News and Events - "DS Article"

Welcome to our newsroom! Here you will find the latest information about our company, projects and people. Browse articles published by our engineers and scientists in national publications and conference proceedings, view our press releases and read through news coverage of Terracon.

If you are a member of the media, you may contact our media relations representative at media@terracon.com.


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The Advantages of Non-Destructive Testing (NDT) Techniques for Evaluating the Compressive Strength of In-Place Concrete

The evaluation of compressive strength of in-place concrete is often needed for projects of new constructions, structural additions and remodeling, and structural repairs. Traditional method of testing the compressive strength of concrete cores retrieved from a structure can be time-consuming and possibly damaging to the structure when a large number of structural members are under evaluation.  Alternative methods that reduce the needs of coring and cause no damage or limited superficial local damage to the structure are becoming more popular.

Non-Destructive Testing (NDT) Techniques

Non-destructive testing (NDT) techniques of evaluating in-place concrete strength measure some properties of concrete which can be correlated to the compressive strength of concrete cores. The well-known guidance of performing in-place concrete strength estimation are ACI 228.1R and ACI 228.2R which are issued by American Concrete Institute (ACI) Committee 228. Pullout testing is recommended by ACI 228.2R as the primary method of estimating the compressive strength of in-place concrete based on accuracy compared to other NDT techniques.

Test Methods for Determining Material Properties of Hardened Concrete in Existing Construction (referencing ACI 228.2R-13 Table 4.1.1)

Property Method
Primary Secondary
Compressive strength (ACI

228.1R)

Cores for compression testing

(ASTM C42/C42M and C39/C39M);

pullout testing (post-installed)

(ASTM C900)

Penetration resistance (ASTM C803/ C803M)
Relative compressive

strength

Rebound number (ASTM C805/

C805M); Ultrasonic Pulse Velocity (UPV) (ASTM C597)

In-place pulloff test (ACI 503.1R:

BS 1881-207) with appropriate

calibration.

NDT Techniques Prevents Project Delays

Pullout-TestThe Terracon team recently worked with a client on a project where a sixteen-story steel framing structure is proposed to be built on an existing twelve-year-old one-story reinforced concrete structure. The concrete mixture design information and concrete material testing records for the in-place concrete were not available to the client. To assist the client in determining if the existing reinforced concrete structure had sufficient capacity to carry the load from the proposed steel structure, pullout testing was performed on three footings and twenty-nine plinths which were identified as critical structural members by the structural engineer of record.

Terracon performed both core strength testing and pullout testing on 15 structural concrete members to develop a job-specific strength correlation. Once the strength correlation was established, the in-place concrete strength of additional members was estimated with pullout testing only. Pullout testing at each additional location was completed in about 40 minutes total from sample preparation to strength estimation The results indicated that the concrete compressive strengths were structurally adequate per the acceptance criteria given by ACI 221.1R. The pullout testing procedure allowed for testing of more locations more efficiently than if only testing core samples. A significant delay in project schedule was avoided.

Pullout testing is just one of the several NDT techniques that Terracon uses to evaluate in-place concrete strength, depending on project variables. Terracon is experienced with field and laboratory testing and statistical analysis, which assures NDT options that can be coordinated with the client and reliable and resourceful services are delivered in a cost-efficient and time-saving manner.


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Terracon Leads Efforts for “Legacy” Oilfield Site Reclamation

Thousands of oil and gas wells have been drilled in North Dakota over the past six decades – many long before the environmental regulations of today were in place. Today, some of the sites damaged from early exploration and production activity, referred to as “legacy sites”  remain and need attention.

In many cases, legacy site locations have been known for years but could not be addressed due to lack of funding or jurisdiction. Now, under state law with funding allocated and legislation in place, abandoned legacy site wells and production facilities where no responsible party remains, can be evaluated and action taken to address environmental impacts. The goal of this funding under the North Dakota Abandoned Oil and Gas Well Plugging and Site Reclamation Fund (AWPSRF) is to repair damages from legacy oil exploration and production to restore the land to productive use.

