Modern sustainable cement and concrete composites: Review of current status, challenges and guidelines

Journal Pre-proof Modern sustainable cement and concrete composites: Review of current status, challenges and guidelines

Natt Makul PII:

S2214-9937(19)30375-6

DOI:

https://doi.org/10.1016/j.susmat.2020.e00155

Reference:

SUSMAT 155

To appear in:

Sustainable Materials and Technologies

Received date:

5 December 2019

Revised date:

9 January 2020

Accepted date:

15 January 2020

Please cite this article as: N. Makul, Modern sustainable cement and concrete composites: Review of current status, challenges and guidelines, Sustainable Materials and Technologies(2019), https://doi.org/10.1016/j.susmat.2020.e00155

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© 2019 Published by Elsevier.

Journal Pre-proof

Modern sustainable cement and concrete composites: Review of current status, challenges and guidelines Natt Makul Department of Building Technology, Faculty of Industrial Technology, PhranakhonRajabhat University, 9 Changwattana Road, Bangkhen Bangkok, 12120, Thailand E-mail: [email protected]

Abstract

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Due to global demand for sustainability, more advanced techniques have been

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recommended for improvement cement and construction purposes. Reduction in emissions for all infrastructural projects can be realized when there will be swift scale-up in the use of

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novel cement alongside the integration of positive policies. This study investigates approaches such as RPC, steel fiber reinforcement, green cement, advanced nano cement and

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self-consolidating and how they contribute to environmental sustainability and reduction of

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costs of construction. An improved multifunctional nanoengineered concrete exhibits advanced functionalities in comparison to the conventional cement. For instance, they have up to 79.5% and 146% respective flexural and compressive strengths. Besides, they have

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advanced thermo-mechanical and electrical performance with a considerable decline in water

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absorption of almost 400%. Such benefits make this study worth analyzing how these approaches can be implemented. The study also analyzes advanced cement repair and

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preservation techniques. Due to the low adoption of these sustainable solutions, this study recommends concerted efforts from different stakeholders including policymakers to implement low-carbon rates.

Keywords: Cement; Concrete; Composites; Sustainable; Challenge; Guidelines

Contents 1. Introduction

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2. Reactive powder concrete

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2.1. Constituents of RPC

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2.2. Long-term performance and mechanical components of RPC

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2.3. Limitations and application of RPC

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2.4. Challenges of RPC

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Journal Pre-proof 3. Fiber-reinforced concrete

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3.1. Functionality

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3.2. Nanoscale improvement of fiber concrete

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3.3. Types of fiber-reinforced concrete

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3.4. Challenges of fiber-reinforced concrete

x x

4.1. Properties of SCC

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4.2. Use of superplasticising admixtures

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4.3. Mix proportions

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4.4. Challenges of SCC

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4. Self-consolidating concrete

5. Green cement and concrete

5.2. Trends towards greener solutions

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5.1. Mix proportions

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5.3. Challenges of green cement and concrete 6. Nano cement and concrete

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6.1. Nano-silica 6.2. Carbon nanotubes

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6.3. Other forms of nanomaterials

x x x x x x x x x

6.5. Challenges of nano cement and concrete

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6.4. Benefits of nanomaterials on cement

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7. Repair cement and concrete

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7.1. Polymer-modified concrete (PMC)

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7.2. Mixture proportions

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7.3. Challenges of repair cement and concrete

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8. Conclusions and guidelines

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8.1. Conclusions

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8.2. Guidelines for development of modern sustainable concrete

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Acknowledgments

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References

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1. Introduction Modern engineering technologies aim at producing ultra-high performance multifunctional concrete materials due to the increased demand for sustainability, durability and cost-effectiveness. Such construction materials are characterized by improved mechanical properties and long-term performance. They also integrate properties that

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encourage multiple applications making them sustainable for future uses. As Dimitar et al. [1] discuss, most of the extant research encourages the adoption of improved concrete that is engineered at the nanoscale. This is because nanoscale engineering encourages the

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enhancement of both physio-mechanical and chemical properties. An improved

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multifunctional nanoengineered concrete exhibits advanced functionalities in comparison to the conventional cement. They also have up to 79.5% and 146% respective flexural and

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compressive strengths. Besides, they have advanced thermo-mechanical and electrical performance with a considerable decline in water absorption of almost 400%. Considering all

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these properties, advanced concrete composites are integral in multifunctional applications such as in marine and chemical exposed environments due to their lightweight nature, high

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durability, affordability and high resistance to corrosion.

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Composite materials are the terminology that resonates with modern cement improvement techniques. As the name suggests, composite materials (a combination of aggregates) provide an intuitive mixture of stiffness, toughness with lightweight and corrosion resistance properties. Such materials are derived from several constituents with different chemical, physical and physical properties. A combination of such aggregates gives unique capabilities, which gives composite materials an edge over other methods of improvement. Additionally, this terminology associates with naturally occurring elements including bones and wood that have respective inorganic hydroxyapatite crystals and fibrous chains of cellulose molecules [2,3]. The unique characteristic of composites from other solid aggregates is that they cannot dissolve, lose their characteristics, or blend. Rather, they amalgamation and synergistically supply their ingredients to enhance the quality of resulting concrete [4]. The distinct composite components in concrete can be observed via a microscope. Thus, such composite material is a blend of filler and base material. Also known as binders, matrix, or

Journal Pre-proof cement, the base materials enclose and bind the reinforcement of other material. The aggregate or filler material is the small fragments, stones, particles, gravels and fibers, whiskers of artificial or organic elements. They are also called reinforcement materials. Since the constituents of a composite material affect their characteristics, there is a need to investigate their categories and unique properties [5]. Considering that such materials are currently used in many applications and fields, scholars and experts have begun to develop different forms of advanced manufacturing techniques to promote efficiency and productivity. Major categories are as discussed as follows:  Reinforcement based composite. The first classification is based on

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reinforcement (examples are fiber-reinforced materials, sheet reinforced materials and particle reinforced materials). Fibers can be taken from organic components or synthetic fibers such as carbon, glass and basalt. Particle reinforced concrete are

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classified into large and dispersion particles. Concrete mixture with sand and

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gravel is one of the largest particle composites. While particle reinforced concrete does not perform better than fiber-reinforced composite, it has the benefit of low

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cost and ease of production. Finally, sheet reinforcement comprises glass. Glass fiber-reinforced concrete is a form of fiber-reinforced concrete comprising “high-

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strength, alkali-resistant glass fiber” dispersed into composite matrices [6] RPC is an example of concrete composite based on reinforcement. It comprises fine grains

steel fibers.

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of cement, sand, quartz and silica fume. It also has elements of superplasticizer and

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 Matrix phase-based composite. The second category of composites is based on the matrix phase. Types include Ceramic matrix composites (CMCs), polymer matrix composites (PMCs) and metal matrix composites (MMCs). Also called inverse composites, CMCs are tailored to overcome the challenge of monolithic ceramics and brittleness. They comprise silicon nitride (SiN), aluminum oxide (Al2 O3 ), carbon and silicon carbide (SiC) fibers. MMCs majorly comprise metallic reinforcements of copper (Cu), magnesium (Mg), titanium (Ti) and aluminum (Al). PMCs have thermoplastics matrix as their constituents interspersed with carbon, glass, metal fibers [7-10].  Nano-scale based composite. The third category of composites is based on scale. The two types include the nano-composites [11-17] and bio-composites. The nanocomposites involve mixing and improvement of materials at the nano-scale [18-24]

Journal Pre-proof that leads to concretes with outstanding qualities. The demand for bio-composites is for biodegradability and environmental sustainability as they can be derived from sugar palm fibers reinforced in the sago starch matrix. Previously, improvement techniques focused on mechanical and physical properties of cement. Modern technologies recommend improvement that is engineered at the nanoscale. This is because nanoscale engineering encourages the enhancement of both physiomechanical and chemical properties. An improved multifunctional nanoengineered concrete exhibits advanced functionalities in comparison to the conventional cement. Furthermore, the

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Association of Equipment Manufacturers confirms that many firms now aim at limiting the

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ecological effects of concrete [25]. Great Britain developed the Concrete Industry Sustainable Construction Strategy in 2008 to provide a unified standard for concrete improvement. In

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2017, the Concrete Industry Sustainability Performance Report revealed that regulating the ecological effects of production and procuring resources contributes to sustainable

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development. Besides, the more sustainable the materials are, the less the wastage, thus,

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reducing costs of construction. Improvement also enhances concrete performance, longevity, consistency, performance and applicability.

