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.
Let’s start with the basics. What is the unconfined compressive strength test? How does it work, and what does it show us? In this blog, we’ll answer these questions and guide you to 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, assesses the unconfined compressive strength (UCS) of a concrete, cement, sand, clay, or soil sample under axial compressive loading. It is one of the simplest and most effective ways to assess the shear properties of a sample. This test is very common in material testing industries because it is easy to set up, fast to complete, cheap, and reliable. It is applicable to cohesive materials like clays or cemented soils but not suitable for dry and crumbly soils, fissured or materials, silts, peats, and sands.
The guidelines for unconfined compressive testing follow the ASTM standard test method. Specifically, ASTM D 2166 is the standard for testing the unconfined compressive strength of cohesive soil. ASTM C39 is the industry standard for testing the compressive strength of concrete. Users must establish proper safety and health practices and check regulatory limitations before using the standard test method.
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. The unconfined compression test is performed under zero confining stress, meaning there is no external pressure applied to the sample. 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. Leave the first cylinder unconfined. 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 the unconfined compression test for materials that have enough cohesion to hold together without external confinement.
Apply unconfined compressive strength tests to materials not laterally confined in use, such as 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 about cohesive soil?
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. During the test, load values and deformation values are recorded to obtain a complete load-deformation curve.
Understanding the unconfined compressive strength of a material helps us calculate its shear properties. We can use unconfined compressive strength to approximate the unconsolidated undrained shear strength (SU) of cohesive soil, clay, or sand. SU equals one-half of the unconfined compressive strength when the specimen’s angle of internal friction is zero. Loading continues until the load values either decrease or remain constant with increasing strain, indicating the sample’s failure.
Unconsolidated undrained shear strength is crucial in the construction industry. It allows professionals to accurately predict the stability of structures built on or in 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 compressive test work?
An unconfined compression test is a simple and effective method to test the maximum load a testing a concrete sample can withstand under axial strain. The test involves applying an axial load along the sample’s longitudinal axis.
To perform the test, the material tester first prepares the specimen. For a concrete sample, they use a concrete cylinder or cube mold to create a standardized sample by pouring and curing the concrete.
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 prepared, the tester places the specimen upright on the loading plate of a Universal Testing Machine or concrete compression machine. They 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)..
This method is particularly useful for determining the unconfined compressive strength of cohesive soils like clays or cemented soils.
When the sample exhibits shear stress, deforms, or breaks apart, the test ends. The tester then records the sample’s maximum axial compressive stress to determine its unconfined compressive strength.
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 compression tests are essential in geotechnical engineering. Engineers use them to calculate the shear properties of materials, crucial for analyzing stability. For example, a geotechnical engineer may use UCS to assess the stability of a slope or embankment.
● Construction and infrastructure: Construction experts use unconfined compressive strength tests to assess material suitability for structures. For example, testing the UCS of soil helps determine if the ground can support a building. Similarly, testing the unconfined compressive strength of concrete helps determine its ability 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.
The test method determines the unconfined compressive strength of cohesive soil in various conditions. It specifies the materials suitable for testing and outlines the method’s limitations.
Where can I find 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 compression 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 unconfined compression 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, Certified MTP has the products and expertise to help you find the best equipment for your material testing needs.