Developing feasible corrective action plans

Terracon was selected by the North Dakota Industrial Commission (NDIC) to provide environmental response services to remediate and reclaim legacy sites damaged by exploration and production activities and unclaimed petroleum hydrocarbon and brine releases associated with former well pads, pits, and seismic boreholes. Terracon’s environmental services include environmental response, site investigation, clean-up, remediation, reclamation, and well plugging, as well as drilling or other geotechnical and materials testing services.

Recently, Terracon was tasked with investigating subsurface impacts of a legacy reserve pit in western North Dakota where obvious surface impacts of stressed and dead vegetation were observed below the historical pit toward a nearby creek. A limited site investigation (LSI) was conducted to determine the extent and magnitude of contamination and to provide corrective action recommendations for remediation and reclamation. Based on the LSI results, initial cost estimates to remove and replace impacted soils, and the site’s priority level based on environmental risk, corrective action was put on hold until additional funding could be made available through the state legislature.

The initial design of the corrective action was revised to limit the amount of soils to be hauled offsite and avoid impacts to the nearby creek. The revised corrective action focused on: source removal of the pit and soils identified to be too contaminated to support vegetation based on electrical conductivity and chloride; mixing “borderline” soils and placing them in the bottom of the excavation; and experimenting with a soluble algae bioremediation product in wetland areas. This redesign reduced the cost to the state by more than $500,000 and resulted in 50,000 tons of soil being removed from the site. Because of the success of previous remediation and reclamation projects under the AWPSRF, the state approved additional funds to address the legacy reserve pit.

Restoring the natural environment

A Corrective Action Plan (CAP) was submitted to and approved by the NDIC. The scope of services in the CAP included the solicitation and selection of an excavation contractor experienced in brine and petroleum hydrocarbon soils remediation and selection of a special waste landfill to dispose of the impacted soil. To accomplish goals of final reclamation desired by the state, landowner, and tenant, environmental consulting oversight was provided during excavation to document site activities, field screen soils for segregation, and directly place backfill for grading. A storm water pollution prevention plan was designed as part of the CAP, and the erosion and sediment controls implemented ultimately proved vital to the project as drought conditions at the beginning of the project gave way to many work days being cancelled due to rain. Drone and GIS services were also provided in conjunction with laboratory analytical results to maintain remediation goals throughout the project to ultimately return the impacted area to productive agricultural use.


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Building an Airport in a Swamp: New Orleans Airport Construction Presents Challenging Site Conditions

The City of New Orleans is known for many things—Mardi Gras, the French Quarter, and Cajun cuisine, to name a few. This unique, diverse city located along a bend of the Mississippi River will celebrate its tri-centennial in 2018. As a tribute to this historic milestone, a new world class airport terminal was planned—the Louis Armstrong New Orleans International Airport.

The airport is one of the critical infrastructure projects that the City of New Orleans initiated after Hurricane Katrina devastated the area in 2005. The $950 million terminal complex will include three concourses encompassing over 1,000,000 square feet of space, 35 gates, a 2,000-space parking garage, and elevated access roads.

Terracon was selected by the global engineering design firm, WSP, to serve on the Quality Assurance (QA) team along with other consultants on behalf of the New Orleans Aviation Board.

Materials Testing Challenges

The new airport site, along with much of the city, is developed on very unstable soils that can be considered swamp land. To build on these subsurface conditions, significant ground improvement is necessary. Deep pile foundations were required to support buildings, as well as wick drains and surcharging to reduce settlement before construction of pavements could begin. Although Terracon’s engineers and technicians typically perform pile capacity and integrity testing on a percentage of piles in a given project, the airport project required testing on more than 7,600 piles.

Ready for the challenge, the Terracon project team quickly developed a strategy to undertake this massive testing requirement. The pile monitoring program developed by the team included almost 1,000 18-inch diameter concrete piles in the elevated roadway, and more than 6,600 14-inch concrete square piles in the main terminal and concourses. Piles ranged from 95 to 110 feet in length.

Value-Added Services

As the project has continued over the past two years, our scope has evolved and grown to include not only QA, but also special inspections, and building enclosure testing services for this massive project.

