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Recently, steel fibre reinforced polymers (SFRP) sheets have been introduced for the repair and rehabilitation of concrete structures. Few researchers studied the behaviour of the concrete columns wrapped with SFRP sheets; however, several critical parameters such as the cost and ductility effectiveness of the SFRP wrapped concrete columns have been lightly addressed. Thus, the main objective of this paper is to study the cost and ductility effectiveness of SFRP wrapped concrete columns and compare the results with the conventionally used carbon FRP (CFRP) wrapped concrete columns. In addition, an analytical procedure to predict the cost effectiveness of SFRP wrapped concrete columns is also suggested, from which, a parametric study was conducted. The parametric study investigated the effect of the concrete strength, the number of SFRP layers, and the size and slenderness effects on the cost effectiveness of the concrete columns wrapped with SFRP sheets. The results from the cost and ductility effectiveness study indicated that the SFRP wrapped concrete columns showed enhanced performance over the CFRP wrapped concrete columns. The suggested analytical procedure proved to be a reliable and accurate method to predict the cost effectiveness parameter of SFRP wrapped concrete columns. The parametric study showed the significant impact of the investigated parameters on the cost effectiveness of concrete columns wrapped with SFRP sheets.

Strengthening of reinforced concrete (RC) columns using fibre reinforced polymers (FRP) is one of the very efficient methods to enhance the strength and ductility of the concrete. The FRP types commonly used for confinement applications are carbon FRP (CFRP), glass FRP (GFRP), aramid FRP (AFRP), and steel FRP (SFRP). There is an ample amount of information available in the literature concerning the behaviour of RC columns wrapped with CFRP, GFRP and AFRP sheet (a few are listed: [

Based on experimental data and research, the behaviour of SFRP wrapped concrete columns subjected to normal room temperature, freeze-thaw, humidity and prolonged high temperature was superior to columns wrapped with the conventionally used CFRP sheets [

Limited information on design procedures related to SFRP wrapped concrete columns is available in the literature [

The experimental program was divided into two categories of columns, the first category was designed to investigate the cost and ductility effectiveness of large-scale concrete columns, and the second category was designed to investigate the cost effectiveness of concrete columns with varying slenderness ratio. The first category consisted of 18-large scale concrete columns with dimensions of 300 mm in diameter and 1200 mm in height. The columns were divided into five groups (A, B, C, D, and E), which varied according to the column type (unwrapped, CFRP wrapped columns and SFRP wrapped columns), column longitudinal reinforcement ratio (non-reinforced and reinforced), environmental exposure conditions (room temperature of +22 °C and freeze-thaw cycling of +34 °C to −34 °C) and the fibre orientation of the FRP sheet (circumferential: 0° and longitudinal/circumferential: 90°/0°). The addition of an inner longitudinal FRP layer was to investigate the durability performance of the concrete. Also, the inner longitudinal FRP layer was intended to regulate the crack distribution and reduce the stress concentration on the outer circumferential FRP layer to increase the strain efficiency of the FRP sheet. A summary of the test matrix of the large-scale columns in the first category is provided in

Test matrix of the large-scale columns wrapped with CFRP and SFRP sheets, with their respective control specimens.

Group | FRP Type | FRP Orientation * | Exposure | Column ID |
---|---|---|---|---|

