# Strengthening of Existing Bridge Structures for Shear and Bending with Carbon Textile-Reinforced Mortar

^{*}

## Abstract

**:**

## 1. Introduction

## 2. CTRM Layer for Bridge Deck Slabs

#### 2.1. Concept

_{tex}= 140 mm²/m and a special epoxy-resin that was complemented with carbon nanotubes (CNT) in order to increase the electrical conductivity (f

_{t,tex}≈ 2200 MPa; E

_{tex}≈ 215,000 MPa) [43]. In addition, a mortar with a maximum aggregate size of 4 mm that is both flowable and stable enough to gaplessly surround the textile reinforcement was used. The mortar was also required to make for a high conveyor capability, on one hand, and to allow the implementation of an inclination of the surface of at least 2.5%, on the other. The mechanical properties of the mortar were determined on prisms with a length of 160 mm and a width and height of 40 mm. The flexural tensile strength f

_{ct,flex}amounted to 10.4 MPa and the compressive strength f

_{cm,prism}to 65.9 MPa, respectively.

#### 2.2. Preparation of the Test Specimens

_{c,cyl}= 58 MPa (h

_{cyl}= 300 mm; D

_{cyl}= 150 mm) at time of tests). After four months of curing, the surface was pre-treated with shot peening to increase its roughness for a better bond in the interface between the existing RC structure and the additional CTRM layer. Afterwards, Smart-Deck was applied. In the first step, the carbon grid was secured in place by plastic dowels that were fixed to the RC slab. The mortar was then cast employing a feed hose that was connected to an automatic mixing unit where water was added to dry mortar stored in a silo. The RC slab was reinforced using different longitudinal reinforcement ratios in each third. The respective segments contained 5.24 cm²/m (using bars of Ø = 10 mm every 15 cm), 10.3 cm²/m (Ø14/15), and 25.13 cm²/m (Ø16/8) steel reinforcement (characteristic yield strength f

_{yk}≈ 500 MPa). No stirrups or other shear reinforcement elements were used. In order to investigate the strengthening effect of Smart-Deck, two segments were sawn out from the slab in the area of the lowest and highest reinforcement ratio, respectively (Figure 2b). Since no CTRM layer was applied on the edge areas, where two segments were located, each test on a strengthened slab segment had a non-strengthened reference test.

#### 2.3. Investigation of the Strengthening Effect

_{V,1}= 56% and η

_{V,2}= 23% (η = (V

_{u,TRC}− V

_{u,RC})/V

_{u,RC}), respectively. The increase in flexural capacity within the bending tests was significantly higher. The flexural strengthening rates were η

_{M,1}= 174% and η

_{M,2}= 91% (η = (M

_{u,CTRM}− M

_{u,RC})/M

_{u,RC}), respectively. Table 2 gives an overview of the test results.

## 3. Strengthening of Webs with a CTRM layer

#### 3.1. Concept and Preliminary Investigations

_{ag}= 2 mm and an unimpregnated carbon grid with an area of a

_{tex}= 55 mm²/m was used as the textile reinforcement material (Figure 8a). The mean tensile strength of the carbon grid in these tests was σ

_{t}= 1136 MPa (Figure 8d).

#### 3.2. Test Specimens and Test Setup

_{w}= 0.22%) which was strengthened with CTRM (CTRM-M-22-7). This member was compared to identical members without strengthening (M-22-7 and M-22-3) from a previous project [48]. Another member without shear reinforcement with CTRM-strengthening was produced (CTRM-I-O-5) which was also previously tested without strengthening (I-O-5) [47]. The test beams were subjected to 1.2 to 3.1 million load cycles using different peak and valley loads. In difference to tests by other authors [42], the strengthening layer was not anchored the compression or tension chord.

#### 3.3. Material Properties

_{cm,cyl}and the splitting tensile strength f

_{ct,split}were determined on cylinders with h = 300 mm and a diameter of d = 150 mm. The cube strength f

_{cm,cube}was determined on cubes with an edge length of 150 mm. The axial tensile strength f

_{ct,ax}was determined on drilled cores with h = 90 mm and d = 45 mm that were either drilled from the web of the beams or a flexural tensile test specimen. The mechanical properties of the shotcrete were determined on prisms with a length of 160 mm and a width and height of 40 mm. After testing the flexural tensile strength f

_{ct,flex}, the compressive strength f

_{cm,prism}was determined from the remaining prismatic samples.

