Performance-based durability testing, design and specification in South Africa: latest developments

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1 Excellence in Concrete Construction through Innovation Limbachiya & Kew (eds) 2009 Taylor & Francis Group, London, ISBN Performance-based durability testing, design and specification in South Africa: latest developments M.G. Alexander & H. Beushausen University of Cape Town, Cape Town, South Africa ABSTRACT: Over the last decade, an approach to improving the durability of reinforced concrete construction has been developed in South Africa. The philosophy involves the understanding that durability will be improved only when unambiguous measurements of appropriate cover concrete properties can be made. Such measurements must reflect the in situ properties of concrete, influenced by the dual aspects of material potential and construction quality. Key stages in formulating this approach were developing suitable test methods, characterising a range of concretes using these tests, studying in-situ concrete performance, and applying the results to practical construction. The paper discusses the latest developments in durability specification practice in South Africa and attempts to show a sensible way forward for practical application of the DI approach. The approach is an integrated one in that it links durability index parameters, service life prediction models, and performance specifications. As improved service life models become available, they can be implemented directly into the specifications. Concrete quality is characterised in-situ and/or on laboratory specimens by use of durability index tests, covering oxygen permeation, water absorption, and chloride conduction. The service life models in turn are based on the relevant DI parameter, depending on whether the design accounts for carbonation-induced or chloride-induced corrosion. Designers and constructors can use the approach to optimise the balance between required concrete quality and cover thickness for a given environment and binder system. 1 INTRODUCTION Deterioration of reinforced concrete is often associated with ingress of aggressive agents from the exterior such that the near-surface concrete quality largely controls durability. The bulk of durability problems concern the corrosion of reinforcing steel rather than deterioration of the concrete fabric itself (Figure 1). The problem is then cast in terms of the adequacy of the protection to steel offered by the concrete cover layer, which is subjected to the action of aggressive Figure 1. The bulk of durability problems concern the corrosion of reinforcing steel. agents such as chloride ions or carbon dioxide from the surrounding environment. For concrete structures, durability is generally defined as the capability of maintaining the serviceability over a specified period of time without significant deterioration. In general, design concepts for durability can be divided into prescriptive concepts and performance concepts. Prescriptive concepts are based on material specification from given parameters such as exposure classes and life span of the structure. However, durability is a material performance concept for a structure in a given environment and as such it cannot easily be assessed through intrinsic material properties. Performance concepts, on the other hand, are based on quantitative predictions for durability from exposure conditions and measured material parameters. As in other countries, durability problems in South Africa derive mainly from inadequate attention to durability with regard to both design and construction. This has resulted in extensive deterioration of concrete, which is mainly related to reinforcement corrosion. In response to this situation, 3 durability index tests, namely oxygen permeability, water sorptivity and chloride conductivity were developed 429

2 (Alexander et al, 2001, Mackechnie & Alexander 2002, Beushausen et al, 2003, Streicher & Alexander 1995, Mackechnie 2002). The concrete surface layer is most affected by curing initially, and subsequently by external deterioration processes. These processes are linked with transport mechanisms, such as gaseous and ionic diffusion and water absorption. Each index test therefore is linked to a transport mechanism relevant to a particular deterioration process. Material indexing of concrete requires quantifiable physical or engineering parameters to characterise the concrete at early ages. The 3 index tests have been shown to be sensitive to important material, constructional, and environmental factors that influence durability. Thus, the tests provide reproducible engineering measures of the microstructure of concrete. The tests characterise the quality of concrete as affected by choice of material and mix proportions, placing, compaction and curing techniques, and environment. In the South African approach, durability indexes are quantifiable physical or engineering parameters which characterise lab or in-situ concrete and are sensitive to material, processing, and environmental factors such as cement type, water: binder ratio, type and degree of curing, etc. Increasingly, design specifications for structures for which durability is of special concern include limiting values for chloride conductivity (marine environment) and oxygen permeability (risk of carbonation-induced corrosion). The paper discusses the latest developments in durability specification in South Africa, using the Durability Index test methods linked to oxygen permeability and chloride