A | Unwrapped | – | 22 °C | NR-CT-RT |

CFRP | 0° | 22 °C | NR-CFRP-RT | |

SFRP | 0° | 22 °C | NR-SFRP-RT | |

B | Unwrapped | – | −34 to +34 °C | NR-CT-EE |

CFRP | 0° | −34 to +34 °C | NR-CFRP-EE | |

SFRP | 0° | −34 to +34 °C | NR-SFRP-EE | |

C | Unwrapped | – | 22 °C | R-CT-RT |

CFRP | 0° | 22 °C | R-CFRP-RT | |

CFRP | 90°/0° | 22 °C | R-CFRP-HV-RT | |

SFRP | 0° | 22 °C | R-SFRP-RT | |

SFRP | 90°/0° | 22 °C | R-SFRP-HV-RT | |

D | Unwrapped | – | −34 to +34 °C | R-CT-EE |

CFRP | 0° | −34 to +34 °C | R-CFRP-EE | |

CFRP | 90°/0° | −34 to +34 °C | R-CFRP-HV-EE | |

SFRP | 0° | −34 to +34 °C | R-SFRP-EE | |

SFRP | 90°/0° | −34 to +34 °C | R-SFRP-HV-EE | |

E | CFRP | 0° | −34 to +34 °C | R-CFRP-AEE |

SFRP | 0° | −34 to +34 °C | R-SFRP-AEE |

* 0°: fibres oriented in the hoop direction; 90°: fibres oriented in the longitudinal direction.

The specimen ID shown in

The second category of columns consisted of 34-circular unreinforced concrete specimens each measuring 150 mm in diameter with varying heights of 300, 600 and 900 mm. The dimensions of the specimens were selected in order to obtain aspect ratios (

The specimen ID shown in

Test matrix of the aspect ratio columns wrapped with SFRP sheets, along with their respective control specimens.

Aspect Ratio ( |
Diameter, |
Height, |
FRP Type | Number of Specimens | Column ID |
---|---|---|---|---|---|

2 | 150 | 300 | Unwrapped | 3 | AR-2-CT-(1/2/3) |

SFRP | 3 | AR-2-SFRP-(1/2/3) | |||

4 | 150 | 600 | Unwrapped | 3 | AR-4-CT-(1/2/3) |

SFRP | 3 | AR-4-SFRP-(1/2/3) | |||

6 | 150 | 900 | Unwrapped | 3 | AR-6-CT-(1/2/3) |

SFRP | 3 | AR-6-SFRP-(1/2/3) |

Test matrix of the aspect ratio columns wrapped with CFRP sheets, along with their respective control specimens [

Aspect Ratio ( |
Diameter, |
Height, |
FRP Type | Number of Specimens | Column ID |
---|---|---|---|---|---|

2 | 150 | 300 | Unwrapped | 3 | AR-2-CT-(1/2/3) |

CFRP | 3 | AR-2-CFRP-(1/2/3) | |||

4 | 150 | 600 | Unwrapped | 1 | AR-4-CT-(1/2/3) |

CFRP | 3 | AR-4-CFRP-(1/2/3) | |||

6 | 150 | 900 | Unwrapped | 3 | AR-6-CT-(1/2/3) |

CFRP | 3 | AR-6-CFRP-(1/2/3) |

It is important to note that the columns were wrapped such that the stiffness in the fibre direction of the SFRP and the CFRP wrapped concrete columns were similar for comparison purposes. Wrapping the concrete columns with one layer of SFRP sheet has almost an equivalent stiffness to the columns wrapped with one layer of the CFRP sheet (the stiffness _{CFRP} = 24.92 MN and _{SFRP} = 25.05 MN, where ^{2}/mm. The first two digits (3×2) indicate the hardwire cord type. The 3×2 cord is made by twisting 5 individual wire filaments together—3 straight filaments wrapped by 2 filaments. The third digit (20) indicates the tape density, in this case 20 WPI (wires per inch). The final digit (12) indicates the width of the sheet; in this case, the sheets were shipped in a 12 inch (305 mm) width. According to the manufacturer, the ultimate tensile strength, modulus of elasticity and strain at failure are 986 MPa, 66,100 MPa, and 1.5%, respectively [

One of the methods to evaluate the effectiveness of the FRP sheet is to consider the cost effectiveness parameter. The cost effectiveness parameter is a function of the strength gain relative to the total cost associated with the construction and the FRP wrapping of the concrete column. Incorporating the cost parameter in the evaluation process is an important criterion for the acceptance of the SFRP sheet as an effective material for concrete enhancement. Thus, the cost effectiveness of the CFRP and the SFRP sheets are presented and compared.

