When engineers design infrastructure, they count on concrete testing to ensure the material they use meets a specified level of strength. Generally, these tests specify the compressive strength of concrete and many other aspects of materials and their installation to ensure that the structures are safe and sound. Engineers also count on, and monitor, systematic processes for ensuring that the implementation of their design leads to the desired result.
Concrete testing falls into two categories: destructive and non-destructive testing. In destructive testing, a concrete sample is crushed until it cracks or is destroyed. Then, the pressure needed to achieve the result is measured, indicating the compressive strength of the concrete.
On the other hand, non-destructive testing detects and assesses flaws, cracks, voids, embedded material (such as rebar), and internal structural properties in buildings by monitoring the response of the concrete to a stimulus. Thereby rendering it unnecessary to destroy the sample, saving a considerable amount of time and money as compared to destructive concrete testing.
Using Non-Destructive Testing To Determine The Compressive Strength Of Concrete
Non-destructive concrete testing is one of the methods to assess the compressive strength of concrete in a constructed building. Since non-destructive testing yields data immediately, builders can determine critical structural characteristics of a building in real-time. This type of concrete testing is generally used when conventional destructive concrete testing is not an option, such as for an existing structure.
Different Non-Destructive Concrete Testing Methods
There are several non-destructive methods for evaluating the quality and compressive strength of concrete.
Visual Inspection
Visual inspections are a standard part of concrete quality assurance. This type of concrete testing is mainly qualitative and limited to inspecting only surfaces. Although it does not provide a quantitative measure of the compressive strength of concrete, it allows the observer to identify flaws and deformities that can undermine stability or render the unit unsuitable for use, which is its most significant advantage.
However, consistency is a significant challenge in visual inspections, since observers may not notice the same conspicuous features all over. Consequently, this concrete testing process calls for a high degree of training to properly assess the compressive strength of concrete structures.
Drop Test
The drop test is only suitable for CMUs. It involves dropping the CMU from shoulder height one or more times onto a hard, flat surface. This type of concrete testing is favorable for areas where standard test methods are not widely available. However, as it is only applicable for CMUs, this limits its scope and makes it unsuitable for testing the compressive strength of concrete in other structures.
Schmidt Rebound Hammer
The Schmidt hammer is a device for measuring concrete’s elastic properties, which correlates to its compressive strength. The sample is struck with a spring-loaded mass, and the size of the rebound is measured. This concrete testing equipment can test a range of structures built through site-cast, precast, or masonry processes. Isolated CMUs can also be tested.
It is an alternative means of determining the compressive strength of concrete in situations where samples are not available for standard destructive testing. However, this concrete testing method takes longer to perform since the hammer requires frequent recalibration. Additionally, because the rebound’s size is largely influenced by the surface hardness where the sample is struck, it is recommended that the test be repeated at multiple positions on the selection.
Furthermore, this type of concrete testing is only useful if there is a connection between the rebound number and the sample made from the same material as the aggregate is found. Since the manufacturer utilizes cube specimens to produce the hammer’s calibration curve, the tool may not yield accurate results when used on a concrete structure made from a different mix.
Ultrasonic Pulse Velocity (UPV)
The velocity with which sound propagates through solids is indicative of the solid’s properties. Knowing this, UPV measures how quickly sound travels through a given sample. In other words, the more robust the material, the faster the sound moves. Conversely, the weaker the concrete, the slower the sound travels.
The UPV transmitter generates an ultrasonic pulse, and the receiver detects the time delay for pulse transmission. Pulse velocity is derived by measuring the distance between the transmitter and receiver. This measurement provides a means for estimating the compressive strength of concrete. UPV testers can detect the thickness and uniformity of concrete and the presence, depth, and shape of voids.
UPV testing can test site-cast, precast, masonry, and isolated CMUs in concrete structures. However, basic UPV devices start at about $4,000 USD and can be quite a bit more expensive with additional features.
Windsor Probe Penetration Test
The Windsor probe implicitly damages the sample undergoing testing, although generally, the concrete element is still usable. It measures the compressive strength of concrete by driving a steel probe into the material and marking the depth of penetration of the probe.
It is suitable for use on large concrete elements built through site-cast, precast, and masonry processes. Isolated CMUs can also be tested if the structure can tolerate the damage caused by the probe. Still, this concrete testing method has three primary disadvantages. First, it requires frequent calibration. Second, the material properties near the probe’s penetration location largely determine the results. Lastly, Windsor Prone Penetration tests are expensive, costing about $5,000 USD.
Pull-out and Pull-off Resistance Tests
The pull-out test gauges the compressive strength of concrete by measuring the force required to remove an insert placed in the concrete during pouring. On the other hand, the pull-off test involves measuring the energy needed to pull off a disk epoxied to the concrete’s surface after curing.
Although classified as non-destructive testing, there is damage to the surface of the concrete in both cases. Additionally, basic testers cost about $2,000 USD. Still, these tests can be considered worthwhile options for testing the compressive strength of concrete in existing structures.
Instrumented Hammers And Modal Analysis
Instrumented hammers of various sizes are used in multiple industrial research applications to impart controlled impulses and monitor responses. The hammer recovers a signal, and in some cases, receiving devices mounted on the testing sample monitors the response.
