ANALYSIS OF EARLY-AGE CRACKING OF CEMENTITIOUS MATERIALS BY COMBINATION OF VARIOUS NON DESTRUCTIVE TESTING METHODS

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1 ANALYSIS OF EARLY-AGE CRACKING OF CEMENTITIOUS MATERIALS BY COMBINATION OF VARIOUS NON DESTRUCTIVE TESTING METHODS Stephan Pirskawetz, Frank Weise and Patrick Fontana BAM Federal Institute for Materials Research and Testing, Berlin, Germany Abstract High performance concrete shows high proneness to autogenous shrinkage and micro cracking due to the low water-binder-ratio and high content of microsilica. An internal restraint of the shrinkage caused by aggregate and reinforcement presence, or the external restraint due to adjacent structural members, causes high tensile stress development in the concrete matrix. The commonly used existing measurement methods enable characterization of the degree of damage only at discrete points in time. The ongoing research work presented in this paper looks into combining the acoustic emission and ultrasonic measurements with the existing experimental techniques (such as shrinkage, tensile strength development, etc.) to potentially better illustrate crack formation in cementitious matrix. 1 INTRODUCTION Autogenous deformations are defined as deformations that occur under isothermal conditions in enclosed cementitious material systems, not subjected to external forces [1]. High Strength Concrete (HPC) with water/binder ratios lower than 0.3 and a high content of micro silica shows intensive self-desiccation, related to autogenous shrinkage. Due to the low permeability of the matrix the self-desiccation cannot be prevented by external curing [2]. The emptying of the pore system starts at large pores and it proceeds to smaller pores with a decreasing content of free pore solution. Therefore, the radii of the menisci forming the boundary layers between the pore solution and the gas filled pore system are decreasing. The resulting capillary tension causes a contraction of the matrix and hence shrinkage [3]. After setting, the restraint of autogenous shrinkage induces tensile stress development. Additionally, these stresses can be magnified by the thermal changes causing the system to expand or contract. In the early stage of hydration residual stresses arise, while the tensile strength of the matrix develops slowly. As such, intensive micro cracking might occur if the deformations are internally or externally restrained. The crack occurrence shortens the life and reduces durability of structural components made of HPC. While internal restraint caused by the aggregate presence cannot be eliminated, external restraint can be controlled to a certain degree. It can be also mentioned there that high tensile stresses may be induced by an exter-

2 nal restraint of the autogenous shrinkage in stiff moulds or by a strong bond to adjoining elements. Previous research has shown that the development of the tensile stresses can be analyzed by the restrained ring test [4] or with temperature-stress testing machines like the ones described in [5] and [6]. These methods allow studying the conditions leading to appearance of visible cracks. Additionally, internal micro cracks can be fixed by special preparation techniques and then examined under the microscope [7]. However, using these methods alone the time when micro cracking starts cannot be determined and further development of cracks due to hydration reaction cannot be observed. To overcome the aforementioned limitations, this work presents the tests results of an ongoing research project that looks into acoustic emission (AE) analysis and ultrasonic testing as suitable techniques for characterization of the damage process in hardening HPC. The AE analysis was used to observe the micro cracking formation and development and to detect the exact time when single cracks occur. This could also allow localizing the origin of these cracks. In order to characterize the development of the microstructure during hardening, the ultrasonic velocity was measured in short intervals. The acoustic measurements were conducted concurrently with shrinkage, stress development, temperature and capillary pressure measurements. 2 EXPERIMENTAL TEST SETUP AND TECHNIQUES The influence of shrinkage on early age cracking was investigated in externally restrained and unrestrained condition. Additionally, both externally restrained and unrestrained samples were tested with and without internal restraint. Figure 1 shows the overview of the experimental program together with the applied measurement techniques. Influenceof shrinkage on early age cracking program without external restraint with external restraint without internal restraint (cement paste) with internal restraint (mortar) without internal restraint (cement paste) with internal restraint (mortar) methods hydration process acoustic velocity temperature shrinkage deformation tensile stress capillary pressure crack development acoustic emission acoustic velocity Fig. 1: Experimental program and applied measurement methods. Cement paste and mortar mixtures were prepared using commercially available portland cement CEM I 52.5 R and silica fume. A superplasticizer was added at 1% and 1.1% by cement weight, for cement paste and mortar, respectively, to achieve workability that is comparable to self compacting concrete. The detailed compositions of the mixtures have been shown in Table 1. Table 1: Composition of the mixtures.

