ENERGETIC QUANTIFICATION METHOD FOR COMPOSITE MATERIALS

Abstract: A method and an apparatus for characterizing composite materials (2) is disclosed. According to invention acoustic waves are transmitted through a composite material (2) and an oscillogram of the acoustic signal is measured and analyzed in order to determine structural and/or mechanical parameters of the composite material (2). The invention is particularly useful to examine concrete (2), mortar or gypsum and in particular Roller Compacted Concrete (RCC). In preferred embodiments an acoustic energy E and optionally other acoustic variables, such as frequency, amplitude, intensity or signal attenuation of the acoustic wave are derived from the oscillogram and are correlated to elasticity, density, strength, internal tension, imperfections, discontinuities, phase changes (gaseous, liquid, solid) and/or setting time of the concrete (2). The invention can be used to monitor the setting and hardening process of concretes (2) such as RCC.

Description: Energetic Quantification Method for Composite Materials TECHNICAL FIELD The invention refers to the field of physical charac- terization of concretes and other composite materials. It is based on the subject-matter as set forth in the preamble of the independent claims. BACKGROUND ART Since the 1940s measurements of acoustic transit times in concrete are used to determine the mechanical strength and to detect cracks, honeycombs and other cavities. However, acoustic measurement techniques have not been used to characterize other physical properties or the hardening process of concrete. Furthermore, pres- ently available strength tests for concrete can only be applied when the hardening process is finished. Setting times of concrete samples are determined by measuring the penetration resistance e. g. with a Proctor needle. However, these tests are destructive and, in particular, inadequate for Roller Compacted Concrete (RCC) , because an essential component of the RCC is sieved out during the sample preparation. Existing methods for determining the volumetric proportions of solid, liquid and gaseous phases in a composite need in general laboratory equipment (e. g. an electron microscope) and are inapplicable for on-site measurements and in particular for monitoring the dynamics of structural changes in hardening concrete. BRIEF SUMMARY OF THE INVENTION It is the object of the invention to provide an improved method and apparatus for characterizing composite materials. This object is achieved according to the inven- tion by the subject-matter as set forth in the independent claims. The invention resides in a method and apparatus for coupling acoustic waves into a composite material and detecting a transmitted acoustic signal, whereby an oscil- logra of the acoustic signal is measured and is analyzed in order to determine structural and/or mechanical parameters of the composite material. This acoustic analysis method allows to perform on-site and real-time measurements of the status and evolution of physical properties of composites, such as mortar, gypsum and Roller Compacted Concrete RCC, that were previously unobtainable . In one embodiment an acoustic energy E and optionally other acoustic variables, such as a frequency, airvpli- tude, intensity or signal attenuation of the acoustic wave are determined and correlated to elasticity, density, strength, internal tension, imperfections, discontinuities, phase changes (gaseous, liquid, solid), setting time and/or acoustic impedance of the composite ma- terial . In another embodiment a series of oscillograms is measured and analyzed in order to monitor dynamic changes in the composite material, in particular during a hardening process . In yet other embodiments additional measurements of temperature, dimensional changes and/or loss of weight of the composite material are made to complement the measurements of acoustic oscillograms. Further embodiments refer to an apparatus comprising a high-speed acquisition board for storing and a micropro- cessor or computer for analyzing the acoustic oscillograms . Other objects, features and advantages of the present invention will become apparent from the dependent claims and the description in connection with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS The description is related to the accompanying drawings, in which, according to invention, Fig. 1 shows an apparatus according to invention for measuring and analyzing acoustic oscillograms ; and Fig. 2 shows experimental curves of acoustic energy versus time characterizing the setting process of Roller Compacted Concrete RCC. In the drawings identical parts are designated by identical reference numerals. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the scope of the present invention a composite material is any material made of several components. The composite contains in general a base material, a ligand and some kind of admixture or addition which improves the performance of the first two. Examples of composite materials are: concrete, mortar, gypsum, epoxy materials, stuccos, and any mixture where a ligand as hydrau- lie cement, fibers, admixtures, additions or other components, which modify the physical or reological properties of the composite, are present. The composite being a heterogeneous material makes use of the properties of its fundamental components in order to achieve special characteristics. Concrete, gypsum and mortar are composite materials employed in the construction industry. These materials are difficult to charac- terize, and it is also difficult to reliably model their behaviour, especially in the transition period from the fresh to the complete hardened state. Therefore the characterization or modeling of composite materials has so far been restricted to materials with less complex matrices . The present invention resides in a method for characterizing composite materials 2 and in particular for measuring physical properties of concrete 2, whereby a wave- form or oscillogram of an acoustic wave having traversed the composite material 2 is detected and the form of or characteristic numbers derived from the oscillogram are correlated to structural and/or mechanical parameters of the composite material 2. This detailed acoustic analysis is advantageous over usual propagation velocity measurements, in that more information about internal characteristics of the composite 2 are obtainable, such as the volumetric proportions of solid, liquid and/or gaseous phases, the hard- ening process, the evolution of the material strength during hardening, etc. In particular the method can also be applied when the acoustic wavelength is comparable to the size of components, inclusions or inhomogeneities in the composite 2, as it can happen in the case of fresh concrete. Disadvantages of hitherto applied methods are avoided, such as need for special environmental meaasurement conditions, waiting for complete hardening of the composite 2, or using extrapolated data from laboratory tests made under ideal conditions. In the following preferred embodiments of the method and apparatus, as exemplified in Fig. 1, are disclosed. An acoustic pulse generator 1 excites an acoustic emitter transducer 3 that sends acoustic waves into the composite 2. The waves are picked up by an acoustic receiver transducer 8 and transformed into an electric signal to be detected and analyzed in the measuring means 7. The measuring means 7 comprise a high-speed data acquisition board or apparatus 4 for storing oscillograms of the acoustic waves and a microprocessor or computer 5 for analyzing the oscillograms. In principle, a single transducer 3, 8 can function as emitter 3 and receiver 8. When impact waves are used the acoustic pulse generator 1 and emitter transducer 3 can be replaced by a sclerometric hammer. The high-speed acquisition apparatus 4 can be a digital oscilloscope 4. Further optional elements of the measuring means 7 are an amplifier 10 for received signals, an acoustic spectrum analyzer 11, as well as a calorimeter 12 for heat of reaction measurements and/or a thermometer, a defor- mimeter 6 and/or a balance 9 for measuring temperature, dimensional and/or weight changes of the composite material 2. The deformimeter or dimensional change tester 6 can be a magnetic displacement transducer (LVDT) 6. The balance 9 can be a digital balance 9. Furthermore the apparatus ■ comprises a coupling system 13, 14, in particular a pneumatic coupler 14 connected to a pneumatic compressor 13, for pressing the acoustic transducer 3, 8 to the composite material 2. The microprocessor or computer 5 shall be designed for controlling the acoustic pulse generator 1 and the measuring means 7. The computer 5 shall be equipped with an IEEE communication board and an A/D converter and be programmed by specific software. 15 signifies the output. The operation of the apparatus according to Fig. 1 is as follows: The wave train generated by the emitting trans- ducer 3, excited e.g. by a high voltage source, falls perpendicularly onto the surface of the composite material 2 and penetrates the composite 2 in form of successive wave fronts, until it reaches the receptor 8, which will transform the mechanic signal into an electric one. This signal is fed to the digital oscilloscope 4, which stores the oscillogram of the wave train that has crossed the composite material volume in that instant, in its memory. The stored signal is sent to the computer 5 practically in real time. The computer 5, programmed with specially designed software, processes the generated diagram stored at the os- cilloscope 4, calculates the energy value E for each measurement and generates a drawing of the result. Fi-=- nally, a curve is obtained, which characterizes with high sensitivity the evolution of the solid-liquid- gaseous phases in the evaluated composite material 2. The computer 5 also registers in a continuous way data coming from the balance or scale 9 that measures weight changes of the probe 2, from a deformimeter 6, which follows the volume changes of the composite material 2, from thermocouples that register the ambient and the ma- terial temperature and optionally from the calorimeter 12. It is important to point out that once the sample has been placed in the system, the computer 5 controls the whole process from then on, until the test is finished, thereby avoiding possible errors by wrong handling of the samples 2. The data recording time, in particular the time intervals between the acoustic and other complementing measurements, can be controlled by the software . At the end of the process characteristic curves of the evaluated