GB2284669A - Determination of in situ stress in concrete - Google Patents

Abstract

The stress existing in the surface layers of a concrete structure 4 is measured by attaching at least one linear strain transducer 2 to the concrete surface and then forming a blind hole 10 in the surface at a point aligned with, and spaced from, each end of the transducer. The perturbation in the strain pattern caused by forming the holes is detected by the transducer. This enables calculations to be made of the original strain, and therefore of the stress in the original structure. The small size of the holes, and the fact that they embrace a rectangular area, rather than a square one, enables the stress determination to be made in far-smaller areas between reinforcement in the concrete, thus giving greater latitude to the location of test sites. The use of a linear array of transducers with intervening holes is envisaged, and a triangular arrangement if the stress orientation is unknown. The subsequent application of radial pressure within a hole, e.g. hydraulically, to restore the transducer reading to its original value, enables the stress to be sensed without knowing the elastic modulus for the material. This pressure can be applied at several depths to give a profile of stress with depth. <IMAGE>

Description

DETERMINATION OF IN SITU STRESS IN CONCRETE This invention relates to a method for determining the amount of stress in the surface layers of a structure made of reinforced concrete, irrespective of whether or not it is pre- or post-stressed.

A stress applied to a body is something which cannot be directly measured, but normally speaking can be calculated without too much difficulty if the standard mechanical properties of the material in question are known and if the strain is measured. The measurement of strain and its use to determine stress is so widespread that it has become an accepted standard method. Further information to be found in "The Strain Gage Primer" by C C Perry and H R Lissner published by McGraw-Hill Book Company.

The determination of in situ stresses in structures is important in a variety of engineering fields, particularly in the civil engineering field.

In situ stresses are stresses which are present in a structure when all live loading is removed. The in situ stress comprises the manufacturing stress plus the stress because of dead load.

In civil engineering the structures are generally constructed of steel or concrete members.

There is a generally accepted technique for use in steel or metallic elements. The strain change is measured arising from the perturbation caused by local stress relief, such as the drilling of a small hole. ASTM Standard E837/81 "Determining Residual Stresses by the Hole-Drilling Strain-Gage Method" discloses this general standard technique, which is discussed further in a paper by E M Beaney and E Procter entitled "A critical evaluation of the centre hole technique for the measurement of residual stresses', 'Strain', January 1974, pages 7-14 and 52.

It is also known, as from GB 1 351 859 and 1 593 907, to mount a strain gauge on a surface and to relieve stress by forming an annular channel around the gauge, to leave the gauge on one end of a core of material from the surface layers of which the stress has been removed.

The standard method of determining in situ stress by local relief of stress, by measuring the resultant strain and evaluating the results, and which is described in the references just identified, is well suited in applications where the material in question is relatively homogeneous, for example steelwork. Examples of such applications are disclosed in US-A-4 248 094 and 4 249 423. However, the technique is not directly applicable to less homogeneous structures, in particular civil engineering structures made of reinforced concrete, masonry or brickwork.

A paper entitled "Direct access to stresses in concrete and masonry bridges" by C Abdunur of the Laboratoire Central des Ponts et Chaussées, published in proceedings of Bridge Management 2 Conference, Guildford, 1993 by Thomas Telford, discloses an alternative method of determining in situ stress in concrete or masonry structures. It proposes a fundamentally different approach which can be simply summed up as forming a slot in the structure rather than a cylindrical hole. A "jack" is then inserted into the slot to pressurise the sides of the slot until the surrounding structure has readopted its original position (as measured by displacement gauges). Although this system has generated a certain amount of interest, it is particularly limited by the fact that a fairly-substantial slot needs to be formed, and special pressurisable "jack" equipment is needed to re-pressurise the slot. The slot must, of course, be machined into the material in a given direction. If the exact direction of the actual stress in the material is not known, more than one slot must be made in order to find the direction as well as the magnitude of the stress in the structure.

In contrast to the proposals for concrete and masonry made in the Abdunur paper, the present invention is based on the known principle of removing the stress in a small incremental test volume of the concrete structure; measuring the resultant change in strain in at least one dimension of that test volume, and preferably in more than one direction, so that the actual direction of the stress can be calculated, and calculating the stress that would need to be applied along the one or more dimensions to restore the original strain. This calculated stress is then regarded as being equivalent to the stress previously in the concrete in the region of the test volume at the start of the testing process.

In implementing this known method to deduce the prevailing stress, at least two strain gauges are placed along lines radiating from a point on the concrete surface, with the adjacent ends of the gauges being equidistantly spaced from that point. A blind cylindrical hole is then produced in the concrete by a coring technique, with the hole being centred on the point. The hole is usually relatively large, being about 75 mm in diameter, and about 50-100 mm deep. The formation of this hole is regarded as removing virtually all the stress in the immediate peripheral zone of concrete, with the consequential movement of the surface layers (the strain field perturbation) being detected by the strain gauges, enabling the necessary calculations to be made.

