The chapter delves into the significance of retrofitting existing buildings due to urbanization and the need for sustainable construction practices. It introduces Textile Reinforced Concrete (TRC) as an innovative material for column confinement, offering advantages such as reduced dead loads and improved durability. The experimental program, involving 12 steel-reinforced concrete cylindrical specimens, demonstrates a significant increase in load-bearing capacity when using TRC. The results show that TRC confinement is particularly effective for components with lower concrete compressive strength, offering a sustainable and environmentally friendly solution for column reinforcement.
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Abstract
In the present paper the results of uniaxial compression tests conducted on textile reinforced concrete (TRC)—confined reinforced concrete (RC) columns are reported. By confining the column with TRC, the lateral expansion of the concrete can be impeded. The resulting multiaxial compressive stress state allows to enhance the components axial capacity. Due to the corrosion-resistant textile, the usual concrete cover in reinforced concrete construction is reduced, which allows slender but at the same time highly load-bearing components to be created. Consequently TRC provides a sustainable, environmentally friendly and lighter option for column reinforcement due to the material savings. The aim of this study is the investigation of various influences on the achievable strengthening effect. The impact of ratio of textile reinforcement and the applied fine grain concrete jacket is evaluated. In addition, the influence of the concrete strength of the strengthened component on the overall increase in load-bearing capacity was investigated. With the help of experiments on TRC reinforced RC columns with a circular cross-section mechanical property and constraint mechanism under uniaxial compression were documented and analyzed. Based on the test results, stress distribution and failure mechanisms of the reinforced specimens is studied. Furthermore, stress-strain relationship of strengthened members is investigated. The results show an increasing in both strength and ductility related to the unstrengthened reference columns. The specimens with a lower compressive strength can achieve a higher degree of reinforcement. A high ductility of the reinforced columns could also be observed.
1 Introduction
1.1 Research Significance
The reuse of existing buildings is becoming increasingly relevant due to progressive urbanization and the resulting demand for residential and commercial space in urban areas. This approach is now being understood as the key to climate protection in the construction industry. In 2014, for example, investments in existing buildings in Germany alone amounted to around €173 billion. This includes monitoring, inspection, maintenance and redesign of structures. Changed and increased requirements, a stricter normative framework, but also damage that has occurred during the service life of the building can lead to the need to improve the load-bearing capacity, serviceability or durability of individual components or of an entire structural system [1].
The standards available to the designer name and classify different retrofitting measures. These essentially include cross-section additions, modifications to the structural system, injections and prestressing technologies. Cross-section additions can be realized by shotcrete or in-situ concrete, while reinforced and unreinforced concrete are possible. In addition, there are newer methods such as confinement with TRC.
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TRC is an innovative building material which has been intensively researched in the last decade. Due to the use of non-metallic, textile reinforcement the concrete cover required to protect the reinforcement from corrosion can be reduced and allows the production of slender, durable components with high load-bearing capacity. Compared to classic reinforced concrete, much lower layer thicknesses and thus lower dead loads can be achieved with even higher load-bearing capacity and lower material consumption.
In order to enable the new type of reinforcement to enter the market, a generally applicable and reliable analytical design model must be created based on certain relevant structural parameters. This must cover the design of load-bearing capacity, serviceability and durability. For this purpose, the behavior of the composite material under different loading situations must be investigated.
1.2 Column Retrofitting
RC columns are an indispensable structural element for transferring vertical loads for the entire structural system. By restoring or increasing the load-bearing capacity of the columns, the overall load-bearing capacity of a building can be effectively strengthened.
In practice, shotcrete is increasingly being used for retrofitting columns. The increase in ultimate load is achieved by a combination of subsequently applied additional layer of concrete and steel reinforcement. Sufficient concrete cover must be ensured to comply durability and protect the applied reinforcement from corrosion. This material application, which is usually not statically relevant, contributes to total layer thicknesses of up to 10 cm, resulting in considerable increases in cross-section and significantly higher dead loads that must be taken into account in the structural analysis. The increase in load-bearing capacity results here from the enlargement of the cross-sectional dimensions and the additional longitudinal and transverse steel reinforcement.
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By confining with TRC systems the mode of action results from the transverse strain restraining effect of the textile reinforcement. The carbon fiber fabrics, which are usually applied unidirectionally, allow forces to be absorbed transversely to the direction of loading and thus impede the transverse strain of the loaded column. The resulting transverse pressure leads to a multi-axial stress state, which significantly increases both the load-bearing capacity and the maximum compression of the whole component.
