Repair and Protection of Existing Steel-Reinforced Concrete Structures with High-Strength, Textile-Reinforced Mortars

Abstract

Numerous concrete monuments built in the High Modern Era (turn of the 20th century until the 1970s) must now be repaired for preservation. Traditional concrete repair according to current guidelines involves considerable material removal, changing the appearance of the existing structure. With a combination of the material properties of high-/ultra-high-performance concrete (HPC/UHPC) with its dense microstructure and corrosion-free textile reinforcement (carbon and basalt), a high-performance mortar repair system can be developed. Such a system allows for concrete repairs with minimal material loss by using very thin layers that are durable and do not change the architectural character of the repaired monument. For the investigation of the load-bearing behaviour of a structural repair system using textile-reinforced, high-performance mortar, 20 mm thick slabs were produced and mechanically characterized. In the next step, the proposed repair system was applied to 70 mm thick old concrete slabs. The results show that a high surface tensile strength of 2.9 MPa was obtained. In a further step, the system will be applied to concrete pillars of transmission tower in Berus, adapted in terms of colour and structure and installed for long-term monitoring.

1. Introduction

In textile-reinforced concrete/mortar (TRC/TRM), technical textiles made of carbon, alkali-resistant (AR) glass or basalt fibres, which are embedded in fine-grained concrete, take on tensile forces, whereas the concrete absorbs the compressive forces. With mechanical properties comparable to those of steel-reinforced concrete, textile-reinforced concrete/mortar thus has the following advantages: thin construction, resource savings, increased durability and potential for very finely distributed crack development in the building material due to the textile mesh size and yarn structure.

Over the past 25 years, the foundations for the use of technical textiles in cement-bound materials have been laid within the framework of numerous large collaborative research projects [1,2,3,4]. The wide range of application possibilities of textile-reinforced concrete/mortar are also demonstrated by numerous approvals and pilot applications, e.g., sandwich elements, pedestrian and cyclist bridges and noise barrier walls [5,6,7,8,9]. Additionally, there is the option to prestress the fibre strands and increase the cracking, as well as ultimate loads (compare [10,11]).

In addition to the use of textile-reinforced concrete/mortar as precast elements and in new buildings, there is a broad field of application in the area of existing buildings. Three aims can be distinguished that can be achieved with textile-reinforced concrete/mortar in building maintenance:

• strengthening of concrete structures or masonry [12,13,14];
• sealing and protecting [15,16,17,18,19,20];
• local reinforced concrete replacement.

Applications of textile-reinforced concretes or mortars in thin layers, predominantly with textiles made of carbon fibres, are used to increase the load-bearing capacity of existing structural components under service conditions. To this end, mortar and textiles are applied in layers over the entire surface of the reinforced concrete structure. This method may be used to reinforce the bending tensile zone and, if necessary, the shear zone of ceilings, beams and columns [12,13] made of steel-reinforced concrete. This method is also used to strengthen masonry [14]. Compared to conventional repair with steel-reinforced concrete, TRM can be carried out with a layer thickness of about 1 cm, meaning that the geometry of existing structural elements can often be preserved. This is particularly important in the field of monument preservation.

In addition to strengthening, TRC/TRM offers the potential to seal reinforced concrete components where unplanned cracks or cracks with inadmissibly large crack widths have occurred [15,16,17,18]. The cracking process (crack distances and width) of RC structures can also be predicted via models (compare [21,22]). To ensure durability, crack widths are limited to permissible widths for the service condition in reinforced concrete construction. Common methods of repairing cracks include filling them with crack fillers or coating them with crack-bridging surface protection systems. Unfortunately, practical experience often shows that these repair methods quickly reach their limits. A developed procedure makes it possible to distribute crack movements in the old concrete to many fine cracks in the repairing layer using textile-reinforced spray mortar on old concrete surfaces [17]. The cracks remain so fine that the textile-reinforced concrete/mortar is impervious to the effects of water. It has been shown that crack movements in the substrate can be distributed in very fine crack widths of around 0.1 mm in textile-reinforced concrete/mortar [17]. The proposed system focuses on hydraulic structures with low-strength old concrete classes without or with minimal steel reinforcement [18] but was also used to seal the steel-reinforced concrete roof of the Mariendom in Neviges, Germany [19]. The high electrical conductivity of carbon textiles in mortar layers applied to steel-reinforced concrete can also be used to polarize the steel reinforcement and thus protect it from corrosion [20].

