Performance Based Design for Durability
The noticeable shift towards performance specifications for durability calls for the adoption of reproducible, reliable and quick measures of transport parameters that are known to affect concrete durability. In addition, the ability to assess these parameters in actual construction, appropriate criteria for limiting values for durability parameters, and penalties or incentives for failing or achieving the necessary durability requirements, are equally important. The Durability Index approach developed in South Africa has been developed to provide a basis for specifying performance for durability of concrete. Three tests that primarily measure the transport properties of cover zone concrete are used to generate performance indicators. This approach has been successfully used in South Africa, and could have a good potential for application in the Indian concrete industry. This article describes briefly the tests followed under the durability index approach, and how the results from these tests could be used to set up performance criteria that can be used for specifying concrete quality to ensure long-term performance.
Durability of concrete
Durability of reinforced concrete structures is a pervasive and universal problem. Many concrete structures deteriorate prematurely, and repair and maintenance costs amount to substantial proportions of public and private sector budgets. Neville1 suggests reasons as poor understanding of deterioration processes, inadequate acceptance criteria of site concrete, and changes in cement properties and construction practices. Durability problems cover a wide range including attack by external destructive agents (e.g. sulphates), internal material incompatibilities (e.g. alkali-aggregate reaction), and aggressive environments such as freeze-thaw. Nevertheless, the greatest threat undoubtedly is corrosion of embedded reinforcing steel, leading to cracking, staining, and spalling of the cover. This in turn can lead to unserviceable structures that may be compromised in respect of safety, stability, and aesthetics.
Corrosion is initiated by a change in the nature of the pore
solution surrounding the steel, due either to the penetration of a de-passivating
carbonation front or, more dangerously, to ingress of chloride ions from
a saline (e.g. marine or de-icing salt) environment. Durability is therefore
largely controlled by the quality of the thin cover layer protecting the
reinforcement. This layer is most susceptible to the negative influences
of poor curing, early-age drying, inadequate compaction, and penetration
of aggressive agents from the environment. The problem reduces to one
of being able to control the cover layer thickness and quality. For designers,
this relates to the ability to quantify cover layer properties for specification
purposes, while for constructors the issue is to implement suitable site
practices that ensure the specified cover properties.
Performance Based Specifications
The primary problem with many present specifications for concrete is that they are too prescriptive, and not sufficiently performance-based. The key to improving reinforced concrete durability is to require that as-built structures meet certain critical performance criteria in respect of probable modes of deterioration. The purpose is to ensure that the structure, during its service life, does not approach some “limit state”, beyond which serviceability of the structure becomes compromised. The goal of performance-based specifications is to ensure that an acceptable probability of adequate durability performance is achieved. A shift from prescriptive to performance specifications is one of the important steps necessary to address the serious shortcomings that are often apparent in current reinforced concrete construction.
Traditionally, concrete compressive strength has been used as an indicator of durability. However, strength is not an adequate indicator because the test does not account for construction processing variables such as placing, compaction and curing. These variables affect the quality of the surface zone of the concrete and, in this way, have a direct influence on durability by controlling the movement of aggressive agents from the environment into the concrete. The important rate-controlling factors of concrete deterioration are therefore the concrete material constituents, the near-surface quality of the finished concrete and the aggressiveness of the environment. There is usually little that can be done to control the environment and strategies for improving the service life of structures have to focus on concrete materials and the quality of construction. Durability specifications are therefore increasingly relying on a measurement of the transport properties of the surface or cover zone of the concrete. These developments are paving the way for crafting innovative performance specifications.
With the advent of performance based specifications, service
life modeling of concrete has acquired a new meaning. While previous safeguards
against durability related problems in concrete were mainly based on provision
of a minimum cement content and limited water to cement ratio, the current
situation demands the adoption of stable and robust indicators of concrete
performance (i.e. durability).
The Durability Index (DI) Approach
In order to address the need for appropriate performance indicators, the Durability Index (DI) approach has been developed in South Africa. This approach is based on the following principles
‘Durability Indexes’ are thus quantifiable physical or engineering parameters (e.g. permeability, water sorptivity), which characterise the concrete in the as-built structure at early ages, and are sensitive to important material, processing, and environmental factors such as cement type, water: binder ratio, type and degree of curing etc. The purpose of material indexing is to provide a reproducible engineering measure of microstructure and properties of importance to concrete durability at a relatively early age (e.g. 28 days).