The cost effectiveness parameter is defined through a strength efficiency scale, _{ff}

As the cost of the concrete, reinforcing steel, epoxy, and transportation are equivalent for all the columns, it is reasonable to ignore these costs for the calculation of the cost effectiveness parameter. Thus, the only varying cost parameters used in Equation (1) is the cost of the confining material and the labour costs.

Based on the 2011 market price (at the time this research was conducted), the cost of the SFRP and the CFRP sheets were 25 $/m^{2} and 45 $/m^{2}, respectively [

In order to calculate the total cost of the FRP confined concrete column, an appropriate labour hourly rate has to be determined. According to a recent study performed by Abdelrahman and El-Hacha [

In 2011, the Government of Canada published the construction union wage rates for all construction professions across the various provinces in Canada [

The percentage increase in strength and the cost effectiveness parameters of the large-scale CFRP and SFRP wrapped concrete columns are presented in

In order to determine the effect of strengthening the RC columns with two layers of SFRP sheets on the cost effectiveness parameter, the results were compared with the RC columns wrapped using one layer of the SFRP sheet. Strengthening the RC columns with two layers of the SFRP sheet having one sheet with the fibres in the longitudinal direction and one sheet with the fibres in the transverse direction (R-SFRP-HV-RT and R-SFRP-HV-EE) had an insignificant effect in terms of the strength enhancement (less than 15% difference), when compared to the RC columns wrapped with one layer of the SFRP sheet (R-SFRP-RT and R-SFRP-EE). This result is expected since the longitudinal layer of the SFRP sheet does not contribute to the axial stiffness of the columns and as a consequence, no significant strength gain is recognized due to the addition of this layer. However, the inclusion of an additional SFRP sheet has reduced the cost effectiveness parameter of the RC columns R-SFRP-HV-RT and R-SFRP-HV-EE strengthened with two layers of SFRP sheet by 40% and 33%, when compared to the RC columns R-SFRP-RT and R-SFRP-EE wrapped with one layer of the SFRP sheet, respectively. Although the percentage increase in the strength of the columns wrapped with one layer and two layers of SFRP sheets were similar, strengthening the columns with an additional layer caused an increase in the total cost of the columns wrapped with two layers of SFRP sheets. The increased cost caused a reduction in the cost effectiveness parameter.

The effect of strengthening the RC column with SFRP sheet, after being exposed to severe environmental condition (R-SFRP-AEE), on the cost effectiveness parameter was determined by comparing the results with the RC columns wrapped with SFRP sheet before being environmentally exposed (R-SFRP-EE and R-SFRP-HV-EE). The column R-SFRP-AEE wrapped with one layer of the SFRP sheet after being subjected to environmental exposure had an enhanced cost effectiveness parameter by 77% and 163%, over the columns R-SFRP-EE and R-SFRP-HV-EE, respectively, strengthened before being subjected to environmental conditions.

The analysis of the CFRP confined concrete columns summarized in

In order to determine the effect of environmental exposure on the CFRP wrapped concrete columns, a comparison between exposed and non-exposed columns was conducted. The CFRP wrapped RC columns subjected to room temperature (NR-CFRP-RT and R-CFRP-HV-RT) had higher cost effectiveness parameter than its corresponding columns exposed to environmental conditions (NR-CFRP-EE and R-CFRP-HV-EE). The RC columns NR-CFRP-RT and R-CFRP-HV-RT had higher cost effectiveness parameter by 34% and 82%, respectively, when compared to the conditioned RC columns NR-CFRP-EE and R-CFRP-HV-EE. The results presented in

The effect of wrapping the RC column with CFRP after being exposed to severe environmental conditions (R-CFRP-AEE) was investigated by comparing the results with the CFRP wrapped RC columns before being environmentally exposed (R-CFRP-EE and R-CFRP-HV-EE). The results show that column R-CFRP-AEE wrapped with one layer of CFRP sheet had a cost effectiveness parameter that is 1.6 and 12 times higher than the cost effectiveness parameters of columns R-CFRP-EE and R-CFRP-HV-EE, respectively.

