Lately, sustainability in construction has been an alarming and growing matter globally and there’s an urgent need to address it. Finding strategies and techniques to reduce energy use, environmental pollution, and building costs has recently become crucial [1]. The development of sustainable buildings has led the path to reduce the deterioration of energy and raw materials, which is crucial in protecting the environment. This invention has progressed and encouraged eco-friendly structure design and practices [2]. Since concrete is now the most frequently used material in the building sector [3, 4], it must be improved to support contemporary, sustainable, and environmental friendly properties [5]. Opaque concrete may be transformed into transparent concrete by fusing optical fibers with a concrete matrix. There are many different types of concrete, which are identified based on the composition, additives, surfaces, manufacturing technique, or type of reinforcing [6-10]. According to the type of concrete being used, some building components must be built or maintained differently. The concrete types can be differentiated according to several criteria: unit weight, compressive strength, application, and special properties. The concrete types are normal concrete, lightweight concrete, aerated concrete, heavy concrete, ultra-high-performance concrete (UHPC), water-impermeable concrete, waterproof concrete, self-compacting concrete, translucent concrete, refractory concrete, recycled concrete, concrete screed, prestressed concrete, textile concrete, fiber concrete, self-cleaning concrete, exposed (fair-faced) concrete, tamped or compressed concrete, amped or compressed concrete, sprayed concrete, vacuum concrete [10]. But light-transmitting concrete (LTC), also known as translucent or transparent concrete (TC), has captured the interest of global academics as well as concrete engineers owing to its relevance, practicality, and special qualities. Concrete is opaque because of its bulk and opaque particles and reinforcements, which prevent light from passing through. However, by incorporating optical fibers with a concrete matrix, opaque concrete might be changed into transparent concrete.

Embedded optical fibers, polymer resins, used glasses, or plastic pipes allow light to pass through translucent concrete (TC), a novel energy-efficient construction material, into the indoor space [11]. Translucent concrete is defined as concrete with the ability to transmit light from one end of its face to the other. Study [12] stresses the significance of innovative construction materials that can reduce the dependence on harnessing energy for artificial lighting. Their research attempts to ensure the construction of transparent concrete utilizing components that are easily accessible while making sure the required qualities aren't compromised. Among the numerous materials usually applied in non-structural walls of buildings, this material could also be applied. There are a number of benefits of translucent concrete, such as the provision of daylighting for buildings, good aesthetics, and the fact that it encourages the construction of green and sustainable buildings.

Fig. (1). LiTraCon block [16].

Transparent concrete, often known as translucent concrete, is a Canadian invention that was first introduced in 1935 and later enhanced by the addition of optical fiberglass [13]. In 1965, James N. Lowe of Britain created concrete wall panels that were installed on church walls and, by inserting stained glass pieces into the concrete mix, light penetrated the building [14]. By combining a significant amount of optical fiber with concrete, Aron Losonzi developed a new type of concrete known as LiTraCon (see Fig. 1) for LiTraCon block), which was introduced as a patent in 2001. The composite was described as produced as a rigid and transparent concrete suitable for floors, sidewalks, and load-bearing walls [15], and the prototype of the translucent concrete panel was successfully developed in 2003 [2].

