GB1560477A

GB1560477A - Repair and/or strengthening of concrete structural members

Info

Publication number: GB1560477A
Application number: GB2995175A
Authority: GB
Original assignee: University of Salford
Current assignee: University of Salford
Priority date: 1976-10-07
Filing date: 1976-10-07
Publication date: 1980-02-06

Description

1(54) REPAIR AND/OR STRENGTHENING OF CONCRETE STRUCTURAL MEMBERS (71) We, UNIVERSITY OF SALFORD, a Corporate Body, established under Royal Charter, of Salford, M5 4WT, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to concrete structural members and more particularly to the repair and/or strengthening of concrete structural members. The term concrete members includes plain, reinforced, partially and fully pre-stressed members of concrete.

Concrete members or structures such as beams, slabs, columns and frames can suffer from a number of defects which may be inherent in the member or which may arise as a consequence of external influences. For example such members may have undergone severe deformation which manifests itself in the form of excessive deflection and/or cracking up to or above the permissible values of deflection and/or width of crack. This defect may occur as a result of the application of loads greater than the working load, loads of long duration, fatigue loading or accidental impact loads. Concrete members may also lose their original strength and stiffness due for example to weathering and/or chemical action on constituent materials, generally concrete and steel, loss of prestress or loss of bond between reinforcement and concrete.Furthermore in some cases a concrete member is to be strengthened due, for example, to a change of use of the structure of which the member forms a part.

According to the present invention there is provided a method of repairing and/or strengthening a concrete member, incorporated in a structure, comprising bonding a preformed layer of fibre reinforced cement to the member by adhesive or by additional concrete. The layer of fibre reinforced cement may be bonded by adhesive directly on to the surfaces of the member and/or th layer may be bonded to a part or all of the surfaces of additional concrete either along with or after the additional concrete has been applied to the member. If desired, reinforcement may be provided in the additional concrete.

The layer of fibre reinforced cement is preformed to suit the shape of the member to which it is to be applied. According to design requirements a single area of the surface of a member may be covered by a layer of fibre reinforced cement or a plurality of such areas may be so covered.

Where improvement of the strength and stiffness of the member is required the fibre reinforced cement layer may comprise high elastic modulus fibres of, for example asbestos or glass, or a combination of high elastic modulus fibres with other fibres. Low elastic modulus fibres, such as fibres of polypropylene or a polyamide, may be used in the fibre reinforced cement layer to improve impact resistance of the concrete member.

The fibre reinforced cement layer containing high modulus fibres has higher flexural and direct tensile strength and in many instances greater impact and fire resistance compared with cement paste, mortar or concrete. The layer therefore provides a form of surface reinforcement which helps in restoring the original strength and/ or flexural rigidity of a concrete member or in resisting further deterioration in strength or deformation which would take place due to continued application of an existing load or due to the application of additional loading.

For example in the case of flexural members such as beams preformed fibre reinforced cement sheets can be bonded to those surfaces subjected to tensile stresses, and in many instances can be extended to cover partly or fully the sides of the members as well. In some instances particularly in the case of accidental damage even a compression face may need some reparatory work and the fibre reinforced cement sheets can be employed for the same.

In the case of a slab, the sheets may be attached to the soffltt and also, if desired, to the top surface particularly over the supports.

For columns, frames and earth and liquid retaining structures a similar technique can be adopted.

Whzr the concrete element is deformed or damaged the repair and/or strengthening work is preferably carried out after reducing the deformation, e.g. by a jacking operation if it is possible in the case of beams, or after carrying out repairs to the damaged portion, for example by filling up cracks with adhesives and/or fillers. For strengthening purposes use may also be made of additional concrete and/or additional steel reinforcement to increase the load carrying capacity of the member and to increase the stiffness of the existing structure before, during or after the application of a layer of fibre reinforced cement.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 shows a part of a concrete beam in side elevation; Fig. 2 is a transverse section through the beam of Fig. 1; Fig. 3 is a transverse section through a different form of concrete beam; Fig. 4 shows the beam of Fig. 3 with fibre reinforced cement sheets applied thereto; Fig. 5 shows the beam of Fig. 3 strengthened with additional concrete and having fibre reinforced cement sheets applied thereto; Fig. 6 shows the beam of Fig. 3 strengthened with additional concrete and steel and having fibre reinforced cement sheets applied thereto; Fig. 7 is a horizontal section through a column; Fig. 8 shows the column of Fig. 7 having fibre reinforced cement sheets applied thereto;; Fig. 9 is a horizontal section through another form of column; Fig. 10 shows the column of Fig. 9 strengthened with a layer of fibre reinforced cement; Fig. 11 is a side elevation of a beam which has failed: and Figs. 12-14 illustrate successive stages in the repair of the beam of Fig. 11.

