DISCONTINUOUS CAPILLARY POROSITY IN CONCRETE – DOES IT EXIST? Michelle R. Nokken, Dept. of Civil Engineering, University of Toronto, Canada R. Douglas Hooton, Professor, Dept. of Civil Engineering, University of Toronto, Canada Abstract Codes and standards set limits on the maximum water to cementitious ratio and other mixture design parameters for concrete exposed to deleterious environments. The aim of these codes and standards is to maximize the service life of concrete structures. Given that the capillary pore system presents the pathway for the ingress of deleterious substances, it follows that the formation of a discontinuous capillary pore system is highly desirable. While T. C. Powers and his colleagues showed this in the 1940’s, there has been little work done since then with modern cementing materials. The time required to achieve discontinuity was based on the degree of hydration of cement under standard laboratory conditions. Since that time, changes in the chemistry of cement and its particle size distribution as well as the use of supplementary cementing materials and chemical admixtures have changed the nature of concrete. Saturated water permeability, as well as other transport properties, was measured for a range of concretes at 28 days of age. In addition, capillary porosity of mortar and paste were measured at the same ages. These properties were used to determine if capillary discontinuity follows the relationship put forward by Powers or if it exists at all due to “interfacial transition zone” effects. 1. Introduction Cement paste is the component of concrete that provides bond between the constituent materials, allowing it to function as a structural entity. Although, the major component of concrete is the aggregate, comprising typically 70% by volume, the cement paste envelops the aggregate particles. This aspect highlights the importance of the cement paste component for the physical and transport properties of concrete. At the time of mixing, cement grains are surrounded by water. The relative volume of voids (water) to solids (cement) controls the initial porosity, as water occupies void spaces at the outset. As the water to cement ratio decreases, the space between the cement grains decreases. As the cement reacts, the hydration products occupy more than twice the volume of the originally unhydrated cement. The structure of the hydrates is subject to debate, but regardless of its physical form, it is composed of the solid hydrates (and crystalline Ca(OH) 2 ) and small voids termed “gel” pores by Powers. Feldman and Sereda [1] had a different model of the physical structure, porosity NOKKEN, Discontinuous Capillary Porosity, 1/16 Fax: (416) 978-7046 E-mail: [email protected]
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DISCONTINUOUS CAPILLARY POROSITY IN CONCRETE – DOES IT EXIST? Michelle R. Nokken, Dept. of Civil Engineering, University of Toronto, Canada R. Douglas Hooton, Professor, Dept. of Civil Engineering, University of Toronto, Canada Abstract Codes and standards set limits on the maximum water to cementitious ratio and other mixture design parameters for concrete exposed to deleterious environments. The aim of these codes and standards is to maximize the service life of concrete structures. Given that the capillary pore system presents the pathway for the ingress of deleterious substances, it follows that the formation of a discontinuous capillary pore system is highly desirable. While T. C. Powers and his colleagues showed this in the 1940’s, there has been little work done since then with modern cementing materials. The time required to achieve discontinuity was based on the degree of hydration of cement under standard laboratory conditions. Since that time, changes in the chemistry of cement and its particle size distribution as well as the use of supplementary cementing materials and chemical admixtures have changed the nature of concrete. Saturated water permeability, as well as other transport properties, was measured for a range of concretes at 28 days of age. In addition, capillary porosity of mortar and paste were measured at the same ages. These properties were used to determine if capillary discontinuity follows the relationship put forward by Powers or if it exists at all due to “interfacial transition zone” effects. 1. Introduction
