USE OF DURABILITY-RELATED TESTS FOR QUALITY CONTROL: VARIABILITY OBTAINED IN A REAL CASE

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1 USE OF DURABILITY-RELATED TESTS FOR QUALITY CONTROL: VARIABILITY OBTAINED IN A REAL CASE Bettencourt Ribeiro and Arlindo Gonçalves LNEC, National Laboratory of Civil Engineering, Portugal Abstract The concrete quality of a large structure (bridge and viaducts) was assessed by testing moulded specimens and in-situ concrete using durability-related tests. In this work the variability of air permeability and capillary suction results is presented. Additionally, results of laboratory-made concrete and in-situ concrete are compared. The data presented for this particular case, which includes tests on 51 pre-cast beams of 35 m long and 244 pre-cast concrete slabs, corresponding to a production period of more than one year, give a measure of the expected variation in real structures for this type of tests. 1. INTRODUCTION The specification of concrete in structural design includes durability requirements. These requirements are usually related with limits on concrete composition and minimum compressive strength values. Sometimes, for longer design working lives or for special structures, performance requirements are established through the specification of durabilityrelated concrete properties. In both cases, the evaluation of the concrete conformity is usually made on standard specimens. However, the quality of in-situ concrete is also dependent on the execution. The safety factors used in structural design usually covers the loss on performance of in-situ concrete, when compared with standard specimens. This is well established for compressive strength but there is a lack of information concerning durability-related tests. This paper presents a broad set of results on air permeability and capillary suction of insitu concrete, and the corresponding ones on standard specimens, which give an example of the relation between potential and actual performance, and of the typical scatter of the results obtained during a large period of production. The presented values were measured during the construction of a bridge and viaduct structures, but most of them correspond to pre-cast beams and slabs. The period of construction with this type of concrete elements lasted more than 15 months. Additionally, concrete of piles was evaluated by tests on cores and results were compared with standard specimens. 2. TESTS DESCRIPTION During the construction of a structure composed by a bridge and viaducts, it was decided to control the quality of some concrete elements by tests performed on site and in laboratory. Two different types of aggressive environments were present: carbonation on columns, beams 515

2 and slabs (exposure class XC4 according to EN 206-1), and slight chemical attack on piles (exposure class XA1 according to EN 206-1). Due to facilities constraints it was decided to perform only air permeability tests on beams and slabs and capillary suction on cores taken from piles. The air permeability test performed on site was intended to evaluate variation of concrete quality related with carbonation resistance. The capillary suction was used to evaluate the chemical resistance of concrete. The durability problem on piles was the ingress of aggressive water. The deterioration mechanism involves the ingression of aggressive ions into concrete, which occurs in the pore structure. This ingression depends on diffusion and capillary suction. The capillary suction test is simple and provides information on variation of pore structure, which allows the identification of potential problematic elements. Air permeability was measured with the Torrent method [1] on the concrete elements without preconditioning. The capillary suction test was done on cores after a preconditioning procedure to reduce relative humidity and humidity gradients of specimens. The preconditioning consisted first on drying the cores during 78 hours at 50 ºC to decrease the internal humidity, and then a period of 2 weeks for humidity redistribution at 50 ºC with the specimens sealed to avoid humidity exchanges. The specimens dimension were 93 mm diameter and 50 mm thick, and the procedure followed the method indicated by the RILEM TC 116-PCD [2]. Figure 1, left, shows a part of the structure and the identification of the type of elements subjected to the tests. The precast concrete beams were about 35 m long and 1.75 m high and the precast concrete slabs were about 4.15 m long, 2.1 m wide and 10 cm deep. The final depth of the slab on site was not 10 cm, once additional concrete was cast in-situ, increasing the total depth to 25 cm. Beam Slab Web Flange Web Figure 1 Left - part of the structure were some concrete elements were subject to in-situ tests; Rigth - schematic representation of beam section and location of measurements The air permeability was measured on the bottom surface of the slabs. The measurements on beams were made on 3 external surfaces as shown in Figure 1, right, and, for each external surface, two readings were taken on two points away. Figure 2 shows piles under construction. The results of capillary suction presented below were obtained on cores taken vertically from the top of the piles. Tests on laboratory made specimens were also conducted. Oxygen permeability by the CEMBUREAU method [3] on concrete cylinders, 150 mm diameter and 50 mm thick, was determined in order to be compared with the in-situ air permeability. Capillary suction was 516