Referring to Figs. 1 and 2 a concrete beam 2 of rectangular cross section and having reinforcement 4 is strengthened by applying a layer 6 of fibre reinforced cement to the under surface of the beam. The layer 6 extends across the full width of the said under surface. Further layers 8 of fibre reinforced cement are applied to the side surfaces of the beam.

Referring now to Figs. 3 and 4 an I section concrete beam 10 having reinforcement 11 and 12 can be repaired and/or strengthened by applying to the under surface of the beam a layer 14 of fibre reinforced cement. The layer 14 extends across the full width of the under surface of the beam.

Further layers 16 of fibre reinforced cement are applied to the side surfaces of the lowermost flange of the beam.

If desired the layers 16 may be replaced, as shown in Fig. 5, by layers 18 which span the flanges of the beam thereby defining cavities 20 which are filled with concrete.

As shown in Fig. 6 instead of extending between the flanges of the beam the layers 18 may, at their upper edges, be spaced from the upper flange of the beam and the cavity 22 so defined filled with concrete and additional reinforcement 24.

A rectangular cross section column 30 as shown in Fig. 7 having reinforcement 31 can also be strengthened and/or repaired by applying to the surfaces of the column sheets of fibre reinforced cement 32 as shown in Fig. 8. Similarly with a column 34 of circular cross section shown in Fig. 9, a layer 36 of fibre reinforced cement can be applied to the surface thereof as illustrated in Fig. 10.

The following Example further illustrates the invention.

EXAMPLE A beam 200 mm deep x 100 mm wide containing 2 Nos 12 mm diameter mild steel bars was manufactured using a concrete mix of 1:1.5: 3:0.55, i.e. 1 part of ordinary portland cement to 1.5 parts of fine aggregates to 3 parts of coarse aggregate, 0.55 being the water/cement ratio. Third point loading was applied on an effective span of 2.75 metres so that the central 0.92 metres was under constant applied bending moment.

The calculated design working load was 10 kN with a global load factor of 2.

The beam was tested in 3 loading cycles, firstly up to design working load of 10 kN then was unloaded and reloaded up to 1.5 times the design working load, then unloaded and reloaded up to failure. The beam cracked at 6 kN during the first loading cycle.

The unloading in each case meant reducing the applied load to zero. The failure occurred at 24 kN. Failure occurred by the beam being unable to take up any more load-the indicator on the loading machine falling back. The failure was preceded by cracks widening and reaching towards compression face, the deflection increasing rapidly resulting in ultimate crushing of concrete on the compression side.

During the test, measurements were taken of the central deflection and crack width at various stages of loading.

At failure the beam had sagged considerably as shown in Fig. 11 and very little recovery of deflection was obtained on unloading.

It was decided to apply the technique of repairing and strengthening to this beam.

The beam was straightened and the cracks in the beam were filled with Epoxy Resin mixed in the ratio by weight of 100 parts resin:65 parts of hardener. Concrete which had chipped during crushing at failure was made good by the use of premix tetrosyl.

Asbestos cement sheet (6 mm thick 100 mm wide and 2.5 m long coated with premix tetrosyl mixed in the proportion of 500 parts of paste to 12 parts of hardener by weight was bonded to the soffit as shown in Fig. 12.

6 mm thick 150 mm wide and 2.4 m long asbestos cement sheets were treated in the same manner as described above and secured to the sides of the beam so that entire tension zone on the soffit as well as the sides was now covered with the surface reinforcement provided by asbestos cement sheets (Figs. 13 and 14).

The beam was reloaded exactly in the same manner as before and deflection measured. The beam failed at the ultimate load of 24 kN-the same load at which the original beam failed.

The central deflection was consistently lower than that of the original beam up to 1.5 times the design working load after which no attempts were made to measure the deflections in either case.

The repaired beam with surface reinforcement cracked at 15 kN compared with 6 kN in the first case. As can be seen from the above described embodiments the fibre reinforced cement does not necessarily cover the entire surface of a concrete member. In the case of beams in situ for example the side surfaces thereof may only be partly exposed thus imposing a limitation on the extent to which the said side surfaces can be covered.

In some cases moreover the side surfaces of a beam are completely unexposed so that fibre reinforced cement can only be applied to the undersurface of the beam.

Where a floor between two beams is hollow but does not permit sufficient access to the sides of the beam for a layer of fibre reinforced cement to be applied to the sides, concrete may be forced into the floor adjacent the beam, fibre reinforced cement being applied, as necessary, to exposed surfaces of the beam. It is, of course, possible for a sheet of fibre reinforced cement to extend beyond a surface of a concrete member and thus cover additionally a part or all of a wall, ceiling or the like supported by the concrete member.