_{yk}= 500 MPa). The mechanical properties of the shear reinforcement are given in Table 5. The beams were pre-stressed using two tendons, each consisting of three 0.6” (15.2 mm) strands of pre-stressing steel St1570/1770 with a cross-sectional area of 3 × 140 mm. The pre-stressing forces at the time of testing and the mechanical properties of the tendons are given in Table 6.

#### 3.4. Test Specimens and Test Setup

_{t}ranged from 1.1 to 2.4 mm. Prior to the application of the strengthening layer, the surface of the webs was cleaned and moistened (Figure 10a). The CTRM was applied layer by layer with three layers of shotcrete and two layers of carbon grid on each web (Figure 10b). The total thickness of the strengthening layer amounted to 25 mm. After strengthening, the shotcrete was moistened for another three days to ensure a sufficient hydration.

#### 3.5. Test Results

#### 3.5.1. Load Regime

_{max}(HL) was then chosen at 110% of the shear crack load to activate the stirrups and was increased if no significant damage occurred after 10

^{6}load cycles. The deflection of the test specimens was measured beneath the loading points by displacement transducers. A digital image correlation system was used to measure the shear crack growth dependent of the load cycles.

#### 3.5.2. Specimen CTRM-I-O-5

_{ult}= 233 kN, whereas the original specimen without CTRM had a remaining capacity of only V

_{ult}= 158 kN (Figure 11b).

#### 3.5.3. Specimen CTRM-M-22-7

^{6}load cycles which can be seen from the progression of the curve in Figure 12a. In contrast, the beam strengthened with CTRM did not exhibit any damage on the stirrups after 2 × 10

^{6}load cycles, after which the amplitude was increased further (Figure 12b). After the increase of the amplitude, some stirrups failed and the deflection grew moderately. Even then, the beam was able to sustain another 1.1 × 10

^{6}load cycles after which the test was aborted. This behavior indicates a considerable load transfer over the CTRM strengthening, relieving the existing stirrups.

^{6}load cycles (Figure 13a). On the other side, the strengthened specimen CTRM-M-22-7 did not show any signs of a progressive fatigue failure even after increasing the amplitude after 2.0 × 10

^{6}load cycles. The remaining capacity of the beam CTRM-M-22-7 amounted to V

_{ult}= 350 kN (Figure 13b). The remaining capacity of the original beam M-22-7 was not determined due to its considerable damage in the stirrups. However, another previous test beam M-22-3 with the same pre-stressing, but subjected to smaller highest loads, had a remaining capacity of V

_{ult}= 264 kN. It can, therefore, be seen that the CTRM strengthening had a considerable effect on the remaining shear capacity for the beams with shear reinforcement as well.

^{6}load cycles in correspondence with the failure of stirrups according to Figure 12a and the increase of deflections according to Figure 13a. For the strengthened specimen CTRM-M-22-7, the crack widths were measured during the first 2 × 10

^{6}load cycles (Figure 14b). The diagram, which is scaled down by one order of magnitude compared to Figure 14a, shows that the shear crack widths are considerably smaller ranging from 0.2 to 0.3 mm. Additionally, the crack widths do not increase exponentially as for the non-strengthened specimen, which illustrates the stabilizing effect of the CTRM-strengthening.

#### 3.5.4. Summary

- Although the strengthening layer was not anchored in the compression or tension chord, a significant strengthening effect was observed. This effect can be explained by the contribution of the horizontal rovings which are activated at crack opening.
- For the specimen without shear reinforcement, additional 180,000 load cycles could be sustained after shear crack formation which results in a much more ductile behavior in comparison to non-strengthened specimens.
- For the specimen with shear reinforcement a significant reduction of stirrup strains was observed, as well as significantly smaller shear crack widths. By this, a progressive fatigue failure was prevented by the CTRM-strengthening.
- A bond failure between old concrete and strengthening layer could not be observed in any of the tests as the surface was sufficiently roughened and cleaned prior to strengthening. However, if the surface is not prepared according to the applicable standards [49], bond failure might occur, neutralizing a potential strengthening effect.

