Concrete strength test is an essential part of the construction process, allowing you to ensure that your structure will stand the test of time.
However, there are different methods for testing the various strength properties of concrete materials. Some are relatively simple tests you can do on-site to get desired strength data quickly. But others are more complex testing techniques requiring performing break tests in concrete testing labs.
What are the common techniques for testing concrete’s strength on-site or in the lab (at home)? This article will answer this question.
We’ll examine specific methods in either concrete strength testing group (in-lab tests and on-site tests), including their pros and cons.
At the end of this article, you’ll know what concrete testing method best suits your project, and you’ll be one step closer to constructing quality concrete structures that stand the test of time.
On-site Concrete Strength Test Methods
The on-site concrete compressive strength test methods do not require taking hardened concrete samples to the lab for testing before getting the desired strength data.
You perform the test on the hardened concrete and obtain strength data right on the job site.
Most of the on-site concrete strength testing methods are non-destructive and do not damage the structural integrity of any existing structures or a concrete slab. However, some involve minimal damage to the concrete’s surface.
Common techniques for testing concrete’s strength on-site include:
Maturity Testing
Maturity testing, performed with concrete cure time equipment, measures the time it takes for concrete to cure at the location of a structure.
A sensor placed in the fresh concrete takes temperature readings at specified intervals. The combined time and temperature readings create a “maturity number” to estimate the compressive strength of the concrete at the structure’s actual location.
The biggest benefit of the ASTM C1074 maturity testing method is that its estimations are much better than laboratory or even field-cured specimens. This method also helps determine which sections of concrete will have the highest and lowest strengths due to temperature inconsistencies caused by shade or direct sunlight exposure. Overall, it enables non-destructive testing of concrete, saving time and money by testing on-site without lab requirements.
Keep reading: Concrete Strength Test: Fixing Failures [Updated 2024]
Pros
- It’s a non-destructive test that does not damage the structure.
- It gives data that allows the calculation of compression strengths in real time.
- The results are very reliable as the sensors are embedded in the in-situ concrete element.
- You don’t waste time waiting for the results of concrete testing.
Cons
- It requires a maturity calibration for each concrete mix.
Schmidt Rebound Hammer Test
In the Schmidt rebound hammer test, concrete test hammers measure the elastic strength of concrete or rock for surface hardness and penetration resistance.
A spring release mechanism impacts the hammer to drive a plunger into the surface of the concrete. The rebound distance from the hammer to the surface of the concrete is then correlated to the concrete’s strength.
The convenience and simplicity of the rebound hammer test have made it one of the most popular forms of solid-state concrete testing. This particular concrete compressive strength test is subjective due to factors like the presence of rebar or subsurface voids. However, the concrete test hammer has proven to give reliable, non-destructive measurements.
Pros
- It is a relatively simple test to do.
- Damage to the concrete’s surface is minimal.
- It can also be used to test the uniformity of concrete.
Cons
- The surface conditions and the presence of large aggregates, rebar, and subsurface voids can affect the result.
- It requires pre-calibration using cored samples.
Ultrasonic Pulse Velocity (UPV) Testing
The UPV testing method involves measuring the travel time of acoustic waves through a concrete slab. The ease at which the waves travel through the concrete provides information regarding its density, elasticity, and resistance to deformation or stress. This information is then used to determine the concrete’s strength.
This test is performed using equipment such as the V-Meter Mk IV Pulse Velocity Meter Complete Set.
Like the rebound hammer, this method can be affected by the presence of rebar. Nevertheless, UPV is an exciting advance in concrete strength testing, and when combined with a rebound hammer, it can produce even more comprehensive results.
Pros
- It is a non-destructive test that does not damage the concrete slab
- It can detect a host of issues within the concrete, such as cracking, voids, and delaminations
Cons
- The presence of aggregates, rebar, and voids significantly affects the test result.
- It requires calibration with multiple samples.
Pull Out Test
The concept behind the Pull-Out concrete strength test is that the tensile force required to pull a specific shaped rod from a concrete it is attached is related to the strength of the concrete.
The test involves casting in place or post-installing a metal rod with an enlarged end into a concrete slab and then measuring the force required to pull out the rod. The stronger the concrete, the more force is required to pull out the rod.
Pros
- It is relatively easy to perform the test
Cons
- It often involves damaging the concrete
- Getting accurate results involves performing many tests at different locations of the slab.
Penetration Resistance Test
When firing a steel probe into a concrete surface, the penetration resistance test concept states that the depth of penetration is inversely proportional to concrete strength.
The penetration resistance test involves using a specialized device (Windsor probe) to fire a hardened steel probe onto the concrete’s surface, followed by measuring the extent of penetration (by measuring the exposed length of the probe). The stronger the concrete, the more it’ll resist being penetrated by the probe, and the lower the penetration values you’ll record.
Pros
- It’s a relatively easy concrete compressive strength test
Cons
- The presence of large aggregate, rebar, and voids affects the test result.
- Getting accurate results requires doing multiple tests.
- It will involve some surface damage (caused by the penetration holes) that will require repairs.
At-home Concrete Strength Test Methods
At-home concrete strength test methods require taking hardened concrete samples to a home lab to test them and obtain strength data.
The testing usually involves crushing/ breaking cylindrical concrete specimens in a compression testing machine. Thus, laboratory construction industry/ home concrete compressive strength tests are usually destructive. That is, they damage the structural integrity of a concrete slab in one way or another.
