Employing nondestructive testing technologies throughout concrete’s life cycle

by nithya_caleb | January 27, 2019 12:00 am

by Roxanne Pepin

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Traditionally, nondestructive testing (NDT) of concrete refers to testing, inspecting, or evaluating concrete materials or components without damaging or destroying the serviceability of the structure or materials. These tests have usually been performed on concrete structures that have been in service for some time, but present-day technology allows the use of nondestructive testing methods during the construction of new concrete elements as well.

Some of the most popular nondestructive testing methods include the pullout, rebound hammer, and ultrasonic pulse velocity (UPV) tests, as well as the use of embedded temperature and maturity sensors for strength determinations. The tests’ purpose is to assess a multitude of properties (including concrete density, elastic modulus, compressive strength, reinforcement location, and surface absorption) affecting the integrity of concrete elements.

NDT can be performed on old and new structures to verify the reliability and workmanship of concrete. The integrity of a building can be determined by monitoring concrete properties such as temperature and strength and detecting cracking, delamination, and the occurrence of corrosion, among other factors.

NDT methods can be applied to assess and report on the condition of concrete at any stage of its life cycle, from the time it is placed to the time it needs to be replaced. Testing is primarily done to ensure the material and structure quality meets design needs, specifications, and building code requirements, ensuring the safety, welfare, and environmental protection of the community during the construction and occupancy of buildings.

Comparing the methods

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When it comes to testing during the placing of concrete, the most commonly used method for evaluating strength is the break test. These tests require concrete cylinders to be subjected to high amounts of pressure in order to assess the material’s compressive strength. Although this test method is said to be the most reliable, results found in the laboratory are often not representative of the in-situ cast concrete. This is due to various factors, including curing conditions, the size of the cylinders compared to the onsite concrete element, and transportation to the laboratory. (Transportation of concrete cylinders can often alter the integrity of the samples due to issues such as cracking and temperature changes.) Further, these tests tend to be less efficient as there is a delay in obtaining results, which in turn drastically increases project costs.

Technological advancements have made it possible to obtain concrete strength data without preforming break tests. Various tools, techniques, and methods can be applied to nondestructively test the properties of curing concrete, such as the pullout, rebound hammer, and UPV test methods mentioned above. These advancements have also allowed for the development and use of embedded wireless sensors connected to the Internet of Things (IoT), a network of connected devices with the ability to transfer data with little to no human interaction. IoT sensors have the ability to gather and transfer information without physical connection or destruction of the concrete.

The three non-IoT NDT methods are described in the following paragraphs.

Pullout test

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The pullout test is invasive, but is still considered nondestructive as it does not affect the integrity of the concrete structure/element. It is used to determine the maximum force needed for an embedded metal insert containing an enlarged head to be pulled from a concrete specimen after being installed into hardened concrete or cast into fresh concrete. The force required to pull the insert out can be related to the compressive strength of the concrete.

Rebound hammer test

As described in ASTM C805, Standard Test Method for Rebound Number of Hardened Concrete, the rebound hammer test involves deploying spring-controlled mass strikes of a hammer onto the surface of a concrete element and measuring the rebound value, which varies depending on the hardness of the curing concrete. The rebound value is then used to determine the compressive strength of the in-situ concrete.

Ultrasonic pulse velocity test

UPV tests are described in ASTM C597, Standard Test Method for Pulse Velocity Through Concrete. They consist of setting two transducers against the surface of the concrete being tested and sending ultrasonic waves from one transducer to the other. In this test, it is the travelling time of the ultrasonic wave that is measured and used to obtain the ultrasonic pulse velocity, which then relates to the compressive strength of the concrete. Typically, a higher speed means a higher quality of concrete in regards to homogeneity, density, and uniformity.

While these methods are often more economical than destructive break tests when it comes to cost and time required for testing, they also tend to be less reliable, as they are greatly influenced by the condition of local materials. For example, the type and volume of aggregates, moisture content, and curing process of the concrete could alter the pulse velocity. Research shows with compressive strength testing, rebound hammer test results can display more than 20 per cent error when compared to results obtained from traditional destructive testing methods (This statistic is derived from Y. Shih, K. Lin, and C. Chen’s “Improving Nondestructive Concrete Strength Tests Using Support Vector Machines, published in Materials in 2015.). Test results have also shown estimations become more accurate when multiple different NDT methods are combined. Embedded sensors offer a significant advantage as well.

Embedded IoT sensors

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Embedded sensors are designed and engineered to provide highly accurate results. They are attached directly to the rebar within the concrete slab or formwork and allow for continuous monitoring of concrete temperature and strength. Newer versions of these sensors connect directly to smartphones and tablets, allowing contractors and engineers to see the data in real time. The ability to embed sensors within the formwork and obtain strength results on the jobsite eliminates the need to transport samples to a laboratory for testing and thus reduces the margin for error.

These real-time sensors provide data on concrete maturity, a function of temperature history in field-cured concrete described in ASTM C1074, Standard Practice for Estimating Concrete Strength by the Maturity Method, and Netherlands Standardization Institute (NEN) 5970, Determination of Strength of Fresh Concrete with the Method of Weighted Maturity. The use of sensors is the only NDT method currently accepted by building codes and standards for formwork removal, post-tensioning, saw-cutting, and similar tasks. It is accepted by the Canadian Standards Association’s (CSA’s) codes and standards, CSA A23.1, Concrete Materials and Methods of Concrete Construction, and CSA A23.2, Test Methods and Standard Practices for Concrete, which are used in the construction of all concrete structures across Canada.

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Wired maturity sensors and thermocouples have allowed contractors and engineers to obtain accurate predictive data on the state of their concrete. In turn, this allows them to make informed decisions relating to project scheduling and operations. New, wireless mobile-based technologies provide even more advantages to workers, as they have been developed to eliminate the hassle of wires on the jobsite and the need to connect to cumbersome data loggers, making data collection and analysis less time consuming. Utilizing such wire-free sensors further reduces the cost and time requirements for concrete quality control by allowing contractors and engineers to view concrete maturity data in real time and proceed to the next steps in a project as soon as the concrete has reached the required strength. This eliminates the need to wait on break test results, which can take up to 24 hours to receive.

Demand for NDT for the condition evaluation and maintenance of aging structures has been increasing over the last several years, especially in North America, where many concrete buildings are nearing the end of their life cycles. Many engineers are increasingly choosing NDT methods over traditional testing methods, as they are proving to be more powerful, reliable, and effective. In-situ tests (such as pullout, rebound, penetration, and dynamic tests) aim to detect the condition of reinforced structures, assess their various properties, and rank their condition.

The benefits of NDT

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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.

[8]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[9].

Source URL: https://www.constructioncanada.net/employing-nondestructive-testing-technologies-throughout-concretes-life-cycle/

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