Abstract:
An evaluation device and an evaluation method for detecting quality of concrete structures comprising a conductive impact device, a sensing film, a receiver, an operation device and an auxiliary circuit, the device and the method is characterized by calculating a penetration time of the conductive film before measuring the depth of a crack in the concrete, and the evaluation work which conventionally requires dual receivers only needs now a single receiver for completion of the work. Therefore, cost and works for evaluation and signal analysis are reduced; this largely enhances the evaluation efficiency and technical level.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to a method and a device for evaluating quality of concrete structures. The device includes a sensing film able to detect the time when a source of waves is generated. With the device, stress waves are generated and the time that the source of waves is generated is recorded. So that the evaluation work which conventionally requires dual receivers only needs a single receiver for completion of the work. 
     2. Description of the Prior Art 
     Conventionally, nondestructive testing techniques for concrete have been developed, wherein, an impact-echo method developed in the middle of 1980 generates stress waves in concrete by impact; the degree of the impact force and the size of the device for impact can be adjusted to control the desired energy and frequency of the stress waves. The impact-echo method uses a signal receiver which is made of inverted conical piezo-electric material, the device makes a point contact with concrete, so that the surface of the concrete needs no grinding for flattening. Application of the impact-echo method in measuring the thickness of concrete plates has been included in the ASTM as a standard test method in 1998. 
     A time-of-flight diffraction technique for detecting the depth of a surface-opening crack in concrete uses steel spheres as impact sources. After impact on the concrete, longitudinal waves (P waves: pressure waves), transverse waves (S waves: shear waves) and Raleigh&#39;s waves (R waves: surface waves) are generated. Wherein, the P waves and S waves are propagated into concrete in the shape of a semi-sphere, while R waves are spread out from the impact point on the concrete surface in the shape of a circle. The source of the waves is generated at the moment when the sphere strikes the surface of the concrete; by virtue that the time when the sphere impacts the surface of the concrete to generate the source of the waves can not be obtained directly, an indirect way was used conventionally to determine the occurring time of the source of waves by locating a receiver close to the impact point. The principle of the indirect way for detecting the vertical depth of a surface-opening crack is as below: 
     The longitudinal waves (P waves) and the transverse waves (S waves) generated by impacting the surface of the concrete propagate inwardly of the concrete; the P waves go faster, hence the wavefront thereof meets the tip of the crack firstly, then the S waves arrive, the incident P waves generate diffraction waves at the tip of the crack to propagate in the shape of a sphere as a new source of waves generated at the tip of the crack. When the diffraction waves are transmitted back to the impacted surface of the concrete, disturbance is induced. To record the time for stress waves traveling from the impact point via the diffraction at the crack tip to arrival at the surface on the other side of the crack, two receivers sensitive to the vertical displacement of particles are respectively disposed at both sides of the surface-opening crack. The waveform of the displacement detected by the receiver at the same side as that of the position of impact is controlled mainly by a downward displacement created by the R waves; thereafter, the waveform is influenced by the disturbances caused by arrivals of the reflecting waves and the diffraction waves. The initial disturbance detected by the receiver at the different side from that of the position of impact is caused by the arrival of the P wave diffracted from the tip of the crack, because the crack that impedes and delays arrival of the R waves. And then, the displacement waveform detected is generated by the subsequent arrivals of the reflecting waves and the diffraction waves. 
     FIG. 1 is a schematic view showing evaluation and detection of the crack. A first receiver  93  is located at a distance HO from the source of impact  91 , the distances between the source of impact  91  and the crack and between the crack and a second receiver  94  are respectively H 1  and H 2 . When the first receiver  93  receives a downward displacement created by the R waves, the signal monitoring system is activated. We hereby provide that arrival time of the R waves is t 1 , the time that the second receiver  94  records arrival of the diffraction waves going around the tip of the crack is t 2 , and the time difference between sensing the arrival of the R waves by the first receiver  93  and sensing the arrival of the diffraction waves by the second receiver  94  is t 2  t 1 . 
