Friday, December 26, 2008

concrete testing ; Aims of in-situ testing

Three basic categories of concrete testing may be identified.

(i) Control testing is normally carried out by the contractor or concrete producer to indicate adjustments necessary to ensure an acceptable supplied material.

(ii) Compliance testing is performed by, or for, the engineer according to an agreed plan, to judge compliance with the specification.

(iii) Secondary testing is carried out on hardened concrete either in, or extracted from, the structure. This may be required in situations where there is doubt about the reliability of control and compliance results or they are unavailable or inappropriate, as in an old, damaged, or deteriorating structure. All testing which is not planned before construction will be in this category, although long-term monitoring is also included.

Control and compliance tests have traditionally been performed on ‘standard’ hardened specimens made from samples of the same concrete as used in a structure; it is less common to test fresh concrete. There are also instances in which in-situ tests on the hardened concrete may be used for this purpose. This is most common in the pre casting industry for checking the quality of standardized units, and the results can be used to monitor the uniformity of units produced as well as their relationship to some pre-established minimum acceptable value. There is, generally, an increasing awareness amongst engineers that ‘standard’ specimens, although notionally of the same material, may misrepresent the true quality of concrete actually in a structure. This is due to a variety of causes, including non-uniform supply of material and differences of compaction, curing and general workmanship, which may have a significant effect on future durability. As a result, a trend towards in-situ compliance testing, using methods which are either non-destructive or cause only very limited damage, is emerging, particularly in North America and Scandinavia. Such tests are most commonly used as a back-up for conventional testing, although there are notable instances such as the Storebaelt project where they have played a major role (1). They offer the advantage of early warning of suspect strength, as well as the detection of defects such as inadequate cover, high surface permeability, voids, honeycombing or use of incorrect materials which may otherwise be unknown but lead to long-term durability problems. Testing of the integrity of repairs is another important and growing area of application.

The principal usage of in-situ tests is nevertheless as secondary testing, which may be necessary for a wide variety of reasons. These fall into two basic categories.

(1) Petersen, C.G. and Poulsen, E. Pull-out testing by Lok-test and Capo-test. Dansk Betoninstitut A/S, 1992.

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