FPrimeC was engaged by an Anhydrous Ammonia manufacturer in Trinidad to perform asset integrity assessment and nondestructive evaluation of cooling tower basin. A preliminary assessment of the tank revealed the following key problems concerning the durability and structural integrity of the concrete structure:
- Degradation of concrete
- Extensive concrete cracking
- Sign of steel corrosion, delamination and spalling.
Formulating an Inspection and Test Plan
Upon reviewing the existing evidence, and further communication with the client, FPrimeC formulated a comprehensive structural condition assessment program to evaluate the structural and durability performance of the concrete cooling tower basin, and to identify all repair and maintenance needs. The main objective of this condition assessment was to:
- Assess the existing condition of the cooling tower basin
- Determine the root cause of the problem
- Identify the extent and severity of defects (corrosion); and
- Provide preliminary repair/rehabilitation options.
The detailed digital inspection followed by a comprehensive multi-technology Non-destructive Testing and Evaluation (NDT-E) program for the condition assessment of concrete walls. The Non-destructive evaluation was validated through a carefully designed intrusive assessment and involved obtaining a limited number of core samples for compressive strength evaluation, as well as NDT-E.
- Digital Inspection (LiDAR capture)
- In-Situ Non-Destructive Testing:
- Ultrasonic Pulse Velocity (UPV)
- Rebound Hammer (RH)
- Ground Penetrating Radar (GPR)
- Half-Cell Corrosion Potential Mapping
- Concrete Core Extraction:
- Compressive Strength
- Surface Electrical Resistivity
- Cross Width Ultrasonic Pulse Velocity
Digital Inspection of Cooling Tower Basin
The cooling tower basin has an overall dimensions of 360 ft x 106 ft and overall height of 11 ft above ground. FPrimeC used a unique digital inspection technology to map the visual condition of the tank using 3D LiDAR capture, laser measure and camera. For this inspection, the ACI Visual Condition Survey of Concrete- Guide (ACI PRC-228.4-23) were used. Where applicable, a standard crack width gauge was used to determine the crack width at multiple test stations along the length of each crack.
The procedure allowed FPrimeC to map out all major anomalies on the surface in an incredibly short timeline. The LiDAR captures were stitched and analyzed using our in-house suit of software.
Nondestructive Evaluation of Cooling Tower Basin
Multi-technology approach was adapted for the nondestructive evaluation of cooling tower basin. Various NDT-E methods were deployed to study the quality, integrity, and uniformity of concrete as well as identifying structural details. Furtheremore, advanced NDT methods were used to study the corrosion in concrete walls.
1. Ultrasonic Pulse Velocity
Ultrasonic Pulse Velocity (UPV) was used to investigate the quality and structural integrity of the concrete, based upon ACI Committee 228 guidelines. The results helped FPrimeC identify areas with potential poor quality concrete.
2. Schmidt Hammer (Rebound Hammer)
The Rebound Hammer test was used to assess uniformity and quality of the concrete. This method has been developed through different standards and guidelines such as the ASTM C 805 “Standard Test Method for Rebound Number of Hardened Concrete”, and ACI 228.1R “In-Place Methods to Estimate Concrete Strength”. This test consists of measuring the rebound of a spring driven hammer mass after its impact with concrete. The rebound test helped FPrimeC engineers identify areas with potential surface anomalies across the large area of the thickener wall.
3 – Ground Penetrating Radar (GPR)
GPR was used to verify the rebar spacing, and depth at various locations across the concrete walls. Furthermore, GPR scans were used to identify the location of potential sub-surface defects such as delamination.
4- Half-Cell Corrosion Potential Mapping (HCP)
The main deterioration mechanism in the concrete elements of the cooling tower basin was rebar corrosion. Therefore, several areas across the walls were tested using Half-Cell Corrosion Potential (HCP) mapping method to determine the likelihood of active corrosion. The results were presented in heatmap format, allowing the client to identify the locations with higher chance of corrosion.
Intrusive Assessment
To provide further information on the condition of the concrete, concrete core samples were extracted from the concrete surface at strategically selected locations. The results of preliminary inspection and nondestructive testing helped identify the most appropriate location for extracting concrete samples. The core samples were taken for additional non-destructive testing, visual examination, and compressive strength.
1. Ultrasonic Pulse Velocity
Ultrasonic Pulse Velocity (UPV) was performed on core samples to assess the overall quality, uniformity and homogeneity of the concrete cores, based upon ASTM C 597 standards. This method involves transmitting high-frequency sound waves through the concrete using transducers placed on its surface and recording the time taken for the waves to travel between them. The UPV measurements were taken with two different adaptations to better assess concrete cylindrical cores. The first adaptation places both transducers on the ends of the cylindrical specimen, evaluating the velocity in the longitudinal direction inside the core.
The second adaptation places the transducers on opposite sides; 180° apart along the curvature of the cylinder, and takes separate UPV reading at specific intervals on the core profiles. This method examines the consistency of the velocity across the depth of the core, which can further indicate if there is much change in quality between the outside and inside face of the tank.
2. Surface Electrical Resistivity for Durability of Concrete
Surface Electrical Resistivity testing was used to assess the penetrability of concrete against chloride ion, based on AASHTO T358, 2015. The test was performed using a common 4-Pin Wenner Probe. The test results allowed engineers get a better understanding of the porosity of concrete materials. Another objective was to identify if the concrete material had a porous structure, or if the materials inside the tank might have caused the deterioration of the tank.
3. Compressive Strength of Concrete
Compressive strength testing on concrete cores involves extracting cylindrical samples from hardened concrete structures. The core samples were trimmed, ground plane, and tested for compressive strength in accordance with CSA A23.2‐14C. The test results helped FPrimeC and the client better understand the impact of the ongoing deteriorations on the mechanical properties of concrete.