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TitleNondestructive Evaluation of Setting and Hardening of CementPaste Based on Ultrasonic Propagation Characteristics( 本文(Fulltext) )

Author(s) KAMADA, Toshiro; UCHIDA, Shinya; ROKUGO, Keitetsu

Citation [Journal of Advanced Concrete Technology] vol.[3] no.[3]p.[343]-[353]

Issue Date 2005

Rights Japan Concrete Institute (日本コンクリート工学協会)

Version 出版社版 (publisher version) postprint

URL http://hdl.handle.net/20.500.12099/28579

※この資料の著作権は、各資料の著者・学協会・出版社等に帰属します。

Journal of Advanced Concrete Technology Vol. 3, No. 3, 343-353, October 2005 / Copyright © 2005 Japan Concrete Institute 343

Scientific paper

Nondestructive Evaluation of Setting and Hardening of Cement Paste Based on Ultrasonic Propagation Characteristics Toshiro Kamada1, Shinya Uchida2 and Keitetsu Rokugo3

Received 30 May 2005, accepted 19 September 2005

Abstract This paper describes the relationship between the setting/hardening properties of cement paste and ultrasonic propagation characteristics from both a macroscopic and a microscopic point of view. The first experimental series was aimed at evaluating changes in the physical properties of high-early-strength cement paste. In the experiment, a single cylinder rotational viscometer was used for viscosity measurement. The second series of experiments was aimed at evaluating changes in the chemical properties of ultra-rapid-hardening cement paste. In the experiment, scanning electron micros-copy (SEM) and powder X-ray diffraction analysis were done to investigate the generation of hydration products. As a result, it was confirmed that the maximum amplitude of the obtained waveform adequately reflects the changes in shear resistance of cement paste. On the other hand, change in ultrasonic wave velocity is well correlated with the for-mation of ettringite crystals as observed by SEM and powder X-ray diffraction analysis.

1. Introduction

Test methods to evaluate the change of the properties of cement-based materials with time after cement-water mixing include the slump test, flow test, funnel flow-through test, Vebe test, rotational viscometer test, Vicat needle test, Proctor penetration resistance test, and strength test. These methods have different ranges of applicable material properties and are thus applicable to different time ranges in the setting and hardening proc-esses of cement, as shown in Fig. 1. Because of this situation, different test methods are being used according to the changes in material properties with time. In the Vicat needle and Proctor penetration resistance tests, the setting and hardening states of the materials in early ages are investigated by means of the change in the material's penetration resistance. The procedures of these conven-tional methods are simple and easy to perform. However, it is difficult to evaluate changes in the physical/chemical properties of the materials based only on penetration resistances. Therefore, the development of methods to measure changes in the physical/chemical properties of cement-based materials in early ages has been required.

Against this background, Whitehurst (1951) attempted nondestructive evaluation using an ultrasonic technique to investigate the changes in material properties with time. Later, some studies evaluated the setting and hardening properties of cement-based material on the

basis of elastic wave propagation behavior in suspen-sions found by Biot (1956). Akashi and Yamaji (1958), for example, found an increase in wave velocity with time but no prominent inflection point in velocity gra-dient. Kakuta and Akashi (1983) measured longitudinal and transverse ultrasonic waves separately and calcu-lated volume modulus and shearing modulus from the velocities of these waves to investigate changes in ma-terial properties associated with setting and hardening. However, the instruments used in the above studies made it difficult to record and analyze the obtained waveform as digital data. Thus, ultrasonic measurement could not be monitored at that time.

Advances in measurement technologies in later years have allowed the obtained waveform to be recorded and analyzed continuously, thus making it reasonably possi-ble to monitor setting and hardening processes continu-ously. Reinhardt et al. (1996a, 1996b, 2000, 2004) evaluated changes in material properties associated with concrete aging by using the velocity of elastic waves and the energy and frequency distribution of obtained waves. However, these studies did not address how these values were related to changes in physical properties in the setting and hardening processes. Grosse et al. (2000) compared ultrasonic velocities with scanning electron microscopy and nuclear magnetic resonance spectros-copy as methods to evaluate changes in chemical prop-erties caused by cement hydration. Their experiment showed a good coincidence between an inflection point in the time-dependence curve of relaxation time obtained by nuclear magnetic resonance spectroscopy and an inflection point in ultrasonic wave velocity gradient. However, the relationship between generation of hydra-tion products and ultrasonic wave velocity was not dis-cussed in detail.

