To study the compression performance of TRM-strengthened columns with small eccentricities, a total of 9 reinforced concrete (RC) columns with end corbels were subjected to compression testing. The test parameters are as follows: the number of textile layers, the ratio of longitudinal reinforcement, and polyvinyl alcohol (PVA) short-cut fiber volume fraction. The experimental results indicated that, compared to the control, columns with three layers of textile exhibited an approximately 10.66% increase in the bearing capacity. However, the effect increased only slightly when the number of textile layers increased to 4. Besides, the effect was improved with the increase in the ratio of longitudinal reinforcement and PVA fiber volume fraction. Finally, based on laboratory tests and related research results, a model for calculating normal section bearing capacity of TRM-strengthened columns with small eccentricities was presented. A comparison of the theoretical and experimental data demonstrated the applicability of the proposed model.

Existing concrete structures often need to be repaired and reinforced due to various reasons, such as design flaws, construction mistakes, function changes, and natural disasters. Therefore, civil engineers have been committed to the research and development of strengthening materials. Fiber-reinforced polymer (FRP) is now widely used as a strengthening material (

In recent years, some new composite materials for strengthening RC structures have been proposed, including textile-reinforced concrete (TRC) (

Currently, many scholars have conducted experimental studies on the performance of RC beams and plates (

Most of the research cited above was devoted to the behavior of TRM-strengthened concrete columns under an axial load, while knowledge of reinforced columns under an eccentric load is lacking. The above analysis shows that the existing research does not involve the influence of the longitudinal reinforcement ratio or PVA fiber on the compression performance of strengthened columns. Since nearly all columns in practical engineering are subjected to a combination of axial load and bending moment, further research is required to explore the compression performance of eccentric compression columns strengthened with TRM. In view of these concerns, this paper conducts experimental research on the strengthening effect of TRM with different numbers of textile layers, ratio of longitudinal reinforcement and PVA short-cut fiber volume fractions. Previous similar experimental campaigns have been developed on this topic and there is a model for calculating the sectional force available in the literature (

There were 9 rectangular RC columns in this test, the overall length of which was 800 mm. The two end corbels had a cross section of 120 mm×250 mm and were 200 mm long. The tested specimens were subjected to eccentric compression with 35 mm of load eccentricity, and the parameters are shown in

Specimen parameters

Specimen number | The diameter of longitudinal reinforcement (mm) | Number of textile layers | Short-cut fiber |
---|---|---|---|

C0 | 10 | 0 | / |

C1 | 10 | 1 | / |

C2 | 10 | 2 | / |

C3 | 10 | 3 | / |

C4 | 10 | 4 | / |

C5 | 12 | 2 | / |

C6 | 14 | 2 | / |

C7 | 10 | 2 | 0.3% |

C8 | 10 | 2 | 0.6% |

Note: the short-cut fiber content is in accordance with volume fraction.

Test setup and steel bars configuration of specimen (units in mm).

The strength grade of the concrete was C40, and the mix proportion was shown in

Mix proportion of concrete

Component | Portland cement PII 52.5R | water | sand | stone | water reducer |
---|---|---|---|---|---|

Content (kg/m^{3} |
475 | 161 | 643 | 1181 | 2.85 |

The strengthening system consisted of a new type of hybrid material made out of textile embedded within fine-grained concrete as a matrix, and the low tensile strength of the matrix was compensated by using a high capacity textile. As shown in

Mechanical properties and geometric parameters of fiber yarns of textile

Fiber type | Number of filaments per yarn | Filament tensile strength (MPa) | Filament elastic modulus (GPa) | Filament ultimate strain (%) | Yarn tex (g/km) | Yarn density (g/cm^{3}) |
---|---|---|---|---|---|---|

Toray carbon (T700S) | 12k | 4660 | 231 | 2 | 801 | 1.78 |

E-glass | 4k | 3200 | 65 | 4.5 | 600 | 2.58 |

Textile material.

The mix proportion was provided in the literature (

Mix proportion of fine-grained concrete

Component | Portland cement PII 52.5R | Fly ash | Silica fume | Water | Silica sand (0–0.6 mm) | Silica sand (0.6–1.2 mm) | Super plasticizer |
---|---|---|---|---|---|---|---|

Content (kg/m^{3} |
475 | 168 | 35 | 262 | 460 | 920 | 9.1 |

The addition of PVA fiber to fine-grained concrete can improve not only its toughness (

Geometric and mechanical properties of PVA fiber

Type | Dtex | Length (mm) | Diameter (mm) | Tensile Strength (MPa) | Elongation (%) | Tensile modulus (GPa) | Density (g/cm^{3}) |
---|---|---|---|---|---|---|---|