This publication focuses on the use of high-strength, textile-reinforced concrete/mortar for the local repair of reinforced concrete components. If steel reinforcement in concrete corrodes as a result of carbonation of the concrete and/or penetration of chlorides, a possible repair principle according to EN 1504 (method 7.2) is the replacement of the damaged concrete. If the corroded reinforcement has partially lost its load-bearing capacity due to a reduction in its cross section, the use of textile-reinforced concrete/mortar for concrete replacement is expedient. TRC/TRM has the following advantages in this context: the dense microstructure of the mortar matrix prevents the penetration of pollutants from the environment, cracks that can occur during stresses beyond the service range are reduced in width and the additional textile reinforcement acts as a reinforcement supplement for the ultimate limit state. These advantages provided the motivation to develop and characterize a suitable textile-reinforced concrete/mortar systems for concrete replacement within the framework of a research project. The focus was on the repair of heritage-protected reinforced concrete structures in which the original substance is to be preserved to the greatest extent possible. A specific application, the repair of the transmission tower of the Berus Broadcasting Hall in Saarland, Germany, is the starting point for this work (see Figure 1) [23].

2. Development of a Specific Textile-Reinforced Cement Mortar

2.1. Compatible High-Performance Cement Mortar

Previous investigations of concretes from historical concrete buildings with minor damage have shown [24] that such concretes often have high cement contents, and therefore, the structure is less porous and denser than comparable standard concretes. However, they still contain a high proportion of capillary pores through which concrete-damaging substances can penetrate. Structurally dense concretes (ultra-high-performance concrete (UHPC)), on the other hand, are based on the concept of optimized packing density, especially of aggregate with a grain size of ≤0.25 mm. In addition, the water/binder ratio, which significantly influences the porosity, is reduced to <0.2. Sufficient processability is achieved by high-performance superplasticizers [25,26,27,28]. The packing density is optimized by introducing suitable filler grains into the grain structure, which can fill the voids between the larger grain fractions, increasing the number of solid, load-transmitting contact points between the individual particles, resulting in a considerably increased load-bearing capacity of the grain structure. Furthermore, the water demand of the grain mixture required to fill the remaining voids decreases continuously. A high packing density results in a reduced water demand for the grain mixture and thus increased strength and reduced lower porosity (see Figure 2). Precise matching of the grain fractions is a basic prerequisite for optimal grain packing to produce a concrete with high resistance, as required in the present research as a basis for a textile-reinforced cement mortar.

Figure 3 shows the difference in pore size distribution between a non-packing-optimized standard concrete, a high-strength concrete (HPC C105) and the comparatively dense structure of packing-optimized UHPC (UHPC C200) with a correspondingly low water/cement ratio, which is practically free of disadvantageous capillary pores.

The extremely low capillary pore content of these microstructurally optimized concretes prevents the penetration of precipitation water into the structure, thus avoiding further reinforcement corrosion. Another advantage of these structurally dense UHPCs is their extremely low carbonation tendency.