## 4. Conclusions

## Acknowledgments

## Author Contributions

## Conflicts of Interest

## References

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**Figure 2.**(

**a**) Application of Smart-Deck; and (

**b**) the position of sawn segments in a demonstrator slab.

**Figure 3.**(

**a**) Detail of a cut surface; and (

**b**) a bridge loaded by a truck and the idealized load setup.

**Figure 6.**(

**a**) Load-deflection-curve of shear tests; and (

**b**) the load deflection curve of the bending tests.

**Figure 7.**(

**a**) Possible are of CTRM strengthening (image edited by the authors, “A71-Thalwassertalbruecke”, author: Störfix, licensed under CC BY-SA 3.0); and (

**b**) the application method.

**Figure 8.**(

**a**) Unimpregnated carbon textile; (

**b**) tensile test setup; (

**c**) pull-out of rovings after failure; and (

**d**) stress-strain relationships of tensile tests with an un-impregnated carbon grid.

**Figure 9.**(

**a**) Strengthened cross-section with shear reinforcement (CTRM-M-22-7); (

**b**) the strengthened cross-section without shear reinforcement (CTRM-I-O-5); and (

**c**) the longitudinal system.

**Figure 10.**(

**a**) Test specimen after shot blasting; (

**b**) the application of the textile reinforcement; and (

**c**) the application of shotcrete layers.

**Figure 11.**(

**a**) Comparison of vertical deflections under cyclic loading for tests without shear reinforcement; and (

**b**) the comparison of the remaining shear capacities of non-strengthened and strengthened specimens.

**Figure 13.**(

**a**) Comparison of vertical deflections under cyclic loading for tests with shear reinforcement; and (

**b**) the comparison of the remaining shear capacities of the non-strengthened and strengthened specimens.

**Figure 14.**Shear crack widths w according to digital image correlation for (

**a**) M-22-7 and (

**b**) CTRM-M-22-7.

Specimen | CTRM (mm²/m) | Rebar (cm²/m) | d_{s} (m) | a (m) |
---|---|---|---|---|

SD-K1-1 | 0 | 5.24 | 0.21 | 1.3 |

SD-K1-2 | 0 | 5.24 | 0.21 | 1.0 |

SD-K2-1 | 280 | 5.24 | 0.215 | 1.3 |

SD-K2-2 | 280 | 5.24 | 0.205 | 1.0 |

SD-K3-1 | 0 | 25.13 | 0.215 | 1.0 |

SD-K3-2 | 0 | 25.13 | 0.205 | 0.7 |

SD-K4-1 | 280 | 25.13 | 0.235 | 1.0 |

SD-K4-2 | 280 | 25.13 | 0.22 | 0.7 |

**Table 2.**Maximum bending/shear capacity M

_{max}/V

_{max}, maximum deflection w

_{max}, and strengthening degree η.

Specimen | CTRM (mm²/m) | w_{max} (mm) | M_{max} (kNm) | V_{max} (Kn) | η (%) |
---|---|---|---|---|---|

SD-K1-1 | 0 | 89 | 43.4 | - | - |

SD-K1-2 | 0 | 71 | 44.9 | - | - |

SD-K2-1 | 280 | 73 | 118.7 | - | 174 |

SD-K2-2 | 280 | 22 | 85.7 | - | 91 |

SD-K3-1 | 0 | 26 | - | 154 | - |

SD-K3-2 | 0 | 32 | - | 268.5 | - |

SD-K4-1 | 280 | 24 | - | 240.9 | 56 |

SD-K4-2 | 280 | 22 | - | 331.5 | 23 |

**Table 3.**Mean values of maximum textile stresses in the tensile tests for different textile materials and sprayed mortars.

