GB2059479A

GB2059479A - Composite prestressing of concrete structures

Info

Publication number: GB2059479A
Application number: GB7935608A
Authority: GB
Original assignee: Trouillard SA
Current assignee: Trouillard SA
Priority date: 1979-10-12
Filing date: 1979-10-12
Publication date: 1981-04-23
Also published as: GB2059479B

Abstract

In a composite prestressing of a concrete structure, there are prestressed reinforcements and unstressed reinforcements. Both prestressed and unstressed reinforcements are made of the same material which is a steel having a breaking strength between 60 and 120 kg/mm<2>.

Description

SPECIFICATION Composite prestressing of structures The invention relates to a method of prestressing invention relates to a method of prestressing structures and to prestressed structures.

In order to provide lasting prestressing forces, particularly in reinforced concrete parts, it is conventional to use steel reinforcements with a high elastic limit and a breaking strength which varies from 110 kg/mm2 to above 200 kg/mm2.

These reinforcements are initially tensioned to the limit of their plastic region and are subjected to different preliminary treatments to ensure their suitability for particular applications (fatigue strength, resistance to corrosion, reduction of the loss of tensile strength).

In certain cases, i.e. in cases which are called composite prestressing, tensioned prestressing reinforcements are used in addition to nontensioned reinforcements to ensure that the part of the structure has particular qualities, for example stability in the case of fire (floors, poles), or reduction of cracking (reservoirs, structures exposed to bad weather).

This combination of two types of reinforcement is also found in cases where the ratio R between the maximum load needed to cause collapse and the normal working load under which the concrete part or structure cannot crack, or cracks only to a limited extent, is higher than 2, which is the approximate ratio for conventional structures subjected to bending stress.

This is particularly so in the case of long, thin poles which are subjected to bending stresses in opposite directions and in which the essential role of the prestressing or of the additional reinforcements is to reduce the deformations which are likely to increase bending. This also applies to parts which may be subjected to particularly high abnormal forces, for example frameworks in seismic zones or railway-sleepers, for which the ratio R may reach values equal to or higher than 4.

Finally, the combination of two categories of reinforcement is used when the main load is the action of shearing stresses in bent parts which have been prestressed according to the pretensioning method. In this case fracture by creeping of a pretensioned reinforcement acted upon by an additional tension in a crack near the support area is a dangerous phenomenon owing to its abruptness and for which the usual rules of adding reinforcing effects do not apply. It is then necessary to use non-tensioned additional reinforcements which are suitably reinforced, particularly since prestressing reinforcements with a high elastic limit generally adhere badly in the case of reinforced concrete with respect to the anchorage of excess tensions on either side of the crack.

This situation applies for example to heavily loaded floors with a poor bearing surface, poles prestressed by pretensioning and embedded in their foundations and railway-sleepers.

The object of the invention is to obtain a prestressing method which can be used in cases of composite prestressing, and particularly in the cases listed above, with reinforcements of a single type owing to the fact they they consist of a steel having particular properties.

For this purpose the invention provides a prestressing method for cases of composite prestressing, wherein reinforcements of a single type are used whose breaking strength is between 60 kg/mm2 and 120 kg/mm2, i.e. between that of usual reinforcing steels of reinforced concrete and that of steels with a high elastic limit (H.L.E.) used for prestressing.

The invention also provides a prestressed structure including both prestressed and nonstressed reinforcements, both forms of reinforcement being made of the same steel which has a breaking strength in the range 60-120 kg/mm2.

Other details and characteristics of the invention will be apparent from the following

description.

In order to manufacture parts or make structures which have composite prestressing, the reinforcements of the above-mentioned special type may be subjected to supplementary treatments.

In certain cases a preliminary work-hardening, tempering or other treatment is carried out, leading to a regularisation of the stress-strain curve of the steel, so as to be able to monitor the prestressing by checking the elongation.

In other cases a preliminary tensioning treatment is carried out, possibly completed by a heat treatment, at a force such that the tension in the reinforcement when the concrete is prestressed is particularly a linear function of the elongation. In general, this treatment prevents loss of tensile strength, susceptibility to brittleness and to corrosion and enhances fatigue strength of the reinforcement.

This preliminary tension is adapted to the residual tension required for each part or for each structure. It also depends on the manufacturing or construction methods (for example heat treatment of the concrete).

CLAIMS 1. A method of composite prestressing, wherein steel reinforcements of a single type are used for both prestressed and non-stressed reinforcements, the breaking strength of the - reinforcements being between 60 kg/mm2 and 120 kg/mm2.

2. A method according to claim 1, wherein a preliminary work-hardening; tempering or other treatment is carried out, leading to a regularisation of the stress-strain curve of the steel, so as to be abide to monitor the prestressing by checking the elongation.

