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

Table of Contents

General
Reinforced Concrete Jackets
New Reinforced Concrete Shear Walls
Steel Bracing
Damping Devices
Fire-Reinforced Polymers (FRPS)
Steel Plates and Steel Jackets
Seismic Isolation
Resins / Mortars
Foundation Strengthening

This post aims to present the most common strengthening techniques for the strengthening of a reinforced concrete building.

Before moving on to the presentation of the methods, some general points regarding the different retrofit techniques are put forward in the current section:

(i) The methods for seismic upgrading are not many, they are only 6-7 in total. In most practical applications it is usually apparent from the beginning that some of the methods cannot be applied, due to architectural, operational or geometrical restrictions, lack of knowledge, or unavailability of the appropriate equipment, e.g. base isolation cannot be implemented in buildings that do not have open spaces throughout their perimeter or in regions where the expertise is still limited. Hence, the designer in every building may in fact have to choose from a set of no more than 2 or 3 candidate methods. It is his/her job to consider and weigh the advantages and disadvantages of each method using engineering judgment, preliminary studies and analytical calculations, and choose the most appropriate one.

(ii) Special attention should be paid to the fact that, when designing strengthening interventions, not all methods and techniques can decrease the vulnerability of the building. Erroneous implementation can strengthen some parts of the building, simultaneously weakening other parts, actually increasing the overall risk. For instance, a concentration of large shear walls on one side of the building might increase, rather than decrease the demand on the vertical members of the other side.

(iii) Different methods have different advantages and disadvantages, and their effect on the global response, the strength, the stiffness, the ductility and the seismic demand may vary considerably, depending on the particular building configuration. There are no standard solutions and recipes that can be applied to any structural type.

(iv) Strengthening techniques can be categorised into two large groups. On the one hand, there are methods that are generally employed at the global level (i.e. considering the entire structure as a single entity) and typically serve to decrease the demand in existing members. The most representative examples in this category are the addition of new shear walls and base isolation. On the other hand, there are methods that are applied on the member level and are mainly used to upgrade the particular characteristics (e.g. strength and/or the ductility) of individual members, e.g. RC jackets or FRP wraps.

(v) Very often a combination of two or more from the available methods might be required. Usually a more ‘global’ method is first applied (e.g. shear walls or braces), and other methods are applied at a second stage to strengthen individual components or parts of the building that still need upgrading.

Concrete jacketing is probably the most widely 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 a jacket using longitudinal steel reinforcement and transverse steel ties outside the perimeter of the existing member.

The jacketing with cast-in-place concrete demands the installation of formwork around the existing column, on which the formwork is tied in order 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 5 cm.

Figure 1: Typical cross-sections reinforced concrete jackets*

Figure 2: Reinforced concrete jackets*

The preparation of the surface of the existing member is critical with jacketing. 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 3: 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 correct 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.

More details on Reinforced Concrete Jackets can be found here.

Shotcrete

The term ‘shotcrete’ refers to both the material and the construction method. The material is a concrete or a high-strength mortar, which is literally ‘shot’ into the forms. The method is the application of this material on site.

Strictly speaking shotcrete (or gunite or sprayed concrete, as it is also called) is not a repair or strengthening method for existing buildings. It is a way of placing and compacting concrete and it has numerous applications, other than retrofit. However, due to the restrictions imposed in existing buildings by their structural and non-structural components, cast-in-place concrete in the majority of cases is difficult, expensive or altogether impossible to apply, which makes shotcrete the most usual way of casting concrete in repair and retrofit applications. In fact, the use of shotcrete is so common when constructing RC jackets that the two terms are often used in an interchangeable fashion in strengthening applications.

More details on Shotcrete can be found here.

Figure 4: Application of shotcrete in buildings*

This method consists of the construction of new shear walls with large dimensions at selected locations in the building perimeter and/or in the interior of the building. The walls can have a very beneficial effect on the seismic performance of existing buildings, providing simultaneously a considerable increase in the strength, stiffness and ductility. One very important advantage of the method is the significant decrease in the demand on existing lightly reinforced members of the building, due to the large dimensions and very large stiffness of the new members.

A typical cross-section of a new shear wall added to an existing building is very similar to shear walls of new buildings, with pseudo-columns with closely spaced stirrups at the two edges, and a lightly reinforced web that is expected to sustain damage in a strong seismic event. The only significant difference is the large number of dowels that are employed for the connection of the new and the existing members, and the safe transfer of the seismic inertia forces from the existing building to the ground, through the new walls.

It is generally preferable that the wall encapsulates two columns of the existing building, to form strong and ductile jackets, which constitute the pseudo-columns at the edges of the new wall. The jackets can be constructed by cast-in-place concrete together with the wall web (employing formworks around the existing members, columns and beams) or separately using shotcrete.

