Single flat jack tests
Single flat jack test measures the change in the stress state in the wall caused by a cutting plane runs in the direction normal to the surface. Release of tension that occurs causes a partial closure of the cut, which is detected by measurements of the relative distance between pairs of points placed in a symmetrical position with respect to the same cut.
A flat jack, realized with thin steel sheets welded, is then inserted in the cutting. The flat jack is connected to the hydraulic circuit of a pump. Internal pressure is then gradually increased to cancel the deflection measured after the execution of the cut.
Test is completed when – increasing the pressure in the cylinder – is obtained the restoration of the initial measurements (relative measures will lead to zero), and the corresponding pressure to the jack is the local stress in the masonry, unless the multiplicative constants kA and kM.
Reference standard: ASTM C 1196-09 and ASTM C 1197-04.
Double flat jack tests
Tests with two flat jacks in parallel (double flat jack test) allow to estimate the elastic limit resistance and deformability characteristics of a representative sample of masonry. Two parallel cuts, preferably carried out along the mortar joints and distant approximately 50 cm, allow the insertion of two flat jacks that are connected to the hydraulic circuit of a manual pump.
The sample of masonry so isolated is subjected to cycles of loading and unloading, with increasing levels load and its deformations are measured using four bases vertical and a horizontal base, constituted by pairs of aluminum dice placed at a distance of approximately 400 mm. Measures between pairs of points are made using special electronic devices with continuous reading. Deformations detected, in function of the pressure applied to the flat jack, allow to determine the modulus of elasticity and draw the curve of load/deformation of the wall structure subjected to compression.
Reference standard: ASTM C 1196-09 and ASTM C 1197-04.
Diagonal tests on site
Diagonal tests are carried out to determine the shear strength of the masonry, to achieve a level of knowledge 3 (LC3), in structural verification.
Test consists in isolating a square panel from a building wall, realizing four cuts with diamond wire saw or circular saw. Test on site is different from that of laboratory for the lower part of the panel which remains attached to the masonry wall; theoretical and numerical analysis indicated, however, that such scarfing, at least in the elastic phase, has negligible influence on the results. Action of compression is applied by hydraulic jacks arranged on one edge of the panel and is countered at the edge opposite. Appropriate devices must be provided at the corners solicited to allow the distribution of the load on the panel. During the test relative displacements of bases positioned along the diagonals of panel are acquired; more suitable measure instrumentation is composed of displacement transducers positioned on each side of the panel.
Reference standard: ASTM E 519.
The direct shear test aims to the determination of the average value of shear resistance for masonry brick blocks in situ. The test consists in the horizontal sliding of the brick block previously isolated inside the masonry wall. The horizontal load is applied by a horizontal jack opportunely inserted in the space of one of the removed units. The shear strength is therefore measured for the bed mortar next to the loaded block and it is calculated considering the joint gross area. It is assumed that it is fully grouted.
Reference standard: RILEM MS-D.6; ASTM C1531 – 16.
Sonic and tomographic tests
Sonic surveys return the velocity of propagation of mechanical waves (sound) through the investigated wall. These tests provide indications of qualitative nature (quantitative if carried out in a comparative manner, for example to identify singular situations of repetitive elements) on the strength of the masonry and on the presence of cavities, cracks or heterogeneity of material intercepted along the transmission path of the wave.
Tomographic sonic surveys represent the methodology that provides a detailed analysis of a particular section of interest; these tests, based on the combination of sonic acquisitions of several directions in a single section, can improve the knowledge by a “mapping” more definite of the speeds obtained.
Sonic investigation technique is based on generation of sonic pulses with frequencies in the field of mechanical noise (20-20000 Hz). Sonic wave is generated on the walls using keystroke with an instrumented hammer, and it is then received by a sensor (piezoelectric accelerometer) located in a different part of the structure. Both devices are connected to a signal amplifier and an analog-digital converter for viewing and recording of data on a laptop.
Reference standard: ASTM C597 – 09.
Surveys with videoscope
Survey with videoscope allows direct visual inspection of cavities and other characteristics otherwise inaccessible, in the thickness of the wall.
The apparatus comprises a micro camera and a video processor. Everything is connected to a laptop computer with appropriate software installed. The probe consists of a bundle of optical fibers for illumination and of a CCD sensor that picks up the light signals and transmits them by cable to the video processor, which processes them into images that are reconstructed on video. Images detected and transmitted to the monitor can be stored in the form of photographs and / or movies that are then transferred to a PC for processing.