As part of this nanoscale improvement [26-32], for example, Chung discusses the

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need for “Multifunctional concretes that serve both structural and nonstructural functions

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have evolved from concretes that are purely structural materials. For example, self-sensing concrete can sense its condition (stress, strain, damage, etc.) without any embedded or

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attached sensor” [33]. However, achieving multifunctional properties is a tedious process that calls for experience in the functional elements. Manufacturers must, therefore, understand electrical and other properties of cement [33]. Concrete smartness is another term that is synonymous with nano-scale improvement. Han et al. [34] define smart concretes and structures as intelligent systems whose behaviors differ from traditional cement. For example, they have self-healing and self-sensing characteristics. Others may positively respond to external stimuli, such as pressure, heat, or stress. Through concrete’s smartness ability, they can respond to the stimuli without human intervention. The self-healing properties allow concrete to respond to air and water restoring the physical and mechanical properties damaged by exposure to such elements [35]. Thus, they promote longevity and resilience. Construction firms attain concrete’s “smartness” by combining with other functional elements, through special processing, transforming the microstructure, or changing

Journal Pre-proof composition design. Smartness processes transform their properties to enhance safety, serviceability, durability, long-term performance, safety and cutting life-cycle expenses. These high-performance characteristics also make concrete resilient and sustainable. As cement becomes more sustainable, quality of life also improves. Examples of highperformance cement properties include “greenness” of cement, autogenous crack-width control and tensile ductility. Digital improvement also contributes to high-performance characteristics of cement. Thus, concrete smartness is a combination of multifunctional concrete based on technological advancement and physical and mechanical properties. The previous section discussed some of the progress that has led to the formation of

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modern concretes. This section will evaluate individual examples of some of the modern concretes that employ each of the above composite classification methods. This study recommends a better identification of how composites are categorized, their primary

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constituents, manufacturing methods and possible applications to substitute conventional

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practices

In an age when natural resources are depleting, a study that investigate s cement,

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which is the most consumed substance in the world, is a novel idea. Cement is not only on high demand but also on a substance that creates pollution. This study informs about the

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possibilities of recycling cement and using environmentally sustainable approaches that make optimal use of this concrete. Conventional concrete has had challenges related to the high

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cost of access, carbon dioxide emissions, cracking, the need for periodic repairs, low

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durability and limitation based on applications. This study informs that technology can change everything, which is already proven useful in the extraction and formulation of waste substances, it can contribute to environmental conservation and reduced costs of production.

2. Reactive powder concrete

Reactive powder concrete (RPC) is an emerging composite matter, which can enable the cement sector to make optimal use of material use, gain economic benefits and establish more long-lasting and stronger structures that are responsive to the environment. An analysis of the “physical, mechanical and durability properties of high-performance concrete (HPC)” and RPC reveals that RPC has improved flexural and compressive strengths with reduced absorption compared to HPC. The physical and mechanical properties of RPC are analyzed alongside other recent improvement techniques to gauge its strength regarding durability, compressive and flexural strengths, as well as permeability. According to the research by Al-Azzawi et al. [36], the

Journal Pre-proof high strength concrete (HSC) and steel fiber concrete (SFC) are examples of these alternatives that seek to improve properties of concrete [36]. For instance, HPC composition is not merely water, aggregates, or cement mixtures. It integrates other mineral elements as well as chemical admixtures with specific properties. HPC’s improvement emanates from the materialization of a recent scientific concrete, mode of admixtures and high-tech scientific tools that evaluate the microstructure of concrete. Such an improvement technique enables HPC to achieve an optimum compressive strength of its present microstructure. Nonetheless, at such strength levels, its course aggregates turn out to be the fragile link. Attaining a high compressive strength is by

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eliminating the coarse aggregates, a technique that has been utilized in RPC. Whereas the HPC has some added benefits through its mode of production of improvement of the microstructure, the lack of a strong compressive strength encouraged the production of better

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alternatives. The current improved form of HPC is characterized as ultra-high performance

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concrete (UHPC). The UHPC has a long-term performance with improved compressive strength. RPC is regarded as the leading improved "High Tech" material. Hiremath and

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Yaragal [37] reveal that as part of an improved UHPC, RPC has the same elements such as those of the UHPC but in an advanced state. Technologies categorize the RPC as a mode of

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technical revolution compared to the familiar concretes. Richard and Cheyrezy were the first experts who initiated the concept of reactive

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powder concrete. Whereas they tested this concrete at France's Laboratory of Bouygues et al.

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[16] discuss that the first structure where this concrete was erected was in Canada, during the construction of the Sherbrooke Bridge. It took seven years since the concept of this concrete was initiated in 1990 to 1997 when it was first globally tested. The Sherbrooke Bridge was a Bikeway that elevated the status of RPC, which was later nominated in 1999 during the Nova Awards. The concrete was later adopted for the containment of nuclear aggregates because of its high resistance to absorption. From the Indian context, the average composition of concrete required for isolation and nuclear waste containment is normal HPC. It has a uniform density, an average elastic modulus (E), improved workability, normal compressive strength and stable longevity. RPC must be evaluated to determine its durability and strength to serve as a nuclear waste containment agent in India.

2.1 Constituents of RPC RPC comprises fine grains of cement, sand, quartz and silica fume. It also has elements of superplasticizer and steel fibers. The superplasticizer, employed at an optimum

Journal Pre-proof level, reduces the water-cement ratio (w/c) as it also enhances concrete’s workability. To attain a dense matrix, aggregates of dry fine powders can be optimized. Thus, the corresponding composition of RPC is highly compact, which gives it ultra-high durability and strength. Average compressive strengths of RPC range from 200 to 800 MPa. There are various principles employed in the improvement of RPC. Based on findings by Richard and Cheyrezy, the following are some of the known principles.  Removal of coarse grains to improve homogeneity  Employing the pozzolanic characteristics of silica fume to increase compressive strength and reduce water absorption.

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 Optimizing the granular mixture that seeks to improve RPC’s compacted density  Utilizing superplasticizer optimally to lower water-cement ratio (w/c) and enhance workability.

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 Applying pressure (before and during setting) that promotes its compaction

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 Using post-set heat-treatment which will boost concrete’s microstructure

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 Incorporating small-sized steel fibers that can augment its ductility

Table 1 reveals the salient characteristics of RPC. It also offers recommendations on

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achieving each of these properties. During the mixing process of RPC, a dense granular structure is formed. This skeletal granular structure can be optimized using packaging

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models. Alternatively, the distribution software that deals with grain size can also improve

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this dense granular mixture. Tables 2 and 3 describe various constituents of RPC alongside their selection parameters and different mixture ratios with different studies, respectively.

Table 1. RPC's salient characteristics [38]. RPC’s properties

Description

Failure eradicated

Recommended values

Improved

Enhanced

Disturbance of the

Elastic modulus values

mechanical

mechanical

mechanical stress

in 50 - 75 GPa range

components

characteristics of the

field.

paste by integrating silica fume Reduction in size

Coarse grains are

Mechanical,

600 µm optimum fine

of granules

substituted by fine

chemical &

sand size

sand. Averagely, a

thermo-mechanical

Journal Pre-proof RPC’s properties

Description

Failure eradicated

Recommended values

factor of 50 is the reduction size Reduction in the

Reducing the content

By any external

Not less than 30% paste

proportion of

of sand

source (e.g.,

volume compared with

formwork).

pores of non-compacted

matrix

sand

Table 2. RPC's constituents [38]. Selection

Types

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Function

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Components

Particle size

parameters

C3 A : 3.8%;

OPC,

1 µm

formation of primary

C2 S : 22%;

Medium

to

hydrates

C3 S: 60%;

fineness

100 µm

Moderate

Natural,

150 µm

hardness

Crushed

to

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Binding material,

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Cement

C4 AF: 7.4%.

Sand

Give strength,

Optimum reactivity

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Quartz powder

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aggregate

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(maximum)

easily found and

600 µm

less expensive Fineness

Crystalline

during heat curing

Steel fibers

Improve ductility

5 µm to 25 µm

Moderate aspect

Straight

ratio

L: 13 - 25 mm Ø : 0.15 - 0.2 mm

Silica fume

Covering pores,

Very less

Procured

0.1 µm

improve

quantity of

from

to

rheology,

impurities

ferrosilicon

1 µm

formation of

industry

secondary hydrates

(highly refined)

Superplasticizer

Reduce w/c

Less retarding

Polyacrylate

_

Journal Pre-proof characteristic

based

Matte and Moranville [39]

Non-fibred

12 mm fibers

Fibered

Silica fume

0.25

0.23

0.25

0.23

0.325

Portland cement

1

1

1

1

1

Compacting pressure

--

--

--

--

--

Sand

1.1

1.1

1.1

Quartz powder

--

0.39

--

Superplasticizer

0.016

0.019

0.016

0.019

0.018

Water

0.15

0.17

0.17

0.19

0.2

Heat treatment

20ºC

90ºC

e20ºC

90ºC

90ºC

--

--

0.175

0.175

0.275

temperature

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1.433

0.39

0.23

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Steel fiber

1.1

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Materials

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Richard and Cheyrezy [38]

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Table 3. Previous research on RPC mixture designs [38].

