Abstract:
A system for monitoring the health of a structure, e.g., a concrete wall, bridge, pillars, using a smart aggregate is disclosed. The smart aggregate includes a piezoceramic transducer(s) and associated communication links. The transducer is embedded into the structure prior to the manufacture of the structure. The disclosed system can monitor internal stresses, cracks and other physical forces in the structures during the structures&#39; life. The system is capable of providing an early indication of the health of the structure before a failure of the structure can occur.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims benefits from U.S. provisional application Ser. No. 60/819,202 filed Jul. 7, 2006, the entire contents of which are hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention is related to the field of monitoring devices and, more specifically, to the field of performance monitoring of concrete structures. 
     BACKGROUND OF THE INVENTION 
     Current early-age concrete evaluation devices, such as the maturity meter, cannot be applied for the detection and evaluation of cracks and damage for the maintenance period. Some other early-age concrete evaluation devices, such as ultrasonic wave velocity meters, require bulky equipment and are not suitable for the health monitoring of in-situ, large-scale concrete structures. 
     The current maturity meter measures the hydration heat of a concrete structure and the hydration time at early-age to estimate the strength development of a concrete structure. An ultrasonic velocity meter evaluates some physical properties of a concrete structure by measuring the velocity of ultrasonic waves propagated inside the concrete structure. 
     Compressive test equipment determines the compressive strength data of concrete by directly compressing and crushing the concrete specimens (structure) but, due to the press method, equipment and other uncertain factors, large amounts of concrete specimens are needed for the test which is time-consuming and effort consuming. 
     The present technological methods to evaluate the strength of concrete at early-age can be classified into two categories: (1) destructive method that crushes the concrete for strength testing and (2) non-destructive testing. 
     Two popular non-destructive methods to evaluate the early-age strength development of concrete are the hydration heat-based method and the ultrasonic wave velocity-based method. Hydration heat-based method evaluates the early-age strength development of concrete by measuring the hydration heat and recording the hydration time. This kind of method cannot be applied to the health monitoring of concrete structure after the concrete strength is fully developed. 
     The ultrasonic velocity-based method applies an ultrasonic meter on the surface of concrete structure to measure the velocity of the ultrasonic waves from the surface to evaluate the concrete strength. The shortcoming of this method is that the variation of the wave velocity of the ultrasonic waves is not sensitive to the strength of the concrete. A ten percent increment of strength may only result in less than one percent increment of the wave velocity. 
     Early-age concrete performance is an important and critical issue for the construction of the concrete structures. The construction speed and the quality evaluation of concrete at an early-age are the major concerns for the construction of civil concrete structures. After the concrete is cured, the detection of the existence and growth of cracks and damage is another important issue for the maintenance of civil concrete structures. 
     It is an object of the present invention, therefore, to extend the lifetime of concrete structures. It is a further object of the invention to enhance the safety of concrete buildings. It is also an object of the present invention to reduce the maintenance effort and cost for concrete structures. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is a novel unified performance-monitoring device (based on piezoelectricity) for concrete structures. A smart aggregate is directly embedded into a concrete structure at the desired location before casting and can be used, not only for early-age strength monitoring of concrete, but also for the health monitoring (crack and damage detection and evaluation) of concrete structure after the concrete strength has been fully developed. 
     A method for monitoring the health of a structure, comprising the steps of: coating piezoceramic transducers with an insulating material; embedding the piezoceramic transducers into a housing; embedding the housing into the structure; inducing a first waveform from a first piezoceramic transducer; and displaying a second waveform received by a second piezoceramic transducer. The structure of this method is composed of concrete. The step of embedding the housing occurs prior to the curing of the concrete. The piezoceramic transducers are composed of lead zirconate titanate. The housing is a cubic concrete block. The insulating material is composed of water-proof insulating layers. 
     This invention reduces the maintenance cost and effort of civil concrete structures and is also capable of giving precaution warnings before the failure of concrete structures. 
     This invention has the advantages of low cost, unified evaluation of concrete from early-age through the life-time, and easy implementation. 
     This invention has the potential to be manufactured in large quantities of commercial product as a meter for early-age performance evaluation and health monitoring (crack detection and evaluation) for civil concrete structures. The commercial product, based on this invention, will have a very competitive price and offer promising profits for civil construction companies, civil maintenance companies and related industrial companies. 
     The commercial market for the invented device is an obvious promising one due to the reason that the invented device is a great necessity for the early-age performance evaluation, the health monitoring during the maintenance period of large-scale concrete structures, such as bridges, buildings, and pillars. The safety and the life-time of the concrete structure are greatly improved by using the invented device. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective illustration of the piezoceramic transducer with waterproof coating. 
