In situ non-destructive testing: the steel and concrete resistance assessment of ancient r/c structures

Transcription

1 Structural Studies, Repairs and Maintenance of Heritage Architecture IX 355 In situ non-destructive testing: the steel and concrete resistance assessment of ancient r/c structures R. Pucinotti 1 & R. A. De Lorenzo 2 1 Department of Mechanics and Materials, University Mediterranea of Reggio Calabria, Italy 2 Architect in Reggio Calabria, Italy Abstract The assessment of the mechanical properties of the materials of reinforced concrete structures by in situ non-destructive investigations are presented here. A series of in situ non-destructive tests has been performed with the purpose of investigating the mechanical properties of the basic materials (steel and concrete) employed in the civil buildings. The penetration resistance methods (Windsor probe system), the surface hardness methods, the ultrasonic method and combined methods were used to control the quality and estimate the strength of concrete in situ. The resistance obtained by the previous non-destructive methods is calibrated with the strength of cylindrical specimens (cores) extracted from some structural elements in the proximities of those considered to perform the non-destructive tests, by the least squares-method. The study gave prominence to: (i) large variations of the mechanics properties of the materials; (ii) the better reliability of the combined methods; (iii) the necessity to calibrate the resistance obtained by the non-destructive methods with the strength of the cylindrical specimens (cores) extracted from some structural elements in the proximities of the non-destructive tests; (iv) for all the methods employed, the sensitivity decreases along with the actual strength. Keywords: reinforced concrete structures, Windsor probe system, surface hardness method, ultrasonic method, combined methods, cores, calibration, seismic damageability, steel, concrete.

2 356 Structural Studies, Repairs and Maintenance of Heritage Architecture IX 1 Introduction The characterization of the materials employed during the construction of ancient structures, turns out of fundamental importance when the study of the structural resistance or the seismic damageability assessment are performed in this type of structures where the ravages of time do not allow a correct evaluation of the mechanical characteristics of the materials. The use of these methods is generally justifiable only if a reliable correlation curve for the specific concrete is opportunely developed. In this paper the criteria and the results concerning a non-destructive investigations carried out on two buildings built approximately on sixtiesseventies in province of Reggio Calabria are reported. 2 Penetration resistance methods The instrumentation used takes the name of "Windsor Probe System" (Figure 1). Figure 1: Windsor probe equipment. The parameter that characterizes the method takes the name of penetration index and it is represented by the length of the part of probe not penetrated in the concrete [2, 5, 6, 7].

3 Structural Studies, Repairs and Maintenance of Heritage Architecture IX 357 For every test three probes were driven into the concrete and the result of the test is the average of the three measures. The reliability of the result depends on the gap among these three values. In Figure 2 the correlation among the values of the experimental strength measured by Windsor Probe System and the core strength are presented R A Car = Rw Rw R 2 = Rcar [MPa] R A Car= Rw R 2 = R W [MPa] Figure 2: Windsor probe results vs. compressive strength. It can be seen that the correlation between the probe penetration and the core strength looks satisfactory where the actually strength are greater than approximately 15 Mpa [10, 11, 13]. The existing correlation has the shape of: A RCar = RW RW (1) if the actually strength is less than 15 MPa, the correlation between the probe penetration and in situ strength becomes more uncertain: A R = 0.116R R (2) 3 Surface hardness methods Car W W The Schmidt rebound hammer (Figure 3) is a simple tool to check the uniformity of concrete [2, 5, 6, 8]. When it is employed to appraise the compressive strength of concrete, it is necessary to build a suitable correlation curve. In fact, the results are strongly influenced by the type and dosage of cement, the dimensions and the nature of the aggregates, the superficial conditions of the concrete and by its humidity and maturity [10, 11, 12].

