Applications of FRP in reinforced concrete columns

The common application the fibre-reinforced polymer is that it is used in increasing the strength of the reinforced concrete columns that are in existence. It is presently popularly known that the ductility and strength of concrete compressive arrangements may be enhanced greatly by transverse wraps of the fibre-reinforced polymer. These easy to install, lightweight and non-corrosive wraps may be applied to improve columns damaged due to corrosion, retrofit seismically insufficient buildings and bridges and increase the weight of a load of low strength members (Cao, et al., 2018).

Products from FRP can attain better or similar reinforcement aim of commonly applied products made of metals such as bonded plates, pre-stressing tendons and reinforcing bars. Development attempts and application of products in FRP components are worldwide to address the various chances for concrete member reinforcements (Setia, 2018). Such attempts include;

1. Modified techniques of construction for improved utilization of strength features of FRP and lower the costs of construction.
2. Techniques of producing high volume to lower the costs of manufacturing.
3. Optimizing the mixture of resin matrix and fibre to make sure that it compacts maximally with Portland cement.

The usual relation amongst every product of FRP illustrated in this paper is the application of continuous fibre such as carbon, aramid and glass confined in the resin matrix, this is the glue that enables fibre to operate together as one element (Pour, et al., 2018). The applied resins are thermoplastic such as polyethene terephthalate and nylon or thermosets such as vinyl ester and polyester. Fibre-reinforced polymer components are distinguished from short fibres applied all over the world recently to strengthen cementitious products that are nonstructural called concrete. The techniques of production of producing continuous fibres combined resin matrix enable the FRP products to be tailored in that maximum concrete reinforcement is attained. The common manufacturing technique widely used now is the process of pultrusion (Vincent & Ozbakkaloglu, 2018).

An important study attempt for the previous 20 years has concentrated on the use of FRP products for the embodiment of the concretes and various behavioural concerns have been evaluated. A number of field uses have been recently implemented all over the world.  Following the present publication of design policy for FRP reinforced-concrete members, there has been a greater effect of FRP on the retrofit and repair companies. Although there are many major areas whereby it seems like the research fraternity have not come to an agreement, and such area is linked to FRP embodied concrete’s analytical modelling.  Number of research in the literature only look at the fully wrapped columns using FRP (Ferrotto, et al., 2018).

Accordingly, the existing design regulations for wrapped columns using FRP, for instance, the   Fib (2), TR 55(3) and ACI 440.2R-08 (8) are maximized to approximate the abilities of FRP partially wrapped specimens. In this research, TR55 (3) and ACI 440.2R-08 (1) does not give data on the effect of confinement of columns of concrete partially wrapping FRP. However, Fib (2) proposes a reduction feature to consider the impact of columns partially wrapped. The research by Fib (2) applies an assumption, given that the impact of steel ties confinement in reinforced concrete column to examining the FRP column effectiveness when partially wrapped (Pour, et al., 2018). Hence there is no experimental or theoretical explanation on FRP-confinement in partial concrete. Following this, a research study in this area was to contrast the effectiveness of confining the FRP columns in partial wrappings to the columns having full FRP wrappings. The similar quantity of FRP was put in wrappings in the same columns of the concrete using distinct wrapping arrangements to attain a maximized wrapping design (Pimanmas & Saleem, 2018).

Manufacturing techniques of FRP

The confining technique of FRP concrete was brought for various situations even for the partial and full confinement. (Huang, et al., 2018). Where a circular column of concrete is wrapped horizontally using FRP all over its perimeter, the FRP jackets release lateral pressure that confines the whole column. Numerous research are done aiming to examine the estimation of the abilities and behaviours of the FRP fully wrapped columns. The spread of the pressure confined is presumed to be the same along the axial axis and inside the cross section of the circular columns (Bai, et al., 2017). Among the existing research, the model is applied in this research is to establish the compressive strengths for FRP full wrapped columns as follows;

That is regards to fco’ together with  fcc’ respectively being the compressive strength of unconfined concrete along with the confined concrete. Whereas fl represents the efficacy pressure for confinement.

In that, E_f  represents the FRP’s elastic modulus while d represents the section’s diameter of the column, ?_fe represents the FRP’s real rapture strain taking note of the hoop direction and t represents the jacket FRP’s nominal thickness. This model is preferred as it offers a reliable accuracy with the simplified form. This simplified model is utilized in order to determine a simple and new strain model as shown in the sections that follow. The model of strain applied while calculating the compressive axial strength from the confined concrete is;

In that, ?_cc depicts an ultimate axial confined concrete tension, ?_co represents the axial tension when stress is at peak of the concrete that is not confined, f_fe is the real FRP’s rupture strength and k is equal 7.6 being the proportioning factor.

As mentioned earlier, the columns of the partially wrapped concrete with FRP have been justified experimentally to raise their ductility and strength. The FRP partial wrapped concrete columns have low efficiency in nature as compared to columns with full wrappings because both the unconfined and confined areas are available. The same approach was used to find out the confining pressure that is effective on the core of the concrete (Pan, et al., 2017). The efficient confinement pressure was applied properly in the area of the concrete core where there is the full production of the confining pressure as a result of the arching activity. In such instance, an effective confinement coefficient (k_e) was developed in order to hold account the partial wrapping as shown below;

Challenges and opportunities in the field

A_c  together with A_e  respectively are the cross-sectional regions and regions of effective concrete core confinement while s is the actual space between the double FRP bands. Accordingly, the strength in compression of the partial wrapped concrete FRP columns with FRP are ascertained as;

K_e is approximated based on illustrated in the below equation and equation 4 depicts the confinement pressure equivalent from FRP, thought to be distributed equally down the column’s longitudinal axis.

An overall 21 cylinders of confined FRP concrete along with 3 unconfined control test examples were developed and put to test beneath monatomic loading. The specimen of the concrete cylinders had a height of 300mm and diameter of 150mm. Every specimen were cast from the similar bunch of concrete containing 54MPa compressive strength in 28 days.

The experiment schedule contains 3 cylinder types so as to able to examine the effectiveness of the confinement between the fully and partially wrapping arrangements when it comes to the maximization of the wrapping arrangement (Ribeiro, et al., 2018).  The specimens notations involve three parts: one shows the kind of the confining FRP product, where “C” and “G” in a respective manner represents CFRP and GFRP. The second segment can be represented by letters “P”, “R” or “F” indicating the name of the sub-class, that is, “p” for the partially wrapped group, “R” is the reference group and “F” represents the fully wrapped specimens. The last segment of notation in these specimen category is the number indicating the total layers in an FRP (Feng, et al., 2018). Information about wrapping arrangements and specimens are shown in the table below:

The FRP loop strains bands were estimated using 3 strain gages having a 5mm gage length. They were placed at the central specimen height and spread uniformly far from the fully wrapped specimens’ overlaps. Inside the specimens’ partial wrappings, 3 gages strains were symmetrically bonded on a tie band while 3 others were bonded in the specimen’s mid-height on a cover band. The specimen’s axial strain was calculated using the longitudinal compressor (Li, et al., 2018). The linear variable differential transformer was put on the compresso-meter’s top ring in order to estimate the axial strain. This instrument is shown in the figure below;

The specimen’s compression tests were done by use of Denison machine of 500 tons testing capacity. Plaster of great strength was used to cover the specimen to make sure that there was complete touch between the specimen and the loading plate. Calibration was keenly conducted to make sure that specimens got mounted at the test machine’s middle position. Initially, every specimen got filled, 30% of the unconfined specimen’s capacity to determine their alignments. If in any case, it was necessary, specimens were loaded again after unloading and realignment (Yu, et al., 2018). Deflection was regulated at a rate of 0.5 mm per minute while tests were being conducted. The readings of the strain gages, LVDT and load were documented using an information logging system.

Effectiveness of fully and partially wrapped FRP columns in compression testing

As usual, fully wrapped specimen containing GF2 and CF3 FRP was unsuccessful by FRP rupture at the central height. The specimens’ failure face when fully wrapped were perceived at around 45⁰ inclination. Meanwhile, specimens partially wrapped, that is, GP40 and CP60 indicated a number of fractures on the face of the concrete at a stress stage which is equivalent to the unconfined concrete’s strength (Huang, et al., 2018). The concrete in the middle of the FRP band close to the specimen’s outer face began to crush when the FRP bands had at the time confined the concrete core. Correspondingly, fractures on the surface of the concrete came up while there was an increase in the load applied. Upon the strain reaching at some high extent the concrete in the middle of the FRP bands split whereas the concrete underneath the core as well as the FRP bands remained confined. There was an explosive failure caused by the FRP rupture at the centre height (Rong & Shi, 2018).

The failure face angle in regards to the specimens fully and partially wrapped was varied greatly. The failure face was perceived at the space in the FRP bands’ middle. This failure face variation relies on the FRP bands toughness as well as the wrapping arrangements. The axial confined concrete’s strain is high as compared to the strength of the unconfined concrete, the failure face of 45⁰ could initially have occasionally happened in the cores of the concrete. However, the FRP bands combated the during high strain levels. Where the FRP bands are not strong enough to prevent fracturing, the failure face occurs at around 45⁰.On the hand, the FRP bands toughness in CP60 specimen had sufficient strength as it rehabilitated the failure face. It is very crucial to indicate that the FRP bands toughness impacts on the tangent modulus concrete confined with FRP. Tangent modulus with less value leads to the collapse of the column stability because the strength of the unconfined concrete is too much (Mansouri, et al., 2018).

