This articles describes how non-destructive testing can help structural engineers in the structural condition assessment of existing buildings, such as parking garages, condominiums, and other constructed facilities.
Structural Condition Assessment usually refers to the process of collecting observations and data about the existing condition of the structure through systematic and scientific methods. According to the Professional Engineers Ontario Guideline for “Structural Condition Assessment of Existing Buildings and Designated Structures” (PEO, Nov 2016), structural condition assessment are within the practice of professional engineering. This primarily fall in two categories:
- Preliminary assessment
- Detailed condition assessment
Depending on the objectives of the condition assessment, physical condition of the structure, and building configuration, the assessment methodology may vary. For example, the assessment can be limited to non-intrusive visual inspection only (mainly in the preliminary assessment), or may involve intrusive and destructive methods to obtain samples (mainly in detailed condition assessment).
Using Non-Destructive Testing for Structural Condition Assessment is a common practice. A multi-technology approach using different NDT methods can help engineers in various engineering applications, and in different ways. In this article, we will briefly review some of the most common challenges engineers face in the assessment of existing structures. We will then review how various NDT methods can help engineers obtain information about different mechanical and durability properties of materials, and structural components.
Structural Condition Assessment
A reliable structural condition assessment heavily relies on rigorous observations, and collecting accurate data that can adequately describe the existing condition of the structure, investigation. In doing so, engineers are looking for signs of defects and distress in structural elements in the first step. Close up visual inspection of elements provides a cost-effective method for rapid screening of the structural system. However, this does not provide any information about the properties of materials, or hidden sub-surface defects. The following summarizes some of the most important issues that is of interest to structural engineers:
1- Structural Details
In order to assess the load bearing capacity of the structural system, engineers must obtain accurate information about the size and configuration of existing elements. Some of these important details are listed below:
- Cross-Sectional Dimensions
- Rebar Size
- Rebar Spacing and Pattern
- Concrete Cover Thickness
- Concrete Element Thickness with one side access only
- Location of pre-stressing/post-tensioning tendons in Concrete Girders and slabs.
Visual inspection and tape measurement can help determine the overall cross sectional dimensions of accessible structural components. However, it does not provide information about sub-surface components.
Non-destructive tests can be used to help engineers get a better understanding of these details. For example, the Ground Penetrating Radar (GPR) can be used to accurately locate steel reinforcement. This is a rapid, cost-effective and 100% non-intrusive method. GPR can help inspectors and engineers verify the as-built reinforcement, identify top and bottom rebar locations (i.e. in slabs, and walls), and find the configuration of transverse reinforcement. For example, if structural engineers need to verify the spacing of the reinforcement in the potential plastic hinging zone, GPR can help verify the spacing of the stirrups.
Another important group of tests are ultrasonic testing of concrete. Ultrasonic methods, such as Ultrasonic Pulse Echo can be used to predict the thickness of concrete elements with one-side access. This is important in tunnel linings, trunk sewers, and abutment walls. Evaluating the thickness of lining can help engineers in evaluating the overall strength with a higher accuracy.
2- Structural Integrity and Defects
Surface and Sub-surface defects can affect structural integrity. It is important to obtain information about potential issues in the structural components. Below is a list of important defects that can affect structural performance.
- Concrete Cracks
- Voids in Concrete
- Delamination in Concrete Slabs, Bridge Decks, etc.
- Bond issues (Old and New Concrete)
- Poor quality materials (Honeycombing)
- Deteriorated concrete (Alkali-Silica Reactions, Weathered Concrete, etc)
Non-destructive test methods for structural condition assessment can be used to evaluate the structural integrity and locate potential defects in structures. Ultrasonic testing of concrete provides a cost-effective approach to evaluating concrete material, and crack depth in concrete structures. Ultrasonic Pulse Velocity (UPV) can be used to evaluate the quality of concrete material, as well as studying the crack depth. The test can also be used to identify subsurface defects, and voids in concrete elements, such as concrete slabs, concrete walls. If engineers are interested in evaluating the quality of concrete at critical locations, such as columns base, or shear walls, this method can help. Another application is when concrete slabs are subject to additional lead or live load. The test provides a great tool to study the reliability of the existing concrete element under new loading condition. In steel structure, ultrasonic testing can be used to evaluate hardness, weld quality, and condition of anchor rods.