Furthermore, correlations are required between indexes, durability results, and actual structural performance, such that the index tests can be used:
Tests Covered Under the DI Approach
Three durability index tests have been developed2-5, namely the oxygen permeability test, the water sorptivity test, and the chloride conductivity test. Each test measures a different transport property of fluids or ions through the concrete cover layer, typically covering the main mechanisms related to deterioration. The tests have been developed and proved in the laboratory, and increasingly are being applied on site in actual construction6,7. They have progressed to the point of being in regular use, and specifications are being written around their site application. At the same time, the performance of structures built using the index approach is being monitored as far as possible to validate the approach and implement improvements.
Oxygen Permeability Test
This involves a falling head permeameter in which oven-dried (50º C for 7 days) concrete samples, generally 68 mm diameter and 25 to 30 mm thick, are placed in rubber collars secured on top of a permeability cell3. The cell is pressurised with oxygen to 100 kPa before being isolated, after which the pressure decay is monitored, from which the Darcy coefficient of permeability, k, may be determined. The oxygen permeability index (OPI) is defined as
Oxygen permeability index = -Log (k) (1)
Oxygen permeability indexes are logarithmic values and range generally from 8 to 11, i.e. three orders of magnitude; the higher the index, the less permeable the concrete. A diagram of the test apparatus is shown in Figure 1.
Figure 1: Schematic diagram of oxygen permeability apparatus
Water Sorptivity Test
Sorptivity is defined as the rate of movement of a wetting front through a porous material. The water sorptivity test involves the uni-directional absorption of water into one face of a pre-conditioned concrete disc sample2,8.At predetermined time intervals, the sample is weighed to determine the mass of water absorbed, and the sorptivity is determined from the plot of mass of water absorbed versus square root of time. The lower the water sorptivity index, the better is the potential durability of the concrete. Sorptivity values typically vary from approximately 5 mm/√h, for well-cured M30-M50 concretes, to 15 – 20 mm/√h for poorly cured M20 concrete. A diagram of the test is shown in Figure 2.
Figure 2: Schematic diagram of water sorptivity test
Chloride Conductivity Test
Streicher developed a rapid chloride conductivity test in which virtually all ionic flux occurs by conduction due to a 10 V potential difference between the two faces of a sample4,9. The apparatus consists of a two-cell conduction rig, each cell containing a 5M NaCl solution so that there is no concentration gradient across the sample and chloride migration is the result of conduction from the applied potential difference – see Figure 3. The concrete disc sample is pre-conditioned by vacuum saturation with a 5M NaCl solution.
Figure 3: Schematic diagram of chloride conductivity apparatus
Diffusion and conduction are related by Einstein’s equation, allowing the conductivity test to be used as an index of concrete diffusivity. The test is sensitive to changes in the pore structure and cement chemistry (mainly binder type), which might appear to be insignificant when using the permeation process10. Typical chloride conductivity index values range from > 3 mS/cm for M20 – M30 OPC concretes, to < 0.75 mS/cm for M40 – M50 slag or fly ash concretes. The lower the index, the better is the potential durability of the concrete.
Quality Assessment Using DI Tests
These durability index tests have been shown to be useful for aspects such as quality control of site concrete, and concrete mix optimisation. However, this approach can also be used for performance-based specifications. From controlled laboratory studies and site data, a matrix of durability index values is being developed that could be used to produce a set of acceptance criteria for performance specifications. Suggested ranges for durability classification of concretes for the three index tests, based on site and laboratory data, for quality of the cover layer, can be prescribed.
This approach could have major benefits for all parties involved
in construction. The current prescriptive approach to durability specifications
is not only vague and sometimes inappropriate but it is often inflexible.
For example, there is often little allowance for imaginative use of cement
extenders, and construction options are unnecessarily limited. Performance-based
specifications allow constructors more leeway in deciding how best to
achieve durability requirements while still having control to ensure satisfactory
Performance Requirements Using Durability Indexes
Framework For A Durability Specification
Figure 4 gives a proposed framework for a durability specification. It recognizes that, in the foreseeable future, specifications will probably require to be of the “mixed” type, comprising both performance and prescriptive elements. Provided these are properly formulated, a well-balanced and effective specification can be achieved, giving assurance of adequate long-term performance, and supplying guidance on how best to achieve such performance.