An overriding factor that determines the quality of the mixture is water. This

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characteristic is the amount of water (water demand) that a certain concrete can require. For

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instance, the rate of voids of the mixture correlates with the amount of water demand and the attached air. Following the selection of a mixture design based on the minimum water demand, one can determine the optimal water using relative density (d 0 /ds). The respective relative density d0 and dS signify concrete's density and mixture's compacted density. That is the concrete without water or air. Relative density shows concrete’s packaging level with an optimal value of one. The mixture design for RPC should have an optimum packaging density. RCP’s Microstructure improvement is achieved by heat treatment. With this curing method, concrete undergoes normal heating at 90°C under typical atmospheric pressure. Heat curing catalyzes a pozzolanic reaction while transforming the microstructure of the residual hydrates produced. Other than heat curing, RCP enhancement can be attained via pre-setting pressurization. This technique specifically seeks to augment the strength capacity of the cement. When concrete attains high strength, it can also become more brittle. Therefore, steel

Journal Pre-proof fibers are integrated into the RPC to improve ductility. The type of steel fibers used is almost 13 mm long with a 0.15 mm diameter. The integration of such fibers is at the rate of 1.5 to 3% by volume, which attracts a cost efficiency of optimum dosage that corresponds with 155 kg/m3 or 2.0% volume. 2.2 Long-term performance and mechanical components of RPC RPC is composed of two types of concrete. The first is RPC 200 while the second RPC 500. While being of the same family, the two have varied mechanical composition capabilities that offer intuitive application possibilities in wide areas. Table 4 shows the two types of RPC. An integrated steel fiber is the reason for the improved flexural strength as

Table 4. Comparing RPC 200 versus RPC 800 [38].

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indicated in table 4.

RPC 800

RPC 200

Heat-treating

250 to 400°C

Pre-setting pressurization

50 MPa

None

CS (using steel)

650 to 810 MPa

None

CS (using quartz sand)

490 to 680 MPa

170 to 230 MPa

Flexural strength

45 to 141 MPa

30 to 60 MPa

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Techniques

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20 to 90°C

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Table 5 represents mechanical ingredients of RPC versus those of a normal

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conventional HPC, whose compressive strength is 80 MPa. When the RPC's fracture toughens, it results in a commensurate increase in the rate of ductility. Density or toughness refers to the amount of energy consumed per unit volume of the concrete’s fracture. Besides RPC’s outstanding mechanical characteristics, RPC’s command an ultra-dense microstructure. This microstructure composition augments its waterproofing and long-term performance properties. Such characteristics are required for isolation and nuclear waste containment services. The ultra-high durability properties of RPC are due to its tremendous low absorption rate, low permeability, high resistance to corrosive agents and reduced shrinkage. Unlike the HPC, RPC does not permit any slight infiltration of liquid such as water and/or gas across its particles.

Table 5. Comparing RPC 200 versus HPC of 80 MPa [38]. Properties

RPC 200

HPC (80 MPa)

Journal Pre-proof FS

40 MPa

7 MPa

CS

200 MPa

80 MPa

Fracture toughness

3010³ J/m²

< 10³ J/m²

Modulus of elasticity

60 GPA

40 GPA

The following properties of RPC give it advantages over chemically reactive regions. These characteristics augment its use in such environments that increase physical wear due to encounters with corrosive elements as represented in table 6.

Table 6. Comparing the durability of RPC versus HPC [38]. Declines 7 times

Abrasive wear

Declines 2.5 times

Chloride ions diffusion

Declines 25 times

Rate of corrosion

Declines 8 times

2.3 Limitations and application of RPC

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Water absorption

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To attain a normal RPC mixture design, traditional concrete’s less expensive constituents are substituted or completely removed in favor of the expensive ingredients. For

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instance, the coarse grain of traditional concrete is replaced by fine sand that serves as its equivalent in the RPC. The Portland cement will act as fine aggregates for RPC, alongside

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the silica fumes. The fine sand is adopted instead of the coarse aggregate, which makes the

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process expensive. The silica fume substitutes the cement. On the other hand, Portland cement substitutes fine aggregate. The mineral element improvement alone contributes to significant expenses compared to normal concrete, which is up to ten times that of HPC. The adoption of RPC should target regions that require considerable reductions in weight savings. It is also applicable in areas that demand complete utilization of RPC's primary properties. Due to its longterm performance properties, RPC can be used as a substitute for steel compressed elements where long term performance is an issue at stake. For example, it can serve as concrete for infrastructural marine projects [40]. Considering that RPC is in a development stage, its durability characteristics are yet to be fully ascertained.

2.4 Challenges of RPC According to Abid et al. [2], “the dense microstructure of RPC makes it more vulnerable to high temperature spalling and cracking. However, the addition of steel fibers improves the

Journal Pre-proof tensile strength, which resists the internal vapor pressure at high temperatures. This protects RPC from spalling.” Similar to most of the concretes, high temperature adversely affects RPC. Accordingly, other than Abid et al. 's study, extant research is non-significant and cannot appropriately demarcate the responsiveness of RPC regarding the hot and residual mechanical properties. Nonetheless, this experiment proves that RPC's functionality increased with the integration of steel fibers. By testing various properties of the resulting fiber-reinforced RPC, Abid et al. [2] concluded that only “2% steel fibers by volume of concrete were sufficient enough to resist spalling of RPC, as no spalling was observed in any sample." Therefore, the improvement of RPC by dispersing steel fibers will increase its strength, durability and widen its

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scope of application.

3. Fiber-reinforced concrete

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Experimental research by Rajak et al. [3] exposed many benefits that fiber-reinforced

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polymer composite provides. Besides the high strength to weight ratio, they also exhibit high stiffness, durability, resistance to reactive activities, fire, to the impact and wear. They also

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have high flexural strength and damping abilities. Such immense functionalities have encouraged the adoption of these composite materials across the automobile industries, in the

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military and marine environment and mechanical and manufacturing development sectors. They also have a wide appeal from the biomedical industry, the construction and aerospace

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sectors. Fiber-reinforced concrete and their wide applications have made these composites a

3.1 Functionality

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reliable replacement or substitute over alloys or solitary metals.

The functionality of fiber-reinforced composite materials largely depends on the manufacturing approaches and their constituent ingredients. Hence, there is a need to study and analyze functional properties of different fibers that are currently used, to recognize their categories as well as the various manufacturing approaches utilized to improve these concretes to come up with optimized properties of the fiber-reinforcement for the specified task. Panthapulakkal et al. [41] analyze different categories of fiber-reinforced concretes. The categorization of composites comprising fibers depends on the length of fibers. For instance, continuous fiber-reinforced composites category is for those composites having long fiber reinforcements whereas discontinuous fiber reinforcement composite category is for composites having short fiber reinforcements. Similarly, the hybrid fiber-reinforced

Journal Pre-proof composite category is “where two or more types of fibers are reinforced in a single matrix structure” [41]. Other than classification, the integration of fibers inside the concrete matrix also differs. For continuous fibers, they can be inserted bi-directionally or uni-directionally. Continuous placement is an effective way considering that the loading of such fibers is easy. On the other hand, discontinuous placement should consider the aspect of fiber lengths. Loading will only increase and be effective when such fibers have a considerable length, which will also contribute to reduced crack development and material failure, particularly for fragile matrices. Agarwal et al. [42] reveal that the orientation and arrangement of fibers will

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determine the residual characteristics, structural behavior and functionalities of composite material. Where their primary properties such as fatigue strength and toughness undergo improvement, they will have positive and durable performance. Material improvement is a

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technique that can enhance both the toughness and strength of these fiber-reinforced

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concretes. They can as well undergo chemical improvement using natural fibers. The study by Dixit et al. [4] and Prasad [25] utilized material improvement where glass, aramid, basalt

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and carbon fibers were inserted into the composite material comprising a fiber-reinforced polymer (FRP), which also improved the final properties of concrete. Due to their improved

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functionalities, the authors established that natural fiber polymer composites (NFPCs) can be

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used in the current construction sector.

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3.2 Nanoscale improvement of fiber concrete It is widely accepted that fiber-reinforced concrete guarantees more tensile strength and toughness. Additionally, these fibers may substitute huge cracks with a dense network of microcracks [43,44]. Nonetheless, as Alrekabi et al. [45] explain, fibers cannot effectively curb all crack initiation at the nanoscale. According to Xu et al. [46] “the fiber-matrix interfacial transition zone (ITZ) at nanoscale plays an important role in determining the mechanical performance of hybrid steel-polypropylene fiber-reinforced concrete at upper scales”. Rutigliano suggests that many scientists acknowledge that concrete has to be engineered at the nanoscale [47]. This is because such an improvement allows for the changing of the mechanical and chemical composition of concrete. Concrete materials have to demonstrate improved longevity and mechanical performance as well as to integrate properties that satisfy multiple applications to match with future emerging structural projects. Thus, the nanoscale improvement deals with the integration of nanomaterials that occur in

Journal Pre-proof crystalline and chemical composition forms. Jeevanandam et al. [5] discuss that those characterized based on electron movement and mode of shapes include the 0D nanoparticles, 1D nanofibers and 2D nanosheets.

