OPERATIVE GUIDE P.I.T. PILE INTEGRITY TEST

Transcription

1 OPERATIVE GUIDE P.I.T. PILE INTEGRITY TEST 1

2 Echotest procedure / PIT Pile Integrity test with MAE ETBT instrument Generals Theory notes Pile Integrity Test (PIT) is a simple non destructive test which can be performed in short time. Echotest, exploiting theory of propagation of elastic waves in a medium, is useful for detecting anomalies such as fractures, discontinuities, variations of pole section and to estimate length of the building. This method, born in Netherlands in the seventies, is well suited for precast concrete piles because of their regularity that will give more reliable measurements. It must be clear this test cannot provide information on bearing capacity of the work, but it can provide only information on quality of concrete. Methodology is essentially based on the physical principle of reflection, which means wave is generated with an impact on pole head and propagates up to pole base recording a signal by a receiver. More specifically, survey consists in generating at the top of the pole a compression wave through an instrumented hammer. Wave propagates with constant speed inside the pole (it is assumed that the medium is mostly homogeneous) until the opposite end where, because of change in density associated with discontinuities between background and concrete, it is reflected. Presence of any cavities, cracks, inclusions or poor quality concrete parts, generates anomalous reflections that overlap the original signal creating a returning signal which contains such information. Reflected signal is registered by a proper accelerometer, fixed at pole end. Analysis and correct interpretation of the signal called in technical terms "Reflectogram", allows to evaluate integrity of the pole. Length can be estimated from the known wave travel time (which travels at constant speed) from one end of the pole to the other. This test allows to investigate many poles in short time and with low cost. It is important to remember that test can be influenced by several factors which can affect the results. In this sense we want to underline that fractures or very large cavities do not allow to investigate the parts / units below them. In any case, strength points of this investigation are speed of execution and not particularly onerous data processing which require operators with a minimum of experience and 2

3 training. Reference regulation - ASTM D5882:2007 Standard Test Method for Low Strain Integrity Testing of Piles ; - Technical constructions regulations of 14 January 2008; - Ultrasonic speed through concrete is similar to that of ultrasonic test (UNI EN 9524). Data recording mode Present paragraph describes instrument functioning and the steps to be done in field for a correct execution of the test. Prepare with proper tools the surface (head of the pile) on which ETBT will be placed. Therefore it is recommended to bring gloves and brushes or iron spatula to remove debris, bentonite layers and angularity from the head of the pile. Without these precautions test can be altered. Test accuracy in determination of pole length directly depends on sonic pulse propagation speed within concrete, a factor that depends on the age and the strength of concrete, varies from pole to pole, and has an error margin of 10%. Travelling through the pile, wave loses energy due to internal frictions and whenever it encounters a change in pole section, a reflected wave that propagates upwards is generated, where it is detected by ETBT accelerometer. Impact point must be chosen close to pole center and the receiver (ETBT device) should be placed at cm from the edge within the armor. It is important to carefully check "coupling" between ETBT and concrete surface on which it is placed; it is recommended to use grout but also adhesive material such as plastic. It can be also used a mass of few kilos to increase adherence. When the head of the pile is hit in desired spot, impact starts signal acquisition coming from the accelerometer; this is shown immediately on Android device screen. To get an 3

4 average more reliable data, it is recommended to repeat test many times (from 5 to 10) so that during processing it is possible to delete any wrong measurement or any data different from average values. Resulting signal is then saved on Android device for later processing. Detailed operations to perform on device are described below. At Echotest Bt application start, screen as picture 1 appears on Android device. Picture 1 : Application home. By clicking on Connect button, list of instruments previously coupled will be displayed (see Pic.2), then scan peripherals detected by Bluetooth system; among that, the one named BT-232 must be searched. Once found and selected it, pairing code must be entered. Pairing-code is 1234 or Green written connected to: BT-232 indicates correct connection. Entering code procedure for connection is necessary only at application first start, then it will be necessary just to select device into the list. 4

