COPING WITH THE EFFECT OF MOISTURE ON AIR- PERMEABILITY MEASUREMENTS

Transcription

1 COPING WITH THE EFFECT OF MOISTURE ON AIR- PERMEABILITY MEASUREMENTS R. Torrent (1), F. Moro (2) and A. Jornet (3) (1) Materials Advanced Services Ltd., Buenos Aires, Argentina (2) Holcim Technology Ltd., Holderbank, Switzerland (3) Istituto Materiali e Costruzioni, DACD, SUPSI, Lugano, Switzerland Abstract The coefficient of air-permeability kt, measured non-destructively by the so-called "Torrent Method", has gained acceptance as a suitable durability indicator of the as-built cover concrete and has been standardized in Switzerland (Standard SIA 262/1:2013 Annex E). The standard adopted the ND electrical impedance method to control the moisture of site concrete. This paper presents data of moisture changes (assessed gravimetrically, by ND Impedance and by Wenner electrical resistivity) of concrete cubes dried in the lab and in an oven at 50 C. In parallel, measurements of kt were conducted on the same cubes. The results confirm that the ND impedance is a suitable method to assess the surface moisture of concrete and that the upper limit of 5.5%, prescribed in SIA 262/1-E, is adequate. In terms of the kt values obtained, it corresponds, roughly, to 3 days drying at 50 C. 1. INTRODUCTION The coefficient of permeability to gases of the concrete cover has been acknowledged as a suitable and practical durability indicator [1]. It is well known that the moisture content in the concrete strongly affects the results of a gas-permeability test, due to its blocking effect on the flow of gas through the pore system of the material [2, 3]. To avoid this problem, laboratory samples (cast specimens or drilled cores) are almost invariably preconditioned by subjecting them to some kind of controlled drying process before measuring gas-permeability [4-6]. The Swiss Code 262 [7] prescribes: The impermeability of the cover concrete shall be checked by means of permeability tests (e.g. air permeability measurements) on the structure or on core samples taken from the structure. Measuring the air-permeability of concrete structures on site poses the problem of the virtual impossibility of artificially preconditioning the measuring area. The tests have to be conducted on a concrete, the moisture content of which is usually unpredictable and unknown, raising doubts on the validity of the test results. This problem was recognized at the early stages of the development of the so-called "Torrent Method" [8] to measure the coefficient of air-permeability (kt), which ended up in 489

2 a Swiss Standard test method [9]. The first attempt of using a surface moisture meter, based on measuring the magnetic dielectric constant, did not work due to the unsatisfactory performance of the available instruments (James H2O meters) in the early 90's [10]. The best solution, found at that time, to cope with the effect of moisture content on the kt measurements, was to combine them with the simultaneous measurement of the electrical resistivity ρ by means of the Wenner method [10, 11]. A nomogram allowed a correction to be made to kt, based on the simultaneous measurement of ρ. Unfortunately, the nomogram is only applicable to concretes made with OPC, the only cement type available in Switzerland in the early 90s. With the advent of composite cements and/or the addition of SCM in Swiss concretes, known to produce a five- or even ten-fold increase in the electrical resistivity, the nomogram became obsolete and unusable. Research work conducted by M. Romer at EMPA [12] with a hand-held instrument (Tramex Concrete Encounter) for the measurement of the moisture of the near-surface concrete, based on electrical impedance measurements, opened a new road to solve the problem. Indeed, the instrument was capable of monitoring the gradual loss of moisture of 200 mm concrete cubes exposed to controlled environments (20 C and 35% or 70% RH), up to 1 year storage. Air-permeability measurements were conducted in the laboratory on the same cubes. Some inter-laboratory tests of air-permeability kt were conducted in Switzerland on a bridge and a tunnel [13], in which the performance of the impedance-based instrument proved more reliable than that of the Wenner resistivity device. The EMPA laboratory results and the inter-laboratory tests, led to a team of Swiss experts to recommend an upper limit of 5.5% to the moisture content, measured by the impedance-based instrument, in order to get meaningful results of the site air-permeability of the concrete cover [13]. Wenner resistivity was proposed as an alternative method [13]; the recommended minimum values of ρ were 10 and 20 kω.cm, for OPC and composite cements, respectively. The new version (1 st August 2013) of the Swiss Standard SIA 262/1 [14], which describes in Annex E how to conduct the site air-permeability test (largely based on [13]), prescribes: 1. The moisture of the concrete shall be measured by an electrical impedance-based instrument (i.e. the alternative Wenner method was abolished) 2. The moisture content of the concrete at the point where the air-permeability is to be measured should not exceed 5.5% 2. OBJECTIVE AND SCOPE OF THE INVESTIGATION The objective of the investigation was manifold: To check the suitability of Electrical Impedance and Electrical Resistivity to monitor the surface moisture of drying concrete To better understand the meaning of the surface moisture readings, based on Electric Impedance Method and their relation with kt To check the adequacy of the limit (m 5.5%) established in Swiss Standard SIA 262/1-E To develop standard guidelines for the measurement of air-permeability on site and in the lab (preconditioning of samples) For that, a series of well-cured concrete samples, made with four different binders and two w/c ratios, were allowed to dry in laboratory and oven environments, checking at intervals the 490

