INFLUENCE OF DIFFERENT CONDITIONING REGIMES ON THE OXYGEN DIFFUSION AND OXYGEN PERMEABILITY OF CONCRETE

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1 INFLUENCE OF DIFFERENT CONDITIONING REGIMES ON THE OXYGEN DIFFUSION AND OXYGEN PERMEABILITY OF CONCRETE Carlo Peretti (1), Andreas Leemann (1), Roman Loser (1), Pietro Lura (1,) (1) Empa, Swiss Federal Laboratories for Materials Science and Technology, Switzerland () Institute for Building Materials, ETH Zurich, Switzerland Abstract Penetration of gases and solutions into concrete can cause damage to the cement paste matrix and additionally induce steel corrosion. As transport properties are critically dependent on the moisture state of the concrete and on possible damage of the microstructure (e.g. cracks), proper conditioning is needed to bring concrete from different sources to comparable conditions before testing. However, the effect of conditioning of the samples prior to transport testing is poorly known. In this paper, the influence of three different conditionings (harsh, intermediate and gentle) on the oxygen diffusion and oxygen permeability of Portland cement concrete (w/c 0.0, 0.0 and 0.) is studied, in order to help creating a standard procedure to investigate transport phenomena. All transport coefficients increased linearly with w/c. Oxygen diffusion was related to the moisture-free porosity volume fraction with an exponential correlation. The scatter in the permeability measurements was higher compared to diffusion, while the influence of dryingshrinkage cracks on transport appeared to be negligible. Based on the results of this research, a possible conditioning standardization for transport testing is proposed. 1. INTRODUCTION The relationship between transport phenomena and durability of concrete is a main focus point of research. The penetration of gas and solutions can cause damage to the cement matrix and to the steel reinforcement, directly or indirectly. Well-known examples are the effects of carbonation, the penetration of chloride ions or the ingress of oxygen on steel corrosion. Many relationships, mostly empirical in nature, have been proposed between the transport properties and the performance of concrete exposed to aggressive conditions [1]. Oxygen is involved in the reinforcement corrosion and can also be correlated to the transport of other gases, like radon or carbon dioxide. For these reasons, oxygen diffusion and oxygen permeability of cement-based materials have been the object of in-depth studies [-

2 1]. However, conditioning of the samples prior to testing has been seldom investigated and often only one type of conditioning was performed. As transport properties of concrete are critically dependent on the moisture state [,1], the main aim of conditioning is bringing concrete from different sources to comparable conditions, so that a fair comparison of the concrete quality can be made. In practice, this can be achieved by emptying the pores that are relevant to transport in field conditions, without producing significant changes in the original pore structure and without creating cracks. Especially when used for standard testing of concrete in the laboratory, conditioning needs to be fast and practical. Lawrence [] found that, after conditioning concrete at 0 C and % RH for a certain time, the oxygen diffusivity did not change anymore. In [1] mortars were conditioned at different RH levels for different periods: oxygen diffusivity increased remarkably for moisture contents lower than % (based on the dry mass as reference), irrespectively of any other parameter. Wong et al. [10] found that drying wasteform grouts at % RH and 0 C to equilibrium was sufficient to empty all the pores relevant to transport, especially for water sorptivity and oxygen diffusion. By lowering the RH further, apparently only small pores with no significant effect on gas transport were emptied. In [], air permeability and water absorption of concretes with different w/c were found sensitive to moisture content only above 0% RH. In [7], oxygen permeability decreased exponentially with increasing moisture content up to 70% RH, while above this value the scatter increased considerably. In this paper, the influence of three different conditionings on oxygen diffusion and oxygen permeability of different Portland cement concretes is studied, in order to help creating a standard procedure to investigate transport phenomena. It was decided to deliberately use one harsh, one intermediate and one gentle conditioning to emphasize their differences.. MATERIALS AND SAMPLE PREPARATION Three concrete mixtures were produced with ordinary Portland cement (OPC, CEM I. R). The w/c was varied between 0.0 and 0. (Table 1). Binder volume and grain size distribution of the aggregates (maximum grain size 1 mm) were kept constant to obtain comparable relative and absolute porosity. Per mixture, cubes were cast and vibrated; stored for days at 0±1 C and 90±% RH, then immersed in water for days. Afterwards cores were drilled and cut to obtain cylindrical specimens (100 mm diameter, 0 mm height). Drilling was performed perpendicular to the direction of casting and the surfaces (approximately the outermost 0 mm in contact with the moulds) were discarded. Conditioning was performed according to the procedures explained in Table. In C1 the samples were oven dried at 110 C for 3 days and immersed in water for days; in C the specimens were stored at 3% RH and 0 C for 7 days; in C3 the cores were stored at 70% RH and 0 C for days, then 10 days at 3% RH and 0 C and finally pre-dried for days at 0 C. In every conditioning, the final steps were oven drying at 0 C for 7 days and 1 day storage in the desiccator. It is noted that drying at 110 C in C1 was employed to represent a quick drying step (often used in practice), that however might change the pore structure. The following immersion in water was necessary to bring the samples to a moisture content similar to the samples in C and C3, as oxygen diffusion and permeability are strongly dependent on the moisture content of the samples.

