There are several established test methods for determining the properties of air entrainment or the resistance of concrete to cumulative freeze-thaw cycling.
These latter tests are particularly interesting in that freeze-thaw testing may serve as an introduction for many specifiers to performance-related design of durable concrete. The durability design method involves consideration of freeze- thaw deterioration mechanisms in a quantitative way and appropriate material parameters may then be determined.
Ultimately it is hoped that our understanding of rate-determining parameters will be adequate to allow full consideration, when framing specifications, of the required service life and an acceptable probability of failure from freeze-thaw. In the interim the introduction of specifications based on relative performance may be a significant step forward from purely prescriptive specifications. The approaches available for determining appropriate parameters are based on past records of satisfactory experience, performance test methods and predictive models.
Performance test methods for freeze-thaw resistant concrete have been referred to in the recently published European specification standard EN206-1. The reference occurs in an annex which briefly mentions possible future developments in durability-related performance specifications in respect of freeze-thaw resistance. Although the references to developments in this area are necessarily somewhat tentative in view of the present uncertain state of know- ledge, they do, nevertheless, signal the drafters’ hope of significant progress in these areas by the time the next major update of the standard is undertaken. Performance test methods for freeze-thaw resistant concrete involve empirical benchmarking of levels of performance in standard test methods. The adequacy of a proposed mix may then be determined by reference to a relevant approval test on the assumption that the latter provides a suitable index of freeze-thaw resistance in service. The test demonstrates the potential success or failure of a particular mix in meeting the performance level defined by the specification. It is envisaged that some performance testing would be conducted specifically for a particular contract. Alternatively, a proposed concrete mix might be deemed to meet the specification where adequate performance has been established by previous tests on similar concretes or weaker mixes than that proposed. The process is still in development and Siebel (1999) noted that the measured scaling in proposed European standardised methods has not yet been calibrated with behaviour in service and so only comparative testing is possible.
Much research is underway on the mathematical modelling of deterioration mechanisms involved in freeze-thaw of concrete. These models, once validated, may be very beneficial in aiding the full implementation of performance-based specifications. The models are aimed at relating the rate of concrete deteriora- tion to key measurable parameters of the concrete, the mix constituents and/or exposure conditions. This would allow optimisation studies to be carried out in the course of mix design or selection.
A simple benchmark level of performance in a standard test method is the mass loss due to scaling in harsh freeze-thaw conditions. The criterion for acceptance of an unknown concrete is the loss of mass equal to or less than that of a reference concrete produced to a mix design and with constituents that have an established reputation in the region of providing freeze-thaw resisting concrete. Reference concrete for non-air entrained mixes would also conform to specified values of minimum cube strength, maximum water/cement ratio and minimum cement content. Air entrained concrete mixes would also have to conform to specified values of water/cement ratio, cement content and air content.
Several test methods are available in both the case of fresh concrete and hardened concrete. The first group of tests is concerned with the testing of fresh concrete prior to use on site to verify that the specified level of air entrainment has been achieved. The second group of tests on hardened concrete, aimed at assessing its anticipated performance in service, involves using concrete prisms, cylinders, cubes, 50mm thick slabs cut from cubes or 70mm thick specimens cast in cube moulds. Test methods include national standards that are widely used internationally, draft methods under development as international standards, tests related to specific aspects of the deterioration process and draft methods specifically targeted at the future need for performance tests allied to performance-based specifications.
It was noted in Section 8.2 that the deterioration process is a multifaceted issue with effects combining to cause failure. This complicates the test process. For example, it may be shown that there is a good correlation between deterioration in freeze-thaw tests and the water absorption of non-air entrained concrete. This reflects one particular parameter ± the degree of saturation. Other tests exploit study of the relative effect on other parameters.