composite material 2 are generated, in which it is possible to observe the variation of hydration with time (ultrasonic energy versus time) , and at the same time it is possible to obtain curves of sample tem- perature versus time. It is possible to characterize in detail the hydrate formation for the case of a mortar or a concrete. The other parameters can complete the information obtained from the acoustic diagnosis according to invention. In summary, a whole package of information of great value and utility for the constructor is pro- vided on the site of construction and essentially in real time. In the following embodiments of the oscillogram analysis are explained in greater detail. The calculation of the acoustic energy starts enunciating the Euler or wave equation ~~~ V 2 Ψ = 1/c 2 * 3 2 Ψ/3t 2 , where Ψ=acoustic wave amplitude, t=time and c=sound propagation velocity. The solution for one axis x in space is given in a general form by the expression Ψ(x, t) =Σ n A n *cos (ω n t-k n x) + B n *sin(ω n t-k n x) where ω n represent the eigenfrequencies and k n the wave numbers of the system. The relation of these two parameters with the mechanical properties of the system is given by: k n /ω n = (p/K) 1/2 = 1/c where p is the density of the medium and K is its Young' s modulus . If the function Ψ(x,t) or |Ψ(x,t)| 2 is obtained experi- mentally, it is possible to extract the different constants of the equation. Therefore, from the Ψ(x,t) function determined for each emitted pulse, the kinetic energy E of the system is calculated: E = y 2 * A 2 * K/p , where A is the integral of the parameters A n and B n of the function Ψ(x,t) . This means, that the calculated value E from the measured function Ψ(x,t) will be proportional to Young's modulus K and inversely proportional to the density p of the material. It has to be pointed out, that the use, calculation and analysis of the wave function Ψ with its parameters (ι) n , k n and A n B n contains more useful information than just the determination of sound speed c, used in the concrete technology since the 40s. The value of the kinetic energy E of the system calculated from that function, al- lows, in a significantly more sensitive and exact way, to obtain information about the state of the medium 2 under evaluation, as it considers and quantifies the most important properties of the incident wave train onto the reception transducer 8. Some remarks related to the above explained model are noted here: The type of composite material 2 and its characteristics define the type of impact or (ultra) sound waves to be used. As the principle of the method and apparatus remains the same for both types of acoustic excitations, they are referred to as sonic or acoustic waves. The acoustic energy is proportional to the square of the oscillation amplitude Ψ of the particles. The energy coming from the source transducer 3 at any moment is always constant, as well as its frequency, wavelength and other acoustic parameters related to the energy, such as acoustic impedance, density or the vibration speed of the particles. Once the sonic waves have penetrated into the composite material 2, all parameters except frequency or period, start changing as a function of the state and characteristics of the composite material 2 and of the ambient conditions of the construction site, which is favourably the measurement site where the apparatus is used, due to its portability. Table 1 shows preferred types of waves and frequencies. The composite materials 2 mentioned in this description typically have a solid, a liquid and a gaseous phase. In all cases the sum of the volumetric proportions of the phases will be one. A wave train which crosses a composite material 2 is build up by a finite series of pressure wave fronts, which can be assimilated to the physical concept of a membrane in three dimensions. The importance of the concept of measuring acoustic oscillograms lies in the fact that the acoustic impedance varies as a function of the structure of the composite 2, and in particular can be higher or lower depending on the aggregates in the composite 2. The wave front, when crossing such aggregates, will loose, in absolute values, a given quantity of energy E or pressure, which will be absorbed or redistributed in the composite 2. This process will repeat a finite number of times until the wave front reaches the reception transducer 8, which in the case of the apparatus is a piezoelectric crystal 8 or a pressure cell 8. Each wave front that belongs to the ultrasonic pulse, which crossed the material 2, excites the piezoelectric crystal 8 of the reception transducer. The piezoelectric crystal 8 responds to the pressure stimulation by oscillating, at the same time generating from this mechanical signal an electromagnetic signal. This continuous series of pressures on the piezoelectric crystal 8 appears on the oscilloscope screen 4. From this oscillogram all the acoustic variables of the composite material 2 can be obtained. The form of the pressure oscillogram is established by parameters such as: frequency, amplitude, intensity and signal attenuation. It determines the energy value, calculated for a delta of time between two limits of the signal, that are fixed according to the analyzed mate- rial 2. The form of the pressure oscillogram and some of its