Abstract:
Displacement measuring means are fixed onto the structural element. A hole is drilled in the measurement zone and a supply pressure is applied to a flat actuator introduced into the hole. The displacements measured are analyzed as a function of the supply pressure to determine the stress from a supply pressure which roughly compensates for the deformation of the element due to the drilling of the hole. The measuring means comprise two arms that are fixed to the element at two respective anchoring points aligned parallel to a measuring direction, and at least two displacement sensors mounted on the arms on each side of the anchoring points to measure the variations in separation between the anchoring points. The arms leave between them a gap through which the hole is drilled at a central position with respect to the anchoring points.

Description:
FIELD OF INVENTION 
     The present invention relates to a method for measuring stress in a structural element. 
     DESCRIPTION OF THE RELATED ART 
     It applies in particular, although not exclusively, to the measurement of residual prestress in a component of a concrete structure subjected to a bending force due to the load on the structure and, on the other hand, to a compressive prestress. Such a component is typically a prestressed concrete beam. 
     A prestressed beam comprises steel cables tensioned between their ends, with enough force that the concrete of the beam is subjected only to compressive stresses, in spite of the bending forces which are due to the loads it supports. 
     Over the course of time, the tension in the prestressing cables tends to decrease, which means that the compressive stresses generated by these cables may become insufficient to compensate for the tensile stresses due to the bending of the beams. These tensile stresses may lead to cracking of the concrete or even to the breaking of the beams. 
     It is therefore useful to be able to monitor the residual value of the compressive stresses generated by the prestressing cables, so as to be able to take appropriate action should that prove necessary. 
     French Patent 2 717 576 describes a method of measuring a residual prestress in a reinforced concrete beam which is subjected to a vertical bending force and to a compressive longitudinal prestress force, any section of the beam having a transverse line, known as the neutral axis of bending, along which the bending forces generate neither tensile stress nor compressive stress. This known method comprises the following steps: 
     determining the position of the neutral bending axis in a given section of the beam; 
     boring along said neutral bending axis, passing transversely through the beam, this boring giving rise to a certain elastic deformation of the beam in its vicinity; 
     measuring the deformation of the beam near the boring, with respect to an initial state prior to the bore hole being pierced; 
     introducing into the bore hole a hydraulic actuator comprising two roughly semicylindrical shells which occupy roughly the entire cross section of the bore hole and which are designed to move apart when the actuator is pressurized, this actuator being arranged in such a way that the two shells can move apart parallel to the prestress force; 
     pressurizing the hydraulic actuator while at the same time measuring the deformation of the beam near the bore hole; 
     recording the hydraulic pressure of the actuator which corresponds to the deformation of the beam due to the bore hole being canceled; 
     and determining the mean residual prestress along the neutral bending axis from the hydraulic pressure value thus measured. 
     One object of the present invention is to improve this method, by allowing better control over the relationships between the stress and the displacements measured. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention thus proposes a method for measuring stress in a structural element, comprising the following steps: 
     fixing displacement measuring means onto the element in a measurement zone; 
     piercing a hole in the element in the measurement zone; 
     introducing an actuator into the hole; 
     applying a supply pressure to the actuator; and 
     analyzing the displacements measured as a function of the actuator supply pressure so as to estimate the degree of stress in the element in the measurement zone. 
     According to the invention, the displacement measuring means comprise two arms that are fixed to the element at two respective anchoring points aligned parallel to a measuring direction, and at least two displacement sensors mounted on the arms on each side of the anchoring points and each measuring a relative displacement, parallel to the measuring direction, of two respective portions of the arms which portions lie facing one another. The arms leave between them a gap through which the hole is pierced at a central position with respect to the anchoring points. 
     Thus, the anchoring points which act as a basis for the displacement measurements are positioned optimally with respect to the hole and to the measuring direction, without this in any way impeding the boring of the hole and the instrumenting of the measurement zone. 
