(This is Part 1 in a two-part blog series – Part 2, Reinforcing Steel and Concrete Quality for Nondestructive Evaluation)
Nondestructive testing methods are used to evaluate concrete properties by assessing the strength and other properties such as corrosion of reinforcement, permeability, cracking, and void structure. This type of testing is important for the evaluation of both new and old structures.
Establishing the quality and properties of hardened concrete is central to evaluating a structure, whether it is decades old or still under construction. In-place concrete testing is a significant part of assessing an existing structure's physical condition and safety during routine maintenance, as preparation for structural modifications, or when changing ownership as a matter of due diligence.
While concrete strength is justifiably the primary factor in evaluating hardened concrete, location and corrosion of reinforcing steel, moisture characteristics, and physical condition of the concrete need to be examined as well. In this blog series, we will first discuss in-place and nondestructive methods and equipment to measure concrete strength, then focus on placement and corrosion of reinforcing steel and evaluation of concrete condition and performance in Part 2.
For projects under construction, testing hardened concrete with nondestructive or other construction materials tests is often used to resolve quality issues encountered during construction. If concrete compressive strength tests are low, there are at least two critical questions that nondestructive tests answer quickly:
- Do the low-strength lab tests reflect the actual strength of the concrete in place?
- Even if the concrete in place does not fully meet the design strength, does it have enough strength to perform safely?
In addition to the ASTM test methods listed throughout this blog post, ACI 437R-19: Strength Evaluation of Existing Concrete Buildings from the American Concrete Institute is recommended as a comprehensive reference. ASTM C823 Standard Practice for Examination and Sampling of Hardened Concrete in Constructions is also recommended as a guide for conducting these evaluations.
Concrete Compressive Strength Test Methods
Whether assessing older concrete or examining areas where strength samples have failed laboratory tests, in-place compressive strength is the most common and persistent question. The best method to evaluate concrete strength depends upon the focus of the investigation. When structural integrity is questioned, selecting accurate and reliable test methods ensures the design requirements are met. By contrast, the objective may be a simple evaluation of the uniformity of concrete throughout a structure.
Concrete Compressive Strength Test Methods
Concrete Maturity strength estimates are developed using the temperature signature of a concrete mix as it generates heat while curing and developing strength.
Temperature sensors are embedded in fresh concrete placed in structures, accumulating concrete temperatures versus time. This temperature history is compared to test data from laboratory samples, and equations are used to calculate the strength-maturity relationship at any time. The maturity method is very useful in various strength assessments. It is often specified to predict safe periods for form removal and reshoring, post-tensioning operations, or putting pavements and structures into service.
Modern, integrated systems developed exclusively for concrete maturity testing use wireless sensors and Bluetooth or Wi-Fi technology to make data collection, computing, and storage easy and convenient. Cloud-based maturity applications ensure continuous monitoring access and make real-time strength reports available for all stakeholders.
Concrete maturity tests require preliminary laboratory preparation of concrete mixes and planning for sensor placement, but the accurate strength estimates this method provides are proven and reliable. Continued readings from the embedded sensors can also monitor performance over more extended periods. Maturity test procedures are defined in ASTM C1074 and AASHTO methods T 276 and T 325.
Rebound Number
Concrete test hammers, sometimes called Schmidt hammers or Swiss hammers, use a spring-loaded mass to strike the concrete surface and record the rebound energy, or R-value, from the impact. The R values can be correlated to compressive strength. These economic instruments excel at quickly assessing large areas of concrete. A single operator can use the test hammer to quickly profile the strength uniformity of a larger concrete structure and define areas requiring more focused testing.
Concrete test hammers yield the most accurate compressive strength estimates when correlated directly to concrete test cylinder results from the same mix. Following the method prescribed in ASTM C805, a reliable estimate of in-place strength can be developed.
The basis of the rebound number is surface hardness, so rough or textured areas require grinding before testing. Damage to the concrete surface from rebound testing is minimal, often leaving just minor dimples on the surface. The digital rebound hammer, a newer version of this device use electronic accelerometers to measure rebound numbers, GPS position recording, and web-based reporting, improving the accuracy and reliability of testing. For a more in-depth discussion of the concrete test hammer, see this previous blog post Rebound Hammer Test: What You May Not Know.
