Nondestructive Evaluation of LNG Domed Roof Slab

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CTLGroup was contracted to identify latent defects in two 200-ft diameter and 14-inch thick reinforced concrete Liquid Natural Gas (LNG) tank domed roofs for a confidential client. Approximately five years old at the time of the investigation, the roofs exhibited cracked and delaminated areas during a routine maintenance inspection. Removal of delaminations exposed significant voiding within the roof slab concrete.

CTLGroup’s investigation involved the following tasks:

  • Visual survey and documentation of observable distress
  • Hammer sounding of selected areas of the concrete to identify near surface delaminations
  • Nondestructive evaluations using impulse response (IR) and ultrasonic shear wave tomography (UST) to identify potential anomalous concrete areas
  • Identification of representative concrete core locations using a combination of IR data and visual survey findings
  • Identification of location of embedded reinforcing steel using ground penetrating radar (GPR) in proposed core locations
  •  Observation of concrete core removal to confirm nondestructive findings

Prior to CTLGroup’s arrival, test grids were marked on the concrete surface. A grid spacing of 50cm was used in areas with suspected deficiencies and grid spacing of 2m was used in areas thought to contain no deficiencies. IR testing was performed at each grid location. Based on IR test results and visual observations, CTLGroup identified locations for removal of concrete core samples. Core locations were selected to represent the range of nondestructive test data. Cores were then used to correlate the test data to site conditions and to physically characterize the nature of the concrete defects. CTLGroup demonstrated that the nondestructive testing could reliably identify significant defects. The testing was then used to mark areas where repairs were necessary.

Repairs were time-critical and there were significant constraints including the remoteness of the facility and the need to accomplish repairs without affecting ongoing operations. CTLGroup provided recommendations regarding available construction methods that could be used to accomplish durable repairs. Conditions encountered during repairs confirmed that IR testing had accurately identified presence of defects and that sound concrete was present at the boundaries of the areas identified for repair.