Additionally, specimens having maximized wrapping arrangements that are not uniform showed a distinct failure criterion from the rest. Where the amount of strain is similar to the unconfined concrete strength level, the concrete remained confined by cover bands and FRP tie bands. As the loading process continues, the slanting cover band strains as well as the tie bands were about to be similar. The failure modes of such specimens are therefore the same as the specimens fully wrapped. The specimen not uniformly wrapped was unsuccessful due to simultaneous FRP rupture at the cover bands as well as the tie bands at the centre of the height (Moretti & Arvanitopoulos, 2018).

Stress-strain correspondence with regards to the specimens already undergone a test are grouped in two major kinds depending on the curves’ shapes of the stress-strain that includes descending branch types and ascending branch types. Where concrete confined FRP column is strongly confined, its strain and compressive strength have greatly increased as compared to the ones of unconfined concrete. However, confined concretes having descending types of stress-strain curves describe the concretes’ stress at the final strain under the unconfined concretes’ compressive strengths. CFRP is developed to represent the ascending type whereas GFRP wrapped specimens are developed to act as a descending branch group (Li, et al., 2018).

Specimen’s stress-strain curves wrapped with two equal layers of GFRP are shown in diagram IV. The specimens wrapped using two equal FRP layers possessed similar stress-strain curves during early levels of loading and encountered small variations during the late test stage. The specimens wrapped using similar FRP quantity, hereby, GP40 and GF2 possessed stress-strain curves similar to the descending branch group whereas the specimens stress-strain curves, that is, GP31 remained the same after attaining the strength required of unconfined concrete and then raised to failure (Thermou & Hajirasouliha, 2018). The specimen’s axial stress GF2 attained the unconfined concrete strength amounting to 54MPa before being held constant till the FRP rupture failure indicated in diagram 4a. The specimen’s GF2 strain and the medium compressive strength of concrete respectively had 0.92% and 57MPa. Specimens GP40 however attained a maximum low stress of 53MPa than the GF2 specimens, they possessed a maximum greater axial strain of 1.18% as compared to the previous specimens. The specimen’s GP40 axial strain raised by 21.31% than the ones for GF2 specimens as it is evident in diagram 4b. Meanwhile, GF31 specimens attained both the axial strain of 1.02% and a maximally greater strength of 60MPa than GF2 specimen as shown in diagram 4c.

Specimens wrapped using 3 equal FRP layers possessed identical stress-strain curves although showed a small variation in their axial toughness during the full process of loading as indicated in diagram 5. CF3 specimens attained the medium axial maximum strain and stress of 2.84% and 122MPa respectively as shown in diagram 5a. The CF3 specimens which were wrapped partially again possessed a low compression strength but greater axial strength than other CF3 specimens. As evident in diagram 5b, CP60 was unsuccessful during the medium axial strain of 3.25% and compressive strength of 116 MPa (Ma, et al., 2018). The CP60 specimen’s axial strength raised by 14% as compared to CF3 specimens. For purposes of comparison of the various wrapping arrangements effectiveness, five specimen’s stress-strain curves were drawn as illustrated in diagram 5e. Referring to this diagram, it is clear that the specimens wrapped partially encountered a maximally greater strain and maximally lower stress than the CF3 specimens.

Conclusion

This paper similar quantity of used FRP in every type of specimens, however, using distinct wrapping arrangements to ensure the proper examination of the effectiveness of the confinement between partially and fully wrapped and suggested a wrapping arrangement which was not uniform for the concrete confined with FRP. The outcome shown in this paper are summarized in the following manner:

The specimens heavily confined with FRP, that is, CP60, CP42, CF3 and CP51, specimens not uniformly and partially wrapped offers a greater axial strength than the other specimens that are wrapped fully.

Those specimens falling in the descending branch category and the specimens wrapped partially contain a diminished compressive strength with a greater strain than the corresponding specimens which are wrapped fully. On the contrary, the specimens which are not wrapped uniformly encountered a greater axial strain and compressive strength as compared to other specimens that had full wrappings (Wang, et al., 2018).

The arrangements with partial wrappings vary the specimen’s modes of failure. Depending on the strength of FRP jackets, the surfaces ‘angle during failure reduce significantly. The real strain FRP jacket rupture is distinct for all wrapping arrangements. The efficiency feature of the strain in most arrangements having fully wrapped FRP is higher compared to other arrangements wrapped partially although is smaller as compared to the arrangements not uniformly wrapped. Lastly, this research examined and gave a number of suggestions on the application of various wrapping arrangements.

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