Cement paste is the component of concrete that provides bond between the constituent materials, allowing it to function as a structural entity. Although, the major component of concrete is the aggregate, comprising typically 70% by volume, the cement paste envelops the aggregate particles. This aspect highlights the importance of the cement paste component for the physical and transport properties of concrete. At the time of mixing, cement grains are surrounded by water. The relative volume of voids (water) to solids (cement) controls the initial porosity, as water occupies void spaces at the outset. As the water to cement ratio decreases, the space between the cement grains decreases. As the cement reacts, the hydration products occupy more than twice the volume of the originally unhydrated cement. The structure of the hydrates is subject to debate, but regardless of its physical form, it is composed of the solid hydrates (and crystalline Ca(OH)2) and small voids termed “gel” pores by Powers. Feldman and Sereda [1] had a different model of the physical structure, porosity
was termed “interlayer pores”, thought to occur between the layered sheets of CSH. Gel pores comprise 26% [2] to 28% [3-5] of the total gel volume. Space not filled with products of hydration is termed capillary porosity. In simplistic terms, “gel” pores are intrinsic to the formation of hydrates of cement, while capillary porosity is the remnant of water filled space [6]. The increase in volume of the hydrates occurs at the expense of this originally water-filled space. Thus, as hydration proceeds, capillary porosity decreases. The use of supplementary cementing materials (SCMs) in concrete has increased markedly in the past few decades. Inclusion of SCMs enhances durability, decreases heat of hydration and generally benefits concrete properties. SCMs increase resistance to transport by two primary means. The first is by reaction with by-products of cement hydration and the other by increasing particle packing efficiency. Since SCMs react in the presence of calcium hydroxide, the reaction is often termed secondary hydration. Residual capillary porosity remaining after hydration of Portland cement decreases both in volume and size. Pores, which were once continuous, can become segmented and blocked by the hydration of SCMs. The inclusion of aggregate does not appear to affect the kinetics of hydration. Shane, Mason et al [7] found that mortar and paste hydrated at the same rate. The porosity, however, is significantly influenced by the addition of aggregate. The pore size distribution of the cement paste fraction of concrete and mortar, as measured by mercury intrusion porosimetry, was shown to be different than that of plain paste hydrated without aggregate [8]. The interfacial transition zone (ITZ) is the boundary of each aggregate particle with the cement paste. Aggregates have a certain affinity for water, or water demand. At the time of mixing, a film of water will form on the aggregates. In addition, due to size differences, there is a restricted amount of cement grains near the surface. These two conditions create a deficiency of cement near the surface of the aggregate, leading to higher local porosity. The ITZ has been observed by scanning electron microscopy [9], X-ray diffraction and microhardness [10] to extend up to 50µm from the aggregate surface. Bentz et al. [11] indicate that the ITZ thickness approximately scales as median cement particle size; while Ping and Beaudoin [12] found that the type and size of the aggregate plays a role in the thickness of the interfacial zone. Winslow et al. [13] found that above a critical aggregate volume, a marked change was observable in the pore size distribution. The authors attribute the difference to the interfacial zone. However, Diamond [14] cast the same mortars and found no evidence that highly porous areas occurred only near the aggregate surface. Differences in the pore size distribution are due to the percolation of porosity throughout the paste and perhaps due to the unconventional reciprocating mixing action [14]. In spite of the limitations of mercury intrusion porosimetry, two useful pieces of information can be gathered from the technique [15]. The threshold radius, the pore radius at which mercury intrusion starts in any appreciable volume, is a qualitative measure of the percolation of the pore system. The method also gives reasonable quantitative measure of total porosity. These two values can be related to transport properties. The threshold radius can be used to evaluate the discontinuity of the pore system. The total porosity measured may be indicative of the capillary porosity. The pressures typically obtainable in MIP, 0.2 to 415 MPa, relate to pore diameters of approximately 8µm and 4nm respectively, depending on the surface tension used. Pore sizes in the range of gel pores are too small (1 - 3nm) to be measured at the pressures achievable using MIP.
