The benefits of NDT
With traditional coring methods, the durability of the concrete structure becomes a major concern as the coring sites introduce weak points within the element. Further, but the physical intervention that these tests require can also damage the structural integrity of any already-distressed structures and elements, especially if the rebar is further damaged. Accuracy can often be compromised as well, as the areas for sampling are limited by the structure’s reinforcement and selection might be less than optimal for tests being performed. Moreover, methods employed for extracting a core sample affect the integrity of the sample. In general, core samples show lower strength compared to replicate (uncored) specimens.
Among emerging technologies and NDT methods, the rebound hammer test mentioned previously is frequently applied to investigate strength characteristics of concrete. It continues to be the most commonly employed method for testing surface hardness and is a cost-efficient and simple way to estimate the strength properties of concrete. However, the results of this test can be altered by the geometric properties and age of the specimen, smoothness of the testing surface, moisture conditions, and more. For these reasons, it is recommended this method be used for testing of variability of strength properties within concrete samples and not as a substitute for compressive strength testing.
Some NDT methods, such as resistivity or chloride permeability, are mainly used to predict the chloride diffusion co-efficient of concrete or moisture transport rates. Resistivity tests (described in American Association of State Highway and Transportation Officials [AASHTO] T 358, Standard Method of Test for Surface Resistivity Indication of Concrete’s Ability to Resist Chloride Ion Penetration) require concrete samples be saturated through vacuum saturation or soaking and placed between two conductive plates and sponges. The voltage drop and current passage is then measured and used to calculate resistance.
Chloride permeability testing is described in ASTM C1202, Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration. It involves placing samples inbetween two chambers (one with sodium hydroxide [NaOH] and the other with sodium chloride [NaCl]), applying 60 V for a period of six hours, and measuring the quantity of charge passing through the sample during that period. Referred to as qualification testing, these methods can be used to directly determine the service life of concrete or to enhance it through changes in mixture compositions.
Onsite testing methods, such as corrosion monitoring, can be used to determine the risk of corrosion in structures. New devices have been developed in recent years that use high-precision sensors to accurately map corrosion rate and potential and measure the in-situ real electrical resistivity, temperature, and relative humidity (RH) of concrete.
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Some advanced corrosion detection devices on the market use a test method that estimates the corrosion rate of steel reinforcement within concrete through a non-invasive approach and without a physical connection to the reinforcement. This test method applies a narrow-current pulse to the concrete reinforcement for a short period of time and records the voltage of the system with a high sampling rate. The voltage recorded is then used to determine the state of corrosion in reinforced concrete structures. With this corrosion assessment information, engineers can determine the residual service life or ways to mitigate corrosion initiation and propagation. This, in turn, improves the longevity of concrete members.
Other NDT strategies, such as the penetration resistance, pull-off, resonant frequency, and impact-echo methods, are gaining acceptance as a means of evaluating the integrity of concrete materials. Each of these tests either involves invasive testing measures (such as driving probes into concrete samples or using levers to pull epoxy-bonded discs from the surface of the concrete) or uses non-invasive measures (such as applying vibrations or impacting the surface of the concrete element being tested). Each one is used to determine different properties.
Conclusion
Using sensors during the placement and curing of concrete can ensure consistency and confirm temperatures remain within the allowed limits. This reduces the possibility of cracking, which could drastically affect the service life of a structure. By avoiding damage to concrete during evaluation, NDT ensures the integrity of infrastructure can be preserved while achieving more efficient maintenance. Leaving structures fully intact during testing also allows them to be tested many times using several different methods throughout their complete life cycles, further ensuring continued integrity.
The ability to perform an unrestricted number of tests on aging concrete structures without damaging them allows engineers to collect and analyze data as required. Conducting these tests throughout the life cycle of a structure and under various conditions allows them to better understand the performance of concrete and changes in concrete properties over time and enables them to improve designs and service life for future structures. The ability to gain this insight and act on it directly contributes to the creation of safer and more secure buildings in the years to come.
Roxanne Pepin is a digital marketing specialist with Giatec Scientific, a global company developing smart technology for the construction industry. She oversees the planning, production, and distribution of communication materials while co-ordinating online marketing efforts relating to digital advertising as well as website and content development. Pepin can be reached at roxanne@giatec.ca.
