EVALUATION OF DRYING SHRINKAGE MICROCRACKING IN CE- MENTITIOUS MATERIALS USING ACOUSTIC EMISSION

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1 EWGAE22 EVALUATION OF DRYING SHRINKAGE MICROCRACKING IN CE- MENTITIOUS MATERIALS USING ACOUSTIC EMISSION TOMOKI SHIOTANI 1,2, JAN BISSCHOP 1 and J. G. M. VAN MIER 3 1 Microlab, Faculty of Civil Engineering and Geosciences, Delft University of Technology, The Netherlands. 2 Research Institute of Technology, Tobishima Corporation, Chiba, Japan. 3 Institute for Building Materials, ETH, Zürich, Switzerland. Abstract To determine the temporal evolution of shrinkage microcracking in cementitious materials, acoustic emission (AE) technique is applied to the materials during drying process. Initial studies on wave propagation are carried out for the materials. Geometry of generated cracks is recorded with FLM (fluorescent light microscopy) both in plain cement paste and glass-particle cement composites. The results indicate two mechanisms of drying shrinkage microcracking; selfrestraint of the specimen/structure and aggregate restraint. In plain cement paste, the cracks due to self-restraint initially develop perpendicular to the drying surface, but may subsequently grow parallel to the drying surface. AE monitoring has revealed that the self-restraint cracks have developed almost instantaneously at the onset of drying. In composites containing glass particles, however, aggregate-restraint cracking continuously develops throughout the experiment. Keywords: cementitious materials, cracking, drying shrinkage, restraint, temporal evolution 1. Introduction Drying shrinkage leads to stresses and cracking in cement-based materials when the deformations are restrained. The mechanism of drying shrinkage cracking strongly relates to the type of restraint that is caused by the material and structure. This paper focuses on only the cracking due to internal restraining of the material. There are two internal restraint mechanisms of microcracking due to drying shrinkage in cement-based materials [1]. The first mechanism is called self-restraining of the material, and is caused by moisture/shrinkage gradients that develop in a specimen/structure. This mechanism is studied in plain hardened cement paste specimens. The second type of internal restraint in cement-based composites is caused by the presence of stiff aggregate particles. Composites containing glass particles are used for studying this type of restraint. In this paper, AE technique is applied to determine the temporal evolution of shrinkage cracking. Fundamental issues on wave propagation within the materials are studied first. Two types of specimens, hardened plain cement paste and cement composite containing mono-sized glass spheres, are used to reproduce shrinkage due to the self-restraint and due to the aggregaterestraint respectively. 2. Experiment 2.1 Materials As mentioned, drying shrinkage micro-cracking is studied in two types of materials: plain hardened cement paste and cement-based composite with mono-sized 6-mm glass spheres. The cement paste consists of ordinary Portland cement (CEM I 52.5R) with a water-cement ratio of.45. The composite consists of the same cement paste with a 35% volume percentage of 6-mm J. Acoustic Emission, 2 (22) Acoustic Emission Group

2 EWGAE22 A 3 # 5 # 6 # 3 # 4 # 1 # 2 A 9 mm # 1&5 # 3 # 2&6 AE sensor, # mm Fig. 1. Cylindrical specimen with the location of AE sensors. glass spheres with smooth surfaces. The Young s Modulus of the glass spheres is 77 GPa. The cement-based materials are cast in cylindrical moulds as shown in Fig. 1. Note that the specimens for the initial studies are cast in smaller prismatic moulds (h x w x l = 4 x 4 x 16 mm). The specimens remain in the moulds for 24 hours at room temperature, sealed with a plastic foil. After demoulding, the specimens are placed for 6 days in calcium hydroxide saturated tap water at room temperature. At an age of 7 days, the specimens are removed from the water and all sides, except the top-surface, are sealed with 3 layers of adhesive tape, creating one-dimensional drying in the specimens. Subsequently, six AE sensors (M65, Fuji Ceramics Corp.) with a 5- khz resonant frequency are placed with wax couplant onto the drying surface in the arrangement given in Fig. 1. The specimens are dried in an environmental cabin ventilated with air of 25% ± 5% RH, at temperature of 31 C (±.5 C). The drying continues for 16 hours. 2.2 AE monitoring The used AE monitoring system is a MISTRAS AE system (Physical Acoustics Corp.). The detected AE signals are amplified 4 db using preamplifiers (PAC, 122). Both parameters and waveforms of AE signals over the threshold of 3 db (ref. db at 1 µv at sensor) are recorded. Three parameters; grade [2], initial frequency [3] and improved b-value [4], are employed to study the fracture behavior during shrinkage. The initial part of the AE waves is extracted from the waveforms because it is least affected by the resonance characteristic of AE sensors and contains the cleanest information about fracture characteristics. A higher initial frequency is expected to occur in a situation where cracks rapidly generate (e.g. mode I), while a lower initial frequency is expected when the cracks develop slowly (e.g. mode II). Grade is defined as a gradient of the AE waveform up to the peak amplitude. A rapid growth/development of cracks 154

