Concrete durability - An approach towards performance testing

Transcription

1 Materials and Structures/Matériaux et Constructions, Vol. 32, April 1999, pp RILEM TC 116-PCD: Permeability of Concrete as a Criterion of its Durability Final Report Concrete durability - An approach towards performance testing RILEM TECHNICAL COMMITTEES Former and present full and corresponding members of the TC as well as members of the research consortium: C. Andrade, Spain; A. Bettencourt-Ribeiro, Portugal; N. R. Buenfeld, UK; M. Carcasses, France; N. J. Carino, USA; F. Ehrenberg, Germany; C. Ewertson, Sweden; E. Garbocczi, USA; M. Geiker, Denmark; O. E. Gjorv, Norway; A. F. Goncalves, Portugal; H. Gräf, Germany; H. Grube, Germany; H. K. Hilsdorf, (chairman ), Germany; R. D. Hooton, Canada; J. Kropp, (secretary , chairman since 1992 and project coordinator), Germany; S. Modry, Czech Republic; Ch. Molin, Sweden; L. O. Nilsson, Sweden; J. P. Ollivier, France; C. L. Page, UK; L. J. Parrott, UK; P. E. Petersson, Sweden; F. R. Rodriguez, Spain; M. Rodhe, Sweden; M. Salta, Portugal; N. Skalny, USA; A.M.G. Seneviratne, UK; L. Tang, Sweden; F. Tauscher, Germany; R. Torrent, Argentina; D. Whiting, USA. FOREWORD Concrete durability has been a key issue for many years due to its high significance in the serviceability of concrete or reinforced concrete structures as well as to its tremendous economic impact in the construction sector. Accordingly, the subject has been studied in all countries using this material and construction technique, thereby focusing on domestic construction traditions with locally available materials that are subjected to local exposure conditions. In consequence, national requirements on durability control emerged from long-term experience in the respective regions. An increasing international cooperation and exchange of knowledge, technology, services and goods, however, demonstrated the need to substitute the regional empirical approach for a more fundamental treatment of durability with internationally recognized requirements on its control. As an example, the creation of the Common Market in the European Union demonstrated the need for supra national standards, and in the Construction Products Directive (CPD) of the European Commission, materials specifications based on performance criteria are preferred over descriptive requirements on the composition, which often obstruct the introduction of new materials and technologies onto the market. In 1989, the RILEM Technical Committee TC Permeability of Concrete as a Criterion of its Durability took up the task of investigating methods for performance testing of concrete in view of its potential durability during the service life of a concrete structure. Based on the fundamental mechanisms involved in the most important corrosion reactions not only for the concrete itself but also for the reinforcement, the transport of species through the interconnected pore spaces of concrete has been identified as a major materials characteristic and, in many cases, a rate controlling parameter for corrosion reactions. Although the volume and continuity of the internal pore system of concrete are determined by parameters such as concrete composition, degree of hydration and the quality of the aggregate/matrix interfacial transition zone, the perviousness of concrete can only be evaluated directly by a permeation test. However, various media, e.g. ions, gases and liquids, as well as different transport mechanisms, must be distinguished. TC 116-PCD has compiled an extensive state-ofthe-art report on the transport of species through concrete, the available methods for an early experimental evaluation of concrete transport properties and existing correlations between concrete transport parameters and the corrosion resistance of the respective concretes [1]. This review is focused on laboratory testing on sepa /99 RILEM 163

2 Materials and Structures/Matériaux et Constructions, Vol. 32, April 1999 rately cast companion specimens for either pre-testing or quality control purposes in concrete production. Increasing attention has also been paid to on-site testing of existing concrete or reinforced concrete structures for the transport properties of the concrete, in particular the concrete cover. More recent developments of speciallydesigned equipment for in-situ testing and their application are presented in [2] and [3]. Within the Technical Committee 116-PCD, a subgroup of 10 research laboratories continued the work on test methods for the characterization of concrete transport properties with an experimental research project. The work program and all experimental results and their evaluation were discussed in the Technical Committee; thus, the conclusions and recommendations emerging from the research program do represent the position of the Technical Committee 116-PCD. Under Contract No. MAT1-CT , the Commission of the European Community granted financial support to conduct an experimental research work which was aimed at developing standard test procedures for hardened concrete with an emphasis on durability control. As contractors and partners of the research consortium, the following European research and testing laboratories participated in different steps of the experimental program according to the agreed work plan: (1) Institut für Massivbau und