3 After conditioning, sample surfaces were investigated with the optical microscope. No cracks were visible at a magnification of 0 (resolution about 10 mm); the presence of internal cracks cannot however be excluded. Finally, transport tests were performed. Length of C3 is different from C1 and C and tests for these two were made while C3 samples were still conditioned. Table 1: Concrete mix composition. Mix ID Cement type Cement content Superplasticizer Vol. of paste Aggr. (kg/m 3 w/c ) (kg/m 3 ) (l/m 3 ) (kg/m 3 ) Concrete OPC 0.0 CEM I. R Concrete OPC 0.0 CEM I. R Concrete OPC 0. CEM I. R Table : Steps in the different conditionings. Conditioning 1 (C1, harsh) Conditioning (C, intermediate) Conditioning 3 (C3, gentle) Oven drying 3 days at 110 C 7 days at 3% RH and 0 C days at 70% RH and 0 C Immersion in water days 10 days at 3% RH and 0 C days oven drying at 0 C 7 days oven drying at 0 C 7 days oven drying at 0 C 7 days oven drying at 0 C 1 day in desiccator 1 day in desiccator 1 day in desiccator 3. METHODS 3.1 General In every test, three samples per conditioning were tested and averages were calculated. For oxygen permeability and diffusivity tests, the mass change before and after the test was comparable to the sensitivity of the balance. When unused, samples were stored in a desiccator to avoid moisture changes. Tests were performed at ±1 C. Since carbonation goes together with a densification of the microstructure, carbonation depth was measured after the end of the transport experiments by chopping off some material and spraying the cracked surface with a phenolphthalein solution. Carbonation depth was found to be always lower than 1 mm. On concrete OPC 0.0 it was undetectable. As such, the influence of carbonation was considered to be negligible for the conducted measurements of the transport properties. 3. Oxygen diffusion The oxygen diffusion test is described in detail elsewhere [,3,9,10,1]. The concrete cores were fitted into a silicon rubber ring in a diffusion cell. The curved surface of the cores was sealed by loading the top plate and silicon rubber ring. Due to the pressure, the latter expands laterally, providing an air-tight grip onto the sample to ensure unidirectional flow. The sample was not loaded directly. The two flat faces of the samples were then exposed to a pure oxygen stream on one side and a pure nitrogen stream on the other side, both at identical pressure (about 1. bar absolute) and temperature. The content of oxygen in the nitrogen stream was measured in ppm(v) at equilibrium with a heated zirconia oxygen analyzer (Servomex 100). Using total oxygen pressure, pressure difference between oxygen and nitrogen flows (precision bar), flow rates and sample size data, the oxygen diffusion coefficient D (m /s) was calculated according to Eq. 1, where Q is the diffusion rate in m 3 /s, L the sample