     Impact is done before sensing the arrival of the R waves by the first receiver  93 , thereby, the initial time of impact shall be obtained by derivation. And this is the propagation time of the R waves from the source of impact  91  to the first receiver  93 , i.e., the value of H O  divided by the speed of the R waves (C R ). Thereby, the time period of the P waves from the source of impact  91  to the second receiver  94  can be obtained from the following calculation formula:                Δ                 t     =       t   2     -     t   1     +       H   o       C   R                 (   1   )                                
     The length of path that the P waves go along equals to the multiplication of the speed of the P waves (C P ) and the time taken, thereby, the depth (d) of the surface-opening crack in concrete is calculated according to the following formula:              d   =           [           (       C   P     ×   Δ                 t     )     2     +     H   1   2     -     H   2   2         2   ×     C   P     ×   Δ                 t       ]     2     -     H   1   2                 (   2   )                                
     Analysis of Difficulty Resided in the Prior Art 
     The prior art has five steps in detecting a crack, it is hereby stated as follows (referring to FIG. 1 ): 
     Step 1 (measuring speed of the R waves C R ): The two receivers  93 ,  94  are allocated on the surface  92  of concrete and has a distance therebetween S, impact  91  is done at a position a suitable distance from the first receiver  93 , the vertical displacement waveform detected shows evident arrival of the R waves. From the detected waveform, the times of arrival of the R waves at the two receivers  93 ,  94  are T R1  and T R2  respectively, thereby, the time period that the R waves propagated from the first receiver  93  to the second receiver  94  is ΔT R =T R2 −T R1  then the speed of the R waves is obtained, i.e., C R =S/ΔT R . 
     Step 2 (measuring speed of the P waves C P ): Before arrival of the R waves, there has been displacement disturbance induced by the arrival of the P waves; however, amplitude of vibration of it is smaller than that of the disturbance induced by the arrival of the R waves. If the front portion of the waveform thereof is partially amplified, the displacement response caused by the arrivals of the P waves at the first receiver  93  and the second receiver  94  respectively are T P1  and T P2 . Similar to the step 1, the speed of the P waves is obtained, i.e., C P =S/T P2 −T P1 . 
     Step 3 (determining H O , H 1  and H 2 ): After the speed of the R waves and the speed of the P waves are obtained-, detecting of the crack is performed in site, H 0 , H 1  and H 2  are given, the displacement waveform obtained by the first receiver  93  allocated at the same side of the source of impact shows the initiating time t 1  of downward disturbance induced by the arrival of the R waves; the second receiver  94  is allocated at the opposite side of the source of impact  91 , the first arriving wave sensed by the second receiver  94  is a diffraction wave going around the tip of the crack, the waveform thereof shows the arrival time t 2  of the diffraction wave. 
     Step 4 (substituting in the formula (1) to obtain the value of At): Deriving from the formula (1), we get the time when an impact is done, and get the time period of propagation Δt that the P waves arrive at the second receiver  94  diffracted via the tip of the crack. Step 5 (substituting in the formula (2).to obtain the value of the depth d): The time period of propagation and the speed of the P waves are substituted in the formula (2) to obtain the depth d of the crack. 
     Examples show that, although the time-of-flight diffraction technique leading in the source of waves by impact can accurately detect the depth of the crack on concrete, it has the following defects: 
     1. The R waves must be detected in the first place before derivation of the occurring time of the source of waves: Calculation for this is complicated, and can increase difficulty and variation in evaluation; thereby, efficiency of evaluation is lowered. 
     2. The technique can not be operated by only one person: When in operating, a person can not maintain the two receivers in positions simultaneously and has the sphere struck on a predetermined position accurately; normally an assistant is wanted, that is, the two persons shall operate together. 
     3. The technique requires a dual receiver; this makes higher complexity of equipment arrangement and cost. 
     SUMMARY OF THE INVENTION 
     The principal object of the present invention is to provide an evaluation method for concrete structures; it can conveniently and accurately detect the depth of a surface-opening crack in concrete. Another object of the present invention is to provide an evaluation method for concrete structures, wherein, the work of evaluation can be completed only with a single receiver; this not only reduces cost, but also simplifies the works of evaluation and signal analysis. 
