Concrete Strength Evaluation through indirect UPV

Transcription

1 Concrete Strength Evaluation through indirect UPV S. Biondi, C. Valente & L. Zuccarino ingeo Engineering and Geology Department, Gabriele d'annunzio University of Chieti-Pescara, Pescara, Italy ABSTRACT: The assessment of concrete properties in existing r/c buildings is a topic of fundamental importance to get reliable safety checks in structural retrofitting or seismic rehabilitation. Among other techniques, the ultrasonic pulse velocity test (UPV) is largely employed to estimate the concrete strength. This is because UPV is a non destructive method, easy to perform and provides reliable results. Several formulas are available in technical literature to relate the UPV measurements to concrete strength. These formulas are robust and reliable above all for direct measurements. Unfortunately these direct measures are a problem for existing buildings where, for a number of reasons, it is often necessary to perform indirect or, at most, semidirect tests. In these cases not definitive relations are available in literature. The present paper aims to contribute to develop a correlation between direct and indirect (or semi-direct) measures in order to use this latter kind of measure for reliable concrete strength estimations. 1 INTRODUCTION 1.1 Research objective The in situ real characteristic of a structural concrete is a fundamental datum for the assessment of capacity of existing structures. Generally the designer involved in structural retrofitting has not at his disposal the original results obtained, via destructive tests, on specimens (cube or cylindrical ones) at the construction time, or it is not possible to guarantee that original specimen strength is actual building strength due to construction difficulties, to environmental actions or to degradation of material. For these reasons an important research topic is the investigation of in situ concrete strength. Accurate strength predictions are possible via semidestructive procedure i.e. via concrete core drillings and laboratory compressive tests. Unfortunately this approach is limited by his punctual nature and, above all, by the structural weakness caused on specimens (concrete cores) by drilling procedure and specimen extraction. So it is very important to get refined non destructive testing procedures on in-situ structure. These procedures are different each other and use different physical criteria in order to determine concrete strength. One of the most efficient procedure is using ultrasonic pulse velocity (UPV) measure in order to determine dynamical elastic modulus of concrete. This dynamical elastic modulus can be related to static elastic modulus and then to compressive strength. These measures are generally combined with the rebound index in order to carry out the so-called Son- Reb tests. The procedure is robust and efficient if it is integrated with complementary compression laboratory tests on sampled cores. It is noted (Biondi, 2008) that direct UPV tests provide reasonably estimates of the concrete strength, whereas both semi-direct and indirect ultrasonic tests can underestimate concrete strength. Also changes in humidity content of the medium, caused by environmental conditions, can have great influence on material response to ultrasonic wave. Due to this problem, a comprehensive research activity that first of all consider the influence of the casting direction for pulse velocity measure and then different humidity and temperature (seasonal) conditions has been planned by a research team at the Laboratory of Structures of Chieti-Pescara University. Some previous results of this activity were discussed in the past (Biondi et al., 2013) regarding casting direction. In this paper the seasonal variability is discussed. Although the UPV technique is well-known and standardized (e.g. European Standard EN :2004), at present time no systematic studies concerning the correlation between direct, semi-direct and indirect measures appear to be carried out. The UPV state of the art is sufficiently consolidated in terms of concrete dependence on chemicalphysical characteristics, (Panzera et al., 2011), and

2 on reinforcement influence in the in-situ case, (Naik et al., 2004). More recently some authors, (Qixian & Bungey, 1996), (Lawson et al., 2011), (Mahure et al., 2011), focused their attention on direct, semi-direct or indirect tests. Among these literature contributes, Qixian & Bungey (1996) observed that a smaller pulse velocity is detected in case of indirect measures as compared to direct measures. This difference is explained in terms of differences both in concrete composition and humidity content between concrete cover and concrete nucleus. These velocity differences are significantly lower than those detected on existing structures in previous studies by one of the authors of this paper, (Biondi & Candigliota, 2008), (Biondi, 