This approach was adopted for the development of a compatible repair mortar. Here, the term mortar is used instead of concrete because in the developed repair material, the aggregates are ≤4 mm, and the material is used as a repair mortar. Based on the properties of the UHPC, as well as the design and the damage pattern of the Berus transmitter structure, a repair mortar with adapted mechanical properties was developed. Relevant properties of the fresh and the hardened mortar were determined through selected laboratory tests, including fresh mortar properties, such as spread, air void content and fresh mortar density, as well as hardened mortar properties. Tested properties of the hardened mortar included compressive strength, colour and pore size distribution (mercury intrusion porosity (MIP)). In a first step, a requirements profile was derived by examining the existing old concrete on the object. The properties of the repair mortar should be adapted to the properties of the old concrete to the greatest extent possible. This also includes adaptation to the visual appearance of the old concrete, such as colour and surface structure, resulting in the choice of a light colour for the raw materials, i.e., CEM III/A 32.5 cement. The minimum grain diameter of the aggregate depends on the textile fabric used. The consistency was adjusted to the workability requirements. Suitable raw materials were selected on basis of the required demands of the repair mortar, such as the selection of a suitable cement and a corresponding aggregate, as well as the additives and admixtures. The old concrete of the Berus transmitter has a very low compressive strength (41.5 ± 8.1 MPa) compared to UHPC. Thus, in the development steps, the very high strength of the UHPC base formulation, which was developed at the University of Kassel, had to be reduced without changing the porosity.

Two mixes were investigated, a UHPC mix (M1) and an HPC mix (M2). Mixture M1 contains CEM III/A 32.5, silica fume and quartz powder as inert fillers and quartz sand with a grain size of 0.125–0.5 mm as aggregate. Mixture M2 consists of a high-strength white cement, CEM I 52.5, with silica fume omitted; a different quartz powder is used, and an additional aggregate with a grain size 0–2 mm is incorporated. A high-performance superplasticizer is used in both mixtures. To reduce the compressive strength, cement was partially replaced by limestone powder. The test specimens were demoulded after 24 ± 2 h and stored under standard climate conditions at 20 ± 2 °C/65 ± 5% rel. humidity until testing.

The spreading dimension according to DIN EN 1015-3 [31] was set in a plastic consistency range between 140 and 200 mm for both mixtures.

Table 1. Test results of the two mixtures.

Mixtures	M1	M2	Unity
Fresh concrete properties
Spreading dimension	160 ± 12	165 ± 9	[mm]
Fresh concrete density	2265	2150	[kg/m³]
Air void content	4.9 ± 0.4	6 ± 0.5	[%]
Hardened concrete properties
Compressive strength	83 ± 7	60 ± 5	[MPa]

shows the results of the individual mixtures.

The fresh mortar properties of the two mixtures are comparable; differences can be observed in terms of compressive strength. Mixture M2 has a lower compressive strength of 60 MPa compared to mixture M1, with a compressive strength of 83 MPa.

Figure 4 shows the pore size distribution of both selected laboratory mixtures. The low porosity and, above all, a very low proportion of capillary pores in the range of 0.01–100 µm are obvious.

The optical appearances of the two investigated mixtures were compared to the existing concrete of the Berus transmission tower by means of colour measurements according to DIN EN 15886 [32]. A spectrophotometer spectro-guide sphere gloss from BYK-Gardner with a standardized light of D65, a 10° image field size and a measuring geometry of d/8° was used for measurements. The results were presented in a CIE L*a*b* colour system, where color is expressed in a Cartesian coordinate system with the following parameters: L* = lightness (black (0) to white (100)), a* = red (+a*) − green (−a*) coordinate and b* = yellow (+b*) − blue (−b*) coordinate.

Figure 5 presents the deviations in the colour scheme of the various materials previously applied during earlier repair measures (M0: old repair mortar at application test area, M0-new: repair mortar in the upper tower area, applied in 2021) as well, as the two developed mixtures (M1 and M2); all data are shown in relation to the existing concrete. The greatest changes in mean values can be detected for M1 in the blue direction (−10.7). The M2 mixture has a colour similar to that of the existing concrete, whereas the changes in lightness are higher than for M1. As a result of outdoor weathering at the Berus transmission tower, the brightness of M2 is expected to adapt over time. This should be observed on the test areas and adjusted in the mixture if necessary.