The actual technique by which a core of concrete is removed from the structure is conventional, and does not form part of the subject-matter of this application it will therefore not be described in any further detail.

The main disadvantage of this technique is that for accurate results it requires that the area encircling the axis of the blind hole has to be devoid of any reinforcement and as far away from the reinforcement as is practical, i.e. in normal circumstances located midway between the surrounding reinforcement, because its presence distorts the extent to which the surface layers can move as the stress relief is carried out in the structure at the time of the test. In very densely reinforced structures, reinforcement may have to be cut.

In structures where the exact position of reinforcement is not known, it may be cut by accident. Neither of these is desirable.

The present invention aims at providing a stressrelieving process which requires a much smaller area between the axes of any reinforcement, which is quicker to carry out than the known technique, and which can be used to produce results more easily and which are more accurate.

Accordingly the present invention provides a method of deducing the stress existing in a concrete structure as claimed in the appended claims.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a plan view of a known technique for deducing the original stress in the surface layers of a concrete structure; Figure 2 is a plan view illustrating the basic technique of the present invention; Figure 3 is a plan view of a concrete surface with an array of strain gauges extending the techniques shown in Figure 2; Figure 4 is a plan view of part of a concrete surface having in it an array of blind holes and strain gauges for determining in situ principal stresses and their directions, and Figure 5 is a diagrammatic cross-section of a blind hole formed in a concrete structure and having placed in it a stack of radial-pressure applicators used in a modification of the present invention.

In the known technique shown in Figure 1, three vibrating wire strain transducers 2 have their ends secured to a concrete surface 4 by setting in end blocks 6 (via grub screws to fix them firmly in place), the end blocks 6 being adhered to the concrete surface by a suitable fastcuring adhesive, such as a dental adhesive. The blocks 6 and strain transducers 2 are removed after the measurements have been made, and re-used. An alternative technique is to use a fully-bonded strain gauge in the form of an etched foil printed on a plastics film support, which film is simply adhered to the surface using a cyanoacrylate adhesive.

As shown in Figure 1, the strain transducers 2 extend along radii from a centre point 8, on the axis of a hole 10 which is formed in surface 4 equidistant from the reinforcement in the concrete, after the strain transducers 2 have been positioned and the adhesive set, by a coring technique. In general practice, the diameter of hole 10 is about 75 mm and its depth is 50-100 mm. As each of the strain transducers 2 is about 50 mm long, the hole 10 and strain transducers 2 extend over a roughlysquare area having sides of about 200 mm. These dimensions are a function of aggregate size, and the stated dimensions apply to concrete containing aggregate of 20 mm maximum size. In order to ensure that the stress calculations are not distorted, it is important to ensure that no reinforcement rods are in this area. This is an undesirable restriction, because in many concrete structures it is necessary or desirable to be able to calculate the stress in parts of the surface in which the reinforcement are closer together than 200 mm, e.g. in pre-stressed beams, and very critical areas such as halfjoints.

As shown in Figure 2, in the basic technique of the present invention, a single strain transducer is similarly positioned on the surface 4 of a concrete structure. In this description, components which are common to two or more figures retain their original references. The strain transducer is aligned along, or in parallel with, the direction in which it is estimated that the maximum stress exists in the structure. Next, in line with the strain transducer 2, and spaced from the end blocks 6, two similar blind holes 10 are formed in the concrete surface by a coring technique. These holes should similarly be formed in a rectangular area free of reinforcement. Each hole 10 in this instance is significantly smaller, having a diameter in the range of 20 to 35 mm and a depth 70 to 100% of its diameter. The formation of these two holes results in the strip of concrete between the holes, and to which the transducer 2 is affixed, changing its shape as the stress in it is at least partially relieved. This change of shape in turn leads to an alteration of the force holding the strain transducer 2 in tension. From the measured strain change along the axis of transducer 2, the stress reduction can be calculated, which in turn leads to a deduction of the stress originally present in that strip of concrete, with appropriate calibration coefficients.

In the known large-hole method of stress relief, and in the small-hole method taught by this invention, the holes should be drilled incrementally so as to obtain a graph strain change with hole depth. This is in order that any secondary stresses (such as arising from concrete shrinkage) can be estimated and allowed for.

In a refinement of the process shown in Figure 2, and as shown in Figure 3, the original combination of one transducer and two holes is added to by alternating transducers and holes spaced apart from each other along an axis along which the stress is to be calculated or deduced. Such a technique is useful for enabling variation of stress, such as along a bridge beam, to be found.

Because the change of strain is affected by a stress acting normally to the line of holes, the accuracy of measurement is improved if there is at least one gauge extending at 900 to the line of holes, and adjacent to one of the holes. The readings from this lateral gauge enable an indication to be obtained of the stress normal to the line of holes, and hence for a correction to be made to the calculated linear stress.