4 height in m, A the cross-sectional area in m, C and C 1 are the oxygen concentrations in the outflow and inflow streams respectively. Q L D (Equation 1) A C C 1 After the oxygen diffusion test, the curved surfaces of the cores where sealed with epoxy resin to ensure unidirectional flow in the following tests. After curing in the desiccator, the sample surfaces were slightly ground (dry process) to remove excess resin. The masses of the samples were recorded after every step. 3.3 Oxygen permeability The oxygen permeability test was performed following the guidelines for OPI test ( Oxygen Permeability Index ) in [13] and [1] using the same samples as for the oxygen diffusion measurements. Samples were placed in a falling head permeameter filled with oxygen, where sealing was obtained with O-rings. The initial pressure difference between samples faces was 1.00 ± 0.0 bar and its decrease was measured over time. The rate of pressure decrease and the sample dimensions allowed calculating, as average, the oxygen permeability coefficient k (m/s). Eq. shows the form of Darcy s equation used for this calculation: m k A P (Equation ) t g z which correlates the gas mass flow rate (kg/s) with the pressure gradient (Pa/m), the gravitational acceleration g (9.81 m/s ) and the cross-sectional area A (m ). The permeability coefficient of a single sample is then calculated as shown in Eq. 3: M M V g d k F (Equation 3) R T A P t 0 where F is the slope of the best fit line produced when ln is plotted versus time P t (provided R >0.99), M M is the molar mass of oxygen (3 g/mol), V is the volume of the pressure cylinder (m 3 ), R is the universal gas constant (8.31 J/(mol K)), d is the sample thickness (m), T is the absolute temperature (K), t is the time (s) and P(t) is the pressure at time t (bar).. RESULTS.1 Moisture-free pore volume fractions The moisture-free pore volume fractions (in %) before the diffusion test were calculated using the mass at the beginning of the first drying step. Water density in the pores is taken as 1000 kg/m 3, which is a good approximation for capillary pores but might be underestimated for the water in the gel pores; in fact water confined in small pores and close to solid surfaces is denser than free water. Standard deviation is shown as error bars in Fig. 1, which reports the moisture-free pores volume fractions. For C and C3 the values are similar and overlap, values for C1 are the highest. For every conditioning, a linear correlation between the oxygen diffusion coefficient and w/c exists (R always higher than 0.98).

5 moisture-free pores volume fraction (%) w/c Figure 1: Average moisture-free pores volume fraction before oxygen diffusion test versus w/c for OPC concretes.. Oxygen diffusion Figure, left, shows the values of the oxygen diffusion coefficient for OPC concretes plotted against w/c. The values of C and C3 are similar for the three concretes, as the error bars overlap, while values for C1 are highest. For every conditioning a linear correlation between the oxygen diffusion coefficient and w/c exists (R always higher than 0.98)..3 Oxygen permeability Figure, right, shows the values of oxygen permeability coefficient for OPC concretes plotted against w/c. Linear correlations exist between w/c and k for every conditioning. R is however always lower than for oxygen diffusion, between 0.93 (C3) and 0.99 (C); relative errors and scatter are higher than in oxygen diffusion and concrete OPC 0. has the highest coefficients. D (m /s/10-8 ) C1 C C w/c k (m/s/10-11 ) C1 C C w/c Figure : Oxygen diffusion coefficient versus w/c for OPC concretes (left) and oxygen permeability coefficient versus w/c for OPC concretes (right).. DISCUSSION The total porosity and volumes of capillary and gel pores for OPC concrete can be calculated with Powers model equations (e.g. reported in [1]). As the degree of hydration was not assessed in this study, for w/c 0.0 a minimum hydration degree of 0.80 and a