To support the project’s extensive materials inspection needs, the team provided field engineers to perform pile integrity testing on 100% of the precast piles supporting the buildings, and pile-driving analyzer (PDA) on 2% (150+ piles) of the precast piles driven for the project. We also provided special inspectors for reinforcing steel and concrete, masonry construction, structural steel connections, spray applied and intumescent fireproofing, and technicians for concrete sampling and testing.

As the project progresses, more testing will be required. We are also performing special inspection of masonry construction for the concourses, fireproofing inspection and roofing observation for the terminal and concourses. On the site, utility installation continues with Terracon technicians performing density tests of backfill, as well as cement treatment of pavement subgrade soils.

The Louis Armstrong New Orleans International Airport is currently scheduled to open in early 2019. We are proud that Terracon’s team of engineers, technicians, and partners are making a major contribution to keeping this momentous project on track for a safe, quality, and timely completion.


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Save with Seismic Site Evaluation: Piedmont Atlanta Hospital's Expansion

The science of structural design serves to protect the well-being of facility patrons and surrounding communities. The International Building Code (IBC) requires that structural design must account for the forces imposed by a potential seismic (earthquake) event. The design is governed by three factors:

  1. The Seismic Design Category (the critical or non-critical nature of the structure),
  2. The Site Class (a function of the subsurface conditions to 100 feet), and
  3. The proximity and magnitude of a probable seismic event.

The first factor, Seismic Design Category, is based on the anticipated use of the structure and occupancy, and leaves no room for interpretation. The IBC usually puts a hospital among the most rigorous design categories. Similarly, the third factor, probability of a seismic event, is based upon published information. The real work goes into determining the second factor; Seismic Site Class. When designing a hospital, it is important to get the second factor, Site Class, right.

Site Class is determined by the shear wave velocity of soil and rock within the upper 100 feet at the site. This can be estimated from Standard Penetration Test N-values (STP N-values), the geotechnical engineer’s experience and laboratory test results. But estimation of such a value must exercise conservative assumptions, and the cost of such conservative estimates may be very large for structures such as hospitals. Direct measurement of shear wave velocity can be accomplished, but at greater expense than simple estimations. When is it appropriate to directly measure this critical parameter, and when will estimations from SPT N-values suffice?

Meeting Challenges with Experience

In 2014, Piedmont Healthcare began planning and designing a new inpatient tower and subsurface parking deck at the Piedmont Atlanta Hospital. This top-rated acute-care community hospital, originally built in 1954, needed a major update. The project, totaling more than $600 million, included approximately 900,000 square feet of new construction and 47,000 square feet of renovations. Terracon’s skilled consulting and specialized field services allowed the seismic design for the hospital to be optimized, and resulted in estimated savings of hundreds of thousands of dollars.

This hospital is a IBC Seismic Design Category IV, requiring the highest level of protection during seismic events. Understanding the impact of conservative estimates on this large structure, the Site Class was initially established during the geotechnical exploration using surface measurement techniques of shear wave velocity, a common approach for typical projects. This geophysical testing approach resulted in a Site Class C. This Site Class requires significant foundation and structural elements to address the stability of the structure in the event of an earthquake.

Define the Problem to Create the Solution

The initial structural designs for the Site Class C conditions would require a significant outlay of funds to meet structural code requirements. Thinking proactively, Terracon collaborated with the structural engineer and architect to consider options that would alleviate this costly condition.

We developed an innovative solution to the challenging seismic evaluation and approached the problem from two fronts; reducing the amount of softer materials below the building and refining the shear wave velocity measurements with more rigorous geophysical site characterization.

The bottom floor level was lowered to within 10 feet from the top of rock. In addition, a more comprehensive downhole seismic test was employed to optimize shear wave velocity measurement. This testing approach utilized a single deep borehole drilled into bedrock using a downhole pneumatic hammer. A temporary casing was inserted to protect a sensitive geophone that was lowered to test depths throughout the profile. The geophone measured the first-arrival times of surface-generated shear waves. By comparing shear wave arrival times at different depths, the average shear wave velocity profile, well into bedrock, was calculated.

Staff from three Terracon offices worked together to perform the downhole seismic testing and successfully determined the necessary conditions were present to recommend a more favorable Site Class B.

