THRESHOLD VALUES OF AIR PERMEABILITY OF CONCRETE COVER - A CASE STUDY IN JAPAN

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1 THRESHOLD VALUES OF AIR PERMEABILITY OF CONCRETE COVER - A CASE STUDY IN JAPAN Kei-ichi Imamoto (1), Kazuyuki Shimozawa (2), Masaru Nagayama (2), Junji Yamasaki (3) and Seiji Nimura (4) (1) Tokyo University of Science, Japan (2) General Building Research Corporation of Japan, Japan (3) Asanuma Corporation, Japan (4) Osaka Institute of Technology, Japan Abstract This paper deals with the air-permeability of concrete cover and its relation to carbonation resistance for Japanese concretes. First, a comparative study was performed with three kinds of air-permeability testers (the single-chamber method, Figg s intrusive method and the Torrent permeability tester), and their capabilities to differentiate the qualities of concretes were examined using six different concrete specimens. Second, the classifications of airpermeability based on the RILEM TC 116-PCD method and the Torrent permeability tester (TPT) method were compared for Japanese concretes. Also, threshold values of the singlechamber method and Figg s intrusive methods were proposed. Finally, these testers were used to conduct on-site assessment to investigate the air-permeability of full-scale reinforced concrete walls. Keywords Air permeability, comparative study, threshold value, carbonation depth 169

2 1. INTRODUCTION In Japan, every year over 400,000 new reinforced concrete (RC) buildings are constructed and about 80 billion euros are spent for the maintenance of existing RC structures. Estimating life expectancies of the buildings and devising strategies to increase structural durability are quite important. It is well known that the quality of concrete cover strongly affects the durability of RC structures. Substances which are harmful for RC structures, such as CO 2 and chloride ions, must reach the steel re-bars through the concrete cover. For this reason, the penetrability of the concrete cover plays a significant role in the durability of concrete structures. The use of concrete with a low water/cement ratio (w/c) is an effective way to make concrete dense to prevent these ingresses. However, it is well known that not only mixture conditions but also degree of compaction, the existence of cracks, and other construction procedures such as curing methods will affect the quality of the concrete cover. Hence, on-site penetrability test methods for the concrete cover are expected to be useful tools in establishing a reasonable maintenance strategy for RC structures. The authors report the results of a comparative study on air-permeability testers and an investigation of the applicability of the testers differentiating grades of Japanese concrete in terms of carbonation resistance. 2. A COMPARATIVE STUDY ON AIR-PERMEABILITY TEST METHODS FOR COVER CONCRETE This study focuses on comparisons of several air-permeability test methods for cover concrete in the same manner as that carried out by TC 189-NEC [1]. In this comparative study, three on-site air-permeability test methods were employed the Torrent permeability tester (TPT) method [2], the Single chamber method (SCM) [3],[4] and Figg s intrusive method [5] modified by Kasai (FIM) [6]. Six wall specimens ( m) with three different design concrete strengths (21, 27 and 36 N/mm 2 ) subjected to two different curing conditions ( A and C ) were prepared. Details on the composition and properties of the concretes are tabulated in Tables 1 and 2. The average of 3 specimens under normal curing is tabulated in the Table. The curing conditions were as follows: A - after the removal of the wooden forms at the age of 3 days, curing at 20 C in water for one month and subsequently of 20 C/60% RH in air for one month, and B - curing at 20 C/60% RH in air for two months prior to the comparative tests. Hence, when the comparative testing began, the age of a wall ranged between 48 and 56 days, all of which had passed with the samples under controlled laboratory conditions (20 C/60% RH). Table 1: Material used Material Properties Cement: C Ordinary Portland cement S: Fine Agg. Pit sand (density: 2.57 g/cm 3, F.M.: 2.80); Crushed sand (density: 2.66 g/cm 3, F.M.: 2.80) G: Coarse Agg. Crushed hardened sandstone (density: 2.69 g/cm 3, solid content: 58%) Admixture Lignin sulfonate acidic air-entraining agent (AE), Polycarboxylic acid-type superplasticizer (SP) 170

3 Table 2: Mixture proportions and properties of concretes w/c Unit content (kg/m 3 ) Slump Air Comp. strength Admixture W C S G (cm) (%) at 28 days (N/mm²) AE SP SP Vacuum pump Silicon ring 2 cm 2 cm Air flow Figure 1: View of the single-chamber method (SCM) 25 mm 50 mm 25 mm Vacuum pump Inner chamber Outer chamber Soft rubber Uniaxial air flow Figure 2: View of the Torrent permeability tester (TPT) 10 mm 50 mm Epoxy resin Vinyl tube Needle Vacuum pump Silicon plug Figure 3: View of modified Figg s intrusive method (FIM) 171

