032 | 1928 |
| 19LHT_0.2% | 1984 | 1989 |
| 24LHT_0.2% | 1914 | 1966 |
| 19PS_0.2% | 1952 | 1931 |
| 24PS_0.2% | 1939 | 1909 |
| 19LHT_0.2%_H40 | 1740 | 1808 |
| 24LHT_0.2%_H40 | 1806 | 1753 |
| 19PS_0.2%_H40 | 1818 | 1786 |
| 24PS_0.2%_H40 | 1792 | 1809 |
| 19LHT_0.4% | 1987 | 1906 |
| 24LHT_0.4% | 1954 | 2046 |
| 19PS_0.4% | 1966 | 2007 |
| 24PS_0.4% | 1983 | 2016 |
| 19LHT_0.4%_H40 | 1809 | 1819 |
| 24LHT_0.4%_H40 | 1823 | 1766 |
| 19PS_0.4%_H40 | 1778 | 1795 |
| 24PS_0.4%_H40 | 1787 | 1818 |
1.21. Compressive Strength
The average 7-day and 28-day compressive strength results of all mix designs are summarized in table. It was observed that for all mix designs compressive has increased from 7 days to 28 days and all the mixtures had compressive strength higher than 30MPa at 28days, which is a normal strength for concrete.
It was observed that the compressive strength for 0.4% CNF reinforced ECC are smaller than 0.2% CNFs reinforced ECC and some of them are smaller than PVA-ECC, however, at 28 days, these values has increased and beyond the plain cement. At 28 days, 24LHT_0.4% possess the highest compressive strength 62.96 MPa, high compressive strength is also exhibited in 19PS_0.4% and 24PS_0.4% with 56.81 MPa and 58.83 MPa respectively. It is impressive that with a concentration increment of 0.2%, the strength of ECC can be enhanced by xx% comparing to PVA-ECC
As for CNF reinforced lightweight ECC, their compressive strengths are very similar to PVA-ECC. At 28 days, the compressive strength for lightweight ECC reinforced with 0.4% CNF is better, thus, it can be concluded that, increase the dosage of CNFs can improve the compressive strength of lightweight ECC. But comparing to CNFs reinforced ECC, the compressive strength decreased significantly.
SEM imaging helped us explain the above phenomenon, Figure shows SEM image of the surface of 24LHT_0.4% at a scale of
20μmand Figure shows the SEM image of the surface of 24LHT_0.4%_H40. Initially, CNFs can be easily observed and identified in the sample surface. It can also be seen that CNFs appears to be embedded into the cement matrix and the hydration products, this provides evidence that good bonding exist between the CNFs and cement matrix. Excellent bonding ensures the load can be transferred between the matrix and CNFs which result in the enhancement in compressive strength. After adding HGMS, CNFs could not be easily found in the surface of the sample anymore, but HGMS can be easily seen in the sample. It seems that HGMS and CNFs interacted on each other, thus the distribution of CNFs and HGMS inside the matrix were very poor. Similar phenomenon can be seen in figure shows the surface of 19PS_0.4%, and figure shows the surface of 19PS_0.4%_H40. In figure, no CNFs was identified within the surface and agglomeration of HGMS can be seen.
| Sample | Compressive strength (MPa) | |
| 7 Days | 28Days | |
| PVA_ECC | 30.54 | |
| 19LHT_0.2% | 39.09 | |
| 24LHT_0.2% | 38.63 | 49.76 |
| 19PS_0.2% | 35.87 | 46.04 |
| 24PS_0.2% | 35.18 | 42.26 |
| 19LHT_0.2%_H40 | 24.84 | |
| 24LHT_0.2%_H40 | 26.82 | 41.15 |
| 19PS_0.2%_H40 | 29.71 | 42.78 |
| 24PS_0.2%_H40 | 25.04 | 40.36 |
| 19LHT_0.4% | 35.31 | 44.92 |
| 24LHT_0.4% | 34.02 | 62.96 |
| 19PS_0.4% | 28.26 | 56.81 |
| 24PS_0.4% | 29.14 | 58.83 |
| 19LHT_0.4%_H40 | 32.75 | 45.44 |
| 24LHT_0.4%_H40 | 32.11 | 45.76 |
| 19PS_0.4%_H40 | 30.34 | 44.78 |
| 24PS_0.4%_H40 | 30.34 | 48.15 |
1.22. Flexural Strength
The average 7-day and 28-day flexural strength results of all mix designs are summarized in table. The largest flexural strength was found in 24LHT_0.4% at 28 days. Distinct from compressive strength, not all mix design’s flexural strength increased with curing time.