Since TC is a brand-new, environmentally-friendly building material, there is not much prior experimental research. The strength and light transmittance of plastic optical fibers (POF) with TC in them and having diameters of 0.3 mm, 0.5 mm, 0.75 mm, and 1.5 mm were examined by Altlomate et al. [17]. The findings demonstrated that the presence of POF had a varied impact on compressive strength (f_c). The direct ultrasonic pulse velocity (UPV) test result revealed that, despite the inclusion of POF, the quality of the TC was outstanding. The f_c and flexural strength (f_f) of cement-based polymethyl methacrylate (PMMA) fibers were investigated by Li et al. [18]. When the amount of fiber rose, both f_c and f_f of TC dropped. Additionally, none of the TC specimens had the same strength as the reference concrete; hence, adding PMMA fibers resulted in a decrease in f_c and f_f, notwithstanding the fibers' volume fraction. According to Li et al. [19], under different curing conditions, the f_c of sulfoaluminate cement-based TC linearly dropped as the number of optical fibers rose. Salih et al.'s [20] investigation of TC reinforced with POF and self-compacting mortar (SCM) was conducted. The f_c and f_f of TC were often reduced by the addition of POF. The strength characteristics of TC, however, appeared to be shifting in response to the fluctuation in diameter and volume of POF. In comparison to 1mm and 2mm diameter PMMA fibers, it was discovered that 2mm diameter PMMA fibers generated TC with a greater f_f and f_c. The evaluation of transparent concrete walls made using a random arrangement of optical fibers for application in Brazil's precast building was another study carried out by Tutikian and Marquetto [21]. The experimental findings showed that as the percentage volume of optical fibers grew, the f_c and tensile strengths (f_c) in the bending of TC dropped.

Research has confirmed that LTC is not only able to transmit light but also to reduce light energy utilization by up to 50% [22] without compromising its compressive strength. Previous investigations of the attributes of LTC found a variation in the translucent materials employed to manufacture LTC, which had a direct impact on the LTC’s characteristics. Moreover, diverse emphases were placed on the assessment and parameters of LTC in previous studies based on the type of LTC and its applications.

LTC could be produced using a large number of translucent materials. However, optical fibers (OFs) made of waste glass and polymer resin are typically used. To produce LTC, materials with high translucency or high light transmittance can be used. Nearly no light is lost by optical fibers because the transmission of light through them is so bright and effective [15, 23-25]. The development of LTC is a great way to transmit light inside structures.

Fig. (2). (a) Structure of optical fiber [26, 27]; (b) Fiber dispersion and modal dispersion [26, 28, 29].

The components of an optical fiber are shown in Fig. (2a). The optical fibers can be classified as:

1. Single-mode and multimode, respectively, depending on their size.
2. Step and gradient index fibers according to their refractive index profiles.
3. Polymer optical fibers (POFs) and silica optical fibers (SOFs), according to their material.

The core of the fiber, which has a refractive index value, and is surrounded by the cladding, which has a higher refractive index, efficiently guides light into the core of the fiber. The core may support many propagation modes as the wavelength of the operation grows, and the fiber can split into single-mode and multi-mode fibers as a result.

Multimode fibers provide a substantially higher light capacity than single-mode fibers, and the light may be guided into the core more precisely. However, having several modes reduces the quality of the light beam that leaves the fiber. Because various modes have different light speeds, the light at the output spreads in the time domain, a process known as dispersion. The core's refractive index profile can be designed to drop gradually from the center to the edge, which will cause light to bend along its transmission path and reduce dispersion in multimode fibers. Gradient index fibers are the name given to these kinds of fibers (Fig. 2b).

More cost-effective, environmentalally-friendly, and sustainable building materials are needed today. Given that concrete is the most widely used construction material in the world, this has spurred a great deal of study in the sector. From the series of research, some researchers have come up with translucent concrete, also known as light-transmitting concrete. This concrete has many potentials that make it a good material that solves the problems faced in construction. However, due to poor awareness, cost, and availability of materials, translucent concrete is still not well known in many parts of the world. Where it is known, the availability of the material and cost of the material that is the main component that transmits becomes a challenge. The current paper reviews and analyzes qualitatively, the properties of translucent concrete, the aggregates, the limitations, and the advantages. Recommendations on the possible way by which the strength and durability of translucent concrete, especially when exposed to harsh environments, can be improved. Although other types of light-transmitting materials are only briefly mentioned or covered in this study, this is done to let the readers know that translucent concrete can also be made from other light-transmitting materials. However, because of its unique qualities that set it apart from other materials, optic fiber received a lot of attention in this study. It is the most popular light-transmitting material identified by numerous researchers, and these characteristics are discussed further in this study. The review study on translucent concrete revealed a good deal of limitations and research gaps. Some of the unmet needs include strengthening translucent concrete, determining the optical fiber percentage that will produce a durable concrete mix, and determining the impact of varying optical fiber ratios on the material's strength and energy-saving qualities.

