TRACING OF CRACK DEPTH IN CONCRETE STRUCTURE USING ULTRASONIC PULSE METHOD

Transcription

1 TRACING OF CRACK DEPTH IN CONCRETE STRUCTURE USING ULTRASONIC PULSE METHOD Jiri Brozovsky (1), Jiri Brozovsky, Jr. (2) and Jiri Zach (1) (1) Brno University of Technology, Faculty of Civil Engineering, Technology Institute of Building Materials and Elements, Brno, Czech Republic (2) VSB-Technical University of Ostrava, Faculty of Civil Engineering, Department of Building Mechanics, Ostrava, Czech Republic Abstract The crack generation in in-situ concrete structures is not unusual. On that ground, there is appropriate to know crack depth, in addition to its opening width. The ultrasonic pulse method is one of way how to detect this. This study describes both findings of crack depth measurement in in-situ concrete structures by means of ultrasonic pulse method and finding confrontation with results as detected in cores. The ultrasonic pulse method shows results comparable to core crack depth measurements at samples taken from points without reinforcement or where cracks have been consolidated as a result of their infilling by cement hydration products. 1. INTRODUCTION Both setting and post-setting shrinkages of cement binder, lead as a result to concrete shrinkage, although the latter is lower than cement binder shrinkage of its own. These changes create stress in concrete construction in case the construction design (i.e. lack of shrinkage joints) disallows shape change; also, some local state of stress can occur in case the surface shrinkage differs from internal concrete mass. The concrete shrinkage depends on a number of factors, in particular as follows: volume of mixing water volume/sort of cement used amount of setting heat due to setting/initial hardening aggregate mixture proportion (concrete rich in fine aggregates shows more shrinkage) influence of the environment in which concrete hardens extent and method of concrete curing

2 As mentioned above, concrete shrinkage depends on many factors; these could lead in real construction to creation of shrinkage cracks during initial hardening, i.e. even before loading of such concrete construction. Cracks in structures are among others weak points in view of the fact that they are the cause of other deteriorations. This paper describes the findings of crack depth tracing in 6 constructions of mass/reinforced concrete. Constructions from 250 to 1,200 mm of thickness, made of concrete class C20/25 to C30/37 to EN [3], have been examined. These constructions showed a number of shrinkage cracks open to various widths (0.05 to 1.50 mm) for these particular reasons: Use of Portland cement; this concrete develops a higher setting heat as compared with blended cements (blast furnace cement of fly ash cement) Aggregate mixture contained a higher portion of fine aggregates than advisable Curing of concrete in its initial setting has been done inadequately (missing data on climatic conditions during casting and subsequent concrete curing in one of the six constructions) 2. METHOD OF CRACK DEPTH MEASUREMENT Before a crack depth measurement of its own, a position of cast-in steel bars has been detected. The PROFOMETER 3 (PROCEQ) electromagnetic bar locator / concrete cover meter has been used for this purpose. To check the reality of crack depth using the ultrasonic pulse method, cored specimens through cracks with withdrawal of test samples have been executed. Nevertheless, some of cored specimens haven t been made through full crack depth. It was because some cracks were situated above the steel bars and it was unreasonable to cut the reinforcing bars. 2.1 Tracing of Crack Depth Using Ultrasonic Pulse Method To measure crack depth using the ultrasonic pulse method, the ultrasonic UNIPAN (Poland) or TICO (PROCEQ) probes (frequency from 54 khz to 100 khz corresponds to a wave length from 25 to 70 mm for concrete under testing) has been used. This device conforms to requirements as specified in EN , Art. 5. The measurement has been done with respect to both ČSN [2] and EN [1]. Before the measurement starts, the ultrasonic device was calibrated by means of a calibration element. Each measuring point sized 140/240 mm or 160/250 mm respectively, has been measured in both directions through crack and through sound concrete. The measured crack was situated in the centre of each measuring base. Also, each measuring base contains only one crack. Measuring base length has been measured with 1 mm accuracy to meet the EN , Art [1] requirement. To ensure good acoustic feedback, each measuring point has been ground down to 1-3 mm to eliminate the effect of a damaged concrete surface, and, moreover, a suitable contact medium has been used i.e. indifferent gel that is commonly used in health service. Crack depth has been detected through surface sounding. See Fig. 1 for measuring set configuration.

3 Ultrasonic pulse transit time has been measured under 0.01 µs deviation. A minimum of four tests of the ultrasonic pulse transit time at one measuring point have been done for one measuring base length. The calculation needed at least 3 valid values of the ultrasonic pulse transit time. The measured values were used for calculation of average and testing of outliers. The measurement has been declared admissible, if minimum 3 values of ultrasonic pulse transit time differ from average value no more than by 2 %, otherwise such measurement was repeated. The ultrasonic pulse velocity (V) was calculated using relation (1) as described in EN [1]: L V = (1) T where: V ultrasonic pulse velocity in concrete [km/s] L length of measuring base T transit time [μs] h uz crack depth was calculated using relation (2) as described in ČSN [2]: huz = V Tu1 Tu2 2 (2) where: h uz crack depth V ultrasonic pulse velocity in concrete without any crack [km/s] T u1 transit time in concrete with crack [μs] transit time in concrete without any crack [μs] T u2 Fig. 1 : Crack depth tracing using surface sounding T exciter; R probe; L measuring base