| Sample | Flexural strength (MPa) | |
| 7 Days | 28Days | |
| PVA_ECC | 4.71 | |
| 19LHT_0.2% | 4.94 | |
| 24LHT_0.2% | 4.67 | 5.76 |
| 19PS_0.2% | 4.56 | 7.14 |
| 24PS_0.2% | 4.51 | 5.01 |
| 19LHT_0.2%_H40 | 4.23 | |
| 24LHT_0.2%_H40 | 4.24 | 5.47 |
| 19PS_0.2%_H40 | 4.66 | 7.04 |
| 24PS_0.2%_H40 | 4.58 | 5.36 |
| 19LHT_0.4% | 7.11 | 6.18 |
| 24LHT_0.4% | 5.73 | 7.82 |
| 19PS_0.4% | 6.53 | 6.58 |
| 24PS_0.4% | 6.25 | 7.56 |
| 19LHT_0.4%_H40 | 5.34 | 6.12 |
| 24LHT_0.4%_H40 | 5.27 | 5.94 |
| 19PS_0.4%_H40 | 6.56 | 6.80 |
| 24PS_0.4%_H40 | 5.96 | 5.59 |
6.0 Conclusion and Recommendations
7.0 Reference
AZoNetwork 2012, Pyrograf III Carbon Nanofibers: Properties and Applications of Type PR-24 Nanofibers. Available from: https://www.azonano.com/article.aspx?ArticleID=3075. [27 September 2017].
Caldarone, MA & Burg, RG 2004, Development of Very Low Density Structural Lightweight Concrete, Special Publication, p. 188[20 October 2017].
Gdoutos, EE, Konsta-Gdoutos, MS & Danogilis, PA 2016, ‘Portland Cement Mortar Nanocomposites at Low Carbon Nanotube and Carbon Nanofibre Content: A Fracture Mechanics Experimental Study’, Cement& Concrete Composites, vol. 70. [17 October 2017].
Li, V 2007, Concrete Construction Engineering Handbook, CRC Press. [27 September 2017].
Li, VC & Kanda, T 1998, ‘Engineered Cementitious Composites for Structural Applications’, Materials in Civil Engineering, vol. 10, pp. 66-69. Available from: https://deepblue.lib.umich.edu/bitstream/handle/2027.42/84890/asceforum_98.pdf;sequence=1. [27 September 2017].
Metaxa, ZS, Konsta-Gdoutos, MS & Shah, SP 2013, ‘Carbon nanofiber cementitious composites: Effect of debulking procedure on dispersion and reinofrcing efficiency’, Cement& Concrete Composites, vol. 36, p. 8. [28 September 2017].
Perfilov, VA, Oreshkin, DV & Semenov, VS 2016, ‘Environmentally Safe Mortar and Grouting Solutions with Hollow Glass Microspheres’, Procedia Engineering, vol. 150. Available from: http://www.sciencedirect.com/science/article/pii/S1877705816313959. [19 October 2017].
PyrografProducts 2011a, A Comparison of Carbon Nanotubes and Carbon Nanofibers. Available from: http://pyrografproducts.com/carbon-nanotubes.html. [28 September 2017].
PyrografProducts 2011b, Pyrograf-III Carbon Nanofiber, Pyrograf Products. Available from: http://pyrografproducts.com/Merchant5/merchant.mvc?Screen=cp_nanofiber. [27 September ].
Sathishkumar, P, Sampathkumar, P, Karthik, M & Vignesh, C 2016, ‘Significance of Various Fibres on Engineered Cementitious Concrete’, International Journal of Science, Engineering and Technology, vol. 4, no. 1. Available from: http://www.ijset.in/wp-content/uploads/2016/02/10.2348.ijset0116284.pdf. [17 October 2017].
TheSilicaFumeAssociation 2014, What is Silica Fume? Available from: http://www.silicafume.org/general-silicafume.html. [18 October 2017].
Tyson, BM, Al-Rub, RKA, Yazdanbakhsh, A & Grasley, Z 2011 ‘Carbon Nanotubes and Carbon Nanofibres for Enhancing the Mechanical Properties of Nanocomposite Cementitious Materials ‘, Journal of Materials in Civil Engineering, vol. 23. Available from: http://ascelibrary.org/doi/pdf/10.1061/(ASCE)MT.1943-5533.0000266. [17 October 2017].
Wang, S & Li, VC 2003, ‘Lightweight Engineered Cementitious Composites (ECC)’, International RILEM Workshop on High Performance Fiber Reinforced Cement Composites (HPFRCC 4), vol. 1, p. 546. RILEM Publications, [18 October 2017].
Wang, S & Li, VC 2017, Polyvinyl Alcohol Fiber Reinforced Engineered Cementitious Composites: Material Design And Performances [27 September 2017].