2.2. Textile Reinforcement

Two materials are considered in this research project for the production of technical textiles: carbon and basalt fibres. Carbon fibres have a higher load-bearing capacity, as well as a higher CO₂ footprint compared to basalt fibres due to the production process and the used raw materials. Thus, a decision must about the repair material based on the requirements of the repair measures.

Textiles with the selected geometries shown in Figure 6 were produced from the fibre materials.

Table 2. Characteristics of the textiles (manufacturer data sheet [33] and own results).

Name	T 1C	T 2B
Fibre material	Carbon	Basalt
Coating	Acrylic/stiff	Acrylic/soft
Distance of the yarns 0°/90° in mm	21.3/21.3	11.9/16.4
Fibre area 0°/90° in mm²/m	42.3/42.3	73/53
Tensile strength of the yarns 1 0° in MPa	2996 ± 174	936 ± 132
Tensile load 1 0° in kN/m	127 ± 7	68 ± 10

¹ mean value ± standard derivation determined in tensile tests with mechanical clamped fibre strands. Testing length = 320 mm; velocity = 6 mm/min; n ≥ 10.

summarizes the material characteristics of the textiles.

The mechanical properties of the two textiles differ due to the fibre materials and the geometry. The carbon textile (T 1C) has a tensile strength of 2996 MPa (see Table 2), which is three times higher than that of the basalt textile (T 2B). Additionally the E-modulus of the carbon fibres (about 200 GPa) is significantly higher than that of the basalt fibres (about 60 GPa). Whereas the geometry of textile T 1C is symmetrical in both directions (0°/90° direction), textile T 2B has a main load-bearing direction (0°). The fine mesh width of the basalt textile leads to a higher fibre area (0° direction), as well as a higher bonding area between the textile and concrete, compared to that of the carbon textile.

2.3. Composite Material

The composite material is referred to herein as a textile-reinforced mortar (TRM). The properties of the composite material depend on the composition its component materials. Therefore, aspects such as load-bearing behaviour, adhesion to the existing structure and the manufacturing process have to be considered. The mechanical properties concerning the load-bearing behaviour can be adapted by the choice of textile and the number of layers. The concrete mainly influences the first crack load and the adhesion to the existing structure. The requirements for the composite material depend on the purpose of the repair measure. The general requirements can be summarized as follows:

• applicable maximum tensile force (kN/m) to enlarge or restore the load-bearing capacity of the existing structure (in the ultimate limit state);
• small crack spacings and crack openings to reduce the penetration of pollutants into the existing structure;
• appropriate processing properties.

To evaluate the suitability of the composite material, tests on the following three material combinations were conducted. The total material thickness is 20 mm for all combinations.

Material combinations:

• M1 with two layers, T 1C;
• M2 with two layers, T 1C;
• M2 with four layers, T 2B.

The materials were tested in terms of adhesion to the existing concrete and tensile load-bearing behaviour.

3. Characterization of Textile-Reinforced Mortar (TRM) as Repair Material

3.1. Specimen Preparation

The test specimens were produced by hand using a lamination process. Two or four layers of textile were applied in horizontal formworks (see Figure 7). The thickness of the test specimens was 20 mm. Plates of 1100 mm × 400 mm were produced for the characterisation of the composite material. After manufacturing, the plates were stored in the formwork covered with foils for one or two days. Until testing, the specimens were stored at 20 °C and 65% rel. humidity under air circulation. After more than 21 days, the specimens were cut out of the plate with a diamond saw. First, the border areas of the plates were removed; then, the specimens were cut out of the inner area.

For analysis of the bond behaviour of the textile-reinforced mortar system on old concrete, adhesive tension plates were produced. 20 mm textile-reinforced mortar was laminated onto 70 mm thick old concrete slabs.

The composition of the old concrete slabs was adapted to the properties of the existing concrete on the structure. Therefore, the existing concrete of the transmission tower was examined in advance using various, mainly non-destructive, analysis methods. At the application test area, the inserted rebars and its concrete cover were detected, the surfaces were analysed and colour measurements were carried out in accordance with DIN EN 15886 [32].

