Method for reinforcing structural elements Technical Field

The present invention relates to a method for reinforcing structural elements, in particular for carrying out operations for the restoration, protection or structural stabilisation of buildings or structures in general. Background Art

It is currently known, in connection with the reinforcement of building structures, to cover a structural element (a pillar, a beam or other load-bearing element) with a high-strength flexible lining so as to provide a resistant outer casing which is able to prevent and oppose the collapse of the structural element, in particular in the event of seismic phenomena or overloads in general.

This lining forms, if there is tendency for the structural element to collapse, an additional reinforcing stage which opposes the collapse of the structural element, maintaining its structural function and integrity.

The abovementioned reinforcing linings perform their function only when the reinforced structure is subject to static/dynamic overloads (for example earth tremors) acting in addition to the loads present at the time of application of said linings.

In other words, the reinforcing operations mentioned above (and the system thus reinforced) remain inert until the stressed state of the structural elements being reinforced changes.

In many cases this "reactive" behaviour may be generally insufficient also because it takes effect with a delay - albeit very small - with respect to the overload and therefore still results in the reinforced structures being affected by deformations, which may also be significant in nature, or by initial Assuring.

Disclosure of the Invention

In this connection, the technical task forming the basis of the present invention is to propose a method for reinforcing structural elements which overcomes the abovementioned drawbacks of the prior art.

In particular, the object of the present invention is to provide a method for reinforcing structural elements which helps increase immediately the strength of the structural element, in particular even before the onset of overloads or seismic phenomena.

The technical task mentioned and the object specified are substantially achieved by a method for reinforcing structural elements, comprising the steps described in one or more of the accompanying claims.

Brief Description of the Drawings

Further characteristic features and advantages of the present invention will emerge more clearly from the description, provided by way of a non-limiting example, of a preferred, but not exclusive embodiment of a method for reinforcing structural elements, in accordance with the accompanying drawings in which:

- Figure 1 is a side view of a device for reinforcing structural elements, used for implementing the method according to the invention and in accordance with a first embodiment;

- Figure 2 is a top plan view of the device according to Figure 1 ;

- Figure 3 is a side view of the device according to Figure 1 in accordance with a second embodiment;

- Figure 4 is a top plan view of the device according to Figure 3.

Detailed Description of the Preferred Embodiments of the Invention

The idea forming the basis of the present invention is to reinforce a structural element (a pillar, a wall which may be load-bearing or not, a beam, a floor, etc.) by means of application of a reinforcing element suitably pre-loaded so that the pre-load of the reinforcing element helps modify the stressed state of the structural element in an advantageous manner for the strength thereof.

In particular, the application of the reinforcing element is intended to impart to the structural element an additional stress which opposes that normally acting on the structural element, so as to improve its capacity to withstand the loads.

The application of the reinforcing element may also be envisaged in other different situations, for example in order to keep under control the Assuring of a structural element or provide perimetral reinforcement (for example for an entire masonry building) or, more generally, to increase the structural strength (for example in view of a change in the intended use of a structure, resulting in greater operating loads).

A method for reinforcing the structural element in accordance with the present invention therefore comprises essentially the steps of subjecting the reinforcing element to pre-loading and stably fastening the pre-loaded reinforcing element to the structural element so as to transmit to the structural element a mechanical stress matching the pre-load of the reinforcing element.

In other words, the reinforcing element is pre-loaded (for example by means of screw, oil-hydraulic or other types of jacks) so as to produce in the reinforcing element a predetermined tensioned state. Then, the pre-loaded reinforcing element is stably applied to the structural element so that, by releasing the mechanical action previously generated by the tensioned state of the reinforcing element, the latter imparts to the structural element its tension thus stored.

The reinforcing element is at least partially made of resistant fibre, preferably in the form of flexible fabric, so as to allow the reinforcing element to adapt to surfaces of any shape, in particular also bends as in the case of arches or vaults.

The reinforcing element is arranged close to the surface of the structural element, said reinforcement is then impregnated with a special binding material so as to secure it stably to the structural element. In this connection, the reinforcing element 1 made of resistant fibre is at least partially permeable to the binding material so that it can be impregnated with the latter. This may be advantageously obtained by constructing the reinforcing element 1 in the aforementioned woven form, the porosity of which allows it to be passed through, and therefore impregnated, by the binding material.

In particular, both the reinforcing element and the structural element are at least partially impregnated with the said binding material so as to obtain stable gluing of the reinforcing element to the structural element at least on a preloaded portion of the reinforcing element.