As has been mentioned previously, the work of T. C. Powers and his colleagues at the Portland Cement Association [16] form the basis for codes and standards regarding the maximum water to cement ratio and curing requirements. It must be remembered that the source of this data is a set of permeability experiments performed on small, mature, leached cement paste samples. Further, the apparatus used was not capable of providing either the high pressure required or the precise measurement of low flows necessary to evaluate low permeability mixtures (i.e. less than 10-14 m/s). A few details of the experiments follow to emphasize these points. The paste samples were cast in test tubes 28.6 mm (1 ⅛ inch) in diameter. The test tubes were stored vertically with the addition of excess water prior to sealing with stoppers. After the desired length of curing, the permeability specimens were taken from the central portion of the length of the sample and tapered with a lathe to form truncated conical specimens to facilitate sealing in the permeability apparatus. No reference is made to the thickness of the permeability specimens, but from sketches and photos in Powers et al [16], they can be estimated as approximately 12 mm (~0.5 inch). In an internal report, Powers [17] gives the thickness as 9.5mm (3/8”). The permeability apparatus supplies pressurized water produced by standpipes of mercury. The pressure differential for most specimens was about 3 atmospheres (0.3 MPa) [16]. The outflow from the apparatus was determined by measuring the change in the water meniscus in a calibrated capillary tube of 1.016 mm diameter by means of a micrometer microscope. The lowest permeability reported by Powers et al. [16] is of the order of 10-14 m/s, given the experimental configuration that would relate to a change in the height of the meniscus of less than a micron a day. Powers and Brownyard [18] “observed that the degree of permeability was controlled mainly by the capillary porosity” [2]. In the course of further hydration, the capillary pores become disconnected and the permeability is controlled by the “gel pores”. Given that the capillary pore system presents the pathway for the ingress of deleterious substances, it follows that the formation of a discontinuous capillary pore system is highly desirable. Powers [19] suggests that moist curing of field concrete past the point of achieving discontinuity is of little value. The theories of permeability in porous media arise from two schools of thought. One is the application of the Poiseuille-Hagen law (used by Hughes [20]); the other considers viscous drag of moving fluid on a particle. Powers and his coworkers [21] took the second approach; which uses Stoke’s law as a basis. (Stoke’s law is also used when determining particle size distribution of small particles, such as silt and clay.) As the concentration of particles increases, the Stoke’s velocity decreases. Steinour [22] determined a function for the variation in particle concentration using data derived from tapioca suspended in oil. The actual function used is proportional to the inverse of the hydraulic radius, dependent on particle concentration and temperature. The theory was extended to hardened cement paste since it is a porous solid with particle connections involving a small fraction of the surface, and can be thought of as a collection of particles. To define discontinuity, permeability experiments were performed on cement paste specimens approximately 600 days of age, leached of all alkalis [21]. The alkali was leached into the curing water placed on top of the cement paste at the time of casting. In their previous reserach, residual alkali was found to slightly decrease permeability. The experiments were designed to minimize variations due to osmotic pressure by using leached specimens. The cement is described as ultrafine with a Blaine of 8000 g/cm2 (sic?). Fine cement hydrated for an extended period, led to completely hydrated specimens free of capillary pores. Four water to cement ratio
pastes were tested at four temperatures. A line was fit to the four data points at each temperature relating the inverse of the hydraulic radius to a function of permeability, leading to the following relationship:
( )C
CTC
CK−
+−−=
− 13.0539log13154.2
)1(10log 2
12 θη (1)
where, K = coefficient of permeability in cm/s, C = particle concentration, i.e., volume of solids per unit volume of specimen, η(θ) = viscosity of the fluid [poises] as a function of temperature [ºC], T = temperature [K]. At 27ºC, the relationship becomes:
( )
−
−=− C
CC
CK1
097.22.41
10log2
12 (2)
Given that porosity of the gel pores is 26%, the capillary porosity, p, at complete hydration is given by:
( ) 26.01 −−= Cp (3)
Solving for K in terms of p yields:
Kpp
pp
= −−+
−
−+
−
10 4 2 2 097
0 740 26
0 740 26
122exp . ...
log.