Baustofftechnologie, University of Karlsruhe, Germany; (2) Instituto Eduardo Torroja, Consejo Superior de Investigaciones Scientificas, Madrid, Spain; (3) COWI Consulting Engineers and Planners AS, subcontracting Ramboll, Hannemann and Hojlund AS, both in Lyngby, Denmark; (4) Laboratorio Nacional de Engenharia Civil, Lisbon, Portugal; (5) Forschungsinstitut der Deutschen Zementindustrie, Düsseldorf, Germany; (6) Department of Building Materials, Chalmers Technical University, Göteborg, Sweden; (7) Laboratoire Matériaux et Durabilité des Constructions, Institut National des Sciences Appliquées, Université Paul Sabatier, Toulouse, France; (8) Institut für Baustoffe, Massivbau und Brandschutz, University of Braunschweig, Germany; (9) Department of Civil Engineering, Aston University at Birmingham, Great Britain; and (10) Swedish National Testing and Research Institute, Boras, Sweden. In the summary report hereafter, an overview of the work completed and the major results are presented. Details on the work plan of the project and the results are given in the interim reports of the individual partners to the project coordinator (TC chairman, Stiftung Institut für Werkstofftechnik, and Hochschule Bremen, Bremen, Germany) and his four Milestone Reports to the European Commission. 1. ABSTRACT In the experimental research project, standard test methods are developed for the measurement of transport coefficients of concrete for gases, water and chloride ions. Since the moisture concentration in concrete is a most important parameter in affecting the transport parameters, in the first work package of the project a preconditioning method was developed to install a controlled water content with a homogeneous distribution in the test specimens. The effectiveness of preconditioning was evaluated in an initial intercomparison testing. Subsequently, a selection of frequently used test methods for laboratory and on-site testing of the gas permeability, the absorption of water and the penetration of chloride ions were applied to a range of concrete grades. This enabled evaluating the suitability and range of applicability of the methods, proposing improvements and correlating the measured transport parameters with durability characteristics. The Cembureau set-up was then selected as a standard method for gas permeability measurements; for the capillary absorption of water, a modified ponding test was recommended. For the measurement of chloride ion ingress, a migration test and an immersion test were evaluated; these tests had been used in the past for pre-testing of concrete mixes and in production control during the erection of advanced structures. However, both methods have not yet been recommended as routine tests in quality control. In a final intercomparison testing, the selected methods showed good agreement among the participating laboratories. Test instructions, which may serve as drafts for European standards on test methods for concrete, are proposed for the preconditioning of test specimens, for the measurement of gas permeability and for the determination of the capillary absorption of water on separately cast companion specimens. 2. INTRODUCTION AND OBJECTIVES In current national and international codes and standards on the production and use of concrete as a structural material, minimum requirements are specified for the quality of the concrete-making materials and their proportioning as well as for placing and handling of the fresh concrete. However, no requirements have been set forth as of yet for the actual performance of concrete under a corrosive attack. For the majority of the corrosion mechanisms encountered in practice, the ingress of aqueous solutions, whether gases or ions, from the environment into deeper sections of a concrete member is required to initiate and support damaging reactions. Thus, the perviousness of concrete for deleterious media controls the corrosion rate and transport parameters which provide quantitative information on the penetration of various media into concrete and could serve as a performance criterion for concrete durability. The objective of the research project lies in developing standard test methods for the determination of concrete transport coefficients, i.e. the coefficient of gas permeability, the capillary absorption of water and the chloride ion diffusion. These three transport mechanisms have been selected because they are either directly involved in corrosion reactions or they exhibit a close correlation with a corrosion rate of concrete [1]. Since the transport capacity of concrete largely depends on its water content [4, 5], the test methods must also consider a standardized preconditioning, which must then result in a controlled average 164

