Concrete is a mixture of cement, sand, aggregate (or small stones), and water. Manufacturers sometimes incorporate addmixtures, or additives, for various purposes. These addmixtures create different changes, which can be as simple as increasing the compressive strength of concrete or as complex as reducing CO2 emissions during the production process.
Concrete’s strength, durability, and versatility make it the most common building material globally and the second highest-consumed material after water. In addition, it is also one of the most cost-effective building materials and is much lighter relative to other rock-like materials.
Cement is the most expensive material in the production of concrete. The cement industry is predicted to surpass $600 billion USD by 2025. China, India, the US, and Canada lead the global concrete and cement market.
As the largest cement and concrete manufacturer in the world, China produces nearly ten times as much as India, the second-largest manufacturer. The US accounts for an estimated 4% of China’s production, while Canada makes about 0.4% relative to China.
Quality Assurance And Concrete Testing
Given that concrete is the most widely using building material, concrete strength is a matter of great concern for engineers. When they design infrastructure they need to be able to count on concrete meeting a specified level of strength.
Engineers and builders depend on manufacturers to produce reliable, high-strength concrete to build a safe and stable building. Concrete testing and quality assurance guarantee that the materials they use in construction are safe so that the structures created from them result in the desired outcome. Quality Assurance is a systematic process for concrete testing to ensure the material’s strength and integrity.
Types Of High-Strength Concrete
To understand the various methods for concrete testing, it’s important to discern the different types of concrete that undergo testing.
Precast Concrete
Precast concrete is poured and cured based on a specified size and shape and then transported to the construction site. Builders primarily use precast for larger structures like the concrete segments in flyovers or elevated sections of sky-trains.
Often, precast concrete is used to build mass-produced products such as traffic barriers, pipes, and culverts. Structures constructed with precast concrete usually lack voids and do not always have embedded reinforcing bars.
Worldwide, the precast concrete market exceeds $80 billion USD, with the US generating about $16 billion in revenue.
Site-cast Concrete
Unlike precast concrete, site-cast concrete (or ready-mix concrete) is delivered to the site as a fluid, mixed, poured into molds, and left to cure. In the US, ready-mix concrete manufacturers produce nearly $35 billion USD every year.
Concrete Masonry and Concrete Masonry Units
Concrete Masonry Units (CMUs) are a type of precast concrete – only much smaller than the typical precast concrete industry standard. They are lightweight and can be handled by a single person without any mechanical assistance.
Builders use Masonry Units for constructing a building, a wall, or other desired structure. Some masonry units are made from stones, bricks, blocks, and occasionally mortar. Others are made from concrete – and are known as CMUs.
What Are The Different Concrete Testing Methods?
American Society for Testing and Materials (ASTM) is the standard model for testing the compressive strength of concrete. Through its rigorous standards, ASTM upholds Quality Assurance, guaranteeing the durability of the material in construction.
Depending on the type of concrete in question, there are different mandates for concrete testing that have been laid out by ASTM.
Precast And Site-cast Concrete Testing
Using Cylinders For Concrete Testing
ASTM standards mandate that for testing precast and site-cast concrete, manufacturers test four sample cylinders of concrete for every 150 cubic yards of concrete produced, or once a week, whichever comes first.
If no cylinders are available for testing, cubes or cores are cut for sampling. Concrete testing is always conducted at a third-party site to eliminate any chance of bias.
The cylinders undergo a concrete compressive strength test, and the force required to crush the sample is recorded. To ensure the pressure is evenly distributed throughout the cylinder, it must be capped or grounded.
Quality assurance testing is similar for both precast and site-cast concrete, but the testing frequency differs. For precast concrete, two cylinders are left to cure for 1 week, while the other two are tested 28 days after curing. On the other hand, site-cast concrete undergoes testing every 3 days, 7 days, 28 days, and 90 days.
Testing is a critical component in the concrete industry and cannot go overlooked, even if it comes at a substantial loss. Globally, an estimated 400 million cylinders undergo concrete testing every year. This large testing sample is meant for the 27 billion tons of concrete produced annually.
Using The Schmidt Rebound Hammer For Concrete Testing
In the event that a manufacturer doesn’t have any cylinders to test or cannot cut a piece of concrete for testing, ASTM C805 outlines that manufacturers can test the compressive strength of concrete using the Schmidt impact rebound hammer.
The Schmidt Rebound Hammer is a tool to safely test concrete without damaging the material. Manufacturers test the compressive strength of concrete by measuring the spring-driven mass rebound after it strikes the concrete. By gauging the rebound number, producers can determine the strength of the concrete.
