Effectiveness of in situ moisture preconditioning methods for concrete

With greater emphasis now being placed on the durability of concrete and the need for on-site characterization of concrete for durability, there is an increasing dependence on the measurement of the permeation properties of concrete. Such properties can be measured in the laboratory under controlled ambient conditions, namely, temperature and relative humidity, and comparisons made between samples not affected by testing conditions. An important factor that influences permeation measurements is the moisture state of the concrete prior to testing. Moisture gradients are known to exist in exposed concretes; therefore, all laboratory tests are generally carried out after preconditioning to a reference moisture state. This is reasonably easy to achieve in the laboratory, but more difficult to carry out on-site. Different methods of surface preconditioning in situ concrete are available; however, there is no general agreement on the suitability of any one method. Therefore, a comprehensive set of experiments was carried out with four different preconditioning methods. Results from these investigations indicated that only superficial drying could be achieved by using any of the preconditioning methods investigated and that significant moisture movement below a depth of 15 mm was not evident.

TECHNICAL NOTE. A BRIEF REVIEW OF METHODS FOR MEASURING THE PERMEATION PROPERTIES OF CONCRETE IN SITU

Numerous methods are available to measure the permeation properties of concrete, which can be classified in terms of the diffusion, absorption and permeability properties. The results from these tests are generally used to infer 'quality' or relative durability. Some of these tests involve the laboratory assessment of a sample of concrete extracted from the structure. However, this Technical Note concentrates on the alternative methods appropriate for use on site. Guidance is given on the choice of an appropriate test method, which in most practical situations depends on the predominant mechanism acting on the concrete under consideration.

In-situ strength assessment of concrete - The European Concrete Frame Building Project (Reprinted from Non-Destructive Testing Civil Engineering, April, pg 583-592, 2000)

A full-scale seven-storey in-situ advanced reinforced concrete building frame was constructed in the Building Research Establishment's Cardington laboratory encompassing a range of different concrete mixes and construction techniques. This provided an opportunity to use in-situ non-destructive test methods, namely Lok and CAPO tests, on a systematic basis during the construction of the building. They were used in conjunction with both standard and temperature-matched cube specimens to assess their practicality and their individual capabilities under field conditions. Results have been analysed and presented to enable comparisons of the performance of the individual test methods employed.