( . ) (4)
Figure 1 shows this equation plotted with data from Powers et al [2] for cement pastes of various water to cement ratios (shown next to the points in the Figure). The line defines discontinuity. Once the paste has hydrated enough to reach this line, it will continue to move along it. Temperature differences are minimal, when 18ºC is used in Equation 1, the two lines are imperceptibly different. The decrease in porosity and permeability with time for a cement paste with 0.71 water to cement ratio can be seen in the Figure. For these experiments, the cement had composition of 45.03% C3S, 25.80% C2S, 13.34% C3A and 6.69% C4AF with fineness of 1800 cm2/g Wagner (approximately 300m2/kg Blaine [23] or 260m2/kg [24]). The highest permeability corresponds to the permeability calculated from bleeding rates. The next point was established on the fourth day and the final point on the twenty-sixth day after casting. Later permeability measurements yielded points slightly below the line signifying capillary discontinuity. However, alkali remained in the specimens accounting for the somewhat lower permeability. Had the specimens been leached of all alkalis, the terminal points for the paste made with the coarser cement would lie on the line. The particle concentration was calculated by non-evaporable water content (hydration) of pastes. (Mercury intrusion porosimetry only became available commercially after the work of Powers [6]).
-0.05 0.05 0.15 0.25 0.35 0.45Capillary Porosity (as fraction of total volume)
Perm
eabi
lity
(m/s
)
Equation 4 Powers et al (1959a) dataPowers et al (1954) 0.71 with time
Discontinuous Capillary Pores
Continuous Capillary Pores
0.80
0.50
1.00
0.90
0.71
0.640.600.55
Figure 1: Definition of Discontinuity for full hydration “The conclusion about continuity or lack of continuity of capillaries rests on conformity or lack of conformity of the data to an equation for the permeability based in the assumption that resistance to flow through a granular body is determined by viscous drags on the individual particles composing that body” [2]. When the flow through the paste does not follow the law, it is assumed that continuous capillary pores exist. The assumption that no continuous capillary pores existed was verified by confirming that the solid surface area calculated by the permeability relationship matched that measured by water absorption. The presence of continuous capillaries would not produce the same result. Table 1 [2] lists the estimated time required to achieve discontinuity of pastes under standard laboratory conditions. The time was based on hydration of ordinary Type I cement (which is certainly lower C3S and coarser particle size distributuion). The degree of hydration, or maturity, required to achieve discontinuity was estimated from the intersection of measurements of permeability with time for pastes at two water to cement ratios (0.64 and 0.71) with the line signifying discontinuity, resulting in the following relationship:
Table 1: Time required to achieve a discontinuous pore structure [2]
W/C Time Required Approximate degree of hydration required
0.40 3 days 0.50
0.45 7 days 0.60
0.50 14 days 0.70
0.60 6 months 0.95
0.70 1 year 1.00
>0.70 Impossible >1.00
2. Materials, Casting and Experimental Methods
Fourteen concrete mixtures form the basis of the test results this paper. Program parameters investigated were water to cementing materials ratio, water content, and inclusion of supplementary cementing materials. Portland cement mixtures with water to cementing materials ratios investigated ranged from 0.30 to 0.90. The large range was used to determine the variation in magnitude of transport properties observed in concrete used for all types of construction from high performance to residential applications. Supplementary cementing materials, such as silica fume, fly ash and ground granulated blast furnace slag, were used at typical replacement levels (7, 20 and 35% respectively). Unit water content of concrete mixtures was investigated independently of w/cm to determine to what extent it influences transport properties. The mixture proportions are given in Table 2. Cementing materials consisted of low-alkali ASTM Type I Portland cement (PC) with Bogue composition of 57.4% C3S, 15.6% C2S, 8.5% C3A, 7.9% C4AF and the same cement blended with approximately 7% silica fume (SF) from the Lafarge Woodstock (Ontario) plant; a CSA Class CI fly ash (FA) from Columbia Unit #1 with 17.5% CaO; and a Grade 80 ground granulated blast furnace slag (SG) from the Lafarge Hamilton (Ontario) plant. The fine aggregate, a local glacial sand, had a density of 2700 kg/m3, an absorption of 0.8%, and a fineness modulus of 2.56. A crushed 10mm