Concrete is considered by many to be a strong and durable material, and rightfully so. But there are different ways to assess concrete strength.
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Perhaps even more importantly, these strength properties each add different qualities to concrete that make it an ideal choice in various use cases.
Here we take a look at the different types of concrete strength, why they matter, and how they impact the quality, longevity, and cost of concrete projects. We also demonstrate the differences in strength between traditional concrete and a newer, innovative concrete technology'Ultra-High Performance Concrete (UHPC).
Terminology: Concrete strength properties and why they are important
Compressive strength of concrete
This is the most common and well-accepted measurement of concrete strength to assess the performance of a given concrete mixture. It measures the ability of concrete to withstand loads that will decrease the size of the concrete.
Compressive strength is tested by breaking cylindrical concrete specimens in a special machine designed to measure this type of strength. It is measured in pounds per square inch (psi). Testing is done according to the ASTM (American Society for Testing & Materials) standard C39.
Compressive strength is important as it is the main criteria used to determine whether a given concrete mixture will meet the needs of a specific job.
Concrete psi
Pounds per square inch (psi) measures the compressive strength of concrete. A higher psi means a given concrete mixture is stronger, so it is usually more expensive. But these stronger concretes are also more durable, meaning they last longer.
The ideal concrete psi for a given project depends on various factors, but the bare minimum for any project usually starts around 2,500 to 3,000 psi. Each concrete structure has a normally acceptable psi range.
Concrete footings and slabs on grade typically require a concrete of 3,500 to 4,000 psi. Suspended slabs, beams, and girders (as often found in bridges) require 3,500 to 5,000 psi. Traditional concrete walls and columns tend to range from 3,000 to 5,000 psi, while 4,000 to 5,000 psi is needed for pavement. Concrete structures in colder climates require a higher psi in order to withstand more freeze/thaw cycles.
Compressive strength is usually tested at seven days and then again at 28 days to determine the psi. The seven-day test is done to determine early strength gains, and in some cases, it may even be performed as early as three days.
But the concrete psi is based on the results from the 28-day test as noted in the American Concrete Institute (ACI) standards.
Tensile strength of concrete
Tensile strength is the ability of concrete to resist breaking or cracking under tension. It affects the size of cracks in concrete structures and the extent to which they occur. Cracks occur when tensile forces exceed the tensile strength of the concrete.
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.
It is difficult to directly test the tensile strength of concrete, so indirect methods are used. The most common indirect methods are flexural strength and the split tensile strength.
The split tensile strength of concrete is determined using a split tensile test on concrete cylinders. The test should be performed according to the ASTM C496 standard.
Flexural strength of concrete
Flexural strength is used as another indirect measure of tensile strength. It is defined as a measure of an unreinforced concrete slab or beam to resist failure in bending. In other words, it is the ability of the concrete to resist bending.
Flexural strength is usually anywhere from 10 to 15 percent of the compressive strength, depending on the specific concrete mixture.
There are two standard tests from ASTM that are used to determine the flexural strength of concrete'C78 and C293. Results are expressed in a Modulus of Rupture (MR) in psi.
Flexural tests are very sensitive to concrete preparation, handling, and curing. The test should be conducted when the specimen is wet. For these reasons, results from compressive strength tests are more typically used when describing the strength of concrete, as these numbers are more reliable.
Additional factors
Other factors contributing to the strength of concrete include:
Water/cementitious ratio (w/cm)
This refers to the ratio of water to cement in the concrete mixture. A lower water-to-cement ratio makes for a stronger concrete, but it also makes the concrete more difficult to work with.
The right balance must be struck to achieve the desired strength while maintaining workability.
Proportioning
Traditional concrete is made of water, cement, air, and an aggregate mixture of sand, gravel, and stone. The right proportion of these ingredients is key for achieving a higher concrete strength.
A concrete mixture with too much cement paste may be easy to pour'but it will crack easily and not withstand the test of time. Conversely, too little cement paste will yield a concrete that is rough and porous.
Mixing
Optimal mixing time is important for strength. While strength does tend to increase with mixing time to a certain point, mixing for too long can actually cause excess water evaporation and the formation of fine particles within the mix. This ends up making the concrete harder to work with and less strong.
There is no golden rule for optimal mixing time, as it depends on many factors, such as: the type of mixer being used, the speed of the mixer rotation, and the specific components and materials within a given batch of concrete.
Curing methods
The longer the concrete is kept moist, the stronger it will become. To protect the concrete, precautions must be taken when curing concrete in extremely cold or hot temperatures.
