operative guide sonic rebound · - measure of ultrasounds propagation speed (uni en 12504-4, 2005)...

Guide SonRebSonic+Rebound

1 MAE advanced geophysics instruments - www.mae-srl.it

OPERATIVE GUIDE

Sonic Rebound SONic + REBound = ultrasounds + sclerometer



Generals

This methodology combines rebound and ultrasonic tests to investigate element (Menditto 2008). In general method allows to estimate compression resistance of concrete, improving reliability of each single methodology, that is reducing influence of factors such as: humidity, inactive corps dimension, inhomogeneity between surface layers and deeper layers, possible processes of carbonation. Hovewer, method brings evaluation of concrete resistance inevitably affected by different uncertainties, depending also on reliability of coefficients used to relate experimental results with concrete resistance. The great number of formulations available in literature has been mainly developed for concretes with recurring resistance, while those of existing buildings often have different features and weaker resistances, therefore may be not reliable. Target of this methodology is to calculate concrete average resistance to compression by combining ultrasonic speed obtained from ultrasonic test, with bounce indicator obtained with rebound hammer tests, compensating in this way mistakes of both tests. Humidity content and concrete age influence in reverse way both tests and so combining “Sonic” and “Rebound” allows to significantly improve reliable of single methodologies. During measures, has been noticed that humidity content causes underestimation rebound hammer indicator and overestimation of ultrasonic speed, moreover the older concrete is, the more rebound hammer indicator increases while ultrasonic speed decreases. Interest into combined method lies in the fact that it presents, compared to other methodologies not destructive or semi destructive, the advantage of simplicity and executive rapidity, allowing to test extended structure portions in acceptable times and costs. For the editing of present guide we referred to scientific articles, reference regulations and

documents spread by companies leader in sector.



Reference regulations

Normative standards which must be respected for a correct execution of measures are listed below; present document refers to different scientific articles and, firstly, documents and volumes spread by Consorzio della Rete of University laboratories of Seismic Engineering (ReLuis): “Guide lines for investigation modalities on structures and soils for repairing projects, improving and reconstruction of condemned buildings”.

- Measure of ultrasounds propagation speed (UNI EN 12504-4, 2005)

- Measure of rebound hammer bounce indicator (UNI EN 12504-2, 2001)

- BS 1881-204, DIN 1045, CP110 Regulations

- Reference regulation is UNI EN 12504-4.



Acquisition and data restitution modality

Instrument calibration represents a main step for a correct execution of the investigation. Rebound hammer should be controlled through the measure of bounce indicator of the instrument located in vertical position on the calibration steel anvil; instrument for ultrasonic measures should be controlled by verifying acoustic probes with ultrasonic speed in a specific sample with known propagation speed. Ultrasonic speed may be accomplished through different instruments: - Cronosonic

- Cronosonic X

- Sonic 15 Working principle of each single instrument is illustrated in their specific manuals; particular use of instruments and software interface are not important in present operative guide. Rebound hammer tests can be realized with digital rebound hammer by CONTROLSGROUP. Below we summarise main operative phases of measures in order to make this document an efficient support for specialists of the sector. Instruments must be used within working conditions declared and data have to be saved for later analysis. Instruments allow to visualize signals obtained in real time on site so that qualitative evaluations can be done.

Once identified structural element to investigate through SONREB method, it is necessary to ensure the presence of reinforcing bars. They can be detected in a direct way by removing concrete covers or, as an alternative (preferably their integration), not invasive methods can be employed such as the use of cover meters. Rebars may significantly affect ultrasonic speed generally increasing it; by repeating the operation on more sections of the element and drawing on its surface a straight line passing through located points, it will be possible to mark the network of rebars. In below image the result of a cover meter investigation.