         FIG. 2  is an exploded view of smart aggregates embedded in a concrete structure. 
         FIG. 3  illustrates the experimental setup for strength testing and health monitoring testing. 
         FIG. 4  shows a concrete bent-cap (structure) with four smart aggregates embedded. 
         FIG. 5  is a chart illustrating the crack width measured by microscope and LVDT. 
         FIG. 6  is a chart showing the damage index curve vs. load of PZT 2 . 
         FIG. 7  is a chart showing the damage index curve vs. load of PZT 3 . 
         FIG. 8  is a chart showing the damage index curve vs. load of PZT 4 . 
         FIG. 9  is a photograph of a test frame setup with the reinforced concrete bent-cap specimen. 
         FIG. 10  is a perspective view of the present invention showing the location of smart aggregates. 
         FIG. 11  is a chart showing the crack width measured by microscope (MS) and LVDT. 
         FIG. 12  is a chart of the time response of PZT 10  with PZT 3  as actuator excited by the sweep sine (100-10 k Hz). 
         FIG. 13  is a chart of the damage index vs. load for PZT 1  with PZT 3  as actuator. 
         FIG. 14  is a chart of the damage index vs. load for PZT 2  with PZT 3  as actuator. 
         FIG. 15  is a chart of the damage index vs. load for PZT 5  with PZT 3  as actuator. 
         FIG. 16  is a chart of the damage index vs. load for PZT 8  with PZT 3  as actuator. 
         FIG. 17  is a chart of the damage index vs. load for PZT 9  with PZT 3  as actuator. 
         FIG. 18  is a chart of the damage index vs. load for PZT 10  with PZT 3  as actuator. 
         FIG. 19  is a photograph of an experimental setup for early-age strength monitoring of concrete specimens. 
         FIG. 20  is a photograph of a Universal compression testing machine for concrete cylinder compressive testing. 
         FIG. 21  is a chart showing the compressive strength of the concrete vs. age. 
         FIG. 22  is a chart showing the amplitude of specimens I, II and III for 60k harmonic response. 
         FIG. 23  is a chart showing the amplitude of specimens I, II and III for 100k harmonic response. 
         FIG. 24  is a chart showing the average value of the amplitude of different harmonic excitation. 
         FIG. 25  is a chart of the average value of the amplitude of different harmonic excitation after the seventh day. 
         FIG. 26  is a chart of the membership function of the input variable (harmonic amplitude). 
         FIG. 27  is a chart of the membership function of the output variable (compressive strength). 
         FIG. 28  is a chart showing the experimental training data and the fuzzy mapping data. 
         FIG. 29  is a chart showing the experimental compressive strength and the estimated compressive strength. 
         FIG. 30  is a chart of the experimental data for healthy monitoring of concrete cylinder specimen. 
         FIG. 31  is a chart of the damage index data for concrete cylinder specimen. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a piezoceramic-based smart aggregate  100  (see  FIG. 1 ) for unified performance monitoring of concrete structures  102  and the method of making the smart aggregate  100 . A piezoceramic transducer  104  is formed from a piezoceramic patch  106  with electric wires  108  and a waterproof, insulating coating  110 . 
     One preferred embodiment of the smart aggregate  100 , as shown in  FIG. 1 , contains an 8 mm×8 mm×0.267 mm piezoceramic patch  106 , the waterproof insulating coating  110 , two soldered electric wires  108  on two sides of the piezoelectric patch  106  that are all embedded in a small cubic concrete block  112 . This is meant by way of example and is not intended to limit the scope of the invention. 
     This smart aggregate  100  of the present invention has three obvious advantages over the current technology for early-age concrete performance evaluation: 
     (1) The smart aggregate  100  can be applied to the evaluation of concrete performance from the beginning of the hydration period through the life-time maintenance period. Other current, early-age concrete evaluation devices cannot be applied for the health monitoring (crack detection and evaluation) during the maintenance period. 
     (2) The present invention  100  is suitable for the performance evaluation of the in-situ, large-scale concrete structures  102  which may be inaccessible for other current devices (not shown) to evaluate the early-age concrete performance. 
     (3) The present invention  100  is very economical. The cost of one invented device  100  is approximately one dollar which is much less than the current transducer (not shown) for early-age concrete performance evaluation. 
       FIG. 2  shows the smart aggregate  100  embedded in the concrete structure  102 . 
