230-PSC : Performance-based specifications and control of concrete durability

Technical Committee 230-PSC

General Information

Chair: Prof. Hans D. BEUSHAUSEN
Deputy Chair: Dr. Luis FERNANDEZ LUCO
Activity starting in: 2008
Activity ending in: 2015
Cluster D

Subject matter

Establish guidelines for the specification of the penetrability and thickness of the concrete cover, as function of the exposure conditions and service life design, and for its compliance control through suitable site and/or core testing
Draft recommendations for the application of such suitable test methods

- Durability of concrete structures in view of reinforcement corrosion
- Specification and testing of transport properties: permeability and conductivity
- Specification and testing of cover thickness

Durability of concrete structures continues to be a matter of concern. As pointed out in the CEB-FIP Model Code (1990), the durability of concrete structures to external aggressive actions depends primarily on the penetrability of the surface layers of a concrete member (the concrete cover or, in short, the 'covercrete').

For the particular, but significant, case of steel corrosion, the service life of the member depends also on the thickness of the concrete cover.
Currently, the specification of durability in the most widespread Codes and Standards (ACI-318 and EN 206-1) is based mainly on establishing constraints to the proportions of the concrete as function of the severity of the exposure. This approach ignores, to a large extent, the different performance of the different cement types and of the mineral components added to the cements or to the concrete itself. Moreover, it is virtually impossible to perform an effective control of the w/c ratio and/or cement content in practice.

The routine control of concrete quality is based on testing (typically for strength) moulded specimens, compacted and cured in a near-perfect, standard manner. As sketched in Figure 1, such specimens do not represent the real quality of the vital 'covercrete'. Thus, the important impact of the application of the correct concreting practices (placing, compaction, finishing, curing, etc.) remains largely ignored.

All this explains, to a large extent, the recurrent problems of lack of proper durability of concrete structures. The obvious solution would be to check the penetrability and thickness of the 'covercrete' in the real structure, as final product, which can only be done on site or on drilled cores.

Previous work of RILEM TCs has identified suitable test methods to measure the 'penetrability' of the 'covercrete:
- in the laboratory, under controlled conditions (work of TC 116-PCD)
- on site, by NDT methods (work of TC 189-NEC).

The latter also investigated the suitability of 'covermeters' to assess the depth of the concrete cover.
The conclusion of the work of those TCs is that today there are several suitable devices on the market, with which site measurements (non-destructively or applied on drilled cores) of the penetrability and thickness of the 'covercrete' can be performed with sufficient accuracy.
The aim of the work of the proposed TC is to develop guidelines on how to apply those methods in practice for the performance-based specification of durability and for checking the compliance of the final product (the finished structure) with the specification.
The work of this TC will support the efforts developed by other organizations (e.g. fib, ACI, ASTM, EN, NRMCA) to move from prescriptive to performance-based specifications.

Terms of reference

The duration of the work of the committee, for tasks 2.1 is estimated in three to four years. Task 2.2 will require one additional year.
The work of the TC will comprise a draft state-of-the-art report on durability indicators and performance-based approaches to durability specifications and control, based on existing information and their application on experimental real cases, completed with the preparation of a final report.

Time frame for major activites
- 2008, March: Approval of TC PSC, invitation of members
- 2008, September: First TC meeting during RILEM week in Italy. Members are invited to present a “situation paper” and to propose scope of work
- 2009, September: TC meeting in Haifa
- 2010: TC meeting in Europe
- 2010: Round Robin Tests in different locations worldwide: Testing of in-situ structures
- 2011: Symposium

The membership should include researchers, laboratories and firms that develop and/or apply the methods. The participation of persons/consultants directly involved in specifications, drafting of codes and standards is extremely important. Representatives of governmental bodies/owners, as main users, will also be welcome.

Detailed working programme

The work will comprise the following tentative stages (to be agreed within the TC):
- Definition of the scope of the TC Work (approach level, complexity, durability indicators, deterioration phenomena/exposure conditions, test methods, etc.)
- Critical review of existing approaches
- Identification of suitable, sound and robust approaches
- Preparation of State-of-the-Art report
- Test of the selected approaches on at least two, if possible three, real cases of concrete structures (different climates, if possible)
- Evaluation of the results of the approaches
- Definitive selection of successful approaches
        Test methods and design criteria
        Test procedures and sampling criteria
        Limiting durability values for relevant exposure classes and conditions
        Compliance control (statistical approach)
- Final report with guidelines on suitable approaches and detailed recommendations on suitable test methods, their proper use, scope and limitations
- Organization of RILEM Workshop
- Publication of RILEM Report

Technical environment

The work of this committee can be considered as a natural extension of the work of TCs 116-PCD 'Permeability as a criterion for durability' and 189-NEC 'Non-destructive evaluation of the concrete cover'.
It should be linked to the following RILEM TCs:
- 202-RWD 'Use of concrete in radioactive waste disposal facilities'
- 205-DSC 'Durability of Self-Compacting Concrete'
- 207-INR 'Interpretation of NDT results and assessment of RC structures'
- 211-PAE 'Performance of cement-based materials in aggressive aqueous environments'
- 214 CCD 'Concrete cracking and its relation to durability: Integrating material properties with structural performance'
- 213 MAI 'Model assisted integral service life prediction of steel reinforced concrete structures with respect to corrosion induced damage'

A liaison to FIB Commission 5 'Structural Service Life Aspects' and, in particular to Task Group 5.2 'Service Life Fundamentals and Design' should be established and held. Same with other organizations with similar aims (e.g. NRMCA)

Expected achievements

The result of the TC’s work will provide useful guidance on suitable 'durability indicators' and their application in performance-based specifications for durability. An important complementary result will be issuing recommendations on: sampling criteria, application of site test methods to evaluate the penetrability (NDT and cores) and thickness of the concrete cover, interpretation of results and principles of compliance control based on site tests.
The recommendations will serve as a basis for harmonization and standardization of the selected approaches and methods.
The products of this TC will consist of a State-of-the-Art report and a Symposium, coupled with an Exhibition (including Practical Demonstrations).

Group of users

The main groups that will benefit from the TC’s work are:
- Designers, consultants and specifiers, that will apply new approaches, tools and criteria to ensure that the design service life will actually be achieved
- Owners of concrete structures, e.g. governmental or private developers
- Laboratories in charge of applying the selected techniques, making use of clear recommendations
- Members of industry, in particular companies involved in the development and sale of instruments that apply sound design concepts

Specific use of the results

Large amounts of money are spent for maintenance, repair and replacement of concrete structures that do not comply with the design service life. The well known Sitter’s 'Law of Fives', indicate that most problems originate due to improper specification, design and construction practices, and that those failures lead to an exponential increase of costs as the damaging processes progress.

The work of the proposed TC will contribute to achieve extended service lives of concrete structures by specifying and controlling the quality of the final product, thus generating a positive chain-reaction in all relevant players: owners, specifiers, contractors, materials’ suppliers, users, etc.