Coring tests on masonry
Mechanical survey, carried out by drilling machine with continuous core, allows the direct observation of the support layer in order to get through the stratigraphy of the masonry and verify the presence and location of discontinuities.
You can make both surveys in order to assess the depth of the laying and the nature of the foundation both coring tests on the walls for the analysis of the internal composition (including using videoscope) and for the extraction of samples of mortar from analysis in laboratory.
Analysis of samples of mortar
Test consists in the extraction of a suitable quantity of mortar from the inner portion of the walls for the physical-chemical characterization (mineralogical-petrographic study). Samples of mortar extracts are then photographically documented, cataloged and analyzed under a polarizing microscope in thin section. The study defines the compositional and textural characteristics of the mixtures by identifying the mineralogical constituents of the binder and aggregate, and assess the conservation status of the mortars.
Sclerometer tests on mortar
This non-destructive test provides information about the mortar quality; it measures the material capacity to resist to penetration. This method is used to evaluate the hardness/strength of the tested mortar.
The recorded rebound defines the value of compression strength, according to conversion tables.
Tests with hammer
Hammer test is a non-destructive survey method capable of providing information on the quality of concrete, by measuring the material’s ability to resist in penetration. The method is used to estimate the cubic resistance to compression of the conglomerate tested, to investigate the degree of homogeneity of the mechanical properties, to delineate areas of poor quality or degraded, not being however the methodology intended as a substitute of the compression test on concrete specimens.
A cylindrical body provided with a rod that protrudes from one end of the housing and is loaded by a spring composes the hammer. The rod is pressed on the surface to be tested up to the limit of its travel. At this point, an internal mass, driven by a spring, hits the ring rigidly fixed to the rod, which in turn, is in contact with the surface of test. The mass, after beating on the rod, bounces to a certain height and is shown by an index placed on a graduated scale. Rebound height is proportional to the surface hardness of the concrete (the link between hardness and resistance is purely empirical and probabilistic).
Reference standard: EN 12504-2/2012.
Tests with covermeter
Tests using covermeter allow the localization of steel in reinforced concrete, the measurement of the concrete cover and provide qualitative data about diameter of the main and secondary bars, in relation to the difference in magnetic properties between these and the concrete. This test, completely non-destructive, consists in the passage of a probe on the surface of the structure. Data obtained are processed and displayed on a display and/or stored in the receiving unit.
Reference standard: BS 1881-204/1988.
Coring tests on concrete and specimens crushing
Taking samples of hardened concrete cores and determining the compressive strength in work is useful when you want to carry out checks on structures.
Sampling of concrete is performed using continuous core in structural elements using a core drill. Each sample shall be clearly identified, noting the specific point of withdrawal and any irregularities.
In certificate laboratory specimens are crushed after correction for the determination of the compressive strength.
Reference standard: EN 12390-3/2009, UNI EN 12504-1/2009.
Depth of carbonation is determined on cores extracted split along planes normal to the surface exposed to the air, or on-site chipping to expose an appropriate area of concrete recently broken. This test can be done by drilling a series of holes in the concrete, to allow the fracture of a fresh surface between them.
The broken surface is cleared by dust and splash through nebulizer with a solution of phenolphthalein with 1% in ethanol. The determination of depth of carbonation should be carried out immediately after collection; if for special reasons you cannot avoid a wait, specimens should be stored in airtight containers.
Reference standard: UNI 9944/1992, UNI 14630/2007.
Pull-out test is a survey technique that allows to evaluate the average resistance of the concrete by the extraction of a bar or an anchor pre-incorporated or post-inserted into the conglomerate. The anchor or the bar is inserted through typing, allowing the enlargement of the radial inside for a perfect adhesion to the walls.
A hydraulic jack is then applied on a circular surface; through the pull of the anchor, the breaking of a concrete cone happens. The force that causes the breaking of concrete is then related, through experimental calibration curves, with the characteristic resistance of the concrete. Measured value interests the surface layer of the structure investigated. Pull- out test is performed in areas without reinforcing bars, suitably identified by covermeter. Typically, you perform three extraction tests for each area under investigation.
Reference standard UNI 10157/1992.
Ultrasonic tests estimate the strength of the concrete trough the speed of propagation of the ultrasonic waves in the same material. This speed is closely correlated with the elastic modulus of concrete, which is in turn correlated with the compression resistance.
Ultrasonic test, therefore, wants to determine the propagation time of a pulse of mechanical vibration in the concrete between one or more pairs of survey points to derive information about the homogeneity of the concrete. The apparatus comprises an electrical pulse generator, a pair of transducers, an amplifier and an electronic timing device for measuring the time interval that elapses between the beginning of the pulse generated by the transmitter and its arrival to the receiver.