limestone with a density of 2670 kg/m3 and absorption of 1.76% was used as the coarse aggregate. Concrete mixtures included an ASTM Type A water-reducer and a naphthalene sulfonate-based superplasticizer were used to obtain workable mixtures. A 20-litre pan mixer was used to mix successive batches of a particular concrete mixture to yield sufficient quantity of concrete (and mortar) for all tests performed. Concrete was cast into 100mm by 200mm cylinders. Mortar recovered from fresh concrete passing a 5mm sieve similar to ASTM C403 was used for mercury intrusion porosimetry. The mortar was cast into 30mm by 45mm cylinders. Similar cement paste mixtures were mixed in a Waring blender. Paste samples were cast in 30mm by 45mm molds, sealed and then rotated to prevent segregation and bleeding. Concrete, mortar and paste cylinders were removed from the molds 18 to 24 hours after casting and stored in lime-saturated water until testing.
0.90 PC 150 167 - - - 162 1200 700 *CSA Type 10SF Blended cement with ~7% silica fume
The following tests were performed on all concrete mixtures at 28 days:
Water permeability in high-pressure cell ASTM C1202 - Rapid Chloride Permeability Test ASTM C1556 - Bulk chloride diffusion Sorptivity Mercury intrusion porosimetry of mortar and paste
After approximately 28 days, one specimen from each concrete mixture was tested in a high-pressure permeability cell. The apparatus the same as that described by El-Dieb and Hooton [25]. The permeability cell is a Hassler type triaxial cell developed to test permeability of rock. The cell is able to withstand the high pressures (up to 24.5 MPa) necessary for measuring flow in materials with permeability as low as 10-16 m/s. Confining water pressure was applied to a 10mm thick neoprene sleeve using a Haskel pump to amplify pressure from laboratory compressed air supply. The confining pressure on the neoprene sleeve also prevented flow around the circumference of the sample. The pressure driving system consisted of a lever arm arrangement with hanging weights providing force to a stainless steel piston. The driving pressure (pressure gradient) used in this study ranged from 5.5 to 7 MPa (800 to 1000 psi). The
confining pressure was at least twice the driving pressure, in the range of 13 to 17 MPa. Driving and confining pressures were measured with pressure gauges for visual confirmation and by pressure transducers connected to a datalogger. Inflow and outflow were measured with linear voltage displacement transducers with micrometer resolution also connected to the datalogger. The outflow piston is 12.7 mm diameter allowing accurate measurements of small volumes of flow. Transparent tubing capable of withstanding 9 MPa (1300 psi) pressure was used throughout to allow for the detection of air bubbles. Replicate specimens were tested at 28 days according to ASTM C1202. Measured coulombs were adjusted for the actual area of the specimens. Two 50mm thick by 100mm diameter concrete slices were removed from the lime-saturated water tank a few days before reaching 28 days of age for testing chloride diffusion, following ASTM C1556-03. The specimens were allowed to air dry for a few hours then coated on the circumference and one flat face with epoxy. After the epoxy hardened, the samples were vacuum-saturated with tap water, wiped saturated surface dry then immersed in 165g/L NaCl solution for 35 days. For sorptivity testing, two 50mm thick by 100mm diameter specimens were dried in a 50ºC oven for 7 days. The specimens were allowed to cool to room temperature in a desiccator prior to recording the initial mass. To simulate one-dimensional flow, the sides of the specimens were sealed with vinyl electrical tape. The samples were placed onto plastic grates in containers of room temperature water, allowing water to wet the entire concrete testing surface but not more than 1-3mm deep. The mass was then recorded after 1, 2, 3, 4, 6, 9, 12, 16, 20 and 25 minutes of exposure of the surface to water. Sorptivity was determined by linear regression of cumulative sorption vs. square root of time data for the time period from 1 to 25 minutes. Mortar and paste specimens for mercury intrusion porosimetry were crushed using a mortar and pestle. Particles passing the 2.5mm sieve and retained on the 1.25mm sieve were kept for analysis. Particles were immersed in propanol for a minimum of 24 hours. Solvent replacement was followed by drying in a 50°C vacuum oven for a minimum of 24 hours. A Quantachrome Autoscan 60 capable of maximum pressure of 415 MPa was used for mercury intrusion. The contact angle was assumed to be 140º. The determination of total porosity and threshold radius are shown schematically in Figure 2.