In addition to the above-mentioned studies, there are

1Associate Professor, Department of Civil Engineering, Gifu University, JapanE-mail: [email protected] Student, Department of Civil Engineering, Gifu University, Japan. 3Professor, Department of Civil Engineering, Gifu University, Japan.

344 T. Kamada, S. Uchida and K. Rokugo / Journal of Advanced Concrete Technology Vol. 3, No. 3, 343-353, 2005

also several studies that measured and utilized the re-flection coefficient of shear waves for the evaluation of cementitious materials in early ages (Valic 2000, Morin 2001, Smith 2002, Voigt 2004). Voigt et al. (2005) compared longitudinal ultrasonic wave velocities with in-situ temperature rises or adiabatic heat releases. As a result, it was reported that temperature measurements were useful to evaluate the setting and hardening prop-erties along with ultrasonic measurements. The authors (2003) had also pointed similar findings in previous studies.

On the basis of currently available research results, in order to investigate the relation between the setting and hardening properties of cement paste and ultrasonic propagation characteristics, two experimental series were performed in this study. Experimental series I was aimed at evaluating changes in the physical properties of cement paste during its setting and hardening from a macroscopic point of view. In the experiment, a single cylinder rotational viscometer was used for viscosity measurement, and the relationship between changes in the physical properties of the cement paste and its ul-trasonic propagation characteristics were discussed. The experimental results were compared with the results of a Proctor penetration resistance test. Experimental series II, on the other hand, was aimed at evaluating changes in the chemical properties of cement paste during its setting and hardening from a microscopic point of view. The rela-tionship between changes in chemical properties, in particular generation of hydration products, and ultra-sonic wave velocity was discussed. In the experiment, scanning electron microscopy and powder X-ray dif-fraction analysis were conducted in parallel with ultra-sonic measurement to investigate the generation of hy-dration products.

2. Experimental Program

2.1 Experimental series I (1) Materials and mixture proportions High-early-strength cement was used in experimental series I. An oxycarboxylate-based liquid retarder was used to confirm the effect of the retarder on ultrasonic

propagation. The percentages of the retarder of cement weight were 0%, 0.2%, and 0.4%. The water-cement ratio of the cement paste was 0.45 regardless of the dos-age of the retarder.

Immediately following the addition of water to the cement, the cement paste was mixed in a mixer for 3 minutes. The retarder was added to the water before the mixing procedure.

The mixed cement paste was poured into forms for the ultrasonic measurements, the Proctor penetration resis-tance test and the beaker for the viscosity measurements. These three tests were carried out always simultaneously.

(2) Ultrasonic measurement The ultrasonic measurement setup is outlined in Fig. 2. A steel form was used, and a hole was made at the center of both sides of the form to allow propagation of the ultra-sonic waves. Ultrasonic sensors (US sensors) were se-cured on brass plates (50-mm long, 50-mm high, and 0.3-mm thick) with sensor holders to maintain contact pressure with the specimen. The distance between the sensors was 35 mm. The sensor distance was chosen so that it would be larger than the wavelength of the ultra-sonic waves.

The ultrasonic pulse output from a multifunction synthesizer was amplified by using a bipolar power supply and then emitted with a voltage of 240 V and a pulse interval of 50 seconds. The ultrasonic wave re-ceived by each US sensor after propagation through the specimen was amplified by a preamplifier and then a main amplifier by 40 dB each. Signals above a threshold were converted to digital data at a sampling frequency of 2 MHz, and the obtained waveform was recorded by an Ultrasonic measurement system (US measurement sys-tem). The US sensors used for pulse transmission and reception have a wide sensitive range of 0 to approxi-mately 500 kHz and a resonant frequency of 140 kHz. The ultrasonic measurement was carried out continu-ously over a duration of 72 hours after mixing and at a room temperature of 20 C.

The ultrasonic propagation characteristics were evaluated in terms of wave velocity, maximum amplitude, and frequency distribution. Wave velocity was calculated

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Fig. 1 Test methods to evaluate the change of the properties of cement-based materials.