3 change in moisture by three methods (gravimetric, electrical impedance and Wenner resistivity), as well as the air-permeability by the "Torrent Method". 3. EXPERIMENTAL 3.1 Mixes, Specimens, Curing and Storage Table 1 summarizes the characteristics of the 8 concrete mixes tested. The three cements were industrially produced from the same clinker. CEM I is an OPC and Cements III-A and III-B contain 45 % and 68% of GGBS, respectively. Mixes I+SF were made with the same OPC as Mixes I, only that 8% of the cement was replaced by Condensed Silica Fume. Mixes contained variable dosages of SP; natural aggregates of low absorption (0.4 %) were used. Table 1: Main characteristics and moist curing lengths of the 8 mixes tested MIX I40 I65 IIIA40 IIIA65 IIIB40 IIIB65 I+SF40 I+SF65 Binder I 52.5N III-A 42.5 N III-B 42.5 N I % CSF w/binder ratio Binder (kg/m³) Slump (mm) Air (%) Moist Curing 104 days 97 days 90 days 83 days Two 150 mm cubes of each mix were cured in a moist room (20 C, > 95% RH) for the indicated periods of time. The long curing periods were deliberately established to ensure a high degree of hydration, so as to minimize further hydration during the drying period. After the moist curing period, one cube of each mix was exposed to laboratory drying (stored in a room with still air at C and 50-65% RH). The companion cubes were placed in a ventilated oven at 50 C (tests of the oven-dried cubes were performed after 24 ± 2 hour cooling in the laboratory drying room). After the tests were completed, the cubes were immediately returned to the oven. 3.2 Test Methods and Instruments Figs. 1 and 2 show the instruments used and a sketch of the location of the testing points. At intervals, the following NDT and (Instruments) were applied on the cubes (Fig. 1): a. Mass M (10 kg/0.1 g Kern Scale) b. Moisture by impedance method m (CMExpert). The instrument's scale goes from 0.0 to 6.9 % c. Wenner Electrical resistivity ρ (RESI) d. Air-Permeability kt (PermeaTORR). The instrument measures above kt = m² Tests b and d (dry tests) were applied on two opposite lateral faces of each cube. Two readings of m% were made on each face, one in a horizontal position (m h ) and the other in a vertical position (m v ), as sketched in Fig. 2. Reported value m is the average of the 4 readings on each cube. One reading of kt was performed on each face. Reported value is the geometric mean of both values of each cube (kt follows a log-normal distribution). 491

4 RILEM International workshop on performance-based specificationn and controll of concretee durability a Test c (moist test) was applied on thee two remaining lateral faces of each cube. Two tests (ρ 1 and ρ 2 ) were conducted along the diagonals of each face. Reported R value ρ is the average of the 4 readings on each cube, dividing the values given by the instrument by a shape factor of 1.72 [14, 15]. kt ρ 1 ρ 2 m h m v Figure 2: Sketch of thee location of testing points Figure 1: Cubes and instruments used When the oven-dried specimens had completed 132 days of drying, d theyy were weighed and dried at 105 C till constant weight, recording the mass M 105 5, and measuring kt and m%. Then, the pieces were submerged underr water at 20 C until constant weight recording the saturated mass M s. This allowed to convert the values of mass M of all the cubes into degree of saturation S, using (1). S = (M - M 105 ) / (M s - M 105 ) (1) 4. TEST RESULTS AND DISCUSSION 4.1 Monitoring the Drying Process in the Lab Figs. 3, 4 and 5 show the change of S, ρ and m with the laboratory drying time for all mixes. S ( ) I40 IIIA40 IIIB40 I+SF40 5 I65 IIIA65 IIIB65 I+SF65 10 t (days ½ ) Figure 3: Change of S with lab drying time ρ (kohm.cm) t (days ½ ) I40 IIIA40 IIIB40 I+SF40 Figure 4: Change of ρ withh lab drying time 15 I65 IIIA65 IIIB65 I+SF

