Carbon Nanofiber Reinforced Lightweight Engineered Cementitious Composites

Table of Contents

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Abstract

1.0 Introduction

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2.0 Literature Review

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Introduction

Historical Background of ECC

Why add PVA in ECC

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CNFs and CNTs in Cementitious Materials

Benefits of Using CNFs Over CNT

Lightweight ECC

Hollow Glass Microsphere

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Conclusion of Literature Review

Gaps in Literature Review

3.0 Experimental Programs

Introduction

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Material

1.1.1 Cements

1.1.2 Fly Ash

1.1.3 Silica Fume

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1.1.4 Silica Sand

1.1.5 PVA Fibres

1.1.6 Carbon Nanofiber

1.1.7 HGMS

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1.1.8 Superplasticiser

Mix Design

Mixing Procedure

Fresh Property of ECC

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Sampling and Curing Conditions

Testing of ECC

1.1.9 Compression test

1.1.10 Flexural test

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1.1.11 Scanning Electron Microscopy

4.0 RESULTS AND DISCUSSION

Workability

Density

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Compressive Strength

Flexural Strength

5.0 Conclusion and Recommendations

6.0 Reference

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7.0

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.

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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).

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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 ).

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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).

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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.

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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

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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 1500^oC. 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)

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Table 3 Properties of HGMS