THRESHOLD PORE RADIUS OF CONCRETE OBTAINED WITH TWO NOVEL METHODS

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1 RILEM International workshop on performance-based specification and control of concrete durability June 2014, Zagreb, Croatia THRESHOLD PORE RADIUS OF CONCRETE OBTAINED WITH TWO NOVEL METHODS Y. Sakai (1), C. Nakamura (2) and T. Kishi (1) (1) Institute of Industrial Science, The University of Tokyo, Tokyo, Japan (2) School of Engineering, The University of Tokyo, Tokyo, Japan Abstract This study proposes two methods for measurement of concrete threshold pore radius, defined as the minimum pore size through which a mass has to pass for penetration to be possible. The first method uses mercury intrusion porosimetry (MIP) analysis in which epoxy resin is applied on the sample to induce abrupt intrusion at the threshold pore radius. The obtained threshold pore radius shows good correlation with water/surface air permeability and confirms that abrupt intrusion occurs at the threshold pore radius. These results indicate that both water permeability and surface air permeability are governed by threshold pore radius. However, this method is time consuming. The second method is developed on the basis of the empirical critical volume fraction for percolation, which assumes that when 16% of the cement paste was filled with mercury in MIP analysis, the corresponding pore radius is the threshold pore radius. The threshold pore radius obtained by this method agrees well with that obtained with the epoxy-coating method, which further supports the reliability of the threshold pore radius obtained by using the first method. 1. INTRODUCTION To ensure accurate verification of newly constructed concrete structures and efficient maintenance of the existing structures, there has been an increased focus on accurate evaluation of concrete durability. Because deterioration of concrete structures is caused by penetration of offending agents through pore networks, it is important to understand the relationship between mass transfer resistance and pore structure of the concrete. Although many researchers have analysed this relationship, no definitive study has been reported thus far. In the present study, we propose two methods for obtaining the threshold pore radius of concrete, defined as the minimum pore size through which a mass has to pass for penetration to be possible, and we investigate their applicability. We are currently conducting further analyses to describe the entire mass transport in concrete on the basis of a typical physical theory by using the threshold pore radius; the results will be published soon. 2. REPRESENTATIVE INDICATOR OF CONCRETE PORE STRUCTURE Many researchers have reported a strong correlation between the threshold pore size of concrete, and air and water permeability. Powers [1] determined that a correlation exists between the volume of capillary pores and water permeability, and Mehta [2] reported a 109

2 RILEM International workshop on performance-based specification and control of concrete durability June 2014, Zagreb, Croatia strong correlation between the threshold pore size and water permeability in which the pore size distribution was measured by mercury intrusion porosimetry (MIP) analysis. Halamickova and Detwiler [3] reported a correlation between the critical pore size, which is an indicator of the pore structure, and both water permeability and coefficient of oxygen diffusion. Goto [4] related the threshold pore size with cement hydration rate. Katz and Thompson [5] proposed a model for estimating the permeability by using threshold pore size and showed a good agreement between the two parameters in rock samples. In their research, the threshold pore size was defined as the inflection point of the cumulative pore size distribution as measured by MIP analysis. However, El-Dieb and Hooton [6] reported that this model is not applicable to cement paste and concrete samples because of the presence of many parameters affecting permeability and the difficulty in identifying the threshold pore size of such samples. Because uniform pore distribution in rocks is generally better than that in concrete, the inflection point is easily identified. However, in samples obtained from mortar or concrete, such an inflection point is not clear. 3. SPECIFICATION OF MATERIALS AND SPECIMENS The specifications of materials used in this study are shown in Table 1, and various mixture components and curing conditions of the concrete specimens are shown in Table 2. In this table, N, L, M and H in the specimen s name indicate cement types of normal, low-heat, moderate-heat and high-early strength, respectively. An air-entraining (AE) water reducer and an AE agent were used in 0.2% and 0.004% amounts, respectively, against the cement weight. The specimens were demolded 24 h after casting, cured under water for 28 days. After this period, all specimens were cured in a room at a temperature of 20 C, where the humidity, which is not controlled, averaged 60%. To represent in-wind curing, a fan was directed towards the specimens to accelerate drying. 4. FIRST NOVEL METHOD TO OBTAIN THRESHOLD PORE RADIUS As previously mentioned in this study, threshold pore radius is defined as the minimum pore size through which a mass has to pass for penetration to be possible. The interpretation of threshold pore radius applied to MIP analysis is based on that given by Winslow and Diamond [7], which is the corresponding pore radius in which the cumulative pore volume curve shows the largest tangent. However, as shown in Fig. 1, the largest tangent is often not clearly defined in concrete samples. One of the reasons is that most part of the sample is already filled with mercury before the mercury intrudes to the threshold pore radius through the interfacial transition zone (ITZ) or entrapped/entrained air. One conceivable method to reduce such bypass is to limit the open area for mercury intrusion. Under such conditions, mercury cannot avoid the smaller pore, and less intrusion will occur before the mercury intrudes to the threshold pore radius. When sufficient pressure is attained, abrupt intrusion occurs at the threshold pore radius. 110