3 TC 116-PCD moisture concentration with a uniform distribution. Corresponding standard preconditioning and testing procedures are mandatory to obtain comparable results from these tests as a prerequisite for drawing correlations of the transport coefficients with concrete durability or deriving a classification of concrete with regard to its durability. Transport coefficients may be used for different purposes, e.g. a comparative tool to rank a number of similar concrete mixes, a characteristic to describe the open pore system of concrete or its potential durability, an input parameter for a numerical modeling of corrosion rates, for service life prediction, or for the evaluation of existing structures with regard to maintenance policies. This research project focuses on transport coefficients as criteria for concrete durability which are available at an early stage, e.g. during pre-testing of concrete mixes and routine testing in production control, as well as on materials coefficients for numerical modeling. Consequently, the test methods being sought should provide an accurate value for a true materials parameter, be suitable for routine testing at an early stage and be sensitive enough to distinguish different concrete grades. 3. WORK PROGRAM The experimental program was subdivided into four consecutive work packages with different scopes. 3.1 Work Package 1: Preconditioning The transport of media through concrete occurs in the interconnected capillary pore spaces, porous interfaces and micro-cracks. Depending on the exposure conditions, these spaces may be partially occupied by water films along the internal surfaces or even saturated with water. While a water saturated state provides for the optimum transport capacity for dissolved ions, the gas flow or the capillary activity vanishes. In contrast, the flow of gases as well as the capillary activity increases with decreasing moisture content while the ionic diffusion ceases for low moisture concentrations. Thus, the moisture concentration of the specimens exerts a significant influence on the observed flow of media. It is mandatory, therefore, to control the average moisture concentration of the test specimens, yet at the same time, a homogeneous distribution of the remaining water content must be assured. The necessary preconditioning of the test specimens should be completed at a concrete age of 28 days; thus, the measurement of transport coefficients could be introduced into the usual testing scheme for concrete. 3.2 Work Package 2: Intercomparison testing I In the first intercomparison testing, the effect of the remaining equilibrium moisture concentration in the test specimens, as a result of the preconditioning, was evaluated. Centrally-manufactured concrete specimens were cured for 3 and 7 days, respectively, and were then subjected to a preconditioning regime. At the age of 28 days, an initial testing at random was conducted on a limited number of specimens; subsequently, the whole set of test specimens was shipped to the partners in a sealed state for testing. At an age of 56 days and 180 days, respectively, the specimens were tested for their gas permeability and the capillary absorption of water. 3.3 Work Package 3: Evaluation of test methods and durability characteristics In Work Package 3, experience should be gained from known test methods that are often employed to measure the gas permeability, the capillary absorption of water and the diffusion coefficient of chloride ions in concrete. These transport cases, had been chosen due to their significance for a wide range of concrete corrosion mechanisms. For each of the selected transport cases, a large number of different test set-ups, equipment and procedures had been proposed by different researchers. The scope of WP3 was to evaluate the suitability of the individual methods and to further develop the procedures, if possible. For the measurement of gas permeability, the Cembureau equipment [6], the Schönlin laboratory method [7], as well as bore hole methods proposed by Parrott [8], Paulmann [9] and Figg [10], were employed. The capillary suction of water was measured both according to a ponding method proposed by Fagerlund [11] and using the measurement procedure comparable to an earlier RILEM Technical Recommendation, CPC 11.2 Absorption of water by concrete by capillarity [12]. Different specimens sizes were used in this testing however. The ISA test [13] and Figg s water absorption method [10] were also employed. The ingress of chloride ions was studied by migration tests [14, 15], basically related to the AASHTO [16] test, as well as by immersion of the test specimens into chloride solutions with subsequent profile analyses [17]. Based on the experience gained from the comparative study, one single test method should be proposed for each of the three transport cases as a standard. On ten different concrete test series included in WP 3, the compressive strength, the abrasion resistance and the depth of carbonation under laboratory conditions up to an age of 1.5 years were measured. 3.4 Work Package 4: Intercomparison testing II The selected standard methods resulting from WP 3, including their proposed amendments, were subjected to a second intercomparison testing with three different concrete mixes. All specimens were centrally manufactured and preconditioned and then shipped to the partners. Tests were conducted at a concrete age of 63 days and 182 days, respectively. The intercomparison should reveal the repeatability and the reproducibility of the selected methods. 165