To determine the compressive strength of the existing concrete, drill cores (two samples, each 50 mm diameter) were taken. Furthermore, the surface tensile strength was measured on different sides of the pillar. Using the results of the compressive strength of 41.5 (±8.10) MPa and the surface tensile strength of 2.9 (±0.3) MPa (see Figure 8), the existing concrete was classified as old concrete class 4 according to Part 1 of the Technical Regulation, “Maintenance of concrete structures” [34].

The results of the tests also allowed the determination of the concrete composition for the reference concrete, which was to serve as a substrate for the developed repair material. In accordance to DIN EN 1766 [35], a type C (0.45) concrete with a maximum grain size of 16 mm, a w/c ratio of 0.45 and a cement content of 375 kg/m³ was selected. With an average 28-day cylinder compressive strength of 40 MPa and an average surface tensile strength of 2.5 MPa, it meets the requirements of a comparative material to the existing concrete (EC) of the structure, making it a suitable substrate.

3.2. Tensile Tests

To examine the tensile load-bearing behaviour of the material combinations, tensile tests were performed on the composite material, comparable to [6,36,37]. For these tests, strips with a width of 80 mm and a length of 800 mm were cut out of the plates (see Section 3.1). The samples were screwed into clamping jaws with an anchorage length of 250 mm (see Figure 9). The clamps were connected to the testing machine via ball joints, and the tests were performed with a velocity of 1 mm/min. The deflection and the cracking process in the 300 mm long free expansion area were measured with an ARAMIS 12M DIC (digital image correlation) system (gom, Germany). All tests were conducted at room temperature. Figure 9 shows the test setup for the tensile tests on the composite specimens.

The main results of the tests are presented in

Table 3. Characteristics of the composite materials; mean values of four single tests each ± standard deviation.

Material Combination	First Crack Load MPa	Max. Load kN/m	Max. Tensile Strength (Textile) MPa
M1—T 1C	3.16 ± 0.49	247 ± 5	2885 ± 62
M2—T 1C	2.63 ± 0.49	245 ± 7	2865 ± 86
M2—T 2B	3.19 ± 0.61	157 ± 5	536 ± 15

. The results show a clear difference between the carbon textile and the basalt textile. As failure mode a tensile fracture of the yarns was observed for all specimens. Due to the higher tensile strength of the carbon fibers the maximum tensile load per meter of textile is also higher (factor 1.56). Comparing the two different mortars, reinforced with the same textile, neither the maximum load nor the first crack load differ significantly. The maximum load depends on the textile and should therefore be similar. The first crack load depends on the tensile strength of the mortar and should therefore be higher for the M1 (compare Table 1) mortar. The low initial crack load of the M1 mortar can be explained by the specimen geometry. Due to the higher cement content of the M1 mortar, shrinkage is also more pronounced. This causes a bending of the specimens which leads to an uneven stress distribution in the uncracked specimen in the tensile test and reduces the cracking loads.

Textile T 2B showed a lower maximum tensile strength in the composite specimens compared to the single-fibre strands (see Table 2) The reason might be a nonuniform loading of the fiber strands due to the low stiffness of the textile (soft impregnation). This low stiffness leads to a lower stability of the textile, which means that the fibre strands are not fully stretched in the mortar. Therefore, the straighter fibre strands are activated more and fail first. A gradual failure on a lower total load level is the result.

Figure 10 shows the tensile force deflection curves of the three material combinations. One exemplary curve is presented for each material combination. All curves can be divided into the three typical parts of the force–deflection curve of a reinforced concrete structure. First, the specimen is uncracked until the tensile strength of the cementitious matrix is exceeded. Then, cracks occur in the specimen on a similar or slightly increasing load level. At this stage, no clear differences between the three material combinations can be observed. At one point, the cracking process is completed, and the curve rises until the specimens break due to a failure of the textile.

