What steps should you follow to strengthen a floor?
The first step before any intervention is to assess the condition of the structure : is it well preserved, and have the beams kept their mechanical properties ? These findings can be made with the help of a surveyor, to test the strength of the concrete and identify the quantity and quality of the reinforcement in the existing joists.
If it is difficult to check beforehand whether the roughness of the surface to be joined provides enough resistance, a connector should be fixed. Indeed, if there is slip between the components of the structure, the load-bearing capacity is reduced and the risk of deformation increases.
One way to reduce the natural tendency to slip is to pour the concrete in two clearly separate stages. This alone is not enough, however, to let the whole act together fully. Hence the importance of positioning a connector, which makes it possible to check the level of cohesion of the materials.
There are 4 surface classifications in terms of roughness. Very smooth, when cast against smooth formwork ; smooth, the most common, for a concrete screed with a vibrated surface ; rough, obtained artificially by mechanical processes ; and indented, which is prepared and then cast using profiled elements made for the purpose. For exposed brick or crumbly smoothing, the contribution should be considered nil from a safety point of view.
In addition, the strength and deformation properties of the connection affect how the composite element will behave. What matters most is the ductility of this connection, because it provides the best load-bearing and bending capacity of the structural assembly. It also influences the even distribution of all the connectors along the whole length of the structure. This shows how important it is to know how a connection behaves in order to design and correctly assess the structural reliability of the floor before any reinforcing slab is added.
The connection is most often made with lugged connectors, with anchors to the various elements depending on the materials ; this is to make installation easier.
Concrete and fibre slabs
New techniques have made it possible to create thin slabs that offer high strength. They are made of fibre-reinforced concrete. This material is now very often used to strengthen concrete and brick floors. It is known as FRC, the English acronym for Fiber Reinforced Concrete. A cement-based mix reinforced with various fibres, this material guarantees high resistance to compression as well as to tension. It is also ductile and withstands shear better than conventional concrete.
As these new materials are not yet classified, their fields of application are not precisely known. They are in any case already used to meet seismic standards and to stiffen thin floors with a limited load.
The effectiveness of such a rigid plane is optimal provided there is a degree of constraint on the existing structure at the double junction of beam and slab and of slab and wall. FRC designers frequently advise carrying out laborious preparation of the surface to be strengthened in order to reinforce brick and concrete slabs. They recommend, for example, dulling the substrate by mechanical abrasion, then cleaning it and starting the strengthening with a primer applied with a roller. Where the surface is not smooth in places, a metal connection is essential so that nothing is left in doubt.
What are the components ?
Floors without a screed : many floors have no upper screed. These are so-called hollow floors, for which a composite concrete slab must be built so that loads can be distributed and the structure meets the requirements of the seismic standards.
Floors with a screed : some floors have screeds that are relatively thin or lightly reinforced. In this kind of configuration, they must be reinforced to stiffen the whole plane and increase the load-bearing capacity.
Floors with low load capacity : for floors characterised by low load capacities, any possible increase in loads must be allowed for. A composite slab is then installed to increase the distance between the internal forces and improve the section's resistance to bending. The more the depth of the section is increased, the more the strength has to be increased.
The dimensions of the existing joists
Take the time to examine the dimensions of the joists already in place. Their width must be sufficient for the connector to be covered laterally by an adequate volume of concrete. This precaution must be taken over the whole embedment depth of the connector. The minimum width of the joist must therefore be 10 cm, or its maximum thickness 2 cm, if the floor has no existing slab. The same thickness requirement applies if the floor has a slab, but its width must be at least 8 cm. If the existing floors have smaller joists, no connector can be installed at all. The type of precast joists makes it easy to tell which case applies. If the floor is made of Bausta elements, there is no problem fixing a connector ; for Varese or Sap floors, which are small in size, it is quite different.
The nature of the existing joists
For existing concrete joists, the compressive strength must be tested. It must give values greater than or equal to Rck 20 MPa. The connector's resistance, its shear and the bending checks all depend on this. If the concrete has carbonated and the steel has then oxidised, it is impossible to position connectors.
The reinforcement of the existing joists
The strength of the lower steel bars of the existing floors to be reinforced must be tested. To do so, their characteristics in terms of diameter, steel type and number should be recorded. Tests carried out in a specialist laboratory make it possible to check the steel's ability to resist up to failure.
Which connectors should you use?
Several connectors are available, and the right one depends on the floor configuration and how the floor will be used. For example, a floor that has to carry heavy loads calls for a CTEM connector. It has a plate at its base that anchors onto the existing slab, giving the best possible mechanical performance.
For lighter applications, the VCEM connector may be suitable. It has a single screw with a threaded section that bites well into the concrete.
Finally, the CTC MINI connector is used to bond several very thin layers of high-performance concrete, in order to create composite slabs with a thickness of at least 20 mm.
Which type of concrete should you use?
Most often, structural concrete of at least class C 25/C 30 is used, with a thickness greater than 4 cm. Above all, take care never to exceed the level of the composite slab, and make sure the floor is properly dampened before the concrete is poured.
Lightweight structural concrete is recommended to reduce the load on the reinforced slab while keeping its optimal mechanical strength. This is the material used in new buildings, as it meets the standards required in any area with significant seismic risk.
The overall thickness of the reinforced slab must be greater than 1/25 of its length. This is a basic rule. A simple calculation is needed to determine the number of connectors to fix. They should be placed close together near the walls and spaced further apart towards the middle of the beam.
The floors must be properly propped before the concrete is poured. If the spaces below cannot be accessed, hangers should be used to suspend the floor.
Fibre-reinforced concrete, for its part, is used to keep the thickness of the work within a range of 20 to 30 mm and to reduce loads.
If there are distribution slabs that are not tied to the walls, or if there is no slab at all, they must either be tied to the walls or bars must be fixed with resin.
Do not forget to add welded mesh at mid-height of the slab. This mesh does not necessarily have to be tied to the slab with connectors.
Finally, it is important to provide for an insulating panel with the right properties of rigidity, strengthening, strength, and thermal and acoustic insulation.
The problem of brick failure
Some floors that are sensitive to significant deflection can bend to the point where the lower plate and the hollow floor block sag. In this case, the floor should be strengthened accordingly, then connectors fixed to the new slab to prevent this brick failure from happening again.
If the floor cannot be strengthened, for example because it is undersized, steel beams must be inserted inside it. Various solutions are possible, from outright demolition to repair using carbon fibre. The steel beams are then placed in place of rows of hollow floor. The beam is then connected to a composite slab with a CTF-type connector and finally properly reinforced with welded mesh.
We are here to help. If you have any questions, do not hesitate to contact us.