2008). A statistical analysis of direct and indirect measures, performed by Yaman et al. (2001) shows that both measures can be considered statistically similar (T test) and can be represented by a normal distribution (i.e. they are defined by average and standard deviation values). In the same paper, as a complementary result, it was also pointed out that the statistical aspect of data depends on uniformity and humidity characteristics of concrete. The in-situ casting direction is considered too in order to evaluate the influence on direct, semi-direct and indirect UPV results, (Turgut et al., 2006). The casting direction enhances the difference between direct and indirect measures while higher is the concrete strength, lower is this difference. Also in that paper, considering laboratory specimens having known casting age, curing conditions, concrete composition and compressive strength, the difference between direct and indirect measures is lower than those differences that have been detected on in-situ structures, above all in the case of low quality structures, i.e. structures having low quality strength (see again Biondi, 2008). In consideration of all those reasons, an experimental campaign was carried out on different concrete specimens. In order to be comparable with actual on site conditions, these specimens have been selected having unknown characteristics in terms of both aging and curing. The specimens were collected from different construction sites and only the casting direction can be determined via a visual inspection. It is to be noted, however, that the influence of this parameter should be carefully evaluated when considering a structural element (beam, slab, column) instead of a specimen. Geometrical difference between a specimen and a real structural element seems to be so significant in order to permit a real understanding of casting direction influence. Obviously segregation and bleeding phenomena or reinforcement influence are much more significant for a column having a dimension of some meters than in the case of a specimen having a dimension of some millimeters. 1.2 Research activities A large experimental campaign was designed to fulfill the research purposes. The experimental activities were replicated in different seasonal conditions. At this time results for summer and winter conditions are available and are discussed in order to complete previous discussions (Biondi et al., 2013). A certain number of concrete samples were tested in laboratory in different environmental conditions. The samples were collected from different construction sites and they could have different (unknown) mix design. On the other side, specimens show an identical geometry. The curing conditions were identical to those of concrete of a building during the construction period and age was approximately 5 years for each specimen at the time of test. Direct, semi-direct and indirect UPV tests were performed and replicated in two opposite seasonal conditions (Summer and Winter) according to an average temperature variation of 20 C. The same testing apparatus was used. In both cases three different humidity conditions were considered for the specimens: a natural humidity condition, a fully wet condition and an intermediate humidity condition. The natural humidity condition, for simplicity herein referred to as dry condition, corresponds to an average humidity condition that one can find inside a building. Obviously, these humidity condition is susceptible to change depending on the season even if for a lower extent than for the outside humidity. One therefore should expect that dry conditions in summer and winter correspond to different humidity rates (i.e. to different specimen masses). In the present paper, natural humidity conditions refer to as stocking in the laboratory without any particular curing or heating provisions. On the contrary, the full wet conditions are artificially obtained. The specimen have been immersed in water for a proper time until they are completely saturated, i.e. until no weight changes can be detected on each specimen for successive measures. Intermediate humidity conditions are those that reveal a mass density for a specimen that is an average value between dry and full wet. For these reasons the intermediate condition is different with respect to winter or summer. Finally, dependence on the path length of UPV impulse and influence of casting direction on transmission velocity have been also considered. The results show that it is possible to obtain a clear relationship between direct and indirect UPV tests and that this relationship is strongly dependent upon the humidity content. This dependence appears as a seasonal dependence and has to be confirmed in real insitu structures. This relation can help in obtaining reliable concrete strength evaluations even when using indirect UPV tests; other factors are less influencing the pulse velocity.