Figure 11 shows the crack patterns of the three material combinations after the cracking process was completed. All material combinations show similar crack patterns. The cracks occur at a distance of about 60 mm. Therefore, all material combinations exhibit comparable composite load-bearing behaviour. The tensile force–deflection curves in Figure 8 support this statement. In the case of the M1-T 1C specimens, the influence of the shrinkage process should be considered. It can be assumed that the higher tensile strength of the M1 mortar leads to wider crack spacing and greater crack openings. If the TRM is used to bridge an existing crack with the aim of protecting it from the penetration of substances (e.g., water), both textiles can be used equally. However, compared to systems that were developed specifically for the purpose of bridging cracks [17,36], the performance is lower. These special systems also require a significantly higher material input, as well as associated effort and costs.

3.3. Determination of the Overlapping Length

If the TRM is used to increase or restore the load-bearing capacity of an existing reinforced concrete structure, the weakest point in the repairing system is crucial for the strengthening effect. Therefore, the overlapping areas of the textiles must also be designed to be as sustainable as possible. Depending on the combination of textile properties, the chosen overlapping length and the mortar properties, a yarn rupture or a splitting failure may occur in the textile plane. These factors affect the bond performance between textile and mortar, as well as the failure mode (compare [38,39,40]). To investigate this, composite samples of the material combination M2-T 1C with overlapping lengths of 10 and 15 cm were tested. The test setup was similar to that shown in Figure 7 (compare also [41]), but the free expansion length was 500 mm. The overlapping area was arranged centrally, and the textile layers were placed directly on top of each other during concreting. All test specimens contained two layers of textile with three fibre strands per layer.

Figure 12 shows the results of the tests on the overlapping length in comparison to the composite tensile tests (see Figure 9). The maximum tensile load (kN/m) that can be transmitted via the overlapping area depends on its length. In the case of the 15 cm overlapping length, the maximum forces slightly exceed the values measured in the composite specimens. For all specimens, a rupture of the textile yarns outside of the overlapping lengths was observed as the failure mode. By reducing the overlapping length to 10 cm, the transmittable forces decrease to 238 kN/m. This value is slightly below the load-bearing capacity in the free length (245 kN/m).

In the tests with an overlapping length of 10 cm, a mixed failure mode of yarn rupture and splitting of the mortar cover were observed (see Figure 13), which can explain the slightly reduced forces compared to the free length and the 15 cm overlapping length. This mixed failure and the results indicate that the required overlapping length to transmit the maximum tensile force marginally exceeds 10 cm. By using an overlapping length of less than 10 cm, a splitting failure in the textile plane by reduced tensile loads can be expected. Choosing an overlapping length of more than 10 cm leads to a change in the failure mode to yarn rupture. Therefore, the full load-bearing capacity of the textile can be utilized.

3.4. Lateral Tensile Tests

The adhesive tensile strength between the existing concrete (EC) and the TRM system was investigated by pull-off tests according to DIN EN 1542 [42]. Cores with a diameter of 50 mm were pulled off of a substrate plate (see Figure 14a). Additionally, the cohesive tensile strength transversal to the textile layer of the TRM system was tested on 20 mm thick cores with a diameter of 50 mm (see Figure 14b). Thereby, the influence of the covering effect of the textile layers [43] and the manufacturing process can be inspected in detail. All tests were performed after 28 d ± 1 d, with a testing velocity of 100 N/s.

The results of the tests are shown in Figure 15. The adhesive bond between the existing concrete and the TRM is the weak point in the whole system. In case of the adhesive tests, the surface tensile strength of the substrate plates and its scatter influence the result. In the case of mortar M2 (σ Adh-M2), all failures were located in the substrate plate or in the bonding zone between the substrate plate and the TRM. However, it can be expected that a higher surface tensile strength of the substrate concrete would lead to an increase in the adhesive bond strength.

