Concrete is considered by many to be a strong and durable material, and with good reason. However, there are several ways to assess the strength of concrete.
Perhaps more importantly, these strength properties give concrete different qualities that make it an ideal choice in many applications.
Here we look at the different types of concrete strength, why they are important and how they affect the quality, durability and cost of concrete projects. We also demonstrate the strength differences between traditional concrete and a newer and more innovative concrete technology: Ultra High Performance Concrete (UHPC).
Terminology: Strength properties of concrete and why they are important
compressive strength of concrete.
This is the most common and widely accepted measure of concrete strength for evaluating the performance of a particular concrete mix. Measures the concrete's ability to support loads that reduce the size of the concrete.
Compressive strength is tested by crushing cylindrical samples of concrete in a special machine designed to measure this type of resistance. It is measured in pounds per square inch (psi). The test is carried out according toASTM-Standard (American Society for Testing and Materials) C39.
Compressive strength is important as it is the main criterion used to determine whether a particular concrete mix will meet the requirements of a particular job.
concrete dogs
Pounds per square inch (psi) measures the compressive strength of concrete. Higher psi means a given concrete mix is stronger, which is why it is usually more expensive. But these stronger concretes are also more durable, meaning they last longer.
The ideal specific psi for any given project depends on many factors, but the minimum for any given project usually starts at around 2,500 to 3,000 psi. Any concrete structure typically has an acceptable psi range.
Concrete foundations and sloped slabs generally require 3,500 to 4,000 psi of concrete. Plates, beams, and cantilevers (commonly found on bridges) require 3,500 to 5,000 psi. Traditional concrete walls and columns typically range between 3,000 and 5,000 psi, while floors require between 4,000 and 5,000 psi. Concrete structures in colder climates require higher psi to withstand more freeze-thaw cycles.
The compressive strength is typically tested at seven days and again at 28 days to determine psi. The seven-day test is done to determine initial strength gains, and in some cases it can even be done in three days.
But actual psi is based on 28 day test results as stated in Normen des American Concrete Institute (ACI)..
tensile strength of concrete
Tensile strength is the ability of concrete to break or crack under load. It affects the size of cracks in concrete structures and the extent to which they occur. Cracks form when tensile forces exceed the tensile strength of the concrete.
Conventional concrete has a significantly lower tensile strength compared to compressive strength. This means that concrete structures subjected to tensile loads must be reinforced with high-strength 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 tensile strength of concrete is determined by a tensile test on concrete cylinders. The test is acc Norm ASTM C496.
flexural strength of concrete
Flexural strength is used as another indirect measure of tensile strength. It is defined as a measure for an unreinforced concrete slab or beam to resist transverse rupture. In other words, it's the ability of concrete to resist bending.
The flexural strength is typically 10-15% of the compressive strength, depending on the specific concrete mix.
There are two standard ASTM tests used to determine the flexural strength of concrete:C78mi C293. Results are expressed as modulus of rupture (MR) in psi.
Bending tests are very sensitive to the preparation, handling and curing of the concrete. The test must be performed with a wet sample. For these reasons, compressive strength test results are more commonly used to describe the strength of concrete as these numbers are more reliable.
additional factors
Other factors that contribute to the strength of concrete are:
Water/cement ratio (w/cm)
This refers to the ratio of water to cement in the concrete mix. A lower water-to-cement ratio makes concrete stronger but also makes it more difficult to work with.
The right balance must be found to achieve the desired strength while maintaining workability.
Offer
Traditional concrete consists of water, cement, air and a mixture of sand, gravel and stone aggregate. The right ratio of these ingredients is the key to achieving higher concrete strength.
A concrete mix with a lot of cement paste pours easily, but cracks easily and does not stand the test of time. On the other hand, too little cement paste leads to rough and porous concrete.
Mix
Optimal mixing time is important for strength. Although strength increases to some extent with mixing time, mixing too long can cause excessive evaporation of water and the formation of fine particles in the mixture. This results in the concrete being more difficult to work with and less strong.
There is no general rule for the optimal mixing time as it depends on many factors such as: B.: the type of mixer used, the rotation speed of the mixer and the specific components and materials in a given batch of concrete.
healing methods
The longer concrete stays wet, the stronger it becomes. To protect concrete, precautions must be taken when concrete is curing in extreme hot or cold temperatures.
The hard facts: Traditional concrete vs. UHPC
A new concrete technology is available that has higher strength properties than conventional concrete in all strength ranges. This innovative material is called Ultra High Performance Concrete (UHPC) and is already being used in many federal and state government infrastructure projects due to its exceptional strength and durability.
UHPC is very similar in composition to traditional concrete. In fact, about 75 to 80 percent of the ingredients are the same.
