Nondestructive Testing of Slip-Form Barriers

The September 8 issue of Engineering News Record reported that flaws, "no good consolidation of the concrete", were uncovered in the slip-form parapet Jersey barriers on the Chesapeake Bay Bridge after a fatal accident in which an 18 wheel truck knocked part of the wall off the bridge. NDT Corporation subcontracted to Maguire Group, Incorporated conducted an investigation of 80 feet of slip-form barrier for the Rhode Island DOT as part of the "Iway" I-195 relocation project in Providence Rhode Island. The DOT had concerns about using slip-form construction particularly in areas heavily reinforced and plated for barrier rail connections. It was believed that voiding might occur in these areas if the concrete was not being vibrated sufficiently.

Prior to construction of the barrier, NDT Corporation and Rhode Island DOT personnel simulated void conditions at barrier rail posts by wire tying pieces of Styrofoam between reinforcing bars and under steel fixing plates (see Photo) where it was believed voiding might occur. Larger void conditions were simulated between barrier post areas by wire tying an empty plastic water bottle and a 2 by 4 by 1 inch thick Styrofoam under horizontal reinforcing.

Sonic/ultrasonic impact echo data was acquired at 0.5 foot intervals along the lines of coverage approximately 6, and 21.5 inches below the top of the barrier on both faces of the barrier. Data were acquired using a system designed and built by NDT Corporation specifically for testing concrete and comprised of a linear array of four sensors and a projectile impact energy source.

Thickness resonant frequencies and sonic/ultrasonic wave velocity values were used to identify internal weak zones. Locations where the thickness resonant frequency are lower than average are indicative of weaker zones where the sonic/ultrasonic wave has diffracted around or traveled through the weak zone. Average compressional and shear wave velocity values recorded at each "shot" location are determined by the concrete strength. Areas of lower than average velocity values are indicative of weaker concrete.

Thickness resonant frequency results were more consistent at the upper barrier lines of coverage because the faces of the barrier are nearly parallel creating favorable conditions for development of resonant frequencies. Sonic/ultrasonic resonant frequency and velocity results indicated other rail post locations had results similar to those obtained at the calibration voids created with Styrofoam indicating voided or weakened concrete. Based on these findings, RI DOT engineers decided not to use any slip-forming on barriers with rail posts.

For additional information call NDT Corporation at 508-754-0417 or check our web site www.ndtcorporation.com.