Transparent concrete's light-transmitting materials are important for light transmittances and prices. The use of POF in daylighting systems has significant potential due to its high transmittance and low cost.

Water, fine aggregate, cement, and roughly 2% to 6% of the volume of the complete sample of optical fiber make up cement-based transparent concrete. The strength, serviceability, and durability criteria must be met for load-bearing and non-bearing transparent concrete panels or facades to handle anticipated ultimate loads with allowable deflection [21]. Additionally, the light transmittance performance must fulfill requirements like the Australian/New Zealand Standard [30] and the minimum illuminance level for human ocular activities in indoor contexts.

According to a study [31], it is practical to omit coarse aggregates from a combination used to create “light-transmitting cement mortar” to achieve higher homogeneity and compaction, which improves TC's light transmission. On the other side, there are worries about overlooking the optical fiber alignment since concrete vibrates. Self-compacting concrete is the greatest option for TC manufacture with good homogeneity to solve this issue. Consequently, much attention is paid to the freshness and hardening properties of self-compacting concrete to make it cleaner and more sustainable, either by replacing fine aggregates with industrial by-products or by adding steel or polymeric fibers [32, 33].

According to the study by Shanmugavadivu et al. [34], the following mix proportions were utilized to create transparent concrete blocks: 360 kg/m³ of cement, 560 kg/m³ of sand, 4.5 kg/m³ of fiber, and 190 L/m³ of water.

Nano-optic fibers and fine concrete are the two main ingredients of transparent concrete. Cement, aggregate, and water form the foundation of the substance, matching the fundamentals of concrete [35]. The binder that sets and hardens over a specific time and gives the product shape is cement, typically Portland cement. The material is strengthened by coarse aggregates like crushed granite or hard basalt fragments. Coarse particles are often absent from translucent concrete because they harm the fibers and prevent light from passing through the concrete block [36]. Despite being chemically inert, fine aggregates have an impact on the material's strength by filling in the gaps and lowering porosity. Sand is a typical fine aggregate.

All acids, alkalis, oils, and other organic contaminants must be absent from the water utilized. Craft clay is further used as a substrate for the optical fibers to combine with the concrete in addition to the desired fast-setting cement for the mixture. Additionally included are superplasticizers like Neoplast, Glenium, Polyplast SP HPC, etc. To boost refractivity, the substance is mixed with crushed, hammered, and ground industrial waste glass [37].

Due to optical elements—typically optical fibers—incorporated in the concrete, translucent concrete can transmit light. The stone allows light to pass through it from end to end. As a result, the fibers penetrate the entire object. Depending on the fiber structure, this causes a specific light pattern to appear on the adjacent surface. Through the material, silhouettes of shadows cast onto one side can be seen [48].

The method of production of translucent concrete is the same as that used to produce traditional concrete and some other varieties [49]. The only difference is the inclusion of optical fibers in the translucent concrete placed in the mold, spaced 2 to 5 mm apart and parallel to one another. The fibers are infused with the concrete as it is poured in thin layers over one another. Smaller layers allow an increased amount of light to pass through concrete. Translucent concrete, which can transmit both artificial and natural light, is made using thousands of fiber strands. Since coarse aggregate tends to damage fiber strands and reduce the concrete block's ability to transmit light, it is not used in production [50].

In addition, the procedures for producing translucent concrete are briefly described in 6 (six) steps as written in the research [37, 51], as well as the steps shown in Fig. (5).

The steps for the production of translucent concrete outlined in Fig. (5) are explained below and demonstrated in Fig. (6).

4.3. Step 3 is Fixing the Fibers in the Mold

Fibers are placed either in organic distribution or in layered distribution. Holes are driven on the wooden or steel plates through which optical fibers are allowed to pass (see Fig. 6c).

Fig. (5). Procedures for the production of translucent concrete.

Fig. (6). Steps for the production of translucent concrete: (a). Preparation of panel; (b). Optical Fibers; (c). Fixing of fibers; (d). Concreting; (e). Cutting and polishing of the surface [52, 53].