WHAT WE CLAIM IS:- 1. A method of repairing and/or strengthening a concrete member, incorporated in a structure, comprising bonding a preformed layer of fibre reinforced cement to the member by adhesive or by additional concrete.

2. A method as claimed in Claim 1, wherein reinforcement is provided in the additional concrete.

3. A method as claimed in Claim 1 or Claim 2 wherein a single area of the surface of a member is covered by one of the preformed layers of fibre reinforced concrete.

4. A method as claimed in Claim 1 or Claim 2, wherein a plurality of areas of the surface of a member are covered by a corresponding plurality of preformed layers of fibre reinforced cement.

5. A method as claimed in any preceding claim, wherein the layer is reinforced with high modulus fibres.

6. A method as claimed in claim 5, wherein the layer is reinforced with high elastic modulus fibres in combination with low elastic modulus fibres.

7. A method of repairing and/or strengthening a concrete member substantially as described herein with reference to Figs. 1 and 2, Figs. 3 and 4, Figs. 3 and 5, Figs.

3 and 6, Figs. 7 and 8 Figs. 9 and 10 or Figs. 11 to 14 of the accompanying drawings.

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Claims

**WARNING** start of CLMS field may overlap end of DESC **.

rapidly resulting in ultimate crushing of concrete on the compression side.

During the test, measurements were taken of the central deflection and crack width at various stages of loading.

At failure the beam had sagged considerably as shown in Fig. 11 and very little recovery of deflection was obtained on unloading.

It was decided to apply the technique of repairing and strengthening to this beam.

The beam was straightened and the cracks in the beam were filled with Epoxy Resin mixed in the ratio by weight of 100 parts resin:65 parts of hardener. Concrete which had chipped during crushing at failure was made good by the use of premix tetrosyl.

Asbestos cement sheet (6 mm thick 100 mm wide and 2.5 m long coated with premix tetrosyl mixed in the proportion of 500 parts of paste to 12 parts of hardener by weight was bonded to the soffit as shown in Fig. 12.

6 mm thick 150 mm wide and 2.4 m long asbestos cement sheets were treated in the same manner as described above and secured to the sides of the beam so that entire tension zone on the soffit as well as the sides was now covered with the surface reinforcement provided by asbestos cement sheets (Figs. 13 and 14).

The beam was reloaded exactly in the same manner as before and deflection measured. The beam failed at the ultimate load of 24 kN-the same load at which the original beam failed.

The central deflection was consistently lower than that of the original beam up to 1.5 times the design working load after which no attempts were made to measure the deflections in either case.

The repaired beam with surface reinforcement cracked at 15 kN compared with 6 kN in the first case. As can be seen from the above described embodiments the fibre reinforced cement does not necessarily cover the entire surface of a concrete member. In the case of beams in situ for example the side surfaces thereof may only be partly exposed thus imposing a limitation on the extent to which the said side surfaces can be covered.

In some cases moreover the side surfaces of a beam are completely unexposed so that fibre reinforced cement can only be applied to the undersurface of the beam.

Where a floor between two beams is hollow but does not permit sufficient access to the sides of the beam for a layer of fibre reinforced cement to be applied to the sides, concrete may be forced into the floor adjacent the beam, fibre reinforced cement being applied, as necessary, to exposed surfaces of the beam. It is, of course, possible for a sheet of fibre reinforced cement to extend beyond a surface of a concrete member and thus cover additionally a part or all of a wall, ceiling or the like supported by the concrete member.

WHAT WE CLAIM IS:- 1. A method of repairing and/or strengthening a concrete member, incorporated in a structure, comprising bonding a preformed layer of fibre reinforced cement to the member by adhesive or by additional concrete.
2. A method as claimed in Claim 1, wherein reinforcement is provided in the additional concrete.
3. A method as claimed in Claim 1 or Claim 2 wherein a single area of the surface of a member is covered by one of the preformed layers of fibre reinforced concrete.
4. A method as claimed in Claim 1 or Claim 2, wherein a plurality of areas of the surface of a member are covered by a corresponding plurality of preformed layers of fibre reinforced cement.
5. A method as claimed in any preceding claim, wherein the layer is reinforced with high modulus fibres.
6. A method as claimed in claim 5, wherein the layer is reinforced with high elastic modulus fibres in combination with low elastic modulus fibres.
7. A method of repairing and/or strengthening a concrete member substantially as described herein with reference to Figs. 1 and 2, Figs. 3 and 4, Figs. 3 and 5, Figs.

3 and 6, Figs. 7 and 8 Figs. 9 and 10 or Figs. 11 to 14 of the accompanying drawings.