3 2 EXPERIMENTAL PROGRAM 2.1 Specimen geometry The experimental campaign is conducted on concrete prisms of identical nominal geometry: mm3, see Table 1., collected from different construction sites with different unknown mix design. The curing, after an initial period at construction sites, is carried out in laboratory (in a closed and unheated stocking room). The curing conditions can be considered as identical to those of a r.c. infilled frame while the specimen age was about five years at the time of tests. The water content is the parameter that simulates maturation of the material. In particular, the saturated full wet specimen can be taken also representative of the condition of the material at 28 days (reference time for design strength), while the dry specimen could be considered as the condition of the material during service life. Figure 1. shows the two specimens that showed maximum (specimen ) and minimum (specimen ) specific mass. A significant difference in superficial porosity is evident. On the contrary, the maximum humidity content corresponds to the full wet conditions artificially obtained via water immersion of specimens. Theoretically, the full wet conditions should not change from summer to winter. Indeed small differences are observed: a bulk density greater in winter than in summer is detected. These differences are insignificant from a practical point of view, however they are reported as measured in Table 2. The bulk density of Figure 2. is computed using data for summer dry conditions; whereas porosity of Figure 3. is computed considering the mass difference between winter full wet conditions and summer dry conditions. These two latter limits will be used throughout the paper to represent the maximum range of measures performed. Table 1. Geometric characteristics of the specimens. Specimen Geometry Volume 3 m mm ρb [kg/m3] (a) % 6% 4% % 0% The geometric characteristics of concrete specimens are summarized in Table 1., while Figure 2. and Figure 3. show the variation of bulk density ρb and porosity φ in concrete specimens. In order to understand correctly the data some comments have to be made. In particular it is to discuss as a condition is assumed as reference condition. We defined two reference conditions: minimum humidity content (called 0% humidity in following figures and tables) and maximum humidity content (called 100% humidity in following figures and tables). The minimum humidity content corresponds to the dry condition measured during summer season. Figure 1. Concrete specimens: (a), (b) Figure 2. Bulk density of concrete specimens (0% humidity, summer dry condition). φ [%] (b) 2200 Figure 3. Porosity of concrete specimens (weight difference between winter full wet condition [100% humidity] and summer dry condition [0% humidity]).

4 For the sake of conciseness and simplicity these two limits have been normalized to 0% ( dry summer conditions) and 100% ( full wet winter conditions). All other measures falling inside this range are normalized accordingly. In this manner the average bulk density of the concrete specimens, in dry conditions, is equal to 2361 kg/m3 ( kg/m3); while the average porosity, computed as a weight percentage, is equal to As shown in Figure 2.&Figure 3., two specimens, P2 & P4 respectively, present values that deviate from average values (minimum and maximum density). Those specimens are shown in Figure Humidity conditions The experimental investigation consists of different measurements on the same specimen according to two different conditions: seasonal temperature and humidity content variation. The UPV tests were performed and replicated in summer and winter with an average temperature respectively of 30 C and 10 C. In both cases three different humidity conditions were considered: dry, intermediate and full wet. As above said summer dry condition is the so-called 0% humidity while the winter full wet condition is the so-called 100% humidity. The parameter that allows to monitoring the water content is specimen mass both in summer and winter. Specimens were weighted in each humidity condition before UPV tests. At first the samples were tested in dry condition, then immersed in water until a complete saturation and tested again, Figure 4. Finally, saturated specimens were exposed to the external environment until an intermediate humidity condition in terms of weight is achieved and then tested again via ultrasonic apparatus. Table 2 shows the comparison, in terms of masses, between wet samples (maximum saturation) and dry samples (minimum saturation). These categories are season dependent. In fact from data, it is possible to observe that if quite same full wet conditions are obtained in different seasons (summer full wet is 95.9% of winter full wet ) a not negligible difference is detected for dry condition between summer and winter (winter mass percentage is 28.1%). This is due to the adopted natural humidity condition, i.e. a condition of a specimen in laboratory without the use of any curing and heating apparatus in order to obtain lower dry condition. At the end of this test campaign, each specimen will be subdivided in three cubic specimens in order to perform compressive tests. Before these crushing tests will be interesting to control the mass variation in dry and wet conditions due to lower geometrical dimensions or to the use of an heating apparatus. 2.3 Ultrasonic apparatus and test procedure Figure 4. Specimens immersed in water for obtaining the full wet condition. Table 2. Measured masses for different conditions Specimen min. max. average Summer (T>30 C) dry full wet mass kg Winter (T<10 C) dry full wet The UPV measurement equipment consists of two 54 khz transducers, with a diameter of 50 mm: one transducer for producing ultrasound pulses and the other for receiving pulse transmitting. System is based on the piezoelectric principle of transconduction: ultrasonic waves are generated by exciting the piezoelectric element in one transducer by an alternating electrical voltage, which causes it to vibrate at its resonant frequency. Any kind of sound can only be propagated in a material medium and it is strongly influenced by that medium. For this reason the velocity of sound, as well as its attenuation, depends in a characteristic way on the nature of the medium. So the UPV can be assumed as a fundamental parameter of medium characteristics and time and distance are test data to be collected for this scope. In the present activity ultrasonic tests were performed in three different manners: direct, semi-direct and indirect tests. The tests were performed on each side of specimen in order to evaluate the influence of concrete casting direction on the ultrasonic pulse velocity. Three directions were considered: one parallel (150 mm length) and two orthogonal (150 and 480 mm lengths) to casting direction.