3.3 Types of fiber-reinforced concrete There are several types of fibers dispersed into concrete mixtures, the primary ones as suggested by Labib include synthetic fiber, waste fiber and steel fiber. Examples of synthetic fibers include carbon, glass, polyvinyl, polyolefin and polypropylene [48].  High-performance fiber-reinforced cementitious composites (HPFRCCs).

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Investigative research by Fediuk et al. into the working of HPFRCCs showed that these uniquely reinforced concrete composites exhibit outstanding functionalities that can self-strengthen and flex before damage or deformation [6]. They also have

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strain hardening capabilities that reflected there is an excessive loading above its

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elastic capacity. This feature is the most attractive thing with HPFRCCs. Strainhardening increases the hardness and compressive strength of the material.

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HPFRCCs also encourage different engineering designs irrespective of the similarity of compositions of such materials. For example, Engineered

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Cementitious Composite (ECC) design emanates from the micromechanics functionality of such composites. Sometimes called “bendable concrete,” Qiu and

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Yang [49] reveal that ECC can be easily improved by molding mortar-based

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aggregates using uniquely identified short random fibers. As shown by Luo et al., appropriate fibers for ECC improvement are typically polymer fibers [50]. Compared with conventional cement whose strain capacity is 0.01%, ECC varies from 3-7%. This strain capacity improves its ductility and can function as metals other than as weak glasses, warranting its application in multiple environments.  Glass fiber-reinforced concrete. This is another form of fiber reinforced concrete comprising “high-strength, alkali-resistant glass fiber” dispersed into composite matrices [6] While existing in such a form, both the matrix and fiber will exist in their natural form. Both the physical and chemical compositions of these elements remain unchanged. This reinforcement also integrates synergistic combinations when each of these elements worked singly.  Ultra-high-performance concrete (UHPC). This is the latest fiber-reinforced composites that attract high compressive strength of concretes of up to 250 MPa.

Journal Pre-proof The research by Fediuk et al. analyzes the functionality of this special fiberreinforced concrete, which also found that its compressive strength can be more than 250 MPa [6]. Accordingly, Lim, Raman and Safiuddin investigated recent progress in concrete engineering and technology, including the UHPC [51]. They established that UHPC is one of the recent methods of concrete improvement, whose functionality and flowability can increase by adopting optimum grading of particles. As stated by Kusumawardaningsih et al. compared with conventional cement, UHPC has high compressive strength and durability [52]. They found out that the benefits of such cement characteristics have made many scholars

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manufacture UHPC with advanced engineering properties. Most of the UHPC has ultra-high strength and durability. Some come with fiber reinforcement while others do not. Further indicates that the average UHPC demonstrates a 28-day

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compressive strength with at least 150 MPa [51]. Nonetheless, the more the

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compression strength increases, the less ductile cement becomes. Hence, synthetic or steel fibers are usually blended in UHPC manufacturing. Thus, this integration

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leads to “ultra-high-performance fiber-reinforced concrete (UHPFC) with ductile behavior and enhanced mechanical properties." Most of the mechanical properties

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improve with the integration of UHPC compared with conventional concrete.  Steel fiber reinforced concrete (SFRC). Another recent improvement has led to the

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production of steel fiber reinforced concrete (SFRC), which has gained much

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traction because of the advantages that fibers provide that enhance the cement’s longevity, resistance to cracks and fatigue, force, anchorage, ductility and tensile strength [53]. Błaszczyński and Przybylska-Fałek suggest, “The use of macro fibers, such as steel fibers, as reinforcement in concrete is well-established in applications such as industrial pavements, precast structural elements, tunnel lining” [7] Using SFRC as a compound matter, the cementitious matrix is blended under a discontinuous reinforcement. Steel fiber is the main material for reinforcement that is unevenly inserted in the mixture. The study by MarcosMeson et al. [8] utilized Portland cement binders as substitutes for steel reinforcement. They placed concrete into a solution with elastic mechanical properties in specific mixture proportions. That is, in the non-cracked condition just as the traditional cement.

3.4 Challenges of fiber-reinforced concrete

Journal Pre-proof The use of steel-reinforced fibers as substitutes for normal concrete has gained traction because of their overall longevity and compressive strength [9]. Steel reinforced fiber prevents the propagation of micro-cracks thus limiting the effect of destruction stress along lines of the defect [10]. Additionally, this concrete performs better on impact loading, which makes it appropriate concrete for military projects [10]. According to Masterbuilder's report [54], fiber eventually bolsters the bonding of the cement matrix reducing the effect of destruction. Thus, steel fiber makes concrete stronger and ductile. Additionally, advanced fiber reinforced concrete has a dense microstructure that reduces water absorption, which eventually reduces the effect of corrosion. The intense

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micro- and macro-structures especially of the UHPC gives it higher bonding capacity. These characteristics also promote durability, which eventually reduces the need for stronger reinforcing aids. It is also an economic use of concrete due to the limited cross-sectional

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dimension. Since it has an overall less structural weight, UHPC has been recommended for

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high-rise structures and applications in areas prone to earthquakes. Irrespective of these benefits, many contractors are objected to the total substitution of steel reinforcement since it

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is yet to address challenges related to exposure of carbonation and chloride.

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4. Self-consolidating concrete

Self-consolidating concrete (SCC), also called the “self-leveling concrete (Super-

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workable concrete and self-consolidating concrete),” is characterized by a highly flowable,

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non-segregating concrete that does not require any mechanical vibration and can compact itself by its self-weight. This type of advanced concrete does not rely on any compaction. Conversely, it can depend on its weight to infiltrate all the needed formwork. It may rely on solutions such as viscosity modifying agents (VMA), High range water reducers (HRWR), or superplasticizers to augment workability. Whereas VMAs regulate the dangers of segregation, superplasticizers promote workability. SCC can be differentiated from conventional cement based on mixture designs. According to the study by Garcia-Taengua, SCC has "lower coarse aggregate contents, increased paste content, lower water-powder materials ratio, an increased superplasticizer dosage and the addition of VMAs when necessary” [55]. The developments in concrete technology have prompted an innovative mode of concrete, referred to as self-consolidating high-performance concrete (SCHPC). This concrete filters throughout the reinforcement material as well as in every pore of the material. Consolidation occurs under self-weight or without the need for a mechanical vibrator [56].

Journal Pre-proof According to Khan et al., this mode of cement gives outstanding filling and passing ability and demonstrates exceptional segregation stability and resistance [57]. While this is achieved in the first state, on the other hand, HPC provides high strength, limited transportability and high longevity in the hardened state. 4.1 Properties of SCC Mostafa et al. define the key characteristics of self-consolidating concrete (SCC), which include the "low yield stress, filling ability, passing ability, segregation resistant, designed rheological workability, high deformability, high durability and high strength dense micro-structure” [58]. According to Navarrete and Lopez, segregation aims at promoting

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homogeneity of the residual concrete [59]. On the other hand, the role of stability as defined

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by Garcia-Taengua is to increase the “resistance to bleeding, surface settlement after casting and separation of the mix constituents during placement or dynamic stability” [55] (Fig.1).

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SCC is among the HPC. It is also referred to as high-performance self-consolidating concrete

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(HPSCC) due to its ability to utilize various substitution elements including fly ash, silica

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fume and nano-silica.

High deformability

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High segregation resistance

Using superplasticizer

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Limiting the coarse aggregate content

Self-flowability and compatibility

Fig. 1. Characteristics of SCC [55].

4.2 Use of superplasticising admixtures Superplasticising and plasticizing solutions have had an integral role in promoting the concrete technology and advancements of the SCC. Before the 1960s, workability improving admixtures relied on lignosulphonates and hydroxycarboxylic acids. These solutions were called plasticizers or water reducers since they would decrease the w/cm ratio by up to 10% without a corresponding effect on concrete’s workability. Later, sulfonated formaldehydebased solutions were created, called, superplasticizers (SP) during the 1970s. SP would lower the w/cm ratio by up to 30%. Superplasticizers developed from sulfonated naphthalene

Journal Pre-proof formaldehyde are sub-classified as naphthalene-based superplasticizers (SNF). On the other hand, those formed from sulfonated melamine formaldehyde are sub-categorized as melamine-based superplasticizers (SMF). On the other hand, those developed from sulfonated naphthalene formaldehyde condensates are sub-classified as SNF. Polycarboxylate-ether-based superplasticizer (PCE) were eventually manufactured, which further improved the workability of cementitious materials by a double impact, which is the steric repulsion and dispersion impact contrary to what SNF and SMF could do since these enhanced the workability of cementitious mixtures only by dispersion impact [60].