A common challenge with this approach for concrete testing is that the hammer itself resonates with the sample under test. This is problematic if the hammer resonances at similar frequencies as the sample and the receiver(s) picks up both resonances. This is specifically a problem when analyzing airborne sound signals.
Additionally, the cost of instrumented hammers ranges from several hundred to several thousand dollars, depending on their size and functionality. A standard computer with signal processing software or commercially available purpose-built processing packages can analyze the results, but these can cost several thousand dollars.
Embedded Wireless Sensors
Sensors embedded in concrete during pouring can relay data on temperature and moisture, which provide more insight into the curing process and development of the compressive strength of concrete than would otherwise be possible. This concrete testing method saves time and money because it signals when poured concrete has cured enough to move on to the next construction project stage. This approach is primarily useful for site-cast and precast concrete.
Nevertheless, sensor batteries only last for about four months, and the readings from embedded wireless sensors can be affected by environmental factors such as sunlight, draughts, and humidity. These triggers may render this concrete testing method ineffective for long-term use.
Advantages And Challenges Of Non-Destructive Concrete Testing
To varying degrees, the non-destructive testings offer significant advantages over standard destructive testing. Specifically, they are non-destructive, or at least less damaging, so the masonry samples can be used for their intended purpose or saved for future testing and validation.
As the required equipment for non-destructive testing is portable, less expensive, and less bulky, it can be performed at the worksite without dedicating a large area solely for concrete testing. There is less necessity for specialized skills, training, and protective gear, which can alleviate concerns regarding the safety of concrete testing equipment, misuse by the operator, or explosive failure during testing.
Non-destructive testing is generally a faster procedure since less preparation is required and results get processed almost immediately. There is generally no need for capping or grinding, and in many cases, it is not necessary to attach transducers to the sample under test.
Beyond mandatory strength testing for quality assurance, non-destructive testing has a variety of applications. It can effectively measure the compressive strength of concrete when conventional methods are not practically available or, in instances when the destruction of the sample is not permitted. Because samples are not destroyed, there are opportunities for double-checking testing results by various organizations using various methods at different times. This promotes consistency and accounts for multiple shapes, sizes, and configurations of concrete.
Non-destructive testings promote experimental development with new mixes and the systematic evaluation of the effect of addmixtures on the compressive strength of concrete. This is especially useful during curing, as it allows manufacturers to improve the production processes and the variability of forces in CMU lots.
But despite its advantages, destructive strength testing is the dominant method of quality assurance in manufacturing and construction. This is due in part to the cost and time in establishing or revising standards for non-destructive testing.
Moreover, the absence of detailed evidence that non-destructive testings are reliable and accurate as compared with destructive testing, increases legal liability.
Additionally, there is resistance in the industry from companies that derive revenue from destructive testing if they are concerned that introducing a new concrete testing method could reduce their profits.
The Strike-It™ Tester: A New Way To Measure The Compressive Strength Of Concrete
There is one non-destructive method that has yet to be covered. Although it is relatively new, it has already proven that it has numerous advantages.
Strike-It™ measures the compressive strength of concrete masonry using acoustic impulse responses. This test can be conducted onsite as it does not require bulky equipment or specialized facilities.
Similar to the instrumented hammers and modal analysis concrete testing method, the sample under test is placed on a soft surface and gently struck with a pendulum at a prescribed location. The sound produced in response to the strike is analyzed to estimate the sample’s compressive strength. One of the best assets of this test is that it takes only a second or two to complete and requires very little preparation. Additionally, the Strike-It™ Tester saves test results, so statistics such as mean and standard deviation of strengths, and counts of passes and failures, can be computed.
Advantages Of Strike-It™ Relative To Other Non-Destructive Testing
- Strike-It™ is easy to use and generates fast results
- Cheaper equipment than other non-destructive testing equipment
- Strike-it makes use of powerful phone and computer hardware, which considerably reduces costs for mass-market applications
- Other testers do not have an integrated data recording and analysis capability, though higher-end testers probably do
- Strike-It™ can determine variances in the overall material properties from testing various points. With other concrete tests, the results of one area are assumed to be representative of the entire selection.
Disadvantages Of Strike-It™ Relative To Other Non-Destructive Testing
- Acoustic resonances used within Strike-It™ depend on the sample’s shape and size under test, so different calibrations are required for each. Other tests are shape-independent
- Strike-It™ is affected by environmental noise, while other tests aren’t
- Unlike other methods, support conditions for the sample under test are sensitive and must be controlled
Non-destructive testing poses several advantages for testing the compressive strength of concrete. The variety of methods gives builders more flexibility to select a testing method that works best for them. Additionally, non-destructive testing is easier to perform, faster, requires less training than destructive tests, and is cost-effective. Most of all, because no destruction is involved, manufacturers do not lose money on testing samples. By and large, these advantages make it a practical choice for smaller builders or those located in remote areas.
Specifically, Strike-It™ with its latest technology is a must-try method of quality assurance.
Despite its advantages, non-destructive testing may not be for everyone. Destructive testing remains the primary concrete testing method in the industry. In addition to the cost of transitioning to non-destructive testing, there are significant concerns about the legal repercussions. A new method of non-destructive testing may not account for all limitations and may fail to detect a structural flaw. Consequently, further evidence is needed to affirm the effectiveness, accuracy, and reliability of non-destructive testing.
Learn more about destructive testing here.