To investigate the influence of the working speed while repairing on the building side, tensile tests were conducted on the TRM system. To this end, individual mortar layers were applied at defined intervals of 0, 20, 40 and 60 min. The single mortar layers were applied “fresh in fresh” without any treatment of the surface of the bottom layer. For each layer, a new charge of mortar was produced. The mortar was applied in a laboratory at room temperature in horizontal formworks. During the manufacturing process, two phenomena were observed on the fresh mortar surfaces. Whereas mortar M1 tends to dry out on the surface, mortar M2 has a tendency to secrete water (bleeding). Therefore, it must be considered that on a building side, the mortar will be applied vertically and at varying temperatures. The results of the lateral tensile tests (Figure 14b) are shown in Figure 16.

The results indicate similar behaviour of the two mortars. The direct application of single mortar layers (0_min series) leads to the highest tensile strength. A break between manufacturing steps significantly reduces the strength. In the case of mortar M1, the reduction is about 40%, whereas the tensile strength of mortar M2 is reduced by about 30% relative to the 0 min series. The length of the break is irrelevant for the result. However, all results were greater than 2.0 and exceed the target value of 1.5 [34]. The results imply that the TRM repairing layer also ensures a sufficient bonding to the existing concrete on the construction site. To verify this, adhesive tensile tests of the repairing layer applied on a 1-to-1 replica of the pillars of the Berus tower will be performed in future studies. To this end, the replica will be freely weathered, and the bond will be regularly checked.

4. Discussion

In the development and investigation of a suitable repair mortar for the Berus transmitter, the focus was on matching the compressive strength of the old concrete. The aim was to achieve a dense structure with low capillary porosity. Through numerous investigations, the two mortar mixtures presented here were evaluated. The workability was adjusted by the spreading ratio. The colouration can also be adapted to the old material by a suitable selection of starting materials. If necessary, the colour can be further adjusted by adding pigments.

The results of the shown study of the proposed TRM systems show that both carbon and basalt textiles can be used to reinforce high-performance, object-specific cement mortars. However, due to the low tensile strength of the basalt material, a higher degree of reinforcement with basalt textiles is required to achieve 1/3 of the load-bearing capacity of carbon textiles, increasing the effort required to applying basalt textiles to the structure. For carbon textiles, we demonstrated that a textile overlapping length of about 15 cm is required to utilise the tensile load-bearing capacity of the textiles, indicating that for the repair of the object, sufficiently large areas must be available for the load application in the textiles.

Furthermore, it was found that waiting times between the individual work steps in the “fresh-in-fresh” process should be kept as short as possible. If the waiting time is longer than 20 min, the adhesive tensile strength between the individual layers decreases. However, the adhesive tensile strength does not deteriorate further with waiting times between the layers of up to 60 min.

5. Conclusions and Outlook

Particularly for reinforced concrete structures built during the High Modern Era, which are also often listed buildings, object-specific approaches to repair are necessary. It is always crucial to find a compromise between the maximum preservation of the as-is substance and a far-reaching durable repair solution. This publication illustrates the performance of textile-reinforced cement mortar in this area of conflict. A high-performance mortar was developed specifically for one application, adapted to the existing optical and mechanical properties, the performance of which was significantly improved in terms of tensile strength and cracking behaviour by means of textile reinforcement. Both carbon and basalt fibre textiles were analysed to cover a wide performance spectrum. The carbon fibre textiles are three times superior to basalt fibre textiles in terms of load-bearing capacity. However, there were no significant differences in performance in terms of crack distribution, transverse tensile strength and adhesion. For mortar development, by varying the starting materials, suitable and structurally dense repair mortars can be developed that are tailored to the application requirements. Further investigations are to be carried out with regard to workability on site in order to gain experience with respect to the mixing technique on the construction site. In a next step, test areas will be set up at the Berus transmitter and at universities to monitor durability and colour adaptation over time. Additional bending tensile tests will be carried out on steel-reinforced beams with and without TRM strengthening layers. The results allow conclusions to be drawn about the combined load-bearing behaviour of the existing component and the repair layer. In addition, tests were carried out to bridge existing large cracks.