286-GDP: Test Methods for Gas Diffusion in Porous Media
Technical Committee 286-GDP
Deputy Chair: Philippe TURCRY
As early as in 1958, Verbeck mentioned that H2O and CO2 Gas Diffusion in Porous media was (GDP) a key physical process of concrete carbonation; 10 years later Hamada proved the diffusive nature of the carbonation of concrete by identifying the square root of time behavior of the “depth of neutralization”. 60 years later, different methods for measuring gas diffusion coefficient of cementitious materials have been developed but no technical consensus exists on those methods. This RILEM TC GDP would like to provide an overview of the available gas diffusion methods for building materials along with their pitfalls, benefits and drawbacks by a group of international experts.
The gas diffusion coefficient is a general indicator of the resistance to gas transfer of a given microstructure of a porous media. It is also a specific durability indicator of cement based materials. The TC would focus on reviewing the literature, benchmarking test methods on both reference inert materials and cementitious materials over a large range of gas diffusion coefficients ([10-11, 10-6] m2.s-1). The group would also focus on calculation method to analyze experimental results, on the evaluation of sources of uncertainty from the procedure, the diffusion cell design or the materials themselves, such as geometries or conditioning.
It is beyond the scope of this group to correlate the results of this test method to the results of actual durability performance of cementitious materials, such as carbonation depth in natural or accelerated conditions. Nevertheless, the field of application of the method will be included in the state of the art review.
Terms of reference
The activity of the TC is planned for three years as the objective is focused on building a consensus on gas diffusion test method by comparing testing principles, by quantifying uncertainties and by simulating tests with numerical models. First a state of the art report based on published literature will be issued on gas diffusivity in porous media, looking at the physics at play, the testing methods, range of values for building materials and usage of values for evaluating material performance. Not many labs are currently equipped with gas diffusion test methods. Therefore a benchmark of methods on reference inert materials and non-ageing cementitious materials is proposed. In the future, gas diffusion could be used as specific indicator of building material performance. The results of this TC will deliver experimental evidence and state of knowledge useful for the definition of future standards.
Confirmed partners are coming from 6 countries (France, Greece, Spain, Switzerland, UK and USA) and prospective partners have been identified in China, India, Hong Kong and Japan.
Detailed working programme
The working programme is organised in six tasks. After a joined effort of the TC GDP on literature review, the other 5 tasks can run for the most part in parallel until completion.
4.1. State of the art
Collecting information on existing gas diffusion tests is a key task. Thus, the first efforts of the TC will focus on a state of the art review. Although a rich literature on gas diffusion in porous materials existed in the 20th century and before, gas diffusion tests for cementitious materials were developed only from the 1970’s.
To our knowledge, the work of C.D. Lawrence was one of the forerunners and remains today the most cited. C.D. Lawrence designed a diffusion test whose principle is still used. This test consists in submitting both opposite sides of a specimen to two flowing gas streams. The gas diffusion coefficient is then deduced from the First Fick Law, when a steady state regime is reached. Another testing procedure consists in measuring the time-evolution of gas concentration at the downstream side of a specimen, while the gas concentration is maintained constant at the upstream side. The diffusion coefficient is calculated using the Second Fick Law from the obtained accumulation curve. In addition to these methods (steady state flux / accumulation), other test principles can be found in literature. For each procedure, benefits and drawbacks will be listed.
Beyond the compilation of diffusion tests designed for cementitious materials, the state of the art review will cover the gas diffusion phenomenon in porous solids. In particular, the following topics will be addressed:
- prevalence of diffusion mechanisms (Knudsen or molecular diffusion);
- effect of pressure and temperature on gas diffusion;
- conversion of gas diffusivity from species A to species B;
- physical sorption of diffusing species on cement matrix;
- effect of internal RH, phase assemblage and microstructure on gas diffusion;
- comparison of gas permeation and gas diffusion.
Applications of gas diffusion tests will be also addressed in the literature review. In the scope of academic research, gas diffusion could be used to enhance the understanding of microstructure of cement based materials. For engineering applications, gas diffusion coefficient could be a relevant durability indicator, for instance in a performance based-approach of the design of reinforced concrete structures. Modelling is the main tool to predict the service life duration of concrete structures. Examples of use of gas diffusion coefficient in analytical and numerical modelling will be provided.
4.2. Benchmark of existing methods on materials
Benchmarking exercises will be organized to compare the various gas diffusion methods used by the members of the TC group. First, reference dried specimens made with inert or non ageing materials will be tested by each participant. In particular, this will permit a comparison between transient and steady-state methods of gas diffusion.
Then, brainstorm will be required to define the cement materials which will be tested in a second phase. The main issues are the following: mix composition, drying state, hydration and carbonation state.