5 The UPV tests carried out on each specimen are direct, VD, semi-direct, VS, and indirect, VI, tests. The direct VD tests include tests along the longitudinal axis of the specimen VD 48 (path length equal to 480 mm, orthogonal to casting direction) and along transversal axes VD 15 (path length equal to 150 mm, both orthogonal and parallel to the casting direction). The indirect VI tests included path lengths equal to 75, 150, 225 and 300 mm on each specimen face (both orthogonal and parallel to the casting direction). Direct, indirect and semi-direct UPV measurements schemes are shown in Figure mm 480 mm Direct UPV: VD 48 - VD mm Indirect UPV, VI 75mm 75 mm Semi-direct UPV, VS 90mm 75 mm Figure 5. UPV measurements schemes for different classes of ultrasonic tests: direct, indirect and semi-direct tests. Direct, indirect and semi-direct UPV measurements are collected at each measuring station for each samples. The average values are calculated. In Figure 5. it is possible to observe that each specimen is ideally subdivided into three parts of 150 mm length (three ideal cubes). For direct and semi-direct UPV tests the transducers (transmitter and receiver) are applied at the intersections of diagonals of each cube faces while for indirect UPV tests the transmitter-receiver configuration is that with the transmitter fixed on the axis of an ideal cube and measurements taken by progressively shifting the receiver by 75 mm steps with the center-to-center distance from the transmitter increasing from mm. The influence of concrete casting direction, parallel (//) or orthogonal ( ), on ultrasonic pulse velocity was evaluated using the direct UPV (VD 15 ) and indirect UPV (VI) methods. For each test were adopted from 1 to 4 measuring points. The tests have been repeated up to a maximum of 3 times. In total 15 tests for VD 48, tests for VD 15 (//) and VD 15 ( ), for VI (//) and VI ( ) have been performed. 3 TEST RESULTS The results of tests are analyzed and compared in order to evaluate the possibility to point out possible relationships between direct, indirect, and semidirect UPV measurements according to the various humidity levels and seasonal temperature variations. The testing program had three principal goals. The first goal was to compare direct, semi-direct and indirect UPV measurements according concrete casting direction. The second goal was to evaluate humidity and temperature dependence. Last but not least, the third purpose was to compare direct, indirect and semi-direct UP velocities and to develop relationships to be used in the case of a difficulty to carry out direct measure due to the insitu conditions. In order to assessing the accuracy of collected data, several repetitions for the same test were carried out. The results show a stable aspect regardless of type of measure (direct, indirect or semi-direct), tests conditions (humidity and temperature) and test apparatus use. Regarding the latter aspect it is to point out that the ultrasonic apparatus that was used was a common apparatus, extensively used in the past for in-situ test on existing structures. 3.1 Casting direction dependence The dependence of the UPV on the casting direction was first considered. From the results of the experimental campaign it is observed that there is no significant relationship between direct UPV, indirect UPV, semi-direct UPV and the concrete casting direction. Average values, coefficient of variation and standard deviation were computed and regression models adopted to compare the data. A sample result referring to specimen is shown in Figure 6. where the two cases of dry and full wet summer conditions are considered for indirect tests. The high similarity between the UPV recorded in the two directions is well assessed by the linear regressions of the data that estimates the pulse velocities. The differences between the angular coefficients are very modest and in any case less than 3%, thus confirming the low influence of the casting direction on pulse velocity. The European Standard (EN :2004) that suggests to use the angular coefficient in order to determine pulse velocity has to be assumed as a correct procedure in common practice.

