Some basic NDT techniques utilized to assess concrete structures include:Once concrete has hardened, it is important to determine whether the final structure is suitable for its designated use. To do this, non-destructive testing (NDT) techniques are ideal as they limit or eliminate any possible damage that could be caused to the concrete structure during analysis.
NDT for the analysis of concrete structures is appropriate for new structures, in which investigators are typically concerned with determining the quality of the structures, as well as for old structures, in which this testing method is often used to assess structural integrity or adequacy.
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- Visual Inspection
- Compression Test
This test determines the strength of concrete cylinders or cubes under standard conditions. Concrete test samples are tested in a laboratory setting using a compression test machine to determine the acceptability of the concrete material.
- Schmidt/Rebound Hammer Test
- Carbonation Depth Measurement (Depassivation)
When carbon dioxide reacts with hydrated cement materials, either as a result of exposure to the atmosphere or other moisture sources, carbonates, such as calcium carbonate, are produced. This test involves the application of a 1% phenolthalein solution to the surface of the concrete, and the depth of the uncolored layer determines the amount of carbonation present in the concrete. Additionally, carbonation depth measurement can also be evaluated by drilling a hole into the surface of the concrete and visually identifying the depth of discoloration.
- Permeability Test
When assessing the durability of concrete, particularly when exposed to harsh environmental conditions or are used for water retaining structures and watertight sub-structures, permeability plays an important role. To measure the potential of liquids, ions, and gases to penetrate into the concrete structure, permeability tests can vary from initial surface absorption tests, the modified Figg permeability test and in situ rapid chloride permeability test.
- Penetrate Resistance/Windsor Probe Test
This test method requires hardened alloy steel probes, a powder-actuated driver or gun, loaded cartridges, a depth gauge for measuring the penetration of probes and related equipment to determine certain concrete properties that exist below the surface. This NDT technique is typically utilized to determine when formwork of concrete can be removed.
- Resistivity Measurement
This NDT technique is a fast, simple and cheap method to determine the potential risk of corrosion or other activity of steel within a concrete sample. The most common way to measure resistivity in concrete samples is through the half-cell electrical potential method or the placement a low resistance path between anodic and cathodic sites to measure current flows. These two NDT techniques can used separately, but are often used in conjunction with each other to ensure adequate corrosion measurement. It is useful to measure the amount of electrical resistivity present within concrete as this property can be a result of the presence of moisture, unwanted salt content, changes in temperature or unequal water/cement ratio and mix proportions.
- Covermeter Testing
Through the use of eddy currents or magnetic induction effects, covermeter instruments are used to measure the presence of any conducting metals within the vicinity of the head of the instrument. This type of NDT method is used for quality control purposes to ensure that reinforcing bars maintain their proper location following concrete placement and to investigate the presence of buried ferromagnetic objects, such as water pipes, steel joints, and lighting conduits.
- Radiographic Testing
- Ultrasonic Pulse Velocity Testing
Electro-acoustical transducers that are held in contact with one surface of the concrete sample produces a pulse of longitudinal vibrations that is then transmitted to the concrete. Following this transmission, the pulse of vibrations undergoes multiple reflections at the boundaries of the different material phases that exist within the concrete sample. The time in which the transmitted waves return to the receiving transducer reflects the uniformity of concrete within the sample.
- Tomographic Modeling
- Impact Echo Testing
This NDT method to measure the propagation of stress waves can be determined through pulse-echo, impact-echo, impulse-response and spectral analysis of surface waves. The presence of stress waves, therefore, provides investigators with useful information on large voids or delaminations in plate-like structures that are present within the test sample.
- Ground Penetrating Radar/Impulse Radar Testing
- Infrared Thermography
References:
- “Guidebook on non-destructive testing of concrete structures” – International Atomic Energy Agency
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