DETECTED REBARS



Ultrasonic pulse velocity is a standard method in diagnostics of concrete buildings and from the analysis of compression P waves in the material it is possible to obtain transit time (time of fly T.O.F.) of ultrasonic waves into material and their transmission speed. More in detail, a longitudinal waves train is produced through an electro-acoustic transducer kept in touch with concrete surface put under investigation. Pulse is generated through a sonic electrodynamics transmitter (using a piezoelectric ceramic pill). Results make possible the use of non- destructive techniques in construction phase of a new building, offering to specialists of sector an efficient instrument for punctual monitoring of the investigated material. It is important to explain that with present survey it is not possible to get information about load capacity of the work but only information about quality of concrete or more in general about investigated means.

Rebound hammer method consists of provoking an impact of a conventional mass against the surface of investigated material and measuring bounce height; measures is expressed in percentage (bounce indicator IR)compared to the distance travelled by moving mass between the moment when it is released and the moment when it hits concrete surface. Considering that kinetic energy of the mass is standard, bounce height depends on the energy dispersed during impact that, in turn, depends on the mechanic energy of concrete surface. Operative steps are described in the UNI EN 12504-2 regulation, which doesn’t provide indications about results interpretation aiming to the evaluation of compression resistance of the concrete in work. A particular of calibration phase for the digital rebound hammer from CONTROLSGROUP is shown in above picture. Please remember that tests are influenced by different factors which may invalidate and threaten results. Most reliable data are obtained when testing not fissured and homogeneous concretes.



Measure of ultrasonic speeds

To get statistically reliable results and not subject to unplanned/punctual natural phenomena, it is recommended to measure of ultrasonic speeds in more parts of the investigated element and to calculate average value so as to have a more careful data estimation. As recommended by reference regulation, it is better to perform measures through direct transmission or “Transparency”. “Cronosonic” instrument is exclusively suited for realizing measures through direct transmissions; indirect and semi direct measures can be done with the versatile instrument “SONIC 15” which, through dedicated sections, can perform also “Pile Integritiy Test” and sonic test with instrumented hammer.

DIRECT INDIRECT SEMI-DIRECT TRANSMISSION TRANSMISSION TRANSMISSION

In case of direct transmission, as stated by UNI EN 12504-4 regulation, distance must be reported with a precision of ±1 %.

In case of semi- direct transmission, distance between the centers of contact surfaces of transducers has to be measured, in this case path measure accuracy depends on probe surface with relation to the distance between faces of the same. In case of indirect transmission, a series of successive measures must be realized by gradually moving receiving probe.

Indirect method is considered the less sensitive and precise and it is used only when the other surfaces of structural element aren’t accessible or when research target is the quality of a determined surface area of concrete. The method allows to investigate reduced thickness of structural elements and for this reason result can be not significant.



Automatic measure particular tool of the available instruments makes investigation simple and fast. Setting up automatic measure, SONIC 15 instrument is arranged for regular intervals measures allowing to visualize different detected speeds in a short time.

More in detail, in automatic mode instrument repeatedly performs measures with temporal pitches variable from 1 to 10 seconds. In this way it is possible to check how transit speed compute varies at each pulse emission; when the measure is stable, it is possible to stop acquisition and proceed with data saving. An example of measure carried out through “SONIC 15” instrument is shown in above

picture.

In general on every structural elements at least 10 measures must be realized on different paths so as to better ensure data quality and compute the average. In case of indirect measures the 10 measures may be determined for example along the

following paths (1-2, 1-3, 1-4, 1-5, 1-6, 2-3, 2-4, 2-5, 2-6, 3-4, 3-5)