3 EWGAE22 corresponds to large values of grade. In contrast, slow development of cracks corresponds to small values of grade. The improved b-value, which is defined as the negative slope of peak amplitude distribution, is used to quantitatively evaluate the fracture state. Improved b-values tend to increase where small-scale fracturing occurs in comparison to large-scale fracturing. When predominantly large-scale fractures are generated in place of small-scale fracture, improved b- values tend to decrease [4]. After experiment, the generated crack-pattern of the specimen is recorded by FLM [1]. Manual crack tracing is subsequently applied to obtain a detailed crack map, which is digitized to extract characteristic crack data. 3. Initial Studies for AE Monitoring 3.1 Wave velocity The velocity of elastic waves within the hardened plain cement paste is studied first. Two conditions of the specimen: before drying (intact state) and after 16 hours drying (cracked state) are prepared. The measurement of the velocity was carried out in the transverse direction of the specimen using AE sensors. With an artificial pulse generator of 24-V peak voltage, signals were excited 3 times. The measured wave velocities in both the intact and cracked specimens are shown in Fig. 2. A slight decrease of wave velocity is observed with progress in microcracking. The difference of wave velocities between intact and cracked specimens is approximately 34 m/s. How does the velocity difference influence the source location? If the distance between AE sensors is 5 cm, the maximum difference of arrival times would be 14.3 to 15.6 µs when the wave velocity ranges from 32 to 35 m/s. This error of the time difference is equivalent to 4.2 to 4.6 mm and the source location error due to variation of wave velocity (i.e., progress of cracks) would be 4.2 to 4.6 mm. Fig. 2. Measured wave velocities. The height of bars shows the average of 3 waves with standard deviations (12.75 for intact and 13.6 for cracked). 3.2 Wave attenuation To obtain the attenuation characteristics of the AE waves, six AE sensors are placed in three dimensions on a plain cement paste specimen. The enlarged view of the arrangement of the AE sensors is shown in Fig. 3. The prismatic specimen is subjected to 16 hour drying and AE signals due to cracking are detected. The attenuation characteristics are determined as follows: 155

4 EWGAE22 AE sensor mm Fig. 3 Enlarged view for the arrangement of AE sensors. Fig. 4. Attenuations of 12 AE sources. 1) 3D source locations are performed; 2) The distances between the source and each AE sensors are calculated; 3) The relation between peak amplitude (db) and propagation distance (mm) is plotted and a linear approximation is carried out, and 4) The attenuation characteristics are obtained. Figure 4 shows the attenuation characteristics of 12 AE sources. With extrapolations, the peak amplitudes of AE sources range around 6 db. This suggests that AE signals with large amplitude are difficult to generate/obtain due to the shrinkage cracking. For a threshold of 4 db, the monitoring area is limited to 5-15 cm. If a comparison is made with conventional concrete [5], AE signals generated by shrinkage cracking is observed to attenuate sharply. 4. Results Figure 5 and 6 show the digitized crack maps on the drying surface and cross-sections of plain cement paste and the composite specimen, respectively. In the plain cement paste, a celllike crack-pattern developed. The crack-widths ranged between 2 and 4 µm. In the composite, there is no clear cell-like crack-pattern as on the surface of the plain cement. However, a similar cell-like pattern is believed to exist as in the cement paste, but since the top layer of the composite specimen was removed by grinding, just lightly to remove the upper cement layer to reveal the positions of the glass spheres, the original pattern can only be inferred. Cracks were generally 156

