From calculating the undrained shear strength of a soil sample to assessing the load-bearing capacity of a concrete beam, the unconfined compression test, or compressive strength test, serves as a powerful testing method for geotechnical engineers and construction experts alike.
But let’s not get ahead of ourselves. What is the unconfined compressive strength test, how does it work, and what does it show us? In this blog, we’ll answer each of these questions and reveal where you can find the best unconfined compressive strength testing equipment for your next critical project.
What is an unconfined compression test?
The unconfined compression test, also known as the uniaxial compressive strength test, is a popular technique to assess the unconfined compressive strength (UCS) of a concrete, cement, sand, clay, or soil sample when it undergoes axial compressive loading. It is one of the simplest, most effective ways to assess the shear properties of a sample. The test is incredibly common in material testing industries because it is easy to set up, fast to complete, cheap, and reliable.
The guidelines for unconfined compressive testing fall under the ASTM standard test method. In particular, ASTM D 2166 is the standard test method for the unconfined compressive strength of cohesive soil, and ASTM C39 is the industry standard for testing the compressive strength of concrete.
Note that the unconfined compressive strength test is typically only used to test the compressive strength of concrete, soil, clay, or sand samples. The sample must be able to hold its own weight, or it risks crumbling. Therefore, unconfined compression tests aren’t suitable for dry soils, crumbly clays, or loose sands.
Confined compression tests vs unconfined compressive tests: What’s the difference?
The primary difference between confined and unconfined compression tests is that the sample in an unconfined compressive strength test is not restrained laterally. Let’s explore what that means in practicality.
Suppose you have two identical, cylindrical cohesive soil specimens of which you want to test the shear strength. You leave the first cylinder unconfined and place the second cylinder in a confining chamber, such as a metal cylinder or cube. The first specimen expands laterally when force is applied to it. The confining chamber restricts the second specimen, preventing it from expanding laterally when placed under axial stress.
The test method you choose will largely depend on the type of material you are testing and its intended purpose. For soils, concrete, and clays used in deep foundation design or underground structures, perform a confined compression test to simulate the intended environment.
Use unconfined compressive strength tests to assess materials not laterally confined in use, like concrete for support beams or soil for shallow foundations.
Keep Reading: Compressive Test: Guide to its Importance and Execution.
What does an unconfined compression test tell us?
In short, the unconfined compression test shows us how much load a material can withstand when it is placed under axial strain. We refer to this ‘ability to withstand load’ as the unconfined compressive strength of the sample.
Understanding the unconfined compressive strength of a material allows us to calculate its shear properties. For example, we can use unconfined compressive strength to gain a reliable approximation of the unconsolidated undrained shear strength (SU) of a cohesive soil, clay, or sand sample. This is because SU is equal to one-half of the unconfined compressive strength when the test specimen’s angle of internal friction is zero.
Unconsolidated undrained shear strength is critical in the construction industry because it allows professionals to accurately predict the stability of structures built in or on soil, clay, or sand. For example, soil with poor undrained shear strength will likely fail if a builder constructs a large structure upon it. This could lead to subsidence and cause long-term safety concerns.
How does an unconfined compression test work?
An unconfined compression test is a simple, effective way to test the maximum load a sample can withstand when it is placed under axial strain. Here’s how it works.
Before performing the test, the material tester prepares their specimen. If they are testing a concrete sample, they can use a concrete cylinder or cube mold to create a standardized test sample by pouring and curing the concrete mixture they intend to test.
If the material tester intends to test a sand, clay, or soil specimen, they can extract it from the ground using a thin-walled sampling tube. The sample can then be removed from the tube and trimmed to the correct length-to-diameter ratio. This is typically between 2.0 and 2.5 L/D.
Once the specimen is extracted, the tester places the specimen upright on the loading plate of a Universal Testing Machine or concrete compression machine. They can then raise the loading plate until the upper plate holds the sample in place before slowly compressing the specimen. The compression machine applies gradual, consistent axial stress along the vertical axis, displaying the resulting load in kilonewtons (kN).
When the sample exhibits shear stress, deforms, or breaks apart, the test ends, and the material tester can record the sample’s maximum axial compressive stress. They can then use this calculation to determine the unconfined compressive strength of the sample.
More Read: Understanding ASTM D 422: Sieve Analysis and Particle Size
Why are unconfined compressive strength tests useful?
Here are some of the primary industries in which unconfined compressive strength testing is useful.
● Geotechnical engineering: Unconfined compressive stress tests are essential in geotechnical engineering. They allow engineers to calculate the shear properties of a material, which is an important consideration when analyzing the stability of an area. For example, a geotechnical engineer may use UCS to assess the stability of a slope or embankment.
● Construction and infrastructure: Construction experts regularly use unconfined compressive strength tests to assess the suitability of materials for use in structures. For example, testing the UCS of a soil sample allows a professional constructor to determine if an area of ground can support a building. Similarly, testing the unconfined compressive strength of concrete allows construction experts to determine how effectively a concrete mixture will be able to support a heavy structure.
● Mining and rock mechanics: The unconfined compressive strength test is incredibly useful in mining operations. For example, when testing whether tunnels, caverns, and underground excavations are stable and safe to use.
Where can I find unconfined compressive strength test equipment?
The most important first step to any successful compressive strength test is having equipment you can rely on.
If you’re searching for the best tools for your next unconfined compressive strength test, choose CertifiedMTP. We stock an exciting range of first-class ASTM standardized equipment designed to get you accurate, reliable results on your next material test.
From soil compressive strength testing machines and extensometers to sample extractors and universal load frames. We have the compressive stress testing equipment to make every test you perform as accurate as the last.
Planning out your next compressive strength test? Consider getting some essentials to make the process go smoothly. Stocking up on calipers, scales, and safety equipment ensures you always have everything you need to get first-class results.
Can’t find what you’re looking for? Don’t hesitate to reach out. Whatever you need, CertifiedMTP has the products and expertise to help you find the best equipment for your material testing needs.