conductivity. Figure 2. test (OPI). Test set-up for the Oxygen Permeability Index 2 DURABILITY INDEX TEST METHODS The Durability Index test methods comprise oxygen permeability, chloride conductivity and water sorptivity. As mentioned above, this publication concerns the application of the former two test methods for design specifications. Test equipment and test procedures are described in detail in the literature developed (Alexander et al, 2001, Mackechnie & Alexander 2002, Beushausen et al, 2003, Streicher & Alexander 1995, Mackechnie 2002) and basic principles are discussed in the following. The Oxygen Permeability Index (OPI) test method consists of measuring the pressure decay of oxygen passed through a 25 mm thick slice of 68 mm diameter core of concrete placed in a falling head permeameter (Figure 2). The oxygen permeability index is defined as the negative log of the coefficient of permeability. Common OPI values for South African concretes range from 8.5 to 10.5, a higher value indicating a higher impermeability and thus a concrete of potentially higher quality. Note that oxygen permeability Figure 3. Test set-up for the chloride conductivity test. index is measured on a log scale, therefore the difference between 8.5 and 10.5 is quite substantial. An empirical prediction model for carbonation was formulated using the oxygen permeability test. Using this approach, 50 year carbonation depths may be predicted for different environments. The chloride conductivity test apparatus (Figure 3) consists of a two cell conduction rig in which concrete 430

3 core samples are exposed on either side to a 5 M NaCl chloride solution. The core samples are preconditioned before testing to standardize the pore water solution (oven-dried at 50 C followed by 24 hours vacuum saturation in a 5 M NaCl chloride solution). The movement of chloride ions occurs due to the application of a 10 V potential difference. The chloride conductivity is determined by measuring the current flowing through the concrete specimen. The apparatus allows for rapid testing under controlled laboratory conditions and gives instantaneous readings. Chloride conductivity decreases with the addition of fly ash, slag, and silica fume in concrete, extended moist curing and increasing grade of concrete. Portland cement concrete for instance generally has high conductivity values with only high-grade material achieving values below 1.0 ms/cm. Slag or fly ash concrete in contrast has significantly lower chloride conductivity values. While the test is sensitive to construction and material effects that are known to influence durability, results are specifically related to chloride ingress into concrete. Correlations between 28-day chloride conductivity results and diffusion coefficients after several years marine exposure have shown to be good over a wide range of concretes (Mackechnie & Alexander 2002). 3 APPLICATION OF THE DURABILITY INDEX APPROACH 3.1 General The sensitivity of the South African index tests to material and constructional effects makes them suitable tools for site quality control. Since the different tests measure distinct transport mechanisms, their suitability depends on the property being considered. Durability index testing may be used to optimise materials and construction processes where specific performance criteria are required. At the design stage the influence of a range of parameters such as materials and construction systems may be evaluated in terms of their impact on concrete durability. In this way, a cost-effective solution to ensuring durability may be assessed using a rational testing strategy (Ronnè et al, 2002). The durability indexes, obtained with the above test methods, have been related to service life prediction models. Index values can be used as the input parameters of service life models, together with other variables such as steel cover and environmental class, in order to determine rational design life. Limiting index values can be used in construction specifications to provide the necessary concrete quality for a required life and environment. Thus, a framework has been put in place for a performance-based approach to both design and specification. Table 1. Environmental Classes (Natural environments only) (after EN206). Carbonation-Induced Corrosion Designation Description XC1 Permanently dry or permanently wet XC2 Wet, rarely dry XC3 Moderate humidity (60 80%) (Ext. concrete sheltered from rain) XC4 Cyclic wet and dry Corrosion Induced by Chlorides from Seawater Designation XS1 XS2a XS2b XS3a XS3b Description Exposed to airborne salt but not in direct contact with seawater Permanently submerged XS2a + exposed to abrasion Tidal, splash and spray zones Buried elements in desert areas exposed to salt spray XS3a + exposed to abrasion These sub clauses have been added for SouthAfrican coastal conditions 3.2 Service life prediction models Two corrosion initiation models have been developed, related to carbonation and chloride induced corrosion. The models derive from measurements and correlations of short-term durability index values, aggressiveness of the environment and actual deterioration rates monitored over periods of up to 10 years. The models allow for the expected life of a structure to be determined based on considerations of the environmental conditions, cover thickness and concrete quality (Mackechnie & Alexander 2002, Mackechnie 2001). The environmental classes are related to the EN 206 classes as modified for South African conditions (Table 1), while concrete quality is represented by the appropriate durability index parameter. The oxygen permeability index is used in the carbonation prediction model, while the chloride model utilises chloride conductivity. The service life models can also be used to determine the required value of the durability parameter based on predetermined values for cover thickness, environment, and expected design life. Alternatively, if concrete quality is known from the appropriate DI, a corrosion-free life can be estimated for a given environment. 3.3 Specifying durability index values Two possible approaches to specifying durability index values are a deemed-to-satisfy approach and a rigorous approach. The former is considered adequate 431