6 maximum of 0.9 were assumed, while the maximum degree of hydration was taken as 1 for w/c 0.0 and 0.. In Figure 3 the lines represent the porosity limits for minimum and maximum degree of hydration, while the points marked as 0 C represent experimental values, referred as the status before oxygen diffusion test. The empty porosity referred as C1 110 C can be interpreted as emptied gel and capillary porosity, calculated with reference to oven drying at 110 C. In C1 the empty porosity after oven drying at 0 C is remarkably higher than for C and C3. This indicates that the capillary pores and part of the gel pores have been emptied. In C and C3, however, it seems that capillary pores are sufficient to account for almost all the empty porosity. However, especially at low w/c, it is not excluded that a small part of the gel pores may be empty as well. Porosity % V/V C1-0 C C - 0 C C3-0 C C1-110 C gel porosity limits capillary porosity limits total porosity limits w/c Figure 3: Ranges of theoretical porosity (total, capillary and gel, according to Powers model) compared with moisture-free porosity after oven drying for different conditionings. The decrease of the oxygen diffusion coefficient with w/c (Figure ) is reasonable and could be explained by the decrease of capillary porosity with decreasing w/c. However, plotting the diffusion coefficient against moisture-free porosity, the data fall on an exponential master curve with a very high correlation coefficient (Figure, left). Gas diffusion would appear to be then dominated by the volume of empty pores, with contributions both of capillary and gel pores. The nonlinear correlation may indicate both that pores of different sizes participate to transport with increasing empty volume, and also that pore connectivity may increase more than linearly with the volume of empty pores. Drying phenomena (including the ink-bottle effect), the tortuosity and the constrictivity of the pore structure and adsorbed water molecules on the pore walls may play a role as well. By plotting the permeability coefficient against the moisture-free porosity (Figure, right), a general trend is found, albeit with much higher scatter than for the oxygen diffusion coefficient. Other factors besides the amount of free porosity, however, must be implied in determining permeability. A possible factor of influence is the size distribution of the empty pores, which could have been coarsened mainly by heating to 110ºC in C1. C1 may also have introduced cracks that are not detectable with 0 magnification, but that nevertheless are able to influence the transport properties.

7 D (m /s/10-8 ) R = % moisture-free pores volume fraction k (m/s/10-11 ) 9 8 R = % moisture-free pores volume fraction Figure : Oxygen diffusion coefficient versus moisture-free pores volume fraction (left) and oxygen permeability coefficient versus moisture-free pores volume fraction (right). Datapoints for OPC concrete with w/c 0.0, 0.0 and 0... CONCLUSIONS Investigating the influence of conditioning on oxygen diffusivity and oxygen permeability of OPC with different w/c, the following conclusions may be drawn: Different conditioning were explored, that brought concretes to different moisture contents without creating apparent cracks that had a significant effect on the measurements. Transport properties correlated well with w/c. Scatter for oxygen diffusion was lower than for oxygen permeability. Moisture-free porosity is the main parameter governing oxygen diffusion and it has a significant contribution to oxygen permeability. The amount of moisture-free porosity was compared to the amount of gel and capillary pores predicted with Powers model. This analysis revealed that both gel and capillary pores, depending on moisture content, contributed to transport. Oxygen permeability is affected by a relatively high scatter. Moreover, it is less sensitive to conditioning and w/c, making it a worse indicator of concrete quality than oxygen diffusion. Conditioning C produces results very similar to conditioning C3, albeit with less expense of time for conditioning. C1 however appears to empty almost all pores and increases transport rates substantially. Based on these considerations, conditioning C should be investigated further for possible standardization. 7. ACKNOWLEDGMENTS We thank Dr Enrico Bernardo of University of Padua for his support. Special thanks to W. Trindler, K. Pfeiffer, K. Burkhard, B. Ingold, L. Brunetti and M. Käppeli for helping with the experiments.

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