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.001 0.01 0.1 1 10Pore Radius [microns]
Intr
uded
Vol
ume
[cm
3
0E+00
5E-06
1E-05
2E-05
2E-05
3E-05
3E-05
4E-05
Inflection point of intrusion curve gives threshold radius
Total porosity
dV/dP curve
Figure 2: Determination of Total Porosity and Threshold Radius for MIP curve
Table 3 gives the results for the tests concerning transport properties. For the Portland cement mixtures, increasing water to cement ratio gives generally increasing water permeability, cumulative charge passed, apparent chloride diffusion and sorptivity. The results for the 0.35 PC mixture are inconsistent. For this concrete mixture, the high-pressure permeability result seems slightly high compared to adjacent water cement mixtures, the ASTM C1202 results are higher than the 0.40 PC (150) mixture, while the chloride diffusion results are lower than the 0.30 PC mixture. The concrete mixtures containing supplementary cementing materials performed better than mixtures containing only Portland cement at similar water to cementing materials for transport mechanisms other than sorptivity. The investigation varying water content (and therefore cement and cement paste content) at the same water cement ratio produced conflicting results. It was expected that as the cement content increased the transport would increase as well. Only the ASTM C1202 results support this concept.
Table 3: 28 day test results
Concrete Mixture High Pressure Permeability [10 –15 m/s]
ASTM C1202 [Coulombs]
Bulk Diffusion [10 –12 m2/s]
Sorptivity [10-5 m/s 1/2]
0.30 PC 8.4 3200 21.9 0.85
0.31 HPC Steam 2.9 350 2.49 0.71
0.35 PC 52 5040 17.2 0.94
0.35 HPC 7.0 490 3.79 0.85
0.40 PC (135) 114 4770 28.3 0.64
0.40 PC (150) 59.1 4510 25.7 0.78
0.40 PC (170) 50 5380 31.9 0.95
0.40 35% SG 8.6 1040 8.14 1.06
0.40 20% FA 22 3420 8.8 1.40
0.40 7% SF - 850 5.21 0.88
0.55 PC 80 5670 37.1 1.08
0.69 PC 5500 8590 41.1 -
0.70 PC 394 6400 - 1.27
0.90 PC 3790 8720 - 1.77 Table 4 gives the total porosity and threshold radius for the mortar and paste as determined by mercury intrusion porosimetry. The water to cement ratio controls initial porosity, but even after 28 days of hydration, total porosity generally increases with increasing water to cement ratio for both mortar and paste specimens. The mortar samples were sieved from fresh concrete and contained aggregate smaller than 5mm. The average fine aggregate content of the mortar
was approximately 55% by volume. When the mortar is corrected to a paste basis by volume, the porosity of the paste is slightly higher than that of the mortar, giving an indication that the sieving procedure retains a portion of the fine aggregate fraction. The porosity is less when supplementary cementing materials are present. The effect of water content can be seen to decrease mortar porosity. Threshold radius decreases with decreasing water to cement ratio. The mortar threshold radius for the 0.55 PC and 0.69 PC mixtures seems abnormally low compared to the other values. Again the effects of slag, fly ash and silica fume substitutions is evident. The 0.40 mortar mixtures with varying water content results seem low for the mortar threshold radii. As expected, the higher paste volume of the 0.69 PC concrete mixture, gives higher transport properties and mortar porosity. The results of this mixture are similar to the 0.90 PC mixture. Here is the higher water to cement ratio is offset by lower paste volume and the addition of powdered silica.
The relationship between mortar porosity and permeability is shown in Figure 3. For this Figure, the Powers data and equation shown in Figure 1 were transformed to mortar porosities and concrete permeabilities based on an average volume basis. (The average paste to mortar ratio, 0.461 and average paste to concrete ratio, 0.267 were used for conversion.) Other than the 0.69 PC and 0.90 PC mixtures, the data from this study are seen to fall near the equation of Powers, indicating that all other mixtures achieved capillary discontinuity at 28 days. Figure 4 gives the correlation to the porosity measured using paste specimens. This time, the equation of Powers remains as the original, but the permeability of the paste from these experiments is
estimated by dividing the concrete results by the paste to concrete volume ratio. In this instance, all mixtures are near the line defining discontinuity. The paste to mortar volume for the high water to cementing materials mixtures was low due to the high proportion of fine aggregate used for these mixtures. From this observation, it is expected that these mixtures are at the limit of obtaining discontinuity. The effect of time on the development of discontinuity is also shown in Figure 4. A marked difference can be seen in the data of Powers et al. [21] and Nokken and Hooton [26] can be seen. The difference may be due to cement composition or the fact that Powers et al did not measure porosity directly, but rather as determined from hydration measurements.
1.E-17
1.E-15
1.E-13
1.E-11
1.E-09
1.E-07
0 5 10 15 20Porosity of Mortar (%)
Perm
eabi
lity
of c
oncr
ete
[m/s
]
0.30 PC 0.31 HPC 0.35 PC 0.35 HPC0.40 PC (135) 0.40 PC (150) 0.40 PC (170) 0.40 35% SG0.40 20% FA 0.40 7% SF 0.55 PC 0.69 OPC0.70 PC 0.90 PC Powers line Powers Data
Figure 3: Relationship to Powers equation for mortar porosity
Data from this paper 0.69 with time [26]Equation 4 0.71 w time [21]
Equation 4
Figure 4: Relationship to Powers equation for paste porosity
Figure 5 shows the relationship between saturated water permeability and water to cementing materials ratio. The frequently referenced data of Powers et al. [16] is shown for comparison. As was mentioned previously, the limitations of the apparatus of Powers et al. [16] was approximately 10–14 m/s. The data for that paper can only be estimated from published charts, giving rise to considerable interpretation at low water to cement ratios. The marked increase in permeability, particularly above 0.60 water to cement ratio, can be clearly seen. To observe this relationship more clearly, Figure 6 shows the permeability on a logarithmic scale.
0
1000
2000
3000
4000
5000
6000
0.00 0.20 0.40 0.60 0.80 1.00
Water to Cementing Materials Ratio
Perm
eabi
lity
[10-1
5 m/s
]
Data from this paper Powers et al. (1954)
Figure 5: Permeability vs. Water to cementing materials ratio
Figure 6: Permeability vs. Water to cementing materials ratio (log scale)
Interestingly, the concrete permeability data from this paper agrees surprisingly well with the data published for paste samples. Those concrete mixtures containing supplementary cementing materials fall below Powers data due to the decreased porosity and the increased sensitivity of the equipment used. The measured permeability of the 0.69 PC concrete mixture appears higher than that of Powers on both scales, likely due to the higher paste content. The importance of capillary porosity is major factor influencing permeability. In the case of the 0.69 PC and 0.90 PC concretes, the capillaries appear at the limit of discontinuity in Figure 3. This feature is apparent when observing the relationship between permeability and water to cement ratio as well. Powers found that capillaries would achieve discontinuity below 0.70 water to cement ratio. But the cement composition and the length and method of curing were unlike that used in this study. Saturated water permeability was correlated to the other properties measured. Good correlations were found between permeability and the ASTM C1202 test (Figure 7, R2 = 0.86), mortar porosity (R2 = 0.82), and paste porosity (R2 = 0.87). Resistance to flow, whether fluid or electrons, occurs through the capillary porosity. The good correlation in this regard is perhaps not surprising. The ASTM C1202 test is recognized to measure resistance to an electrical current, not resistance to chlorides. The relationship between apparent chloride diffusion and the ASTM C1202 test, not shown here, was not as significant (R2 = 0.76), but still reasonable. There was one considerable outlier for this relationship: the 0.40 20% FA mixture, which is unexplained.