Fig. (7). SEM images of light transmitting cement-based material: (a) Matrix with optical fibers; (b) cross-section of optical fibers; (c) fiber/matrix interface [18].

Through a series of experiments, the study [54] attempted to determine the strength and durability properties of translucent concrete by using a water permeability test. It was discovered that transparent concrete with 4% optical fiber is more durable than conventional concrete and more durable than transparent concrete with 2.5% plastic optical fiber by 28% [54].

5.2. Stress-strain Behavior for Translucent Concrete

The slope of the stress–strain curve for concrete under uniaxial loading determines the static modulus of elasticity under tension or compression. The elastic property of a material is a gauge of its stiffness, but this curve for concrete is nonlinear [72]. As a result, stronger concrete has a more elastic static modulus [73]. A crucial factor expressing concrete's ability to deform elastically is its modulus of elasticity. The modulus of elasticity is important because, for a given strain, a higher modulus produces a higher tensile stress, which means that stronger concrete exhibits a lower strain. As a result, the length of the member under compression affects the descending branch of the stress-strain relationship [74].

In comparison to concrete boards without optic fiber, the study [75] looked at the stress-strain behavior of translucent concrete boards of various dimensions. Figs. (8, 9, and 10) show the stress-strain diagram for all types of boards. Based on these data, it can be concluded that the embedding of POF in the mortar causes a reduction in the static modulus of elasticity in translucent concrete for all types of boards because the POF has a low modulus of elasticity that is less than 2.1*10-2 GPa [76]. Following [72, 77], the POF causes the mortar to become more porous, which causes the modulus of elasticity to decrease.

Fig. (8). Stress-strain behavior for the translucent concrete board of 15 mm height [75].

Fig. (9). Stress-strain behavior for the translucent concrete board of 25 mm height [75].

Fig. (10). Stress-strain behavior for the translucent concrete board of 50 mm height [75].

Fig. (11). Schematic diagram of the setup of light transmittance test [78].

Since light transmission and transparent concrete may disclose the shadows of any surrounding objects put on the brighter side of the wall, they can be used in the building of jails, banks, and museums to provide safety, monitoring, and security [91]. However, there could be some limitations to the use of translucent concrete in places like banks and museums. This limitation ranges from the high cost of using optic fibers in the concrete, which makes translucent concrete expensive, to the manpower in this area of construction. The poor strength recorded by some researchers when the optical fiber is incorporated into concrete could be a thing of concern for this type of concrete to be used in security-sensitive areas like banks and museums. Residential and commercial buildings consume the most energy for electric lighting. The building industry is responsible for 34% of the world's energy use [92]. Around 19% of the total provided power worldwide is used for artificial lighting [93]. The need for electric lighting has been growing significantly because of population increase, urbanization, and the construction of towering structures. Natural sunlight is impeded from passing through when high-rise buildings are constructed close to one another because of the neighboring structures' obstructions. Buildings' indoor environments are always kept bright throughout the daylight thanks completely to artificial light, which uses a lot of electricity. Utilizing natural light indoors decreases the demand for artificial lighting, lowers energy costs, and improves occupant comfort levels. It has been demonstrated that indoor spaces with sufficient natural light lighting reduce stress on occupants, enhance visual comfort, and result in higher staff retention [94].

Although transparent concrete isn't used much today, some potential uses are being considered for implementation [50]:

1. Illumination of interior building spaces. Buildings and basements have long been associated with drab and depressing concrete. Transparent concrete has the potential to alter the stereotypical perception of concrete by allowing light to pass through it, changing the interior of buildings so they appear more light-filled, airy, and spacious.
2. The people inside can see when someone is standing outside, thanks to the translucent concrete used for front doors in homes and offices.
3. Translucent concrete can be used as flooring on a surface illuminated from below. It appears like typical concrete pavement during the day, but at dusk, the paving blocks start to shine and change color.
4. Sidewalks may be constructed by using transparent concrete and with lighting underneath, creating a passable surface that would employ more safety for pedestrians during the night, which is at a very high risk.
5. Transparent concrete can also be used as interior walls, facades, and dividing walls based on panels.
6. Creativity with the designing, logos and other aesthetic structures can also be made using transparent concrete. It takes an artsy mind to design these structures. It can also be used to design floors and furniture.
7. Subways, which are sometimes too dark, can also be illuminated. A stylish reception desk can also be made using it.