4 358 Structural Studies, Repairs and Maintenance of Heritage Architecture IX Figure 3: Schmidt rebound hammer Rcar [MPa] Ir R Car = e R 2 = Ir [MPa] Figure 4: Rebound number vs. compressive strength. In figure 4 the Rebound Number vs. Compressive Strength is reported for a building built approximately on sixties-seventies in province of Reggio Calabria. Figure 4 reports the interpolation curve obtained by regression: R A Car I r = e (3)

5 Structural Studies, Repairs and Maintenance of Heritage Architecture IX Ultrasonic methods The Ultrasonic Pulse Velocity System (Figure 5) has been using from more than 50 years [1, 2, 4, 5, 6, 8]. It is a very suitable method for the study of the quality of concrete by means of the measure of the speed of propagation of ultrasonic waves. Figure 5: Ultrasonic pulse velocity system. They can be used for the appraisal of the uniformity, the location of defects or cavity inside both reinforced or masonry structures, for the determination of the depth of fractures and for the assessment of the elastic module and therefore of the compressive strength of the concrete. The parameters that influence the speed of propagation of ultrasonic waves are: (i) the entity of the load; (ii) the age of the concrete; (iii) the form and the dimension of the structure; (iv) the presence of metallic reinforcements; (v) the ratio water/cement; (vi) the state of strength, (vii) the temperature; (viii) the humidity of the concrete [11, 12]. When the method is used to assess the compressive strength of the concrete it is necessary the construction of an appropriate correlation curve. In Figure 6 in situ Ultrasonic Velocity is correlated with the Compressive Strength where a large increment of resistance is recorded when varying the ultrasonic speed. 5 Combined methods The property of the concrete can be improved using simultaneously more nondestructive methods under conditions that [9]: (i) each method provides information about different properties that affect the strength of concrete, (ii) special sample preparation should not be required, (iii) tests should be quickly carry out, (iv) test methods should provide strength estimations to a similar level of accuracy, (v) tests should not affect the structural performance of the unit under test.

6 360 Structural Studies, Repairs and Maintenance of Heritage Architecture IX Rcar [MPa] VL,s R Car = e R 2 = VL,s [m/s] Figure 6: Ultrasonic velocity vs. compressive strength. The availability of the two instruments of measure (Rebaubd Hammer and Ultrasonic Probe) has allowed the use of the well-known combined method SonReb [6]. The advantage in the use of SonReb technique is that the variation of some properties of concrete produces opposite effects on the result of each component test. Then, the use of SonReb method reduces the effects of (i) aggregate size, (ii) cement type and content, (iii) water-to-cement ratio, (iv) moisture content. In Figure 7 the compressive strength of cores (Core), is compared with those obtained by Rebound Hammer method (Reb), ultrasonic method (Ultr) and Combined method (SonReb) opportunely calibrated [11, 12]. The results showed the greater reliability of the combined method when compared with an individual method. The follow law of correlation among compressive strength, Rebound Hammer index and ultrasonic speed has been obtained: RSon Reb = 1.003Vm Im (4) Eqn (4) has been extracted from the expression in which the values of the constants a and b have been deduced by the least-squares method. 6 Steel strength valuation Some specimens of steel reinforcements were extracted from the structural elements and submitted to laboratory tests to estimate their mechanical properties (Figure 8). In Table 1 the resistances to tensile stress of steel reinforcements are presented. In particular are reported: the nominal diameter,

7 Structural Studies, Repairs and Maintenance of Heritage Architecture IX 361 the actual diameter, the yielding tension, the final strength and the maximum deformation. 40 Core Reb Ultr SonReb 30 Rcar [MPa] Figure 7: Table 1: Correlation between Core, Reb, Ultr and SonReb. Mechanical properties of steel reinforcements. 7 Conclusion Specimen Φ nom Φ real f y A [mm] [mm] [Mpa] [Mpa] [%] T T T The indirect determination of the concrete actual is very difficult to perform. It must be carried out by skilled workers of consolidated experience. When opportunely calibrated, the penetration method (Windsor probe) provides, on the other hand, good results in the field of the medium-high strength, but it is not reliable for values of strength smaller than 15 MPa. That s way the need to support the penetration method with destructive or semi-destructive tests used to calibrate the measured values of strength. The rebound hammer method is not intended as an alternative to determine the strength of concrete. With reference to the Ultrasonic Pulse Velocity method, the appraisal of the resistance of the degraded concrete turns of difficult because of the insufficient sensibility of the ultrasonic speeds in the field of the low strength. f u