3 also measured on the same specimens in order to compare their results with the values obtained on cores. Before the tests, the specimens were subjected to the preconditioning described above, in order to decrease and homogenise the relative humidity. The preconditioning started at 7 days and oxygen permeability was measured at 28 days. The capillary suction was also measured at 28 days. Top of the piles 3. RESULTS AND DISCUSSION Figure 2 Piles under construction 3.1 On-site variation during production Concrete mixtures for beams, slabs and piles are presented in Table 1. Table 2 shows statistical information of air permeability measurements on beams. Slabs were supplied by 2 different producers and the respective air permeability statistical values are presented in Table 3. The age of testing varied not due to planning but because construction works constraints. However, all beams and slabs were cured for 24 hours and then exposed to the outside air. Table 1: Concrete mixtures Constituents Piles Beams and slabs Cement CEM IV 32,5 R (kg/m 3 ) Cement CEM I 52,5 R (kg/m 3 ) Fine aggregate (kg/m 3 ) Coarse aggregate (kg/m 3 ) 1035* 1000** Water (kg/m 3 ) Plasticizer (kg/m 3 ) Superplasticizer (kg/m 3 ) * maximum aggregate size = 25 mm; ** maximum aggregate size = 12.5 mm The influence of the testing age on air permeability of beams is evaluated in Figure 3. As can be seen, no correlation was found. It means that after the curing period the improvement of surface concrete is not significant for this high strength concrete (strength class C40/50 according EN 206-1). Table 2 shows the average values of air permeability of beams, as well as the standard deviation and the coefficient of variation. For each beam 4 measurements were made on the webs, 2 per each web, and 2 measurements on the bottom flange. It can be seen that the standard deviation and the average values are of the same order of magnitude (values of coefficient of variation around the unity). These high values of the coefficient of variation may be due not only to changes of concrete quality but also to variations on the age of testing and changes in relative humidity of surface concrete (due to environment humidity fluctuations). In Figure 3, as referred above, we can see that no apparent relation exists 517

4 between age of testing and air permeability of beams. The humidity was indirectly evaluated by resistivity measurements but no correction of air permeability was needed. The bottom flange, however, was not exposed to the sunshine and was more protected from the wind. This fact may lead to less evaporation on concrete surface of flanges (which implies higher relative humidity), which should be responsible for the lower air permeability of this part of the beam, when compared with the webs. Comparing Figure 4 with Figure 3, higher values of air permeability were obtained on slabs (see also Tables 2 and 3). This is due not only to the different components used in the two concretes but also to some cracking in slabs related to handling after demoulding. The small depth of the slabs, compared with the span, and the consequent small rigidity, was responsible for the appearance of micro-cracking at surface, which increased significantly the air permeability of concrete. By observing Figure 4, no major differences are evident on the results scatter from the two suppliers. The air permeability averages of the slabs delivered by the two suppliers are similar, and one order of magnitude higher than the results obtained in beams. It can also be seen that the coefficient of variation of air permeability is around 1, as it was obtained in the case of the measurements made on beams. The apparent relatively high coefficient of variation is related with the scale of the values, and does not necessarily represent high differences, once the values are in the same order of magnitude. Statistical values of capillary suction measurements of concrete cores are presented in Table 4. The age of specimens was about 4 months, and the strength class required for concrete was class C35/45. The results presented show the average suction rate between the beginning of the test and 8, 24 and 48 hours. As usual a small decrease of the suction rate with time is obtained, due to some saturation of the specimen. The coefficient of variation is about 0.3. kt (x10-16 m 2 ) Age of testing (days) Web Flange Figure 3 Air permeability of beams versus age of testing Table 2: Torrent air permeability: statistic of tests on beams Web Flange Web+flange Number of results 51x4=204 51x2=102 51x6=306 Average kt (x10-16 m 2 ) Standard deviation kt (x10-16 m 2 ) Coefficient of variation