4 Materials and Structures/Matériaux et Constructions, Vol. 32, April Materials For the experimental investigations, five different concrete mixes were designed which cover a range of concrete qualities typically encountered in the structural application of concrete. Not considered were extreme concrete compositions, e.g. low grade concretes with a high porosity, such as concrete with a honeycombed structure, or high strength concretes with very low water cement ratios. The strength class of the cement was 32.5 N/mm 2. In Table 1, characteristics of the concrete mixes are summarized: Table 1 Composition of concrete mixes Mix Type of cement Cement content Water cement [kg/m 3 ] ratio A CEM I B CEM I C CEM III A C CEM III A D CEM II B - V E CEM II A - D * * (where commercially available, this cement corresponded to CEM II 52.5 R). After casting of the fresh concrete in molds and compaction, the top side was covered with a plastic sheet until demoulding. The curing under wet burlap continued up to an age of 3 and 7 days, respectively. Test specimens for migration or immersion tests were not specially cured but stored under water until testing. In Work Packages 1 and 3, the participating partners produced their own test specimens on the basis of Table 1, using the locally available cement of the individual types and the locally available aggregates. Thus, rounded material as well as crushed rock was employed. Also, the grading of the aggregates followed national tradition, i.e. the maximum aggregate size varied between 16 and 32 mm for the different member states. In order to obtain a sufficient workability of the fresh concrete, plasticizers were used in some laboratories. 3.6 Size and shape of the test specimens For the gas permeability and the capillary absorption of water, the attempt was made to apply the same geometry of the test specimens, i.e. circular discs with a diameter of 150 mm and a thickness of 50 mm. Bore hole techniques and some capillary absorption experiments required concrete cubes with a side length of either 100 mm or 150 mm. Specimens for migration tests were cut from cylinders with a diameter d = 100 mm, length l = 200 mm which were then cut into discs of the appropriate height. 4. RESULTS 4.1 Preconditioning Drying methods The aim of the preconditioning consists of a uniform moisture distribution with a well-defined and pre-set average moisture concentration of the test specimen which is below the original moisture content after the curing regime. The necessary drying of the concrete must be accelerated in order to allow the testing within the 28-day period. The drying of concrete is accelerated by an elevated temperature that increases the diffusion coefficient for water vapor as well as by a low moisture concentration of the ambient air resulting in a steeper moisture gradient. Limitations exist however for the application of these drying conditions: elevated temperatures under moist conditions will enhance the hydration, but under dry conditions they may cause internal micro cracking of the concrete and the degree of the resulting pre-damage may depend on the concrete quality, e.g. increased micro cracking can be expected in dense and high-quality concretes. On the other hand, low ambient moisture conditions may cause large variations of the water content across the drying specimen. Two drying methods have been investigated, both applying an elevated temperature to accelerate the drying process, but the applied temperature was limited to only 50 C. The parameters of the drying regimes were fixed on the basis of desorption isotherms which were measured at 20 C and 50 C in the range of 55 to 95 percent relative humidity: Method I: The basic aim of drying method I was to precondition the test specimens to a moisture concentration which is in equilibrium with a 65 percent relative humidity at 20 C. In order to approach the equilibrium condition within a 28-day period, the drying process must be accelerated but, at the same time, no steep moisture gradients along the drying path in the specimens should develop. Therefore, it was attempted to maintain a high moisture concentration in the outermost zone of the concrete specimens which did not fall below the final equilibrium moisture concentration after the preconditioning. From the measured desorption isotherms, it followed that 85% relative humidity at 50 C corresponds to the same equilibrium moisture concentration in the concrete specimens as 65% relative humidity at 20 C, see Figs. 1 and 2. Test specimens were kept in ventilated containers at 85% relative humidity and 50 C over saturated salt solutions or in ventilated climate chambers, and the drying process was monitored by periodic weighing of the specimens. In Fig. 3, the drying behavior of 4 different test series is presented by the remaining relative water content of the specimens as a function of time. w wt w w w = ( ) o inf inf w(t) = water content at time t w o = original free water content w inf = equilibrium water content. 166