The comparison between the cost effectiveness parameter of the columns wrapped with CFRP and SFRP is summarized in

Percentage increase in the strength of the SFRP and CFRP strengthened concrete columns.

Cost effectiveness of the SFRP and CFRP strengthened concrete columns.

The analysis of the results for varying slenderness ratios of 2, 4, and 6 of RC columns wrapped with CFRP and SFRP sheets, in terms of the percentage increase in strength and the cost effectiveness parameter are summarized in

In order to determine the efficiency of the FRP sheet in enhancing the cost effectiveness parameter, a comparison between the columns wrapped with CFRP and SFRP sheet with varying slenderness ratios was performed. The results presented in

Percentage increase in the strength of the aspect ratio columns wrapped with SFRP and CFRP sheets.

Cost effectiveness of the aspect ratio columns wrapped with SFRP and CFRP sheets.

Another method to evaluate the effectiveness of the various types of FRP sheets to confine RC columns is to consider the ductility effectiveness parameter. Similar to the cost effectiveness, the ductility effectiveness parameter is a function of the ductility gain relative to the total cost associated with the construction and the FRP confinement of the concrete column. The ductility of the columns was measured as the total area under the stress-strain curve up to the failure load of the experimentally tested SFRP or CFRP wrapped concrete columns. The failure load is defined as the load corresponding to the rupture of the confining FRP sheet. The ductility effectiveness parameter is very important when considering strengthening the columns subjected to seismic or eccentric loading. Thus, in this section, the ductility effectiveness of the columns confined with SFRP and CFRP sheets are evaluated and compared.

The ductility effectiveness is defined through a ductility efficiency scale, _{ff}

In the comparison of the ductility effectiveness parameter, only the labour cost and the cost of the CFRP/SFRP sheets are considered due to the reasons already mentioned above. It is important to note that in this section, only the ductility effectiveness parameter of the large-scale columns is presented. As previously mentioned, the data for the aspect ratio of the columns wrapped with CFRP sheets were tested by Bisby and Stratford [

The results of the ductility effectiveness study, in terms of the percentage increase in the ductility and the ductility effectiveness parameter for the large-scale SFRP and CFRP wrapped RC columns are summarized in

The efficiency of the FRP sheets in enhancing the ductility effectiveness of the concrete columns is determined by comparing the ductility effectiveness parameter between the concrete columns wrapped with CFRP and SFRP sheets. It is important to mention that during the experimental testing, it was noted that the SFRP wrapped non-reinforced concrete column subjected to environmental exposure (NR-SFRP-EE) experienced premature failure as an accidental eccentricity was applied from the loading plates of the testing machine. Thus, if this column was to be ignored, the results from

Percentage increase in the ductility of the SFRP and CFRP wrapped concrete columns.

Ductility effectiveness of the SFRP and CFRP wrapped concrete columns.

One of the most critical components in the design process is to determine the cost effectiveness parameter. The cost effectiveness parameter provides information to the designer regarding the effectiveness of the confinement material, relative to its strength and the total cost involved. However, there is no information available to the designer in the literature or in the current design codes/guidelines to predict the cost effectiveness parameter. Thus, one of the objectives of this investigation presented in this section is to develop a procedure to analytically predict the cost effectiveness of concrete columns wrapped with SFRP sheets.

In order to predict the cost effectiveness parameter, the initial step is to predict the ultimate confined concrete strength of columns wrapped with SFRP sheets. Recently, Abdelrahman and El-Hacha [

The accuracy of the proposed procedure to analytically predict the cost effectiveness of the SFRP sheet can be validated by comparing the analytical results with the experimental data.

Cost effectiveness parameter from the analytical and experimental data.

