By Frank Cox, ECS Mid Atlantic
In many ways, a building is like a cake. Its structure consists of different ingredients and layers. When one ingredient differs from the recipe, the entire cake is compromised. This is also true for building structures, and pinpointing the issues can be challenging for those tasked with fixing them. Often, what starts out as a small crack can reveal a host of structural issues when the layers of the building are peeled back.
“The building has good bones,” is a phrase often used to describe a building that is being slated for adaptive reuse. How do we know that the building elements are structurally sound? That’s where structural diagnostics comes into play. Leveraging a combination of advanced diagnostic tools, laboratory testing and professionals experienced with new design, construction, forensics and instrumentation, diagnosing a known or unknown issue can be performed—one layer at a time.
Field diagnostic testing
Structural diagnostics routinely requires the use of non-destructive evaluation (NDE) equipment to evaluate a variety of building materials, including concrete; steel; masonry; wood; composites, such as fiber-reinforced or glass-reinforced polymers and soils beneath the building floor or foundation.
NDE tools commonly used by structural diagnostic professionals include ground-penetrating radar (GPR), ultrasonic testing and a small hammer. Believe it or not, many structural issues can be identified using a light sounding on a concrete surface using a small hammer. For example, a hollow sound is bad, whereas a ringing sound is good.
GPR is used to locate the presence of reinforcing steel or other embedded items within a concrete structure or voids below slabs or pavements. A GPR scan is typically the first step to identify gross deficiencies. Suspect areas on concrete structures identified to have deficiencies are then further investigated using tools such as ultrasonic echo tomography (UET).
UET is used to evaluate the condition of hardened concrete by measuring time of an ultrasonic shear-wave transmitting in the hardened concrete. UET tests such as ultrasonic pulse velocity and impulse response can locate voids or honeycombing in the concrete.
A useful testing method for evaluating steel is half-cell potential testing. It determines the amount of corrosion of rebar embedded in concrete by measuring electrical resistance when current is passed through. Ultrasonic testing is also used as a non-destructive method to measure section loss on steel beams or other structural members.
Laboratory testing
Field diagnostic testing is most often followed by collecting samples for specialized laboratory testing. Reasons to conduct lab tests include evaluating tensile strength and chemical analysis of steel, petrographic examination of concrete to determine the condition of concrete evidence of deleterious chemical reactions and chemical analysis of building materials to determine contamination from exposure to chemicals.
The list of investigation tools and techniques is very long and employing them can give diagnostic professionals the information needed to evaluate a structure or identify the extent of damage or deterioration and make informed engineering design decisions.
Structural diagnosis case study
This structural-diagnosis project involved a donated building at the heart of a community-revitalization project. The intent was to adapt the building for use as a commercial kitchen space for residents and farmers to use to support the community.
Stripped of all interior finishes, this three-story building began its life as a retail and warehouse space and has undergone many generational changes, which was evident by different structural and non-structural components. Exposed reinforcing steel, chipped and spalled concrete and deteriorated beams were found throughout, raising concerns about the structure’s integrity.
Our evaluation began by taking concrete core samples from structural members to determine compressive strength of the concrete. Cores were taken from beams, columns and slabs. Additionally, ground penetrating radar was used to locate reinforcing steel within the structural members and to determine the depth of concrete where repairs or additional slabs had been installed over time.
Finally, steel samples were extracted from exposed rebar in several locations to test for tensile strength and determine section loss. Reinforcing steel bars of different sizes showing varying stages of corrosion were sampled.
Structural diagnostics enabled engineers to advise the client on how to design and engineer the building for its new, intended purpose.
Making the case for structural diagnostics
Structural diagnostics is an important method used to paint an overall picture of the condition of a structure. It can also be used to pinpoint singular issues and provide information to remedy them. Without assessing the structure and getting into the different layers, valuable information and potential issues can be easily missed, making it difficult to repair or design effective projects.
Simply put, hiring an experienced firm staffed by seasoned professionals who are skilled at applying specialized tools offers the opportunity to non-destructively peel back a structure’s layers and dive deeper to identify potential issues, providing the analysis and information required to remedy them.
Frank Cox is Facilities Associate III at ECS Mid Atlantic.
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