3 mixture silica fume (by cement weight) superplasticizer (by cement weight) sand (0,25 2 mm) (by paste volume) w/c w/b cement paste 10 % 1,0 % - 0,30 0,27 mortar 10 % 1,1 % 33 % 0,30 0,27 The stresses development under restraint conditions was measured in a hydraulic testing machine with a mould divided into three sections: 1) end piece fixed bearing, 2) measuring sections and 3) end piece movable bearing, as shown in Fig. 2. The measuring section between the two end pieces of the mould had a cross sectional area of 4 cm x 4 cm and a length of 34 cm. The dog-bone like geometry of the sample allowed stress concentrations in the measuring section. The three parts of the mould were allowed to move independently due to approximately 1 mm wide joints. Both end pieces of the mould were fixed in a hydraulic testing machine. One was screwed on the crosshead (shown on the left in Fig. 2), while the other one was mounted to a horizontal movable slide (shown on the right in Fig. 2). The measuring section of the mould was placed on the frame of the testing machine loosely. In order to transfer the force from the testing machine into the specimen, anchors were screwed into the massive front ends of the mould. The position of the slide and hence the distance between the end pieces of the mould were controlled by the hydraulic cylinder. A load cell was applied between the cylinder and the slide in order to measure the force needed to restrain the shrinkage. The measured forces were divided by the cross sectional area of the measuring section in order to calculate the resulting stresses. end piece fixed bearing measuring section end piece movable bearing ultrasonic sensor AE-Sensors ultrasonic sensor anchors steel wire with LVDT anchors Fig. 2: Test setup (with empty mould) for measuring the stress development under restraint conditions

4 The measuring section of the mould was lined with 1 mm thick polyester fleece and 0.05 mm thick plastic foil. The foil also covered the joints between the three parts of the mould, allowing to cast continuous sample. This lining served multiple purposes, as it allowed to: seal the specimen, reduce the friction between the specimen and the measuring section of the mould, and it acoustically decoupled the specimen from the mould. After casting, the mould was covered with plastic foil and a cover plate and then sealed with a duck tape. In order to measure linear shrinkage of the specimen, two thin steel wires were embedded in the measuring section. In order to embed the wires, two pairs of openings were drilled 260 mm apart. This way, wires could be coupled to four linear variable displacement transducers (LVDTs) mounted on the outside of the mold. The wires were placed into the empty mould and they were just fixed by the plastic foil. This allowed free movement and measuring the deformations directly after casting. The free shrinkage was measured in an identical mould. In order to minimize the external restraint the three parts of the mould were supported on ball bearings and were able to move against each other in the axial direction. In order to monitor and compare crack development under free and restraint conditions, acoustic events were measured on both specimens. When a crack develops, a sudden material displacement is generated at the origin of the crack. It propagates as an elastic wave through the material and can be detected by piezo-electric sensors on the surface. From the analysis of these signals, conclusions can be drawn on the process of cracking during hardening. Three acoustic emission sensors were integrated in each cover plates of the measuring section of the moulds. They were coupled acoustically to the specimen by a film of Vaseline (approximately 1 mm thick). Two acoustic emission sensors were integrated in the front edges of the moulds. They were operated as ultrasonic transmitters and ultrasonic sensors in order to measure the ultrasonic velocity inside the specimen during hydration. They were coupled to the specimen by a film of Vaseline as well. The temperatures of the specimen were measured by thermocouples. In order to measure the capillary pressure a small tube with an inner diameter of 1 mm was fixed through a side wall of the moulds and connected to a pressure transducer. Before casting these tensiometers were filled out completely with degassed water. 3 Results Figures 3 and 4 show the measurement results for paste and mortar, respectively. Acoustic emission measurements events were counted only if they were localized between the three sensors of the measurement section (Fig. 2). This excludes events from cracking that may occur in the area of the anchors in the front edges. As mentioned before, the definition of autogenous shrinkage includes deformations before setting. However, cracking starts if tensile stresses can be transferred by the material. Therefore, the measured deformations were set to zero at the time when a significant tensile stress has developed in the specimen with external restraint. All tests were performed at a constant room temperature of 20 C. However, the measured deformations and stresses were influenced by the thermal effects related to heat of hydration.