The Value of Partnership

Following work on the design phase, Terracon’s Materials professionals addressed specific needs of construction materials testing and special inspections. Our partnering approach, knowledge of site conditions, the use of experienced field personnel, and multi-year commitment to working with the design and construction team, all contributed to Piedmont Healthcare’s selection of Terracon for construction materials testing and inspection services.


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A Historic Homecoming: Relocation of the Livermore Depot

Without tracks, trains, passengers, or freight, is a railroad depot still a railroad depot? Old buildings, no matter their significance, are always inextricably connected to the ground they sit on. However, when a historic building loses its context, it can also lose its significance.

Redevelopment can often mean a dismal end for historic buildings, even if they are recognized by the community as an important local landmark. Yet, Terracon’s historic preservation specialists in the Facilities division are adept at creating innovative strategies to care for historic structures and plan for their future maintenance. The Terracon team jumped at the opportunity to collaborate with the general contractor to relocate the California Southern Pacific depot in Livermore, Calif. The depot, which was originally built in 1892, was to be safely moved to a nearby location adjacent to the long-since rerouted tracks.

Preparing for a safe departure

The first step in the move process was an evaluation of the depot’s existing historic fabric, and execution of a treatment plan to ensure the depot’s safe and intact arrival at a new location as soon as possible.

Although documentation about how to move a train station didn’t exist, the team did find records in the Southern Pacific Railroad architectural catalog from 1892 providing details about the balloon frame structure. Used as a passenger train station until the 1940s, the building later handled freight and served several commercial uses. The train tracks next to the depot were rerouted in the 1970s. When Terracon’s preservation team arrived on site, the train station appeared to have lost its identity.

After visiting two other historic #18 depots along the west coast, the Terracon preservation team created plans to survey, inventory, and record every piece of historic fabric removed from the building, without damaging the material. Ultimately, the intention was to create a manual to return each historic component to its exact location, and correctly reassemble the building on its new site.

Discovering New Ways to Preserve Architectural Details

Several surprises were found during the deconstruction process, and at times, it seemed the 125-year-old building just did not want to be taken apart. One discovery included the identification of the previously unknown original interior redwood tongue and-groove siding which was dormant, but intact, behind the modern sheetrock walls.

As it needed to be removed to accommodate the interior bracing framework, Terracon devised an experimental method to remove the century-old square nails without damaging the siding boards themselves. Our historic preservation specialists were on site for nearly two weeks, working closely with thebcontractor’s crews to instruct them how to preserve nearly every component of the structure.

To avoid disruption from street closures and power line relocation, an overnight move was planned for the depot. Terracon was part of the on-hand team to ensure the depot remained undamaged during the journey, which took nearly five hours.

More than forty years after the tracks were relocated, the depot followed. Now situated directly alongside the rails, the building that stewarded train passengers long ago will once again serve as a modern train station.

Traditional historic preservation convention relies on the notion buildings belong in the places they were built. Yet, when a small train station has lost its purpose, to restore its original use in a new and rightful location, is nothing short of a homecoming.


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Geotechnical Engineering in the 21st Century

Do you have the latest smart phone? Have you checked out the new crowd-sourcing app or trending Twitter hashtag? Change is fast-paced these days. Contrast that for a moment with geotechnical engineering.  For more than one hundred years, geotechnical site characterization has followed a standard process with little change. When we want to know about the subsurface, we drill soil borings, collect soil samples while performing the standard penetration test (SPT), perform laboratory tests, and develop a report that tells what we did and what we found. After all, what could change with that tried and proven approach?

It is time for geotechnical engineers join the pace of technological change. If technology can tell our car when it is safe to change lanes, or even change lanes for us, perhaps we should start to think about how it can change how we go about geotechnical characterization.

Creating a Powerful Resource with Big Data

Terracon is leading the way in changing the traditional approach to geotechnical engineering with the power of technology. There is a wealth of information about the subsurface geology, hydrogeology and geotechnical characteristics on the internet. Governmental agencies are sharing what they know about the subsurface. All it takes is the time to look and the discovery can be amazing. It doesn’t stop there. Geotechnical engineers are starting to understand the power of their big data. Terracon has more than two million soil boring logs in just about every geologic formation across the USA, and is capturing at least 100,000 more each year. We pull them out of long-term storage and put them into our geographic information system (GIS) retrieval platform. What used to take hours to find, now takes just seconds. All of this data, combined with the information that is shared by government agencies are powerful tools when we embark on a geotechnical characterization.