Preferably, the said binding material is an epoxy resin.

By leaving a thin gap between the reinforcing element and the surface of the structural element to which it is applied it is possible to ensure efficient gluing since the binder which impregnates the reinforcing element also fills the gap and reaches the structural element, forming a solid connection.

The binder, once it has cured, absorbs and transmits the tangential forces between the reinforcing element and the structural element.

Preferably, moreover, the resistant fibre from which the reinforcing element is made consists of a high-strength material, in particular chosen from steel, carbon, glass, aramid, basalt, hemp or flax. These materials allow the transmission of high forces even in the case of small application surfaces.

The resistant fibre is a dry fibre, namely a fibre which has not been previously treated or soaked with materials intended as binders and/or reinforcing agents. In this way, the resistant fibre is highly prone to be impregnated effectively with the binding material applied at the moment of fixing of the reinforcing element to the structural element..

Preferably, fastening of the pre-loaded reinforcing element to the structural element is carried out while keeping the reinforcing element subject to the external pre-loading action. In particular, the pre-loading acting on the reinforcing element is preferably maintained for the entire duration needed for complete curing of the binder, and the pre-loading action is stopped only when the binder has cured sufficiently to transmit the tangential forces between the reinforcing element and the structural element. In this situation, in fact, when the external pre-loading action is no longer present on the reinforcing element, the latter may keep stored the elastic deformation energy resulting from the deformation previously imparted and may transfer to the structural element a tangential action corresponding to the stored deformation energy.

In greater detail, the application of the reinforcing element on the structural element consists of the following steps, performed in succession:

- stably fastening a first portion of the reinforcing element to the structural element, preferably arranging the first portion of the reinforcing element directly against the surface of the structural element;

- exercising a traction on a second portion of the reinforcing element so as to subject to traction the intermediate part of the reinforcing element included between the aforementioned first and second portions;

- impregnating the reinforcing element, after the pre-loading step, preferably at least in the said intermediate part thereof.

It is then sufficient to wait for complete curing of the reinforcing element in order to obtain the desired containing effect of the structural element.

Figures 1 to 4 show an example of application of the aforementioned reinforcing method, which is intended to reinforce a beam, and in particular show a device 10 used to carry out the method described above. The teachings which arise from this example of embodiment, however, may in any case be applied also to other situations, for example for reinforcing pillars or floors (for example in order to increase the strength in view of a change in intended use) or for operations intended to keep under control Assuring in external walls, internal walls, vaults, arches or other structural elements.

In accordance with the accompanying figures, 1 denotes the reinforcing element which in the specific example of embodiment is made of an average- density dry steel fibre.

The reinforcing element 1 is in the form of a strip or band having a main longitudinal dimension along which the containing forces are exerted on the structural element _, (denoted by the reference number 2), once applied thereon, for the purposes of gluing by means of an epoxy resin.

The device 10 comprises an anchoring plate 11 which keeps a first portion 3 (preferably a first end) of the reinforcing element 1 pressed against the surface 6 of the structural element 2. The anchoring plate 11 can be fastened removably to the structural element 2, for example using first threaded members 12 and dual- component epoxy resin, and therefore performs stably fixing of the said first portion 3 of the reinforcing element 1.

Preferably, the first portion 3 of the reinforcing element 1 is impregnated with epoxy resin 7 in order to improve the adhesion to the surface 6 of the structural element 2.

The device 1 also comprise a grip element 13 for stably gripping a second portion 4 of the reinforcing element 1 (preferably a second end). As can be seen in Figure 2, the grip element 13 has two parts 13a, 13b having the function of clamping between them the aforementioned second portion 4 of the reinforcing element 1. The two parts 13a, 13b can be clamped together by means of second threaded members 14.

The grip element 13 may also have two opposite eyelets 15 which can be engaged by respective guide pins 16 (for example threaded pins screwed into the structural element 2) which cause the grip element 13 to follow a predetermined movement path.

The device 1 also comprises a tensioning apparatus 17 connected to the grip element 13 so as to move it towards and away from the anchoring plate 1 1.

The tensioning apparatus 17, in the embodiment shown, is of the screw type and, in the specific case, comprises a base plate 18 supporting a threaded rod 19 which on one side is connected to the grip element 13 and on the other side supports a tightening nut 20.