     The method makes it possible in a particularly advantageous way to constantly record the displacement and supply pressure measurements while the hole is being pierced and the supply pressure is being applied to the actuator, this allowing in-depth analysis of the results. 
     In a preferred embodiment, the hole comprises a slot orientated at right angles to the measuring direction, symmetrically with respect to an axis passing through the anchoring points, the actuator being a flat actuator introduced into the slot. 
     The fact that the slot and the measurement axis are at right angles to each other and the fact that the slot is centered with respect to the anchoring points improve the reliability of the displacement measurements and improve their correlation with the looked-for stress. 
     This slot may pass all the way through the element, but that is not essential as long as it is deep enough. 
     The flat actuator may be supplied with hydraulic fluid by a manually operated pump and associated with means of measuring the supply pressure. 
     The flat actuator may be introduced into the slot with the insertion of at least one wedging plate which makes the distribution of the force exerted by the actuator uniform over the extent of the slot. 
     The displacements analyzed advantageously represent a variation in separation between the two anchoring points, which variation is obtained from a mean of the displacements respectively measured by the sensors. In a preferred embodiment, additional displacement measuring means are fixed to the structural element at two anchoring points lying outside of the measuring zone and aligned in the measuring direction and having, between them, a distance roughly identical to the distance between the two anchoring points lying in the measurement zone. These additional measurement means provide a corrective term that represents a variation in separation between the two anchoring points lying outside the measurement zone, said corrective term being subtracted from said mean of the displacements in the analysis step. 
     In some particular embodiments of the method: 
     the displacement sensors have a measurement accuracy of the order of one micron; 
     the supply pressure of the actuator is increased until a supply pressure is achieved that roughly compensates for the deformation of the element that is due to the piercing of the hole, then the supply pressure has gradually reduced while continuing to record the displacement measurements, and a degree of compression in the measurement zone is estimated from the supply pressure which has roughly compensated for the deformation of the element; 
     the change in the measured displacements is recorded as a function of the supply pressure of the actuator, and if a supply pressure which roughly compensates for the deformation of the element due to the piercing of the hole is not achieved, the change in the measured displacements is extrapolated so as to estimate the degree of stress in the element in the measurement zone; if extrapolation is toward high pressures, it can then be determined that the measurement zone is in a state of compression, while it can be determined that the measurement zone is in a state of tension, if extrapolation is toward negative pressures; 
     once the measurements have been taken, an actuator containing a substance under pressure is left in the hole; 
     use is made of a measurement zone situated roughly along the neutral axis of the structural element; 
     use is made of at least two measurement zones situated roughly symmetrical with respect to the neutral axis of the structural element, and in which a stress is evaluated along the neutral axis using a mean of the stresses measured in said measurement zones. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Other specifics and advantages of the present invention will become apparent from the description hereinafter of some nonlimiting embodiments, with reference to the appended drawings, in which: 
     FIG. 1 is a front view of a beam on which stress measurements will be taken according to the invention; 
     FIG. 2 is a schematic front view of displacement measuring means lying in the stress measurement zone; 
     FIG. 3 is a schematic front view of other displacement measuring means lying outside of the stress measurement zone; 
     FIG. 4 is a sectional view of the measurement zone; 
     FIG. 5 is a block diagram of a control and measurement sequence that can be used in the method; and 
     FIG. 6 shows an example of a graph generated according to the method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates the application of the invention to the measurement of the residual prestress in a prestressed reinforced concrete beam  10 . On its top face, this beam  10  is subjected to variable loads which tend to cause it to bend, in additional to its self-weight. In consequence, in the upper part of the beam, the loads induce compressive (positive) stresses in the concrete, whereas they induce tensile (negative) stresses in the concrete of the lower part of the beam. 
     The line B in FIG. 1 indicates the neutral axis of the beam, that is to say the axis for which the stresses induced by the loads it supports and by its self weight change sign. For the concrete to work properly, prestressing cables longitudinally compress the beam so that the overall stress along the neutral axis B corresponds to a positive compressive stress. 