5 TC 116-PCD Fig. 1 Desorption isotherms for concrete A at 20 C and 50 C, respectively. Fig. 2 Desorption isotherms for concrete at 20 C and 50 C, respectively. 167 At w = 0, the specimen has achieved the equilibrium moisture condition. For very porous concretes with high water cement ratios, the accelerating effect of the elevated temperature brought the specimens close to their moisture equilibrium within a 28-day period, but for low ratios of w/c = 0.4 the method failed because the specimens were still far from their equilibrium condition after 28 days, see Fig. 3. Method II: The second drying method included the acceleration of the drying process both by increasing the diffusion coefficient of water vapor at an elevated temperature and by a steeper moisture gradient across the specimen. However, a second step in the preconditioning regime must then redistribute the remaining water profiles according to a homogeneous distribution. From the desorption isotherms, the necessary liberation of water of the individual concrete series was determined as the difference between the original water content and the equilibrium water content at 55, 65, 75, 85 and 93 percent relative humidity values, see Figs. 1 and 2. The test specimens were then dried in ventilated drying chambers at 50 C and approximately 10 percent relative humidity until the pre-set weight loss was achieved. Depending on the original water content and the porosity of the specimens, as well as on the desired degree of drying, the necessary duration of drying was between a few hours and approximately 3 weeks. Due to the low ambient humidity, moisture profiles occurred at the end of the drying phase. In a second step, the specimens were then kept in sealed containers over saturated salt solutions of appropriate relative humidity in order to redistribute the moisture profiles in the specimens. At an age of 28 days, the remaining moisture profiles were measured in terms of relative pore humidity in cavities as well as in terms of absolute water content by crushing small samples taken at various depths. These tests showed that the pre-drying should be limited to an equilibrium with 75 percent relative humidity in order to prevent excessive moisture gradients, see Fig. 4. For an equilibrium with a lower relative humidity of the ambient air, i.e. a higher degree of pre-drying, the remaining difference in water content between a specimen s surface zone and central portion was higher than 1 percent by dry mass of concrete. For all further experiments, the test specimens were pre-dried to an equilibrium with 75 percent relative humidity according to drying method II Effect of water content on transport properties The effect of the internal moisture concentration on transport properties was studied for concretes A and B, cured for 3 and 7 days, respectively. The test specimens

6 Materials and Structures/Matériaux et Constructions, Vol. 32, April 1999 were preconditioned according to method II in the range of 55 to 93 percent relative humidity and then tested at an age of 56 days. Fig. 5 shows the results of the gas permeability tests with the Cembureau method and Fig. 6 demonstrates the results of the capillary absorption tests. These results reveal the drastic drop in the transport coefficients with increasing moisture content, and for elevated moisture concentrations in equilibrium with 93 percent relative humidity, the tests revealed that no reliable measurements of the two transport coefficients were possible any longer. In a number of tests, no flow could be measured at all and for all measured values a high scatter of the results was observed. 4.2 Intercomparison testing I Fig. 3 Drying curves of concrete test specimens at 50 C and 85 percent relative humidity, w wt ( ) = w inf w w o inf The first intercomparison testing concentrated on the effectivity of the preconditioning by repeated measurements of the gas per- Fig. 4a Profiles of relative pore humidity across the thickness of the test specimens. Fig. 4b Profiles of relative pore humidity across the thickness of the test specimens. 168

7 TC 116-PCD Fig. 5 Effect of equilibrium moisture concentration on the gas permeability of concrete. The investigation was limited to concrete mix A cured for 3 days. Figure 7 presents the results of the permeability measurements at an age of 56 days. Due to the large number of required test specimens for all partners, the concrete was mixed and poured in 4 consecutive batches. Although the same materials were used and the same proportioning was attempted, a statistical analysis of the results of a pre-testing in the manufacturing laboratory revealed that the concrete batches exhibited differences in their air permeability. The results of the random pre-testing for each concrete mix are indicated in the diagram by the horizontal bars; the columns represent the results obtained in the participating laboratories on other specimens of the same batch. Considering the magnitude of the measured value, a satisfactory level of agreement was observed. In cases of increased deviations, the respective laboratory carried out further calibration tests with a porous body of known permeability. Figure 8 shows the results on the capillary absorption of water after 1, 4 and 24 hours of contact with water, for concrete ages of 56 days and 90 days, respectively. Since no difference in the absorption values was observed, it may be assumed that the specimens had effectively reached their moisture equilibrium during the preconditioning. Fig. 6 Effect of equilibrium moisture concentration on the absorption of water. meability and the water absorption after 56 days and 180 days, respectively. The required number of test specimens were manufactured, cured, preconditioned and subjected to a random pre-testing in one laboratory; then, the specimens were shipped in a sealed state to the participants. By virtue of the results of Work Package 1, the specimens were preconditioned to an equilibrium moisture concentration with 75 percent relative humidity at 20 C. After the initial testing at an age of 56 days, the specimens