Using the framework in Figure 4, the following sections provide examples, gleaned from current South African practice, of possible performance and prescriptive requirements for durability specifications.
“Deemed to Satisfy” Approach
This approach would probably be adequate for the bulk of construction. It has an analogy in structural design codes in the “deemed to satisfy” rules associated with, for example, span/depth rules for deflection checks. The approach involves requiring as-built structures (and possibly also laboratory trial specimens) to conform to limiting criteria for durability indexes. If conformance is achieved, the structure is “deemed to satisfy” the durability requirements. This approach can be coupled with penalty measures for cases where non-conformance occurs. Incentives for excellent performance can also be introduced.
An example is given in Table 2, taken from an actual durability specification currently in use in South Africa11, which has found acceptance with authorities, consulting engineers, and constructors.
Table 2: Acceptance limits for durability indexes11
FRAMEWORK FOR (PERFORMANCE-BASED) DURABILITY SPECIFICATION
Figure 4: Framework for
a durability specification
A limitation of the approach above, if used on its own, is that it does not recognise the “matrixing” effect of binder type and exposure environment, particularly in relation to chloride environments. To illustrate this, consider Table 3. The exposure classes in the table are those suitable for South African marine conditions, and the various binder blends are all in regular use in South Africa. It is immediately obvious that limiting chloride conductivity values depend both on the exposure conditions and the binder type. The values in Table 3 for any horizontal row can be regarded as giving approximately equal “protection” against chloride ingress, but a single nominal value is an oversimplification.
A further important design and specification aspect, not addressed
in the above approach, is that of concrete cover to reinforcement. This
aspect also has an influence on the economics of construction12. Larger
covers would require less “durable” concrete to provide protection
to the steel, but at the same time, would increase the cost of construction.
Thus, in critical cases, it will normally be necessary to optimise concrete
type and steel cover, subject to the exposure conditions and the economics
of construction. An example of this approach, which also introduces a
“design life” allowance, is given in the next section.
Table 3: Allowable maximum chloride conductivity
values (mS/cm) at 28 d
Marine Exposure Zones are those for SA
conditions as follows:
“Service Life” Approach
This approach represents the direction in which reinforced concrete design and specification is likely to progress. The essence of the approach is to fully “matrix” the key elements of:
Table 4: Maximum 28 day chloride conductivity
values (mS/cm) for 50 year design life in SA marine conditions (for
avoidance of corrosion activation at 50 years)
Note: For conditions indicated by the light grey shading, the
indicated binder types may be used, but w/b should not exceed 0,55 for
any marine zone.
Since most current specifications deal with prescriptive requirements, these will not be covered in detail here. As mentioned earlier, prescriptive requirements have merit in the following respects: they can assist constructors in achieving the performance requirements demanded of the structure, by giving guidance on “best practice”; and they can cover particular requirements necessitated by local conditions, e.g. materials, environment, etc.
The prescriptive section of a specification may need to cover
the issues given in the “Framework” diagram in Figure 4, with
particular attention to aspects such as: achievement of dense concrete;
avoidance or elimination of voids and cracks; control of bleeding and
plastic settlement or shrinkage; control of thermal cracking; limiting
concrete temperature at time of placing; requirements for binders and
aggregates including grading requirements; control on maximum or minimum
water content; control on placement and compaction procedures; and requirements
The paper has attempted to sketch a way forward for performance-based durability specifications, using the so-called “Durability Index” approach. This involves obtaining reliable measures of physical and engineering parameters (i.e. durability indexes), which relate to transport mechanisms (e.g. permeation, sorption, ionic diffusion) that govern reinforced concrete deterioration. These measures need to be obtained on actual “as-built” structures, so as to characterise in-situ performance. Durability index values, once “matrixed” to account for effects of binder type, exposure environment, concrete cover, and Service Life, can then be used to specify limiting criteria to be achieved in construction.
A framework is also suggested to help provide the basis for a
durability specification, incorporating both performance and prescriptive
requirements. Provided the criteria for these elements are well formulated,
effective specifications can be produced that could greatly assist in
achieving long-term durability with reasonable assurance.
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