The usage of LTC in the construction sector has increased during the previous five years. Fig. (12) displays instances of LTC applications in different buildings from across the world. The Al-Aziz Mosque in Abu Dhabi is another example of a newly constructed edifice that uses LTC technology. When the mosque was opened in 2015, optical fibers were used to change the illumination. LTC panels with a total surface area of 525 m² and dimensions of 1.8 x 1.4 x 0.3 m were used [95]. Another building built using the LTC approach was the Italian Pavilion at the 2010 Shanghai World Expo in China. Instead of using optical fibers in this LTC, a precast concrete panel was covered by the addition of specialized plastic resins (polymer-based material) to a creative mortar. 3774 LTC panels were used to cover 1,887 m² or over 40% of the building's envelope. Each plate is 1 x 0.5 m, weighs 50 kg, and is 5 cm thick [96].

In elegant architecture, translucent concrete is used as a façade material and for interior wall cladding. Several design products have also been made from translucent concrete [97].

Fig. (12). Examples of applications of light-transmitting concrete: (a) Abu Dhabi, United Emirates, completed in 2015, 30 mm and 40 mm thickness, 525 m2, (b) Shanghai, China, completed in 2010, 1887 m2, (c) Izmir, Turkey, completed in 2015, 20 mm thickness, 300 m2, (d) Berlin, Germany, completed in 2014, 20 mm thickness, 60 m2, (e) Tbilisi, Georgia, completed in 2011, 15 mm thickness, 300 m2, (f) Aachen, Germany, completed in 2012, 102 m2 [17, 95, 96].

With the growing need for green buildings and alternative building materials, translucent concrete has a bright future. Because sunlight can easily pass through translucent concrete to provide visual clarity for the occupants even at night when the moon is out, translucent concrete is environmentally friendly, sustainable, and less expensive for the government and individuals to use during the day. Translucent concrete is a great way to save money and energy. Translucent concrete serves as a superior replacement since it is approximately as robust as conventional concrete blocks and stronger than glass [99].

Apart from the results from the research by Altlomate et al. [17], the majority of investigations generally showed that the addition of POF reduced the strengths of transparent concrete. However, most of them agreed that lengthening the curing time increases compressive strength. Among these experiments, Halbiniak and Sroka [100] looked at the impact of compressive stresses applied parallel or perpendicular to the POF orientation on transparent concrete specimens. They verified that when the parallel and perpendicular loading orientations were on the POF orientation, respectively, the loss in compressive strength of the transparent concrete relative to the reference concrete (without POF) was 38% and 19% [100]. As a result, most of the research concluded that the POF configuration with perpendicular loading is preferred. Numerous industry experts have projected that transparent concrete will revolutionize the market as a whole and replace regular concrete as an affordable and environmentally friendly material.

To make translucent concrete an accessible solution for both commercial and residential buildings, producers are currently putting a lot of effort into developing transparent concrete at a reduced cost [95]. LTC offers excellent qualities but is high priced because it requires pricey optical fibers and has limited widespread commercial implementation [19]. So, to make judgments that are cost-effective, economic analysis is essential. According to a review of the literature, there is a paucity of investigation of the quantity of optical fibers and the amount of light passing through concrete walls with different percentages of optical fibers [22]. The amount of electrical energy that will be conserved and the amount of light that will pass through various types of light-transmitting concrete in different places such as galleries, offices, or other space walls (with 3-15% optical fibers continent) are thus unknown. In a prior study, high-performance light-transmitting concrete samples with five optical fiber contents (3%, 5%, 7%, 10%, and 15%) were made by integrating innovative matrix materials to reach the cost-effective quantity [62]. Compared to earlier studies, concretes prepared for this study contained more optical fiber, and the best option was chosen. These percentages were chosen based on those that were utilized in other publications [18, 26, 62, 64, 66, 101, 102]. Additionally, it was thought that increasing them by 1.5 or 2 times would make the variations in the light flowing through them more noticeable and observable. Additionally, it was not possible to produce concrete that contained more than 15% fiber optics due to constraints, including the large number of optical fibers needed and the reduction in the distance between the fibers [62].