The hard facts: Traditional concrete versus UHPC
A new concrete technology is available that has greater strength properties than traditional concrete across all strength ranges. This innovative material is called Ultra-High Performance Concrete (UHPC), and it is already being implemented in many state and federal government infrastructure projects given its exceptional strength and durability.
UHPC is very similar to traditional concrete in its composition. In fact, roughly 75 to 80 percent of the ingredients are the same.
What makes UHPC unique are integrated fibers. 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. Each of these integrated fibers create a progressively stronger end product, with steel and stainless steel delivering the greatest gains in strength.
Here is a closer look at how UHPC compares to traditional concrete:
'UHPC has a tensile strength of 1,700 psi, while traditional concrete typically measures between 300 and 700 psi.
'UHPC can deliver more than 2,000 psi in flexural strength; traditional concrete normally has a flexural strength of 400 to 700 psi.
After just 14 days of curing, UHPC has a compressive strength of 20,000 psi. This number increases to 30,000 psi when fully cured for 28 days. Some mixes of UHPC have even demonstrated a compressive strength of 50,000 psi.
Other benefits of UHPC include:
'Studies have shown UHPC to withstand more than 1,000 freeze/thaw cycles, while traditional concrete begins to deteriorate in as few as 28 cycles.
'UHPC can absorb three times more energy than regular concrete. Under impact loading, UHPC was twice as strong as regular concrete and dispersed up to four times as much energy. This makes the material a great candidate for earthquake-resistant bridges and buildings.
'Due to a higher density than traditional concrete, it is harder for water to penetrate UHPC.
'UHPC can be stretched out into thinner sections under tensile stress, unlike regular concrete.
'UHPC lasts more than 75 years versus 15 to 25 years for traditional concrete.
It is no surprise that UHPC is being used in many American infrastructure projects to repair the country's aging bridges and roads. The material increases the lives of bridges, decreasing the overall lifecycle cost of these structures. UHPC has lower maintenance demands given its increased lifespan, further contributing to its lower lifetime cost.
Ideal uses for UHPC include:
When assessing a given concrete mixture for a project, it is important to know the various strength properties of that mixture. Knowledge of these numbers and what each type of concrete strength property delivers to a project is the key to choosing the right concrete mixture.
Concrete innovations such as UHPC outperform traditional concrete in all areas of strength, making it a smart choice for any and all concrete projects. The reduced maintenance and increased longevity of UHPC delivers a win-win of superior strength and lower lifecycle costs.
Photo provided by Peter Buitelaar Consultancy and design by FDN in Eindhoven the Netherlands.
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Many people in the concrete industry still think that high strength cures all. This is, unfortunately, not true. High-performance concrete is certainly a tempting solution for increasing your concrete's abrasion resistance. After all, its high compressive strength does make it tough and durable against abrasion. And it's not even that hard to make. Increase the cement content. Keep the water-cement ratio low. Maybe add some silica fume. And the test results are pretty decent.
Sounds pretty good, right?
Well, don't get too eager! High-performance concrete may have some benefits, but they come at a price.
To delve into that aspect, we first need to talk about the two main methods for creating high-performance concrete: adding silica fume and adding extra cement. Both are popular ways of strengthening concrete, but neither are efficient for increasing concrete abrasion resistance.
When Made with Silica Fume, This Concrete Offers Diminishing Returns
So, let's say you want to go with the silica fume route. What are the likely results?
Silica Fume Does More to Increase Compressive Strength Than It Does to Increase Abrasion ResistanceAccording to a study done by the Amirkabir University of Technology in Iran and the Yamaguchi University in Japan, the results are not overly impressive when it comes to silica fume increasing abrasion resistance.
As expected, however, they did see good results for it increasing compressive strength. For instance, when adding 10% more silica fume to a concrete mix, the study's researchers noted that the mix's initial 35 MPa (5,000 psi) jumped to 50 MPa (7,000 psi).
To determine the impact that silica fume had on the abrasion resistance of this concrete, the researchers conducted a modified version of the ASTM C418 test method. That allowed them to measure the mass loss of their 40-day concrete samples, showing an increase in compressive strength. And for every percent increase, there was a 0.33% increase in abrasion resistance. That eventually led to a maximum increase of about 13%.
This is certainly an improvement in a mix's abrasion resistance. But it is a minor one.
