Nondestructive Evaluation (nde) Methods for Quality Assurance of Epoxy Injection Crack Repairs

A concrete bridge over Interstate 70 east of Denver, Colorado was impacted by a fork lift being transported by a truck moving at a speed of approximately 75 mph. The forklift impacted the east face of the prestressed concrete I-beam bridge girder, resulting in severe cracking and concrete spalling on both sides of the eastmost girder. Polymer-modified repair mortar was used to repair spalled areas and epoxy injection was used to repair the cracks. Various types of nondestructive tests including Impact Echo, Ultrasonic Pulse Velocity and Spectral Analysis of Surface Waves were performed after the repairs were completed to provide quality assurance of the epoxy injection filling of the cracks and concrete repairs. Although three nondestructive evaluation (NDE) methods were used in the investigation and all three provided useful information for the quality assurance process, this paper focuses on the primary nondestructive testing program consisting of a combination of Impact Echo and Ultrasonic Pulse Velocity tests. The combination of these two methods proved to be very effective in locating internal unfilled cracks. The results from both methods correlated well and taken together the NDE results better defined the extent of isolated, unfilled cracks. Introduction A concrete bridge over westbound Interstate 70 just outside Denver, Colorado, was recently impacted by a forklift carried by a truck moving at a speed of approximately 75 mph. The impact resulted in cracking and concrete spalling on both sides of the easternmost prestressed concrete Ibeam bridge girder. The damage was most severe on the east side of the girder where the impact occurred, as shown in Fig. 1, although significant cracking was also seen on the west side of the girder as well. Epoxy injection and a concrete patching material were used to repair the cracks in the damaged girder. After the repairs were complete, a nondestructive testing program was developed to help assure the quality of the repairs by checking for areas of unfilled cracks. The nondestructive testing program used in this investigation consisted of Impact Echo (IE), Ultrasonic Pulse Velocity (UPV) and Spectral Analysis of Surface Waves (SASW). Since a combination of IE and UPV tests was found to be most effective in characterizing unfilled cracks in this repaired concrete bridge girder, this paper focuses on the combination of these two methods. The SASW tests were used in a more limited role in this investigation, primarily in areas with unfilled vertical cracks to increase the confidence level in the quality assurance program. The nondestructive tests used for this quality assurance cannot determine the bond strength of the epoxy material used in the repairs, but can identify areas of unfilled cracks. Repair procedures, brief backgrounds of the IE and UPV test methods, the nondestructive field investigation and the results are presented herein. Fig. 1 Damage on the Eastmost Concrete Girder before the Repairs REPAIR PROCEDURE The impact on the bottom flange of the bridge girder caused the girder to rotate inward toward the center of the bridge. The rotation was resisted by the concrete diaphragm attached to the web of the girder, causing a large amount of spalling and cracking to occur in the region of the diaphragm. The full depth and partial depth spalled web areas were repaired by chipping out all loose or damaged concrete using light (15 pound) electric chipping hammers so that the repaired areas were approximately rectangular in shape. Edges of the repaired areas were cut perpendicular to the girder a minimum of 3⁄4” deep. The repair area was pre-saturated with water and the surface allowed to airdry to a saturated surface dry condition. Master Builders Emaco S77 structural repair mortar was installed using the form and pour method and cured for seven days. After the web repairs were complete, the bridge girder was pre-loaded and the bottom flange repairs were completed as detailed above. After the web and flange repairs were complete, approximately 800 lineal feet of cracks were injected with Master Builders Concresive 1380 Injection resin (Fig. 2 shows beam prior to epoxy Fig. 2 The Damaged Girder Prior to Epoxy Injection Repairs Fig. 3 The Damaged Girder with Injection Ports in place injection, Fig. 3 shows injection ports in place). As part of the overall quality control process, inspections were performed by the structural engineer prior to placement of repair mortar and at other critical times during the repair work. NONDESTRUCTIVE QUALITY ASSURANCE PROGRAM After the repairs were complete, a quality assurance program was developed using a combination of two nondestructive tests: Impact Echo (IE) and Ultrasonic Pulse Velocity (UPV). This part discusses general backgrounds of IE and UPV tests, nondestructive testing field investigation, and nondestructive testing results. Impact Echo (IE) Impact Echo is a stress wave method using a small steel impactor to generate an impact on the face of a member, and a nearby receiver to pick up echoes of the impact. The IE test requires only one-side access to the structure. The resonant echoes from the impact of the displacement responses in time domain are recorded by a displacement transducer mounted in contact with the test surface next to the impact location. Resonant echoes from member thicknesses and/or flaws are not readily apparent in the time domain but are more easily identified in frequency domain. To accomplish this, the linear frequency spectra of the displacement response is calculated by performing a Fast Fourier Transform (FFT) analysis on the received signals to determine the resonant echo peak(s). A simplified diagram of the method is shown in Fig. 4. The relationship between the resonant echo depth frequency peak (f), the compression wave velocity (VP) and the echo depth (D) is expressed in the following equation: