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.

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.

Architectural Concrete Repair Recommendation

Carrasquillo Associates reviewed as-cast white architectural columns exhibiting discoloration at the edges of the formwork. The columns were to be removed and replaced. Instead, CA provided recommendations to develop a repair mixture to correct and minimize the appearance of the discoloration, which would be satisfactory to the architect.

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.

Concrete Repairs under Severe Exposure at Wastewater Treatment Plant

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.

Forensic Investigation into Distress of Rigid Pavement

This project consisted of various segments of approximately 9.7 miles of Portland Cement Concrete Pavement (PCCP) on Road US 64/87, located in Union County near Des Moines, New Mexico. The project was divided into East Bound Lane and West Bound Lane sections each consisting of two traffic lanes. CA was retained to identify the possible cause(s) and/or contributing factor(s) of the distress in the PCCP, as well as the repairs made during and after construction. The investigation included on site evaluation, photographic documentation, analysis of technical documents, among others. CA was able to identify segments of most distress associated with concrete placement and location, and the cause(s) of the distress associated with the PCCP.

Alkali-Silica Reaction (ASR) in Pier Structure

Carrasquillo Associates (CA) was retained in 2012 to investigate concerns about the structural integrity and long term performance of a pier structure in Honduras and to determine the deterioration mechanisms affecting the durability, the structural safety and useful life of the structure. The investigation into the structure included visual inspection, crack monitoring, concrete sampling, petrographic examination, chloride testing, structural analysis, and half-cell testing among other items. Based on the testing performed, the internal concrete expansion damage mechanism was identified and the remaining service life of the structural performance of the structure was assessed. Further, several alternatives for restoring the service life and the facility were evaluated. This allowed the client to better understand the cause of the distress that was being observed and what needed to be done to allow the structure to continue to operate safely moving forward.

Slab-On-Grade Distress Investigation and Analysis

The project consisted of two aircraft assembly hangars consisting of 8 in. thick cast-in-place slabs-on-grade with 0.5 in. wide and 2 in. deep joints.  The facilities were constructed during the period from 2004 to 2005.  CA was retained to conduct an investigation and determine the cause(s) of the distress occurring at the joints that consisted of the joint sealant protruding up to a 0.5 in. above the surface of the slab-on-grade.  CA’s investigation consisted, among others, of several site visits, concrete core sampling at the joint sealant locations, shore hardness testing, evaluation and different joint sealant, test program, and condition surveys.  CA determined the cause(s) of the distress and provided recommendations for remediation of the distress including material selection and installation procedures and recommendation for joint detailing for future construction.

Alkali-Silica Reaction Residential Home

CA was retained in order to investigate the long term performance and possible remediation alternatives for the cast-in-place concrete slab-on-grade foundation at the residence affected by alkali-silica reaction (ASR). Shortly after placement, the foundation exhibited signs of ASR by exuding the slippery gel product throughout the slab causing a safety hazard for the owners. CA was tasked with determining the extent of the damage as well as provide a set of guidelines for repair. CA’s investigation consisted of document review of concrete delivery records, photographic evidence of the site, litigation documents, concrete sampling,  petrographic examination, and three site visits to the residence.

NDT Assessment, and Repair of Concrete Basement Wall

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.