Q.: The Problem Clinic in the June 1993 issue of Concrete Construction, described the use of pull-out tests to determine the in-place strength of concrete. What about the break-off test?

A.: The break-off test also can be used to measure in-place strength of concrete in the field. In North America, there is less experience with this method, compared to pull-out tests, probe penetration, and the rebound hammer. The break-off test has been available commercially in the United States since 1986. The test procedure has been standardized as ASTM C 1150-90, "Standard Test Method for The Break-Off Number of Concrete." Reportedly, the device has been used widely in Europe for testing pavements and slip-formed structures.

The break-off test measures the force required, or break-off number, to fracture a small cylinder formed by reusable sleeves inserted in fresh concrete, or later cut by a special diamond drill bit and counterbore. Typically, break-off numbers from five individual tests are averaged. The manufacturer provides a generic correlation between the break-off number and results of compressive-strength tests, but this relationship is best developed for specific local combinations of concrete materials. Results of the break-off test are said to correlate better with concrete tensile strength than those obtained by pull-out tests.

In the United States, the break-off tester is available from: SDS Co., P.O. Box 844, Paso Robles, CA 93447 (805-238-3229; fax 805-238-3496).