5 Picture 2:Devices previously connected To proceed with echometric test, some information are required. For future reference to the investigated pile, it is recommended to identify it through site name and a brief description. These data will be used later, when saving recordings, to generate file name. To start a new test, press button New. Screen to enter pile identification data will be displayed as in picture 3. Picture 3:Pile identificative data entering. In addition, it is necessary to enter pile estimated length, in order to facilitate reading of 5

6 reflected pulse, and average propagation speed of compression waves into investigated concrete. It can be estimated according to concrete type and its maturity. If user has accessories for ultrasonic measures, it is recommended to measure speed directly through contact probes for a better characterization. Once entered and saved information, they will be displayed on main screen. Moreover, on graphic a vertical cursor will appear, in pink color and named P, which indicates entered pile length as in picture 4. If entered data are not correct, they can be changed through New button. Picture 4 : Entered data visualization. After entering identification values, speed and length, device must be configured; by clicking Setting button, configuration menu will be accessed (pic. 5). 6

7 Picture 5: Configuration window. Setting parameters are described below. Timebase, which indicates sampling interval of signal received by accelerometer, determines also observation time after hammering. Recommended setting is microseconds. It is recommended to set it so to have P cursor at 3 quarter of the screen. Trigger, which determines necessary intensity to start recording: instrument sensitivity decreases at this percentage increasing. It is recommended to set it at 30%, given seismic vibrations usually present on site. Compensation, which allows to apply an amplification, increasing exponentially in time. Signal generated by accelerometer at hammering is hundreds of time higher than signal reflecting few milliseconds later, it is clear that applied amplification cannot remain constant during recording. It is recommended to start with cursor placed between a quarter and middle of its travel (compensation 45%). Low-pass filter (LPF), which is necessary to remove high frequencies disturbing recording. In most cases it is recommended to activate low-pass filter to remove higher frequencies noise. Moving cursor towards right, a more and more low shear frequency is 7

8 selected. Gain varies recording wave amplitude. 2.5 gain is recommended. Invert which, if activated, allows to invert recording polarity, that is to say turn semi-wave from positive to negative and vice-versa. Once set necessary parameters detailed above it is possible to start recording by clicking Start button on the application, in order to arm trigger and prepare it to hammering. On device trigger led will brighten to confirm status. At each hammering device will receive an acquisition and related reflectogram will be displayed, confirming it through a beep and a vibration. Before proceeding with a second hammering, wait for trigger rearmament, which can be tested with a second vibration (if active), with trigger led lightening on ETBT and Start button in green color. At each hammering a reflectogram will be added in recording list, while that present in main screen is average of all recorded reflectograms. It is possible to include or exclude a reflectogram from average by clicking on it once opened the list (pic.6). Picture 6: Window displaying sequence of performed recordings. Reflectogram 4 is excluded so it won t be computed in average computing; to include it, just click again on it. 8

9 The estimated length of the pile can be acquired on field by touching the screen on graph in echo matching point. You will see a red vertical cursor with the distance in meters of its position. When test is finished, it is possible to immediately visualize on site test report containing all data of the survey. When it is possible to find return point (or points) with sufficient accuracy, save data acquired by pressing "Save" button on the Home and choose a name or leave the one set above. File will be saved in *.wav extension in the SD-card of Android device, in the folder Echotest (directory:/mnt/sdcard/echotest). Moreover it is possible to add descriptive notes to recording. By clicking on button Menu screen shown in picture 7 will be displayed. Pic.7 Menù Supplied software Echowin Net allows to elaborate survey and do proper evaluations about status and depth of investigated pile. 9

10 PILE PROFILE DESCRIPTION REFLECTOGRAM Pile: base Base: fixed Length: as estimated Pile: full Base: free Length: as estimated Pile: full Base: fixed Length: shorter Pile which broadens Pile which narrows Pile which broadens locally Pile which narrows locally Irregular profile 10

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