4 2.2 Tracing of Crack Depth on Cored Specimen To trace crack depth, cored specimens Ø 50 to 150 mm have been sampled. Their maximum length was limited by location of reinforcement bars, because it wasn t allowed to break reinforcement. Maximal crack depth has been measured on cored specimens with accuracy to 1 mm - see Fig. 2. Fig 2 : Tracing of crack depth on cored specimen 3. TEST RESULTS See the following text for results of crack depth measurement by the ultrasonic pulse method and on test samples as well. Crack depth mass been measured at points where cracks do not interfere whole construction s thickness, or at points without water leakage through a respective crack. Six various mass/reinforced concrete constructions have been examined see [6 to 11]. 48 cored specimens has been examined in total, 36 ones were without crack through the whole length of cored specimen; measured values served for the assessment of the ultrasonic pulse method. Remaining samples (mainly holes terminated above reinforcing bars) were interfered thoroughly. Bore length deviation (in %) as detected by the ultrasonic pulse method in relation to actual crack length, was calculated using relation (3). h h Δ h =.100 (3) h UZ

5 See Table 1 for results of crack depth measurement by the ultrasonic pulse method and on test samples as well. used symbols: h crack depth on cored specimen h UZ average crack depth as detected using the ultrasonic pulse method on 3 measuring bases Δh value difference between core sampling and the ultrasonic pulse method [%] Table 1: Concrete crack depth measurement results ( [Ref. 6-11] for author s results) crack name h h UZ mass concrete Δh [%] crack name h h UZ reinforced concrete TV Tr Δh [%] TV Tr TV Tr TV Tr TV Tr TV Tr TP TK TP TK TP TK TP TK TP TK TP TC TP TC TVO TC TVO TC TVO TC TVO TC TVO TC Based on results listed above, and using the method of least squares, a correlation between crack depth detected by both the ultrasonic pulse method and cored specimens has

6 been elaborated. See Fig. 3 for this correlation. The recognized linearity features a narrow correlation with 0.95 ratio. A practically usable correlation [4] features the ratio above Fig 3: Correlation between crack depth detected by both the ultrasonic pulse method and cored specimens h = 0,947.h UZ + 7,3 r = 0,95 h h UZ 4. CONCLUSIONS Based on these records, we can state as follows: - Recognized correlation as detected by both the ultrasonic pulse method and cored specimens, features practically usable narrow correlation with 0.95 ratio. - The ultrasonic pulse method shows comparable results of crack depth measurement in places without steel bars. - Average value in crack length as detected by the ultrasonic pulse method differs from values detected by cored specimens by 8.2 %; maximum crack length differs by % ( respectively), these values are acceptable in practice. - Lesser differences have been detected with mass concrete (average difference 7.4 %; maximum values % or % respectively). In general terms, it is possible to state that performed measurements proved a factual efficiency of the ultrasonic pulse method for crack depth tracing in concrete structures.

7 ACKNOWLEDGEMENTS The work was supported by the GAČR 103/04/0169 project and by the MSM plan: Progressive Building Materials with Utilization of Secondary Raw Materials and their Impact on Structures Durability. REFERENCES [1]. EN Testing concrete Part 4: Determination of Ultrasonic Pulse Velocity [2]. ČSN Non-Destructive Testing of Concrete Structures [3]. EN Concrete Part 1: Specification, Performance and Conformity [4]. EN testing concrete in structures Part 1: Cored specimens Taking, examining and testing in compression [5]. Janko, J.,: Statistical Tables, NČSAV, Prague, 1958 [6]. Drochytka R.: et al.: Progressive Building Materials with Utilization of Secondary Raw Materials and their Impact on Structures Durability. Brno University of Technology, Final report of the project VVZ CEZ MSM: , Brno Brozovsky J.: Subtask 3 (in Czech). [7]. Matušina, J. et al: Survey of Košice Airport s Runway System. Report to HS 89/19. VAAZ/VSP Brno, October 1989, p. 196 (in Czech). [8]. Matušina, J. et al: Geological-Engineering and Civil Engineering Survey of Building No. XXXII of the LO Trenčín. Final report to HS 89/26. VAAZ/VSP Brno, October 1989, p. 54 (in Czech). [9]. Coufal, J. et al. Civil Engineering Survey of the Vodochody Airport. Report to SD 93/19, [10]. VSP/VA Brno, , p. 72 (in Czech). [11]. Brožovský, J., Bydžovský, J. Audit of Selected Communication and Areas except the EDE Area. Report No. 05 / 05 / 515 for ČEZ a.s. company, VUT FAST Brno, ÚTHD, 2005, p. 38 (in Czech). [12]. Bydžovský, J., Brožovský, J., Civil Engineering Survey and Redevelopment Technology of Reinforced-Concrete Basins of the Waste Treatment Plant in Ostrava, Stavexis s.r.o. company, Report No. 204/05, 2005, p. 58 (in Czech).