5 Table 3: Torrent air permeability: statistic of tests on slabs Suplier 1 Suplier 2 Suppliers 1+2 Number of concrete elements Average kt (x10-16 m 2 ) Standard deviation kt (x10-16 m 2 ) Coefficient of variation Table 4: Capillary suction: statistic of tests on cores taken from piles Rate 0-8h Rate 0-24h Rate 0-48h Number of concrete elements 36 Average kg/(m 2.h 1/2 ) St. deviation kg/(m 2.h 1/2 ) Coefficient of variation On-site variation within the elements In order to evaluate the variation of the results in the same element, statistical values were determined using the 4 measurements done on the webs of each beam. The results on the flange were disregarded due to the different exposure conditions. Another reason for not taking into account the results of flanges was the very low air permeability obtained in this part of the beam, which is relatively more affected by the less accuracy of the equipment in this range of values. Average, standard deviation and coefficient of variation of each beam were determined and the statistical values for 51 beams are presented in Table 5. Within element variation may be expressed by the coefficient of variation. The average value of the coefficient of variation for the 51 beams was This may be considered a high value, however, in this particular case, the concrete element is large and incorporates concrete from different batches. Table 6 shows statistical values calculated using the differences between the two measurements made on the same part of the beam. This means that from each beam we have 3 pairs of values, two pairs from the webs and one pair from the bottom flange. With each pair of values, the absolute difference was calculated, kt, and Table 6 was filled with the statistical determinations made with the 153 results from the 51 beams. The relative difference presented in this Table corresponds to kt divided by the average of the two values. These differences, kt, should not be interpreted as a measure of the reproducibility because the two measures were taken on different places, located several meters away, possibly corresponding to different batches. However, as the face of the element was the same, the concrete had equal exposure conditions. An average difference of x m 2 and an average relative difference of 0.40 were obtained. The values in italic in Table 6 correspond to the statistical results without the measurements on flanges. In this case the average relative difference decreases to 0.34, which is manly due to the higher air permeability of the webs. Differences in a order of 40 % of the measured average seems to be high, but we must take into account the high variability of in-situ measurements and the low values of the air permeability. 519

6 Information on repeatability and reproducibility of in-situ air permeability is scarce. Coefficient of variation of about 0.4 is considered poor for compressive strength [4], but this value was found in a study with the Figg test [5]. Within-specimen variability of Torrent method obtained in a research study was 6.6 % [6] which is much lower than the obtained in the present work. The test results presented in the Tables below includes sources of variation other than the usual ones when repeatability and reproducibility are analysed, as for instance relative humidity changes, which have great influence on the air permeability. Despite this fact, coefficients of variation of about 1 for entire production and about 0.3 for the single elements may be found in practice, were many different sources of variation are possible. kt (x10-16 m 2 ) kt (x10-16 m 2 ) Age of testing (days) Age of testing (days) Figure 4 Air permeability of slabs versus age of testing: left - suplier 1; right - suplier 2 Table 5: Air permeability of the web parts of the beams: statistic of results variation Average n=4 Stand. Deviation n=4 Coeff. of variation n=4 Number of beams 51 Average kt (x10-16 m 2 ) St. deviation kt (x10-16 m 2 ) Coeff. of variation Table 6: Difference between air permeability measurements Difference between 2 measurements Relative difference between 2 measurements Number of measurement pairs 153 (102) Average kt (x10-16 m 2 ) (0.012) 0.40 (0.34) St. deviation kt (x10-16 m 2 ) (0.015) 0.41 (0.34) Coeff. of variation 1.31 (1.27) 1.02 (1.00) 3.3 Actual and potential performance The results of the tests on standard cylinders are presented in Tables 7 and 8. Table 7 shows the results of oxygen permeability of concrete used in beams. The results presented in this table correspond to 6 cylindrical specimens 150 mm diameter and 50 mm thick, prepared from 3 standard cylinders 150 mm diameter and 300 mm length. From each standard sample, 2 specimens were prepared for testing. Table 8 indicates the average values of capillary 520