The inspector must use a flat and dry testing surface of 150 mm or larger diameter when using the Rebound Hammer. Inspectors should never test frozen concrete, since it skews rebound numbers. Additionally, they should not perform tests over steel reinforcements when cover thickness is 20 mm or less in diameter.
Inspectors should keep the plunger perpendicular to the surface and record the hammer’s orientation relative to a 45-degree angle. If the hammer is upturned, a positive angle should be used. Conversely, if the inspector uses the hammer at a bowed point, a negative angle must be recorded.
When using a Rebound Hammer, the inspector presses the plunger against the concrete, opening the latch and releasing the hammer. After impact, the hammer recoils, causing the sliding marker to shift from the hammer mass and record the rebound distance. Inspectors must take ten readings with 25 mm between each impact point while leaving 50 mm between each impact point and the concrete’s edge.
CMU Testing
Like precast and site-cast concrete, CMUs must also adhere to strict ASTM standards of concrete testing, often involving the destructive testing methodology.
During testing, the CMU is capped, creating a smooth, flat surface that evenly distributes the compression machine’s force. Usually, a sample of 5 CMUs is tested for concrete strength and is considered representative of the strength of the remaining CMUs.
When testing CMUs with mortar, a prism is constructed from several CMUs and then tested. In other circumstances when the product is too large for a compression machine, a smaller sample can be cut for concrete testing.
ASTM standards mandate that manufacturers test 6 CMUs every 10,000 produced, or 12 for every 10,000 to 100,000. For larger lots (where CMUs are made with the same mix in the same way), 6 CMUs must be tested for every 50,000 CMUs in that lot.
The US National Concrete and Masonry Association (NCMA) estimates that about 1.1 billion CMUs are produced in the US and 100 million in Canada. Knowing that every 6 out of 500,000 CMUs are tested, the US and Canada test a respective 130,000 and 13,000 CMUs annually. Global cement volume used in the manufacture of CMUs indicates that producers should test about 6 million CMUs each year. However, not every country follows ASTM standards, and hence that is unlikely the case.
Destructive Vs. Non-Destructive Concrete Testing
Whether you’re testing for precast concrete, site-cast concrete, or CMUs, the industry is split on the methods that should be used for testing concrete strength.
Destructive Concrete Testing
Conventional Destructive Testing is widely considered the “gold standard” for measuring the compressive strength of concrete and masonry. Whereas non-destructive testing (NDT) is more accessible, and a rather convenient testing method.
Destructive testing is used to test standard concrete and masonry units, cores cut from larger products, and cylinders that are poured for testing when large batches of concrete are mixed.
However, this method has several drawbacks, namely, cost.
Testing equipment, which starts at $10,000 for a single machine, is often difficult to access for those in remote areas or with limited resources. Moreover, if the site is far from the testing facility, transporting the testing samples draws more time and resources away from the project at hand.
In addition to the upfront cost of purchasing the equipment, manufacturers must consider extra expenses for installation, maintenance, and additional features. These machines also require specialized training and protective gear, further adding to the initial costs.
Non-Destructive Concrete Testing
Non-destructive testing on the other hand, is more accessible and far more convenient. The Schmidt Rebound Hammer mentioned earlier is one example of an NDT, but other methods include:
- Visual inspections
- The Strike-it™ Tester
- Ultrasonic Pulse Velocity
- Windsor Probe Penetration Test
- Pull-out and Pull-off resistance tests
- Instrumented Hammers and Modal Analysis
- Embedded Wireless Sensors
Unlike destructive testing, non-destructive testing can be applied in a variety of situations, such as cases when conventional processes are not readily available or when destructive testing is not permitted.
Since the sample remains intact after the testing process, many organizations using various non-destructive testing methods can assess the same batch at different times. This promotes consistency among samples of varying shapes, sizes, and configurations. Overall, non-destructive testing is cheaper, faster, more accessible, typically requires less specialized training and equipment, and can usually be performed onsite.
However, despite its advantages, the industry continues to primarily utilize destructive testing for quality assurance for a variety of reasons, such as:
- The cost and time required to develop or modify standards
- A shortage of sound evidence proving the reliability and accuracy of non-destructive tests
- Fears of legal repercussions if a flaw is discovered in the testing process
These concerns are valid and indicative that more convincing objective evidence is necessary for the industry to fully accept the different non-destructive testing methods of concrete testing.
Each method has its benefits and disadvantages, which are discussed in detail in Measuring The Compressive Strength Of Concrete With Destructive Testing Methods and Measuring The Compressive Strength Of Concrete Using Non-Destructive Concrete Testing Methods. Getting a better understanding of each approach promotes more-informed decisions, which ultimately leads to a better quality assurance for your projects.