6 Distance (µm) Distance (µm) a) b) 0 0 y = 4662 x R² = y = 4808 x R² = % (dry - summer) Time (µs) y = 3754 x R² = y = 3713 x R² = % (full wet - summer) // // Time (µs) Figure 6. Casting direction dependence VI tests for summer minimum ( dry - 0 % humidity) and maximum ( full wet % humidity) humidity conditions Direct UPV - VD 15 Humidity level= 0% (Summer) % (Winter) Summer Winter Direct UPV - VD 15 Humidity level= 95.9% (Summer) - 100% (Winter) Summer Winter Indirect UPV - VI Humidity level= 0% (Summer) % (Winter) 3.2 Humidity and temperature dependence As previously discussed, different humidity and temperature conditions were taken into account. The UPV tests were performed and replicated in summer (T = 30 C) and winter (T = 10 C) according to an average temperature variation of 20 C. In both cases 3 different humidity conditions were considered: dry, intermediate and full wet. For each season, each condition is different in terms of humidity ratio. So the dry winter condition reveals a 28,1% humidity in respect to dry summer condition. The main results for the whole set of specimens are summarized in Figure 7. Four histograms: two for direct UPV, VD 15, and two for indirect UPV, VI. In each case both low humidity ( dry ) and high humidity ( full wet ) results are reported. If the seasonal variation is assumed dependent on temperature, it is possible to detect an almost uniform increase of UPV equal to 5% in winter conditions as compared to summer conditions. Particularly interesting is that the same ratios are shown by single specimens. This aspect underlines the stability of the results. An exception is for the case VD 15 dry tests where only 1% increase is observed and the behavior is less stable with some fluctuations for the single specimen: water tends to uniform the medium response to the ultrasonic pulsing signal Summer Winter Indirect UPV - VI Humidity level= 95.9% (Summer) - 100% (Winter) Summer Winter Figure 7. UPV: Humidity and temperature dependence for each specimen in both summer and winter condition Concerning to humidity an expected result is pointed out in Figure 7.: the UPV increases with the humidity level. The increase is greater in the VI case than in the direct one, VD 15. Specifically, the increase is about 15% for VD 15 and 30% for VI and it does not appear to be related to seasonal conditions. But an unexpected result is pointed out too: a 28.1% humidity (i.e. weight) difference in dry condition (winter to summer) is less relevant than a 4.1% humidity difference in full wet condition (summer to winter). This aspect has to be investigated deeply.

7 3.3 Direct, semi-direct, indirect dependence In Figure 8. direct (VD 48 and VD 15 ) and indirect (VI) ultrasonic pulse velocities are reported for winter conditions considering defined boundary humidity ratios ( dry, 28.1%, and full wet, 100%). UPV Winter (T<10 C) 28.1% 100% UPV Winter (T<10 C) VD48 48 VD15 15 VI VD48 48 VD15 15 Figure 8. Direct to indirect UPV comparison for winter tests on each specimen UPV Winter (T<10 C) VD48 VD15 15 VI VS 28.1% 100% VI 4898 Figure 9. Direct, semi-direct and indirect average values for winter dry (28.1%) and full wet (100%) conditions Table 3. UPV average values for different testing conditions Humidity Summer Winter VD 48 VD 15 VI VD 48 VD 15 VI VS dry full wet dry 3% - 22% 3% - 17% 8% full wet 4% - 3% 4% - 2% 6% In Figure 9. and Table 3. the whole set of the results is shown in terms of average values. In particular in Table 3 the percentage ratio between a current value and the VD 15 dry or full wet value is calculated. From these data it is possible to point out some interesting observations. It is possible to observe that VD 48 velocities are generally lower than VD 15 ones. The ratio between two case is stable and approximately equal to This value is observed regardless the humidity level. However VD 48 tests are characterized by a greater uncertainty than VD 15 ones. In the first case the standard deviation is about 2.0% whereas in the second case reduces to about 0.7%. This fact could be due to irregularity in concrete specimens. This aspect will be investigated when specimens will be subdivide in three identical concrete cubes. The VI tests produce results systematically lower than the direct tests and relation between direct and indirect UPV is influenced by humidity level. In dry conditions the VD 15 /VI ratio is equal to 17% for winter. This ratio decreases at about 11% in the intermediate conditions and reduces at 2% in full