       FIG. 3  is a preferred embodiment of an experimental testing system  120  for strength testing and health monitoring testing. The system  120  includes two smart aggregates (embedded in a concrete cylinder specimen  306 ) that are attached to industry standard devices (such as a function generator  300 , a power amplifier  302 , and an oscilloscope  304 ) via the electric wires  108 . The smart aggregate  100  can be used as either an actuator  100   a  or sensor  100   s  as depicted in  FIG. 2 . The function generator  300  and the power amplifier  302  generate a signal to the smart aggregate  100   a  to induce a mechanical force (shown in  FIG. 2 ). The mechanical force is detected by the smart aggregate  100   s  and the smart aggregate  100   s  provides a signal to the oscillator  304 . The mechanical This test setup is meant by way of example and is not meant to limit the scope of the invention. 
     Method of Creating 
     To protect the piezoelectric patch  106  from water and moisture, the patch  106  is coated with waterproof coating layers  110  as shown in  FIG. 1 . The smart aggregate  100 , as shown in  FIG. 2 , is manufactured by embedding the coated, piezoelectric patch  106  into a small, cubic concrete block  112 . The smart aggregate  100  is then positioned at a pre-determined place in the concrete structure  102  before casting, as shown in  FIG. 2 . This invention  100  is then used to conduct early-age strength monitoring and health monitoring after the concrete strength is fully developed. 
     The present invention uses a novel treatment of the piezoceramic transducer  104 . The piezoceramic transducer can be constructed from various ceramic materials, such as lead (plumbum) zirconate titanate (PZT). The piezoceramic transducer  104  is first coated with water-proof insulating layers  110  and then embedded into a cubic concrete block  112  to form the smart aggregate  100 . The smart piezoceramic-based aggregates  100  are then directly embedded into the concrete structure  102  to evaluate the performance of the concrete in the structure  102 . 
     Test Results for the Invention 
     Concrete cylinders with smart aggregates were tested. The strength monitoring experimental data verified the effectiveness of the invention to monitor the strength development of concrete at early ages. The health monitoring experimental data verified the effectiveness of the invention to be applied to the health monitoring of the concrete structure. 
     The following figures show the results of the testing which are an impressive improvement over current methods: 
       FIG. 4  shows one test of four smart aggregates  100  embedded into a concrete bent-cap concrete structure  102 .  FIG. 5  is a chart of the test results (measured by microscope and LVDT) showing Crack Width vs Load V. 
       FIGS. 6-8  are charts showing the results of the Damage Index vs. Load for actuator PZT 1  and sensors PZT 2 , PZT 3  and PZT 4 , respectively. 
       FIG. 9  is a photograph of a test frame setup with the reinforced concrete bent-cap specimen  102  and four hydraulic actuators (A-D). 
       FIG. 10  is a view of the test frame setup showing the location of the smart aggregates (PZT 1 -PZT 10 )  100 . 
       FIG. 11  is a chart of the crack width measured by microscope and LVDT. 
       FIG. 12  is a graph of the sensor voltage vs. time showing the time response of PZT 10  with PZT 3  as actuator excited by the sweep sine (100-10 k Hz). 
       FIGS. 13-18  are graphs showing the Damage Index vs. load for PZT 1 , PZT 2 , PZT 5 , PZT 8 , PZT 9  and PZT 10 , respectively, with PZT 3  as the actuator (sweep sine 10-100 Hz). 
       FIG. 19  is a photograph of an experimental setup for early-age strength monitoring of concrete specimens using an Agilent Function Generator, a Quickpack Power Amplifier, a Multifrequency LCR meter, a LeCroy Digital Oscilloscope and three concrete cylinder specimens. 
       FIG. 20  is a photograph of a Universal compression testing machine for concrete cylinder compressive strength testing. 
       FIG. 21  is a graph showing the compressive strength vs age (days) for the testing done on the Universal compression testing machine. 
       FIGS. 22-23  are graphs showing the amplitudes of specimens  308   a, b, c  (see  FIG. 19 ) for 60k and 100k harmonic response, respectively. 
       FIGS. 24-25  are graphs showing the average values of the amplitude of different harmonic excitation for various days. 
       FIGS. 26-27  are graphs showing the membership function of input (harmonic amplitude) and output (compressive strength) variables, respectively. 
       FIG. 28  is a graph showing the experimental training data and the fuzzy mapping data and  FIG. 29  shows the experimental compressive strength and the estimated compressive strength. 
       FIG. 30  is a graph of the experimental data for health monitoring of concrete cylinder specimen (sensor voltage vs. time). 
       FIG. 31  shows the Damage Index data for the concrete cylinder specimen. 
     The above described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.