Approximate values of “apparent” V speed of ultrasounds in concrete are: for bad quality concretes V < 3000 m/s, for average quality concretes 3000 m/s ² V ² 4000 m/s, for good quality concretes V > 4000 m/s). User must prepare for the measure and so has to prepare instrument in such a way to easily manage data acquisition operations through the instrument he intends to use. Before probes application, surfaces of the investigated structural elements must be clean, smooth and plain. If there is plaster or cover material of beams or pillars, it must be removed and avoid measures in visibly homogeneous areas or with surface defects. It is possible to refine surface with manual or electric abrasive grinder. First operation to carry out is fixing ultrasonic probes to the structural element to investigate. A good acoustic contact must be ensured by using coupling means provided together with instrument (hydrosoluble gel) or other like vaselina, grease, soft soaping, kaolin/glycerin paste. Also grout can ensure an excellent contact. It is necessary to apply always same compression on probes because it can influence visualized time value which may change according to the applied compression; in particular if the compression is excessive time value decreases. After measuring probes distance and entered it on the instrument that is used through specific functions (refer to Instrument Manual) it is possible to connect ultrasonic probes to specific cables. Distance may be detected through a callipers or a template and, for better detail, it is possible to detect also probes position according to a reference system.



At this point user can proceed to crossing speeds measurement phase and data save for the next elaboration phase. As recommended by reference regulations, it is important to check homogeneity of speed values (difference between maximum and minimum value must be within the range 200 ÷ 300 m/s). Measures which involve transmission speed into concrete over than 4800 m/s or lower

than 2500 m/s must always be verified /or repeated.

It is possible to use different kind of probes in relation to the structural work that user wants to investigate and to used methodology (probes distance). We describe high efficiency ultrasonic transducers of active type that are mainly required

by customers; they are produced by MAE based on the highest qualitative standards to

provide a long-term efficiency without signal decay.

SCU23DU15 Probes technical specifications: - Min voltage excitation: Vp 300 - Max voltage excitation: Vp 2000 - Resonance frequency: 23 Khz - Operating temperature range (°C): -20 a +80 - Dimensions (Diameter) PVC cap: 38 mm - Dimensions (Diameter) cylinder: 30 mm - Total height: 90 mm - Weight: 235 g - Sensitive ceramic element - Stainless steel case - Sealing: welded hermetic - Connector: ODU / 6 poles + 1 unshielded pole - Cable length: 2.5 mt



Survey itself must be considered wrong or less valid if preliminary operations of pairing

between concrete and each transducer surface are not observed. In this sense it is recommended

to ensure contact with another user’s help.

Responsibility of accepting or refusing the results of an integrity test is exclusively of the person who performed survey.

If possible, measures close to rebars, parallel to pulses propagation direction, must be avoided. It is necessary to pay particular attention to the presence of rebars which can be negligible only if correlation between sum of the diameters crossed by waves train and total length of the way is lesser than 0.06 (for rebars perpendicularly placed along the way) or lesser than 0.30 (for rebars placed in parallel along the way).

SCU70DU15 Specific technical probes: - Min voltage excitation: Vp 300 - Max voltage excitation: Vp 2000 -Resonance frequency: 70 Khz - Operating temperature range (°C): -20 a +80 - Dimensions (Diameter) PVC cap: 38 mm - Dimensions (Diameter) cylinder: 30 mm -Total height: 50 mm -Weight: 190 g -Sensitive ceramic element -Stainless steel case - Sealing: welded hermetic - Connector: ODU / 6 poles + 1 unshielded pole - Cable length: 2.5 mt



Please remember that survey goal is to measure transit time of longitudinal P waves (compression) identified in regulations as Time of Fly (TOF) or First Arrival Time (FAT). When performing test it is recommended to repeat measures until you get a value lower than the time; this indicates that thickness of coupling means has been reduced to the minimum. Before proceeding with measure of the element to research, user could test instrument through provided sample so as to check proper functioning especially when anomalous data are recorded or there are doubts about setting parameters. Test on sample with known and defined speed is a preventive control to guarantee that there isn’t Probe Delay if different times from that indicated on sample are recorded. To further references, please refer to appendix A of regulation UNI EN 12504 4:2005.



Determination of rebound hammer bounce index

Survey operating procedures are described in the UNI EN 12504-2, which provides no

indications about results interpretation for the estimation of concrete compression

resistance. Bounce index involves many indeterminations and probable accuracy in the

estimation of compression resistance that is about ± 25% suggest the limited use to the

evaluation of concrete uniformity. Rebound hammer measures are finalized to the

determination of concrete resistance through the measure of surface hardness, through

evaluation of a metal sphere bounce contained in a proper cylinder. These are the most

used tests, although values they provide, if not combined to the ultrasonic test (SonReb

method),often result to be not very significant.