5 EWGAE22 Fig. 5 Digitized crack-map in the plain cement. Fig. 6 Digitized crack-map in the composite. 157

6 4 8.E+6 EWGAE22 Cumulative AE events Composite Plain cement paste 6.E+6 4.E+6 2.E+6 Absolute energy (aj).e+ Fig. 7. Cumulative AE events and absolute energy. perpendicular to the boundary of glass spheres, indicating that they were partly caused by aggregate restraint. Self-restraining also took place in this composite (see Fig. 6b-d): a significant part of cracking had an orientation perpendicular to the drying surface. Horizontal location (mm) # 1, 5 # 3 # 6, 2 Horizontal location (mm) legend 1,8, (pvs) # 1, 5 # 3 # 6, 2 # 4 (a) AE events in the plain cement. (b) AE events with signal strength in the plain cement. Horizontal location (mm) (c) AE events in the composite. # 1, 5 # 3 # 6, 2 # 4 Horizontal location (mm) # 4 # 1, 5 # 6, 2 (d) AE events with signal strength in the composite. Fig. 8. One-dimensional source locations as function of drying time. # 3 # 4 158

7 EWGAE22 Figure 7 shows the cumulative AE events (black lines) and cumulative absolute energy (dotted lines) as function of drying time for plain cement paste and the composite. In comparison to the plain cement paste, approximately 3.5 times more AE events (accompanied by 4.5 times more absolute energy) are recorded in the composite after 16 hours drying. In both specimens, a rapid increase of AE events is observed in the first hour of drying. In the plain cement after this initial activity during approximately one hour drying, no further AE events are recorded. In contrast, AE events are continuously recorded, and a stepwise increase of absolute energy is found in the composite throughout the 16-hour drying experiment. Figure 8 shows 1D source locations as a function of drying time for plain cement paste and composite. Source locations projected along line A-A in Fig. 1 are shown along the y-axis in Fig. 8. Figure 8b and d are shown with the scale of signal strength using the first arrival AE signals. In the plain cement (see Fig. 8a), AE events are dispersed along the A-A-axis in the first hour of drying. In the composite, more AE events are recorded from the start of drying and continue to be generated throughout the drying experiment. An important aspect is also found when the comparison is made between AE sources (see Fig. 8a) and those with signal strength (see Fig 8b) in the plain cement. There are a number of AE sources observed in instantaneous drying in Fig. 8a, although they could not be found in Fig. 8b. This implies that the instantaneously generated AE sources had very small signal strength. Initial Reverberation Threshold Fig. 9. Three extracted components of AE waveform to obtain individual frequencies. Average frequency Average frequency (khz) avg I-freq avg r-freq avg a-freq R =.998 R =.936 R = Average grade (db/µs) Fig. 1. Relationships between average frequency and grade. 159

8 EWGAE22 Figure 1 shows the relations between average frequency and average grade AE signals obtained from shrinkage cracks of 16 hours drying are used for constructing the chart. Three components of the waveform as shown in Fig. 9 are extracted to obtain the frequencies. Note that these are apparent frequencies based on threshold-determined parameters such as ring-downcounts, rise time and duration. As mentioned before, the initial part of the AE waves (initial frequency) and gradient of the AE waves up to peak (grade) are supposed to be closely related each other. Consequently, the correlation coefficients are.998,.936 and.258 for initial frequency, average frequency and reverberation, respectively. These results show explicitly the close relationship between initial frequency or average frequency and grade. As expected, reverberation remains essentially unchanged with grade Grade (db/µs) Grade (db/µs) (a) Grade in the plain cement. (b) Grade in the composite. Improved b -value Improved b -value (c) Improved b-value in the plain cement. (d) Improved b-value in the composite. Fig.11. Grade and improved b-value as function of drying time. Figure 11 shows the grade and the improved b-value with respect to the drying time. The grade sharply decreases only for the first one-hour of drying in the plain cement, whereas repetition of rising and falling values is found in the composite. The same trends are also observed in the charts of the Ib-value (see Fig. 11 c and d). Finally in Fig. 12, the AE activity for the first one-hour of drying in the plain cement is enlarged showing Ib-value and initial frequency. A sudden drop is found after.1 hour (6 minutes) drying in both parameters, which suggests that a large scale fracturing and a slower deformation due to cracking have started at that moment. 16

