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Water vapour and liquid permeability measurements in cementitious samples
2018/09/28
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Permeability is one of several key properties which are indicators of durability. The mechanisms which cause permeability to affect durability are described by Claisse1 and include the transport of chlorides dissolved in water causing corrosion of steel reinforcement.
There are numerous methods available to measure it but some actually measure other properties which are assumed to correlate with permeability and are therefore indirect (such as the the ‘rapid chloride permeability test’,2 which measures electromigration) and few of the others actually yield results for permeability itself. For example the initial surface absorption test (ISAT),3 which measures both absorption and permeability is useful for comparing materials but the standard report from the test gives an ISAT value, not a permeability. Similarly the water penetration test in EN12390-84 records penetration depth, not permeability. The advantages of knowing the permeability in standard units are listed here.
(a) The results from one test can be compared with another.
(b) The results can be used in theoretical work to calculate durability of structures using, for example, finite element modelling of the transport processes.
(c) The permeability must be known in order to calculate the performance of structures in applications such as waste containment.5 Indirect measurements of permeability are of little use for this.
The literature on the permeability of concrete is extensive. Previous work by the authors has been reported6–9 to compare standard tests by calculating permeability values from them. The objective of this paper was to analyse an experiment which measures a permeability for water vapour and demonstrate that this may be related to gas and liquid permeabilities obtained using other test procedures. Water vapour transport is a key process in many durability-related processes such as carbonation. The permeability itself is a macroscopic property and will be controlled by other microscopic properties such as connectivity, tortuosity and pore size distribution and uniformity.
Research significance This work is intended to give an improved understanding of the transport mechanisms that take place during laboratory testing. The particular emphasis of the work is to show how the fundamental property of permeability may be obtained and also to focus on vapour transport during drying as a means to measure it. The work will be of interest to researchers who are measuring or modelling durability or to practitioners who are designing containment structures for fluids or waste materials and need to know the permeability as part of the design. The analysis methods which are presented may also be used to confirm the reliability of any individual test.
Experimental methods
Four mortar mixes were used in this investigation and the mixture proportions and strengths are shown in Table 1. The test specimens (dimensions are detailed in each test procedure) were cast and kept under wet hessian for 1 day before demoulding. All samples were then cured in water at 208C until testing at 28 days. All testing was carried out at 20~ 28C.
Drying test
Samples were cast in cylindrical moulds 30 mm diameter by 50 mm long. The moulds were laid on their sides for setting in order to keep their ends under identical conditions. After curing, polyvinyl chloride adhesive tape was wound round the curved surfaces of the cylinders to prevent moisture evaporation except from the top and bottom ends (Fig. 1). The cylinders were then placed in a glass desiccator connected to a vacuum pump with a 0–25 mbar (0–2.5 kPa) pressure gauge on it. The vacuum pump was run until the air in the desiccator became dry, as indicated by the colour change of silica gel in the desiccator (this took up to 30 h). The colour changes at a water content of 8% by mass, which occurs at a humidity of 15% at atmospheric pressure, i.e. a partial vapour pressure of 0.3 kPa.
The vacuum pressure was monitored during the drying process. At different times samples were taken out of the desiccator and weighed and then split down the axis and the depth of drying at each end was measured visually. No precautions were taken to prevent carbonation of the samples but since the experiments only lasted for a few days the weight gain from this process was not considered to be significant.In addition to these tests some specimens were dried n an oven for 3 days at 1058C to calculate porosity.
High-pressure permeability test he water permeability was measured in a modified oek cell manufactured by ELE International, USA The cell and the modifications are shown in Fig. 2. All f the components identified in the figure except the ell and the oil are modifications for concrete permeability esting. Specimens were cast as 100 mm ubes and 55 mm diameter cores were cut through them nd approximately 40 mm long samples were cut from he central part of the cores. The apparatus comprised a tainless-steel triaxial cell in which oil was used to pply pressure to the curved surface of the specimens hrough a rubber sleeve. The test method was based on eeding the water to the lower surface of the specimens t high pressure (6–8 MPa) while the oil pressure was aintained about 1 MPa higher to prevent flow around he specimens. Due to the high pressures used the flow hrough the samples became constant only approximately h after the start of the test for most of the amples. The flow rate was determined by measuring he rate of water flowing from the upper surface using graduated measuring cylinder.
Initial surface absorption test nitial surface absorption test (ISAT) measurements ere carried out using the method defined in BS188111 n 100 mm cubes which had been dried for 3 days at 058C. A cap of known area (6360 mm2) was clamped o the test surface. Two pipes led from the cap. One cted as a reservoir that can be isolated by a tap. The other was connected to a calibrated capillary tube to easure the rate of absorption of water into the surface f specimen under the cap on closure of the tap. The low was recorded at intervals up to 2 h. ater absorption test ome 100 mm cubes from mix C were oven dried at 058C and then immersed in water to a depth of 20 mm nd the mass gain was recorded at intervals up to 2 h. est programme he programme of testing is shown in Table 2.
Methods of analysis of results
Transport processes o analyse the results it was necessary to determine hich processes were transporting the water during the ests. The main process that was considered in this aper was pressure driven flow measured by permeability.1 he permeability may be defined in terms of a ad of water ?h in the following manner:
where P is the pressure drop and e is the viscosity in a s.