4 2.1 Test results and discussions The capabilities of the methods were evaluated using Student s t-test to analyze the differences between the means of the results of the compared sets, as shown in Table 3. The null hypothesis is that one set has a mean penetrability higher than that of the other, as indicated in the Rating row of the Table. In this analysis, six data were used for each specimen. If the result of the statistical test allows us to reject the null hypothesis at a level of significance <1%, then the differentiation capability of the test for the particular sets compared is highly significant (+++). If the result of the statistical test allows us to reject the null hypothesis at a level of significance between 1% and 5%, then the differentiation capability of the test for the particular sets compared is significant (++). If the result of the statistical test does not allow us to reject the null hypothesis at a level of significance 5%, then the differentiation capability of the test for the particular sets compared is not significant (--). Table 3 indicates that the on-site air-permeability tests were capable of significantly differentiating the penetrability of the cover concrete in which the k_cembureau given by the RILEM-PCD method were 112, 168 and m 2 for w/cs of 0.44, 0.54 and 0.63, respectively. Among them, the TPT method yielded the best result. In the case of SCM, the results of statistical tests did not allow us to reject the null hypothesis at a 5% level of significance. This might be due to the existence of surface cracks. Although further investigations are required on this aspect, overall, the comparative test proved that the three methods were capable of evaluating the penetrability of the concrete cover on-site in a reliable and statistically significant manner. Table 3: Results of on-site air-permeability tests Variable tested Comp. strength Compared sets A21 A27 A27 A36 C21 C27 C27 C36 Rating A21 < A27 A27 < A36 C21 < C27 C27 < C36 SCM T Significance TPT T Significance FIM T Significance Variable tested Curing condition Compared sets A21 C21 A27 C27 A36 C36 Rating A21 > C21 A27 > C27 A36 > C36 SCM T Significance TPT T Significance FIM T Significance Example of notation: A21 (Design strength of 21 N/mm 2 cured under condition A ) 172

5 3. CARBONATION RESISTANCE OF JAPANESE CONCRETE AND ITS RELATION TO PERMEABILITY COEFFICIENT Carbonation is a significant factor in the deterioration of RC buildings in Japan. The concrete cover is a pathway for harmful agents such as CO 2, which initiate steel rebar corrosion. The Architectural Institute of Japan (AIJ) classifies the expected lifetime of RC buildings [7]: Long-term ( Durable ) with life span of 100 years, Standard ( Normal ) of 65 years and General ( General ) of 30 years. These criterions are based on the age at the carbonation progress of 30mm which value is reference cover thickness of RC building in Japan. Values of air permeability of 15 kinds of concretes with w/cs ranging from 0.3 to 1.0 were measured. The concrete specimens ( cm) were kept at air-controlled room of 20 C/60%R.H. for 2 years and at indoor condition for 2 years. Based on measured carbonation depths at 4 years, ages at the carbonation progress of 30 mm were estimated according to the square root theory. Materials used in this experiment were same with those listed in Table.1. kts were converted from the values of k_cembureaus using the relationship between kt and k_cembureau using 3 concrete mixtures shown in Table.2 (k_cembureau kt ). Figure 4 shows the relationships between the carbonation depths and permeability coefficients of concretes measured by each method. Threshold values of air permeability by FIM and SCM corresponding AIJ criterions are shown in (b) and (c) of Figure 4. For each test, variances of air permeability testers were not considered; 20% for SCM and 40% for FIM.Also, the threshold values of k_cembureau [9] and kt [10] paired with AIJ criterions are plotted in (a) and (d) in the figures. Estimated age at carbonation depth of 30mm (The criterion of AIJ) increases with the decrease with permeability coefficients of each test method. As shown in (a) of Figure 4, most of the permeability coefficients are located within the Average range according to the k_cembureau classification, in spite of the wide distribution among AIJ classifications along the y-axis. It should be noted that no carbonated concrete at the age of 4 years is also classified as Average (w/c=0.3). However, in (d) of Figure 4, the kt values are distributed within the Very good to Bad range, while the carbonation depths fall within the Durable to Poor range. Although only a limited number of materials were used in this study, the kt classifications correlate closely to the quality grades of Japanese concretes. These test results will provide information useful in the establishment of strategies to assess the structural durability and life expectancy of RC buildings. 173

6 Estimated age at carbonation depth of 30mm (Years) (a) w/c=0.3, no carbonation K=8x10-18 m 2 y = x R 2 = 0.84 Low Average High w/c=0.50 AIJ: Durable AIJ: Normal AIJ: General AIJ: Poor w/c=0.60 w/c=0.45 w/c=0.40 w/c=0.80 w/c=0.65 w/c= Air permeability K Estimated age at carbonation depth of 30mm (Years) (b) AIJ: Normal AIJ: General AIJ: Durable y = 20.98x R 2 = 0.78 AIJ: Poor Air permeability index by FIM kpa/s Estimated age at carbonation depth of 30mm (Years) (c) AIJ: Durable AIJ: Normal AIJ: General y = x R 2 = AIJ: Poor Air permeability index by SCM (kpa/s Estimated age at carbonation depth of 30mm (Years) Very good AIJ: Durable AIJ: Normal AIJ: General AIJ: Poor (d) Good Normal Bad y = x R 2 = 0.84 Very bad K T ( m 2 Figure 4: Air permeability versus estimated age at carbonation depth of 30mm 174