3- Strength of Concrete Materials
A key aspect in detailed structural condition assessment is to evaluate the capacity of structural members. In addition to the cross sectional dimensions, engineers often look for methods for helping them in estimating the strength of materials. In concrete structures, development and use of in-place methods to estimate concrete strength has been a popular topic over the past two decades. Several attempts have been made to correlate strength of concrete with results obtained from different NDT methods.
The most widely used method is to Rebound Hammer (Schmidt Hammer) test. While it is reported that no theoretical correlation exists between concrete strength and rebound number, the test is often used for comparative evaluation of strength. Moreover, by developing a project-specific correlation between rebound number and concrete strength, the number of intrusive tests can be significantly reduced.
Another major test which is not 100% non-intrusive, is the pull-out testing, where test results can be used to evaluate the compressive strength of concrete.
4- Corrosion Inspection and Monitoring
- What is the current state of structure?
- What is the rate of corrosion? (kinetics of corrosion)
- What is the remaining capacity and service life?
Non-destructive testing has long been used in the corrosion inspection and monitoring of reinforced concrete structures. (NDT Methods of Corrosion Inspection) Researchers have developed various tools and methods for evaluating corrosion activity in reinforced concrete structure.
Half-Cell Corrosion Potential mapping is a quick non-destructive method for evaluating the likelihood of corrosion in concrete structures. It can be used to assess the probability of having active corrosion. The simplicity of the test makes it popular for inspection of large areas, such as parking garage slabs, and bridge decks. The test can also be used to verify the quality of a repaired concrete structure.
Corrosion Rate measurement will add a new piece of information to the equation. corrosion rate measurement can be used to evaluate how fast corrosion is happening. This is normally reported as cross section loss per year. Corrosion rate can be determined through intrusive and non-intrusive procedures. Most commercial devices work based on estimating the polarization resistance, where the change in the half-cell potential is measured under applied current/voltage.
Surface Electrical Resistivity is another important test that provides information on the conductivity of concrete materials. The test can help evaluate the resistance of concrete against aggressive agents, such as chloride ion.
The information obtained from these tests can be used to determine the remaining service life of the structure.
5- Sub-Surface Voids
Voids in concrete Slabs on Grade is a common issue in older concrete buildings. The underneath soil might have been washed out over time, leaving voids and hollow areas that might affect safety and reliability. Impulse-Response method is a great non-destructive method for void survey in concrete slab on grade.
Another significant issue is presence of voids and hollows underneath concrete lining in tunnels, and trunk sewers. Water flow can wash out the soil underneath the lining, leaving void. Ultrasonic Pulse Echo and Ultrasonic Tomography can be used to evaluate the extent of sub-surface voids.
While non-intrusive methods are generally preferred, there are some cases where engineers need to take a closer look at the details. Taking core samples or cutting steel rebar from concrete structures becomes necessary in order to evaluate certain mechanical properties of materials.
Multi-Technology Approach to Structural Condition Assessment
Using a combination of Non-destructive Testing for Structural Condition Assessment can help engineers collect critical information about the existing defects with a better accuracy. The main advantage of these tests is that they are repetitive, and can be used for periodic inspection and monitoring. If one or two of the tests point to an existing issue, the accuracy of diagnosis will be improved. Moreover, these tests can complement each other in good ways. For example, GPR scanning and Ultrasonic Tomography can be combined to collect information about the structural details such as thickness of element, configuration of reinforcement, and presence of sub-surface defects.
Another great example, is multi use of half-cell corrosion potential mapping, surface electrical resistivity, and cover thickness measurement that can help engineers involved in bridge condition survey.