3 RILEM International workshop on performance-based specification and control of concrete durability June 2014, Zagreb, Croatia Table 1: Specifications of materials used in study. Cement (Sumitomo Osaka Cement) Sand Gravel Admixture material Admixture chemical (BASF) Ordinal Portland cement (Density: 3.15 g/cm 3 ), Low Heat Portland Cement (Density: 3.24 g/cm 3 ), Moderate Heat Cement (Density: 3.21 g/cm 3 ) Fujigawa river sand ( Surface dry density: 2.62 g/cm 3, Water absorption: 2.1%, FM: 2.55) Chichibu Ryogami crushed stone ( Surface dry density: 2.72 g/cm 3, Water absorption: 0.5%, Maximum diameter: 20 mm, FM: 6.67) Flyash (JPec, Blain value: 3400 cm 2 /g), Blast furnace slag ( Nippon Steel & Sumikin BFS Cement, Blain value: 4250 cm 2 /g) AE water reducer (Lignin sulphonic acid series, Density: 1.25 g/cm 3 ), AE agent ( Alkyl ether series, Density: 1.04 g/cm 3 ) Table 2: Mixture proportions of concrete specimens prepared in laboratory. Specimen W/B Curing Unit : kg/m 3 Threshold name (%) condition W C FA or S G pore (nm) N Water N Water N Water FB Water FC Water BA Water BB Water N Wind FB Water FB Wind BB Water BB Wind L Sealed M Sealed H Sealed Epoxy resin (SHO-BOND CORPORATION) was adopted to limit the open area of the sample because the hardened resin is not porous, so it will not greatly affect the result. In addition, it is sufficiently stiff and strong to retain its integrity during MIP analysis, and its high viscosity does not generally permit permeation into the sample. To confirm the effects of the epoxy resin on the mercury intrusion curve, MIP analysis was conducted on a hardened epoxy resin ball 5 mm in diameter. Throughout this study, MIP analysis was performed with a porosimeter (Autopore III, Micromeritics). The obtained results are shown in Fig. 2. Mercury intrusion appeared when the mercury was pushed to pores less than 10 nm, probably due to 111

RILEM International workshop on performance-based specificationn and controll of concretee durability 11-13 June 2014, Zagreb, Croatiaa Cumulative pore volume (ml/ml) Pore radius(nm) Figure 1:

4 RILEM International workshop on performance-based specificationn and controll of concretee durability June 2014, Zagreb, Croatiaa Cumulative pore volume (ml/ml) Pore radius(nm) Figure 1: Example of measured pore size distribution of sample taken from concrete. Cumulative intruded mercury volume (ml/g) Pore radius(nm) Figure 2: Measured poree size distribution of epoxyy resin-coated samples. 2 (a) 4 mm (b) 16 mm (c) 100 mm 2 Figure 3: Epoxy-coated sampless in which small areas have h been left exposed. m 2 elastic deformation of the resin. The effects of the open areas was w analysedd with sample N55-1. Nine cubic 10-mm-pieces were takenn from the concrete specimen, immersed into acetone for 24 h and dried by using the D-Dry method for 24 h. Epoxyy resin was then applied with a toothpick to leave exposed areas of 100 mm 2, 14 mm 2 and 4 mm 2, as shown in Fig. 3. The sampless were then cured forr three dayss in a room at a temperature off 20 C, where the humidity, which is not controlled, averaged 60%, to sufficientlyy harden thee epoxy resin. Three sampless were prepared in each case, andd the results are shownn in Fig. 4.. In the graph, bars indicate the maximum and minimum values, and the larger plots reveal thee average value. As shown in the figure, the measured threshold pore radius of the uncoated u specimens and that of coated specimens with 100 mm 2 exposed varied widely. However, whenn the open area was less than 16 mm 2, variation was negligible. These results indicate that a smaller open area is desirable to achieve reproducibility. The effects of the drying period were determined by examining mortar with water cement (W/C) and cement sand (C/S) ratios of 55% and 30%, respectively. The specimens were demolded 24 h after casting, cured under water for 28 days and dried for six months in a room at a temperature of 20 C, in which the uncontrolled humidity averaged 60%. The proceduree used for sample preparation in n MIP analysis was similar to that used in the previous test with exceptions of an exposed area of 4 mm 2, and MIP analysiss was conducted 24 h, 48 h and 72 h after application of o the epoxy resin. The results are shown in Fig. 5. When the drying period was short, the curve showed an abrupt increase at approximately 20,000 nm probably due to elastic deformation of the epoxy resin. However, after 72 h of drying, the increase was minimal. These results indicate thatt a three-day drying period is sufficient after application of epoxy resin for minimizing the effects of deformation in the resin. Finally, MIP analysis was used to confirm the abrupt intrusion in the threshold pore size by examining the colours of the split surfaces of the samples att several intrusion points. The 112

RILEM International workshop on performance-based specificationn and controll of concretee durability 11-13 June 2014, Zagreb, Croatiaa Threshold pore radius (nm) Full 100 mm 2 16 mm 2 4 mm 2 Figure

Cumulative intruded mercury volume (ml/g) Dryingg period after coating 244 hours 488 hours 722 hours Pore radius(nm) Figure 5: Measured pore size distribution of o epoxy resin.

Pore radius (nm) Figure 7: Tangents of pore radii shown s in Fig. 6.

5 RILEM International workshop on performance-based specificationn and controll of concretee durability June 2014, Zagreb, Croatiaa Threshold pore radius (nm) Full 100 mm 2 16 mm 2 4 mm 2 Figure 4: Threshold pore radii of samples with various degrees of open areas. Cumulative intruded mercury volume (ml/g) Dryingg period after coating 244 hours 488 hours 722 hours Pore radius(nm) Figure 5: Measured pore size distribution of o epoxy resin. Intruded mercury volume (ml/g) Coated Normal Δ(intruded mercury) Δ(log pore radius) Coated Normal Pore radius (nm) Figure 6: Comparison between samples with and without coating. Pore radius (nm) Figure 7: Tangents of pore radii shown s in Fig. 6. same mortar as that used in the previouss test was applied; the open area was 4 mm 2, and the sampless were dried for threee days afterr application of the epoxy e resin. Fig. 6 shows the intrudedd mercury volume for samples with and without coating. Because each pair of curves shows nearly the same behaviour, it can be deduced that this method is reproducible. As shown in Fig. 7, the tangents of the curves with and without epoxy-coatine ng measured by the proposed method showed peaks at approximately 45 nm and 120 nm, respectively, and the threshold pore radius was 45 nm. To confirm the correctness of the obtained threshold pore radius, in which the abrupt intrusion of mercury occurred at approximately 45 nm, the intrusion was stopped at various pressures. The samples were then split with a hammer and chisel, and the splitting surfaces were observed. The measurement was stopped at pressures of 8.58 MPa, MPa and MPa, which correspond to pore radii of 164 nm, 1000 nm and 30 nm, respectively. The split surfaces are shown in Fig. 8. Inn case of normal samples, the colour of the surface gradually darkened as the mercury intruded into smaller pores. Conversely, coated samples showed s abrupt changes between 100 nm and 30 nm; in other words, abrupt intrusion occurred between 100 nm and 30 nm, n which corresponds to the extracted threshold pore radius of 45 nm. These results indicate that the abrupt intrusion of mercury was induced at the threshold pore radius in the epoxy resin-coated samples. The validity of these results as representativr ve indicators of pore structure is discussedd in the following section. 113

RILEM International workshop on performance-based specificationn and controll of concretee durability 11-13 June 2014, Zagreb, Croatiaa 164 nm 100 nm 30 nm 164 nm 100 nm 30 nm (a) Normal sample (b))