Semisolid Solid

T. Kamada, S. Uchida and K. Rokugo / Journal of Advanced Concrete Technology Vol. 3, No. 3, 343-353, 2005 345

by dividing the sensor distance by the propagation time, which was given by the difference between the time of pulse input and the time when the signal detected by the reception-side US sensor exceeded the threshold. The maximum amplitude was obtained as the largest voltage amplitude in the obtained waveform, and the frequency distribution was calculated by fast Fourier transform (FFT) of the obtained waveform.

(3) Viscosity test The single cylinder rotational viscometer shown in Fig. 3was used to measure the change in specimen viscosity with time. For the viscosity measurements, the fresh cement paste was placed into a 500-ml beaker having a diameter of 90 mm and a height of 120 mm. The viscos-ity measurements were started one hour after mixing of the cement paste and then repeated at intervals of 10 to 30 min according to the state of the paste. Because ce-ment paste is a non-Newtonian fluid, the results obtained by the rotational viscometer indicate apparent viscosity (ISO 2555).

Using ISO 2555 (1989) as a reference, apparent vis-cosity at each time was measured with the following procedure. A spindle with a diameter of 3.18 mm was vertically immersed in the fresh cement paste and was then rotated at 10 rpm for 1 min. Then, apparent viscosity was measured two times consecutively to confirm that the difference between the two values was within 3%. The average of the two measurements was adopted as a measured value of apparent viscosity.

The upper limit of measurable apparent viscosity is

400 Pa s for the rotational viscometer, the spindle, and the rotational speed used in the test.

(4) Proctor penetration resistance test A specimen for the Proctor penetration resistance test was poured into a steel form with a length of 400 mm, a height of 100 mm, and a width of 100 mm. Four types of penetration needles having sectional areas of 100, 50, 25, and 12.5 mm2 were used. The test started by using a

Fig. 3 Determination of viscosity.

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100-mm2 needle, and the needle was sequentially re-placed with one having a smaller sectional area as the cement paste hardened. In each test run, a needle appro-priate for the specimen's hardened state was first mounted on the penetration resistance test equipment. Then, the needle was inserted into the specimen as much as 25 mm, and the elapsed time (in hours) and the re-quired force (in N) were recorded. Penetration resistance (in N/mm2) was derived by dividing the force (in N) by the cross sectional area of the needle (mm2). According to the ASTM C403-85 (1985), the penetration resistances corresponding to the values of 500 psi (3.5 MPa) and 4000 psi (27.6 MPa) are defined as initial and final set-tings, respectively.

2.2 Experimental series II To confirm the applicability of the present method to cement that shows rapid hardening, ultra-rapid-hardening cement was used in experimental series II. The mix proportions of the cement paste consisted in a wa-ter-cement ratio of 0.30 and amounts of retarder and superplasticizer of 1.0% and 1.5% of the cement weight, respectively. An US measurement system was used for ultrasonic measurement, similarly to experimental series I. The ultrasonic measurements were started when water was added to the cement, and automatic measurement was continued until the cement paste was hardened and the change in wave velocity became very gradual. In the case of automatic measurements, the pulse interval was 0.8 seconds. Then, manual measurements were per-formed every few hours until the lapse of 12 hours. The sensor distance was set to 27 mm in experimental series II. The other methods and conditions of the ultrasonic measurements were the same as those in experimental series I.

To investigate the hydration products generated in the specimen, scanning electron microscopy (SEM) was conducted intermittently in parallel with the ultrasonic measurements. SEM samples were prepared at certain time intervals by replacing water in the cement paste with acetone to stop the hydration reaction. Powder X-ray diffraction analysis was performed simultaneously with SEM by using samples in which hydration was stopped by acetone. CuK-alpha radiation was used for the analysis, with a scanning rate of 2 degrees/min and diffraction angles ranging from 2 to 70 degrees.

3. Results and discussion (Experimental series I)

3.1 Ultrasonic wave velocity The relationship between ultrasonic wave velocity and elapsed time is shown in Fig. 4. Time zero in the figure means the time when cement and water were mixed; only air existed in the form at that time. The measured veloc-ity of approximately 340 m/s at time zero in the figure indicates the wave velocity in the air. After a certain time, ultrasonic waves could be obtained continuously. The

wave velocity then increased gradually up to levels close to those found in water (approximately 1500 m/s) and an inflection point appeared. The period between time zero and an inflection point is defined as stage 1 in this study.