Probe Penetration
The Windsor probe method uses a powder-actuated charge (a "blank" firearm cartridge) to drive a hardened steel probe into the concrete surface. Advanced versions of the device feature electronic probe measurement, logging, and calculation of compressive strengths.
For a single test, three probes are driven into a test area. The exposed lengths of the probes are measured and used to provide an average penetration resistance. Since the hardness of the aggregate directly affects the penetration, the manufacturer provides a Moh's hardness test kit for the aggregate and a chart to provide corrected resistance values.
As noted in ASTM C803, accurate concrete strengths can be estimated if a correlation between penetration resistance and compressive strength of a similar mix design has been performed. The test is relatively easy to perform, but the powder-actuated mechanism and high-energy impact require compliance with strict safety measures and personal protective equipment. The device is designed with safety features to prevent operation unless firmly placed in the firing position. Steel probes and blank powder charges are consumables for each test performed with this device. The Windsor probe test leaves some repairable surface damage caused by the penetration holes.
Pulse Velocity
Ultrasonic Pulse Velocity Instruments (UPV) use transmitting and receiving transducers to send and receive timed pulses of ultrasonic energy through a concrete section. The signal's transit time indicates the concrete's density and is slowed by voids, inclusions, and discontinuities. This method is optimal for gathering related information on uniformity and detecting internal defects like honeycombing and cracking. ASTM C597 describes procedures for the use of pulse velocity meters on concrete.
The chart below shows a general relationship between concrete quality and pulse velocity.
| Pulse Velocity, (Meters/Second) | Concrete Quality |
|---|---|
| >4,500 | Excellent |
| 3,500 to 4,500 | Good |
| 3,000 to 3,500 | Fair |
| <3,000 | Poor |
The benefits of UPV are not limited to assessing concrete uniformity or detecting voids. Like concrete test hammers, pulse velocity instruments can be used to assess the quality of concrete over large areas to select areas for further testing.
Mathematically blending the test values from a pulse velocity instrument with the R values of a concrete test hammer has been shown to produce highly accurate estimations of compressive strength, especially when correlations with core samples are possible. The SONREB (SONic-REBound) method is an algorithm combining the results from pulse velocity and concrete test hammer test methods. Combining the results of the two methods offsets some of their shortcomings and allows a more comprehensive assessment beyond just surface hardness.
There are currently no ASTM or AASHTO test methods published for the SONREB method. Numerous studies and research papers are available online that discuss the method and suitable equations for compressive strength estimates. Creation and adoption of a standard method is ongoing in the European Union, notably in Italy, where seismic testing of structures is required by law.
Coring
The extraction of drilled cores from hardened concrete is essentially the opposite of nondestructive evaluation. Still, the quality of the information they provide makes it essential to consider them in strength assessments.
Core samples tested in the laboratory provide simple, accurate, and straightforward data on actual concrete compressive strength. Although some correction for orientation and unbound aggregates on their perimeter must be made when testing, core samples are the benchmark for in-place concrete strength determinations. The obvious problem is that core sampling cosmetically damages the structure being tested, and there is some potential for unintended structural damage as well.
With some planning, core sampling can be performed in areas where the risk of structural damage is minimal and visual blemishes can be repaired. A few core samples taken to correlate with nondestructive strength tests ensure greater accuracy and enhance the credibility of an assessment. This approach is more efficient and economical, and reduces damage to the structure.
Concrete coring equipment requires some initial expenditure, but the cost of consumables is low relative to the number of samples taken. ASTM C42 covers the sampling, preparation, and testing of concrete cores.
We hope this blog post has given you an idea of what is needed to assess the quality and strength of hardened concrete in place. For more information on NDT instruments, visit our Non-Destructive Testing Equipment page. Please contact our testing experts for questions or help with your application.
Additional Resources
Gilson Videos
Testing Sandards and Methods
ASTM C918 Measuring Early-Age Compressive Strength and Projecting Later-Age Strength
American Concrete Institute (ACI) Guides & Reports
ACI PRC 214.4R Guide for Obtaining Cores and Interpreting Compressive Strength Results
ACI PRC 228.1R Report on Methods for Estimating In-Place Concrete Strength