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4.3 Mix proportions

The cement used in the mix was 53 grade of Ultra Tech brand with 3.15 gravity, 7 % fineness and with 38 minutes initial setting time. The coarse aggregate utilized had a

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maximum size of 12.5 mm. The physical properties of coarse aggregate and fine aggregates

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included respective 0.45% and 1.0% water absorption capacity, 2.68 and 2.61 gravity and 6.62 and 2.72 modulus fineness. The study utilized Conplast SP430 as the admixture. The

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tables 7 and 8 show this mixture proportions. The modification of the conventional concrete is based on the EFNARC standards, which regards the hardened and fresh properties of the

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mixes [3].

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Table 7. Mix proportion by weight [61].

Grade Cement RHA

SSC

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Mixture

M30 1 0.10

Coarse aggregate (CA)

1.62

Fine aggregate (FA)

2.23

Water

0.42

The specific gravity of cement (SP)

0.025

Table 8. Mix proportions in fresh state [61]. Test

Mixture

TM-3

TM-2

TM-1

FM

Slump Test

Slump (mm)

720

705

715

710

T50 Sec

3.39

3.18

3.34

3.23

Journal Pre-proof Test

Mixture

TM-3

TM-2

TM-1

FM

Vfunnel

T0 Sec

6.24

6.08

6.22

6.12

T5min Sec

8.27

8.14

8.20

7.18

H2/H1

0.93

0.89

0.92

0.90

T20 Sec

3.18

2.58

3.15

3.12

T40 sec

5.25

5.13

5.20

5.18

L-BOX

Table 9 shows the test results of concrete in the fresh stage. The level of workability of SCC needed depends on” w/c ratio, the content of cementing material, superplasticizer and

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content of coarse aggregates and fine aggregates” [61]. TM refers to the trail mixes required to create obstruction clearance, self-compatibility and flowability. FM refers to the final mix,

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which came after all the mixing and improvement. It demonstrated improved strength compared with other trails. Therefore, SCC cement can enhance both the compressive,

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splitting and flexural strengths of concrete.

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Table 9. Compressive strengths of mixes [61]. Properties

TM-3

TM-2

TM-1

FM

38.64

38.37

37.92

39.13

Compressive Strength, 7 days (MPa)

28.84

28.51

28.37

29.27

Flexural Tensile Strength, 28 days (MPa)

4.70

4.52

4.26

4.84

Splitting Tensile Strength, 28 days (MPa)

3.41

3.31

3.15

3.58

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Compressive Strength, 28 days (MPa)

4.4 Challenges of SCC

One of the problems facing the construction industry is to implement projects in harmony with the ecosystem by integrating the concept of sustainable development, which requires the implementation of high performance and ecologically friendly materials manufactured at affordable prices. Since concrete is the predominant building material, it is necessary to identify cheaper cement substitutes. Extant studies on many complementary concrete materials especially HPSCC, which include silica fume, PBSand fly ash among others, have proved that both the fresh and hardened properties of cement eventually decline accompanied by a reduction in costs, as the study by Ogiri proves. The experiment by Qasrawi further demonstrates that it is possible to manufacture SCC utilizing steel slag aggregates, which contribute to more sustainable and durable green solutions [62].

Journal Pre-proof As explained by Ng et al. [60], the inclusion of micro silica (MS), shale ash (SA) and fly ash (FA) limits carbon dioxide emission as well as energy consumption. These substances can be utilized as supplementary binder materials, which eventually achieve low watercementitious materials (w/cm) ratios and high compressive strength of SCC admixtures. Polycarboxylate-ether-based superplasticizer (SP) positively correlates with reduced watercement ratio (w/c), content. Moreover, the study recommends that rationalizing the SP dosage considering the specific surface area of cementitious materials as opposed to traditional methods of dosing the SP using the mass content of cementitious materials.

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5. Green cement and concrete

The cement industry is confronted with many setbacks that include declining raw materials, the ever-growing demand for concretes, an attenuated economy, the depletion of

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fossil fuel reservesand threats from the ecosystem. The internal pressure is about attaining

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sustainable and greener cement techniques. For instance, each ton of ordinary Portland cement (OPC) creates a commensurate produced quantity of CO 2 , thus, the focus is replacing

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OPC with low carbon substitutes [63]. Hence, composites produced using “locally available minerals and industrial wastes that can be blended with OPC as a substitute, or full

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replacement with novel clinkers to reduce the energy requirements is strongly desirable” [64]. Limiting energy required and carbon pollutions during concrete production can be attained

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using alternative cement but first by analyzing characteristics of the binder with modern technologies.

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This pressure for utilization of sustainable “green” solutions also emanates from the desire for modern complex designs and paradigms in high-performance civil infrastructure. Such projects require materials with outstanding longevity, mechanical performance, affordability, resilienceand constructability [65]. Additionally, the affordability of fly ash and slag has made cement solutions more accessible [66]. Nonetheless, this is majorly for economic purposes but not for ecological ambitions. To help contractors achieve greener cement solutions, an approach was developed that translates all ecological expenses in a single gauge called the MKI - Milieukosten tool [67].

5.1 Mix proportions To determine the role and importance of green concrete, tests were evaluated at 3, 7, 14, 28, 60 and 90 days. The researchers used 228 samples for testing “60 cube specimen for compression, 60 cylinder specimens for compression, 60 cylinder specimens for split tensile,

Journal Pre-proof 48 specimens for flexural strength” [61]. The objective of this experiment was to investigate the properties of concrete using quarry dust as a complete substitution of sand. Workability tests were enhanced by integrating superplasticizer. The mix proportions are as shown in the table below. “The CC = Control concrete without fly ash Quarry dust concrete (QCC) without fly ash QCFA1 = Quarry dust concrete with fly ash 10% QCFA2 =Quarry dust concrete with fly ash 15% QCFA3 =Quarry dust concrete with fly ash 20% [61].

CC

QCC

Cement (kg) QCFA1

QCFA2

Superplasticizer (S.P) %

0

2.0

2.0

2.0

2.0

Fly ash (F.A)%

0

0

10

15

20

Fly ash (kg/m3 )

0

0

53

80

106

Cement (kg/m3 )

530

530

477

450

424

Quarry dust (kg/m3 )

0

725

725

725

725

Sand (kg/m3 )

740.25

0

0

0

0

water(lite/m )

212

212

212

212

212

Coarse aggregate (kg/m3 )

901.53

901.53

901.53

901.53

901.53

Superplasticizer (lite/em3 )

658

10.6

10.6

10.6

10.6

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3

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Mixes

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Table 10. Mix ratio of concrete [61].

Samples were tested to evaluate their strengths including tensile, flexural and

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compressive strengths. Results show that the strengths of quarry dust improved, they were equal and even more than the conventional concrete when incorporated with fly ash. Therefore, a greener solution of quarry rock dust can serve as an equal or better replacement of sand.

5.2 Trends towards greener solutions Based on the Paris Agreement on climate change, many countries are focused on achieving zero carbon. By 2030, carbon emission needs to decline by 16%. The assumptions about carbon capture and storage (CCS) technologies can only be realized when steeper reductions are implemented [62]. At the same time, investors now expected firms to report their exposure to climatic risks.  Reduction in fossil fuels. Many countries are likely going to move away from the use of fossil fuels in concrete manufacturing. India and China, to be specific, have

Journal Pre-proof a huge opportunity to embrace sustainable lower-carbon fuels. European concrete companies have demonstrated more than 90% reliance on non-fossil fuels. The bottleneck is to ensure that volumes of biomass are accessible and manufactured from sustainable alternatives. Presently, the industry significantly depends on waste-derived biomass, albeit, adopting alternative fuels could later prompt the industry to embrace wood pellets.  Clinker substitution. This is another alternative for greener concrete solutions. This process deals with the substitution of the clinker content in concrete with other sustainable materials. Clinker substitution would enact a larger functionality as

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what is envisaged. Based on the 2018 Technology Roadmap, attaining 0.60 global clinkers by 2050 can considerably mitigate a range of 0.2 gigatonnes (GT) of carbon dioxide by this time. The content of clinker should decrease more for

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individual applications for further reductions of 70-90% of carbon dioxide

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emissions. On the internal scale, such could depict 1.5 GT of carbon dioxide saved

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by period.