The base plate 18 is fixed, in particular may be stably connected to the structural element 2 by means of third threaded members 21 and dual-component epoxy resin.

The threaded rod 19 is slidably inserted inside a guide channel 21 of the base plate and thus, by regulating the tightening nut 20, it is possible, via the threaded rod 19, to move the grip element 13 away from the anchoring plate 11.

The tightening nut 20 constitutes, therefore, the actuating member of the tensioning apparatus 17.

In accordance with different embodiments not shown, the tensioning apparatus 17 may be hydraulic, electric or generally motor-driven. By adjusting . the tightening nut 20 it is therefore possible to perform tensioning of the reinforcing element 1 until a desired pre-load is achieved.

The attainment of a desired level of tensile stress may be verified by means of use of a displacement transducer 22 which is fitted to the reinforcing element 1 and provides a visual indication of the degree of elongation achieved.

Then, by impregnating the intermediate part 5 (included between the two portions 3, 4) of the reinforcing element 1 with epoxy resin, complete gluing thereof to the structural element 2 is obtained (in the figures only the epoxy resin part optionally applied during fixing by means of the anchoring plate 11 is shown).

Once curing of the resin has taken place, and therefore when the system is stable, it is possible to perform transverse cutting of the reinforcing element 1 on one side along the edge of the anchoring plate 11 and on the other side along the left-hand edge of the grip element 13 used for connection to the tensioning apparatus. The anchoring plate 11 and the base plate 18 may be removed in order to allow, for example, the continuity of the operation for adjacent zones of the structure to be reinforced.

Figure 3 shows a different embodiment of the device 1.

This embodiment differs in that it has a load cell 23, arranged between the tensioning apparatus 17 and the grip element 13.

The load cell 23 has the function of showing the value of the traction force applied by the tensioning apparatus 17 on the grip element 13 and, therefore, on the reinforcing element 1.

By reading off the elongation value supplied by the displacement transducer 22 at the moment when the desired traction force is reached (shown in real time by the load cell 23), the application of successive reinforcing elements 1 may be performed also using by way of comparison only the devices 1 of the type without load cell 23.

All the subsequent identical repetitions of the procedures, in fact, may be performed using an apparatus without load cell, verifying merely the deformation produced on the reinforcing element 1 by tightening of the nut 20 and comparing it with the reading previously obtained. This is done not only in order to simplify the operations, but also to avoid prolonging the time involved and the need for costly cells during the whole of the time necessary for curing of the resin.

The use of the load cell 23 is moreover particularly advantageous since it may be used to determine, during the first application or during subsequent applications, the tension useful or suitable for solving a particular structural problem (for example reduction of a crack in the structural element) and to then apply the same tension (without having to measure it again with the load cell) during the subsequent applications.

In one example of embodiment the reinforcing element 1 has a length "LI " of 1800 millimetres and a width "L2" of 300 millimetres.

The present invention achieves the preset object, overcoming the drawbacks associated with the prior art.

The reinforcing element according to the present invention allows the structural element to which it is applied to be relieved of stress, thereby increasing the strength in respect of any overloads. By way of example, in the case of reinforcement of a beam made of (prestressed or ordinary) reinforced concrete, a calculated reduction in the tensile stress acting on the steel forming the original reinforcement and of the compressive stress acting on the concrete is obtained.

The use of an initially dry fabric made of fibre (carbon, aramid, glass, steel, basalt, hemp, flax, etc.) allows the same to be impregnated with the binder (epoxy resin) and therefore stably secured to the structure.

Once curing has been achieved, the force applied to the two ends of the fabric would then be released, thus immediately transferring tangentially, via the resin, the tensioning stresses applied previously to the fabric from the outside.

As a result a structural element with a behaviour different from the original behaviour is obtained, where the reinforcing element applied participates, immediately and actively, with a degree of co-operation which correspondingly increases the greater the tensional stress applied during pre-loading.

The advantages arising from the method are notable, allowing the reinforcement, restoration and stabilisation of elements which otherwise would probably be demolished.

In the case of masonry structures, which are the most vulnerable in the event of seismic phenomena, by using pre-tensioned reinforcing elements, as already described above, it is possible to control actively the fissures which are present and to which the pre-tensioned reinforcing elements have been applied, as well as prevent and/or oppose the dynamic forces since the system does not respond in a "reactive" manner as in the case of "ordinary" application of the composite materials, but instantly, precisely because of the "responsiveness" of the pre- tensioned fabrics.