Segments of a concrete bridge over Kosi River were impacted by a hydraulic lift trolley being shifted to side bays of stacking yard. The self weight of each segment was approx. 90 MT and during shifting to side bay on hydraulic lift trolley, due to change in supports of the segments of the pre-stressed concrete bridge, resulting in severe cracking. Epoxy injection WEBAC4120 was used to repair the cracks. Various types of non-destructive tests including Ultrasonic Pulse Velocity, Scanning for reinforcement and Rebound hammer were performed pre and post the repairs were completed to provide quality assurance of the epoxy injection filling of the cracks.

Repair Procedure

•     Clean the cracks. The first step is to clean the cracks that have been contaminated; to the extent this is possible and practical. Contaminants such as oil, grease, dirt, or fine particles of concrete prevent epoxy penetration and bonding, and reduce the effectiveness of repairs. Preferably, contamination should be removed by vacuuming or flushing with air or other especially effective cleaning solutions.

•     Seal the surfaces. Surface cracks should be sealed to keep the epoxy from leaking out before it has gelled. Where the crack face cannot be reached, but where there is backfill, or where a slab-on-grade is being repaired, the backfill material or sub base material is sometimes an adequate seal. A surface can be sealed by applying an epoxy, polyester, or other appropriate sealing material to the surface of the crack and allowing it to harden.

•     Install the entry and venting ports.

Two methods are in general use:

a.   Fittings (Mechanical wedge shaped packers) inserted into drilled holes. This method was the first to be used for the repair of the cracks. The method entails drilling a hole into the crack, approximately 10 mm in diameter and 15 to 25 mm below the surface of the section.

b.   Bonded flush fitting. When the cracks are not V grooved, a method frequently used to provide an entry port is to bond a fitting flush with the concrete face over the crack. The flush fitting has an opening at the top for the adhesive to enter and a flange at the bottom that is bonded to the concrete.

•      Mix the epoxy. This is done either by batch or continuous methods. In batch mixing, the adhesive components are premixed according to the manufacturer’s instructions, usually with the use of a mechanical stirrer, like a paint mixing paddle. Care must be taken to mix only the amount of adhesive that can be used prior to commencement of gelling of the material.

•      Inject the epoxy. Epoxy injection requires a high degree of skill for satisfactory execution, and application of the technique may be limited by the ambient temperature. High pressure electrical operated diaphragm pump was  used. The pressure used for injection must be selected carefully. Increased pressure often does little to accelerate the rate of injection. If the crack is vertical or inclined, the injection process should begin by pumping epoxy into the entry port at the lowest elevation until the epoxy level reaches the entry port above. For horizontal cracks, the injection should proceed from one end of the crack to the other in the same manner. The crack is full if the pressure can be maintained. If the pressure cannot be maintained, the epoxy is still flowing into unfilled portions or leaking out of the crack.

•      Remove the surface seal. After the injected epoxy has cured, the surface seal should be removed by grinding or other means as appropriate.

Nondestructive Quality Assurance Program

After the repairs were complete, a quality assurance program was developed using a Ultrasonic Pulse Velocity (UPV) test.

Ultrasonic Pulse Velocity (UPV) Method

The UPV method involves measuring compression wave velocity and amplitude in concrete by measuring the direct travel times and amplitude of compression waves. In general, low velocities and amplitudes indicate poorer concrete quality (or cracks). This method requires 2-sided access to the structure for direct method and on same surface for indirect method. Typically, two 54 kHz UPV transducers used as a source and a receiver are greased-coupled to the concrete and placed at two locations on a structure with a known distance between them.

Conclusion

Non-destructive testing is proving to be very useful in both initial concrete condition assessment and in the quality assurance process for concrete repairs. The non-destructive testing program used for quality assurance of epoxy crack injection consists of Ultrasonic Pulse Velocity method. Once a possible crack was defined, Ultrasonic Pulse Velocity tests with angular paths were performed to provide more information on the location and extent of the unfilled cracks.