Everything You Need To Know About Concrete Strength – LBM Research Associates

Amazing! Everything You Need To Know About Concrete Strength – LBM Research Associates

Information about Everything You Need To Know About Concrete Strength – LBM Research Associates

Phil Puccio

Concrete is considered by many to be a strong and durable material, and rightly so. However, there are several ways to assess concrete strength.

Perhaps more importantly, these strength properties each give the concrete different qualities, making it an ideal choice for different applications.

Here we take a look at the different types of concrete strength, why they are important, and how they affect the quality, longevity, and cost of concrete projects. We will also demonstrate the strength differences 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 recognized measurement of concrete strength to assess the performance of a particular concrete mix. It measures the ability of concrete to withstand loads that reduce the size of the concrete.

Compressive strength is tested by breaking cylindrical concrete samples in a special machine designed to measure this type of strength. It is measured in pounds per square inch (psi). The test is carried out according to the ASTM (American Society for Testing & Materials) Standard C39.

Compressive strength is important as it is the main criterion in determining whether a particular concrete mix will meet the requirements of a particular job.

Concrete psi

Pounds per square inch (psi) measures the compressive strength of concrete. A higher psi means that a given concrete mix is ​​stronger, so usually more expensive. But these stronger concretes are also more durable, meaning they will last longer.

The ideal concrete psi for a given project will depend on a number of factors, but the bare minimum for any project usually starts at 2,500 to 3,000 psi. Every concrete structure has a normally acceptable psi range.

Concrete foundations and slabs on the level typically require 3,500 to 4,000 psi of concrete. Suspended slabs, beams, and girders (as often found in bridges) require 3,500 to 5,000 psi. Conventional concrete walls and pillars typically range from 3,000 to 5,000 psi, while the pavement requires 4,000 to 5,000 psi. Concrete structures in colder climates require higher psi to withstand more freeze-thaw cycles.

Compressive strength is typically tested after seven days and then again after 28 days to determine psi. The seven-day test is done to determine early strength gains, and in some cases it can be done as early as three days.

However, the specific psi is based on the results of the 28-day test, as in the American Concrete Institute (ACI) standards.

Tensile strength of concrete

Tensile strength is the ability of concrete to break or tear under tension. It affects the size of cracks in concrete structures and the extent to which they occur. Cracks occur when the tensile forces exceed the tensile strength of the concrete.

Conventional concrete has a significantly lower tensile strength compared to compressive strength. This means that tensile concrete structures must be reinforced with tensile materials such as steel.

It is difficult to test the tensile strength of concrete directly, so indirect methods are used. The most common indirect methods are flexural strength and splitting tensile strength.

The splitting tensile strength of concrete is determined by a splitting 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 a further indirect measure of tensile strength. It is defined as the measure of an unreinforced concrete slab or unreinforced beam to withstand flexural failure. In other words, it’s the concrete’s ability to withstand bending.

The flexural strength is typically 10 to 15 percent of the compressive strength, depending on the specific concrete mix.

There are two standard tests from ASTM that are used to determine the flexural strength of concrete:C78 and C293. Results are expressed in terms of a modulus of rupture (MR) in psi.

Bend tests are very sensitive to concrete preparation, handling and curing. The test should be performed when the sample is wet. For these reasons, when describing concrete strength, results from compressive strength tests are more likely to be used, as these figures are more reliable.

Additional factors

Other factors that contribute to the strength of concrete include:

Water / cement ratio (w / cm)

This refers to the ratio of water to cement in the concrete mix. A lower water-cement ratio makes the concrete stronger, but also makes it more difficult to process the concrete.

The right balance has to be found in order to achieve the desired strength while maintaining processability.


Conventional concrete consists of water, cement, air and an aggregate mixture of sand, gravel and stone. The right proportion of these ingredients is the key to achieving higher concrete strength.

A concrete mix with too much cement paste can be easy to pour – but it will break easily and won’t stand the test of time. Conversely, too little cement paste leads to a rough and porous concrete.


The optimal mixing time is important for strength. While the strength increases with mixing time to a certain point, mixing too long can actually lead to excessive water evaporation and the formation of fine particles in the mixture. This makes the concrete harder to work with and less strong.

There is no golden rule for the optimal mixing time as it depends on many factors, such as:

Curing methods

The longer the concrete is kept moist, the stronger it will be. Precautions must be taken to protect the concrete when curing concrete in extremely cold or hot temperatures.

The hard facts: traditional concrete versus UHPC

A new concrete technology is available that has better strength properties than conventional concrete in all strength ranges. This innovative material is called Ultra-High Performance Concrete (UHPC) and is already used in many federal and state infrastructure projects due to its exceptional strength and durability.

UHPC is very similar to traditional concrete in its composition. In fact, around 75 to 80 percent of the ingredients are the same.

Integrated fibers are what makes UHPC unique. These fibers are added to the concrete mix and make up 20 to 25 percent of the end product.

The fibers vary from polyester to fiberglass rods, basalt, steel and stainless steel. Each of these integrated fibers creates an increasingly stronger end product, with steel and stainless steel providing the greatest strength gains.

Here’s a closer look at how UHPC compares to traditional concrete:

  • Tensile strength-UHPC has a tensile strength of 1,700 psi, while conventional concrete typically measures between 300 and 700 psi.
  • Flexural strength—UHPC can provide flexural strength in excess of 2,000 psi; conventional concrete typically has a flexural strength of 400 to 700 psi.
  • Compressive strength—The advanced compressive strength of UHPC is particularly important compared to conventional concrete. While conventional concrete typically has a compressive strength in the range of 2,500 to 5,000 psi, UHPC can have a compressive strength up to 10 times that of conventional concrete.

After only 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 blends of UHPC have even shown a compressive strength of 50,000 psi.

miscellaneous Benefits of UHPC contain:

  • Frost / dew resistance—Studies have shown that UHPC can withstand more than 1,000 freeze-thaw cycles, while conventional concrete begins to deteriorate after just 28 cycles.
  • Impact resistance—UHPC can absorb three times more energy than normal concrete. Under impact, UHPC was twice as strong as normal concrete and distributed up to four times as much energy. This makes the material an excellent candidate for earthquake-proof bridges and buildings.
  • Moisture resistance—UHPC is more difficult for water to penetrate due to its higher density than conventional concrete.
  • Ductility– Unlike normal concrete, UHPC can be stretched into thinner sections under tension.
  • Longer service life—UHPC lasts more than 75 years versus 15 to 25 years for conventional concrete.
  • Lighter weight—Although UHPC is stronger, it uses less material so the end structure is lighter, reducing the need for grip and support.

Unsurprisingly, UHPC is used in many American infrastructure projects to repair the country’s aging bridges and roads. The material increases the service life of bridges and lowers the overall life cycle costs of these structures. UHPC has lower maintenance requirements due to its longer lifespan, which further contributes to its lower lifetime costs.

Ideal applications for UHPC are:

When evaluating a particular concrete mix for a project, it is important to understand the various strength properties of that mix. Knowing these numbers and the importance of each type of concrete strength property to a project is key to choosing the right concrete mix.

Concrete innovations like UHPC outperform conventional concrete in all strength ranges, making it a smart choice for any concrete project. The reduced maintenance and longer life of UHPC offer a win-win situation through superior strength and lower life cycle costs.

Photo provided by Peter Buitelaar Consulting and design by FDN in Eindhoven, the Netherlands.

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Original Source: https://www.lbmresearchassoc.com/2019/12/19/everything-you-need-to-know-about-concrete-strength/
Category – Concrete

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