Subsequently, the wave velocity increased gradually from approximately 1500 to 3000 m/s and then plateaued at another inflection point. This behavior is explained by a decrease in the volume of water and an increase in the amount of hydration products with progress of cement hydration in the cement paste. The time period between

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the first inflection point and the second inflection point is called stage 2, and the time period after stage 2 until the end of measurement is called stage 3. When different retarder dosages are compared ((a), (b), and (c) in Fig. 4), the stage boundaries are delayed as retarder dosage in-creases. This indicates that different setting states in cement paste can be recognized by looking at ultrasonic wave velocity and the time of each inflection point in velocity gradient. Similar findings were obtained in a previous study using retarder-added concrete (Reinhardt and Grosse 1996a).

The relationship between ultrasonic wave velocity, apparent viscosity and elapsed time is shown in Fig. 5,which also shows the stage boundary defined by ultra-sonic wave velocity (Fig. 4). The time axis in Fig. 5covers the range between zero and 15 hours, during which the apparent viscosity was measured. At any re-tarder dosage, the apparent viscosity increased abruptly and reached a maximum in stage 1 ((a), (b), and (c) in Fig.5). This phenomenon implies that, as hydration pro-gressed, aggregation and cohesion of cement particles accelerated (Nawa et al. 1986) and shear stress resistance of the cement paste increased. The ultrasonic wave ve-locity also increased in stage 1 but more gently than apparent viscosity. In stage 1, the time differential of apparent viscosity increased with time whereas that of wave velocity decreased with time.

Changes in ultrasonic wave velocity and Proctor penetration resistance with time are comparatively shown in Fig. 6. In any case, penetration resistance showed the timing of initial and final settings (3.5 and 27.6 MPa, respectively) in stage 2, during which wave velocity increased monotonously. This phenomenon has already been pointed out by the authors (2002) and Voigt et al. (2005). A comparison between the change of ap-parent viscosity with time in Fig. 5 and the change of Proctor penetration resistance with time in Fig. 6 shows that apparent viscosity increased abruptly before the initial setting in all cases. The apparent viscosity meas-ured with the rotational viscometer in this study is thus considered to be a suitable index for evaluating the shear stress resistance of the cement paste prior to its initial setting determined by the penetration resistance test.

3.2 Maximum amplitude The relationship between the maximum amplitude ratio, apparent viscosity and elapsed time is shown in Fig. 7.The maximum amplitude ratio is defined as the ratio of the wave amplitude to the maximum amplitude observed within the measurement period in each test case.

As shown in Fig. 7, the maximum amplitude ratio was small at the start of ultrasonic wave measurement, in-creased abruptly and marked a clear inflection point in stage 1, and plateaued in stage 2. This trend is in agree-ment with the results obtained by Akashi and Yamaji (1958) for the maximum amplitude of longitudinal waves.

In Fig. 7, the inflection points of both the maximum

amplitude ratio and apparent viscosity are delayed as the retarder dosage increases, and the maximum amplitude ratio and apparent viscosity exhibit similar gradient pat-terns. It is inferred that the maximum amplitude ratio increased suddenly as the shear stress resistance of the cement paste increased and the energy of propagated ultrasonic waves increased abruptly in stage 1.

3.3 Frequency distribution The changes in frequency distribution with time when no

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retarder was added are shown in Fig. 8. A frequency component very close to the resonant frequency of the US sensor (140 kHz; B in Fig. 8) was dominant in stage 1. An increase in the shear stress resistance of the cement paste in this stage is also affirmed by the abrupt increase in maximum amplitude ratio.

A frequency component around 29 kHz (A in Fig. 8)

became dominant at the lapse of 7 hours and 47 min in stage 2. After a further lapse of time, the peak frequency shifted to around 35 kHz and the spectral intensity in-creased.

In stage 3, the intensity of the spectral peak around 35 kHz (A in Fig. 8) and the resonant frequency of the US sensor (B in the figure) remained stable.

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4. Results and discussion (Experimental series II)

The change in ultrasonic wave velocity with time is shown in Fig. 9. The time axis in Fig. 9 covers the range between zero and 4 hours, during which the ultrasonic wave velocity leveled off. The wave velocity was in the range of 1100 to 1600 m/s after the start of measurement and then gradually increased to approximately 1800 m/s. The wave velocity measured in this time region was larger in experimental series II than in experimental

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series I. This is due to the difference of the water-cement ratio of the cement paste. This time period (elapsed times: 0 min to 1 hour 40 min) is defined as stage 1.