Glass windows have been serving as a source of light in the room. This type of lighting system has its material readily available and cheap. Translucent concrete has more strength than glass windows, which means it is often important to provide more protective methods in structures bearing glass windows because it could serve as easy access for criminals to access the building in certain regions. The cost of acquiring the materials to produce translucent concrete surpasses the cost of glass windows. From the earlier reviewed works, the reason for the poor use of translucent concrete is due to its high cost optical fiber is the main contributor to this high cost.

It is safe to conclude that researchers will need to discover affordable production methods for transparent concrete before it can be considered a viable substitute. Research into less costly light-transmitting materials is strongly advised.

It is necessary to improve the strength and durability of translucent concrete especially when exposed to harsh environments hence, this study suggests the use of fiberglass mesh (Fig. 13) as external confinement of the concrete and incorporation of fibers in the translucent concrete mix to enhance the strength of the concrete.

Fig. (13). Concrete fiberglass mesh [103].

Composite masonry mesh is a fiberglass rod of circular cross-sections, which are perpendicular and firmly fixed to each other at the connection points. The cells in the grid are most often square, and the minimum size is 50x50 mm, but products with larger cells are also produced. Composite mesh is usually produced in the form of rolls or cards (sheets). The width of the grid can be up to 2 m, while the length can be any. The main task of the fiberglass mesh is to strengthen the walls of the building, providing resistance to various types of damage [104].

Using conventional light-transmitting concrete is a clever strategy to maximize the sun's energy and a way to save non-renewable energy. Translucent concrete is a cutting-edge construction material. Translucent concrete has strong light-guiding qualities, and the optical fiber volume-to-concrete ratio is proportional to transmission. Translucent concrete may be used as a lightweight structural material to illuminate interior construction areas in addition to its many aesthetic uses. To increase its potential, more study is required. Most of the thermal and energy-saving research focuses on experimental or modeling studies of the possibility of integrating optical fibers with concrete to bring sunlight into interior spaces and analyze their ratios in concrete. Tries to find the ideal ratios to reduce the demand for industrial lighting and heating loads to save the most energy possible. Numerous studies on the mechanical characteristics of transparent concrete have compared its characteristics to those of ordinary concrete, highlighting the compressive strength value as one of the most important characteristics of the substance. Most of the research, however, indicates that further study is necessary since the mechanical characteristics of transparent concrete's strength are still inferior to those of regular concrete. Some studies looked at how adding fibers to transparent concrete might affect the mechanical characteristics in relation to the fiber ratios. To increase the strength performance, several of these studies used experimental research to examine the impact of optical fiber ratios on concrete mixtures with diameters ranging from 0%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 10%, and 15%. As predicted, the results showed that compressive strength declined as the proportion of optical fibers increased and that the ideal proportion for mechanical qualities is less than 5% of fibers per volume of concrete. As previously noted, the ideal ratio of fibers in concrete volume is less than 5% for improved mechanical qualities and 6% and above for energy savings. A proper understanding of the right percentage of optic fiber to be included in the concrete is essential. The pattern of placing the fibers is of high importance. From the review study, the diameters of the optical fibers mostly used were 0.25 mm, 0.5 mm, 0.75 mm, 1 mm, and 2 mm. Therefore, there is a gap in the literature about the optimal optical fiber ratios for concrete to obtain optimum energy savings and higher mechanical properties. It was discovered that adding POF to concrete drastically reduced its flexural strength. (In general, the pattern shows that flexural strength decreases as the POF volume ratio rises.