In those forms of the invention shown in Figures 2 and 3, it is assumed that only a single stress is to be calculated, and its likely axis is known. In cases where the direction and magnitude of stress are unknown, a strain transducer array as shown in Figure 4 can be used.

In this array, the strain transducers 2 are arranged in an equilateral triangle configuration, with the blind holes 10 being formed at the apices of the triangle. The individual strain changes in the concrete body aligned with each strain gauge enables calculations to be made by standard formulae, after modification by the appropriate calibration coefficients, to determine the actual direction in which stress pre-exists in the surface layers of the concrete, and also its magnitude.

In that modification of the technique shown in Figure 5, the surface 4 of the concrete structure 12 has had the strain transducer 2 secured to it in end blocks 6 as described above. After the blind holes 10 have been formed in the concrete 12, one or more pressure-applying devices 14 are inserted into each hole, with each device being connected through a multiple inlet conduit 16 with a source of pressure fluid, which may be an hydraulic oil, of which the instantaneous pressure is known. The holes can then be re-pressurised until the strain transducer has been restored to its original state prior to the holes being bored.

Each device 14 comprises rigid circular end walls having compliant cylindrical or curved side faces. In practice, each device 14 would fit neatly in hole 10 with little space between the side walls and the inside of the hole 10. After the stress-relieving step has been effected, and the necessary readings taken of strain transducer 2, pressure fluid is supplied to each device 14, which acts to reply stress to the cylindrical surfaces of holes 10. This in turn leads to movement of the adjoining concrete structure until the original strain along that particular axis of the structure is restored, as indicated by the reading on the transducer. The pressure of the fluid in each device 14 at this stage is then equivalent to the strain produced in the concrete structure, and hence to the stress giving rise to that strain.

The form of the invention shown in Figure 5 may be modified by the use of two or three independent pressure devices, each of which can be pressurised independently.

In this way radial pressure can be exerted on the walls of hole 10 at different levels. The different pressure levels read off when the original strain is restored enable variation of the original stress with depth to be calculated, knowing the dimensions of the devices 14.

The technique described with reference to Figure 5 enables the stress state to be determined, and its relationships to the depth from the surface, without needing to know the mechanical modulus for the material.

Accordingly it will be seen that the present invention provides a simple technique by which the stress in a relatively-small area of a concrete surface can be calculated accurately in a relatively-short period, in that the positioning of a single strain transducer on, and the formation of two small holes in, a concrete surface take far less time than the accurate positioning of multiple strain gauges and the formation of a single large hole.

Claims (7)

1. A method of determining the in situ stress in a concrete structure, including the steps of: fixing to the surface of the structure a linear strain transducer in line with the principal axis of a presumed stress; taking initial readings from the transducer; forming blind holes in the structure in line with, and adjacent to, each end of the transducer; taking second readings from the transducer, and calculating the initial stress in the concrete from the strain change in the concrete extending between the two holes.

2. A method as claimed in Claim 1, in which the stress existing along an axis of the surface of significantly-greater length than that covered by the overall length of two holes and one strain gauge is deduced by adding the steps of: fixing to the surface of the structure, along the axis along which stress is to be determined, at least one further transducer; obtaining initial readings from the or each further transducer; forming a further blind hole in the structure in line with, and adjacent, the 'free' end of the or each further transducer; obtaining second readings from the or each further transducer; calculating from these second readings the reduction in stress in the concrete extending between the further hole and the hole at the other end of the further transducer, and iteratively adding further gauges; forming further blind holes, and taking readings as necessary to deduce the stress formerly existing in each volume of the concrete extending along the line of blind holes.

3. A method of deducing the stress existing in a concrete structure, including the steps of: fixing to the surface of the structure three like linear strain transducers in an equilateral triangle configuration; taking initial readings from each of the transducers; forming three blind holes in the structure at the apices of the configuration, whereby each hole is adjacent to, and is spaced from, an end of two transducers; taking second readings from all three transducers; calculating, from the changes in strain shown by the sets of readings, the reduction in stress in the concrete extending beneath each transducer, and from that the direction and magnitude of the in situ stress in the area of concrete enclosed by the configuration of transducers.

4. A method as claimed in any preceding claim, including the steps of: inserting into at least two blind holes a pressure vessel in the form of a cylinder with rigid end walls and a resilient curved wall; increasing the pressure in the vessel until the reading of the transducer between the holes has returned to its initial value, and calculating from the pressure in the vessel the deformation applied by the vessel to the hole in which it is placed, and from that a deduction of the original stress.

5. A method according to Claim 4, wherein a stack of such vessels is inserted into each hole; the vessels are successively pressurised, and measurements taken to enable the variation of stress with depth to be calculated.

6. A method as claimed in any preceding claim in which the length of the or each transducer, and/or the dimensions of each blind hole, are a function of the average size of the aggregate in the concrete.

7. A method of deducing the stress existing in a concrete structure, substantially as described herein with reference to Figures 2 to 5 of the accompanying drawings.