Understanding Concrete Strength: From PSI To Tips For Pouring Concrete
Updated: Dec 9, 2020
Why would you use concrete in a construction or home improvement project?
Because concrete is considered one of the most durable and low maintenance construction materials available!
That means that when you use concrete - as a contractor or homeowner, you expect the concrete to do its job. You expect it to be strong and durable.
Redoing a project because of concrete failure is like a bad dream. Worse yet, someone could get injured when concrete fails.
In other words, you really want to make sure your concrete is strong enough to do its job!
But how do you measure concrete strength? And how do you strengthen concrete if you need to?
We hear your questions, and at Gra-Rock, we’re ready to put our experience in the concrete industry to use and help you understand concrete strength.
You can read the whole article or click on the section that interests you.
Let’s dive in!
Forces Put on Concrete
A big part of understanding concrete strength is understanding the different types of forces or stresses placed on a slab of concrete.
First, there is compressive stress. Compressive stress is a force that is placed upon an object that shortens or compresses the object. For example, if an elephant steps on your toe, you would 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 that is 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 when it comes to 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 will be exerted upon it.
Concrete has to be able to withstand the compressive, shear, and tensile strength placed on it, or it will fail.
And you don’t want failed concrete!
The only fool-proof way to make sure your concrete can withstand the forces put upon it is to test the concrete.
But how can you test the strength of concrete?
How Concrete Strength Is Tested
To understand concrete strength, a firm understanding of some terminology is needed.
When it comes to concrete strength, you’ll hear the term “concrete psi” used frequently. But what exactly does it mean?
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.
In its base form, psi is pretty simple:
If an object is receiving five psi, five pounds of pressure are being exerted on that object per one inch of surface area.
We actually deal with psi all the time - even when we don’t know it. Did you know that the atmosphere exerts about 14.7 psi on your body at all times?
The only reason we aren’t crushed by air pressure is that there’s also air inside our bodies, pushing outward at the same pressure!
But what does psi have to do with concrete?
Concrete strength is most commonly measured in psi, so it’s very important!
Let’s explore how we can determine concrete psi.
Testing the Compressive Strength of Concrete
The compressive strength of concrete is measured by psi.
Compressive strength is tested by compressing cylindrical concrete specimens in a special machine designed to measure this type of force.
In short, the machine compresses the cylinders until they crack or break or completely.
Testing is done according to the American Society for Testing & Materials standard C39.
When the concrete is done being tested, it 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 is not 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 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 as compared to compressive strength.
This means that concrete structures undergoing tensile stress must be reinforced with materials that have high tensile strength, such as steel. (For more information on reinforcing concrete, read our blog: Concrete Rebar: Everything You Need To Know.)
If testing the compressive and tensile strength of concrete makes your head swim, don’t worry. We’ve only given a general overview here, and it's the engineers who use specialized machinery and intricate formulas to determine the strength of concrete.
Your job is to understand how much stress will be placed on your concrete and purchase concrete that stands up to the pressure.
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
The tensile strength of concrete can be quite difficult to determine. That’s why we use the results from compressive strength tests when describing the strength of concrete.
For the rest of this article, we will refer to the compressive strength of concrete (concrete psi) when talking about concrete strength.
So, what is an ideal concrete psi for some common structures?
The psi required for most residential projects like sidewalks and driveways is around 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 wherever heavy loads are expected to be or stored or loved, like RV pads.
Concrete used in warehouses, factories, and other large-scale commercial properties often require 4,000-5,000 psi.
Concrete with a psi over 6,000 is usually found in nuclear power plants and other areas where radiation contamination is possible. (concrete with a compressive strength greater than 6,000 psi is considered high-strength concrete.)
There’s nothing wrong with having concrete with a higher psi rating than you need. However, be aware that the higher the concrete psi rating, the more expensive the concrete is.
But, as we said earlier, engineers are the ones determining concrete psi.
So, how can you know what the psi is when it's time to order a load of concrete?
It's simple: Call your local concrete provider, tell them the project you have in mind, and the psi required to finish the job. They will take care of mixing the concrete and delivering it to your home or job site!
How To Make Concrete Stronger
There are some things you can do 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 a stronger concrete, but it also makes the concrete more challenging to work with. You must find the right balance to achieve the desired strength while maintaining workability.
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. Though the concrete may seem hard, it will fail more quickly if it is not able to 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 concrete. Concrete poured on a hot day with little humidity may set too quickly, leading to shrinkage and premature curing. (If you’re interested in how to pour concrete in hot and cold weather, read our blog: Guide To Pouring Concrete in Any Weather.
This doesn’t mean that more water is always better when pouring concrete. Remember, concrete porosity is also a problem - and too much water makes pockets in the concrete and doesn’t allow it to bond.
That’s why 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. By wetting the concrete’s exterior during the curing process for three days, the concrete develops a strong 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.
In fact, compressive strength of 5,000 psi in concrete is considered quite high in most settings. This can be achieved using a premium concrete mixture and traditional reinforcing materials.
However, if you need extremely 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 and account 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 the tensile and compressive strength capacity of traditional concrete.
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.
And while UHPC isn’t a feasible solution for most applications, it is worth checking into if you are 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.
We’re excited to offer you all the benefits of working with us:
Hundreds of proven mixes and designs
Over 15 years of concrete experience
Quality concrete to ensure longevity and optimal performance
1-2 business day delivery
Available concrete hand tools for rent
We sincerely hope we can help you with whatever project you are working on!