This method combines the advantages of shotcrete and CFRP confinement. On the one hand, due to the additional concrete application, the old concrete can be repassivated. This process restores the alkaline environment, stops the carbonation of the carbonation of the old cross-section and thus prevents further corrosion of the existing reinforcement. On the other hand, the fire protection properties can be improved in parallel with the increase in load-bearing capacity. Another significant advantage is the corrosion resistance of the textile reinforcement material. The concrete cover required to ensure durability is eliminated, which both saves material and reduces the space required for the reinforcement measure. TRC thus provides a sustainable, more environmentally friendly and lighter option for column reinforcement.
2 Experimental Investigation
2.1 Experimental Programm
The experimental program, shown in Table 1 includes 12 steel-reinforced concrete cylindrical specimens, which are divided into two groups with different compressive strengths. All specimen have a diameter D of 200 mm and height H of 1000 mm and have been tested under a longitudinal force load. The main objectives of the experimental program were (a) to analyze the possible increase in ultimate load and (b) the exploration of the influence of the concrete compressive strength on the reinforcing effect. In order to simulate a component in need of rehabilitation in the existing structure the core concrete compressive strength of the first series (C20) was chosen to be low. From the literature, it is known that the core concrete compressive strength has a great influence on the confining pressure of the reinforcement system, which is why it was increased in the second series (C55) up to a target compressive strength class of C55. Specimen R of each series represent the reference unconfined specimens consisting only of the reinforced core concrete. Specimen with the Label 2L20 are equipped with a 20 mm thick layer of fine-grained concrete in which two layers of carbon textile are embedded.
Table 1.
Experimental Program
Series
Specimen
label
No. of
spec
H/D
Confinement
fc0
[N/mm2]
fcm,m
[N/mm2]
C20
R
3
5
reference
27.67
-
2L20
3
5
2 layers, 20 mm fine-grained concrete
30.76
94.96
C55
R
3
5
reference
*
-
2L20
3
5
2 layers, 20 mm fine-grained concrete
*
81,62
*material characteristic value to be determined
2.2 Specimen Preparation
Each test series was cast out of the same recipe designed to obtain a cylindrical compressive strength (fc0) of 20 MPa and 55 MPa. The cement content was 240 kg/m3 in C20 and 330 kg/m3 in C55 while the water cement ratio (w/c) was 0.75 in C20 and 0.48 in C55. The cement:sand:gravel proportions in the concrete mixtures were around 1:3.22:4.80 in C20 and 1:2.34:3.46 in C55 by weight and the maximum size of the coarse aggregate was 16 mm in both series. With the aim to determine the mechanical properties of the concrete, three cylindrical specimens measuring 150 × 300 mm for each series were made out of the same batch and tested on the same day as the main specimens. The compressive strength of the core fc0 and the fine-grained concrete fcm,m is shown in Table 1. Furthermore, the specimens were provided with a steel reinforcement content of 2.36 cm2/m. Six rebars with a diameter of Ø12 mm as longitudinal reinforcement and every 10 cm cross-sectional reinforcement out of Ø6 mm curved rebars were used. The tensile strength of the steel rebars was 500 MPa and a young's modulus of 200 GPa. The concrete cover at all of the specimen was 15 mm.
Before any confinement works, the surface of the specimen needed to be prepared. A middle roughness of the surface of around 1 mm could be achieved by sandblasting the specimen until the aggregate with a diameter of >4 mm was visible. Furthermore 24 h before confining, the specimens were prewetted and covered with foil. The surface was wetted again and cleaned of dust 20 min before confining. To ensure uniform loading, all specimen were capped with a fine-grained concrete.
2.3 Confining Materials
The confining materials, which were used are regulated by the german general technical approval (Z-31.10-182 (2016)). It designates the use of the textile reinforcement TUDALIT-BZT1-TUDATEX, which a bidirectional warped mesh impregnated with a film-forming dispersion based on Styrene-butadiene rubber. According to the approval, only the yarns in the warp direction with the red knitting thread (Table 2) may be used for reinforcement. Important mechanical properties are summarized in Table 2.
As a mortal matrix, the fine-grained concrete TF10 CARBOrefit® is being used. This concrete has a maximum grain size of 1 mm and has been specially developed for the processing of carbon reinforcements in the hand lay-up and spray process. The concrete mixture, which is available as ready-mixed concrete, has a characteristic minimum compressive strength equal to 80 MPa. The compressive and flexural strengths was measured using three prisms for each confined specimen.
Table 2.