Figure 5: New shear walls in existing RC buildings*

RC INFILLING

An alternative to the construction of new shear walls that encompass the columns and beams of the existing frame is RC infilling. With this method a reinforced concrete wall is constructed inside the RC panel and it is connected with the adjacent columns (to the left & right) and beams (up and bottom) with a series of strong dowels that are designed to undertake the inertia forces developed during the earthquake and ensure a monolithic connection with the existing frame.

With respect to the construction of new shear walls that encompass the existing frame members, RC infilling is significantly cheaper and much less disruptive. The main drawback of the method is related to the capacity of the existing members that surround the infill. In order to be able to transfer the shear forces and overturning moments from floor to floor, they need to possess a minimum capacity; otherwise, they will suffer significant local damage in the case of strong seismic events.

Figure 6: RC infilling [Chrysostomou et al. 2014, Poljanšek et al. 2014]

Steel bracing offers similar advantages to new shear walls, increasing the strength, the stiffness and the ductility of the building. The braces are directly fitted to the concrete frame, inside the existing bays. They contribute to the lateral resistance of the structure through the axial force developing in their inclined members. The diagonals are pinned to steel plates that are anchored at the corners of each concrete bay with epoxy resins. Similarly to the case of the new RC walls, the braces should be placed in symmetrical positions, so as to not introduce unwanted torsion in the building, and, if possible, reduce in-plane irregularities.

Since in the general case the braces are attached to existing unstrengthened concrete members of the building, the method is not suitable, when the beams and columns do not possess a minimum strength; if this is not the case the concrete elements can be strengthened with composite materials, or more often with jacketing.

Energy dissipation devices can be easily combined with steel braces, efficiently increasing the damping during the dynamic excitation. It is noted however that if dampers are employed, the steel braces should be designed so that they do not increase significantly the stiffness, otherwise the efficiency of the damping mechanisms, which require large-deformations to be cost-efficient, is compromised.

Figure 7: Steel braces*

Damping devices are used to reduce the amplitude of vibration, the deformations and consequently the demand imposed on the structural members by dissipating energy during large earthquake events. There are passive, active and hybrid dissipation systems.

Most of the systems (the viscous, viscoelastic, friction and metallic yield dampers) can be easily combined and integrated with steel bracing members, providing increased strength, stiffness, ductility and energy dissipation capacity. Alternatively, the dissipative devices can be installed independently from other strengthening interventions and contribute to the structural response by dissipating energy.

Figure 8: Retrofit of an RC frame employing steel braces with fluid viscous dampers [Staaleson Engineering, P.C. 2021]

Fibre Reinforced Polymer (FRP) composites comprise of fibres of high tensile strength within a polymer matrix such epoxy, vinylester or polyester thermosetting plastic, but most commonly epoxy resins. The fibres are usually made of carbon (CFRP), glass (GFRP), aramid (AFRP), or rarely basalt, although other fibres such as paper or wood or asbestos have been used in the past.
FRP materials in structural engineering are treated as additional reinforcement, the only difference being the initial strains that are present in the concrete and reinforcement, due to the dead load at the time of applying the FRP.

Due to their high tensile strength and low weight (compared to the conventional structural materials, and in particular steel), FRPs have become an important structural material for use in the construction industry as internal or more frequently external reinforcement.

Fibre-reinforced polymers can be used in the strengthening of existing buildings with variable techniques: FRP wrapping, FRP Laminates, near surface mounted (NSM) FRP reinforcement, FRP strings and lately sprayed-FRP. FRP wrapping is typically used, in order to increase the confinement, the ductility and the shear capacity of walls, columns and beams with the fibres placed in the direction of the hoops. FRP laminates are mainly used to increase the bending capacity of RC members, typically in beams or slabs. NSM FRP reinforcement is used for flexural strengthening, providing increased strength and stiffness, but it can also be used for shear strengthening. FRP strings are used as near surface reinforcement, and more often as fibre connector and anchorage of FRP wraps. Finally, sprayed FRP provides increased shear strengths and deformation capacities and can be used for the seismic strengthening of existing RC member.

Figure 9:  FRP Wraps*

Figure 10: FRP laminates*

Figure 11: FRP strings*

Single steel plates or straps bonded to the concrete members can improve their flexural strength, similarly to FRP laminates. Likewise, plates, straps and angles welded together to form a jacket can increase the shear strength, improve the behaviour of lap spices and provide ductility through confinement without significantly affecting the stiffness of the existing system, in a similar fashion to FRP wraps.

Strengthening of reinforced concrete members using external bonding of steel plates was one of the most popular methods and very common in retrofit applications some decades ago, however it gradually lost popularity to other more reliable and easier to use methods, in particular FRP wrapping and FRP laminates.