9 EWGAE22 Averaged initial frequency (khz) Improved b -value Initial frequency Improved b -value Fig. 12. AE activity for the first one-hour of drying showing Ib-value and initial frequency. 5. Discussion The microscopy result shows that cracking in plain cement paste is due to self-restraining only, while cracking in composite is the result of combined self-restraint and aggregate restraint stresses. The AE event data show that in the plain cement paste most micro-cracking was generated within 1 hour, while micro-cracking in the composite continued throughout the drying experiment of 16 hours. The grade, initial frequency and the Ib-value analysis confirm this early behavior; large scale fracturing has already started at 6 minutes and continues to occur up to one hour in the plain cement paste. Similarly in the composite, large scale fracturing has started just after 6 minutes. Consequently, the self-restraint cracking in the plain cement paste is a very early drying phenomenon, while cracking due to aggregate restraint is a continuous process upon further drying. The development of the moisture gradient seems to be the main cause for the early crack-growth due to self-restraint in plain cement paste. The moisture gradient is steepest at the onset of drying, and starts to flatten already from (at least) 8 hours onwards [6]. This means that from this moment onwards, self-restraint stresses become less and cracking comes to a halt. In the composite, an initial peak in cracking is also observed, which can be explained by a significant contribution of self-restraint stresses. At later ages, aggregate restraint is the cause of cracking in the composite. Aggregate restraint cracking is not related to the moisture or shrinkage gradient and depends only on the local magnitude of matrix shrinkage. For the AE activity, which was generated during the initial 6 minutes of drying, the mechanisms producing the AE activity could not be ascertained. However, these initial AE signals have very small energy (from the observed absolute energy, signal strength and Ib-values), and they are caused by rapid phenomena judging from the observed initial frequency and grade. This AE mechanism will be clarified in a future paper. 161

10 6. Conclusions EWGAE22 Early behavior of micro-cracking due to drying shrinkage in cementitious materials has been studied by means of acoustic emission. The results are compared to findings from fluorescent light microscopy, and are summarized as follows: (1) Self-restraining caused cracking in the very early stage of drying in both the plain cement paste and the composite; surprisingly and quite unexpectedly, drying cracks already were shown to form after 6 minutes of drying. (2) Aggregate restraining occurred continuously throughout the drying experiment of the composite up to 16 hours. (3) AE parameters such as grade, initial frequency and improved b-value provide valuable information on fracture mechanisms. Acknowledgement T. Shiotani received a TU Delft Senior Fellowship, which is gratefully acknowledged. References [1] Bisschop J, van Mier J G M. How to study drying shrinkage micro-cracking in cementbased materials using optical and scanning electron microscopy, Cement Concrete Research, 32, , 22. [2] Shiotani T, Ohtsu M, Ikeda K. Detection and evaluation of AE waves due to rock deformation, Construction and Building Materials 15, Elsevier Science, pp , 21. [3] Shiotani T, Bisschop J, van Mier J G M. Temporal and spatial development of drying shrinkage cracking in cement-based materials, Engineering Fracture Mechanics (in press). [4] Shiotani T, Yuyama S, Li Z W, Ohtsu M. Application of AE improved b-value to quantitative evaluation of fracture process in concrete materials, Journal of Acoustic Emission, [5] Shiotani T, Miwa S, Monma K. Characteristics of elastic waves induced by raindrops, Proc. 6th Domestic Conference on Subsurface and Civil Engineering Acoustic Emission, MMIJ, pp (in Japanese). [6] Bisschop J, Pel L, van Mier J G M. Effect of aggregate size and paste volume on drying shrinkage microcracking in cement-based materials, Creep, Shrinkage and Durability Mechanics of Concrete and Other Quasi-Brittle Materials, Proc. CONCREEP-6, Elsevier Science, pp. 75-8,