A quick web search will show that there are tons of non-destructive testing methods to choose from. With so many tests available, how do you know which method and equipment is right for you? This blog post will outline the importance of non-destructive testing, eight top methods, when you should use them, and what equipment you need.
Why is non-destructive testing of concrete important?
Testing hardened concrete in-place is often necessary to determine the suitability of a structure 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, cracking, and void structure. This type of testing is important for evaluation of both new and old structures. For new structures, the principal applications are mainly used to determine the quality of materials. Testing existing structures is usually related to an assessment of structural integrity.
When is non-destructive testing of concrete used?
Non-destructive testing can also be used as an initial step to subsequent coring and more invasive measures such as:
- Gauging characteristics of pre-cast, cast-in-place, or in-situ construction
- Determining the acceptability of supplied material and components
- Locating and categorizing cracks, voids, honeycombing and other defects in a concrete structure
- Determining the concrete uniformity prior to core cutting, load testing or other more expensive or disruptive tests
- Monitoring strength development related to formwork removal, cessation of curing and load application
- Determining the position, quantity or condition of reinforcement
- Confirming or locating suspected deterioration of concrete resulting from such factors as overloading, fatigue, external or internal chemical attack or change, fire, explosion, environmental effects
- Assessing the potential durability of concrete while monitoring long-term changes in properties
What Methods/Equipment Should I be Aware of?
Windsor Probe - The Windsor Probe offers rapid, accurate determinations of concrete compressive strength. This method uses a hardened steel probe driven by a powder-actuated charge to penetrate the surface of concrete. A manufacturer provides a chart of Moh’s hardness for aggregate versus penetration depth in order to obtain the concrete strength.
When to use - The Windsor Probe is a useful option for estimating the compressive strength of concrete for general assessment of concrete quality and relative strength in different parts of a structure. It’s small enough for field use and operation is simple with little training required.
Concrete Test Hammer - The Concrete Test Hammer is used to estimate strength and assess the in-place uniformity of concrete, and can delineate regions of poor quality or deteriorated concrete. A spring-loaded hammer is released to impact against a piston in contact with the concrete surface, and a sliding indicator records the distance the hammer bounces back on a linear scale. The rebound number is then cross-referenced to an actual compressive strength value to establish relative and proportional strengths of the same concrete mix between different areas of a structure.
When to use - The Test Hammer is best used to establish a relative strength profile of a structure. Ideally, one technician can quickly canvas large areas with potential strength problems and narrow down specific areas for more rigorous testing using this instrument. Areas with lower rebound numbers can then be economically assessed with cores, penetration tests, or pulse velocity measurements while areas showing higher strengths can be bypassed.
Coring – Coring is the common and most widely accepted practice to extract samples from hardened concrete for direct strength determinations. Although technically a “destructive” method, when used with care, cores can often be extracted from locations that will not affect the integrity of the structure. Core samples offer the most definitive results for determining compressive strength of any of the methods listed here, but they can result in cosmetic damage and are labor-intensive to extract.
When to use – Coring is often the end result of an assessment program that begins with use of Concrete Test Hammers, Windsor Probes, or other non-destructive methods. Cores are frequently considered the final word in strength determinations of hardened concrete.
Concrete Maturity Testing - Concrete gains strength over time and generates heat as it cures. Logging temperatures of in-situ concrete over time, then applying standard mathematical equations to the data allows the operator to establish a correlation with laboratory samples of known strengths. Maturity Meters collect temperatures from probes buried in the fresh concrete and log them together with the time collected. A value is calculated with this data as either equivalent age or time-temperature factor and is used to estimate compressive strength.
When to use- Maturity testing is a good option if you need a simple and reliable way of estimating early-age strength of concrete for safe removal of formwork, and to reduce delays in putting pavements and structures into service.
Crack Monitors - Concrete Crack Monitors measure crack widths in concrete structures such as bridges, buildings and roads. The overlapping upper and lower plates are marked and the opening and closing of the crack can be monitored in increments.
When to use – Crack monitors can be used to measure cracking periodically in the field for easy, precise determinations of structural foundation movement.
Moisture Emission Testing - Every year, millions of dollars in damages to coatings and flooring systems occur as a result of moisture migration through concrete slabs and structures. Moisture Emission Test Kits determine moisture emission over time through concrete floor slabs.
A container of moisture-absorbing calcium chloride is weighed and placed under a plastic dome sealed to the concrete surface with a self-adhering gasket. After the test cycle, the plastic is cut open and the dish of calcium chloride is removed, sealed and weighed. The weight gain and exposure time values are used to compute the test result, expressed in pounds of moisture emitted per 1,000 square feet in 24 hours. A Moisture Meter can also be useful for instantly measuring moisture content on concrete floor surfaces prior to the application of floor coverings.
When to use – A Moisture Emission Test Kit or Meter is useful when determining moisture deep beneath a concrete surface. This type of floor moisture test is also valuable when assisting contractors in identifying suspect areas that might need further testing, deeper in the slab.
Concrete Humidity Measurement Systems - Excessive moisture in concrete floors can lead to expensive floor covering or coating failures such as de-bonding, warping, blistering, and increased potential for mold growth. Relative Humidity (RH) Measurement Systems offer a complete profile of moisture content throughout the entire slab. The operator simply drills a borehole to the specified depth and electronic moisture sensing probes periodically measure humidity levels. After the test is complete, it’s easy to fill the holes with standard cementitious.
When to use - A Relative Humidity Measurement System is useful for measuring humidity and other factors including temperature, dew point and concrete surface moisture testing to ASTM F2659.
Rebar Locators and Covermeters – Rebar Locators and Covermeters are used to find reinforcing bars, welded-wire mesh, and metal wall ties in structures. Their primary function is to establish the vertical location of bars to help avoid damage to reinforcing elements during cutting or coring. Advanced models enable estimates of bar size and depth for assessing existing structures for integrity or compliance to design specifications.
When to use - These methods are useful for uncovering the exact size, location and depth of reinforcing steel and subsurface metalwork for quality control and effective recovery of test cores.
We hope our guide has made it easier to choose your ideal method for the non-destructive testing of concrete!