RELATIONSHIP BETWEEN THRESHOLD POREE SIZE ANDD WATER/SURFACE AIR PERMEABILITY Because the threshold pore is the smallest pore through which a mass can penetrate the concretee specimen, the threshold

6 RILEM International workshop on performance-based specificationn and controll of concretee durability June 2014, Zagreb, Croatiaa 164 nm 100 nm 30 nm 164 nm 100 nm 30 nm (a) Normal sample (b)) Epoxy-coated sample Figure 8: Split surfaces after mercury intrusion. Numbers below figures indicate the smallest radii of the intrudedd pores. 5. RELATIONSHIP BETWEEN THRESHOLD POREE SIZE ANDD WATER/SURFACE AIR PERMEABILITY Because the threshold pore is the smallest pore through which a mass can penetrate the concretee specimen, the threshold pore size is expected to have a correlationn with masss transfer resistance. Water permeability, surface air permeability and threshold poree radius of concrete specimens with various mixture components and curing conditions were measured, as shown in Table 2. The preparation and procedure used for MIP analysis are described in the previous section; however, coarse aggregates were excludedd from the samples. s A water permeability test was conductedd using a water/air permeability tester (CH-155, were 3.8-cm-thick concrete specimens cut from a cylinder specimen that was φ 10 cm 20 cm. Prior to the test, the specimens were Maruto Testing Machine Co.) with an outpu method of 2.5 MPa. Test samples vacuumed and saturated with water for 244 h. The water used forr the test was degassedd for 3 h. In this test, the amount of water permeating through the concrete specimenn was measured, and the water permeability k wl wass calculatedd using the following equation: k wl = lμq/(aδp), (1) where l, μ, Q, A and ΔP are the specimenn thickness, liquid viscosity, flow rate, cross-sectional area of the specimen and pressure gradient, respectively. Thee surface air permeability was measured with Permea-TORRR (Materialss Advanced Services). In the torrent method, a cup is attached to the concrete surface, the cup is vacuumed, and relationshipr p between time t and pressuree recovery because of the t air flow is measured and analysed to calculate the surface air permeability. The cup consists of two chambers to make the air flow one-dimensional [8]. Test samples of the cylinder specimens s of φ 15 cm 30 cm and their bottom surfaces rather than their casting surfaces were measured. The relationship between the threshold pore size obtained by the new method and air and water permeability are shown n in Fig. 9. A good correlation exists, which indicates that the surface air permeability and water permeability are governed mainly by the threshold pore size. In contrast, the correlation off the threshold pore size obtained with a normal sample, s in which air and water permeability were poor, showed determinant coefficients of approximatelyy 0.5 or lesss [9]. These results indicate that extraction of threshold pore size failed with normal samples. 6. SECOND NOVEL METHOD TO OBTAIN THRESHOLD PORE RADIUS In the previous section, the epoxy-coating method was established to obtain the threshold pore radius of concrete, which showed good correlation with water/surfacw ce air permeability. However, the application and drying of epoxy resin is timee consuming, and information 114

7 RILEM International workshop on performance-based specificationn and controll of concretee durability June 2014, Zagreb, Croatiaa Surface air permeability ( m 2 ) Threshold pore radius (nm) Water permeability ( 10 9 cm/s) Threshold T pore radius (nm) Figure 9: Correlation between air/ /water permeability andd threshold pore radius. obtained by MIP analysis with normal samples, such as total pore volume,, is lost because the coating blocks some of the pores. Therefore, an additional method was developed on the t basis of the empirical critical volume fraction for percolation. Scher and a Zallen [10] reported that in three dimensions, when 16% of the total volume of an object is i occupied,, percolation occurs regardless of the lattice shape.. Thus, in the second novel method, MIP analysis was conducted on a normal, uncoated sample, and the pore radius obtained when w 16% of the cement paste was filled with mercury is considered to be the threshold pore radius. The sample volume was measured by the porosimeter, and a volume of the sand was removed on the basis off the mix proportion becausee the sand in a 5-mm-cubic sample is tooo large to treat the sample as homogeneous. The relationship of the threshold pore radius obtained by the method proposed in this section and that by using the epoxy-coating method is shown in Fig. 10; both agree well quantitatively. The results of a cement paste specimen with a W/CC of 40% are also plotted. Two cylinder specimens of φ 100 cm 20 cm were kept sealed for 28 dayss and six months after casting, respectively. The procedure for sample preparation of MIP analysis was same as that used in the the previous method but smaller samples, 3-mm-cubic, were used. Similar results were obtained with samples taken from the concrete specimen, which indicates i the validity of removing the sand volume to calculate 16% of the sample volume. This agreement supportss the adequacy of the threshold pore radius obtained withh the epoxy-coating method. As shown in Fig. 11, the correlation with water permeability was also good. However, the volume fraction of 16% is simply an approximat ed value and sometimes varies [10]. Further study is required to conclude that the critical volume fractionn for percolation in concretee is 16%. Threshold pore radius: epoxy-coated samples (nm) Cement paste samples Threshold pore radius: critical volume fraction (nm)) Figure 10: Comparison of obtained threshold pore radii. Water permeability ( 10 9 cm/s) R 2 =0.91 Threshold pore radius: : critical volumee fraction (nm) Figure 11: Correlation between threshold pore radius and waterr permeability. 115