What makes UHPC unique are the embedded fibers. These fibers are added to the concrete mix and make up 20 to 25 percent of the final product.
The fibers range from polyester to fiberglass, basalt, steel and stainless steel rods. Each of these integrated fibers creates an increasingly stronger end product, with steel and stainless steel offering the greatest strength gains.
Here's a closer look at how UHPC compares to traditional concrete:
- tensile strenght—UHPC has a tensile strength of 1700 psi, while conventional concrete typically measures between 300 and 700 psi.
- flexural strength—UHPC can provide flexural strength in excess of 2000 psi; Conventional concrete typically has a flexural strength of 400 to 700 psi.
- compressive strength—UHPC's improved compressive strength is particularly significant compared to traditional concrete. While conventional concrete typically has a compressive strength in the range of 2,500 to 5,000 psi, UHPC can have compressive strengths 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 UHPC blends have even demonstrated a compressive strength of 50,000 psi.
OthersUHPC advantagescontain:
- Freeze/thaw resistance—Studies have shown that UHPC can withstand over 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 loading, UHPC was twice as strong as regular concrete and dissipated up to four times more energy. This makes the material a great candidate for earthquake resistant buildings and bridges.
- moisture resistance—Due to a higher density than traditional concrete, UHPC is more difficult for water penetration.
- ductility—UHPC can be stretched into thinner sections under tension, unlike normal concrete.
- longer durability—UHPC lasts more than 75 years compared to 15 to 25 years for conventional concrete.
- lower weight—While UHPC is stronger, less material is required, so the final structure is lighter and foundation and support requirements are reduced.
It's no surprise that UHPC is being used in many American infrastructure projects to repair the country's aging bridges and highways. The material increases the lifespan of the bridges, which reduces the overall life cycle costs of these structures. UHPC has a lower maintenance requirement due to the longer lifetime, which further contributes to lower life cycle costs.
Ideal applications for UHPC include:
- bridge girder
- bridges
- seismic columns
- Accelerated bridge construction
- road infrastructure
When evaluating a specific 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 traditional concrete in all strength ranges, making it a smart choice for any concrete project. The reduced maintenance and longer lifespan of UHPC offers a superior strength advantage and lower life cycle costs.
Photo courtesy of Peter Buitelaar Consulting and Design by FDN in Eindhoven, The Netherlands.
FAQs
What is the resistance of a concrete? ›
Wet concrete has a resistivity of around 1 Ω-m which progressively increases as the cement sets.
What determines the strength of concrete? ›Concrete strength is affected by many factors, such as quality of raw materials, water/cement ratio, coarse/fine aggregate ratio, age of concrete, compaction of concrete, temperature, relative humidity and curing of concrete.
What is the most important determinant of concrete strength? ›The water-cement ratio (W/C ratio) is the most important determinant of concrete strength. Lowering the proportion of water to cement increases concrete strength and durability, decreases workability, and increases cost.
How do you test concrete resistance? ›One of the most common new methods is what is called Rebound Hammer Test. This method required a tool that uses a spring release to drive a plunger-tipped hammer down into the concrete that is being tested. The distance the hammer is rebounded is measured and assigned a score between 10 and 100.
How is concrete resistance measured? ›Two-probe method is used to measure the bulk electrical resistivity of concrete. In this technique, the concrete sample is placed between the two electrodes, and the potential is measured after applying the current. Normally, a wet sponge is used to provide contact between the electrode plates and concrete surface.
What are the two most important factors affecting strength of concrete? ›- Raw Material Quality. The initial factor that must be addressed when creating concrete is the raw materials that you wish to use to make said concrete. ...
- Water/Cement/Aggregate Ratio. ...
- Compaction. ...
- Temperature. ...
- Curing. ...
- Age.
Pozzolans, such as fly ash and silica fume, are the most commonly used mineral admixtures in high-strength concrete. These materials impart additional strength to the concrete by reacting with portland cement hydration products to create additional C-S-H gel, the part of the paste responsible for concrete strength.
What is the most important test for concrete? ›Compressive strength test is the easiest, most economical or most accurately determinable test. The variability of concrete is best studied by means of compressive strength tests.
What ASTM do you use to test concrete? ›ASTM C39 determines the compressive strength of cylindrical concrete specimens such as molded cylinders and drilled cores. It is limited to concrete having a unit weight in excess of 50 lb/ft3 (800 kg/m3).
What are the three 3 testing that usually use in fresh concrete or mixture? ›- Slump Test. A concrete slump test is conducted in-field on fresh concrete mix. ...
- Compressive strength test. ...
- Drying shrinkage test.
What are the 3 types of concrete? ›
This article will explore the three most common types of concrete: cast-in-place, precast, and prestressed. Cast-in-place concrete is the most common type of concrete used in construction.
