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What’s the Best Concrete Strength Testing Method for Your Project?

  • June 12, 2024

Concrete is a popular commercial construction material mainly because of its strength and durability.

However, many factors can negatively affect the strength of concrete. For the sake of quality control, every concrete structure should undergo testing procedures to ensure it is safe and will withstand the test of time. 

There are several testing options available to concrete contractors. Which is the best? 

Read on to find out. 

Table of Contents

1. What is Concrete Testing?
2. Testing Fresh Concrete
3. 7 Types of Concrete Compressive Strength Testing
4. How to Determine the Best Compressive Strength Test for Your Project


What is Concrete Testing?

Concrete testing is when procedures are conducted to check for deficiencies in concrete pouring, curing, or construction.

In this article, we will discuss testing methods that determine the strength of new concrete. 

(The creep test is another concrete testing method that serves another purpose.) 

Why Test The Strength of Your Concrete?

Testing the strength of your concrete ensures the structural integrity of your building, necessitating fewer repairs down the road. 

These tests will also help with quality control and ensure compliance with building codes and industry standards. Knowing the strength of your concrete will determine its load-bearing capacity, allowing you to design accordingly.

It also prevents overdesign and excessive material use if your concrete proves stronger than expected.

Additionally, testing ensures the professionals pick the best concrete mix design for certain applications. Some concrete elements, such as beams, columns, and pillars, require the strongest concrete mix. 

Lastly, but most importantly, testing the strength of your concrete ensures the safety of your building’s future occupants. 


Testing Fresh Concrete

Testing can be done on a fresh sample of concrete, or it can be done in separate cast cylinders. These tests are done before the concrete is poured onsite.

Sampling

Tests done according to ASTM C172 protocols involve laboratory testing performed by an ACI-certified technician.

Professionals take samples from the ready-mix truck, but not from the first or last 10% of the mix. These samples must be protected from contamination and evaporation. 

The tests performed aren’t to test the strength of the concrete but to ensure the mix has the necessary characteristics to be suitable for the project at hand:  

Temperature

One sample can be placed in a container to check the temperature. To be accurate, the sample must be checked within five minutes of receiving it. 

A thermometer must be surrounded by 3 inches of concrete on all sides. After being inserted for 2 minutes, the temperature is to be read before removing the thermometer from the mix. 

Air Content and Unit Weight

Another sample is measured for air content and unit weight using a Type B pressure meter. 

The formula to follow is:

D = (Mc – Mm) / Vm

(Density equals the weight of the measure while holding concrete, subtracting the weight of the empty measure and dividing it by the volume of the measure.) 

Concrete Slump Test

This test must also be done within 5 minutes of receiving a concrete sample. The concrete mix is placed inside a slump test cone compacted by a rod. The cone is removed, and the slump of the concrete is measured.

Field Testing

Another test that can be done before the concrete is poured is in the field according to the procedures explained in ASTM C31. This test verifies the concrete’s compressive strength and involves casting test cylinders of the concrete. 

Four concrete cylinder samples are cured in molds and tested under a hydraulic press to measure strength.

Two cylinders are tested after 7 days of curing, and two are tested after 28 days. To ensure accurate results, these cylinders should be carefully handled and kept in a controlled environment. 


7 Types of Concrete Compressive Strength Testing

The cylinder testing process discussed above is one of the oldest known methods of testing in-situ concrete strength. However, it isn’t always the best technique for testing every project’s concrete. 

The following seven compressive strength testing methods should be considered for speed and accuracy:

1. Penetration Resistance

Under ASTM C803, this test involves driving a probe into the concrete surface and measuring the penetration depth to estimate its compressive strength.

The testing instrument should be calibrated for the type of concrete used and the aggregate to provide an accurate measurement. 

Pros

This test is non-destructive, meaning it will not cause any deformation or weakness in the concrete. Only small holes from the probe will need to be patched up. 

The penetration resistance instrument is compact and portable, enabling this test to be performed on-site. It also provides a quick assessment, keeping your construction schedule on target.

Cons

One big drawback is that it is limited in measuring surface strength estimation. Also, the accuracy of this test can be affected by the type of aggregate in the concrete and the condition of the concrete surface.

As previously mentioned, the probe requires calibration with standard strength tests for accurate measurements.

2. Ultrasonic Pulse Velocity

This concrete strength testing method, ASTM C597, uses a tester to measure the time an ultrasonic pulse travels through the concrete.

The strength is estimated based on the pulse velocity. If there is a large difference in pulse velocity within the same concrete structure, this is a good indication of deteriorated concrete that needs repair. 

Pros

UPV is another non-destructive test that can not only measure strength, but also assess internal defects and uniformity. It is the most useful for quality control and early crack detection.

Cons

This test requires coupling agents and calibration. The test results are influenced by moisture content and temperature, which only indirectly measure the concrete strength. This testing method needs to be correlated with other methods for accurate measurements.

3. Rebound Hammer

Also known as the Schmidt Hammer Test, the testing protocol outlined in ASTM C805 uses a spring-driven hammer to impact the concrete surface and measure the rebound distance to estimate strength.