Textile | Shotcrete; d_{ag} = 4 mm | SPCC; d_{ag} = 2 mm |
---|---|---|

carbon fiber + epoxy resin | 2397 MPa | 2928 MPa |

AR-glass fiber + epoxy resin | 1640 MPa | 2076 MPa |

carbon + styrol-butadien (type 1) | 935 MPa | 1198 MPa |

carbon + styrol-butadien (type 2) | 362 MPa | 276 MPa |

unimpregnated carbon | - | 1136 MPa |

Specimen | Concrete | Shot Mortar (SPCC) | |||||
---|---|---|---|---|---|---|---|

f_{cm,cyl} (MPa) | f_{cm,cube} (MPa) | f_{ct,ax} (MPa) | f_{ct,split} (MPa) | E_{cm} (MPa) | f_{cm,prism} (MPa) | f_{ct,flex} (MPa) | |

I-O-5 | 29.4 (6) | 34.9 (10) | 2.80 (10) | 2.54 (3) | 22,200 (6) | - | - |

CTRM-I-O-5 | 42.3 (6) | 47.0 (6) | 2.98 (13) | 3.44 (6) | 26,790 (6) | 53.8 (10) | 6.30 (5) |

M-22-3 | 35.3 (6) | 38.6 (9) | 2.68 (15) | 2.43 (4) | 24,833 (6) | - | - |

M-22-7 | 32.0 (7) | 35.4 (9) | 2.55 (15) | 2.59 (5) | 23,900 (7) | - | - |

CTRM-M-22-7 | 43.0 (6) | 47.2 (4) | 3.10 (14) | 3.25 (6) | 25,140 (6) | 44.6 (8) | 7.12 (4) |

Specimen | f_{y;0,2} (MPa) | f_{t} (MPa) | E_{s} (MPa) |
---|---|---|---|

M-22-3 | 587 | 626 | 200,777 |

M-22-7 | 587 | 626 | 200,777 |

CTRM-M-22-7 | 595 | 633 | 203,800 |

Specimen | P_{mt} (kN) | σ_{cp,mt} (MPa) | f_{p0,2} (MPa) | f_{pt} (MPa) | E_{p} (MPa) |
---|---|---|---|---|---|

I-O-5 | 320 | 1.78 | 1764 | 1950 | 190,000 |

CTRM-I-O-5 | 327 | 1.82 | 1764 | 1950 | 190,000 |

M-22-3 | 320 | 1.78 | 1764 | 1950 | 190,000 |

M-22-7 | 314 | 1.75 | 1764 | 1950 | 190,000 |

CTRM-M-22-7 | 329 | 1.83 | 1764 | 1950 | 190,000 |

Specimen | V_{crack} (kN) | Load Cycles × 10³ | V_{max} (kN) | V_{min} (kN) | ΔV (kN) | |
---|---|---|---|---|---|---|

N_{i} | ΣN_{i} | |||||

I-O-5 | 176 | 1000 | 1000 | 103 | 47 | 56 |

1011 | 2011 | 102 | 35 | 67 | ||

CTRM-I-O-5 | 188 | 1000 | 1000 | 141 | 79 | 62 |

180 | 1180 | 171 | 109 | 62 | ||

M-22-7 | 145 | 1853 | 1853 | 160 | 103 | 57 |

CTRM-M-22-7 | 185 | 2000 | 2000 | 204 | 147 | 60 |

1100 | 3100 | 204 | 118 | 86 |

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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**MDPI and ACS Style**

Herbrand, M.; Adam, V.; Classen, M.; Kueres, D.; Hegger, J.
Strengthening of Existing Bridge Structures for Shear and Bending with Carbon Textile-Reinforced Mortar. *Materials* **2017**, *10*, 1099.
https://doi.org/10.3390/ma10091099

**AMA Style**

Herbrand M, Adam V, Classen M, Kueres D, Hegger J.
Strengthening of Existing Bridge Structures for Shear and Bending with Carbon Textile-Reinforced Mortar. *Materials*. 2017; 10(9):1099.
https://doi.org/10.3390/ma10091099

**Chicago/Turabian Style**

Herbrand, Martin, Viviane Adam, Martin Classen, Dominik Kueres, and Josef Hegger.
2017. "Strengthening of Existing Bridge Structures for Shear and Bending with Carbon Textile-Reinforced Mortar" *Materials* 10, no. 9: 1099.
https://doi.org/10.3390/ma10091099