7 4. AIR-PERMEABILITY OF CONCRETE COVER OF JAPANESE RC BUILDING Three full-scale RC building walls 240 cm in width, 360 cm in height and 20 cm in thickness were constructed, as shown in Figure 6. The materials used for the concrete mixtures are listed in Table 1. The wooden forms used for the walls were removed one day after the casting of the concrete. The front sides of the walls were exposed to air and sunshine, and the rear sides were sealed with vinyl sheets, for seven days. The fresh and mechanical properties of the walls are tabulated in Table 2. The air-permeability was measured at the top, centre and bottom of each wall, and the results are displayed in Figure 5. The k_cembureau values were measured in between the positions where the TPT and other tests were performed. The RILEM-PCD tests were carried out in laboratory on cored specimens with 15 cm in diameter and 5 cm in thickness. Figure 6 shows the test results of the air permeability distributions of the walls. Both RILEM-PCD and TPT methods were able to differentiate the quality of the walls. It should be noted that the permeability of actual RC walls tends to be much higher than the laboratory data shown in Figure 4. The difference between the surface conditions of the sealed curing (front side) and those of the air-dried curing (rear side) of the walls was not pronounced in the test results obtained from the SCM. This might be due to the fact that the SCM method evaluates the permeability of concrete nearer the surface of the concrete cover than those evaluated by the other test method. Anyway, future research to evaluate air permeability of the whole wall should be needed, considering its large variance. kt, SCM, FIM k_cembureau Front side 360cm 240cm 240cm 240cm 20cm Figure 5: View of full-scale RC walls and tested points of air-permeability 175

8 Front side (Air-dried curing) k_cembreau ( x m 2 ) Rear side (Sealed curing) k_cembreau ( x m 2 ) 1 Low 10 Average 100 High Low 10 Average 100 High 1000 RILEM TC-PCD RILEM TC-PCD Front side (Air-dried curing) kt ( x m 2 ) 1 Good 10 Normal 100 Bad 1000 Rear side (Sealed curing) kt ( x m 2 ) 1 Good 10 Normal 100 Bad 1000 TPT TPT Front side (Air-dried curing) A.P.of SCM (kpa/s) Rear side (Sealed curing) A.P.of SCM (kpa/s) AIJ durable 2. Normal 3. General 4. Poor SCM AIJ durable, 2. Normal 3. General, 4. Poor SCM Front side (Air-dried curing) A.P.of FIM (kpa/s) Rear side (Sealed curing) A.P.of FIM (kpa/s) AIJ durable 2. Normal 3. General 4. Poor FIM AIJ durable FIM 2. Normal 3. General 4. Poor Figure 6: Air permeability of full-scale RC walls 176

9 5. CONCLUSIONS AND OUTLOOK A comparative study was performed with three kinds of air-permeability testers (the SCM, FIM and the TPT) and their capabilities to differentiate the qualities of concrete were examined using six kinds of concrete specimens. Among them, the TPT method yielded the best results in terms of accuracy. Furthermore, the classifications of air-permeability based on the kt values correlated closely to the different grades of Japanese concretes. In this study, authors proposed threshold values of air permeability indexes of SCM and FIM. On-site assessment based on air-permeability testing will provide useful information upon which to base strategies for estimating life expectancy and increasing structural durability. Further discussion should be needed to evaluate the air permeability of RC structure considering its large variance. REFERENCES [1] RILEM Report 40, Non-Destructive Evaluation of the Penetrability and Thickness of the Concrete Cover - State of the Art Report of RILEM Technical Committee TC 189-NEC Non-Destructive Evaluation of the Concrete Cover, Edited by R. Torrent and L. Fernandez Luco, [2] Torrent, R. J., A two-chamber vacuum cell for measuring the coefficient of air of the cover concrete on site, Materials and Structures, 25 (1992) [3] Schönlin and Hilsdorf, Evaluation of the effectiveness of curing of concrete structures, ACI SP-100 (1987) [4] Imamoto, K. et al., Applicability of single-chamber vacuum cell for the evaluation of the airpermeability of concrete walls, Int. Conf. on Concrete Repair, Rehabilitation and Retrofitting (ICCRRR), South Africa, [5] Figg, J.W., Methods of measuring the air and water permeability of concrete, Magazine of Concrete Research 25 (85) (1973) [6] Kasai, Y., Matsui, I. and Nagao, M., On site rapid air-permeability test for concrete, ACI SP-82, pp , In Situ/Non-destructive Testing of Concrete, V.M. Malhotra (Ed.), ACI, Detroit, [7] Architectural Institute of Japan, Japanese Architectural Standard Specification (JASS 5 Reinforced Concrete Work), [8] JIS A 1153:2003 Method of accelerated carbonation test for concrete. [9] Concrete Society Technical Report No.31, Permeability Testing of Site Concrete: A Review of Methods and Experience, Aug [10] Torrent, R. and Frenzer, G.: A method for the rapid determination of the coefficient of permeability of the covercrete, International Symposium Non-Destructive Testing in Civil Engineering (NDT-CE), 1995,