It was imperative that the concrete in the tower of a cable stay concrete bridge under construction have a minimum compressive strength of 6000 psi and be free of entrapped air voids. The nondestructive test method used involves sending an acoustic signal through a plastic concrete mass with an internal vibrator. The signal is picked up by an acoustic transducer at the form and sent to an oscilloscope. From changes in frequency and amplitude an index is obtained of the density of the plastic concrete. Tests were first conducted on a mockup test block to calibrate the equipment and evaluate the method.
The test block consisted of fresh concrete within a steel and wood form approximately 11 feet long, 7 feet wide and 3 feet high. The dimensions allowed acoustic tests to be made at distances from vibrator to pick-up similar to those used in concrete placements on the actual tower structure. Concrete of the same mix design to be used in the tower structure was delivered in 3-cubic-yard bottom dump buckets. After the concrete was placed in the bottom 18 inches of the form and vibrated in accordance with job procedures, the acoustic tests were conducted. The work of compacting the concrete during the test was done by the contractor's concrete placement labor force using standard immersion vibrators.
The high frequency immersion vibrator was inserted into the plastic concrete mix for approximately 2 to 3 seconds or until a constant amplitude and frequency signal response appeared on the oscilloscope and was recorded on the magnetic tape. The vibrator was never allowed to stay in the concrete mass longer than 5 seconds. The acoustic transducers for sensing the energy signal were held on the form by hand. To make one set of tests this procedure was repeated nine times, starting at 8 feet away from a form surface and ending with the vibrator 6 inches in from the form. Many sets of tests were made in each layer.