Stelios Antoniou

Dr. Stelios Antoniou
Head of Structural/ Earthquake Engineering Department at Alfakat
Co-founder, Managing Director and R&D Director at SeismoSoft

Application

Concrete jacketing is probably the mostly used technique for the strengthening of RC members. It is constructed either with cast-in-place concrete or, more often, with shotcrete. The method involves the addition of a layer of reinforced concrete in the form of the jacket using longitudinal steel reinforcement and transverse steel ties outside the perimeter of the existing member (Figure 1).

figure-1b-beforefigure-1a-after

Figure 1: Reinforced concrete jackets: typical cross sections, before and after casting of the concrete*

The jacketing with cast-in-place concrete demands the installation of formwork around the existing column, on which the formwork is tied so that to withstand the poured concrete. The thickness of the jacket usually exceeds 10 cm, in order to allow the casting of the concrete without voids and gaps. On the contrary, shotcrete allows for jackets of thickness as low as 5cm, although typically the jackets are 7.5 cm thick or more, in order to allow for a cover of adequate thickness, the positioning of the longitudinal and transverse reinforcements, and some space between the new rebars and the existing member.

The preparation of the surface of the existing member is critical with jacketing. It is essential that the existing member has a clean, sound concrete base, in order to achieve good bonding conditions with the jacket. The connection of the new and the existing concrete is further enhanced with the roughening of the surface and the introduction of steel dowels (Figure 2).

Figure 2:Roughening of the surface of the existing member and introduction of dowels*

The new vertical steel bars and stirrups of the jacket are then installed according to the designed dimensions and diameters, paying particular attention to the good closing of the hoops. Since many times it is not possible to bend the hoops at 135o angles, due to the presence of the existing member and the small thickness of the jacket, very often welding is required, as in (Figure 3).

Figure 3: Closing of the stirrups through welding*

Special attention should be paid to the beam-column joint regions, where the jackets are generally extended. In order to splice the longitudinal rebars between adjacent floors, typically the creation of small holes in the concrete slab is required, either with a small demolition hammer or a portable electric drill. Vertical holes in the adjacent beams also need to be made, which can only be done with an electric drill, in order to avoid causing serious damage to the beam. Special care should be taken in the placement of the 4-5 stirrups at the level of the beam, since a series of holes should be made in order to pass through the parts of each individual stirrup. The different parts are then connected through welding (Figure 4).

Figure 4: Lap splicing in the jacket between floors and reinforcement arrangement in a beam-column joint*

Sometimes the new and the existing rebars are connected by welding employing U-shaped steel connectors or steel plates (Figure 5), and this method can be used as an alternative to the steel dowels. However, nowadays steel dowels are considered to be a better method, since the welding of the often corroded, existing reinforcement is unreliable, or even impossible.

Figure 5: Steel connectors between existing and new rebars*

Because of the significant increase in the stiffness of the new member with respect to the existing one, and in order to avoid stiffness discontinuities, the jackets need to cover the entire length of the member. This means that at the ground storey the column jackets should not stop at the ground floor level, but rather they should be extended until the upper surface of the footing, where the longitudinal rebars are anchored inside the existing footing with epoxy resins (Figure 6). In several cases this may lead to large scale excavations, especially in buildings that have at the ground level several weak columns that are closely located together.

Figure 6: Continuation of the jacket until the existing footing*

In a variation of the method, and when only the shear strength and the deformation capacity of the member are of concern, a selective interventions scheme may be applied, whereby the jacket (both concrete and reinforcement) could be terminated without being anchored in the beams or the slabs at its ends, leaving a gap of the order of 1-2cm. This method however, although effective, is more cumbersome and expensive with respect to similar alternatives (i.e. FRP wrapping) and is not so common in practical applications.

In cases where the construction of a closed (4-sided) jacket is not feasible (e.g. columns at the perimeter of buildings that are adjacent to other existing properties, or beams in locations without access to the upper floor), 3-sided jackets may be applied, provided that the jacket is well connected to the existing member through dowels or welding, and the stirrups are adequately closed or anchored (Figure 7). 2-sided and 1-sided jackets should be avoided and they are even not permitted by some Standards (e.g. Greek Interventions Code). This is because it is not possible to anchor the stirrups in a sufficient way and connect effectively the jacket with the existing member, hence the new member may not behave monolithically.

Figure 7:Closing the stirrups of the jacket inside the existing member*

It is noted that, when strengthening vertical members with jackets the sides of the existing member where the jacket is applied need to be fully uncovered for the passing of the reinforcement and the construction of the jacket. This requires the demolition and re-construction of several non-structural parts of the building, such infills, floors, tiles, suspended ceiling, doors or windows, and can cause increased costs, disruption in the operation of the building and architectural problems. All these factors should be considered, when deciding to strengthen a building with jackets, especially when the building is in operation.

Advantages and Disadvantages

With reinforced concrete jackets a considerable increase in the strength, both flexural and shear, and the vertical bearing capacity of the member can be achieved. Furthermore, there is a significant increase in the ductility and the deformation capacity of the member, through the confinement and the anti-buckling action of the new stirrups. The bearing capacity and the flexural strength of the member are enhanced, due to additional longitudinal reinforcement, whilst the improvement in the shear strength and the ductility are achieved through the additional transverse reinforcement.