This approach is not only effective but also a cheaper alternative. There is no need to

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substitute fuel sources or to invest dearly for new tools. Therefore, clinkers should be

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substituted alongside other more sustainable alternatives such as carbon-negative and novel solutions. The leading bottleneck is an absence of clinker alternatives and the consumer

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demand for the low-clinker concretes. Whereas many researchers express criticism over the possibility of an increased in CCS, the figure below paints a possibility of CCS becoming the next sustainable option based on current modeling exercises. Still, carbon-capturing technology remains a viable option for enhancing the Beyond 2°C Scenario (B2DS). Therefore, CCS could accompany other alternatives such as novel and carbon-capturing techniques. A major problem facing CCS is the cost of technology compared with the cost of other levers. Increased emission from concrete products emanates from high consumer demand. This demand can be regulated by taking a new approach to design by utilizing higher-quality concretes. Alternatively, the efficiency of the use of concrete in construction places can be enhanced. Additionally, concrete can be replaced with other materials. Finally, the amount of recycled and reused concrete can be increased (Fig. 2). Utilizing a range of such demand-side techniques in strategic growth sites including Africa, India and China will significantly

Journal Pre-proof reduce carbon emission in favor of greener alternatives. Nonetheless, these measures depend on multiple factors and motivations other than just the cement industry. 2450

MtCO2 /year

2400 2350

Thermal energy efficiency

2300 Reduction of clinker to cement ratio Fuel switching

2250

2150

2014

2020

2025

2030

2035

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2200

2040

Year

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5.3 Challenges of green cement and concrete

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Fig. 2. Low-carbon transition in the cement industry [68].

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Demand for green concrete is increasing although encountered with several hindrances. There are weak carbon policies in most of the countries. Reduction in emissions for all infrastructural projects can be realized when there will be a swift scale-up in the use of

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novel cement alongside the integration of positive policies. Some projects can achieve more

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than 90 net-zero emissions. Some can isolate emitted carbon emissions by sequestering the emitted carbon and transforming it into carbon negative. As Lehne and Preston [69] put it,

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currently, most of these sustainability products have not attained commercial viability. Advances into this field call for coordinated initiatives for large-scale demonstration and research and development. Consumers should also be trained to construct using novel products.

6. Nano cement and concrete The use of nanomaterials for enhancement of the longevity and mechanical components of cement-based materials (CMBS) has gained traction in the recent past due to their ability to improve the durability and mechanical properties of cement composites [70]. A popular perspective is that the cement’s ability will improve by admixing nanomaterials in CBMS using two common approaches. The first technique involves the application of fillers while the second uses cement hydration [71]. The influence of nanotitanium’s photocatalytic can also enact a similar influence. This approach covers the CBMS’s surface with self-

Journal Pre-proof cleaning capability when exposed to ultraviolet waves. In as much as nanomaterials may increase defects in some CBMS’s properties, for example, limiting workability because of the more water required by ultrafine granules, their gains on the technical composition of CBMS has been evident. Frequently utilized nanomaterials include “nanoscale spherical particles (nano-SiO 2 , TiO 2 , Al2 O3 , Fe2 O3 , etc.), nanotubes and fibers (carbon nanotubes and carbon nanofibers) and nanoplatelets (nanoclays, graphene and graphite oxide)” [72]. Extant studies reveal that there is a high demand for nanotechnology applications in the concrete paste, cement and mortar. They are also utilized in the newly manufactured nanomaterials and nanomaterial- modified materials. There are two basic incidences

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approached for the integration of nanotechnology in the CBMS. The first direction involves

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creating new products “engineered at the nanometer scale for the concrete industry.” The second direction involves “characterizing and understanding the materials at the nano- (and

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sometimes micro-) scale through the use of atomic modeling and advanced characterization

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techniques” [73].

Another form of nanomaterials is the smart concretes such as graphene that limits the

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discharge of carbon dioxide by half and the same time creates sustainable construction resources for roads, houses, bridges, dams and other structures [74, 75, 76]. Smart materials

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can also be used to manufacture different types of sensors, which is linked to signal transmission and diagnostic monitoring. The role of such sensors is to monitor the strain and

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stress of the health of structures. They can detect damages, cracks, or even determine early

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age curing behaviors. Therefore, sensing is an integral component of smart cementitious materials. Carbon nanofibers and nanotubes can be used as reinforcement materials. Residual cement made from such carbon elements exhibits high compressive strength with improved mechanical properties such as the chemical and electronic constituents. Additionally, concrete based elements with self-sensing or piezoelectric sensors can provide actual tracking of vehicles that appropriate for smart traffic control. Most of these elements tracking elements are composed of multi-walled carbon nanotubes. Another application of smart cement comes with the integration of iron oxide nanoparticles. The resulting aggregate has improved piezo-resistive properties that equally contribute to actual tracking. At the same time, it improved the modulus of elasticity and strength for oil well structures. The benefit with tracking and real-time data collecting using smart objects, particularly in large-scale applications, will allow and promote present “big data” approaches such as multi-resolution and sampling.

Journal Pre-proof Recently, nanotechnology has proved to be a salient solution concerning ecological improvements as it reduces dependence on non-renewable energy and contributes to significant energy savings. This technology enhances cement performance using nanomaterials. Some of the nanomaterials come in the form of nanofibers, nano-chemical additives and nano-particles embedded in concrete aggregates. Nanomaterials concrete has a lower volume capacity, which is a positive performance characteristic. Types of nanomaterials used for improvement include carbon nanotubes (CNTs), nano-alumina (nano Al2 O 3 ), graphene and graphene oxide, nano-silica (nano-SiO 2 ), nano-titanium oxide (nano TiO2), nano-ferric oxide (nano Fe2 O3 ), nano clays. The essence of each of these methods is to

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promote 'Green Concrete' that enhances the sustainability of concrete.

6.1 Nano-Silica

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These are examples of nanomaterial composites utilized in engineering applications that

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can substitute micro-silica and silica fume. These elements can react with lime during the cement hydration process creating a C-S-H gel that augments the longevity and mechanical

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strength durability of concrete. When integrated into cement composites, they improve the cement paste hydration process creating a stronger microstructure. Conversely, they should

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be used minimally to avoid agglomeration considering that they have high surface energy. The high surface energy creates a dispersion that is not even. The figure below displays

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several materials employed in concrete fabrication [77].

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Journal Pre-proof

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Fig. 3. Nanomaterials used in the concrete formation [77]. Nano-silica can be used in a variety of contexts for the improvement of cement. For example, Flores-Vivian et al. [77] employed nano-silica materials to improve the Portland

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cement. The objective was to transform the rheological characteristics, promote strength and

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enhance longevity. A 0.25% weight nano-silica content was used alongside the Portland cement. Similarly, other researchers indicated the use of “stabilized nano-silica particles

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(between 3 and 200 nm in size) in Brazilian-type CP V ARI PLUS Portland cement" [78]. These experts fabricated three types of concrete mixes, "a reference concrete, a concrete added with stabilized nano-silica and concrete including stabilized nano-silica with silica fume" [56]. The tested results 3, 7 and 28 curing days. The nano-silica improved cement had an augmented and more compressive strength by 27%, 20% and 11% across the curing days compared with the control Portland concrete. When nano-silica was modified with silica fume, the compressive strength increased further by 28%, 37% and 24% across the curing days compared with the control Portland concrete. Therefore, cement mixed with nano-silica integrated with silica fume performed better concerning the compressive strength.

6.2 Carbon nanotubes As another type of nonmaterial cement, Carbon nanotubes (CNT) enhance the engineering properties of construction materials alongside providing the benefits of

Journal Pre-proof ecological sustainability [76]. The carbon nanotubes are microscopic particles combined with a strong bond. Their size range from “diameter 20-40nm, length 1-10 m, density 0.15-0.35 g/cm3, surface area 35 m2 /g with carbon purity above 90% and it is typically 100 times stronger than the steel so adding carbon nanotube in any concrete mix can reduce or eliminate the usage of the steel reinforcement” [79]. By utilizing carbon nanotubes as the use of reinforcement materials, they transfer the macroscopic properties to the nanoscopic level as they serve as fillers generating denser concrete. They can impede the formation of cracks at an early age, increase durability and enhance the bond interaction quality between matrix and cement paste. They have intuitive thermal and electronic properties as well as good elastic

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behaviors with high strength and ultra-high aspect ratio. These characteristics give them advantages as reinforcement materials.

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An experiment to describe the integration of Carbon nanotubes (CNT) on cement was done by Lushnikova and Zaoui [80]. The study examined the impact of various types of

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CNTs integrated into cement specimens. The researchers employed molecular dynamics simulations to establish the effect of such carbon nanotubes o the mechanical characteristics

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of C-S-H. Such key characteristics evaluated comprised the shear modulus, bulk modulus, elastic constants and Poisson ratio. Findings showed an enhancement of all the evaluated

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mechanical components. Therefore, the CNTs are nanomaterials that can improve the mechanical properties of concrete.