Noted economist and Nobel Laureate Professor, Robert J. Shiller, is quoted as saying “It amazes me how people are often more willing to act based on little or no data than to use data that is a challenge to assemble.”  To avoid this challenge at Terracon, we are combining the power of computers, GIS-based archiving and retrieval systems and even machine learning to start our geotechnical site characterization process with predictive analysis techniques.  These techniques incorporate what we already know, how consistent that information is and, bolstered by the opinion of our local, experienced geotechnical practitioners, what confidence we have in our predictions from the information available.

Predicting Conditions with Confidence

The day is coming when we will be able to implement machine learning technology to develop a virtual soil boring log for any given point within the United States.  In the interim, geotechnical engineers must activate their data, implement a program of staying current in their use of public domain data, and predict what will be encountered at a project site, along with predicting confidence in that estimate. With such predictions, and with an understanding of technologies that can be used to supplement common soil borings, some intrusive in nature, others using geophysics and nonintrusive, the geotechnical engineer can develop a smart work plan. That smart plan will confirm (or modify) the predicted conditions and gather the information necessary to make that foundation design with higher confidence.

Geotechnical engineers have run with our SPT hammer long enough. It’s time to let technological innovation help us do our work better, quicker and with more confidence.


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We have an App for that: Bringing Efficiency to Process

Environmental AssessmentSaving time and money on your next environmental site assessment just got a little easier. Terracon’s new Environmental Site Assessment (ESA) Field App allows assessors to be more efficient while conducting the site reconnaissance and completing report writing for Phase I ESAs.

This mobile app, developed by Terracon, is the first of its kind in the environmental industry. It can be used for most of Terracon’s Phase I ESA projects. Technology associated with the app creates opportunities for Phase I practitioners and other Terracon professionals to streamline assessment processes, efficiently coordinate project deliverables, and communicate issues to clients faster. The mobile app is estimated to save 25,000 to 30,000 total labor hours per year and won the EDR 2017 PRISM Technology Innovation Award.

“We are fortunate to have an incredibly dynamic team who enjoy rising to the challenge of creating efficiencies that help us serve our clients better each day.” John Sallman, National Director, Terracon Environmental Services.

 

Prior to the development of the app, each assessor would take a site reconnaissance form, note pad, camera, or mobile phone into the field to complete each reconnaissance. Then, back at the office, photos would need to be uploaded to the project file and notes taken would be transcribed into the applicable sections of the Phase I report.

Now with Terracon’s ESA Field App, all the assessor needs is a mobile phone! Once on site, our assessor collects necessary data to populate several sections of the ESA report, including the interview, site observations, adjoining properties, and if applicable, additional services such as limited sampling of building materials. The content of the app is structured based on the ASTM standard requirements. Additionally, the assessor can take representative photos and begin a site diagram, all within the app.

With necessary questions and user-friendly answering options, the app allows the assessor to streamline site reconnaissance and be confident the necessary information was collected in the field. Once the required questions in the app are answered, the data can be uploaded to Terracon’s server. The ESA report draft can be generated, with information entered in the interview, site observation, adjoining properties, and additional services sections of the app transferred to the applicable sections of the report. Photos taken in the app will be uploaded to the project file and the site diagram features will be uploaded to Terracon’s GIS Toolbox.

“It was really fun to be part of the team that was given the time and creative freedom to design this app,” said Emily Blakeway, field scientist in Terracon’s Seattle office. Emily was part of the app development team that included several assessors and IT personnel from Terracon offices across the country.


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Using Collaboration and Communication to Reduce Geotechnical Project Risk

One of today’s buzzwords is “Risk.” We all have experience with the prime example of risk and risk mitigation: insurance. Simply stated, insurance mitigates financial risk by paying some money to make sure that we never have to pay a lot of money if something goes wrong.