The significant increase of the member’s strength with the introduction of jackets can be observed by means of a simple example with SeismoStruct [Seismosoft 2022b] of an existing member and the corresponding jacketed section. Typical member sizes and reinforcement patterns are employed, i.e. an existing 25/25 column with 4Ø18 rebars and Ø8/30 hoops is strengthened with a 10cm jacket with 8Ø20 and Ø10/10, as in Figure 4 9. The strength under bending increases from 25kNm to 320kNm, whilst the shear strength increases from 20kN to 235kN!

Furthermore, because of the increase in the section’s dimensions, an increase of the member stiffness is also achieved. For instance, in the example of Figure 9 the increase in the elastic section stiffness is again tenfold. This increase in the stiffness of the strengthened members of an existing building usually causes a decrease in the demand of the members that remain unstrengthened. However, in the typical cases of buildings of the 60s or 70s this decrease in the demand is mediocre, with respect to the considerable lack of resistance of the existing members and typically it is not sufficient, so that to leave these members unstrengthened. As a result, when RC jacketing is selected as the main method of retrofit, typically it requires the strengthening of all or almost all the vertical members of a building, at least at the lower floors, where the shear demand on the columns is higher. As a result, the use of jackets is advantageous in buildings that undergo radical refurbishment, when the cost for the damage on non-structural elements from the strengthening interventions is relatively small.

Figure 9:Comparison of the shear capacity for a typical C25/25 reinforced concrete column with its jacketed counterpart (a) existing column: 4Ø18mm, Ø8mm/30, materials S220 & C12/15: Shear Capacity Ø 20kN (b) jacket: 8Ø20mm, Ø10mm/10, materials S500 & C25/30: Shear Capacity Ø 235kN*

Furthermore, because of the increase in the section’s dimensions, an increase of the member stiffness is also achieved. For instance, in the example of Figure 9 the increase in the elastic section stiffness is again tenfold. This increase in the stiffness of the strengthened members of an existing building usually causes a decrease in the demand of the members that remain unstrengthened. However, in the typical cases of buildings of the 60s or 70s this decrease in the demand is mediocre, with respect to the considerable lack of resistance of the existing members and typically it is not sufficient, so that to leave these members unstrengthened. As a result, when RC jacketing is selected as the main method of retrofit, typically it requires the strengthening of all or almost all the vertical members of a building, at least at the lower floors, where the shear demand on the columns is higher. As a result, the use of jackets is advantageous in buildings that undergo radical refurbishment, when the cost for the damage on non-structural elements from the strengthening interventions is relatively small.

On the contrary in buildings in operation the disadvantages of jacketing all vertical members are very significant (often insurmountable), for several reasons: (i) generally it is a ‘dirty’ and disruptive method (Figure 10), (ii) the increased cost for the non-structural damage caused for the jacket construction, walls, plaster, ceilings, tiles, (iii) the architectural problems that the jackets can impose, e.g. jacketing a column adjacent to an opening would require the relocation, replacement or adaptation of the window or door.

Figure 10: Jacketing, especially when it is done with shotcrete, is a dirty and disruptive task*

One final disadvantage of jacketing that cannot be ignored is its increased construction cost, with respect to the most widely used alternative, FRP wrapping. All the same, it should be noted that RC jackets feature some significant advantages over FRP wraps, providing enhanced strength and stiffness in both bending and shear, contrary to the latter that in its usual application (FRP wraps with the fibres perpendicular to the axis of the member) increases only the shear strength, the deformability and the confinement, but not the bending capacity.

Design Issues: Modelling, Analysis and Checks

The modelling and the checking of the jacketed (non-uniform) sections in a structural model can be easily done, under the following simplifying assumptions:

  1. The jacketed member behaves monolithically, with full composite action between old and new concrete. Hence, the Euler–Bernoulli hypotheses that plane sections remain plane and normal to the member axis are also valid for the composite (existing concrete + jacket) section.
  2. The fact that axial load is applied to the old column alone is disregarded, and the full axial load is assumed to act on both the existing and the jacketed part of the section.
  3. If the two different concrete material cannot be accommodated by the structural analysis package, the concrete properties of the jacket are assumed to apply over the full section of the element.

The member flexural and shear capacities are then multiplied with some modification coefficients that take into account the interaction between the surfaces of the new and the existing concrete, and decrease the total (jacketed section)+(existing section) capacity of the member; for instance, according to EC8 the decrease of the total shear strength is 10% (VR* = 0.90VR, see EC8-Part 3, A.4.2.2.).

One final key issue in jackets is the ability of the section to transfer forces between the new and the old concrete, through the interface between the two concretes, dowels or welding. Since welding should generally be avoided due to possible corrosion in the existing reinforcement, a good strategy towards this end is to design the dowels between the new and the existing concrete to carry the entire shear force in the interface, ignoring the transfer between the interface, in order to be on the safe side.

References

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One Comment

  1. Sunil September 5, 2022 at 20:50

    Nice & informative write-up
    Thanks

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