5 3.1 Cement Paste Results Significant amount of shrinkage was measured in cement paste specimen without external restraint. Under restraint conditions, significant tensile stresses developed 4 hours after water addition. This is the time when the first cracking might have started, however the acoustic activity remained low at that time indicating no microcracking. A distinct acceleration of the development of tensile stresses was observed 5 hours after addition of water to cement and the first acoustic hits were registered from both specimens at that time. The maximum stress development rate was reached 6 hours after the addition of water. At the same time, shrinkage derivative for the specimen under unrestraint conditions reached a maximum. According to previous work by Fontana [8] this is the time when a solid matrix begins to form. The resistance against deformation increases with the developing stiffness of the matrix and hence the rate of shrinkage decreases. At this time, the acoustic activity in both specimens increased and it was more pronounced in the specimen with external restraint. This can be explained by the onset of microcracking which is more pronounced under external restraint conditions. Lura et al. [9] interpreted the acoustic emissions in this stage of hydration as cavitation that is caused by self desiccation. This model doesn t explain the difference in the acoustic activity between the restrained and the unrestrained specimen. The tensile stress reached a local maximum after 7 hours and decreased after that, even though the measured shrinkage was still increasing slightly. During this period, the tensile stress was reduced by relaxation at a faster rate than it was built up by restrained shrinkage. It was also observed that the acoustic emission activity decreased in case of both specimens. After 8.5 hours the tensile stress started to increase again while the shrinkage decreased slightly. Subsequently, a continuous increase of the shrinkage and the tensile stress was measured until the end of the test. The tensile stress exceeded the tensile strength of the specimen with external restraint and after 101 hours a rupture occurred. The acoustic emission activity was low in both specimens, showing no significant differences. In the paste, which is a brittle material, the rupture occurred suddenly without a preliminary microcracking that can be detected by acoustic emissions. Similar phenomena were previously observed in numerical simulations on acoustic emissions released by concrete under compressive load and different degrees of homogeneity [10]. The consumption of free pore solution by the hydration reaction can be observed by the measurement of the capillary pressure [11]. In the cement paste the capillary pressure reached a minimum of N/mm² within the first 7 hours after addition of water. This can t explain the tensile stress of around 0.55 N/mm² that was measured at the same time. The capillary pressure, which is considered as the driving force for autogenous shrinkage, can t be measured by sensors described above. Within next 1.5 hours, the measured pressure increased to the ambient pressure. It is estimated that a gas filled pore system developed in this time period and the sensor lost its direct contact to the pore solution. The gas filled pore system slowly reached the balance to the ambient conditions. This might explain the temporary reduction of the measured tensile stress that is described above. The ultrasonic velocities that were measured during hydration of the specimens with and without external restraint showed no significant difference. No conclusions can be drawn on the basis of these measurements on an intensified microcracking before the rupture in the restraint specimen occurred. As the result of the large distance of 560 mm between the ultrasonic transducers and the high attenuation of the fresh cement paste, the first measurement of the

6 ultrasonic velocity was possible not until 5 hours after addition of water. It was expected that the ultrasonic velocity during hydration would increase quickly and monotonically first and then, as it approaches the ultrasonic velocity of the solid cement paste, asymptotically. Contrary, the presented results show a temporary decrease of the ultrasonic velocity around 7.5 hours after addition of water. This might be explained by the development of a gas filled pore system as mentioned above. However, this assumption has to be further investigated.

7 tensile stress [N/mm²] tensile stress derivation of tensile stress [N/(mm² h)] derivation of strain [mm/(m h)] strain [mm/m] strain with restraint strain without restraint capillary pressure [mbar] pressure temperature temperature [ C] accumulated hits ultrasonic velocity [m/s] with restraint without restraint time after water addition [h] Fig. 3: Test results for cement paste