As for the sonic tests speed measurements of the pulse can be performed by placing two transducers on opposite faces (direct transmission), or on adjacent faces (semi-direct transmission), or on the same face (indirect transmission) of a structure in concrete. Test points are selected with a covermeter that identifies armor surface.
Reference standard: EN 583-1/2004, UNI EN 12504-4/2005.
Tensile tests on metal bars
When you want to carry out checks on structures, it is a good idea to remove pieces of steel bars, examine and determine the tensile strength in work. After identifying the reinforcing bars by covermeter, we scarify the surface of concrete structures, with small power hammers, for the unveiling of the bars and finally the cut of the same with angular grinder.
In authorized laboratory, the tensile test on the bars extracted is used to determine the mechanical properties of the material. In particular the yield stress (conventional at 0.2 % of elongation or actual), the breakdown voltage for the determination of the characteristic strength and ductility of the material through the determination of elongation at break are determined.
Reference standard: EN 10002-1/2004.
SONREB method is based on the combination of results obtained, in the same areas, from tests with hammer and ultrasonic probes, correlating the rebound index (Rebound) with the velocity of ultrasonic waves (SOnic), with the compressive strength of concrete. A calibration of the relationship between these three variables is carried out by statistical regression of experimental values.
The validity of SONREB method derives from the compensation of inaccuracies of the two non-destructive methods used. Moisture content underestimates the index obtained by hammer and overestimates the ultrasonic velocity, and, increasing age of concrete, the index obtained by hammer increases while the ultrasonic velocity decreases too.
Executing compressive tests on samples taken in the same places where non-destructive tests were carried out, we determine the values to be assigned to the three correlation coefficients between resistance, index rebound and ultrasonic velocity.
If the number of results of compressive tests on cores is limited, you can use formula proposed in literature, identifying the one that best fits to the results of compressive tests.
Reference standard: EN and UNI EN 12504-2/2012 12504-4/2005 .
Measurement of the corrosion potential
The survey consists in the measurement of the corrosion potential on R.C. structures exposed to atmospheric agents. The field potential is mapped with the aim of value the corrosion state of the bars.
First, the electrical continuity of the bars is verified and, later, the electrical continuity of the bars with the connection point is checked.
The reference electrode is placed in electrolytic contact with the concrete surface: the reference electrode is connected to the negative terminal, the reinforcing steel to the positive terminal of the voltmeter. Measures are performed shifting the reference electrode in different zones of the structure surface following a reference grid previously draw on the tested surface.
Reference standard: UNI 10174/1993.
Test is based on the relief of the puncture resistance opposed by the timber. This resistance is evaluated by measuring the energy used by the instrument to allow the advancement of a thin steel tip at a constant speed. The puncture resistance is concentrated on the tip of the needle, because this has a double thickness compared to the stem. The electronic control of the motor ensures a constant speed of the needle. Speed must be adapted to the specific characteristics of wood density to examine.
Resistograph provides a listing of the sections and allows to estimate the size of any degraded areas not visible from the outside, so as to determine the real-resistant section of the structure. Graphs, obtained by the test report, show on the abscissa axis the depth of penetration and on the ordinate axis a percentage value, referred to the energy consumed by the instrument in response to the density of the studied section.
Tests with hammer on wood
Hammer on wood allows to estimate the mechanical properties of wood elements. The purpose of this test is the knowledge of the conservation of the wooden material, in case of historical monuments or buildings where it is often impossible for reasons of conservation, extract microcores by the element, not to compromise the integrity of the section.
The instrument substantially corresponds to a modified hammer in which is added on the rod of percussion a tip consisting of a steel needle with circular section. This tip is put into the wood element with a predetermined number of strokes. It is possible to correlate the penetration of the needle to the mechanical characteristics, the nature of the wood material and to its conservation, correlating the impact energy and the corresponding penetration.
Reference standard: UNI EN 408/2012.
Visual grading of the wooden elements
UNI 11119/2004 shows the classification rules and methods of measurement of quantifiable characteristics on wooden structural elements in place.
For assignment to a category it is necessary that all the features and / or defects are in the specified limits.
From these indications standard shows, for wooden elements of different wood species and categories:
– The maximum stresses, which can be taken for the application of the method of the admissible tensions;
– The average values of flexural elastic modulus, which can be used for the calculation of deformation of the structure in the limit states.
All values are taken from the text “Tecnica delle Costruzioni in Legno” by Guglielmo Giordano, where they remained unchanged in subsequent editions (first edition in 1946 – fifth edition 1999).