Figure 7: Relationship between Permeability and ASTM C1202
5. Conclusions
The transport of fluids into concrete is controlled primarily by the capillary porosity. Decrease in water to cementing materials ratio and the replacement of cement with supplementary materials will decrease the volume and size of the capillary pores. Frequently referenced work by Powers et al. [2, 16] on cement paste was compared to data generated using modern cement composition and concrete mixture design. It was found that the relationship between water to cementing materials ratio for this study using concrete was similar to the published data of cement paste. Further, the discontinuity relationship holds for the concrete mixtures studied. The maximum water to cement ratio for the limit of discontinuity in concrete would seem to be below that suggested by Powers (0.70), depending on paste content. The effect of time on permeability and porosity can be seen in the development of discontinuity. Over the 28 days of curing, the 0.69 PC mixture approached the line Powers used to define discontinuity. It appears that the addition of aggregate affects the development of capillary discontinuity only at higher water to cement ratios. Although numerous microstructural observations of the interfacial zone have been published, evidence of a permeable interfacial transition zone does not seem to be apparent in the measured transport properties measured in this study. Due to the debate concerning the interfacial zone [14], more studies need to be completed with respect to transport properties measured in the composite concrete material. Saturated water permeability results correlated well to mortar and paste porosity, and the paste threshold radius, highlighting the importance of porosity on water transport. Permeability values also related well to the ASTM C1202 values, indicating that although the test has been highly criticized that it remains a useful rapid indicator of permeability and hence durability.
The authors would like to thank the Natural Sciences and Engineering Council of Canada and the Portland Cement Association for providing funding for this project. The kind assistance of Ms. Mirela Saraci at the University of Toronto is greatly appreciated for performing the mercury porosimetry intrusion experiments.
7. References
1. Feldman, R.F. and P.J. Sereda, A model for hydrated Portland cement paste as
deduced from sorption-length change and mechanical properties. Materiaux et Constructions, 1968. 1(6): p. 509-520.
2. Powers, T.C., L.E. Copeland, and H.M. Mann, Capillary continuity or discontinuity in cement pastes. Journal of the PCA Research and Development Laboratories, 1959. 1(2): p. 38-48.
3. Neville, A.M., Properties of concrete. 1996, New York: John Wiley & Sons. 4. Hansen, T.C., Physical Structure of Hardened Cement Paste. A Classical Approach.
Materials and Structures, 1986. 19(114): p. 423-436. 5. Hearn, N., R.D. Hooton, and R.H. Mills, Pore structure and permeability, in
Significance of tests and properties of concrete and concrete-making materials, P. Klieger and J.F. Lamond, Editors. 1994, ASTM: Philadelphia. p. 240-262.
6. Young, J.F., A review of the pore structure of cement paste and concrete and its influence on permeability, in Permeability of Concrete, D. Whiting and A. Walitt, Editors. 1988, American Concrete Institute: Detroit. p. 1-18.
7. Shane, J.D., et al., Effect of the interfacial transition zone on the conductivity of portland cement mortars. Journal of the American Ceramic Society, 2000. 83(5): p. 1137-1144.
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