LABORATORY TESTS OF COMPRESSIVE CREEP IN CONCRETE
Contents
1. 1991 2002 DataTaker DT5 6xx User s Manual Series 3 2002 DataTaker Powerful Data Logger and Flexible Data Acquisition and Data Logging Systems www datataker com SOLARTRON Metrology Complete Catalog C2006 01 E 2006 SOLARTRON Metrology Analogue Electronics Basic Operation of the DC DC Transducer Application Note 502355 Issue 2 s d TRAFAG Sensors and Controls Display Control Switch DCS 8864 Datasheet 6 2 2 4 2006 LNEC Specifications Concrete measurements of creep in compression E399 1993 LNEC 1993 ICEM15 72. 1s expected and had to be corrected with further analogue filtering at the entrance to the data acquisition system in this application digital filtering would require sampling rates higher than 10 kHz which was obviously out of scope Electromagnetic disturbances were mitigated by means of 1 short length cabling the data acquisition cabinet is placed near the centre of the plant which is distributed by two rooms 2 separation of signal circuits from power circuits 3 careful grounding and 4 signal cable screening Good quality and stable power supplies of 10V 10V were chosen in addition both supply voltages are also monitored by the data acquisition system in order to make suitable corrections since the sensitivity of LVDTs is proportional to the excitation voltage Each LVDT carries an individual sensitivity value obtained by calibration in the factory under a given excitation and environmental conditions which are not always met in the applications For the present system an in situ calibration with a traceable precision instrument was needed It is highly recommended that an individual calibration is made for each LVDT chain including the LVDT itself the respective cabling and filtering as well as its exclusive channel in the data acquisition system CONCLUSIONS The design of a creep test facility namely the actuating machines and the instrumentation the selection and adaptation of sensors as well as of data acquisition equip
3. its measurement needs highly sensitive measurement devices that should also be easy to calibrate and require a detailed analysis of the metrological chain from the sensors to the data storage Furthermore load as well as ambient temperature and relative humidity may be measured with moderate cost instrumentation Test conditions are of particular importance with regard to the temporal and spatial uniformity of the load applied to the specimen in the long run as well as to uniformity of environmental conditions temperature and relative humidity The ease of the devices placement for measuring the deformation alongside with the means for load application and balancing require substantially demanding operational conditions All these requirements were taken into account in the selection of instrumentation in the design of the machines and in the monitoring and data acquisition tools Palma et al 2010 Gustavo Coelho et al 2010 as described next EQUIPMENT FOR ACTUATION MEASUREMENT AND DATA ACQUISITION Each prismatic specimen is compressed in a high stiffness reaction structure see Fig 1 by means of an oil filled flat jack the pressure being maintained by an hydraulic accumulator the load up to 15 MPa is measured with a pressure transmitter inserted in the hydraulic circuit The reaction structure is constructed from press braked steel plates in order to obtain optimized profiles regarding to permissible loads and deformations The pr
4. 15 International Conference on Experimental Mechanics PAPER REF 3762 LABORATORY TESTS OF COMPRESSIVE CREEP IN CONCRETE SAMPLES DEVELOPMENT OF SOLUTIONS FOR PROBLEMS OF ACTUATION MEASUREMENT DATA ACQUISITION AND LOAD BALANCING Jo o Palma Paulo Morais Gustavo Coelho A Bettencourt Ribeiro Laboratorio Nacional de Engenharia Civil Lisboa Portugal Email jpalma Inec pt ABSTRACT This paper reports a study carried out to develop an automatic facility for compressive creep testing of concrete specimens in laboratory The solutions for actuation measurement and data acquisition were developed after the identification of the main metrological requirements and operational constraints The whole transduction chains from the sensors to the data storage was be analysed in detail In addition a software tool was developed as an aid to the delicate process of force application and load balancing The system has been tested and some results are presented INTRODUCTION The Concrete Division of the Materials Department of LNEC carries out laboratory compressive creep tests of concrete specimens of 600 x 150 X 150 mm by submitting them to constant axial compression Over periods ranging from a few months to several years During a test deformation measurements are made on two opposite sides of the specimen along with load measurements Given that the deformations are typically in the order of several tens of microns in 400 mm length