     The present invention will be apparent after reading the detailed description of the preferred embodiment thereof in reference to the accompanying drawings, wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing the positions of an evaluation device for detecting cracks and two sensors used in the prior art; 
     FIG. 2A is a schematic perspective view of the system of the present invention; 
     FIG. 2B is a schematic view of the impact device of the present invention; 
     FIG. 2C is a schematic view showing an auxiliary circuit is used together with the impact device of the present invention; 
     FIGS. 3A, B and C are views respectively showing the positions of the evaluation device of the present invention and the results obtained in the first test; 
     FIGS. 4A, B and C are views respectively showing the positions of the evaluation device of the present invention and the results obtained in the second test; 
     FIGS. 5A, B and C are views respectively showing the positions of the receivers of the present invention for detecting the crack and the results; 
     FIG. 6 shows a flow chart of the present invention in evaluating the crack. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 2 A, B and C, the evaluation device of the present invention is composed mainly of a conductive impact device  10  a sensing film  14 , a receiver  20 , an operation device  30  and an auxiliary circuit  40 . The detailed characteristics of the evaluation device are as follows: 
     1. The conductive impact device  10  is flexible and has a 10 cm long connecting section  11 , and an impacting end  12  connecting with one end of the connecting section  11  and a knob  13  in opposition to the impacting end  12 . 
     2. The sensing film  14  is soft and thin, and includes a conductive film  141  and a non-conductive plastic film  142  which is stuck at the point to be impacted. The conductive film  141  is connected with a third wire  16 ; when the conductive impact device  10  knocks down to contact the conductive film  141 , a first wire  15  will be connected with the third wire  16  to form a closed circuit from an open circuit. 
     3. The receiver is used to receive the signals caused by arrivals of P waves, S waves and R waves, to transmit the signals of the waves out of a second wire  21 . It is tested twice before crack evaluation, in order to calculate a penetration time r and speed of the P waves C P . 
     4. The operation device  30  (such as a notebook computer) includes a data acquisition card  31  (analog/digital converter) to receive the signals from the first wire  15 , the third wire  16  and the second wire  21 . The operation device  30  can calculate the penetration time T and the speed of the P waves C P  before the crack evaluation, then calculate the depth of the crack. 
     5. The auxiliary circuit  40  (as shown in FIG. 2 C) is connected to the first wire  15  and the third wire  16  parallelly connected with each other; the third wire  16  is serially connected with an electric power source  41  (such as a DC battery) and a closed circuit display element  43  (such as a lamp bulb or a buzzer for indicating the power on state). The first wire  15  and the third wire  16  are parallelly connected with an electric resistant  42  therebetween to form a closed circuit (loop), the signal of difference of voltage between the two ends of the resistant  42  is stably transmitted into the analog/digital interface card  31 . 
     The present invention provides the sensing film  14  sensitive to the occurring time of the source of sensible waves, together with the conductive impact device  10  of which the impact end is-made of a steel sphere with a diameter around 3-20 mm. The first wire  15  is connected to the auxiliary circuit  40  and then to the analog/digital interface card  31 , such as is shown in FIGS. 2A to  2 C. When the steel sphere on the impact end- impacts the surface of concrete  92 , the impact end is pressed down to contact the sensing film  14 , so that the conductive film  141  of the sensing film  14  form a conductive loop with the first wire  15  and the third wire  16  to generate signals of voltage. However, the impact end can only really contact the surface of concrete  92  when it is continued to be pressed down to extend through the conductive film  141  and the non-conductive plastic film  142 , thereby, the actual initial time of steel sphere to start to contact with concrete is later than the time receiving the initial voltage signal. And the time difference -is called the penetration time τ. 
     The voltage signal is sent to the auxiliary circuit  40  via the first wire  15  and the third wire  16 , and is transmitted to the analog/digital interface card  31 , finally to the operation device  30 . Thereby, the contact tine-history waveform induced when the steel sphere impacts the surface of concrete can be traced and recorded, and the occurring time of the source of waves can then be obtained. 