Characteristics of the textile reinforcement TUDALIT-BZT2-V.FRAAS according to Z-31.10-182 (2016) [2]
Properties of a coated yarn
Carbon yarns
in warp direction
Structure of mesh
Number of filaments per yarn
3200/3300 tex
Fiber cross-sectional area
Textile
Yarn
140 mm2/m
1.8 mm2
Tensile strength
Mean value
Characteristic value
1980 MPa
1890 MPa
Modulus of elasticity
Mean value
Characteristic value
170 GPa
166 GPa
Ultimate strain
Mean value
Characteristic value
1.28%
1.24%
Coating
Styrene-butadiene rubber (SBR) -
Lefasol VLT-1
2.4 Instrumentation of the Specimens and the Experimental Setup
All of the 12 specimens were stored for more than 28 days under controlled termperature and humidity conditions (20 ℃ and 60% relative humidity) until the testing. The wrapping started with the application of the first layer of fine-grained of 5 mm; after, the first ply of carbon mesh was applied and slightly pressed into the mortar. The textile was then wrapped around the specimens under slight tension, and the next layer of concrete was applied in parallel (Fig. 1(a)).
These processes of wrapping and applying the fine-grained concrete were repeated until the required number of layers was achieved. As a concrete cover, a final 5 mm layer of fine-grained concrete was applied. By using additional stencils, uniform total layer thicknesses of 20 mm could be realized (Fig. 1(b)). In addition, to prevent premature debonding failure of fibers, an overlap length of 50 cm was provided in the confined specimen. Furthermore, one layer of CFRP was applied to avoid a failure in the column head and foot (Fig. 1(c)).
Fig. 1.
(a) wrapping process; (b) uniform total layer thicknesses; (c) test setup
×
All tests were performed using a servo-hydraulic compression testing machine with a maximum load carrying capacity of 6,000 MPa. The tests were done on deformation-controlled mode, primarily to allow accurate analysis of the processes of load transfer to the reinforcing layer and failure. The test speed was set at 0.01 mm/s according to empirical values. Axial and lateral displacements were measured by external linear variable differential transducers (LVDT) mounted on two opposite sides of the specimen. The test setup is shown in Fig. 1(c).
3 Experimental Results
The evaluation of the test results showed that from confining the columns with TRC an significant increase in load-bearing capacity in relation to the unconfined specimen can be attained. With identical initial conditions, specimen with a lower core concrete compressive strength achieved a higher percentage increase in load-bearing capacity. An increase by an average of 67% compared to those with a higher concrete compressive strength class with 56% could be achieved. As expected, a dependence of the core concrete compressive strength of the component to be reinforced with the effectiveness of the reinforcement becomes apparent. The test results are showed in Table 3.
By confining a column with TRC the lateral expansion can be limited by the textile absorbing the axial loads as tensile stress in hoop direction. Once activated due to volume increase during load the textile provides confining pressure, which is continuously increased with gaining axial load. If the formation of lateral expansion is limited by a higher concrete compressive strength, this results in a lower as tensile stress in textile. A lower increase in load capacity is the consequence.
Table 3.
Experimental Results
Series
Label
Age
H
Fmax
Δ F
Strength
increase
COV
–
–
[d]
[cm]
[kN]
[kN]
[%]
[%]
C20
R-1
R-2
R-3
107
107
107
100.7
101.0
100.8
1125.2
1115.6
1080.5
1107.1
0
1,7
2L20-1
106
100.3
1892.4
2L20-2
2L20-3
106
106
100.4
100.4
1794,9
1856,1
1847.8
66,9
2.2
C55
R-1
294
100,4
1845,4
R-2
294
100,2
1969,8
1901,1
0
2,7
R-3
294
100,9
1888,2
2L20–1
295
100,6
2895,3
2L20–2
2L20–3
295
295
100.4
100.4
3216,4
2822,7
2978,1
56,7
5,7
The experimental results are shown visually in Fig. 2, while radial strain is negative and axial strain is positive. The increase in stiffness before macro cracking compared to the unreinforced specimen is noticeable. This is due to the increased cross-section and the much higher compressive strength of the fine-grained concrete.
Fig. 2.
Diagrams of the experimental results
×
4 Summary
In this paper, a part of a more extensive experimental program is showed. The retrofitting of RC columns with TRC increased the load-bearing capacity compared to the unreinforced specimens. The collected results show that with lower concrete compressive strength a higher strengthening effect can be observed. Also a typical behavior for TRC could be observed. After the axial compressive strength of the core concrete is reached, the textile can be gradually activated by reaching the tensile strength of the fine-grained concrete characterized by elongated cracks. The compressive load acting on the component can thus be converted into the textile. A renewed absorption of the load is then the consequence.
Acknowledgements
The authors would like to thank the University of Applied Sciences Leipzig (HTWK Leipzig) for the sponsorship of this research. Additionally, PAGEL Spezial-Beton GmbH & Co. KG is gratefully acknowledged for providing the fine-grained concrete mixture.
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