8 RILEM International workshop on performance-based specification and control of concrete durability June 2014, Zagreb, Croatia 7. CONCLUSIONS The results of this study are summarised in the following points: Two novel methods for measuring the threshold pore radius of concrete are proposed. In the first method, MIP analysis was performed on epoxy-coated specimens. The desirable procedure was studied, and abrupt intrusion was confirmed to occur at the threshold pore radius. The obtained threshold pore radius showed good correlation with water/surface air permeability. The second method was implemented on the basis of the empirical critical volume fraction for percolation by assuming that the mercury amount corresponding to 16% of the paste volume also corresponds to the threshold pore radius. The obtained threshold pore radius agreed well with the results obtained by the epoxy-coating method. The correspondence between the two methods supports the reliability of the threshold pore radius obtained by the epoxy-coating method. ACKNOWLEDGEMENTS Some experimental works in this paper were conducted with financial support by the Express Highway Research Foundation of Japan. REFERENCES [1] Powers, T. 'The physical structure and engineering properties of concrete', Research and Development, Bulletin of Portland Cement Association, (1958) 90. [2] Mehta, P. and Manmohan, D., 'Pore size distribution and permeability of hardened cement paste', 7th International Congress Chemistry of Cement (3) (Paris, 1980) [3] Halamickova, P., Detwiler, R., Bentz, P., Garboczi, E., 'Water permeability and chloride ion diffusion in Portland cement mortars: Relationship to sand content and critical pore diameter', Cement and Concrete Research 25(4) (1995) [4] Goto, T., 'Modelling of cement hydration reaction and strength development', Doctoral dissertation in the University of Tokyo (1996). [5] Katz, A.J., Thompson, A.H., 'Prediction of rock electrical conductivity from mercury infection measurements', J. Geogr. Res. 92(B1) (1981) [6] El-Dieb, A.S., Hooton, R.D., 'Evaluation of the Katz-Thompson model for estimating the water permeability of cement-based materials from mercury intrusion porosimetry data', Cement Concrete Research 24(3) (1994) [7] Winslow, D., Diamond, S.A., 'A mercury porosimetry study of the evolution of porosity in Portland cement', J. Mater. 5 (1970) [8] Torrent, R.J., 'A two-chamber vacuum cell for measuring the coefficient of permeability to air of the concrete cover on site', Mater. Struct. 25(6) (1992) [9] Sakai, Y., Nakamura, C., Kishi, T. and Ahn, T., 'Interpretation of non-destructive test results for evaluation of mass transfer resistance of concrete members', The 5th International Conference of Asian Concrete Federation (2012). [10] Scher, H. and Zallen, R. 'Critical density in percolation processes', J. Chem. Phys. 53 (1970)

PROPERTIES OF PORE STRUCTURE MEASURED BY STEP-BY-STEP MERCURY INTRUSION POROSIMETRY TEST

Cementitious Composites, 11-13 April 212, Amsterdam, The Netherlands PROPERTIES OF PORE STRUCTURE MEASURED BY STEP-BY-STEP MERCURY INTRUSION POROSIMETRY TEST Ryo YOSHIDA (1) and Toshiharu KISHI (2) (1)