NDTnet 1998 Aug, Vol.3 No.8

Abstract

There is an increasing demand for suitable non-destructive test methods, which can be applied in civil engineering, in particular to the investigation of reinforced concrete. This is coupled to an increasing awareness of damage mechanisms, structural and safety-related deficiencies in existing structures and the need to locate reinforcement and pre-stressing cables. The test methods are also required to determine the condition of the concrete and what influence this has on the various measurement parameters.

Modern NDE techniques for testing concrete include those based on stress waves, i.e. acoustic methods using relatively low frequency inputs, and high frequency radar. The acoustic methods discussed here use transient stress waves created by mechanical impact on the concrete surface. The first of these is the Impact Echo method and works on the principle of transmission and reflection of compression waves. The second is named SASW (Spectral Analysis of Surface Waves) and as the name implies it is based on the dispersion of Rayleigh waves, i.e. the variation of surface wave velocity with depth in layered systems. The main application of the former technique is in measurement of concrete thickness, depth to cracks and voids and location of relatively large objects in concrete. The latter (SASW) is used to study the physical characteristics of concrete and soils and to obtain information, which can be used to calculate their elastic moduli.
Radar has been developed mainly for geophysical surveys and has seen extensive use in this field. Modern developments include high frequency antennae (1000 MHz) which give sufficiently good resolution to locate reinforcing at depths outside the range of traditional electro-magnetic covermeters. Using special processing software the data can be improved to increase reliability and ease of interpretation.

In this paper some examples are given of site investigations on reinforced concrete structures using these methods. Closer studies of model concrete blocks were used to obtain information about method sensitivity and capability. The work on site relies heavily on the use of two or more techniques and demonstrates the advantages and indeed necessity of combining techniques to improve reliability of data and in providing perspective to the problem case.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

Publication Contact:
7th ECNDT, Park Alle 345, DK-2605 Broendby, Denmark, Fax: +45 46 26 70 11, Email: 101373.3414@compuserve.com