This month's articles focus on UT and NDT in Civil Engineering, especially in testing of concrete. As usual, this will be the focus of our monthly forum. This issue also includes articles from other fields in NDT which are welcome to be discussed on this forum as well.

Profil: William B. Streett

One article related to NDT for concrete is " The Impact-Echo Method" by Mary J. Sansalone and William B. Streett. In conjunction with this article we are pleased to introduce this month Mr. Streett, who will serve as a coordinator for comments on concrete for the February 1998 Forum of NDT.net. He is now our Virtual Session Chairman.
Mr. Streett doesn't claim to be an expert in NDT as it applies to all areas of Civil Engineering, however his colleague Professor M. Sansalone is knowledgeable about many aspects of field, and will assist in responding to any comments that arise on this topic.
Impact-echo is an acoustic method for nondestructive evaluation of concrete and masonry, invented at the U.S. National Bureau of Standards in the mid-1980's, and developed at Cornell University, in Ithaca, New York, from 1987-1997.

Impact-Echo is not strictly an ultrasonic method. It employs stress waves generated by the impacts of small steel spheres on the surfaces of concrete structures. The frequencies of the resulting stress waves typically extend from about 1 kHz to 70 kHz, and in most routine testing the important frequencies are less than about 30 kHz. At wave speeds of about 4000 m/s, the resulting wavelengths are around 10 cm or longer, and the stress waves travel through concrete as though it were a homogeneous elastic medium. They are reflected by internal flaws and interfaces and by external surfaces, but they are not reflected or scattered by the by the natural, small-scale inhomogeneities in concrete.

But enough of this brief introduction.
Now it's your turn, let's start our discussion.