acoustic variables (as e.g. its energy) allows to characterize with high precision those characteristics of the main part of the composite material 2, that are related to the internal structure, the state of the ten- sions, the behavior of the transition zones between in- elusions and bonding material, the presence of imperfections, among others. From a series of oscillograms, it is possible to calculate for each oscillogram an ultrasonic energy value E so that a series of energy values E versus time is obtained, which finally describes the evolution of the phases . It has to be noted that the ultrasonic energy is calculated by a numeric integration of the pressure oscillo- gram of the wave train over a time interval. The inferior limit is given by the time it takes the front of the wave emitted by the emitting transducer 3 to reach the reception transducer 8. The superior time limit corresponds to the time of the first peak, after the one with highest amplitude, when the form of the oscillogram has stabilized. As long as the same composite material 2 is examined, always the same value for the superior limit is used. If the procedure is repeated during some time, for exam- pie the time it takes to set and harden a volume of concrete 2, a series of histograms are obtained which are like the sonography of the material's state at any instant, from the plastic state (viscoplastic) , to the hardened (solid) state. From the series of oscillograms and from the time it is possible to obtain curves of the different acoustic variables, for example the energy or, in addition, the propagation velocity. An experimental example is shown in Fig. 2, where the ultrasonic energy is given as a function of time for two curves 1 and 2, that exemplify the setting behaviour of two variants of Roller Compacted Concrete RCC. The experimental parameters are: 32 °C, 75% relative humidity. Curve 1: cement weigth 70 kg/m 3 , water 4.8%, gravel (19- 38 mm) 249 kg/m 3 , gravel (4.8-12.5 mm) 543 kg/m 3 , crushed sand 566 kg/m 3 , fine sand 362 kg/m 3 ; curve 2: same composition as in curve 1, except that a setting retarder admixture is added with a dosage of 2% of the cement weight. F. I. signifies the initial setting and F. F. the final setting. The curves 1 and 2 describe very sensitively the evolution of the liquid and gaseous phases to solid (hydration process and crystal formation) . This type of measurements can also detect the tension state, the presence of internal discontinuities of the material, and the behavior of the aggregate-paste transition zone for the case of concrete. As well, a simplified acoustic energy measurement method is presented. The acoustic energy E for a given time interval Δt can be calculated as where E=acoustic energy (in erg) , Aι=potential difference between a base state and an i-th maximum excitation, provoked by the wave front, of a piezoelectric transducer or crystal 8 (in mV) , Δt is the time difference (in μs) , and K=constant that relates the mechanical amplitude of the displacement of the piezoelectric crystal 8 to the electromagnetic response. The constant K also includes characteristics of the matrix like density, and characteristics of the emitter 3, like signal frequency or period. Besides the determination of the volume percentage of solid, liquid and gaseous phases in arbitrary composite materials 2, the apparatus and method of the present invention are very appropriate for the characterization of the materials 2 used in the field of construction, such as concrete, mortar and gypsum. In these materials a setting and hardening process takes place, that determines their future characteristics, such as mechanical strength and durability. Therefore a precise knowledge of the setting process is very desir- able and in fact necessary. The presented acoustic tech- nique provides a new method for determining these setting times by using a physical principle completely different to the existing ones. In special concretes 2 , such as Roller Compacted Concrete and others, the tradi- tional techniques cannot be applied to determine these setting times. In contrast the measurement of setting times with ultrasonic energy is able to solve this problem and to sensitively characterize the dynamics hydration of these concretes 2. Other important applications of the presented method in such materials are: Quantification of the maximum content of additions in a cement or concrete, Determination of the optimum time for the joint cutting in concrete pavements, depending on the ambient conditions, Determination of the tensile states in concrete structures, Determination of the residual strength in a structural element, Determination of the setting and hardening evolution curves of composite materials like concrete or gypsum, Determination of the hydration processes in gypsum and concrete, Determination of the behavior of the transition zones in composite materials, especially of the behavior of the transition zone between aggregate and cement paste in concrete, Determination of the optimum water/cement ratio from the hydration and consistency of the concrete, Determination of the minimum water curing, Determinations of the behavior of admixtures in composite materials like gypsum or concrete, and Determination of the generation of hydration products coming from the cement additions.