7 suction obtained on each standard cylinder and the average capillary suction on the 3 standard cylinders. Comparing the average results of air permeability, laboratory versus in-situ measurements, of concrete applied in beams, the difference is very small, 0.035x10-16 against 0.037x10-16 m 2 or 0.046x10-16 m 2, respectively for web+flange or only web results. These differences don t reproduce the quality decrease from standard specimens to in-situ concrete, once different tests were performed and the conditioning of specimens was distinct. However, it is interesting to identify the same order of magnitude of average values, which indicates that the method used for evaluating concrete in-situ is promising. As expected, the scatter of the results is high on site, due to the different factors as, for instance, influence of workmanship. Additionally, the presence of cracking is well identified, as was noted by the difference between results from beams and slabs, which is in favour of using this air permeability test for quality control. The presented average results of capillary suction, on cores (Table 4) and on standard specimens differ significantly. This may be due not only to the workmanship but also to the higher level of drying obtained in cores. In fact, the same drying procedure on cylindrical specimens of different diameter, 150 mm or 93 mm, induces distinct relative humidity inside specimens. It is expected lower RH in the small specimens, which leads to higher capillary suction. Additionally, the usual methods of construction of piles are not favourable to obtain dense concrete, which contributes to the difference between on site concrete and laboratory concrete. Table 7 Oxygen permeability of standard cylinders x10-16 m 2 Cylinder Average result of the standard cylinder Average result of the sample Table 8 Average capillary suction of standard cylinder Cylinder Rate 0-8h Rate 0-24h Rate 0-48h kg/(m 2.h 1/2 ) kg/(m 2.h 1/2 ) kg/(m 2.h 1/2 ) Average Table 9 Statistic of results presented in [7] (kt x10-16 m 2 ) Average St. deviation Coef. of variation Number of measurement days 5 Slab locations Slab locations slabs Locations The values obtained in the present work were compared with the results of Bryan Adey and al., where air permeability of two existing slabs was measured [7]. The air permeability 521

8 measurements were taken on 7 locations at 5 different dates. Location 1 to 3 corresponded to one slab and locations 4 to 7 to another slab. Table 9 indicates the variation of measurements on the same day and on the same slab. Comparing the average coefficient of variation in the present work for the webs, (Table 5), and the coefficients of variation of the two different slabs, 0.85 and 1.35 (Table 9), we may conclude that the variation found in the present work is not too high. The same is applied if we compare the coefficients of variation of the entire production in present work, ranging from 0.78 to 1.28 (Tables 1-3), with the coefficient of variation of the 2 slabs in Table 9, CONCLUSION During the construction of a structure composed by a bridge and viaducts, it was decided to control the quality of some concrete elements by tests performed on site and in laboratory. In this work, a broad set of results on air permeability and capillary suction of in-situ concrete was presented, as well as the correspondent ones on standard specimens. On site coefficient of variation of air permeability was around 1 for the entire production and about 0.3 for the same element, which were lower than those reported by others. For capillary suction, coefficient of variation was about 0.3 for the entire production. The comparison between actual and potential performance indicated values of the same order of magnitude for air permeability and a deep difference for the capillary suction. However, preconditioning of the specimens should have had a significant influence on the results, lowering the differences on air permeability measurements and increasing the variation on capillary suction tests. REFERENCES [1] Rommer, M., Comparative test Part I Comparative test of penetrability methods in Materials and Structures, 38, 2005, [2] RILEM TC 116-PCD, Recommendations of TC 116-PCD: Tests for gas permeability of concrete. B. Measurement of the gas permeability of concrete by the RILEM-CEMBUREAU method, C. Determination of the capillary absorption of water of hardened concrete, in Materials and Strucures. 32, [3] Kolleck, J.J., The determination of permeability of concrete by Cembureau method: a recommendation ; in Materials and Structures, 22, 1989, [4] Bungey, J.H., Millard, S.G., Grantham, M.G., 'Testing of Concrete in Structures', 4nd Edn (Taylor&Francis, New York, 2006). [5] Paulmann, K., Molin, C., On-site test methods ; in RILEM Report 12, Performance Criteria for Concrete Durability, E&FN Spoon, 1995, [6] Torrent, R., Basheer, M., Gonçalves, A.F., Chapter 3. Non-destructive methods To measure gaspermeability ; in RILEM Report 40, Non-Destructive Evaluation of the Penetrability and Thickness of the Concrete Cover, RILEM Publications S.A.R.L., 2007, [7] Adey, B., Roelfstra, G., Hadjin, R., Brühwiler, E., Permeability of existing concrete bridges, 2 nd International PhD Symposium In Civil Engineering, Budapest,