Concrete in Aggressive Aqueous Environments

Toulouse, France

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Special sessions

PSC Technical committee : Performance-based specifications and control of concrete durability

Chairman : Dr. H. Beushausen, Secretary : Ing. L. Fernandez Luco

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

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

Significance
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.