POSSIBILITIES OF REINFORCEMENT OF SUBTLE TRANSLUCENT CONCRETE PANEL

. Textile-reinforced concrete not only has many advantages that lead to less consumption of primary resources, but also enables the creation of thin and subtle elements. The high performance of such elements allows us to create interesting architectural features, such as translucent concrete. This article focusses on the possibility of reinforcing translucent concrete panels.


Introduction
Concrete is one of the most used building materials, but due to its massive production, concrete has a huge impact on the environment (especially because of the cement and steel used as reinforcement).On the other hand, concrete offers many benefits (such as mechanical properties or the ability to maintain the stability of the internal environment) that make it almost irreplaceable in many cases.This is the reason why it is so important to optimise concrete [1].
There are several ways to improve concrete and reduce its environmental impact -for example, it is possible to partially replace cement using fly ash, silica fume, or slag, reduce the amount of water (that leads to higher strength which allows lower concrete thickness) or replace traditional steel reinforcement with alternative noncorrosive reinforcement that has a lower environmental impact.Furthermore, no concrete cover is required to protect the reinforcement, which also leads to a lower thickness of the concrete.Reducing the thickness of concrete has an impact on the reduction of used materials, lower load, which means smaller dimensions of the supporting structures, and easier handling [1].
Textile reinforced concrete (TRC) combines the above improvements.It is reinforced by technical textiles made of carbon, alkali-resistant AR-glass, basalt, or natural materials such as flax or hemp, which can be impregnated using a polymer matrix, e.g., epoxy resin.Technical textiles are in the form of orientated rovings and are embedded in a concrete matrix, which is usually high-performance fine-grained concrete [2][3][4][5].
Another material that combines the above improvements is fibre-reinforced concrete (FRC), where dispersed reinforcing fibres (made of steel, polymers or, for example, glass) instead of traditional reinforce-ment.These fibres can contribute to higher strength and ductility, but also to enhance fire resistance [5,6].
Overall, there are many ways to improve concrete and thus reduce its environmental impact, but it is also possible to combine multiple functions (e.g., load bearing, acoustic, or lighting) in one material.For example, luminescent or acoustic concrete is being developed and light-transmitting concrete is already available (under the name of Litracon, Lucem, or Licrete).Light transmission is ensured by several elements -usually optical fibres, polymers, or glass [7][8][9].
Based on that, it was decided to explore the possibilities of light-transmitting concrete reinforcement using alternative reinforcement to reduce the thickness of concrete and thus reduce its environmental impact.Different types of rovings (carbon, AR-glass, and flax) were tested, and a plexiglass (or polymethyl methacrylate -PMMA) elements were chosen to ensure light transmission.
A fine-grained concrete mixture was used for the preparation of samples, which is described in the Table 1.
Reinforcement was prepared as impregnated rovings with epoxy resin.The used rovings are described in the Table 2.
[kg m −3   To ensure a 10 mm distance between the plexiglass elements (Figure 1), 10 mm wide wooden bars were placed between them.During the preparation of the translucent TRC samples, the rovings were impregnated with epoxy resin, placed in the space where the plexiglass elements are cut and then sprinkled with fine silica sand.After 24 hours, the hardened epoxy resin was able to keep the required distance between the plexiglass elements so the wooden bars could be removed and the same process could be repeated on the other side of the sample (Figure 2).The prepared inserts were placed into the mould and weighted down and then the concrete mixture was poured.All the samples were later demoulded and left in water until the day of testing.

Experiment and results
The compressive test according to CSN EN 12390-3 and the flexural strength test (three-point bending test) according to CSN EN 12390-5 both at the age of 28 days were performed to measure the mechanical properties of the concrete mixture.The translucent samples were prepared for the flexural strength  test (four-point bending test) with constant loading speed of 1 mm min −1 which was partly according to CSN EN 12390-5.
The average compressive strength on the cubes was 110.69 MPa, and the average flexural strength on the prisms was 17.24 MPa.In Figure 3 force-displacement curves of the translucent TRC samples are presented.The translucent TRC samples achieve high flexural strengths.The highest strength is achieved by the carbon-reinforced

Figure 2 .
Figure 2. Elements with one layer of carbon rovings impregnated with epoxy resin and silica sand (right).

Table 3 .
Results of the four-point bending test.