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Investigation of Post-Tensioned Slab Cover in Mid-Rise Building

CA investigated allegations of improper top cover in eight levels of post-tensioned concrete slabs at a mixed-use office building containing ground floor retail space. During construction of the building, the concrete cover over the distributed post-tensioning tendons was allegedly inadequate at the drop beams. Eight levels of post-tensioned slabs were remediated by placing a cementitious, self-leveling topping over the entire slab to increase the top cover. CA conducted an investigation to vet the accuracy of approximately 800 top cover measurements performed by others and to identify possible trends and/or causes of any areas of improper top cover. CA performed ground penetrating radar (GPR) to measure the remediated top cover and the slab thicknesses. Rigorous calibration procedures consisting of physical measurements were employed to maximize the accuracy of the GPR top cover measurements. Using elevation survey measurements from before and after the application of the topping, CA determined the original as-built cover. CA’s investigation revealed systemic inaccuracies of the top cover measurements reported by others. Only approximately 10 percent of the measurements were less than the minimum allowable top cover.

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Retrofit of Reinforcement Concrete Cantilevered Seating Structure

CA investigated the structural adequacy of an as-built cast-in-place concrete cantilever seating structure in two levels of a sports venue’s grandstand. The cantilever section consisted of an almost 4-foot overhang. Sometime after placement, it was discovered that the cantilever’s tensile reinforcement was placed incorrectly. CA used ground penetrating radar scans to determine the actual reinforcement position, performed a structural analysis to determine the as-built capacity of the cantilever, and designed a retrofit to restore the capacity. CA performed a load test on the first section of repair to document the effectiveness of the retrofit solution. CA maintained a full-time field engineer to oversee and document the entirety of the grandstand repairs.

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Evaluation and Repair of Concrete Slab Placed in Inclement Weather

Carrasquillo Associates investigated and characterized the surface distress occurring at an elevated post-tensioned concrete slab in a parking garage that is part of a high-rise building under construction in downtown Austin, Texas. CA’s investigation included an examination of the extent of distress or unsound concrete in an area within one level of the parking garage equal to approximately 10,000 square feet, where rain had affected the fresh concrete surface soon after it had been placed. CA conducted a site visit to the parking garage to perform a visual examination and condition survey, and to obtain core samples from the concrete deck. The cores were studied by a concrete petrographer, who determined the extent of rain damage within the depth of each core. CA utilized this determination in developing a repair plan for the affected concrete surface to restore its serviceability and long-term durability. This project provides an example of CA’s expertise in recognizing a structural or materials problem and its root cause, assessing how it will affect the structure, and offering a repair solution that efficiently and reliably restores the structure.

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Convention Center CMU Wall Repair

Carrasquillo Associates designed repairs for a 50 ft. tall exterior concrete masonry unit (CMU) wall. During construction, it was discovered that the wall was constructed without the proper amount of reinforcement, essentially making it unreinforced. CA computed the loads acting on the wall from self-weight, cladding, and wind. Multiple wall sections were considered due to the varying cladding, geometry, and parapet details. Nearly the entirety of the wall was overstressed due to the wind load. CA’s repair involved adding a steel structural support to reduce the wall’s span and thus reduce flexural demands. This significantly reduced the overstressed areas such that localized areas could be strengthened with fiber reinforced polymer sheets. While developing the repair, CA collaborated with the Structural Engineer of Record to ensure that the added steel support would not overload the existing steel columns and braced frames.

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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.

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Reinforced Concrete Pavement Cracking

Carrasquillo Associates investigated cracking in a new reinforced concrete pavement. The cracking, consisting of regularly spaced longitudinal cracks occurring immediately over the reinforcement, was observed to have occurred a few days after the placement. CA’s analysis revealed an error in the dosage of retarder in some batches of the ready-mix concrete. In these batches, the retarder dosage was based on the entire weight of cementitious materials, rather than just the portland cement, resulting in excessive retardation. Excessive retardation made the concrete pavement susceptible to cracking due to shrinkage and dimensional movements. This propensity for cracking was exacerbated by the non-typical shallow top cover specified in the plans and drawings. CA’s investigation revealed that areas with the proper retarder dosage exhibited infrequent cracking, as compared to the extensive cracking in the remainder of the pavement where the retarder dosage was incorrect.

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Cementitious Underlayment Deficiencies in Hospital Operating Rooms

CA investigated the quality of a cementitious underlayment installed as part of the finish out of the 11th and 12th floors of an existing 33-story hospital building. The elevated concrete structural slabs of these floors were covered with a variable thickness cementitious underlayment material, with the intent to achieve an adequate floor surface that would later receive final floor finishes such as tile or carpet. The owner observed that this underlayment exhibited an inconsistent appearance, variable color, a dusty/powdery surface, and a poor bond with the finished flooring. CA evaluated the underlayment and assessed the expected performance of the floor, specifically its integrity and ability to resist wear due to long-term loading from wheeled medical equipment. CA’s investigation included a walk-through, visual condition assessment, core sampling, petrographic examination, and bond pull-off testing. CA’s analysis resulted in the removal and replacement of the underlayment to ensure a quality long-term floor system in the operating rooms and associated support rooms.

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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.

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GPR Scanning for Artwork Design and Construction

Carrasquillo Associates (CA) performed work related to ground penetrating radar (GPR) scanning for artwork to be installed. Prior to construction, CA scanned and mapped areas around the perimeter and approximately 900 sq. ft. of the art zone’s interior to determine appropriate places for through drilling between double tee stems and embedded anchors in double tee stems. After reviewing available construction documents, drawings, and project related material, CA completed extensive scanning of the parking deck using high resolution GPR equipment. CA mapped the location of precast double tee stems and embedments in the parking deck. Embedments and items found on the underside of the slab were identified as “no drill zones” and the extents of the double tee stems were also mapped. The project was a great success with over 900 sq. ft. scanned, and only one drilled hole encountering an unforeseen embedment.

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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.