3. A method according to claim 1, wherein a preliminary tensioning treatment is carried out, at a force such that the tension in the reinforcement when the concrete is prestressed is a linear

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

Claims

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

SPECIFICATION Composite prestressing of structures The invention relates to a method of prestressing invention relates to a method of prestressing structures and to prestressed structures.

In order to provide lasting prestressing forces, particularly in reinforced concrete parts, it is conventional to use steel reinforcements with a high elastic limit and a breaking strength which varies from 110 kg/mm2 to above 200 kg/mm2.

These reinforcements are initially tensioned to the limit of their plastic region and are subjected to different preliminary treatments to ensure their suitability for particular applications (fatigue strength, resistance to corrosion, reduction of the loss of tensile strength).

In certain cases, i.e. in cases which are called composite prestressing, tensioned prestressing reinforcements are used in addition to nontensioned reinforcements to ensure that the part of the structure has particular qualities, for example stability in the case of fire (floors, poles), or reduction of cracking (reservoirs, structures exposed to bad weather).

This combination of two types of reinforcement is also found in cases where the ratio R between the maximum load needed to cause collapse and the normal working load under which the concrete part or structure cannot crack, or cracks only to a limited extent, is higher than 2, which is the approximate ratio for conventional structures subjected to bending stress.

This is particularly so in the case of long, thin poles which are subjected to bending stresses in opposite directions and in which the essential role of the prestressing or of the additional reinforcements is to reduce the deformations which are likely to increase bending. This also applies to parts which may be subjected to particularly high abnormal forces, for example frameworks in seismic zones or railway-sleepers, for which the ratio R may reach values equal to or higher than 4.

Finally, the combination of two categories of reinforcement is used when the main load is the action of shearing stresses in bent parts which have been prestressed according to the pretensioning method. In this case fracture by creeping of a pretensioned reinforcement acted upon by an additional tension in a crack near the support area is a dangerous phenomenon owing to its abruptness and for which the usual rules of adding reinforcing effects do not apply. It is then necessary to use non-tensioned additional reinforcements which are suitably reinforced, particularly since prestressing reinforcements with a high elastic limit generally adhere badly in the case of reinforced concrete with respect to the anchorage of excess tensions on either side of the crack.

This situation applies for example to heavily loaded floors with a poor bearing surface, poles prestressed by pretensioning and embedded in their foundations and railway-sleepers.

The object of the invention is to obtain a prestressing method which can be used in cases of composite prestressing, and particularly in the cases listed above, with reinforcements of a single type owing to the fact they they consist of a steel having particular properties.

For this purpose the invention provides a prestressing method for cases of composite prestressing, wherein reinforcements of a single type are used whose breaking strength is between

60 kg/mm2 and 120 kg/mm2, i.e. between that of usual reinforcing steels of reinforced concrete and that of steels with a high elastic limit (H.L.E.) used for prestressing.

The invention also provides a prestressed structure including both prestressed and nonstressed reinforcements, both forms of reinforcement being made of the same steel which has a breaking strength in the range 60-120 kg/mm2.

Other details and characteristics of the invention will be apparent from the following

description.

In order to manufacture parts or make structures which have composite prestressing, the reinforcements of the above-mentioned special type may be subjected to supplementary treatments.

In certain cases a preliminary work-hardening, tempering or other treatment is carried out, leading to a regularisation of the stress-strain curve of the steel, so as to be able to monitor the prestressing by checking the elongation.

In other cases a preliminary tensioning treatment is carried out, possibly completed by a heat treatment, at a force such that the tension in the reinforcement when the concrete is prestressed is particularly a linear function of the elongation. In general, this treatment prevents loss of tensile strength, susceptibility to brittleness and to corrosion and enhances fatigue strength of the reinforcement.

This preliminary tension is adapted to the residual tension required for each part or for each structure. It also depends on the manufacturing or construction methods (for example heat treatment of the concrete).

CLAIMS 1. A method of composite prestressing, wherein steel reinforcements of a single type are used for both prestressed and non-stressed reinforcements, the breaking strength of the - reinforcements being between 60 kg/mm2 and

120 kg/mm2.
2. A method according to claim 1, wherein a preliminary work-hardening; tempering or other treatment is carried out, leading to a regularisation of the stress-strain curve of the steel, so as to be abide to monitor the prestressing by checking the elongation.
3. A method according to claim 1, wherein a preliminary tensioning treatment is carried out, at a force such that the tension in the reinforcement when the concrete is prestressed is a linear

function of the elongation.
4. A prestressed structure including both prestressed and non-stressed reinforcements, both forms of reinforcement being made of the same steel which has a breaking strength in the range 60-120 kg/mm2.

Amendments to claims filed on 3rd December 1979.

In claim 1 line 3 delete "breaking" and insert "ultimate tensile".