8 3.2 Test Results for Mortar The results from the measurements of the autogenous shrinkage, the tensile stress, the temperature, the capillary pressure and the ultrasonic velocity of the mortar are similar (of the same quality) to the results from the cement paste. The fraction of hydration heat releasing binder is lower in the mortar and therefore the maximum temperature is lower. Due to the internal shrinkage restraint of the binder by the aggregates the measured shrinkage is lower as well. However, contrary to the shrinkage development lower than in cement paste, the tensile stress developed faster in the mortar. During the test no rupture occurred and after demolding, the specimen showed no cracks visible to the naked eye. Similar to the results obtained for cement paste specimen, the development of the ultrasonic velocity in the mortar showed a temporary decrease. All in all the increase of the ultrasonic velocity was faster than in the cement paste and the velocity was higher at the end of the test. This corresponds to the higher compressive strength of the mortar that was estimated on separate specimen. Normally, an increasing compressive strength corresponds to an increasing ultrasonic velocity. The acoustic emission activity of the mortar is different from activity in the cement paste. The first acoustic events were registered at the same time after addition of water but the quantity was significantly lower. The main reason is the higher attenuation of the acoustic waves in the fresh mortar. At the beginning of the hydration the difference in impedance between the cement paste and the aggregates is large. Therefore, the acoustic waves are strongly dispersed and only a small part of the acoustic energy arrives at the sensors. The difference in impedance decreases while the strength of the matrix increases. As such, an increasing number of acoustic events can be located during the hydration. During the entire test the acoustic emission activity was higher in the specimen with external restraint. Based on these findings, it can be concluded that microcracking is more pronounced in the specimen with external restraint.

9 tensile stress [N/mm²] tensile stress derivation of tensile stress [N/(mm² h)] derivation of strain [mm/(m h)] strain [m/mm] strain with restraint strain without restraint capillary pressure [mbar] pressure temperature ultrasonic velocity [m/s] temperature [ C] accumulated hits without restraint with restraint Time after water addition Fig. 4: Test results for mortar

10 4 Summary Preliminary results showed that acoustic emission analysis in combination with ultrasonic testing enables characterization of the damage process in hardening high performance concrete. The comparison of the deformation and stress is essential to the results interpretation. However, additional work is needed to determine if only cracking is responsible for the acoustic emissions, or there are also other phenomena occurring concurrently. Also, the measured development of the ultrasonic velocities raised some questions. For example, the common technique of simply measuring the pulse transit- time might be not sensitive enough to detect the microcracking. Future work will use the coda wave interferometry (CWI) to increase the sensitivity of the ultrasonic measurements. Additionally, the temporary decrease of the ultrasonic velocity and hence the dynamic elastic modulus during the early stage of hydration will be further investigated. For this purpose the elastic modulus will be measured in short intervals during hardening and compared to the ultrasonic velocity results. Literature [1] Jensen, O. M. : Autogenous Phenomena in Cement-Based Materials, Kopenhagen: Nyt Teknisk Forlag 2005, ISBN [2] Bentz, D. P. and O. M. Jensen: Mitigation strategies for autogenous shrinkage cracking, Cement and Concrete Composites 26 (2004), [3] Lura, P., O. M. Jensen and Klaas van Breugel: Autogenous shrinkage in highperformance cement paste: An evaluation of basic mechanisms, Cement and Concrete Research 33 (2003), [4] ASTM International: ASTM Standard C : Standard Test Method for Determining the Age at Cracking and Induced Tensile Stress Characteristics of Mortar and Concrete Under Restrained Shrinkage. [5] Springenschmid, R., Breitenbücher, R., Mangold, M.: Development of the cracking frame and the temperature-stress testing machine, Thermal Cracking in Concrete at Early Ages, RILEM Proceedings 25, E&FN Spon, London 1995, [6] Igarashi, S.-i., A. Bentur and Kovler, K.: Autogenous shrinkage and induced restraining stresses in high-strength concretes, Cement and Concrete Research 30 (2000): [7] Lura, P., Guang, Y., Tanaka, K. and Jensen, O. M.: Microcrack Detection in High- Performance Cemetitious Materials, Proceedings of the 4 th Seminar on Selfdesiccation and its Importance in Concrete Technology, Gaithersburgh 2005, [8] Fontana P., Einfluss der Mischungszusammensetzung auf die frühen autogenen Verformungen der Bindemittelmatrix von Hochleistungsbetonen, Schriftenreihe des DAfStb, Heft 570, Berlin: Beuth-Verlag [9] Lura, P., J. Couch, Jensen, O. M. and Weiss, J.: Early-age acoustic emission measurements in hydrating cement paste: Evidence for cavitation during solidification due to self-desiccation, Cement and Concrete Research 39 (2009), [10] Zhu, W. C., Zhao, X. D., Kang, Y. M., C. H. Wie, C. H. and Tian, J.: Numerical simulation on the acoustic emission activities of concrete, Materials and Structures (2010), [11] Georgin, J. F., Le Bihan, T., Ambroise, J. and Pera, J.: Early-age behavior of materials with a cement matrix, Cement and Concrete Research 40 (2010),