Which grade of concrete is high strength? ›Concrete with over 50 MPa is categorised under the high concrete grade. Its compressive strength varies between 6,525 and 9,425 PSI. The high concrete grade is applied mainly to construct high-rise buildings and structures around water bodies.
What are the most important parameters for concrete? ›The significant physical parameters of concrete, affecting the work of dynamically loaded reinforced concrete structures, include dynamic coefficient of concrete elasticity, vibration energy absorption, dumping coefficient, and fatigue strength.
How can you tell if concrete is weak? ›Perhaps the easiest thing to do is to tap around visibly weak areas with a hammer. If the head of the hammer bounces, the concrete's compressive strength is good. If the hammer indents and pulverizes the surface, the concrete is unsound and should come out.
How strong is 7 days concrete? ›What is compressive strength after 7 days and 14 days? Compressive strength achieved by concrete at 7 days is about 65% and at 14 days is about 90% of the target strength.
What are the two main tests done on concrete? ›When it comes to concrete, the two main tests are the tests for compressive strength on the hardened concrete in the laboratory and slump tests on the wet concrete on-site. Compressive strength tests indicate the capacity of the concrete to support the load of the building or structure.
What is the tolerance for concrete strength? ›International Concrete Abstracts Portal
One tolerance limit for strength, in use since 1971, is f c- 500 psi (3.4 MPa).
Two primary factors influence this response: the degree of hydration of the cement and the degree of saturation of the sample.
How may the resistance of concrete be improved? ›It is possible to increase the abrasion resistance of concrete by changing void content and the porosity with addition of some of the SCMs such as GGBFS, FA, SF, and super plasticizers (Haque and Kayali, 1998; Shannag, 2000).
What single factor causes the most problems in concrete? ›Steel Corrosion
Corrosion of embedded steel is the most common cause of concrete problems.
What causes low concrete strength? ›
Low strength concrete is frequently linked to an error during production, such as adding too much water to offset placement delays or because of a customer request for wet concrete. Another common error is too much air content for the design mix.
What does FC mean in concrete? ›f'c = compressive strength of concrete (MPa) after 28 days of curing. Source publication. +3.
What are the 3 components of concrete? ›Concrete is made up of three basic components: water, aggregate (rock, sand, or gravel) and Portland cement. Cement, usually in powder form, acts as a binding agent when mixed with water and aggregates.
What is the highest PSI concrete ever? ›Laboratories have produced strengths approaching 60,000 psi (480 Mpa). High-strength concrete can resist loads that normal-strength concrete cannot. Several distinct advantages and disadvantages can be analyzed.
What is 6000 psi concrete used for? ›PSI 6000 can be used for any application requiring concrete in a minimum thickness of 50 mm (2”), such as slabs, footings, steps, columns, walls and patios. Each - 25 kg (55 lb) bag of QUIKRETE® PSI 6000 will yield approximately 12.2 L (0.43 cu ft) of mixed concrete.
How do you make super strong concrete? ›You can add more Portland cement to bagged concrete to make it stronger. You can also add hydrated lime. To make the strongest concrete, the sand should be sourced from volcanic lava that has a high silica content.
What should I look for when inspecting concrete? ›Structural problems are indicated by cracking, exposure of reinforcing bars, large areas of broken-out concrete, misalignment at joints, undermining and settlement in the structure. Rust stains that are noted on the concrete may indicate that internal corrosion and deterioration of reinforcement steel is occurring.
What is the best slump for concrete? ›A good slump for most flatwork placed by hand: five inches. For slabs struck off with mechanical equipment: two to four inches.
How often are strength test required for concrete? ›Tests are normally made at the standard ages of 7 and 28 days. High early strength concrete may require additional beams to verify opening strengths.
What are the standard strength grade of concrete? ›Standard strength grades range from 15MPa, 20, 25, 32, 40, and 50 MPa (Mega Pascal) and are available in 20mm, 14mm or 7mm maximum aggregate size to facilitate conveyance and placement.
What is the difference between ASTM D and ASTM E? ›
ASTM International standards are designated with a specific prefix which denotes the broad scope of the standard. For instance: ASTM D refers to miscellaneous materials. ASTM E refers to miscellaneous subjects.
What is the difference between ASTM A and ASTM B? ›The A prefix in an ASTM specification indicates Ferrous material; the B prefix indicates a Non Ferrous material. The ASTM standards are the most widely used in the United States for steel products.
What are the two major requirements of fresh concrete and why are they important? ›There are two sets of criteria that we must consider when making concrete; 1) Long-term requirements of hardened concrete, such as, strength, durability, and volume stability, 2) Short-term requirements, like workability.
Which test Cannot be performed on fresh concrete? ›Slump test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work. It is not a suitable method for very wet or very dry concrete.
