A quick web search will reveal that there are countless non-destructive testing methods to choose from. With so many tests available, how do you know which method and device is right for you? This blog post will outline the importance of nondestructive testing, the eight main methods, when to use them, and what equipment is required.
Why is non-destructive testing of concrete important?
Field testing of hardened concrete is often required to determine its structural suitability for its intended use. Non-destructive testing methods are used to evaluate concrete properties by assessing strength and other properties such as corrosion of reinforcement, permeability, crack and void structure. This type of testing is important for evaluating both old and new structures. For new structures, the main application is to determine the quality of the material. Testing existing structures is often associated with assessing structural integrity.
When to Use Nondestructive Testing of Concrete?
NDT can also be used as a first step in subsequent coring and more invasive measures such as:
Gage properties for precast, cast-in-place or on-site construction
Determining the acceptability of supplied materials and components
Locate and classify cracks, voids, honeycombing and other defects in concrete structures
Determine concrete uniformity prior to core cutting, load testing, or other more expensive or destructive testing
Monitor strength development in relation to form removal, cessation of cure and application of loads
Determine the location, amount or condition of reinforcement
Identify or locate suspected deterioration of concrete caused by overloading, fatigue, external or internal chemical attack or changes, fire, explosion, environmental influences, etc.
Assess the potential durability of concrete while monitoring long-term changes in performance
What methods/equipment should I know about?
Windsor Probe - The Windsor Probe quickly and accurately determines the compressive strength of concrete. The method uses a hardened steel probe driven by a powder-driven electric charge to penetrate the concrete surface. To obtain concrete strength, manufacturers provide a graph of the Mohs hardness of the aggregate versus the depth of penetration.
When to Use - The Windsor Probe is a useful option for estimating the compressive strength of concrete and can be used for an overall assessment of concrete quality and relative strength in different parts of a structure. It's small enough to use in the field and easy to operate, requiring little training.
Concrete Springback - Concrete springback is used to estimate strength and assess the in-situ uniformity of concrete, and can delineate areas of poor quality or degraded concrete. The spring-loaded hammer is released so that it strikes the piston in contact with the concrete surface, and a slide indicator records the distance the hammer has returned on a linear scale. The rebound numbers are then cross-referenced with actual compressive strength values to establish the relative and proportional strength of the same concrete mix between different areas of the structure.
When to Use - A test hammer is better used to establish the relative strength curve of a structure. When desired, a technician can quickly map larger areas of potential strength issues and narrow down specific areas for more rigorous testing with this instrument. Areas of lower rebound values can then be economically assessed by core, penetration testing or pulse velocity measurements, while areas of higher strength can be bypassed.
Coring – Coring is a common and widely accepted method of extracting samples from hardened concrete for direct determination of strength. Although technically a "destructive" method, when used with caution it is often possible to extract core from locations where it will not compromise structural integrity. Core samples provide the most definitive results for determining compressive strength by any of the methods listed here, but they can cause cosmetic damage and are laborious and time-consuming to extract.
When to use – Coring is often the end result of an assessment program that begins with the use of a concrete test hammer, Windsor probe, or other non-destructive methods. The core is often considered the last word in determining the strength of hardened concrete.
Concrete Maturity Test - Concrete gains strength over time and generates heat as it cures. The temperature of the concrete in the field is recorded over time, and standard mathematical equations are then applied to the data, allowing operators to correlate with laboratory samples of known strength. Concrete maturity meters collect temperatures from probes buried in fresh concrete and record them with the time of collection. This data is used to calculate a value as the equivalent life or time-temperature coefficient and used to estimate the compressive strength.
When to use it - Maturity testing is a good choice if you need an easy and reliable way to estimate the early strength of concrete to safely remove formwork and reduce the time it takes pavements and structures to be in service.
Crack Monitor - Concrete Crack Monitor measures the width of cracks in concrete structures such as bridges, buildings and roads. Overlapping upper and lower plates are marked and crack opening and closing can be incrementally monitored.
When to Use – Crack Monitors can be used to measure cracks periodically in the field to easily and accurately determine movement of a structure's foundation.
Moisture Emission Testing - Millions of dollars are lost annually to paint and flooring systems due to the migration of moisture through concrete slabs and structures. Moisture Emission Test Kits determine the moisture emission of concrete slabs over time.
Weigh a moisture-absorbing calcium chloride container and place it under a plastic dome sealed to the concrete surface with a self-adhesive gasket. At the end of the test period, the plastic was cut open, the calcium chloride dish was removed, sealed and weighed. Weight gain and exposure time values are used to calculate test results expressed in pounds of moisture released per 1,000 square feet over a 24-hour period. A Moisture Meter can also be useful for measuring the moisture content of concrete floors on the surface immediately before the application of floor coverings.
When to Use – Moisture emission test kits or meters are very useful when determining moisture deep within the concrete surface. This type of floor moisture testing is also valuable when assisting a contractor in identifying suspect areas that may require further testing.
Concrete Moisture Measurement System - Excessive moisture in concrete floors can lead to costly floor covering or coating failures such as debonding, warping, blistering, and increased potential for mold and mildew. A relative humidity (RH) measurement system provides complete information on the moisture content throughout the slab. The operator simply drills the hole at the specified depth, and the electronic moisture-sensing probe periodically measures the moisture level. After testing, the holes can easily be filled with standard cement.
WHEN TO USE - The relative humidity measurement system can be used to measure humidity and other factors including temperature, dew point, and concrete surface moisture testing according to ASTM F2659.
Rebar Locator and Coverer – The Rebar Locator and Coverer is used to find rebar, wire mesh and metal wall ties in structures. Their main function is to determine the vertical position of the reinforcement to help avoid damage to reinforcing elements during cutting or coring. Professional models can estimate the size and depth of reinforcement to assess the integrity of existing structures or meet design codes.
When to use - These methods can be used to discover the exact size, location and depth of rebar and subsurface metalwork for quality control and efficient recovery of test core.