Figure 12: Steel plates*

Figure 13: Steel jacketing in a column*

Base isolation, also known as seismic isolation, is a state-of-the-art method that constitutes one of the most effective means of protecting a structure against earthquake forces. A collection of structural components, called the isolators, are used to decouple to a large extent the superstructure from the base (foundation or substructure) that rests on shaking ground, thus protecting the building’s integrity.

The main characteristics of a seismic isolation system are the limited stiffness at the isolators’ level, which leads to the significant period elongation of the structure to fundamental periods of up to 2.5 sec or more. This leads to a significant reduction in the acceleration passed to the superstructure, the inertia forces and the earthquake force demand. Consequently, the lateral deformations, and the interstorey drifts are considerably smaller, thus leading to light or very light damage to the structural and the non-structural components even in very large earthquake events.

The main concept behind using base isolation for retrofit is that, instead of strengthening the structural members to withstand the imposed seismic action (as is done with all the other methods), base isolation takes the opposite approach, that is to reduce the seismic demand instead of increasing the capacity. Since controlling the ground motion that is imposed to the structure is impossible, the structural protection is done by modifying the demand by preventing/reducing the motions being transferred to the superstructure from the foundation level.

Figure 14: Installation of a lead bearing (elastomeric) isolator [Wikipedia, 2021]

Epoxy resins and repair mortars are commonly used materials related to the repair and strengthening of reinforced concrete buildings. They are mostly used to repair localised damage, hence they do not constitute a method for the strengthening and upgrading of existing buildings. However, together with all the other methods they are employed for the repair of individual structural members.
The use of epoxy resins by injection is the most common solution for cracks repair. Compared to concrete, epoxy resins have very high compressive and tensile strength, and they are used to ensure the efficient transfer of strengths and the recovery of the structural rigidity, due to the strong adhesion between epoxy resins and concrete.
Repair mortars are used for the repair and restoration of damaged concrete sections. It is noted that one of the most common reasons for the disintegration of concrete is corrosion, for which reason repair mortars are commonly used together with corrosion inhibitors.

Figure 15: (a) Cracks repair with epoxy resins, (b) Repair with corrosion inhibitors and repair mortars*

The dynamic response of footings is a very complex problem requiring skill in Soil Mechanics, Foundation Engineering, Structural Dynamics and Soil Structure Interaction. Furthermore, strengthening interventions at the footings is a very disruptive and expensive work, which requires excavations in the entire building at the ground level, and usually the evacuation of the building. Moreover, in recent earthquakes there have been very few cases of failures in the foundation system, and these were mostly attributed to reasons irrelevant to the vibration and the structural response, e.g. soil liquefaction or slope stability.

For all these reasons, the strengthening of the foundations is difficult to justify technically and economically, with the exception of cases of very large deficiencies in the foundation system, such as the lack of reinforcement, or even the total lack of entire footings. Unfortunately, these cases are not rare in buildings constructed before 1980, due to poor workmanship and the absence of supervision. In such cases, the existing footings are strengthening either with the increase in their size with reinforced concrete jackets (usually with cast-in-place concrete) or with the construction of connecting beams and strip footings.

Figure 16: Serious structural deficiencies at the foundation system*

Figure 17: Foundation strengthening with (a) jackets and connecting beams, (b) strip footings*

Figure 18: Strengthening interventions at the foundation level can be very disruptive*

• Chrysostomou C., Kyriakides N., Kotronis P,. Georgiou E. (2014). RC infilling of existing RC structures for seismic retrofitting. 2nd European Conference on Earthquake Engineering and Seismology, Aug 2014, Istanbul, Turkey. HAL archives, HAL Id: hal-01080302.
• Poljanšek M., Taucer F., Ruiz J.M., Chrysostomou C., Kyriakides N., Onoufriou T., Roussis P., Kotronis P., Panagiotakos T., Kosmopoulos A. (2014). Seismic Retrofitting of RC Frames with RC Infilling (SERFIN Project). JRC Science and Policy Reports, Joint Research Center.
• Staaleson Engineering, P.C. (2021). Retrofit Steel Chevron Braced Frame with Fluid Viscous Dampers. Available at: https://www.staaleng.com/seismic_strength/default.html.
• Wikipedia (2021). Seismic base isolation. Last modified date April 27, 2021. Available at: https://en.wikipedia.org/wiki/Seismic_base_isolation
• *All the pictures, which are in the article and are not referenced, are courtesy of Alfakat (www.alfakat.gr), an official partner of Seismosoft.
• Structural Assessment, Strengthening & Retrofitting carried out using SeismoSoft Earthquake Engineering Software.