5. e of sending alarm events via SMS messaging to system operators was implemented using a GSM modem through a serial RS 323 when thresholds are overcome dedicated alarm routines embedded in the logging program send a SMS to report the event to the operators enabling to promptly implement corrective procedures METROLOGICAL CHAINS AND CALIBRATION NEEDS A pressure transmitter rated for 250 bar 25 MPa with 4 20 mA current analogue output and 0 5 FS accuracy is used for load monitoring Trafag 2006 The LVDT is a suitable choice for the measurement of the small deformations presently below 100 um with high stability in the long term Pallas Areny 2001 Solartron 2006 Since there are specially demanding accuracy objectives in the measurement of these quantities a lot of effort should be put on each component of the entire metrological chains ICEM15 5 Porto Portugal 22 27 July 2012 from the physical sensor up to the production of digital measurements namely the mounting device the LVDT itself the signal cabling and the data acquisition channel The concerns about the mounting devices were presented before The LVDT has to be more deeply analysed in particular the consequences of its transduction principle consisting of a transformer working with alternating current at 5 kHz the output signal is obtained by peak detection of the differential voltage followed by smoothing as a consequence a residual ripple content
6. ement corresponding to better than 0 1 um a sampling rate of less than 1 reading hour is appropriate for compressive creep testing Any pressure losses have to be prevented or in case of occurrence to be timely corrected The actual data acquisition system allows to detect pressure losses and to send SMS alerts The compression system has no electric supply needs An UPS was provided for the measurement and data acquisition instruments which are sensitive to electric power outages and fluctuations it yields more than one hour autonomy The UPS was placed inside the metallic cabinet with the instrumentation power supplies and the data acquisition system see Fig 3 ICEM15 Porto Portugal 22 27 July 2012 b Fig 4 a Test machines with specimens and instrumentation b The metallic cabinet with the data acquisition system the UPS and the power supplies LOAD BALANCING AND DIRECT STATE MONITORING The initial loading operation is a delicate procedure and must comply with the required specifications LNEC Specification E399 1993 When a target specimen is initially loaded the deformation measurements in two opposite sides may not differ by more than 25 of the average Failure to do so the specimen must be unloaded and repositioning of the specimen is required before repeating the loading operation In order to achieve appropriate balancing conditions to the target specimen the operator needs instant information of the pres
7. etallic parts used In this sense for the construction of the rod stainless steel was used a solution that meets good corrosion resistance properties with a linear coefficient of expansion close to the material used in the construction of test specimens about 12x10 mm mm C at 20 C tA SSS QTE Ws Fixing screw arp anode ara Extension rod N h zzry i N i Pzzz77 IEA LVDT uide screw azz Lock screw qeza TA Ger Support Fig 2 a Schematic representation of LVDT mounting device b Detail view of mounting device installation on specimens The LVDT mounting devices are applied on the faces of the test specimen at the ends of threaded rods that are inserted in the moulds used for casting of the specimens The rods fully pass through the specimen between opposing sides protruding from the face 10 mm The relative positioning of the rods in distance and parallelism of the axes is ensured by the mould within tolerances compatible with the accuracy required for construction of specimens being of the order of magnitude of 0 5 mm The data acquisition system DataTaker 2002 serves the purpose of automatic logging of the measurement data from all instrumentation deployed in the concrete specimens It consists of an autonomous system without a PC allowing to start and conclude tests in different machines independently at any time A resolution of 130 uV is used in channels for deformation measur
8. ment and software is a complex task having multiple matters of concern as described in this paper Special tools had to be provided in order to ease the balancing and set up operation as well as for direct state monitoring without interfering with the automatic logging Additional alarming resources to be triggered in the occurrence of undesired loading or ambient conditions were also considered The highly demanding measurement of deformations in the order of a few tens of micrometers strongly recommend the individual calibration of each entire deformation measurement chain for the purpose of achieving the best measurement quality and should be put in practice at short term A calibration study should be pursued in the forthcoming developments Experimental results obtained so far reveal the functionality of this lab facility as well as its stability REFERENCES Palma J Morais P Esteves Coelho G Data Acquisition System for Creep Tests of Concrete Samples Under Compression Report no 121 2010 CIC in portuguese LNEC 2010 Esteves Coelho G Palma J A monitoring program for Creep Test of Concrete Samples under compression Datapicker version 3 0 4 Report no 232 2010 NSE in portuguese LNEC 2010 15 International Conference on Experimental Mechanics Pallas Areny R Sensors and Signal Conditioning 2nd edition Wiley 2001 DataTaker Pty Ltd DT50 DT500 DT600 Series dataTakers User s Manual Series 3 UM 0076 A0