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Similarly, the study by Sedaghatdoost and Behfarnia [81] investigated the effect of

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CNTs on the mechanical constituents of the Portland cement through the integration of multiwalled CNTs (MWCNTs) at the range of 0 and 0.15% by weight of original samples. They heat the samples at temperatures of 200-800 °C. The integrated 0.1% weight of MWCNT enhanced the flexural, tensile and compressive strength by 11.2%, 8% and 35% correspondingly. Additionally, the cement paste gained stability with a denser form because of the presence of MWCNT.

6.3 Other forms of nanomaterials Nanomaterials can also be integrated into the cement materials through the following forms. Graphene-Based Nanomaterials are another form that includes the "graphene family nanomaterials (GFN) such as graphene, graphene oxide (GO), reduced graphene oxide (rGO) and graphene nanosheets (GNS)" [82,83,84,85]. These elements have outstanding thermal, chemical and mechanical characteristics. Hence, GFN can augment the durability and

Journal Pre-proof structural strength of cement since they also comprise self-sensing and self-cleaning capabilities. Additionally, Nano-Alumina (Al2 O3 ) is another form of nanomaterials element that can quicken the formation process of C-S-H gel. It is known to improve the compressive strength and durability of cement materials [86]. That said, Nano-Titanium Oxide (NanoTiO2 ) is known to contain self-cleaning properties that “can allow a photocatalytic degradation of pollutants (e.g., VOCs, CO, NOx, aldehydes and chlorophenols) from industrial and automobile emissions." Although, such an impact could reduce because of the

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carbonation effect [87,88].

6.4 Benefits of nanomaterials on cement

Experiments prove that the integration of nano-silica elements reduces the

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permeability effect of cement. For instance, the research by Nair and Rahim established that

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“the overall performance of concrete with or without fly ash was significantly improved with the addition of variable dosage of nano-silica" [89]. The mechanical properties of these

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elements also improved with the addition of other elements of nanomaterials such as nano aluminum. On the other hand, the concrete self-compaction and pore structure improved with

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the addition of nano-titanium oxide. Most of these elements will increase the porosity, enhance the bonding of aggregate and cement matrix and increase the density of cement.

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Carbon nanotubes can reduce the formation and development of micro-cracks in concrete.

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They also contain high mechanical properties with considerable aspect ratios (length to diameter ratio) that create stronger cement composites [90]. These materials have strainsensing characteristics that can assess their electrical functionalities under applied loads [91]. Such characteristics enhance the strain-sensing capabilities of concrete structures, which make them suitable for structural health monitoring and recognition of damage.

6.5 Challenges of nano cement and concrete A major setback for the use of nanomaterials in CBMS is the poor dispersion capacity. The level of dispersion determines concrete's effectiveness, composite's properties and admixture nanomaterials functionality. A method that can determine the dispersion level of nanomaterials in concrete aggregates is yet to be established. Instead, studies rely on the mechanical properties of CBMS, which is an indirect approach. They evaluate whether such nanomaterials form agglomeration in cementitious materials. This is because nanomaterials with poor dispersion agglomerate, creating clusters since they encounter strong van der

Journal Pre-proof Waals force. The formed clusters may degrade the mechanical characteristics of concrete composites [92,93,94,95]. For example, the hydrophobic type of carbon nanotube (CNT) cannot evenly disperse in concrete materials [96]. The research by Rocha and Ludvig revealed a perfect way that can reduce aggregation levels of nanomaterials in the CNT. The appropriate dosage for integrated CNT that can reduce the aggregation is 0.05 wt %. Using this kind of dosage, the dispersion of CNT is not based on surface treatment "but dispersed in a non-aqueous isopropanol media with the aid of ultrasonication for 2 h." As recommended by Rocha and Ludvig, two methods are widely used to improve its dispersion and reduce aggregation. The first is noncovalent while the

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second is the covalent approach [97]. The covalent functionalization deals with “inducing chemical functional groups on the sidewalls of CNT, thereby improving the adherence of CNT to the cement matrix.” Such an approach will eventually improve CNT’s dispersion.

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Nonetheless, Shao et al. are cynical about this approach, who state that CNT, hence

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negatively influencing the performance of such cementitious materials. Thus, a new noncovalent functionalization approach that improves CNT’s surface with other

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nanomaterials or surfactant is encouraged [97].

A possible example of a surfactant is the superplasticizer. Meng et al.'s study indicate,

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"that the polycarboxylate ether (PCE) superplasticizer can improve the dispersibility of CNT and reduce its aggregative tendency in the water” [94]. Their research utilized the

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cyclodextrin- modified polycarboxylate superplasticizer (PACD). In their study, one specific superplasticizer, “cyclodextrin-modified polycarboxylate superplasticizer (PACD),” was used

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as a surfactant to disperse multiwalled CNT (MWCNT). The figure below shows MWCNT’s dispersion mechanism using PACD. It demonstrates a double impact of steric impediment and electrostatic repulsion that both prevent CNTs from moving closer to each other. Other authors employed a sol-gel technique, which eventually improved the dispersion stability of nano-silica CNT.

7. Repair cement and concrete Structural components including slums, beams and slabs should be under repair and preservation at moments when exposed to deformation. Engineering professionals use various repair materials (RMs) based on factors such as the extent and type of crack, existing reinforcement and the cost of the repairing material. Several steps are involved with the repair and preservation of such materials. The first step deals with the identification of the problem. For example, the cause of the problem and the renovation need. Identification of

Journal Pre-proof such issues will go way ahead in informing the best approach for repair. The second step involves the identification of concrete, existing reinforcement that is damaged such that it must be eradicated. If the damage exceeds 50% of the original structure, it should be abolished. The third step deals with the removal of deteriorated cement without affecting the existing reinforcement and cement. The fourth step deals with the preparation of the cement's surface such that bonding cannot be affected. Modern repair materials integrate fibers into concrete aggregates to avoid obstacles weak concrete and low tensile strength. There are various types of fibers utilized, among them is steel fiber which is commonly adopted. For instance, steel fiber-reinforced concrete

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(SFRC) is an appropriate solution in the renovation of fire-damaged RC concretes [98]. Modern progress in SFRC has prompted the creation of Ultra High Performance FiberReinforced Concrete (UHPFRC), which is "a cementitious material with higher compressive

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strength (> 150 MPa), tensile strength (> 10 MPa) and ductility than NC [30]. The production

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of UHPFRC cement involves the integration of fine particles, steel fibers and admixtures into the concrete mixture with a low water-cement ratio [76]. Currently, Carbon Fiber Reinforced

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Polymer (CFRP) and Glass Fiber Reinforced Polymer (GFRP) are the common materials utilized for the renovation of deformed RC structures. CFRP “is an advanced non-metallic

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composite material made of a polymer resin reinforced with carbon fibers. It has many superior performances, such as high strength, lightweight, no corrosion and high fatigue

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resistance” [99]. GFRP’s, on the other hand, is a modern method of impropving the flexural

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strengths of existing reinforced concrete element. They have a high resistance to chemical, salt water, acidic rain and the environment. GFRP has high strength, it is lightweight, durable and with the capacity to mold any shape [100]. The benefits of such materials that include “ease of application, high strength to weight ratio, excellent mechanical strength and resistance to corrosion and chemical attack” makes them relevent for use in repair cement and concrete [98]. Such materials lack a stronger density, which is an integral element since little weight is incorporated into the renovated structure and can be expediently utilized without the need for larger machinery. GFRP and CFRP exhibit higher tensile strength that is up to eight times higher compared with steel reinforcements in RC (Table 11).

Table 11. Comparing CFRP and GFRP concrete properties [98]. Material

Steel

Concrete

GFRP

CFRP

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500

3-6

1,518-2,300

Tensile elastic modulus (GPa)

≈180

≈25

76

165- 238

Density (kg/m3 )

7,850

2,240 -2,400

≈1,800

≈1,500

The type of affected RC member determines the choice of technique for repair and renovation. For instance, flexural members including beams and slabs may involve repairs that are either placed at the tensile section or the soffit. Some experts have tried to enact such repairs at the top surface but had minimal benefits [101,102].

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7.1 Polymer-modified concrete (PMC)

Also called the latex-modified concrete (LMC), this is the common repair material for

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concrete. Experts use it to repair concrete substrates or for making overlays to resurface. This technique resembles the conventional PCC considering that the addition of polymer modifier

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correlates to other admixtures to the mixture, which is mortar or PCC. The application of PMC works with immersion in water whereby the polymer coalesces with the hydrated

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cement forming “a comatrix of hydrated cement and polymer film throughout the mixture.’ The generated film eliminates the loss of water keeping it ready for the hydration reducing

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7.2 Mixture proportions

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the need for length curing periods.