By including viscosity in the equation the coefficient should theoretically be the same for all fluids. This definition was used in the analysis in the present study. It should also be noted that equations (1) and (2) can only be applied to a steady state in which the pressure is constant. For all of the work in the present study equation (2) was integrated with respect to time and the condition of constant pressure through the time step was therefore met as the time step approaches zero. Other potential transport processes include concentration-driven flow measured by the diffusion coefficient,electromigration driven by an electric field and thermal migration driven by a temperature gradient.1 The flux from these processes may be increased by capillary suction or osmosis and they may be inhibited by absorption.
The drying test
The rate of loss of moisture from the specimens is governed by the movement of vapour from the drying front to the surface. There are two possible transport processes to consider for this test: pressure-driven flow and moisture diffusion. The differences between theprocesses is discussed by Neville.11 The pressure at any point in the system will be made up of contributions from several different gases and vapour. The effect of them will be additive and any pressure measurement will record the total. A change in one partial pressure will not affect the others. At the drying front the pressure of water vapour will depend on the equilibrium with the adjacent liquid and will be determined by the temperature and surface tension. The pressure outside the sample will be determined by vacuum pumping and will be substantially lower. Thus there is a pressure drop and the flow caused by it will be controlled by the permeability.
Diffusion is driven by a chemical concentration gradient and would typically be relevant to a liquid with a higher concentration of salt at one position than another. It is also used to measure the movement of one gas through another and could be the main mechanism to transport vapour through air from a drying surface. In the present experiment, however, there was virtually no air present with the pressure in the desiccator reduced to 0.1 kPa (0.001 atmospheres) and the diffusion coefficient for vapour through air could not therefore be relevant. The transport was therefore controlled by permeability and described by the Darcy equation
where M is the cumulative mass loss (kg); r is the density of liquid water (kg/m3); and ¨ is the crosssectional area through which the transport is taking place (m2).
The partial pressure of water vapour above a liquid surface at 208C is 2 kPa. This pressure is correct for pure water but will have been affected by the presence of the dissolved ions in the water. This would be expected to lower the vapour pressure and lead to a slight reduction in the flow.
The measured pressure in the desiccator was initially 0.6 kPa but fell to 0.1 kPa during the test. Of the pressure in the desiccator the partial pressure due to vapour was initially approximately 1% of the total(which would be the case in a room at a humidity of 50%). Thus the vapour pressure in the desiccator was below 1% of 0.6 kPa and the drop in pressure from the drying front to the concrete surface was close to 2 kPa. Not all of the pores will dry at exactly the same pressure. It has been shown12 by measuring gas permeabilities at different humidities that the Kelvin equation gives a good indication of the pore sizes in concrete that will sustain a meniscus. This shows, however, that the smallest capillary pores (0.01 m) will not sustain a meniscus below 90% relative humidity. Thus all the pores will empty of liquid within 10% of the distance over which the pressure drops.
Using standard gas constants for a molecular mass of 18 the constant WF was calculated to be 1.25 3 103 by assuming the vapour was an ideal gas.
The viscosity of water vapour e ? 2 3 105 Pa s.The porosity was calculated from the weight loss on drying using the ‘volume products of hydration’ method12 in which the cement and water were assumed to combine in fixed proportions and fixed values were assumed for the specific gravity of unhydrated and hydrated cement.
Equation (5) may be seen to have a similar form to the equation presented by Vuorinen13 and Valenta (cited by Neville11) for water intruding into concrete under pressure:
The absorption and ISAT For the ISAT the transport process will be pressuredriven flow but in this case the pressure driving it will arise from the capillary suction at the wetting front. The analysis has been given by the present author6 and the following relationship was derived
Results and discussion
When considering results for permeability testing Neville11 states ‘reporting the order of magnitude ... is adequate. Smaller differences in the value of the coefficient of permeability are not significant and can be misleading’. Oven drying of the samples causes microcracking and will also have contributed to the spread of data. The results are shown in Figs 3–8 and the spread of data predicted by Neville may be seen in addition to the trends from which the conclusions are drawn. Figure 3 shows the average values for water permeability.
It may be seen that the methods of analysis give consistent results from the different experiments and also the expected increase of permeability with water/cement (w/c) ratio. Figure 4 shows all of the permeability results for mix C. The first three series (absorption, ISAT and high pressure) were very close. The vapour permeability would be expected to be substantially higher but the results from the measured drying depths may be seen to fall over a very wide range. The results from mass loss were, however, grouped in an expected range and were therefore indicated to be far more reliable than the results from drying depth. The results from the drying depth measurements were generally lower than those for mass loss, indicating that significant drying may have occurred from regions which still appeared to be wet when the samples were inspected. Since the larger pores would dry first this implies that the visible moisture was in the smaller ones (possibly below 0.1 m).
The reduction in permeability which is normally observed during testing with water may be caused by sedimentation causing blocking of pores and would thus not be expected to occur during vapour transport.
Figures 5–7 show the permeability plotted against time for mixes A, B and D for the high-pressure cell and drying experiment data. These support the observation that the mass loss data was far more consistent than the observations of drying depth. They all also show the drying depth data giving lower results. The drying front was readily visible on the tested samples but a further disadvantage of this method would be that on some samples (e.g. white cement) it could be very difficult to see. The mass loss was observed to be the more reliable test in these laboratory trials but this conclusion might not be valid in other circumstances
such as site testing.
The high-pressure cell data, which was for liquid rather than vapour was consistently lowest and the reasons for this are discussed below.
The new European standard test for permeability4 relies on a visual observation of a wetting front but this is during wetting, rather than drying as in the experiments reported here. Thus the observed poorer quality of data recorded in this way would not be relevant to the EN test.
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