STRENGTHENING EXISTING CONCRETE STRUCTURES WITH HILTI’S TOOLKIT OF SOLUTIONS

However, these potential savings are strongly predicated upon the structural engineer’s ability to select and the construction industry’s ability to provide and install appropriate strengthening solutions that address identified local and / or global deficiencies. While most strengthening projects will usually incorporate multiple solutions, some are ruled out due to architectural, operational, or geometrical limitations, a lack of knowledge in design and / or execution, unavailability of appropriate equipment, which narrows the list of potential solutions. This selection is further influenced by the advantages and disadvantages accompanying these solutions and are not “silver bullets” miraculously resolving structural deficiencies. Adding another layer of complexity is the potential for flawed implementation of these solutions that may lead to strengthening a particular part of the structure but weakening another, illustrated by the following two examples, one at a local and the other at the global level:

• Local: thickening the slab with a concrete overlay but ignoring the additional loads on the supporting beams.

• Global: a high concentration of shear infill walls on one side of the structure increasing the load demand on the other side, achieving the opposite effect.

At a “local” level, deficiencies may include the individual concrete members such as beams, columns, slabs, walls, and foundations lacking sufficient resistance to preclude failure in tension, compression, bending, shear, punching shear, torsion, and other effects caused by new loading demands. Solutions for individual members include, among others:

• Concrete overlays or jackets as seen in Figure 1,

• Post-installed reinforcement,

• Steel jackets,

• Near surface mounted (NSM) or bonded plates,

• Fibre-reinforced polymer (FRP) wraps and strips,

• External post-tensioning.

Figure 1: An example of concrete overlay / jacketing in columns

Strengthening on a “global” level typically includes addressing issues related to the whole structure, such as seismic, fire, fatigue, and wind by introducing solutions such as:

• Shear infill walls as shown in Figure 2,

• Steel braces,

• Micro-piles,

• Base-isolation,

• Energy dissipation / damping devices.

Figure 2: Example of a shear-infill wall between columns

Assuming a building previously housing offices now changed to commercial use from a change in ownership, higher footfall will increase load on, say, the floor that must be resisted by all structural components – slabs, columns, and the foundations. Typically, after verification, an engineer may find that a flat slab may lack sufficient resistance in both bending and punching shear or, in some cases, only punching shear. Recall that in most design standards, such as Section 6.4 of EN 1992-1-1:2004 [2], the resistance of a concrete member in punching shear depends on the following five parameters:

• Concrete strength,

• Effective depth to the flexural reinforcement from the top of the compression fiber,

• Size of the support and the control perimeter

• The amount of longitudinal reinforcement

• The amount of punching shear reinforcement

Employing a range of potentially available “local”-level solutions to improve one or several of these parameters enhances punching shear resistance by varying amounts, yet this incurs a trade-off in terms of invasiveness, cost, availability, and other parameters. Some modifications may not be even feasible, such as increasing the concrete strength of an existing beam. Others, such as adding more supports through additional columns to reduce punching shear demands will require load transfer to the foundations. Therefore, the Hilti range of solutions to enhance parameters (1) to (5) is summarized in Table 1:

Table 1: Potential solutions to strengthen concrete deficient in punching shear

Directly increasing the amount of punching reinforcement results in a proportional increase of the punching resistance, and the solutions available presently in the industry are typically minimally invasive and will reduce disruption to other members. The use of all other solutions typically results in an under-proportional increase to the punching resistance, apart from concrete overlay, which is accompanied by its own trade-offs

In recent years, the development and sufficient maturity of post-installed anchoring technology have led its use in applications beyond steel-to-concrete fastenings and concrete-to-concrete connections. One use in strengthening is in concrete overlay where both bonded and mechanical fasteners function to reinforce the interface between the existing and new concrete.

Another use of the bonded anchor system in strengthening is in a recently developed Hilti strengthening solution “HIT-Punching Shear” which directly increases the resistance to punching shear loading of reinforcement concrete members, as illustrated in Figure 3, akin to cast-in stirrups.

Figure 3: Hilti HIT-RE 500 V4, HAS(-U) rods, and filling set used as post-installed punching shear reinforcement

This solution is installed in a similar way to a bonded anchor: i.e., drilling to a fixed embedment depth perpendicular to the concrete surface, thoroughly cleaning the debris from the boreholes, and injecting the mortar and then inserting the rods. Once the mortar cures, the nuts can be torqued up to a maximum specified value.

Note: Unless explicitly considered in design, drilling and cutting through the flexural reinforcement should be avoided wherever possible to prevent further weakening of the structure. If this cannot be avoided, for instance to facilitate drilling in densely reinforced areas, additional measures with the explicit agreement of the Engineer-in-Charge of design are required to compensate for a loss of flexural reinforcement.

The new Hilti HIT-Punching Shear strengthening solution consists of the following components:

The solution pairs the HIT-RE 500 V4 injection mortar with Hilti HAS series of threaded rods in sizes M12, M16, M20, and M24, each available in carbon- and stainless-steel for indoor or outdoor use. The steel elements are completed by the Hilti Filling Set that consists of a sealing and spherical washer, a nut, and an optional locknut, also available in both carbon- and stainless-steel for each rod diameter.

The versatility of this system covers the following parameters:

• Slab thickness between 200-1100 mm, with a minimum effective depth of 160 mm.

• Installation from top or bottom side of the slab with defined rod length ranges as function of the slab thickness and rod diameter.

• Concrete strength between C20/25 & C50/60.

• Dry or water-saturated concrete, and in waterfilled holes.

• Maximum short- and long-term temperatures of +40°C and +24°C, respectively.

• Members subjected to static and quasi-static loading.

Hilti’s cloud-based design software PROFIS Engineering includes a new dedicated module for assessing and strengthening concrete members deficient in punching shear that assists structural engineers when evaluating the resistance of existing members and strengthening them, thereby ensuring a safer and more efficient workflow. The new PROFIS Engineering Punching Shear Strengthening module enables:

• Selection between rectangular and circular column, wall ends, and wall corners on slabs and definition of its material properties & geometry.

• Verification of the existing concrete’s resistance to EN 1992-1-1:2004.

• Strengthening according to aBG Z.15-5.387 [5] in conjunction with DIN EN 1992-1-1/NA:2013, with a choice of four reinforcing diameters in carbon- or stainless-steel and a free input of the spacing and edge distances.

• Free manual input of the radial spacings and the number of strengthening elements in each perimeter.

• Generation of a comprehensive design report with all verifications, reinforcement detailing, and installation instructions.