What is blue test for concrete? ›"The methylene blue test has been shown to be an effective indicator of the amount and type of clay present in aggregate and can therefore help to distinguish between harmful and beneficial fines," says Koehler, a C09.
What is 3000 psi concrete used for? ›3,000 PSI. The residential workhorse of concrete, 3,000 PSI can be used for driveways, patios and sidewalks. Its durability will help shrug off the freeze-thaw cycle of harsh winters. This is a fine choice for any general construction use.
What is concrete basics for beginners? ›In its simplest form, concrete is a mixture of paste and aggregates. The paste, composed of portland cement and water, coats the surface of the fine and coarse aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete.
What is Type 1 and Type 2 concrete? ›Type I is a general purpose portland cement suitable for most uses. Type II is used for structures in water or soil containing moderate amounts of sulfate. Type II(MH) is a moderately sulfate resistant cement that also generates moderate heat during curing.
What are the 7 grades of concrete? ›Concrete grades include; C7/8 Concrete, C10 Concrete, C15 Concrete, C20 Concrete, C25 Concrete, C30 Concrete, C35 Concrete and C40 Concrete.
What grade of concrete is 1 2 4 mix? ›The proportions selected for cement concrete are as per the design and strength requirements. The proportion can be 1:2:4 (M15 concrete) or 1:1.5:3 for M20 concrete.
What is a 1 2 3 mix for concrete? ›
Concrete is made from cement, sand, gravel and water.
In making concrete strong, these ingredients should usually be mixed in a ratio of 1:2:3:0.5 to achieve maximum strength. That is 1 part cement, 2 parts sand, 3 parts gravel, and 0.5 part water.
The single most important indicator of strength is the ratio of the water used compared to the amount of cement. Basically, the lower this ratio is, the higher the final concrete strength will be.
What is important to increase the strength of concrete? ›Increasing Concrete Strength
Adjusting the cement type and quantity. Reducing the water/cement ratio. Utilizing supplementary cementitious materials (SCMs) Altering the aggregates - type and gradations.
Usually, the compressive strength of concrete varies from 2500 psi (17 MPa) to 4000 psi (28 MPa) and higher in residential and commercial structures. Several applications also utilize strengths greater than 10,000 psi (70 MPa).
What is the approximate resistance of dry concrete? ›The electrical resistivity of concrete ranges over greater extents. Wet concrete behaves as a semiconductor, with resistivity in the range of 105 ohm-mm. Whereas dry concrete has resistivity in the range of 1012 ohm-mm. Hence oven dry concrete acts as an insulator.
What is the average strength of concrete? ›The compressive strength of normal concrete varies between 25 and 50 MPa. Above this level, the term High Performance Concrete is used.
Does concrete have an R value? ›Concrete has an R-value of between 0.1 and 0.2, but when combined with the built-in EPS foam, Fox Blocks boast an impressive R-value of 22. In addition to providing strong walls, ICF walls achieve such high R-values because of the different construction methods.
Which concrete grade has highest strength? ›High concrete grade
Its compressive strength varies between 6,525 and 9,425 PSI. The high concrete grade is applied mainly to construct high-rise buildings and structures around water bodies.
to be stable up to 250°C. On further heating, its structural behaviour is impaired. There is a marked fall in strength and increase in plastic deformation. It disintegrates completely at 500°-600°C.
What is the strength of 1 2 4 concrete? ›
The proportion of 1:2:4 concrete denotes the ratio of cement: sand: coarse aggregates by volume unless specified. Minimum compressive strength of concrete of 1:2:4 mix proportion should be 140 kg/sq.cm or 2000 lbs/sq.in on 7 days.
What is the strength of 1 2 3 concrete? ›At 30 minutes old, a yard of 4-inch slump 1-2-3 concrete will typically contain about 6½ sacks of cement, 1850 pounds of stone, 1220 pounds of sand, and 300 pounds (36 gallons) of water. At 28 days, its 0.49 w/c will deliver a compressive strength of 4500 psi.
What is the R-value of a 4 inch concrete slab? ›Typically, the type of concrete used for floor slabs has an R-value of 0.1-0.2 per inch of thickness.
What is the R-value for a 4 inch thick block of concrete? ›| MASONRY AND CONCRETE | THICKNESS | R-VALUE (F° · SQ.FT. · HR/BTU) |
|---|---|---|
| Common Brick | 4" | 0.80 |
| Face Brick | 4" | 0.44 |
| Concrete Masonry Unit (CMU) | 4" | 0.80 |
| Concrete Masonry Unit (CMU) | 8" | 1.11 |
The R-value listed by PCA for a normal-weight (144 pcf) 6-inch concrete wall is 1.23 and includes inside and outside air-film resistances, which contribute to overall thermal resistance.