Understanding Concrete Strength: From PSI To Tips For Pouring Concrete
Updated: Jul 21
Are you wondering if you should use concrete for your home improvement project?
Concrete is a popular choice for various projects since it's one of the most durable and low-maintenance construction materials available.
But nothing in life is entirely foolproof, and sometimes concrete fails. Redoing a project because of concrete failure is a huge headache. Worse yet, if your concrete fails, someone could get seriously injured.
That's why it's vital to ensure your concrete is strong enough to do its job!
But how do you measure concrete strength, and how do you make concrete stronger?
You can read the whole article or click on the section that interests you.
Let's dive in!
Forces Put on Concrete
To understand concrete strength, you must understand the different types of forces or stresses placed on a concrete slab.
First, there is compressive stress. Compressive stress describes the force placed on an object that shortens or compresses it. For example, if an elephant steps on your toe, you experience compressive stress!
Second, there is shear stress. Shear stress occurs when forces are applied perpendicularly to one another. If you lock your fingers together and pull against yourself, you are experiencing shear stress.
Finally, there is tensile or flexural stress. Flexural stress is a force exerted upon an object that lengthens or stretches that object. When you jump into a swimming hole using a rope swing, you exert flexural stress on the rope.
Concrete handles compressive stress and shear stress well, but it performs poorly regarding tensile strength. In fact, the tensile strength of concrete is only about 10-15% of its compressive strength.
When considering the strength of concrete, you must consider all the forces that it will endure.
If it can't withstand the compressive, shear, and tensile strength placed on it, it will fail.
To prevent concrete failure, it's important to test the concrete strength.
Let's explore how it's done!
How To Test Concrete Strength
Before we get into the details of testing concrete strength, we need to review the terminology. You'll hear the term "concrete PSI" used frequently. But what exactly is PSI?
Understanding Concrete PSI
PSI is an abbreviation for pounds per square inch and is the most common unit for measuring pressure in the U.S.
If an object receives five PSI, five pounds of pressure are exerted on every square inch of surface area.
We actually deal with PSI all the time, even when we don't know it. For example, the atmosphere exerts about 14.7 PSI on your body all the time. The only reason we aren't crushed by air pressure is that air is inside our bodies, pushing outward at the same pressure. The two forces balance each other so we don't get crushed or explode!
But let's get back to concrete. Since concrete strength is usually measured in PSI, we need to understand how to find the correct concrete PSI.
Testing the Compressive Strength of Concrete
The compressive strength of concrete is measured in PSI.
Compressive strength is tested by compressing cylindrical concrete specimens in a particular machine designed to measure this type of force.
In short, the machine compresses the cylinders until they crack or break completely.
Testing is done according to the American Society for Testing & Materials standard C39.
After being tested, concrete is given a PSI rating. The ideal PSI concrete rating depends on the concrete's use, but concrete almost always has a PSI rating of 3,000 or more.
That's compressive strength. But how is the tensile strength of concrete tested?
Testing the Tensile Strength of Concrete
The tensile strength of concrete isn't recorded in pounds per square inch. Instead, it is calculated in flexural strength or modulus of rupture.
When we discover the flexural strength of concrete (how much it bends before breaking), we are indirectly finding the tensile strength of that concrete.
Since flexural strength measures how much concrete can bend before it breaks, the test is done on a concrete beam rather than a cylinder.
In center-point loading, the load or pressure on the beam is applied at the center of the beam. The maximum stress occurs at the center of the beam under the load location.
In third-point loading, the load is applied at two points. Maximum stress occurs over the center 1/3 portion of the beam.
Remember—traditional concrete has a significantly lower tensile strength than compressive strength.
This weakness means that concrete structures undergoing tensile stress must be reinforced with high-tensile strength materials, such as steel. The steel helps correct the concrete's weakness. (For more information on reinforcing concrete, read our blog: Concrete Rebar: Everything You Need To Know.)
If testing concrete's compressive and tensile strength makes your head swim, don't worry. We've only given a general overview here, and the engineers use specialized machinery and intricate formulas to determine the strength of concrete. Most normal people don't need to know everything about the physics.
You must understand how much stress will be placed on your concrete and purchase concrete that with an appropriate PSI.
So how strong does concrete need to be for a driveway, sidewalk, beam, or bridge?
Let's find out!
Ideal Concrete Strength For Common Structures
It's difficult to determine the tensile strength of concrete. So, it's common practice to use the results from compressive strength tests.
For the rest of this article, we will refer to the compressive strength of concrete (concrete PSI) when discussing concrete strength.
So, what is an ideal concrete PSI for common structures?