wet conditions. Similar comparison for semi-direct tests. In this case the UPV reduction is lesser and almost constant in the range 8%-6%: so the full wet VI average value (5110 m/s) is greater than the corresponding VS average value (4898 m/s) while in dry condition the VS average value is greater than the VI average value. Similar conclusions can be observed in the case of summer conditions. One topic to be better investigated is relation between direct and indirect UPV for different humidity level. The ratio VD 15 /VI is shown in Figure 10. VD 15 / VI 1, , , , , y = x R² = Summer + Winter Summer Winter Humidity [%] 1, % 20% 40% 60% 80% 100% Figure 10. VD 15 direct to VI indirect UPV correlation for different humidity level (0% summer dry condition) Table 4. Linear regression coefficients (S = summer, W = winter) y = ax + b S W S + W a -0,195-0,208-0,197 b 1,211 1,228 1,216

8 For the sake of correctness we have to reaffirm that humidity level, as we define it, is a conventional one. In fact it ranges from dry summer conditions (conventionally 0%) and full wet winter conditions (conventionally 100%). In Figure 10. it is possible to observe that the ratio VD 15 /VI reduces as long as the humidity level increases. The reduction can be modeled by a linear regression. Three linear regressions are considered, Table 4, one for summer conditions, one for winter conditions and one for both cases. In any case similar regression coefficients are detected. Clearly those coefficients have to be refined if different humidity levels would be considered. 4 CONCLUSIONS The present experimental activity was carried out with the aim to contribute to development of a correlation between direct and indirect (or semi-direct) UPV to be used for in-situ tests on existing RC structures. For this purpose ultrasonic pulse velocities were measured, using direct and indirect tests, on ten concrete specimens. On the one hand results show that it is possible to obtain a good correlation between direct and indirect UPV tests; on the other hand this correlation is strongly dependent on humidity rate. In the paper three different conditions have been studied: the so-called dry, intermediate and full wet. The reference condition for minimum humidity content is assumed to be the dry condition in summer season. On the contrary, the reference condition for maximum humidity content is assumed to be the full wet condition in winter season. The full wet conditions are obtained, in each season, via water immersion of specimens. Direct, semi-direct and indirect UPV tests were performed and replicated in two opposite seasonal conditions (summer and winter) according to an average temperature range of about 20 C. In order to assess the degree of accuracy of results, several readings and repetitions for the same test were carried out. The results are stable and repeatable regardless of the type of measure (direct, indirect, semi-direct), temperature and humidity conditions of the tests. In order to evaluate test impact on common practice, a normal pulse velocity apparatus was used. First of all it is detected that concrete casting direction is quite irrelevant on different UPV tests: difference between perpendicular casting directions does not exceed 1%. This result is clear and stable for specimen geometry. It is possible that a different result could be obtained if a structural element is considered due to macro phenomena in concrete (segregation, bleeding, reinforcement interaction). Considering different measure distances, the average direct UPV, VD 15, is 3% higher than the average direct UPV, VD 48 for dry condition, while it increases to 1% for full wet condition, regardless the temperature. The center-to-center transmitterreceiver distance is quite irrelevant in indirect tests. The ratio between direct UPV, VD 15 and indirect UPV, VI, is equal to 1.22 in summer (T > 30 C) for to the minimum humidity content ( dry ), while it reduces to 1.17 in winter (T < 10 C). On the contrary, for the maximum humidity content of the specimens ( full wet ) this ratio tends to unity both in summer (1.03) and in winter (1.02). The indirect UPV tests are more dependent on humidity, particularly in case of winter test (T < 10 C). Finally a relation between direct and indirect UPV for different humidity level is pointed out. A simple linear regression model can be used in order to define ratio between direct UPV, VD 15 and indirect UPV, VI, results. 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