4 for the majority of reinforced concrete construction and represents the simpler method in which limiting DI values are obtained from a design table, based on binder type and exposure class, for a given cover depth (50 mm for marine exposure and 30 mm for carbonating conditions). The rigorous approach will be necessary for durability critical structures, or when the design parameters assumed in the first approach are not applicable to the structure in question. Using this approach, the specifying authority would use the relevant service life models developed in the concrete durability research programme in South Africa. The designer can use the models directly and input the appropriate conditions (cover depth, environmental classification, desired life, and material). The advantage of this approach is its flexibility as it allows the designer to use values appropriate for the given situation rather than a limited number of pre-selected conditions Examples for the deemed-to-satisfy approach This approach mimics structural design codes: the designer recommends limiting values which, if met by the structure, result in the structure being deemedto-satisfy the durability requirements. The carbonation resistance of concrete appears to be sufficiently related to the early age (28 d) Oxygen Permeability Index (OPI) value, so that OPI can be used in a service life model. The environments that require OPI values to be specified in the South African context are XC3 and XC4 (Table 1), with XC4 considered the more critical because steel corrosion can occur under these conditions. Two design scenarios with standard conditions and required minimum OPI values are shown in Table 2. Chloride resistance of concrete is related to its chloride conductivity, and therefore this index can be used to specify concrete performance in seawater environments. Table 3 presents chloride conductivity limits for common structures (50 years service life). Different values are given for different binder types, since chloride conductivity depends strongly upon binder type. The horizontal rows give approximately equal performance (i.e. chloride resistance) in seawater conditions for the different binders. Binder types are restricted to blended cements for seawater exposure, since CEM I on its own has been shown to be insufficiently resistant to chloride ingress Example for the rigorous approach As an example of practical implementation of the rigorous approach, consider the case of specifying a marine structure for a 50-year design life, subject to the environmental conditions given in Table 1. Combining the relevant durability index of chloride conductivity with the appropriate service life model yields the data given in Table 4. It should be noted that the DI values are presented here for purposes of illustration only. The relative values are more important than the absolute values as these will vary in response to regional and environmental variations. Table 3. Maximum Chloride Conductivity Values (ms/cm) for Different Classes and Binder Types: Deemed to Satisfy Approach Common Structures (Cover=50 mm). Binder combination EN206 Class 70:30 50:50 90:10 CEMI:FA CEMI:GGBS CEMI:CSF XS XS2a XS2b, XS3a XS3b Table 4. Limiting DI values based on rational prediction model: maximum chloride conductivity (ms/cm) (50 year life). Table 2. Deemed to Satisfy OPI values (log scale) for carbonating conditions. Common Structures Monumental Structures Service Life 50 years 100 years 100 years Minimum Cover 30 mm 30 mm 40 mm Minimum OPI

5 The table shows the trade-off between material quality (i.e. chloride conductivity) and concrete cover, with lower quality (represented by a higher conductivity value) allowable when cover is greater. The dependence of the conductivity on binder type is also illustrated, with higher values permissible for blended binders at any given cover, based on their superior chloride ingress resistance. These higher values translate into less stringent w/b ratios. Therefore, a conservative approach is recommended at present, with mixes for which the concrete grade may be less than 30 MPa, and/or the w/b may be greater than 0.55, not being recommended. However, in these cases, the particular cover and binder can be used, but the conductivity value will be over-specified. 