8 362 Structural Studies, Repairs and Maintenance of Heritage Architecture IX Figure 8: Steel reinforcement. The Ultrasonic Pulse Velocity method is very suitable for the study of the uniformity of concrete and provides a convenient means of estimating the

9 Structural Studies, Repairs and Maintenance of Heritage Architecture IX 363 strength of in situ concrete, but it requires an appropriate correlation curve (mostly when the evaluation of strength of in situ concrete is required). The use of the combined methods (SonReb Method) increases the accuracy of the estimation of the in situ compressive strength of concrete, with smaller dispersions with regard to that compared to the use of the single methodology. It is better the use of 3 parameters combined methods (ex. SonReb method + Windsor probe method). Such an indication is already contained in RILEM 43 CND. The study has evidenced the importance of the correct mechanic characterization of the in situ materials and how its quality influences, together with the structural design, the assessment of seismic damageability of ancient r/c structures. In fact the non-destructive investigations have put out: (i) great variations of the mechanics property of the materials even in structural elements similarly aged; (ii) large variations of the mechanics property depending on the maintenance of the structural elements (deterioration condition). Further analytical and experimental studies are necessary to calibrate each parameter for the evaluation of the correct degree of deterioration of structural elements. References [1] British Standards Institution BS Recommendations for Nondestructive methods of test for concrete, Part 5. Measurement of the velocity of ultrasonic pulses in concrete, [2] Di Leo A., Pascale G., Viola E.. Statistical Approach to Assess Hardened Concrete Strength by means of non-destructive methods, I.A.B.S.E. Symposium on Strengthening of Building Structures: Diagnosis and Therapy, Venezia, [3] Viola E., Pascale G., Di Leo A.. Core Sampling Size in N.D.T. of Concrete Structures, International Conference on In-Situ Non-destructive Testing of Concrete, Ottawa, [4] HALABE U. B.. Condition assessment of reinforced concrete structures using electromagnetic waves, Ph.D. Thesis, MIT, [5] British Standards Institution BS Guide to Assessment of Concrete Strength in Existing Structures, [6] Malhotra V. M., Carino N. J.. Handbook on Nondestructive Testing of Concrete, CRC Press, [7] Malhotra V. M., Carette G. G.. Penetration Resistance Methods. Handbook on Nondestructive Testing of Concrete, CRC Press, [8] Braga F., Dolce M., Masi A., Nigro D.. Valutazione delle caratteristiche meccaniche dei calcestruzzi di bassa resistenza mediante prove non distruttive. L Industrua Italiana del Cemento, 3, pp , [9] RILEM 43-CND. Draft recommendation for in situ concrete strength determination by combined non-destructive methods. Materials and Structures, 26, pp , 1993.

10 364 Structural Studies, Repairs and Maintenance of Heritage Architecture IX [10] Pucinotti R., Nucera F.. Prove non Distruttive sui Calcestruzzi: Rassegna Critica di alcune Indagini Sperimentali. Il Giornale delle Prove non Distruttive Monitoraggio Diagnostica, 2, pp.34-44, [11] Pucinotti R. Criteri per la Progettazione di Operazioni di Diagnosi Strutturale; Conferenza Nazionale sulle Prove non Distruttive Monitoraggio Diagnostica - 10 Congresso Nazionale dell AIPnD. Ravenna Italy, [12] R. Pucinotti, R. A. De Lorenzo (2003). Nondestructive in situ testing for the seismic damageability assessment of ancient R/C structures, Book of Proceedings, 3rd International Conference on NDT, Chania, Crete, Greece, pp [13] R. Pucinotti, M. Versaci, R. A. De Lorenzo (2003). A Neuro-Fuzzy approach to correlate probe penetration test results and compressive strength of R/C structures, Book of Proceedings, 3rd International Conference on NDT, Chania, Crete, Greece, pp