The present invention relates to a method for measuring stress in a structural element.
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.
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.