The ultrasonic wave velocity increased abruptly after stage 1 until the lapse of approximately 2 hours and then plateaued with a clear inflection point; this time period is defined as stage 2. As is the case with experimental series

I, the increase in wave velocity is explained by an in-crease in the amount of hydration products in the cement paste. The wave velocity was nearly constant at 4000 m/s after the lapse of 2 hours until the end of the measure-ment; this time period is defined as stage 3. The pla-teaued level of velocity was larger in experimental series II than in experimental series I because the water-cement

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ratio of cement paste was larger in experimental series I than in experimental series II.

Figure 10 shows SEM images obtained after different elapsed times. In stage 1 (elapsed times of 4 min to 1 hour), hydration products were hardly found and only cement particles were observed. After the lapse of 1 hour and 40 min in stage 2, acicular crystals were observed. The number of acicular crystals then increased abruptly over the next 20 min until the lapse of 2 hours in stage 2.

However, the acicular crystals showed little change during stage 3 (elapsed times of 4 to 12 hours). Hydration products other than acicular crystals were rarely found in the SEM.

To identify the hydration products observed as acicular crystals in the SEM, X-ray diffraction analysis was car-ried out; the results are shown in Fig. 11. There was little difference in the diffraction pattern between the elapsed times of 4 min and 1 hour in stage 1. During stage 2,

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diffraction peaks at approximately 9 and 15 degrees grew markedly with time. However, there was no change in the diffraction pattern during stage 3. The X-ray diffraction patterns obtained for each stage are concordant with the SEM results. By comparing the spectral peaks (ap-proximately 9 and 15 degrees) obtained in this study and the JCPDS (Joint Committee on Powder Diffraction Standards) card published by the International Centre for Diffraction Data, the hydration products found by the SEM and the X-ray diffractometry were identified as ettringite crystals (Uchida et al. 2003).

Comparison of Fig. 9 with Fig. 10 and 11 indicates that the abrupt formation of ettringite crystals and the sudden increase in the ultrasonic wave velocity both occurred during the time period from 1 hour and 40 min to 2 hours in stage 2. It was considered that the cement paste texture became denser owing to the formation of acicular ettringite crystals, so that initial increase in strength was achieved (Nagataki and Yamamoto 2000) and the ultrasonic wave velocity increased.

The cause of the wave velocity increase is thus clearly associated with the formation of ettringite crystals in the cement paste.

5. Conclusions

Changes in the physical and chemical properties of ce-ment paste during its setting and hardening were inves-tigated in association with its ultrasonic propagation characteristics from macroscopic and microscopic points of view. The following conclusions were obtained in this study. (1) In experimental series I, apparent viscosity meas-

ured with a rotational viscometer increases sharply when the ultrasonic wave velocity remains stable at approximately 1500 m/s. Therefore, within the limits of this study, evaluation of changes in apparent viscosity of cement paste using wave velocity is thought to be difficult.

(2) The maximum amplitude ratio is considered to be an ultrasonic wave propagation index that reflects the increase in apparent viscosity associated with changes in physical properties during the setting and hardening of cement paste. The apparent viscosity is an index for evaluating the shear stress resistance of cement paste. Maximum amplitude is therefore a characteristic value of propagation that adequately reflects changes in shear stress resistance.

(3) Frequency distribution, like maximum amplitude ratio, is a physical property index for evaluating the shear stress resistance of cement paste. Unlike maximum amplitude, frequency distribution does not depend on elastic wave energy or the states of setting and hardening.

(4) In experimental series II, ettringite crystals were formed abruptly in a stage where ultrasonic wave velocity increased suddenly. It is considered that the chemical properties of the cement paste changed

abruptly and the cement paste texture became denser owing to the formation of acicular ettringite crystals, so that ultrasonic wave velocity increased. The authors intend to extend their study to mortar and concrete and pursue a method to estimate set-ting time on the basis of existing physical and chemical knowledge. With the aim of developing a nondestructive method to evaluate the setting and hardening behavior of concrete quantitatively, the authors also intend to clarify the meanings of the start of ultrasonic wave propagation, the gradients of wave velocity and maximum amplitude when they increase abruptly, and the points at which they begin to plateau.

Acknowledgements The authors gratefully acknowledge the financial support of the Japan Society for the Promotion of Science (Grant No. 15656105).

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