3.4 Establishing limiting values for concrete mixtures To establish limiting DI values for concrete mixtures and evaluate compliance with durability requirements, the following two aspects need to be considered: Statistical variability of test results (hence selection of appropriate characteristic values for Durability Indexes) Differences between as-built quality (in-situ concrete) and laboratory-cured concrete The consideration of the two above aspects is discussed below and illustrated by an example in Table Characteristic values versus target values The values determined in Tables 2, 3 and 4 are characteristic values to be achieved in the as-built structure, Table 5. As-built chloride conductivity values (ms/cm) vs. potential target values (hypothetical case). not target (average) values. The material supplier must aim at target values that will achieve the required characteristic values with adequate probability. The variability inherent in concrete performance needs to be considered when evaluating the test results, similar to the approach that is adopted with strength cubes. Since durability is a serviceability criterion, the limitations may not need to be as stringent as for strength. It is proposed that a 1 in 10 chance be adopted at this stage for the Durability Index tests with a margin of 0.3 below for the OPI, and 0.2 ms/cm above for the chloride conductivity test As-delivered concrete quality versus as-built concrete quality A clear distinction must be drawn between material potential and in-situ construction quality. Although specifications are usually only concerned with asbuilt quality, the processes by which such quality is achieved cannot be ignored. There are two distinct stages and responsibilities in achieving concrete of a desired quality. The first is material production and supply, which could be from an independent party such as a ready-mix supplier. A scheme for acceptance of the as-supplied material must be established so that the concrete supplier can have confidence in the potential quality of the material. The second stage is the responsibility of the constructor in ensuring that the concrete is placed and subsequently finished and cured in an appropriate manner. It is ultimately the as-built quality that determines durability and the constructor has to take the necessary steps and precautions in the construction process to ensure that the required quality is produced. If the as-built quality is found to be deficient, the specification framework must have an internal acceptance scheme that is able to distinguish whether the deficiency arises from the as-supplied material or the manner in which it was processed by the constructor. To enable this, a two-level quality control system has been proposed in South Africa, with testing of both material potential and as-built quality. Material potential is represented by as-supplied concrete specimens with a laboratory-controlled wet curing period (5 days), while as-built quality is determined using in-situ sampling of concrete members. As a general rule, concrete in the as-built structure may be of lower quality compared with the same concrete cured under controlled laboratory conditions described above. To account for the improved performance of laboratory concrete over site concrete, the characteristic values for the durability indexes of the laboratory concrete should be: For OPI: a margin of at least 0.10 greater than the value determined in Sect For chloride conductivity: a factor no greater than of 0.90 times the value determined in Sect

6 4 CLOSING REMARKS The paper describes the development of the Durability Index approach to addressing problems of reinforced concrete durability in the South African context. The approach is an integrated one in that it links durability index parameters, service life prediction models, and performance specifications. As improved service life models become available, they can be implemented directly into the specifications. Concrete quality is characterised in-situ and/or on laboratory specimens by use of durability index tests, covering oxygen permeation, water absorption, and chloride conduction. The service life models in turn are based on the relevant DI parameter, depending on whether the design accounts for carbonation-induced or chloride-induced corrosion. Designers and constructors can use the approach to optimise the balance between required concrete quality and cover thickness for a given environment and binder system. More work remains to be done, in particular generating correlations between indexes and actual structural performance. Only in this way will the usefulness of the approach be assessed. of Concrete, Vol. VI, Ed. J. P. Skalny and S. Mindess, American Ceramic Society, Beushausen, H., Alexander, M.G., and Mackechnie, J. (2003), Concrete durability aspects in an international context, Concrete Plant and Precast Technology BFT, vol. 7, 2003, Germany, pp Mackechnie, J.R. and Alexander, M.G. (2002), Durability predictions using early age durability index testing, Proceedings, 9th Durability and Building Materials Conference, Australian Corrosion Association, Brisbane, 11p. Mackechnie, J.R., (2001), Predictions of Reinforced Concrete Durability in the Marine Environment Research Monograph No. 1, Department of Civil Engineering, University of Cape Town, 28 pp. Mackechnie, J.R., and Alexander, M.G., (2002), Durability Predictions Using Early-Age Durability Index Testing, Proceedings of the Ninth Durability and Building Materials Conference, Australian Corrosion Association, Brisbane, Australia, 11 pp. Ronnè, P.D., Alexander, M.G. and Mackechnie, J.R. (2002), Achieving quality in precast concrete construction using the durability index approach, Proceedings: Concrete for the 21st Century, Modern Concrete Progress through Innovation, Midrand, South Africa, March Streicher, P.E. and Alexander, M.G. (1995), A chloride conduction test for concrete, Cement and Concrete Research, 25(6), 1995, pp REFERENCES Alexander, M.G., Mackechnie, J.R. and Ballim, Y. (2001), Use of durability indexes to achieve durable cover concrete in reinforced concrete structures, Materials Science 434