     In FIG. 1, the line A indicates an axis of symmetry of the beam  10  and the point C denotes its intersection with the neutral axis B. 
     To measure the residual stress in the concrete at the neutral axis (point C) use is made, in the example depicted in FIG. 1, of two measurement zones Z arranged symmetrically with respect to the point C. 
     In each measurement zone Z, a value of the compressive stress parallel to the direction B is determined. To obtain the value of the residual stress at the point C, all that is required is for an arithmetic mean of the two measured values to be calculated. 
     If the point C is accessible, it is also possible to take just one measurement in its immediate vicinity. 
     A first step in the stress measuring method consists in defining two anchoring points X in the measurement zone Z. These two anchoring points are aligned parallel to the measuring direction B and their distance is defined precisely using a template  11  with two openings through which holes are pierced, in which holes anchor bolts are installed. 
     Each measurement zone Z is associated with a reference zone Z′ in which two other anchoring points X′ are defined, these too being aligned parallel to the direction B and having between them the same distance as there is between the anchoring points X. The anchoring points X′ may be positioned using a template  12  similar to the one used in the measurement zone Z. 
     The next step consists in equipping the measurement zone Z with displacement measuring means such as those depicted in FIG.  2 . 
     These means comprise two arms  15  fixed respectively in their central part to rods sealed into the holes pierced in the concrete at the anchoring points X. The arms  15  run in a direction which is generally at right angles to the measuring direction B. Their ends are bent inward, and displacement sensors  16  are inserted between the facing portions  17  of the bend ends of the arms  15 . 
     The sensors  16  may be electromechanical feelers with a measurement accuracy of the order of one micron in a range of displacement of ±1 mm. 
     The two sensors  16  are arranged symmetrically with respect to the axis B′ passing through the two anchoring points X, which is parallel to the measuring direction B. Thus, the arithmetic mean (d 1 +d 2 )/2 of the two displacement measurements d 1 , d 2  supplied by the sensors  16  represents a measure of the variation in separation between the two anchoring points X. 
     Additional displacement measuring means (FIG. 3) are installed in the reference zone Z′. These means comprise two platelets  18  fixed respectively to rods sealed into the holes pierced in the concrete at the two anchoring points X′. A displacement sensor  19  similar to those  16  provided in the measurement zones Z is arranged between the two platelets  18 . The displacement value d 3  supplied by this sensor  19  represents the variation in separation between the two anchoring points X′. 
     The next step in the method consists in piercing a hole  20 ,  21  in the concrete element  10  in the measurement zone Z. This hole is pierced through the gap left free between the two arms  15  of the displacement measuring device, as illustrated in dotted line in FIG.  2 . 
     This hole comprises a slot  20  directed at right angles to the measuring direction B. This slot  20  is centered between the two anchoring points X, and the axis B′ passing through these two anchoring points X passes through the middle of the slot  20 . The slot  20  may have a thickness of the order of one centimeter and a length of about ten times that. 
     At the two ends of the slot  20 , the hole made in the measurement zone Z has two cylindrical bore holes  21 , the diameter of which is, for example, of the order of a few centimeters. 
     To drill the hole  20 ,  21 , the bore holes  21  are made first of all using a hydraulic concrete core drill, then the slot  20  is made using a hydraulic concrete cutter. The core drill and the cutter may be mounted on a chassis that is anchored to the concrete structure. 
     The hole  20 ,  21  may pass right through the beam  10 . In some cases, it may penetrate the concrete to a sufficient depth without passing through it. 
     The next step (FIG. 4) consists in introducing a flat actuator  24  into the slot  20 . This actuator may consist of two metal plates welded together along their periphery. An injection orifice, not depicted, causes the space between the two plates to communicate with a hydraulic circuit. As shown by FIG. 4, a wedging sheet  25  of appropriate thickness (or a stack of several sheets) may be placed, with the flat actuator  24 , in the slot  20 . This plate  25  allows more uniform distribution of the thrust exerted by the flat actuator  24  across the extent of the slot  20 . 