Claims: PATENT CLAIMS 1. A method for characterizing composite materials (2), in particular suited for measuring physical properties of concrete (2), whereby an acoustic wave is coupled into the composite material (2) and a trans " - mitted acoustic signal is detected, characterized in that a) an oscillogram of the acoustic signal is detected and b) the oscillogram is analyzed in order to determine structural and/or mechanical parameters of the composite material (2) . 2. The method according to claim 1, characterized in that a) from the oscillogram an energy E and, in particular, a frequency, amplitude, intensity, signal attenuation and/or impedance of the acoustic wave are determined and/or b) a series of oscillograms is measured and analyzed in order to monitor changes in the composite material (2), in particular during a hardening process . 3. The method according to one of the previous claims, characterized in that a) the determined parameters of the composite material (2) are a Young's elasticity modulus K, a density p, a mechanical strength, a state of ten ¬ sion, a presence of imperfections, a presence and behaviour of internal discontinuities, a volume percent of a solid, liquid and/or gaseous phase, a state of a hydration process, a state of a crystal formation and/or a setting time and/or b) the composite material (2) is concrete (2), mortar or gypsum, and in particular Roller Compacted Concrete (RCR) . 4. The method according to one of the previous claims, characterized in that a) an energy E of the acoustic wave is calculated -£>y a numeric integration of the oscillogram of the acoustic signal over time, b) in particular that an inferior time limit is given by an acoustic transit time through the composite material (2) and a superior time limit corresponds to the time when the form of the oscillogram has stabilized. 5. The method according to claim 4, characterized in that the energy for a given time interval Δt is calculated according to the equation where E=acoustic energy, Aι=potential difference between a base state and an i-th maximum excitation of a piezoelectric transducer (8), K=constant relating an electromagnetic to a mechanical response of the piezoelectric transducer (8) and ∑i designates a summation over i=l...m oscillations during the time interval Δt . 6. The method according to one of the previous claims, characterized in that a) from a series of oscillograms the acoustic energy E versus time is measured, b) in addition temperature, heat of reaction, dimen- sional changes and/or loss of weight versus time are measured and c) the correlated structural and/or mechanical parameters of the composite material (2) are monitored. 7. An apparatus for characterizing composite materials (2), in particular suited for measuring physical properties of concrete (2), comprising an acoustic pulse generator (1), at least one acoustic trans- ducer (3, 8) for coupling acoustic waves into and out of a composite material (2) and measuring means (7), characterized in that ~~ a) the apparatus is designed for implementing the method according to one of the previous claims and b) the apparatus is portable. 8. The apparatus according to claim 7 , characterized in that a) the measuring means (7) comprise a high-speed acquisition apparatus (4) for storing oscillograms of the acoustic waves and b) the measuring means (7) comprise a microprocessor or computer (5) for analyzing the oscillograms. 9. The apparatus according to claim 7, characterized in that a) the high-speed acquisition apparatus (4) is a digital oscilloscope (4) and/or b) the measuring means (7) further comprise an acoustic spectrum analyzer (11) and/or c) the microprocessor or computer (5) is designed for controlling the acoustic pulse generator (1) and the measuring means (7) . 10. The apparatus according to one of the claims 7-9, characterized in that a) the apparatus comprises a calorimeter (12) for heat of reaction measurements and/or a thermometer, a deformimeter (6) and/or a balance (9) for measuring temperature, dimensional and/or weight changes of the composite material (2) and/or b) the apparatus comprises a coupling system (13, 14), in particular a pneumatic coupler (14) connected to a pneumatic compressor (13), for pressing the acoustic transducer (3, 8) to the compos- ite material (2) .

Inventors: BOLLATI MARCOS RUBEN, BOLLATI, MARCOS RUBEN
Applicants: BOLLATI MARCOS RUBEN, SIKA AG, BOLLATI, MARCOS RUBEN, SIKA AG, VORM. KASPAR WINKLER & CO.
Application year: 1999
Main classification: G01N29
IPC codes: G01N29, G01N33