Pros

This test is non-destructive, leaving everything intact after the procedure is complete. 

It is quick, easy, and portable, allowing the test to be done on-site. It can also be performed vertically, horizontally, or at an angle as long as the hammer is calibrated in that position. 

Its cost-effectiveness makes it a popular test for those under a strict budget.

Cons

This is another test that provides only surface strength, which may not represent overall concrete strength. Additionally, results may be affected by surface conditions and carbonation. It also requires correlation with other tests for accuracy.

4. Drilled Core

ASTM C42, the Drilled Core testing method, involves extracting a core sample from hardened concrete and testing it for compressive strength.

Pros

This strategy directly measures concrete compressive strength, making it reliable and widely accepted. It can assess in-situ concrete properties.

Cons

Even though this is a direct measurement of strength, it is also destructive (removes part of the structure). The test is also time-consuming and labor-intensive, requiring specialized equipment and proper sample handling for accuracy.

5. Pullout Test

The pullout test measures the force required to pull an embedded metal insert out of the concrete, according to the ASTM C900 protocol.

Pros

This is another test that provides a direct measurement of in-situ strength. It can be correlated with compressive strength. This procedure is useful for early-age concrete.

Cons

The pullout test is semi-destructive, leaving a hole in the concrete.

Careful installation of the pullout assemblies is required before the concrete is poured, which requires planning. 

This method is also more labor-intensive compared to non-destructive methods.

6. Cast-In-Place Cylinders

ASTM C873, otherwise known as cast-in-place cylinder testing, is a method that involves curing concrete into cylindrical molds at the construction site under standardized conditions. The cylinders are then tested for compressive strength.

Pros

These cylinders act as a representative sampling. Also, this method uses standardized testing to ensure accurate strength measurement. As well, this process’s flexibility leaves wiggle room around the testing parameters. 

Cons

To get results, these tests are labor intensive. There is also typically a delay in test results, adding unnecessary time to the schedule.

The curing conditions of the cylinders can differ from those of the in-situ concrete and nullify the results.

Additionally, handling and transporting these cylinders is risky, since they can be easily chipped, cracked, or broken. 

Multiple samples are needed for all the necessary tests to be completed.

7. Wireless Maturity Sensors

The last test on this list is the ASTM C1074, which uses wireless maturity sensors. The sensors are placed on rebar embedded in concrete to monitor temperature and time, estimating strength based on maturity.

Pros

This is a non-destructive test that provides strength analysis, real-time data, and continuous monitoring. It is also useful for early-age strength prediction and curing control.

Cons

The sensors require initial calibration with standard strength tests. These sensors and data loggers can be costly. Since this technique is so technical, installation and monitoring require specialized expertise.


How to Determine the Best Compressive Strength Test for Your Project

With so many different options for testing the compressive strength of concrete, it may be difficult to determine which to use for your project. 

Consider the following factors before making your decision:

Schedule

Methods that take longer may negatively impact the construction timeline. 

The destructive tests, such as drilled core and cast-in-place cylinders, take significantly longer because they require testing in an off-site lab.

Methods that require calibration, like the ultrasonic pulse velocity test and pull-out test, will take longer since calibration needs multiple specimens.

Accuracy

Of course, each modality differs in accuracy levels. Faster methods aren’t usually as accurate, and neither is the easiest to complete. 

As is usually the case, the longer and more difficult a process, the better the results. 

Resources

Some methods require trained persons to complete accurately. Do you have people with this expertise on the team?

Other strategies require special equipment and materials. Do you have the necessary testing materials?

What are you familiar with? Which method do you feel confident using? Testing using a method inaccurately would be a waste of time and money.

Cost

Consider laboratory fees, the price of testing equipment, and even labor costs when choosing your testing method. 

Will you need to buy testing equipment? Does it fit into your budget? The answers to these questions may provide insight into your best option.

Project Requirements

Professionals exploring the project requirements before concrete testing

The purpose and parameters of your project can also influence the testing requirements for your concrete. 

The performance criteria, structure dimensions, intended use, and exposure conditions can dictate the chosen extent, frequency, and testing method. 

Combined Methods

As you read through the various procedures, you may have concluded that no perfect method for testing concrete compressive strength exists. 

Your conclusion would be correct, making using more than one testing method wise. 

Each has its own benefits and limitations. Combining testing methods will increase quality control and result accuracy. 

One popular combination is rebound hammer testing and ultrasonic pulse velocity testing. The former is good for testing aging concrete, while the latter works well on new concrete. 

Getting a multi-angled view of your slab can help to get a more rounded idea of the strengths and weaknesses within it. 


Conclusion

Concrete strength testing is essential to your structure’s long life and safety. 

Since testing is an expense for your project, and it takes time and expertise to conduct it correctly, finding the best method for your project may take some research.

Why not discuss it with a team with years of experience in various concrete testing methods? Contact FMP Construction today to discuss the testing needs for your concrete project.

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