As stated in UNI regulations, bounce index has to be considered as average on the number of beating done in the same measurement area (it’s wrong to repeat them on exactly the same physical point) which surface has been conveniently prepared (without roughness). Areas on which rebound hammer beatings are done must be internal to the areas of only concrete circumscribed by rebars network individuated through cover meter measures. Criteria to choose beating points in general should be to avoid joint, crawl spaces and

porous areas; walls with minimum thickness of 10-15 cm and pillars with side lesser than

12 cm might distort rebound hammer indications since their elasticity. Cleaning of

investigated surface is the first preparatory phase for the measure. Below image shows

the employ of abrasive rock which is usually used for the cleaning.



You may avoid to carry out measures in correspondence of detached or clearly

deteriorated concrete.

Also in the case of very old and superficially very hard concretes, it will be necessary to grind surface for a depth equal to carbonated layer thickness for an area sufficient to do from 5 to 10 sclerometric beatings. For the grinding it is recommended a flexible of 750 W with grindstone of about Ø 120 mm and

speed of 6000 rotation/minute.

More in detail, on every face of concrete surfaces it is better to perform at least n°9 measures or not overlapped beatings distant not less than 25 mm between each other or from the border of possible present lacks and rebars, preventively located. Beating points are composed of intersection points of grid lines. The image extracted from “Manual of not destructive test”, by SIDERCEM Institute of

Research and Experimentation on Construction Materials, reports an example of uniform

grid with points in which it is possible to carry out measures.

Test result is provided in terms of average bounce index Ir and it is reported for each test point. If, for each point, more than 20% of all measures moves away from the average for more than 6 units, the whole measures series has to be rejected. Bounce index must be evaluated by carrying out measure with the rebound hammer in horizontal position. If this is not possible, in order to determine the equivalent bounce index measured in horizontal, please refer to information curves specified by instrument manufacturer.



This operation further reduces reliability of survey results.

In the end we remind you that rebound hammer functioning must be periodically

verified and calibrated (when the instrument doesn’t allow calibration and it results not calibrated,

it has to be replaced), carrying out measures on a specified mass called setting anvil or

comparison mass. Preliminarily to every reading, it is necessary to activate at least three times the

instrument to check correct working.

During the test, rebound hammer axle must be always perpendicular to structure surface because a possible inclination would influence negatively results, because bounce of the beating mass can be more or less disturbed by the gravity. Usually, inclination established by the diagrams attached to the instrument are related to the following corners: α = 0° (pillars, girders ecc.); α = + 90° (horizontal bases, soffit of girders and frames); α = - 90° (foundations plinth, road bases ecc.). So it is necessary, according to the kind of structure to test, to pay great attention to the

correct use of the instrument in order to get satisfying results.



As orientation purpose, average values used to express concrete quality with described

methods are listed below. Rebound hammer index is converted into cubical compression

resistance through correlation curves related to rebound hammer direction of use.

SonReb measure determination

Method application requires the evaluation of local values of the ultrasonic speed V and of the bounce index S, from which it is possible to get Rck concrete resistance through expressions such as:

Rck= a ×Sb×Vc S= sclerometric bounce index V= ultrasonic speed a, b, c = parameter of no-dimensional correlations to determine with experimental

measures.



To facilitate technicians in computing operations, CSXreader software has been

developed, with tool to select the different relations present in the scientific literature; it is

important to remind that no one of these expressions has general validity, especially when

low quality concretes are analyzed.

Software allows to save calculated value of compression (Rck) resistance and computing

parameters, by clicking on the button “Save” in such a way that in future opening these

values will be automatically visualized by the software. Software allows to export files in

format .csv e .txt

operative guide sonic rebound · - measure of ultrasounds propagation speed (uni en 12504-4, 2005)...

Documents