Kuralon K-II Rec15 | 15 | 12 | 0.04 | 1600 | 6 | 40 | 1.3 |

Before the reinforcement, the surface of the test region of the RC column was chiseled within the height range of 400 mm. In order to prevent the occurrence of stress concentration in the column edges, a corner radius of 20 mm was then applied to the specimens. The whole reinforcing process of the specimen was as follows: First, clean and wet the surface of the column. Second, apply fine-grained concrete to the surface of the column, with a thickness of approximately 2 mm. Third, lay the textile horizontally along the test region of RC column with a lap length of 200 mm (the available anchorage length of the FRCM should exceed the minimum development length of 152 mm according to the ACI 549.4R-13 (

Strengthening process.

In this experiment, a 7,000 kN pressure tester was used for loading. The test setup is provided in

In this test, the specimens were measured under static loading conditions and positioned according to geometric alignment. Preloading was designed and implemented before the start of the test to not only eliminate the influence of the bearing offset but also ensure normal operation of the measuring instruments and the test equipment. A multistage loading was adopted for the test, and the loading rate was 10 kN/min. When the load was 90% of the theoretical bearing capacity, the load of each stage was increased by less than 5% of the limit load. After the completion of each stage, the loading was maintained for 10 minutes. The relevant data cannot be recorded until the readings of the test instruments are stable.

A summary of test results is shown in

Test results of specimens

Research factors | Specimen number | Ultimate bearing capacity (kN) | Increase rate of bearing capacity (decrease rate) | Failure mode |
---|---|---|---|---|

Number of textile layers | C0 | 580.0 | contrast column | first type |

C1 | 608.0 | 4.8% | first type | |

C2 | 621.4 | 7.1% | first type | |

C3 | 641.8 | 10.7% | second type | |

C4 | 642.5 | 10.8% | second type | |

Ratio of longitudinal reinforcement | C2 | 621.4 | contrast column | first type |

C5 | 624.4 | 0.5% | first type | |

C6 | 636.8 | 2.5% | first type | |

PVA short-cut fiber | C2 | 621.4 | contrast column | first type |

C7 | 623.5 | 0.3% | first type | |

C8 | 629.8 | 1.4% | first type |

Failure modes of columns.

After reinforcement, the failure of the column was delayed, and the ductility was enhanced. When the column was strengthened with 3 layers, the damage of the TRM occurred in the upper part of the column, and the transverse cracks on the tension side moved upward. The reason for this phenomenon was that the increase in the number of reinforcement layer improves the ring hoop action of TRM during the compression process. Therefore, the unreinforced part of the corbel was relatively weak, and the deformation was too large, causing damage to the upper part of the column and crushing the internal concrete. However, the ratio of longitudinal reinforcement and PVA short-cut fiber volume fraction had little influence on the failure mode of TRM-confined columns.

The load-strain curves of the representative specimens C0, C2 and C5 are shown in

Load-strain curves of TRM-strengthened columns: (a) C0; (b) C2; (c) C5.

As shown in

Load-midspan deformation curves under different conditions: (a) numbers of textile layers; (b) ratio of longitudinal reinforcement; (c) PVA short-cut fiber volume fraction.

The longitudinal reinforcement ratio of eccentrically loaded columns C2, C5, C6 were 1.01%, 1.45% and 1.97% respectively. As shown in

As shown in

The tensile stress of the fine-grained concrete and concrete is not considered.

The average strain in measurement distance of the column strengthened with the TRM basically satisfies the plane section assumption; the constraint of the textile of the rectangular section is not heterogeneous, and only the effective constraint is considered for the safety and convenience of calculation.

As the TRM reinforcement layer is thin, the increase in section thickness is neglected in order to simplify the calculation; there are effective bonds between the fine-grained concrete and textile materials, and no debonding failure occurs.

There is no relative slippage either between the concrete and steel bars or between the fine-grained concrete and textile.

The limitation of compressed elements’ transverse strains is achieved by FRP and TRM wrapping, so the basic principle for both systems was similar (

Equivalent circular cross section.