Most residential projects, such as sidewalks and driveways, require 2,500-3,000 PSI.
Structural components like beams and footers require a PSI of 3,500-4,000. Concrete in this range is also a good choice for concrete slabs for storing RVs or other heavy loads.
Concrete used in warehouses, factories, and other large-scale commercial properties often requires 4,000-5,000 PSI.
Nuclear power plants and other areas of possible radiation contamination need a PSI of over 6,000. (Concrete with a compressive strength greater than 6,000 PSI is high-strength concrete.)
Using concrete with a higher PSI rating than you need is fine. Just remember that it will be more expensive!
Getting the correct PSI is simple: If you call your local concrete provider and tell them what PSI you're looking for, they will mix the concrete and deliver it to your home or job site! They're also a good resource if you're unsure what PSI you need for a project.
How To Make Concrete Stronger
You can do things as a homeowner or contractor to strengthen your concrete. But before we explore that, we need to know what affects concrete strength.
Factors Affecting Concrete Strength
Several factors affect the strength of concrete. Let’s take a look at a few of them:
Water-to-cement ratio: A lower water-to-cement ratio makes for stronger concrete but makes it more challenging to work with. Finding the right balance to achieve the desired strength while maintaining workability is necessary.
Concrete porosity: Voids in concrete can be filled with air or with water. Air voids are an obvious and easily-visible example of pores in concrete. The more porous the concrete, the weaker it will be.
Strong aggregates: Aggregates are the larger stones used in concrete, bonded together by the cement. Weak aggregates make weaker concrete, while strong aggregates result in stronger concrete.
Curing: If concrete is allowed to dry out while curing, the hardening process will stop, and the concrete will weaken. Though the concrete may seem hard, it will fail more quickly if it cannot cure completely (concrete sets within 24-48 hours but won’t cure for around 28 days).
Other materials: Some concrete additives and materials like steel reinforcing bars or reinforcing fibers increase concrete strength.
Tips For Increasing the Strength of Concrete
Considering the factors that affect concrete strength, let’s discuss how you (not the mixer or engineer) can make the concrete stronger.
First, consider the weather when pouring your concrete. Concrete poured on a hot day with little humidity may set too quickly, leading to shrinkage and premature curing. That means weaker concrete. (If you’re interested in how to pour concrete in hot and cold weather, read our blog: Guide To Pouring Concrete in Any Weather.)
The solution to pouring strong concrete in hot weather is more than just adding more water. Remember, concrete porosity is also a problem - and too much water makes pockets in the concrete and doesn’t allow it to bond.
Our second tip is not to use excessive water when pouring concrete.
Instead, mist concrete 2-3 times a day for three days after you pour it. Wetting the concrete’s exterior during the three-day curing process helps the concrete develop a robust internal bond.
Third, vibrate the wet concrete. Vibrating does two things to strengthen the concrete:
It encourages the wet concrete to filter into voids in hard-to-reach places, such as the space below a basement window.
It removes tiny bubbles from the wet concrete, making the final product more solid.
Fourth, always reinforce your concrete.
Traditionally, concrete has been reinforced with rebar or steel mesh. Both of these materials work well, and we recommend using them.
The compressive strength of 5,000 PSI in concrete is considered relatively high in most settings. This can be achieved using a premium concrete mixture and traditional reinforcing materials.
However, if you need extra strong concrete, there is another step you can take.
Ultra High-Performance Concrete
Ultra High-Performance Concrete (UHPC) is a new concrete technology with greater strength properties than traditional concrete across all strength ranges.
How is this strength achieved?
UHPC achieves incredible strength by using integrated fibers in its makeup. These fibers are added to the concrete mix, accounting for 20 to 25 percent of the end product.
The fibers vary from polyester to fiberglass bars, basalt, steel, and stainless steel. These integrated fibers create a progressively stronger end product, with steel and stainless steel delivering the most significant strength gains.
In the end, UHPC doubles traditional concrete's tensile and compressive strength capacity.
After just 14 days of curing, UHPC has a compressive strength of 20,000 PSI. This number can increase to 30,000 PSI when fully cured for 28 days.
UHPC isn’t a feasible solution for most small applications. It is generally only used when building a bridge or constructing something that requires enormous strength.
Concrete is known as a reliable and durable construction material—and most of the time, it is!
However, concrete is not immune to cracking or breaking. And if you don’t know how strong the concrete needs to be for your next project, you might end up with concrete that doesn’t meet your expectations.
Thankfully, you can now confidently assess how strong your concrete needs to be and take practical steps to ensure the concrete is as strong as possible.
For more information on pouring concrete, check out our related blog posts:
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