Common at-home/ in-lab concrete strength testing techniques include:
Core Drilling
Core drilling involves using a core drill to cut a square-inch cylindrical sample out of a concrete slab and crushing the core sample in a machine at home (laboratory) to determine the concrete’s strength.
The core samples from coring are considered more accurate than field specimens because the concrete has been subjected to the curing conditions of the in-place slab. In fact, many codes and guidelines consider core drilling the only approved way of determining concrete strength.
Pros
- It provides accurate results reflective of the in-situ concrete.
Cons
- It’s a destructive test, as cutting a chunk out of a slab may cause structural damage to the structure.
- You need to repair the holes created after removing the concrete cores for testing.
Cast-in-Place Cylinders
The cast-in-place cylinder technique enables testers to follow the ASTM C873 standard test method to compressively test cylindrical concrete specimens. This technique facilitates easy removal of concrete samples from a concrete slab and transport to a laboratory for strength testing.
When pouring your concrete mixture, you include a non-absorbent cylinder mold, with its rim level with the surface of your slab. You’ll also pour concrete into the mold, which remains in the slab.
Once the concrete hardens, the cylindrical concrete specimens are removed from the cylinder molds and taken to a laboratory for a concrete compressive strength test.
Pros
- The samples are subjected to the same curing conditions as the in-place slab, so they provide more reliable compressive strength test results than field-cured samples.
- It makes obtaining concrete samples for lab testing easier and cheaper than core drilling.
Cons
- It’s a destructive test that damages the structural integrity of the concrete slab.
- The holes left after the concrete cylinders/ samples are removed must be repaired.
Combined Methods of Concrete Strength Testing
Combined methods of concrete strength testing refer to using a combination of two or more techniques to measure a concrete strength.
However, combining concrete compressive strength test methods allows you to confirm strength data using different methods, resulting in a more comprehensive overview of your concrete slab.
The accuracy of your test results also increases, as the second test may account for influencing factors of the first test. Thus, combined concrete compressive strength testing methods allow for better concrete quality control and quality assurance of a structure.
Common combined methods of concrete strength testing are:
Rebound hammer and Ultrasonic Pulse Velocity (UPV) Tests
The combination of rebound Hammer and UPV is the most widely used combined NDT method for evaluating concrete’s strength.
The “Rebound Hammer X UPV” combined method provides highly accurate results of concrete’s strength by using correction factors that consider the influence of aggregate type, rebar positions, etc.
For example, since the presence of rebars can affect the result of the rebound hammer test, you may start with the ultrasonic pulse velocity test to identify the position of the reinforcements. Then, you can steer clear of the steel rods when testing using the rebound hammer, resulting in more accurate concrete strength data.
Maturity Testing and Core Drilling
The maturity method helps you determine if your concrete is properly cured and ready for a break test to confirm its strength.
Taking concrete temperature readings (via embedded sensors) is essential when curing concrete, as it helps determine if your slab is curing properly. Using these readings to calculate the compressive strength of your concrete in-situ concrete is what is called the maturity method.
However, this testing method is considered theoretical. So, it is recommended that after your concrete cures properly (after 28 days of pouring the slab), you should perform cylinder break tests to confirm the strength of your in-situ slab. This is where the core drilling method comes in. You need to cut out a concrete cylinder from the slab and take it to a laboratory to determine the actual strength of the in-situ concrete.
How do you Choose the best Compressive Strength Method for your project?
The best compressive strength method for your project depends on your specific needs and preferences (such as whether you want a test that will not damage your slab, is easy to perform, provides accurate results, etc.).
Damage to Concrete Slab
If you need tests that will not damage your concrete slab, non-destructive tests like the maturity method and ultrasonic pulse velocity test are the best options.
You may also consider the rebound hammer, penetration resistance, and pull-out tests, as these result in only minimal damage to the concrete’s surface.
Ease of Testing
If you need tests that are easy to perform, the best options are the rebound hammer and penetration resistance tests.
However, these concrete strength testing techniques are considered less accurate than other methods because they only examine the curing conditions directly below the concrete’s surface and do not assess the conditions at the center of the concrete material.
Accurate Results
If you need accurate results, the best techniques are those that examine the concrete’s core, such as the ultrasonic pulse velocity method and core drilling method. Combined methods (which include one technique that examines concrete cores) are also excellent.
However, these tests are relatively difficult to perform (e.g., UPV entails lengthy calibration as it requires many sample specimens).
Takeaway: Perform concrete strength testing to ensure the quality of your structures
Concrete strength testing refers to measuring the compressive strength of your concrete to ensure that your structure will stand the test of time.
The different methods for testing concrete’s strength can be divided into two main categories: on-site and laboratory/ home testing.
Moreover, the choice of a method depends on your specific needs and preferences, as each method has its own advantages and disadvantages.
Whatever your chosen testing method, getting accurate results depends on following standard test procedures and precautions. It also depends on using reliable equipment. This is where Certified MTP comes in!
Certified MTP is a comprehensive provider of top-quality testing equipment and lab supplies for testing materials such as concrete, cement, aggregate, soil, asphalt, and more. All our products are designed with durability and accuracy for optimal performance.
Ready to ensure the quality of your concrete? Browse our full selection of concrete testing equipment and accompanying accessories.
If you want additional help deciding which method is right for you, send us an email today.