Considering the weakening of the textile layer due to the section shape and column corner and referring to the simplified method in reference (_{f}

where _{l}

_{e}

_{s}

_{f}

_{f}

_{f}_{f}

(_{e}

According to references (

where _{c}

_{g}

_{g}

(_{s}

The presence of the fiber bundles spacing will result in uneven longitudinal restraint. Compared with the continuous wrapping of the fiber sheet, the lateral confining strength is reduced to some extent, that is [

where

(_{f}

It is noted in the literature (

It is noted that the confinement of the textile layer around the concrete is similar to the hoop action of stirrups. Referring to the results of a concrete cylinder under three-directional pressure, a rectangular column is still subjected to the compressive stress around it. The compressive strength of the confined concrete _{c}

where _{c0}

The calculating diagram for the column strengthened with TRM is shown in

Calculating diagram of column strengthened with TRM.

where

_{s}

_{1} is the ratio of the stress value to the _{c}

_{1} is the ratio of the height of the compression zone in the equivalent rectangular stress block to the height of neutral axis assumed by the plane section assumption, and the value is 0.8 (

_{u}

_{y}

_{s}

_{0}, and _{b}

_{0} is the sectional effective height, _{0}
_{s}

_{i}

_{s}

Combined with the above formula, the theoretical bearing capacity value of the column can be calculated by the following steps:

For a given external eccentricity _{i}

Calculate the equivalent lateral confining strength _{l}

Use the _{l}_{c}

_{s}

In light of the equilibrium condition, the ultimate bearing capacity is evaluated by equations [

In this chapter, the effect of PVA fiber on the loading capacity of TRM-strengthened columns is very small, so its influence is not considered. According to the derived calculation formula,

Comparison of calculated and experimental values

Specimen number | Number of reinforcement layers | longitudinal bar diameter (mm) | Eccentricity (mm) | Calculated value (kN) | Experimental value (kN) | Relative error |
---|---|---|---|---|---|---|

C1 | 1 | 10 | 35 | 593.4 | 608.0 | 2.4% |

C2 | 2 | 10 | 35 | 656.7 | 621.4 | 5.7% |

C3 | 3 | 10 | 35 | 719.9 | 641.8 | 12.2% |

C5 | 2 | 12 | 35 | 690.5 | 624.4 | 10.6% |

C6 | 2 | 14 | 35 | 728.5 | 636.8 | 14.4% |

C7 | 2 | 10 | 35 | 656.7 | 623.5 | 5.3% |

C8 | 2 | 10 | 35 | 656.7 | 629.8 | 4.3% |

C-1H-16 [24] | 1 | 12 | 16 | 2096.0 | 1956.8 | 7.1% |

C-2H-16 | 2 | 12 | 16 | 2144.2 | 2043.65 | 4.9% |

C-1H-32 | 1 | 12 | 32 | 1714.6 | 1596.0 | 7.4% |

C-2H-32 | 2 | 12 | 32 | 1753.7 | 1812.2 | 3.2% |

This paper examined the viability of textile-reinforced mortar to upgrade RC columns under eccentric loading, and a test was designed to provide better insight into the effect of the number of textile layers, the ratio of longitudinal reinforcement and the PVA short-cut fiber volume fraction on the compression performance of small-eccentricity columns. In addition, this paper proposed a preliminary model for TRM-confined RC columns. Due to the small number of specimens, more tests are needed to improve the model in the future. Based on the results of the experimental study, the following conclusions were obtained:

As a result of strong restrictions, the TRM composites allow an increase in the energy absorption capacity of reinforced concrete columns under eccentric compression. Compared with the unconfined columns, the increasing rate of the carrying capacity of the confined specimens ranged between 4.83% and 10.78%.

The compression ability of concrete inside the column confined with TRM is enhanced, and an increase in the longitudinal reinforcement ratio has little influence on the strength. The ratio of the longitudinal bars is not the key factor to determine the development of cracks in TRM-strengthened concrete columns, so the deflection is reduced less.

The column confined by double strengthening layers that incorporate 0.6% PVA fibers exhibits an approximately 1.35% increase in the load carrying capacity over the unreinforced column.

The failure modes of the TRM-strengthened columns are dependent on the number of textile layers, while the ratio of longitudinal reinforcement and PVA short-cut fiber volume fractions are not influential. For single or double layer confined columns, the failure is due to partial stripping of the TRM from the internal concrete in the compression zone. When the reinforcement includes three layers, the damage of the TRM occurs in the upper part of the column and the degree of exfoliation is reduced.

Referring to the mechanical properties of a rectangular column confined by FRP, a model for calculating the bearing capacity of small-eccentricity columns strengthened with TRM has been presented. The proposed calculation model is shown to agree with the existing test results presented in this paper, showing the applicability of the finished model for TRM-strengthened concrete columns. In future research, the test of columns with different sizes will be carried out to improve the model proposed in this paper.

The authors gratefully acknowledge the financial support from the Program of the Fundamental Research Funds for the Central Universities (2017XKZD09). The experimental work described in this paper was conducted at the Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Civil Engineering in the China University of Mining and Technology. Helps during the testing from staffs and students at laboratory are greatly acknowledged.