5 I40 IIIA40 IIIB40 I+SF40 I65 IIIA65 IIIB65 I+SF65 m (%) t (days ½ ) Figure 5: Change of m with laboratory drying time Fig. 4 confirms that the electrical resistivity presents, for the same drying period, significantly lower values for the OPC (CEM I) concretes than for the other binders. Moreover, with the exception of the OPC concretes, in most cases it was not possible to get readings of ρ beyond 25 days of laboratory drying. On the other hand, the electrical impedance-based instrument was able to follow the whole drying process for all the mixes at least for 132 days (Fig. 5). As expected, the degree of saturation dropped more markedly for the concretes with higher w/c ratio (Fig. 3). On the contrary, the value of m dropped less markedly for the OPC concretes than for the other binders (Fig. 5), which may indicate some dependency of the reading on the binder type. 493

6 4.2 Relation between m and S Fig. 6 shows the relation between m and S, for all the specimens, including those under laboratory and oven drying. It is worth mentioning that all the cubes showed that: after drying to constant mass at 105 C (S = 0.0) m = 0.0% (bottom of instrument's scale) after saturation to constant mass under water (S = 1.0) m = 6.9% (top of instrument's scale) m (%) I40 I65 IIIA40 IIIA65 IIIB40 IIIB65 I+SF40 I+SF65 Oven S = S Figure 6: Relation between m and S (framed symbols correspond to oven-drying) Fig. 6 shows the continuous link existing between S and m along the whole drying process. With the exception of mixes I40, I65 y I+SF65, the relation S-m is very similar for the remaining 5 mixes which also show continuity between the samples under laboratory drying and the companions under oven drying (the latter framed in a box in Fig. 6). Mixes I40, I65 y I+SF65 show lower values of S for the same m and the points corresponding to the laboratory drying samples of these mixes are shifted to the left in Fig. 9 chart, with respect to those under oven drying. Notice that, at the initiation of the test, with the samples just removed from the moist chamber, S is below 1.0 and, for mixes IIIA40 and I+SF40 also m is below 6.9% (Fig. 5). The S corresponding to the limiting value of m = 5.5 % range between 0.75 and It must be borne in mind that the saturation degree corresponds to the bulk of the cube, whilst the surface moisture corresponds to a layer about 20 mm thick. The degree of saturation of that layer must certainly be lower than the bulk value, due to the moisture gradient that is established during drying. 494

7 4.3 Relation between kt and m Fig. 7 shows the relation between kt and m, including both the cubes under laboratory and oven (50 and 105 C) drying. The values of kt below m² are not accurate, being beyond the limit of sensitivity of the instrument. kt (10 16 m²) I40 IIIA40 IIIB40 I+SF40 Limit I65 IIIA65 IIIB65 I+SF65 Oven m (%) Figure 7: Relation between air-permeability kt and surface moisture m The air-permeability increases (monotonically) several orders of magnitude with drying, from near-saturation (m values close to 6.9%) till 105 C oven-dried (m values equal 0.0%). kt 3d oven (10 16 m²) I40 IIIA40 IIIB40 I+SF40 Other I65 IIIA65 IIIB65 I+SF E kt natural drying until m <5.5% (10 16 m²) Figure 8: Relation between kt after 3d at 50 C and kt under laboratory drying until m < 5.5% (empty circles correspond to data from other binders' mixes still under drying) 495