Beyond the diffusion tests with “usual” gas such as O2, diffusion tests will be also carried with reactive gas, such as CO2 or water vapor. For the latter, tests could be carried with the cup-test method. This will allow comparing gas diffusion coefficient and vapor permeability in dry conditions
4.3. Gas diffusion cells and procedure
Results of gas diffusion test can be affected by any pressure gradient between up and downstream compartment. Thus, works will be conducted to assess the effect of low pressure difference for each test method, resulting from the setup or multi-species diffusion. For that purpose, diffusion cells could be modified and pressure sensors could be installed if necessary. In the same manner, RH control probes could be installed in the diffusion cells, in order to investigate the influence of RH on diffusion.
4.4. Specifications of samples and materials
This task will include the following topics:
- effect of drying on diffusion coefficient and role of isotherm hysteresis;
- reactivity of the cement matrix with diffusing gas species;
- choice and calculation of the representative elementary volume with respect to specimen size;
- specimen geometry.
4.5. Uncertainties: ranking and evaluation of possible sources
Reliable tests require a knowledge of uncertainties. Among the uncertainties sources, the following will be assessed:
- pressure gradient;
- material variability;
- lateral leakage (in the case of 1D test);
- uniformity of water saturation degree within the specimen after a preconditioning period;
- comparison of 1D and 2D numerical of diffusion test (for instance, to assess the effect of sealing problem).
Different methods are considered to evaluate those uncertainties, including the analysis of existing dataset shared by participants and the use of numerical models. For example, the role of pressure or liquid saturation gradient can be investigated with numerical models. Uncertainty evaluation will also rely on experimental data from tasks 4.4 and 4.5. Furthermore, the range of measurement (i.e. upper and lower limits) have to be justified for each method and accompanied with a precision.
4.6. Calculation methods
Results of all existing diffusion tests are used to calculate the effective gas diffusion usually from Fisk’s Laws. This task will focus on different calculation methods of the diffusion coefficient. Moreover, numerical simulations of the various diffusion tests will be carried out to address the following issues :
- effect of the initial gas concentration in the specimen;
- effect of gravity, gas density or gas homogenization in the downstream compartment;
- choice of approach for gas diffusion modelling (one gas species diffusion or multiple gas species inter-diffusion);
- effect of entry parameters such as porosity, water saturation, reference gas diffusion coefficient;
- need to account for gas equilibrium with pore water for steady state or transient analysis.
Many former RILEM TCs have focused on testing methods for durability performance assessment of cementitious materials in various exposure classes. In the late 90s, TC 116-PCD focused on gas permeability of concrete as a criterion of its durability, and the TC 178-TMC focused on testing and modelling chloride penetration in concrete. Later, the TC 230-PSC (Performance-based specifications of concrete durability) focused on permeability and conductivity as an indicator of mass transfer processes.
The proposed activity is also related to the one of current TCs. In particular, the TC 281-CCC is focusing on carbonation of supplementary cementitious materials. Some materials tested in that TC could be also evaluated within the proposed TC GDP. The TC 262-SCI, on the initiation of chloride-induced reinforcement corrosion, would be interested in method to evaluate the availability of gas at the steel concrete interface as criteria for depassivation with or without macro-cell corrosion current.
The activity of this group should also be coordinated with other projects and communicated to other organizations. In particular, the FIB commision 8 (COM8) is focusing on durability. Coordination with the subgroup 8.3, focusing on service model (fib MC2010) as a tool for service life design, and with the subgroup 8.6, focusing on threshold and reliability index, will be of key importance.
Also, the results of this group would be of interest to technical bodies of the European committee for standardisation (CEN): CEN/TC104/SC1 focusing on concrete performance and CEN/TC 51/WG 12 focusing on special performance criteria. A joined working group of the latter two TCs is evaluating test methods for durability assessment.
In France, a large project PERFDUB is working on a methodology for justifying durability of concrete (and concrete structures) with a performance-based approach: a correlation between gas diffusion coefficient and other durability indicator could be therefore envisioned.
The TC will deliver the following achievements:
- a state-of-the-art report addressing the relevance of gas diffusion, the physical principles, the various test methods, factors influencing measurements, a compilation of available data and examples of applications;
- a testing campaign on inert porous materials and reference cementitious materials;
- a comparison of existing test methods with source of uncertainties;
- an harmonized method of analysis to calculate gas diffusion from raw experimental measurements;
- summary of TC findings in one or more journal publications;
- an international workshop for dissemination of information.
Group of users
Academics, testing laboratories, industrialists, practitioners and technical bodies of standardization committees.
Specific use of the results
In the context of performance assessment of durability: gas diffusion could be used as an indicator for corrosion initiation for XC class but it could also be used for the assessment of corrosion propagation period in wet environment, for the design of cathodic protection, or imbibition and drying cycles in XC, XS, XD class. Oxygen diffusion could also be a relevant indicator for the oxidative aging of asphalt binders.
The collection of results would be used as an underlying set of technical evidence for future standardisation body for gas diffusion test method.
Published documentation could also serve a reference technical documentation for laboratories (academics, industries, service companies) when developing their own setup.