The parametric study included in this section involves investigating several factors that affect the cost effectiveness of the SFRP confined concrete columns. The parameters investigated were the concrete strength, number of SFRP layers, size and slenderness effects. In order to determine the cost effectiveness of the SFRP confined concrete columns, Equation (1) was used. The numerator of this equation can be determined experimentally or through analytical equations. The denominator of Equation (1) is based on estimating the total cost involved with the construction and the confinement of SFRP concrete columns. As mentioned previously, the only varying parameters associated with the total costs are the material cost of the SFRP and the labour cost. These costs are dependent on the size of the column and the number of SFRP layers.

Summary of the material, labour and the total cost of the SFRP confined concrete columns.

Diameter, |
Height, |
No. of SFRP layer(s) | Labour Hours | Total Area (m^{2}) |
Material Cost ($) | Labour Cost ($) | Total Cost ($) |
---|---|---|---|---|---|---|---|

150 | 300 | 1 | 1.00 | 0.17 | 4.28 | 33.00 | 6.78 |

2 | 1.50 | 0.31 | 7.82 | 57.32 | 8.21 | ||

3 | 2.00 | 0.45 | 11.35 | 77.35 | 8.90 | ||

150 | 600 | 1 | 1.25 | 0.34 | 8.57 | 41.25 | 49.82 |

2 | 1.75 | 0.63 | 15.64 | 57.75 | 73.39 | ||

3 | 2.25 | 0.91 | 74.25 | 74.25 | 96.96 | ||

150 | 900 | 1 | 1.50 | 0.51 | 12.85 | 49.50 | 62.35 |

2 | 2.00 | 0.94 | 23.46 | 66.00 | 89.46 | ||

3 | 2.50 | 1.36 | 34.06 | 82.50 | 116.56 | ||

150 | 600 | 1 | 1.25 | 0.34 | 8.57 | 41.25 | 49.82 |

200 | 800 | 1 | 1.75 | 0.58 | 14.57 | 57.75 | 72.32 |

300 | 1200 | 1 | 2.00 | 1.25 | 31.27 | 66.00 | 97.27 |

In order to understand the slenderness effects on the cost effectiveness parameter of the SFRP confined concrete columns, the slenderness ratio was varied between 2, 4, and 6. In addition, for each slenderness ratio, the concrete strength was varied between 20, 30, and 40 MPa. According to

The effect of the slenderness ratio on the cost effectiveness parameter from

Cost effectiveness of the SFRP confined concrete columns with varying slenderness ratios and concrete strengths.

Experimental research [

The analysis obtained from increasing the number of SFRP layers and the slenderness ratio, along with varying concrete strengths is presented in

A general trend can be observed from

Cost effectiveness of SFRP confined concrete columns with 20 MPa concrete strengths and varying numbers of SFRP layers and slenderness ratios.

Cost effectiveness of SFRP confined concrete columns with 30 MPa concrete strength and varying numbers of SFRP layers and slenderness ratios.

Cost effectiveness of SFRP confined concrete columns with 40 MPa concrete strengths and varying numbers of SFRP layers and slenderness ratios.

The increase in the number of SFRP layers directly enhances the confinement pressure. The enhanced confinement pressure leads to higher strength, thus increasing the cost effectiveness parameter of the SFRP confined concrete column. It should also be noted that the percentage increase in the cost effectiveness parameter decreases with the increase in the number of SFRP layers. Despite the fact that the concrete strength is increased as the number of SFRP layers increases, the lower percentage increase in the cost effectiveness parameter is achieved because the total cost involved in the construction of the SFRP confined concrete column also increases.

