Municipal Water Tank Roof Failure

CA investigated the structural integrity of a 275,000 gallon municipal water tank in Vermont. The water tank consisted of a cast-in-place concrete foundation slab, cast-in-place concrete walls, and a precast, hollow-core plank roof with a continuous cast-in-place concrete topping slab over the planks. After only about ten years in service, distress was noted on the underside of the precast, hollow-core planks in the form of cracking and fractured and fallen sections of concrete. CA investigated the distress and alleged failure of the water tank. CA focused on the structural system, construction, and condition of the water tank as well as determining the cause(s) of the observed distress. CA reviewed construction documents, construction specifications, previous engineering and petrographic summary reports, and relevant published literature and industry standards. CA’s field observations, sampling, and petrographic examination revealed that the overall tank structure was in good condition and that the distress in the roof structure was due to an internal sulfate attack mechanism. CA determined that the distress mechanism emanated from planks’ manufacturing process.

Inspection and Rehabilitation of Underground Tunnel System

The Govalle Tunnel is an approximately 8 mile long, 96-inch diameter, cast-in-place concrete tunnel at an average depth of 100 feet that conveys wastewater generated in central and southeast Austin, Texas to the South Austin Regional Wastewater Plant. Inspections performed by the City of Austin in 2002 identified rehabilitation needs for the tunnel system. Rehabilitation work included structural repair of large diameter and deep shafts and laterals using a cementitious repair material followed by the application of an epoxy protective system. The purpose of these repairs was to remediate the impacts of concrete deterioration due to exposure to sulfates and steel reinforcement corrosion and to protect from future deterioration. CA was retained by the City of Austin to determine the cause(s) and/or contributing factors resulting in the failure of the rehabilitation work using an approved repair mortar. Among the most relevant issues investigated were the quality of the substrate surface preparation, repair materials characteristics and properties, repair practices, and behavior, performance and curing requirements of the repair mortar at different temperatures.

Concrete Basement Wall Assessment and Repair

Carrasquillo Associates (CA) assessed the condition of a 22 in. thick reinforced concrete basement wall and pilasters during construction after irregularities were observed on the surface of the concrete wall. CA determined the extent of the deficiencies and developed a repair protocol for the wall. CA’s investigation of the wall included a visual survey, non-destructive testing (NDT) using ultrasonic echo 3D tomography, and concrete core sampling for visual examination and compressive strength testing. CA non-destructively scanned the wall using ultrasonic echo 3D tomography, a method that uses ultrasonic shear waves to locate internal defects, reinforcement, and the back surface of the concrete. The majority of the NDT testing data did not indicate the presence of internal defects within the wall. Verification cores and compressive strength testing from several of these areas confirmed the absence of internal defects and/or low strength concrete. At CA’s recommendation, the project team approved the repair of the wall.

Assessment and Repair of Cast-in-Place Concrete Framing

Carrasquillo Associates investigated deficiencies observed on the surface of the concrete for the second floor cast-in-place concrete framing, which consisted of conventionally reinforced beams, girders, column tops, and the second floor slab. CA reviewed the condition of the cast-in-place concrete to determine the extent of the deficiencies, to develop a repair protocol, and to selectively observe and document the repair work. CA’s investigation included a visual condition survey, non-destructive testing (NDT) using Ultrasonic Pulse Velocity (UPV) equipment, and concrete core sampling. CA’s visual condition survey identified several types of possible deficiencies. These included surface honeycombing, poor consolidation, internal voids, and visible lift lines and/or cold joints. CA demonstrated that the majority of the observed deficiencies were near surface and did not impact the structural integrity of the concrete structure. Based on CA’s recommendations, the project team decided that all of the observed deficiencies could be repaired.