were stored in climate-controlled chambers at 75 percent relative humidity until the second testing at an age of 180 days Evaluation of test methods and durability characteristics The different test methods pre-selected for the comparative study were allocated to the participants so that each test method was employed by at least three laboratories; a comparison of the experiences and the obtained results could therefore be made. It was also within the scope of Work Package 3 to report typical values for the observed transport characteristics measured with different methods. Therefore, the test methods were applied to the whole range of concrete mixes A, B, C, C, D and E, with curing under plastic sheets for 3 and 7 days, respectively. The preconditioning of the test specimens for laboratory tests was conducted according to method II. In order to calculate the necessary loss of water, one point of the desorption isotherm had to be measured first, i.e. the equilibrium moisture content at 75 percent r.h. In Work Package 3, methods suitable for on-site testing were also employed. For these methods, additional test series were not preconditioned by accelerated drying, but rather were kept in a climate-controlled chamber at 75 percent r.h. until testing. Further application of transport parameters in durability studies was demonstrated with the help of correla-

8 Materials and Structures/Matériaux et Constructions, Vol. 32, April 1999 Fig. 7 Results of the intercomparison testing of gas permeability: the figures on top of the horizontal lines represent the results of pre-testing in the manufacturer s laboratory. Fig. 8 Results of the intercomparison testing on the capillary absorption of water. Fig. 9 Correlation between carbonation depth (1 year) and coefficient of gas permeability. tions between the measured transport parameters and durability characteristics. On the ten concrete test series, the rate of carbonation in a laboratory environment and the abrasion resistance measured in a grinding wheel test were observed. The experience gained from the individual methods for gas permeability tests showed that the Cembureau method is very reliable, easy to handle and exhibits very good repeatability. This method is recommended therefore as a standard test method for gas permeability measurements. All bore hole methods frequently failed due to leakage of the pressurized or evacuated bore hole either due to failure of the sealing or because of pre-damage introduced while drilling. For laboratory tests, it has been recommended therefore to cast the cavity instead of drilling [4]. Furthermore, the flow path of the permeating gas is not known in most cases; therefore, these methods may only allow a relative ranking of concretes based on an index instead of calculating a true materials parameter. Nevertheless, these methods may be used for on-site testing on a comparative basis, but it must be kept in mind that they are very vulnerable to moisture profiles in the near-surface zone, e.g. the concrete cover. A simultaneous measurement of the moisture concentration, as foreseen in Parrott s method [4], is mandatory for the interpretation of the results. Furthermore, the composition of the concrete in the surface zone is different from the deeper sections; results on the bore hole techniques do not represent the properties of the bulk concrete, and they do not closely correlate with results from Cembureau tests. For the capillary absorption of water, the modified Fagerlund test is recommended; it is easy to conduct and shows only very little scatter of the test results. For the measurement of chloride ion diffusion, no standard test method which could meet the set of requirements can be recommended as of yet. The immersion test yields an apparent diffusion coefficient for chloride ions since binding of chlorides by the cement s hydration products is not considered in the calculation of the diffusion coefficients from total chloride concentration profiles. The method requires too much time and labor for routine testing in production control, but it may represent a powerful tool for research or pre-testing of selected concretes. In the migration tests, concepts have been developed to include further analytical steps aside from recording the electric charge 170

9 TC 116-PCD passing; thus, the calculation of an apparent or an effective diffusion coefficient for chloride ions will be possible. Basically, the tests on the chloride ion diffusion exhibited a low scatter of the results within each laboratory, and they may serve already for a relative ranking of different concrete mixes. The compressive strengths of the ten concrete test series of the program covered a range from 20 MPa to approximately 80 MPa and as a general trend, the results showed that the transport of species through concrete decreases as the compressive strength increases. This observation is explained by the porosity of the concretes which controls both their strength and permeability. In order to demonstrate the testing concept, the observed durability characteristics were correlated with the measured transport characteristics. In all participating laboratories, a close correlation has been found for the rate of carbonation not only with the gas permeability, but also with the water absorption, e.g. as given in Figs. 9 and 10. Fig. 10 Correlation