Reference standard: UNI 11119/2004, UNI 11035/2010.
Wood bending strength tests
For the determination of the bending strength of the timber the test piece shall have a minimum length of 19 times the depth of the section. Where this is not possible, the span of the beam shall be reported.
For the determination of the bending strength of the timber the test piece shall be symmetrically loaded in bending at two points over a span of 18 times the depth.
Load shall be applied at a constant loading-head movement so adjusted that maximum load is reached within (300±120) s. The maximum load (Fmax) of the bending test shall be recorded.
The loading equipment used shall measure the load to an accuracy of 1% of the load applied to the test piece or, for loads less than 10% of the applied maximum load, with an accuracy of 0,1% of the maximum applied load.
Reference standard: UNI EN 408/2012.
Identification of timber species
With the aim of the study of timber species with microscope, some wood samples from the beams are picked up for analysis in the laboratory.
The samples are then photographically documented, cataloged and subjected to test under an optical microscope in polarized transmitted light. Three diagnostic sections are prepared: transverse (perpendicular to the grain), longitudinal tangential (tangent to the growth rings) and radial longitudinal (parallel to radius of the growth rings).
Reference standard: UNI 11118/2004 and 11130/2004 UNI.
Pull-off test is a survey technique that allows the measurement of the tensile strength or tear resistance of a material applied to a substrate, such as wall cladding (finishing mortars, plasters or other). Test is made by practicing on the surface of the material to investigate circular incision of 50 mm diameter, with a depth at least equal to the thickness of the coating, using a special core barrel, to isolate the area under investigation. Then we proceed pasting on the surface a metal disc, on which we apply, perpendicularly, a traction force, by a jack fitted with a calibrated pressure gauge, until a fracture occurs on the plane of contact between the coating and the support.
Pull-off test is useful to determine the tensile strength and tear of plaster, mortar or other types of coatings for masonry or concrete walls.
Reference standard: UNI EN 1215-12/2002, ASTM – C1538 – 04, UNI EN 1542/2000, UNI 12636/2001.
Reference recommendation: CNR- DT 200 R1/2013.
Pull-out tests on FRP/FRCM spikes and Helifix ® bar
This methodology is used to determine the tensile strength of fiber flakes in an existing substrate. The system was developed to allow the application of the tensile load by an hydraulic jack. The force-displacement graph and the typology of failure are determined through the pulling force.
Instrumentation allows a precise measurement of the tensile force impressed, using a load cell, and the measurement of displacements by an adequate number of displacement transducers applied on the sample.
This test is a semi-destructive technique of investigation, useful for assess the grouting quality. The minimum number of tests is three.
This semi-destructive technique allows the measurement of the tensile strength or of the tear resistance of a material applied to a substrate, through the application of a load parallel to the fibers direction.
The execution is carried out in one free edge of the structure on which the reinforcement is applied (“direct” test). Test requires the availability of a suitable free portion of the composite material (ie not glued) in continuity of the pasted material. On it, the shaving action should be applied using as a contrast the edge of the building.
The bonding quality and the preparation of the surface may be considered acceptable if at least 80% of tests (at least two of three in case of only three tests) provide a breakout force of intensity not less than 85% of the value of the maximum design force.
Reference recommendation: CNR- DT 200 R1/2013.
Thermography is a non-destructive technique that operates using the band of infrared radiation. Each material emits energy in the form of electromagnetic radiation, as characterized by its own thermal conductivity and specific heat. Thermography detects and subdivides the infrared radiation emitted spontaneously by the individual points of a warm body, in a certain moment; detection is recorded by a special instrument that provides a thermal image of the object through scales of colors or shades of gray. A temperature range, which is generally in the order of fractions of a degree centigrade, corresponds to each color or tone of gray scale.
The instrument measures the energy flow E at a distance, without any physical contact with the surface examined. For this feature, thermography is considered a particularly non-destructive investigation technique.
Reference standard: UNI 10824-1 / 2000.
Load tests are performed on structural elements in order to verify experimentally their behavior under the exercise actions: acceptance tests, carried out before putting on or put into operation the structures, in order to verify the responsiveness to project forecasts, or analysis tests, in order to verify the behavior of a structural element already in work, can be.
Test consists in loading the slab until total load required. Displacement transducers measure failure during loading and the residue at the exhaust.
Monitoring vertical displacements by means of geometric levelling
High precision geometric leveling is the topographic method, which allows to determine the differences in height with the maximum precision reached in topography.
The instrument used by us is the high precision digital level Leica DNA03, with beam splitter, coupled with a leveling rod in invar. The accuracy of the method is high; differences in height with precision of 0.3 mm can be determined under well-controlled conditions.