     The hydraulic circuit is depicted schematically in FIG.  5 . The hydraulic fluid from the reservoir  27  is sent under pressure to the flat actuator  24  by a pump  28 . By way of example, the supply pressures may range up to about 200 to 300 bar. A pressure gauge  29  situated between the pump  28  and the actuator  24  is used to measure the actuator supply pressure. To reach high pressures with a gradual pressure rise, the pump  28  is advantageously manually operated. 
     FIG. 5 also shows a computation device  30  consisting, for example, of a portable computer of the PC type, which gathers the various parameters measured by the sensors  16 ,  19  and  29 . The computation device  30  exploits the displacement and pressure measurements to evaluate the stress exerted in the measurement zone Z. This exploitation may be done in real time, which means that the stress measurement is available immediately. 
     The data recorded by the computation device  30  correspond to the change in the separation between the two anchoring points X as a function of the supply pressure P applied to the flat actuator  24 . The separation (d 1 +d 2 )/2 measured between the two anchoring points X is corrected using the separation d 3  measured between the anchoring points X′. The relevant displacement variable is therefore (d 1 +d 2 )/2−d 3 . 
     An example of the change in this variable (d 1 +d 2 )/2−d 3  as a function of pressure is illustrated by the graph in FIG.  6 . Curves I and II correspond respectively to the rise in pressure in the flat actuator  24  and to the fall in pressure. The rise is halted when the measured variable (d 1 +d 2 )/2−d 3  reaches the displacement value d 0  that corresponds to the value recorded before the hole was pierced. The pressure P measured at that instant corresponds to the looked-for compressive stress. 
     If the displacement value d 0  has not yet been reached when the actuator  24  is supplied with its maximum pressure, then the computer  30  extrapolates the curve obtained, which is approximately linear, to obtain a measure of the stress given by the X-axis value of the point of intersection of the extrapolated line with the y-axis value d 0 . 
     The variations in displacement as a function of supply pressure are also recorded while the hole  20 ,  21  is being pierced, and this makes it possible to observe the behavior of the structure and possibly to estimate how deep the holes need to go, this being the depth beyond which the additional displacements measured at the anchoring points are no longer significant. 
     The fact of arranging the flat actuator  24  in a slot  20  perpendicular to the measuring direction B allows reliable measuring in that direction, avoiding the geometric configuration of the hole causing other undesirable stresses to be taken into consideration. The bore holes  21  at the ends of the slots  20  limit parasitic stresses at the ends and make the slot  20  easier to cut. They may also make it easier for the equipment to be fitted. 
     The making of the hole  20 ,  21  does not generally disturb the structure, given its small size. However, if such disturbance is feared, this may be overcome by leaving in the hole, once the measures have been taken, an actuator containing a pressurized substance. This substance is, for example, a resin injected into the flat actuator  24  at a pressure corresponding to the measured residual stress, which is left to cure in the actuator which will remain in situ. 
     It should be noted that the method may also be applied when the measurement zone is in a state of tension rather than of compression. In this case, the relaxation that follows the piercing of the hole tends to cause the arms  15  to move apart rather than to move closer together as is the case with compressive stresses (FIG.  6 ). Furthermore, when the actuator is supplied, it introduces an additional separation which moves the measurement point (d 1 +d 2 )/2−d 3  even further away from the reference value d o . This does not prevent the degree of stress in the measurement zone from being estimated using the aforementioned process of extrapolation. Quite simply, extrapolation is toward the negative pressures (rather than toward the higher pressures). The opposite X-axis value at which the extrapolated measurement straight line reaches the y-axis value d 0  in a diagram according to FIG. 6 gives an estimate of the tensile stress. 
     Furthermore, the method can be applied to all types of structure, which are not necessarily made of concrete, for example to stonework structures.