It's Just Not the Biggest Contributor for Your Concrete's Protection Against AbrasionIn support of this view, another study done by a structural designer from Tacoma Engineers Inc. and a professor at George Brown College suggests a similar outcome. They noted that silica fume's contribution to abrasion resistance did not have a significant impact. In fact, it was more important to consider the mix's aggregate type and water-cement ratio.
They did also note that of course, with an increase in compressive strength, there was some increase in abrasion resistance.
So you can expect to see some benefits in abrasion resistance with a greater increase in compressive strength with silica fume. But it may not be worth it if you need better abrasion resistance.
Then, what about adding cement instead?
High-Performance Concrete with Extra Cement Is Better But Costly
Again, like with silica fume, you'll see a great increase in compressive strength when using extra cement. Scotland's University of Dundee confirmed this with their study in . During the study, researchers found they could increase cement content to raise a concrete mix's 27 MPa (4,000 psi) to 50 MPa (7,000 psi).
It also showed some promise of doubling the mix's abrasion resistance. Still, for it to work, you'd need to increase the cement content by 80%.
That might not seem like much of an issue initially. But it won't take long for you to see the problem when you look at the bill for your concrete. Adding so much cement has a literal high price attached to it, making it the least appealing way to boost your concrete's resistance to abrasion.
Worse Still, It Can Come with a Number of Unwanted Side Effects
No matter whether you went with silica fume or extra cement, you are very likely going to see some downsides.
These Can Include Application ConcernsIf you went with adding silica fume as it's the more budget-friendly of the two options, you may end up facing application concerns. However, it depends on the amount you're using. If you add less than 10% to your concrete mix, your concrete's application will likely go on as usual. But if you add more, your concrete will become sticky, making it harder to place and finish.
Moreover, sometimes, the best overall result comes from using a concrete mix that is easy for the contractor to work with. In these cases, you might be better off avoiding silica fume.
Or They Might Include an Increase in Heat of Hydration, Which Can Lead to Other Structural ConcernsIf you went with adding extra cement instead, you will have a more pressing concern to worry about: heat of hydration. After all, increasing your mix's cement paste means increasing the mix's hydration. And doing that generates more heat within the concrete due to the chemical reaction between the water and extra cement.
It can add stress to large concrete masses as they cool, leading to the concrete cracking. On top of that, more cement paste also creates more shrinkage. And that can cause concrete slabs to curl. That can in turn cause joint spalling, joint filler failure, and more.
Another Concern Is the Additional Pollution That Comes from an Increase in Cement ContentThe addition of more cement isn't just a concern to structural integrity. It's also a concern to the surrounding environment. Around two thirds of a ton of C02 is released for every ton of cement manufactured. So to add even more cement to your mix is only increasing your structure's carbon footprint, which contributes to extreme weather events and other symptoms of climate change.
You Don't Have to Deal with These Issues
While it might seem like it currently, you don't actually have to choose between either form of high-performance concrete. If you're looking to significantly increase your concrete's abrasion resistance and avoid the potential downsides of adding silica fume and extra cement, Hard-Cem® should be your go-to solution.
Hard-Cem Increases Abrasion Resistance without Application ConcernsAs an integral hardening admixture, Hard-Cem is a unique solution for increasing abrasion resistance. It's the only one of its kind on the market, and it's capable of doubling your concrete's abrasion and erosion resistance.
There's no real concern when applying it either. You just add Hard-Cem to your concrete mix during batching, and that's it! The admixture will go on to increase the abrasion resistance of your concrete's paste and reduce abrasive wear and tear. It leaves no impact on your concrete's workability or any other aspect, so the contractor can place and finish it like they are accustomed to.
It Can Even Make Your Project More SustainableWhat's more, Hard-Cem can even decrease your structure's carbon footprint! Because it raises the concrete's surface wear resistance to counter abrasive and erosive forces, it mitigates the need to replace or repair your concrete over time. For instance, after seeing rapid abrasive damage in their mine's original concrete, the owner of the New Afton Mine in British Columbia, Canada, found that Hard-Cem helped them avoid at least two concrete replacement cycles. That let them reduce the concrete's lifetime carbon footprint by 49.5%. And it was all thanks to making sure their concrete was durable enough.
For a Less Risky, More Green Approach, Hard-Cem Is Worth It
It's not a conventional choice, but Hard-Cem has had more than 15 years to prove its worth. And it has done so many times for over 7 million m2 (80 million ft2) of concrete across North America. This has produced multiple success stories over the years, making Hard-Cem a proven and welcome addition to abrasion resistance solutions.
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