Risk is certainly relevant to the geotechnical profession. It comes into play when characterizing the subsurface soil, rock, and groundwater conditions (which, it turns out, can be quite variable), evaluating foundation design options, and selecting a particular design addressing the pertinent construction and performance problems to the satisfaction of the owner’s risk tolerance. But who “owns” the risk, and how is it exposed and mitigated?

 Preparing you to make informed decisions

Communication is a critical aspect of risk awareness and mitigation. At Terracon, we have developed a collaborative platform to inform our clients of the site conditions, foundation options and risks associated with the options. We want our clients to be well informed. With the right information, you can make the decisions that impact your construction costs and schedules, and adequately address your risk tolerance.

Using our web-based GeoReport platform, we inform you of subsurface condition, and associated compatibility considerations for your planned structure, and suggest various foundation designs and means of construction, noting the risks associated with each. For instance, a larger foundation, which may be more expensive, and slowly constructed, can reduce the obvious risks; however, a well-informed owner can select a foundation choice consistent with his or her risk tolerance, perhaps one that shortens construction time or reduces cost and maximizes value. To accomplish this interaction, vibrant collaboration among the consultant, owner, and other appropriate team members is essential.

The Hidden Risk

Geotechnical risk considerations can include unforeseen construction delays, unexpected subsurface conditions, poor foundation performance (such as excessive movements), and end-user safety.  There is also another risk that is always present, yet rarely discussed: the risk of excessive foundation costs and associated construction time for foundation elements that were not necessary in the first place. Overly conservative designs are the result of insufficient designer/owner collaboration and insufficient risk assessment. Just as it is important to understand risk exposures when buying insurance to avoid being over- or underinsured, collaboration is essential for a more complete understanding of your risk tolerance and can lead to more efficient designs and improved long-term performance.

Learn more about GeoReport

For a quick introduction to GeoReport, how it quickly provides the information we gather, our geotechnical recommendations for design and construction, and how it allows all stakeholders to think through the options together, watch this 80-second video.

 

We encounter risk in all we do, and that certainly applies to geotechnical aspects of construction. At Terracon, we are building a better way to work with you, our clients, as a team to understand and mitigate risks on your next project.


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Structural Steel and Welding Inspection – A Critical Service for Building Construction

FAA tower with rainbowStructural steel and welding inspection is a critical service for building construction. Poor welding or misalignment of steel components can adversely affect the structural performance or safety of a building. This is important for all buildings, but especially for critical structures such as hospitals and schools that must be able to withstand seismic events.

To protect buildings and their occupants, Terracon’s approach on structural steel projects is to form a close working partnership between our project inspectors and structural engineers.

Communication Yields the Best Results

Communication on these complex projects is key. Pre-fabrication and pre-erection meetings should be mandated to be sure that all team members (general contractor, inspector, fabricator, erector, and structural engineer) understand the requirements. These meetings are a good opportunity for the structural engineer to review the important aspects of the structural steel design, such as critical components like brace frames and moment frames.  Opening communication between inspectors and structural engineers saves time, allowing the inspector to help clarify project requirements, when questions come up during construction.

Welding procedures, applicable to the project, must be submitted and approved by the structural engineer.  Many structural engineers do not have the expertise to review welding procedures, and will utilize other consultants to advise and recommend acceptance.  Terracon has in-house welding experts to perform this welding procedure review.

Details are Critical

It is very important that structural components such as brace frames, moment frames, and other elements are installed in the proper location, within specified tolerances.  For upgrades in existing structures, there can often be existing conditions that interfere with the placement of the new structural steel members.

Inspectors must carefully detail any fit-up issues in the inspection report. The inspector needs to make sure that the contractor accurately describes nonconforming issues in Requests For Information (RFI) forms and submits them to the structural engineer for clarification and/or approved fixes.  Inspectors are required to be certified by the American Welding Society and/or the International Code Counsel and also need to be certified to perform, when required, nondestructive testing, such as ultrasonic or magnetic particle testing on welds.  These certifications require a high level of training and expertise.