between carbonation depth (1 year) and water absortion. Fig. 11 Results of the intercomparison testing of gas permeability. The first column in each block represents the result of pre-testing in the manufacturer s laboratory; the last column gives the mean value of the intercomparison testing Second Intercomparison Testing Experimental results The second intercomparison testing concentrated on those test methods that emerged from Work Package 3 as recommended standard test methods. From Work Packages 1 and 2, pre-conditioning method II with an equilibrium moisture content at 75 percent r.h. was recommended as the standard. For the measurement of the gas permeability, the Cembureau method [2] was selected with minor modifications in the test parameters; for the measurement of the capillary absorption, the recommended method relates to Fagerlund s method [7], with amendments in specimen size and testing parameters. It was felt that insufficient experience is available at the present stage to recommend one single standard test method for the determination of the chloride ion diffusivity that meets all requirements. However, potential methods, which were further developed in Work Package 3, have been recognized. Therefore, corresponding tests as described in [10, 11, 13] were also continued in the frame work of intercomparison testing. On a limited scale, the Schönlin gas permeability test [3] was also considered because of its potential advantages in rapid testing. Three different concrete test series were included, i.e. mixes A, B and C, each cured for 3 days. After curing, the specimens for gas permeability and water absorption testing were preconditioned according to method II. Specimens for chloride diffusion testing were water saturated before their testing. After their shipment to the participating laboratories, the specimens were tested at an age of 63 days and 182 days, respectively. An initial evaluation of the test results shows that the Cembureau gas permeability is a very sensitive method for monitoring variations in the properties of the test specimens; consequently, a considerable scatter of the results is encountered due to unavoidable inhomogeneities in the material. However, the spread of the results obtained on different concrete qualities is also large; thus a clear distinction between different concrete qualities is provided. A further analysis showed that the scatter of the results observed between the participating laboratories is in the same range as the scatter of the results observed on different specimens in one laboratory. Fig. 11 presents the

10 Materials and Structures/Matériaux et Constructions, Vol. 32, April 1999 Fig. 12 Results of the intercomparison testing of the capillary absorption of water. The first column in each block represents the result of pre-testing in the manufacturer s laboratory; the last column gives the mean value of the intercomparison testing. Fig. 13 Repeatability of the test methods in Work Package IV at an age of the concrete test specimens of 63 days. results of the gas permeability tests at an age of 63 days. The resolution of the capillary suction measurements is less pronounced; however, this method exhibits only a small scatter of the test, results. Similar to the observations made for the gas permeability tests, the deviation between the participating laboratories is in the same range as the scatter of the results within one laboratory. Although the method is less sensitive, the very low scatter of the results allows distinguishing between different concrete qualities. In Fig. 12, the absorption data are shown for tests at an age of 63 days Repeatability and reproducibility of the test methods The test results from the intercomparison testing in the participating laboratories were subjected to an analysis of the achieved accuracy of the test methods. Based on the definitions and principles set forth in ISO 5725: 1994 [14], partner 10 investigated repeatability and reproducibility for the proposed standard test methods as well as for the Schönlin gas permeability and methods on the chloride ion penetration [15]. Migration data from partner 2 were not included in the data analysis because no parallel testing took place using exactly the same procedure. Fig. 13 shows the repeatability of the individual test methods. With reference to the definitions of ISO 5725, repeatability may be understood as the variability of the test results within one laboratory. In Fig.14, the reproducibility is shown for the test methods. In contrast to repeatability, the reproducibility describes the variability of the test results between the participating laboratories. The analysis shows that the variability of the test results between the laboratories is in the same range as the variability of results obtained in one laboratory for all test methods considered. The variability of the results is high for gas permeability measurements because of their high sensitivity to differences in concrete properties. In this connection, it must be assumed that no homogeneous set of test specimens was available for concrete mix C. The observed reproducibility of the Cembureau method in the range of 30% is acceptable in view of the method s high resolution. The absorption of water exhibited only a small variability in the test results which remained in most cases below 10%. The observation of the chloride ion diffusion was completed with rather low scattering of the test results using one particular method. A direct comparison of the different methods is not possible because the measured parameters include different implications. More research is needed in this field before a routine test method may be recommended. A new RILEM Technical Committee, TC TMC - Testing and Modeling the chloride penetration in concrete took up the challenge to investigate more deeply into mechanisms of chloride contamination of concrete. An outline of the planned work program is presented in [20], and the progress of work may be tracked on the TC home page under:

11 TC 116-PCD Fig. 14 Reproducibility of the test methods in Work Package IV at an age of the concrete test specimens of 63 days. 5. CONCLUSIONS In an experimental program, different test methods were evaluated with regard to their suitability for routine testing of concrete transport parameters. The results support the Cembureau gas permeability method and a modified Fagerlund method for the testing of the capillary absorption of water as routine test methods. Corresponding instructions are developed as RILEM Technical Recommendations [21, 22]; these may also serve as a basis of draft standards for testing hardened concrete. The test results strongly depend on the preconditioning of the test specimens. A standard procedure for preconditioning is therefore also mandatory. A method has been developed and the corresponding instructions are prepared for a RILEM Technical Recommendation or draft standard on preconditioning [23]. The measurement of concrete transport parameters is very sensitive to the test procedure, in particular to the water content of the test specimens. Comparable results may be obtained only if the same testing procedure is strictly followed. Therefore, consideration of the present test instructions is intented to provide a common basis for further research in which relations with durability characteristics must be included. On the basis of such correlations, limiting values for concrete transport parameters may be developed and compliance criteria may be derived. 6. REFERENCES [1] Performance Criteria for Concrete Durability, J. Kropp, H. K. Hilsdorf [Ed.], RILEM Technical Report No. 12, F &N Spon, London, [2] Basheer, P. A. M., Long A. E. and Montgomery, F. R., Clam tests for measuring the in-situ permeation properties of concrete, 173 Non-destructive Testing and Evaluation International 12 (1995) [3] Torrent, R., Frenzer, G., A method for the rapid determination of the coefficient of permeability of the covercrete, Proc. Int. Symposium on Non-Destructive Testing in Civil Engineering, Berlin, Germany, [4] Jacobs, F., Permeability to gas of partially saturated concrete, Mag. Concrete Research 50 (2) (1998) [5] Abbas, A., Écoulements gazeux dans les bétons partiellement saturés : Application à la mesure de perméabilité, Thesis, (in print), Institut National des Sciences Appliquées de Toulouse, France, Déc [6] Kollek, J. J., The determination of the permeability of concrete to oxygen by the CEMBU- REAU method - a recommendation, Mater. Struct. 22 (1989). [7] Schönlin, K. F., Permeabilität als Kennwert der Dauerhaftigkeit von Beton, Schriftenreihe Institutes für Baustofftechnologie, Universität Karlsruhe, [8] Chen Zhang Hong, Parrott, L. J., Air permeability of cover concrete and effect of curing, BCA Report No. C/5, October [9] Paulmann, K. and Rostasy, F. S., Praxisnahes Verfahren zur Beurteilung der Dichtigkeit oberflächennaher Betonschichten im Hinblick auf die Dauerhaftigkeit, Institut Massivbau, Baustoffe und Brandschutz, TU Braunschweig, [10] P-6000, Poroscope operating instruction, NDT James Instruments Inc., Chicago. [11] Fagerlund, G., The critical degree of saturation method of assessing the freeze/thaw resistance of concrete, Mater. Struct. 10 (58). [12] RILEM Technical Recommendation CPC 11.2: Absorption of water by concrete by capillarity, Ibid. (June 1982). [13] BS Test methods for hardened concrete other than compressive strength: Part V - Initial surface Absorption of Water, 1970 [14] Andrade, C., Calculation of chloride diffusion experiments from ionic migration experiments, Cement and Concrete Research 23 (1993) [15] Tang Luping, Chloride transport in concrete - Measurement and prediction, Publication P-96:6 Chalmers University of Technology, Department of Building Materials, Göteborg, Sweden, [16] ASTM-C : Standard test method for electrical indication of concrete s ability to resist chloride ion penetration. [17] Nord Test, NT Build 443: Concrete testing, hardened concrete, Chloride penetration. [18] ISO 5725:1994: Accuracy (trueness and precision) of measurement methods and results. [19] Tang Luping, Precision analysis according to ISO 5725:1994: Internal report to the coordinator of MAT1-CT93-001, Swedish National Testing and Research Institute, Boras, Sweden, 1998, unpublished. [20] RILEM Annual Report 1997, RILEM, Cachan, France, [21] RILEM Technical Recommendation: Measurement of the gas permeability by RILEM - CEMBUREAU method. [22] RILEM Technical Recommendation: Determination of the capillary absorption of water of hardened concrete. [23] RILEM Technical Recommendation: Preconditioning of concrete test specimens for the measurement of gas permeability and capillary absorption.