One of the major factors that affect the cost effectiveness parameter is the size effects of the SFRP wrapped concrete columns. The column sizes included in this study were 150 mm × 500 mm, 200 mm × 800 mm, and 300 mm × 1200 mm. The column sizes were selected to maintain a constant aspect ratio of 4 for comparison purposes. The main purpose of this investigation was to determine the size effects along with varying concrete strength of 20, 30, and 40 MPa, on the cost effectiveness parameter of concrete columns wrapped with SFRP sheets. The results of the size effect study are summarized in

The results clearly indicate that within the same size group, columns with lower concrete strengths achieved higher cost effectiveness parameter due to aforementioned reasons. The columns with 20 MPa concrete strengths can achieve higher cost effectiveness parameters by up to 28% and 42% over the columns with 30 and 40 MPa concrete strengths, respectively. A general trend can be observed regarding the fact that while maintaining a constant concrete strength, an increase in the size of the columns significantly reduces the cost effectiveness parameter of the SFRP wrapped concrete columns. The results in

The small scale-specimens are mainly affected by the restraining influence of the end bearing plates, which leads to non-homogeneities and produces results that are not representative of the large-scale columns. Thus, the size effect is an important parameter to be investigated in relation to the cost effectiveness parameter. The size of the specimen significantly affects the confinement pressure applied to the concrete. As the size of the specimen increases, the confinement pressure decreases resulting in lower strength enhancement. In addition, increasing the size of the specimens involve higher costs. The combined factors of lower strength enhancement with higher total costs leads to lower cost effectiveness as the size of the column increases.

Cost effectiveness of SFRP confined concrete columns with varying column sizes and concrete strengths.

A cost and ductility effectiveness study was conducted on concrete columns wrapped with CFRP and SFRP sheets experimentally tested under uniaxial compression loading. An analytical procedure was also proposed to predict the cost effectiveness parameter of concrete columns wrapped with the SFRP sheets. The effect of several parameters such as the concrete strength, the number of SFRP layers, and the size and the slenderness of the SFRP wrapped concrete columns were analytically investigated.

Based on the current study, the cost and ductility effectiveness investigation showed that the SFRP wrapped columns were always superior to the columns wrapped with CFRP sheets. The results presented in this paper highlight the importance of cost considerations, while determining the overall efficiency of the FRP sheet for confining the concrete column. The strength and ductility enhancement of various fibre orientation of the FRP sheet (0° for the circumferential direction and 90° for the longitudinal direction) can be tremendous. However, when the cost criteria are considered, the overall beneficial value of the strengthening system can be negligible. Thus, an optimum design for certain FRP strengthening configurations can be attained using cost and ductility effectiveness analysis.

Comparison between the cost effectiveness parameters determined experimentally with those analytically predicted showed good agreement. The proposed method provides a new tool for designers to investigate the cost effectiveness parameter of SFRP wrapped concrete columns, allowing for optimum and efficient strengthening designs. However, further investigation is required to develop an analytical procedure to predict the ductility effectiveness of the SFRP wrapped concrete columns. Despite the wide database regarding FRP wrapped concrete columns; there is a lack of a reliable and accurate models to predict the axial strain of SFRP confined concrete columns. The proposal of a critically needed model is an initial step towards understanding the ductility effectiveness behaviour of SFRP wrapped concrete columns.

The authors would like to express their gratitude to Hardwire for providing the SFRP sheets, Sika Canada for providing the CFRP sheets and the epoxy adhesive, and Lafarge Canada for providing the ready-mix concrete. The authors are also grateful to the University of Calgary for the financial support to this research project.

The authors contributed equally to this work. The authors designed the experimental program, analyzed the data, developed the analytical model, and wrote the manuscript.

The authors declare no conflict of interest.

A brief section of the data on the construction union wages rates [

The procedure proposed to analytically predict the cost effectiveness of the SFRP sheet is as follows:

Determine the mechanical properties of the concrete and the SFRP sheets to be used in the design.

Determine the confinement pressure (_{l}_{f}_{SFRP}), the modulus of elasticity of the FRP sheet (_{f}_{g}

Calculate the reduction factor (

Determine the ultimate confined concrete strength of the SFRP wrapped column (_{l}

Determine the percentage strength increase of the SFRP wrapped concrete column, with respect to the unwrapped concrete column.

Determine the total cost involved with the construction and the FRP confinement of the concrete column.

Calculate the cost effectiveness parameter (_{ff}

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