A welding inspector must measure the size and length of every structural weld.  Field welds on brace frames are critical.  These welds are often welded out of position or can be undersized if there is a gap between the tube and gusset plate.  The fillet weld size must be increased by the size of the gap.  The alignment tolerances of the brace frame tubes are also critical and are usually detailed on the structural drawings.  There is often an erection bolt through the tube and gusset plate.  If the erector can’t get the erection bolt through the hole, there is likely an alignment issue that requires an RFI.

When welding new structural steel to existing structural steel, the existing steel must be cleaned to bare steel prior to welding.  Welding through coatings such as paint and galvanizing can lead to weld cracking and lack of weld fusion.

Bringing it All Together

When structural steel inspection is a required part of the project, it is important to build a collaborative team.  Structural steel and welding inspectors must have a high level of expertise and good communication skills. At Terracon, our materials professionals add value to your project at any stage. By joining your team early in the design process, we can identify, evaluate and recommend the right materials selection, welding procedures, and nondestructive testing, optimizing them for the project, which can speed construction and reduce costs.


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What’s Under the Hood (Concrete)?

Using ultrasonic echo tomography is a very useful structural diagnostic technique for diagnosing surface and internal concrete defects.

Ultra Sonic Echo Testing

Cracks, spalls, and surface defects such as voids, honeycombing, exposed rebar, etc., are typical symptoms of distress associated with reinforced concrete structures. These symptoms are typically identified as a precursor to a potentially larger issue hiding under the concrete surface. To an owner, this can mean unanticipated costs, potential impacts to building operations, and even safety concerns. The process of finding what has caused the defects will include an evaluation of distress symptoms such as cracks, voids, and surface defects. This effort requires a combination of visual assessment, and field and lab testing to develop a prognosis of the issue. One very useful and powerful tool used for assessment is ultrasonic echo tomography.

Defects in new or existing construction come from improper construction practices, improper design details, accidents, or sometimes natural disasters. These defects can impact the integrity of the structure and lead to sustainability and usability issues.

What is Ultrasonic Echo Tomography?

Ultrasonic echo tomography is a non-destructive test method used for evaluating the condition of hardened concrete by measuring the time of ultrasonic shear-wave transmitting in the hardened concrete. A shear-wave which is transmitted in an isotropic medium will be partly reflected when it reaches another medium with a different acoustic impedance. The amount of energy reflected depends on the significance of difference in the acoustic impedance of the two media. The effectiveness of ultrasonic echo testing in identifying defects inside concrete has been proven in many field applications.

3-D Ultra Sonic Testing

Diagnosis of A Reinforced Concrete Aeration Tank

This project involved a newly constructed cylindrical aeration tank that was designed to hold wastewater. The tank had an inner diameter of 63 feet, and a height of 18 feet. Our client observed concrete construction defects, including map pattern cracking, cold joints, and honeycombing on the exterior face of the reinforced 1-foot thick concrete wall of the tank after the forms were removed. These defects raised serious concerns related to the water-tightness of the concrete structure and its structural integrity.

Ultra Sonic Testing

To identify the probable causes of the observed defects, and determine if subsurface defects existed that were not visible on the surface, Terracon performed Ground Penetrating Radar (GPR) test and ultrasonic echo tomography test on the wall (on both interior and exterior faces) at several locations. The results of GPR test indicated inconsistent and less than specified concrete cover thickness near the exterior face of the wall which had contributed to the map pattern cracking mimicking the pattern of the reinforcing steel cage. Based on the 3-D models obtained from ultrasonic echo tomography test, the defects appeared to be limited to the vicinity of the exterior surface of wall, with no additional subsurface concrete flaws.  This was verified by through-wall coring performed on the concrete wall.

The findings allowed the client to determine that the new tank structure did not require demolition. They could be confident that the extent of observed surface defects were limited in nature. The structural engineer of record proposed a surface sealing repair coating to verify the water tightness of the structure. Significant demolition cost and schedule impacts were avoided.

Ultrasonic echo tomography is just one of the non-destructive techniques that can be utilized in evaluating concrete structures. With each unique project, a combination of a reasonable structural/materials evaluation and use of appropriate advanced non-destructive testing technology can help save time and money for all stakeholders.