8 The values of kt, measured on the laboratory dried cubes as soon as m < 5.5%, are very similar to those obtained after 3 days of drying at 50 C, see Fig. 8. The values of m for the samples dried 3 days in the oven ranged between 4.2 and 5.0 %. Starck [16] found small influence of m on kt, for samples in outdoors and lab exposure, for m in the range %. 4.4 Drying conditions and kt assessment Fig. 9 shows the kt values obtained for the 8 mixes reported, under different drying conditions: laboratory drying until m<5.5%, 3 days in oven at 50 C and in oven at 105 C till constant mass m <5.5% C 1 kt (10-16 m²) I40 IIIA40 IIIB40 I+SF40 I65 IIIA65 IIIB65 I+SF65 Figure 9: Effect of drying conditions on the measured kt values of tested mixes Fig. 9 shows the strong effect of the intensity of drying on the measured kt values. Drying at 105 C increases the air-permeability several orders of magnitude, yielding values which are not representative of those that may be obtained under site conditions. See for instance mix I+SF40 which, under mild drying conditions is the less permeable, becoming indistinguishable from the other mixes with w/b= 0.40 when dried at 105 C. Therefore, drying at 105 C, with the associated risk of damaging the microstructure of the concrete, is not advisable for kt measurements in the lab. 496

9 5. CONCLUSIONS a) The impedance method is a good NDT to follow the effect of drying on the surface moisture of concrete m. b) For each mix, m relates well with the degree of bulk saturation, with continuity between laboratory and oven-dried specimens, although the relation varies with the type of mix. c) 105 C oven-dried specimens have m = 0.0% (bottom of scale) and water saturated samples have m = 6.9% (top of scale). d) The impedance method is suitable to monitor the effect of surface moisture on kt test results. e) The Wenner method is not suitable to check surface moisture for same purpose. f) The limit established in the Swiss Standards for surface moisture (m 5.5%) seems adequate and corresponds, in terms of measured kt, to 3 days drying at 50 C in the oven used in this investigation. g) Hence, drying in an oven at 50 C and watching that the surface moisture is within %, may be a robust and realistic conditioning procedure for lab specimens to be tested for kt. On the other hand, strong drying at 105 C is not recommended. h) The PermeaTORR follows well the evolution of kt with changing moisture conditions (good repeatability). ACKNOWLEDGMENT To Dr. Christian Paglia, Director of Istituto Materiali e Costruzione (SUPSI) for the support provided to this investigation. REFERENCES [1] "Performance Criteria for Concrete Durability", Kropp J. and Hilsdorf, H. K. (Eds.), RILEM Report No. 12, [2] Parrott, L. J., "Moisture conditioning and transport properties of concrete test specimens", Materials and Structures, v. 27, 1994, [3] Jacobs, F., "Permeability to gas of partially saturated concrete", Magazine of Concrete Res., v. 50, 1998, [4] RILEM TC 116-PCD, "Recommendations: Tests for Gas Permeability of Concrete", Materials and Structures, v. 32, 1999, [5] Spanish Standard UNE 83966, "Acondicionamiento de probetas de hormigón para los ensayos de permeabilidad al aire y capilaridad", Julio 2008, 5 p. [6] Alexander, M.G., Ballim, Y. and Mackechnie, J. R., "Concrete durability index test manual", Research Monograph No. 4, Univ. of Cape Town and Witwatersrand, South Africa, 1999, 16 p. [7] Swiss Code 262:2003, "Concrete Construction". [8] Torrent, R.J., "A two-chamber vacuum cell for measuring the coefficient of permeability to air of the concrete cover on site", Materials and Structures, v. 25, 1992, v. 25, [9] Swiss Standard SIA 262/1-E:2013, "Luftpermeabilität am Bauwerk" (Air-permeability on site) [10] Torrent, R. u. Ebensperger, L., Studie über Methoden zur Messung und Beurteilung der Kennwerte des Überdeckungsbetons auf der Baustelle, ASTRA Report 506, Bern, Suisse, Januar 1993, 119 p. [11] Torrent, R. u. Frenzer, G., ibid, ASTRA Report 516, Oct 1995, 106 p. [12] Romer, M.,., Effect of moisture and concrete composition on the Torrent permeability measurement, Materials and Structures, v. 38, 2005,

10 [13] Jacobs, F., Denarié, E., Leemann, A. und Teruzzi T., Empfehlungen zur Qualitätskontrolle von Beton mit Luftpermeabilitätsmessungen, Office Fédéral des Routes, ASTRA Report 641, December 2009, Bern, Suisse, 53 p. [14] Morris, W., Moreno, E.I. and Sagües, A.A., "Practical evaluation of resistivity of concrete in test cylinders using a Wenner array probe", Cement and Concrete Res., v.26, 1996, [15] Fernández Luco, L., Personal communication. [16] Starck, S., The integration of non-destructive test methods into the South African Durability Index approach", MSc Thesis, Univ. Cape Town, South Africa, Feb