9. ofiles are welded ICEM15 1 Porto Portugal 22 27 July 2012 to each other forming a closed frame The test specimen is installed inside the frame directly contacting the frame on one end and the flat jack on the other end Between the flat jack and specimen is placed an interface plate to better distribute the load The flat jack consists of two welded steel plates spin formed into a half toroidal section Reaction structure Interface plate with the specimen Accumulator Pressure Flat jack transmitter Spacer Valve block Filling block Hydraulic piping Fig 1 Schematic representation of a creep test system The load application system consists of a block of valves a hydraulic accumulator the flat jack and hydraulic piping and connecting accessories The accumulator contains nitrogen and oil under pressure It has an initial volume of oil that can absorb the expected deformation of the specimen and to make up for any small leaks of oil or nitrogen The valve block allows to temporarily isolate the flat jack from the rest of the circuit in order to perform hydraulic joint maintenance adjust the pressure of nitrogen or add oil The toroidal form of the flat jack allows a wide range of deformation without significant variation of its mechanical characteristics The relationship between the pressure applied inside and the fo
10. rce applied to the specimen remain substantially constant until the deformation equals the initial diameter of the toroidal section The deformations are measured with 75 mV V mm sensitivity linear variable differential transformer LVDT transducers equipped with special mechanical interfaces to provide protection and fixing points 400 mm apart a stroke of 2 5 mm intentionally exceeding the maximum expected deformation range was chosen in order to cope with the manufacturing tolerances of the specimen fixing points as well as to provide some clearance for handling The LVDT mounting device is represented schematically in Fig 2 a The device is mainly composed by a support and an extension rod The LVDT body is inserted inside the support and fixed with a lock screw allowing for minor adjustments of the mechanical zero The LVDT core is attached to one end of the extension rod The relative movement between the 15 International Conference on Experimental Mechanics two parts is limited by a plate attached to the rod which 1s endowed with two slots Two guide screws attached to the support slide inside the slots in the plate By using this arrangement it is ensured that the set can be treated as a single component even when it is not attached to the specimen preserving the integrity of the core of LVDT and facilitating the initial assembly and adjustment operations The design of this device took into account the thermal expansion of the m
11. s a ead HMI Software Fig 4 Compressive Creep Test HMI illustrating DataPicker software detail during a test set up phase The program was developed for Microsoft Windows platforms and connects to the Data Acquisition System through a serial RS232 port It allows sampling rates up to 1 per second without interfering with the actual data logging in progress The acquisition mode can be done manually by clicking get values or in auto mode selecting a sampling period All acquired raw values in volts are stored in a log file and catalogued by date for further analysis and or record purpose Furthermore the HMI provides a platform for access to the current state of all measuring devices therefore allowing assessing monitoring and record state conditions throughout the laboratory test process In compressive creep tests the pressure applied to the specimen should be kept constant during all test period within 3 of the initial value in order to validate the compressive creep results Ambient monitoring is also crucial since the requirements specifications defines tight values for room temperature 20 2 C and room relative humidity 50 5 Permanent monitoring of all specimens throughout their test periods not only allows us to trace tests results but also provides immediate detection and alarming capacity at the occurrence of undesirable conditions Therefore an unsupervised monitoring system capabl
12. sure being applied as well as the deformations in two opposite sides of the specimen Thus aiming to expedite the loading operating a software module was written to provide a Human Machine Interface HMI to the autonomous data acquisition system Depicted in figure 4 the software module DataPicker easily accesses data measurements without interfering with the actual data acquisition system and logging process from any other test machines already in operation The operator is able to select a specific concrete sample and automatically query the acquisition system for load and deformations values for that particular concrete specimen Both deformations and load values are acquired and converted according to their unique parameters These parameters are stored in a file that holds the conversion factor for each instrumentation previously calibrated in order to accurately convert the acquired values in milivolts to their measured physical value respectively in microns and bar 15 International Conference on Experimental Mechanics nnna Instrumentation wiring interface Ensaios de Hu ncia Data Picker Connected to PortaCOM1 1 i i i i i i i Endere os do DataT aker LYDT A LVDTB PRESSAD Data Logging amp _ RS 232 Interface ser Load mace Acquisition rate options Modo de Leitur 7 ere ee eer erry Arrei i o 3 0 4 CIC NSE 2009 TX RX 17 Sai
Download Pdf Manuals
Related Search