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To make repairs for concretes, this mixing ratio uses cementitious coatings, polymermodified mortars concrete deck overlays and polymer- modified cementitious coatings. Mixture ratio resembles the conventional PCC, with the exception that polymer levels of 1025% by mass of the Portland cement are used; a typical value is 15%. Higher levels usually do not provide improved properties commensurate with the increased cost, since the polymer is the most expensive component of the mix. As stated, “for concrete, 415 kg/m3 Portland cement, fine-to-coarse aggregate ratios of 55:45-65:35, w/cm of 0.25-0.40 are recommended” [102]. The recommended mass for mortar as follows. The “cement, 100; fine aggregate, 150450; polymer powder or latex non-volatile content, 10-20; anti-foam agent, 0.02-0.10; and total water, 25-40. For coatings, the following proportions are recommended in parts by mass: cement, 100; polymer powder or latex non-volatile content, 10-20; anti-foaming agent, 0.02-0.10; and total water, 25-65” [102]. There is also a need to use the antifoaming agents since they regulate the air content of PMC. The only exception is when the polymer comes

Journal Pre-proof with a reliable antifoaming agent. Examples of such antifoaming agents include blended cement, calcium aluminates and Types I, II and III Portland cement. The common examples of PMC include the Styrene-butadiene (S-B) copolymers, Acrylic ester homopolymers (PAE) and copolymers with styrene (S-A), Vinyl acetate copolymers (VAC) and Redispersible powders.

7.3 Challenges of repair cement and concrete High-performance fiber-reinforced cementitious composites (HPFRCCs) is one of the best repair materials because it has outstanding functionalities such as longevity, mechanical

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performance and ease of application. Such properties make it the best alternative for the successful renovation of fire-damaged RC structures. RM manufactured from OPC provides the best strength compared with those of substrate concretes. Therefore, they increase

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compatibility with the original material.

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8. Conclusions and guidelines for development of modern sustainable concrete

8.1 Conclusions

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This study provides a comprehensive review of the development of modern concrete solutions. The development of modern concrete has undergone a long route. Starting with

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Reactive Powder Concrete (RPC) towards the nano-scaled concrete and green solutions,

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research on improvement focuses on key ingredients of each of these elements including the durability, availability of the raw resources, economic feasibility and ecological friendliness. Due to the demand for increased sustainability, ecological friendliness and functional reliability of nanomaterials, this study emphasizes on nanomaterial cement and the examples considering the high demand for them. Modern concrete applications need higher durability and mechanical strength. The best way to accomplish this is by integrating nanomaterials to cement-based materials. Recently, nanotechnology has proved to be a salient solution concerning ecological improvements as it reduces dependence on non-renewable energy and contributes to significant energy savings. This technology enhances cement performance using nanomaterials. Some of the nanomaterials come in the form of nanofibers, nanochemical additives and nano-particles embedded in concrete aggregates. Nanomaterials concrete has a lower volume capacity, which is a positive performance characteristic. Types of nanomaterials used for improvement include carbon nanotubes (CNTs), nano-alumina (nano Al2 O3 ), graphene and graphene oxide, nano-silica (nano-SiO2 ), nano-titanium oxide

Journal Pre-proof (nano TiO 2 ), nano-ferric oxide (nano Fe2 O3 ), nano clays. The essence of each of these methods is to promote ‘Green Concrete' that enhances the sustainability of concrete. The following is a summary of key improvement processes.  Reactive Powder Concrete (RPC). According to Abid et al. [2], “the dense microstructure of RPC makes it more vulnerable to high temperature spalling and cracking. However, the addition of steel fibers improves the tensile strength, which resists the internal vapor pressure at high temperatures. This protects RPC from spalling.”  Self-consolidating concrete (SCC). The development of self-consolidating cement

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was based on the invention that both the fresh and hardened properties of cement eventually decline accompanied by a reduction in costs, as the study by Ogiri

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proves. The construction industry has been faced with the problem of implementing projects in harmony with the ecosystem by integrating the concept of

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sustainable development, which requires the implementation of high performance and ecologically friendly materials manufactured at affordable prices. Thus, such a

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remarkable finding meant that a modern form of cement, the Self-consolidating concrete (SCC), would be developed characterized by a highly flowable, non-

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segregating concrete that does not require any mechanical vibration and can compact itself by its self-weight. This type of advanced concrete does not rely on

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any compaction. Conversely, it can depend on its weight to infiltrate all the needed

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formwork. It may rely on solutions such as viscosity modifying agents (VMA), High range water reducers (HRWR), or superplasticizers to augment workability. The advantage with this improvement as explained by Ng [60] is that the inclusion of micro silica (MS), shale ash (SA) and fly ash (FA) limits carbon dioxide emission as well as energy consumption. These substances can be utilized as supplementary binder materials, which eventually achieve low water-cementitious materials (W/CM) ratios and high compressive strength of SCC admixtures.  Green cement and concrete. Another mode or improving cement is through the replacement of ordinary Portland cement with carbon alternatives. This pressure for utilization of sustainable “green” solutions also emanates from the desire for modern complex designs and paradigms in high-performance civil infrastructure. Such projects require materials with outstanding longevity, mechanical performance, affordability, resilience and constructability [65]. This approach is

Journal Pre-proof not only effective but also a cheaper alternative. There is no need to substitute fuel sources or to invest dearly for new tools. Therefore, clinkers should be substituted alongside other more sustainable alternatives such as carbon-negative and novel solutions. The leading bottleneck is an absence of clinker alternatives and the consumer demand for the low-clinker concretes.  Repair cement and concrete. The benefits of such materials include “ease of application, high strength to weight ratio, excellent mechanical strength and resistance to corrosion and chemical attack” [98]. Structural components including slums, beams and slabs should under repair and preservation at moments when

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exposed to deformation. Engineering professionals use various repair materials (RMs) based on factors such as the extent and type of crack, existing reinforcement

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and the cost of the repairing material.

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8.2 Guidelines for development of modern sustainable concrete

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Reduction in emissions for all infrastructural projects can be realized when there will be a swift scale-up in the use of novel cement alongside the integration of positive policies. Approaches such as RPC, steel fiber reinforcement, green cement and advanced nano cement



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construction.

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and self-consolidating contributes to environmental sustainability and reduced costs of

Steel fiber integration. Another guideline for the development of modern

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sustainable concrete is the addition of fiber-reinforced polymer composite. Besides the high strength to weight ratio, such a procedure promotes cement with high stiffness, durability, resistance to reactive activities, fire, to the impact and wear. They also have high flexural strength and damping abilities. 

Integration of mineral and chemical admixtures with specific properties. Other than relying on steel fibers, water, aggregates, or cement mixtures for improvement, a recent form of improvement requires the integration of chemical admixtures and other mineral elements with specific properties such as the case of RPC cement. HPC’s improvement emanates from the materialization of a recent scientific concrete, mode of admixtures and hightech scientific tools that evaluate the microstructure of concrete. Such an improvement technique enables HPC to achieve an optimum compressive strength of its present microstructure.

Journal Pre-proof 

Using carbon alternatives. Another mode or improving cement is through the replacement of ordinary Portland cement with carbon alternatives.

Cement improved using these approaches has multiple advantages over the concrete cement. For instance, steel-reinforced fiber prevents the propagation of micro-cracks thus limiting the effect of destruction stress along lines of the defect. The fiber eventually bolsters the bonding of the cement matrix reducing the effect of destruction. Thus, steel fiber makes concrete stronger and ductile. Furthermore, steel-reinforced concrete eases the speed of construction cutting related expenses and saving time. It also simplifies the construction

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process considering that such concrete has simpler joints that are less prone to fabric

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positioning.

Nano-scaled concretes present benefits of high adaptability and longevity. Such a

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concrete avails a new opportunity for unique prospects and regulation of structural design, functionality, shape, composition, functionality and embedment of elements. For example, it

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incorporates additives including nanomaterials, sensors and fibers, which are indispensable

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for durable and flexible concrete options. Nano-materials are those cementitious components derived from nanotechnology. Nanomaterials are known to have high tensile strength, fire resistance and self-cleaning capacities. When other elements are integrated into

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nanomaterials, they increase the cement durability, reduce the force of wear and tear, increase

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permeability and make them lighter. These elements have a tremendous contribution to roofing and facade solutions. As the world moves towards environment conservation, nano-

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insulating materials provide the much-needed sustainability to the ecosystem. Other than allowing architects to model various shapes and designs from the printed concrete, nanoscaled concrete also closes the gap between conventional cement and masonry placement techniques. It abolishes the need for traditional formwork and molding processes. Therefore, designers and engineers have the freedom to construct structures without depending on formwork.

Declaration of Competing Interest The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

Journal Pre-proof This research has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement no. 777823 and the Faculty of Industrial Technology at the PhranakhonRajabhat University. The contributions of the PNRU laboratory staff are greatly appreciated.

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