     FIGS. 3 to  5  show an example of using the present invention to detect the depth of a surface-opening crack in concrete. They are only for illustration and not for giving any limitation to the scope of the present invention. The method of crack evaluation of the present invention including the following four steps (as shown in FIG.  6 : 
     Step 1 ( 61 ) (detecting the penetration time τ and the speed of the P waves C P ): To determine the penetration time τ and the speed of the P waves C P , it is required that a single receiver  20  performs two impact tests at different positions to get two sets of data for calculating the penetration time τ and the speed of the P waves C P . FIGS. 3 and 4 give the results obtained from two tests. The first test is shown in FIG. 3A, the impacting sphere  12  is kept a distance 0.20 m from the receiver  20 . FIG. 3B shows the recorded contact time-history waveform of steel sphere, FIG. 3C shows the waveform recorded by the receiver  20 . The receiver  20  is then moved to a distance 0.30 m from the sphere (as shown in FIG. 4) to perform the second impact test. FIG. 4B shows the recorded contact time-history waveform of steel sphere, FIG. 4C shows the waveform recorded by the receiver  20 . As shown in FIGS. 3B and 3C, during the first test, the time T P1  that the P waves arrive at the first test position of the receiver  20  is 62.0−1.0=61.0 microseconds. And as shown in FIGS. 4B and 4C, the time T P2  that the P waves arrive at the second test position of the receiver  20  is 87.2−0.8=86.4 microseconds. 
     According to: distance=the speed of the waves×time, we get: 
     For the first impact test: 0.2=C P ×(61.0−τ) 
     For the second impact test: 0.3=C P ×(86.4−τ) 
     With the two equations, we can get that the speed of the P waves C P  is 3937 m/s, and the penetration time τ is 10.2 μs. 
     Step 2 ( 62 ) (determining H 1  and H 2 ): FIG. 5A shows an example using a steel sphere capable of recording the occurring time of the source of waves to detect the crack on the concrete. The impacting end,  12  is kept a distance (H 1 ) 0.15 m from the crack. The receiver  20  is allocated at the- other side a distance (H 2 ) 0.15 m from of the crack, the receiver  20  is used as the second receiver  94  in the prior art, the distance Ho and the first receiver  93  in the prior art are omitted here. 
     Step 3 ( 63 ) (obtaining the time period of propagation Δt by means of the formula Δt=t 2 −t 1 −τ): after impact, the contact time-history waveform of the impact of the sphere recorded and the waveform recorded by the receiver  20  are respectively depicted in FIGS. 5B and 5C. It has been known that the penetration time τ=10.2 μs, and the occurring time of the source of waves obtained from the transient reacting waveform induced by the impact in FIG. 5B is 4.0 μs (t 1 ), and the time period that the P diffraction waves arrive at the receiver  20  via the tip of the crack obtained from FIG. 5C is 185.0 μs (t 2 ); thereby, the time period that the P waves arrive at the second receiver  20  started from the source of impact and diffracted via the tip of the crack can be calculated from Δt=t 2−t   1−Δ= 185−4.0−10.2=170.8 μs. 
     Step 4 ( 64 ) (substituting the above results in the formula (2) to obtain the value of the depth d): The time period of propagation and the speed of the P waves 3937 m/s obtained are substituted in the formula (2) to obtain the depth 0.3001 m of the crack. The value is very close to the real depth 0.30 m of the crack. This example shows that the initial time of impact can be obtained by deducting the penetration time τ from t 1 . Therefore, as is shown in FIG. 6, only four steps are required to obtain the object of crack evaluation on concrete. 
     The key point of the evaluation method and the evaluation device of the present invention resides in deduction of the penetration time τ when in application to evaluation of cracks in concrete. Wherein, the sensing film (such as the products of U.S. 3M company) is cheap and can be obtained conveniently, no order for the goods is necessary. It only needs to use a single receiver to be operated by only one person, and it is no necessity to derive the occurring time of the source of waves from the speed of the R waves. Cost of the hardware and software for the present invention is low, and the steps for evaluation thereof is simple; the present invention is substantially contributive to the quality evaluation of concrete. 
     The above statement is only for illustrating a preferred embodiment of the present invention. It will be apparent to those skilled in this art that all equivalent modifications and changes without departing from the spirit and features of the present invention shall fall within the scope of the appended claims. Having now particularly described and ascertained the nature of the invention and in what manner the same is to be performed, we declare that