Geometric leveling is used in testing and/or control of structures, buildings or in monitoring of monuments (towers, bridges, historical buildings, etc.) and slopes in landslide.
Geometric leveling is also used for technical purposes, such as the design and construction of roads, railways, aqueducts, sewers, etc.; in these cases the reference dimensions for the construction of structures are to be determined with high accuracy to predict, for example, the flow of water or design drainage lines. Another field of application of geometric leveling is the determination of basic points distributed in vast areas to which you can connect other operations of leveling.
Moisture measurement of mortar through “carbide method”
The carbide hygrometer allows to measure in-situ the moisture in building materials (concrete, aggregate, mortar) or equivalent, using the method of reaction of calcium carbide.
The instrument consists of a sealed chamber in which you insert the material taken with calcium carbide (CaC2). The calcium carbide combining with water that depends on the moisture of the sampled material, produces acetylene. More gas is formed higher is the pressure in the test chamber, because the reaction takes place in a closed environment. Pressure is measured by a manometer, which provides directly the percentage of moisture with a graduated scale and by relations of correlation.
Together with the gravimetric method (thermobalance), the carbide higrometer is the only “direct” test that can provide the water content of a solid in a quantitative manner.
Pull-out tests on anchors in concrete and masonry elements
This semi-destructive technique allows the measurement of the tensile strength of anchors in concrete and masonry elements.
The test is carried out by the use of an hydraulic jack, which apply a concentrated load on the anchor via a threaded bar connected to the load cell.
The tests are carried out applying a monotonic loading up to braking the anchor by manual pumping of oil into the jack. At the same time is made a controlled recording of data read from the PC.
Determination of the tension in the chains
Control of tension in the chains of historic buildings is a useful tool for understanding the conditions of thrust of vaults and arches subtended by the same, also for a control repeated in order to re-evaluate the conditions previously found (monitoring). Analysis of the shot in the chains can be performed using non-destructive surveys (static and dynamic tests).
Both in static and in dynamic analysis the main problems appear to be the constraint conditions. Analysis methods used envisage two limit solutions that relate to the constraint of the ends of pure or interlocking hinge. Clearly the two limit situations create a band in which is positioned the real situation.
Identification tests are aimed at the characterization of the dynamic modal response of the structure. Results obtained are directly related to physical and structural parameters of building subjected to investigation, as the geometry (mass distribution), the stiffness and the constraint conditions. Models derived from structural identification are used to assess the current behavior of the building and pose as a useful predictive tool against exceptional actions, such as earthquakes.
Modal identification techniques operate in time or frequency domain. The first procedure consists on the extraction of FFT (Fourier Transform – Fast Fourier Transform) from signals recorded and on the subsequent correlation between the various channels, while the second group consists on the parametric “fitting” of a mathematical model. Surveys consist of vibration measurement of several points of the building under study in order to identify the natural frequencies and mode shapes associated with them, through excitation environmental (traffic…) or forced (vibrodyne…). From experimental modal parameters you can proceeds with the identification of the FEM models.
Ceilings hollow brick deterioration and verification of the false ceilings
We proceed according to the methodology indicated below, to acquire adequate knowledge of the ceilings that may be affected by the hollow brick deterioration:
The thermography is used for detecting the warping of the ceiling and the presence of anomalies (water infiltration, the presence of moisture …).
– Investigations on the ceilings
Investigations are carried out using videoscope, to “explore” from the intrados of the slab and recognize the type of construction, evaluating the presence of technological vulnerabilities.
– Beating of ceilings
To verify the presence of brick elements potentially subject to break and fall down, we proceeded executing a manual percussion of the ceiling on a regular mesh (about 50×50 cm) by a hollow aluminum rod: the response of the noise anomaly is the index of the presence of hollow brick deterioration.
– Instrumental analysis (acoustic auscultation)
In areas where we find a “worst” or “doubt” response by manual typing, we proceed with a mechanical percussion with the signal recording.
– Inspection of the anchors of the false ceilings
Inspection are made to acquire adequate knowledge of the structures and anchoring surface of false ceilings. Visual inspection and photographic survey are conducted, also checking the anchoring of non-structural elements anchored to the ceiling.
For each type of false ceiling (when geometrically possible), load tests by application of lumped masses in a single fixing to the slab are carried out. The load is maintained for few minutes to check the tightness of the anchoring system.
– Report with colour map
Appropriate technical report with the areas subjected to hollow brick deterioration with a colour scale according to the hazard detected is drawn up.