How to Evaluate the Compressive Strength of Concrete

The civil, construction and infrastructure industry rely heavily on concrete as one of the main foundations for structures of all shapes and sizes. In any building structure, maintenance, or repair project, selecting the correct grade and mix design of concrete in conjunction with the steel reinforcement design can make all the difference in the life of the asset, even after reaching design life. 

The Concrete Compressive strength test determines the maximum compressive stress that when under a gradually applied load, a given solid material (concrete cylinder) will sustain without fracture.

We explore all things concrete, how compressive strength is measured, and why it is important in construction.  

What is the Compressive Strength of Concrete & Why It can Increase with Time. 

Compressive strength refers to the strength of hardened concrete when measured by a compression test, which entails crushing cylindrical concrete in a compression testing machine.

It tests the capacity of concrete to withstand a load before experiencing failure. There are many tests applied to concrete, but the compressive strength test is one of the more crucial tests as it provides contractors information on the strength characteristics of concrete, and how this potentially increases over time.

In most cases, concrete is recognised as achieving full strength at 28 days. Hence the need to perform concrete testing on-site during a pour. It is estimated that concrete reaches 75% of this 28-day compressive strength in 7 days, and its strength will remain stable or even increase over time. 

Over time, the Concrete’s compressive strength can continue to increase due to a chemical reaction called hydration. In making concrete, cement and water form a paste that coats each particle of stone and sand in the mix. Through hydration, this cement paste then hardens and gains strength. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules. If water is available hydration will occur providing the concrete with the opportunity to increase in strength for potentially years.

How Compressive Strength of Concrete is Measured

The Australian Standard for determining the Compression Strength of Concrete (AS 1012.9) test uses a 200mm high 100mm diameter cylinder. The sample of concrete is compacted into the mould on-site, generally 2 samples are taken as a pair, and when applicable transported to a Laboratory for curing to Australian Standards. The cylinders are demoulded and placed into a controlled environment bath.

At the interval (age), the cured concrete cylinder (pair) is then placed into a Compressive Strength test rig individually after being weighed and measured. Force is applied until failure and the results recorded. Calculations are performed which then determine the Compressive Strength to the nearest 0.5 MPa (Megapascal) and an average of the pair calculated. The result then compared to the strength grade of the concrete (e.g., 25MPa) and a determination of pass or fail according to the specification requirements are made. 

Why Compressive Strength of Concrete is Important

The compressive strength parameter of concrete is used in the design of concrete structures and when evaluating the condition of existing structures. It is a piece of information in construction, infrastructure, and concrete repairs all parties want to know and understand before going deep into design, construction, resolution, and analysis. 

Structural Engineers use this parameter in conjunction with steel in modelling different structural properties, such as tensile strength, and to predict the flexural and shear capacity of the elements.

Having a concrete with high compressive strength, you potentially achieve: 

• higher modulus of elasticity, 
• higher tensile strength, 
• reduced creep, and 
• increased durability leading to a longer life expectancy of the asset.

Concrete displaying all these characteristics will ensure it has improved durability while preventing the structure from experiencing premature fatigue or deterioration, essential factors to improve safety, reduce maintenance, and increase the serviceable life of the asset. 

The Formula for The Compressive Strength of Concrete 

Referencing AS 1012.9, the compressive strength of a concrete specimen (cylinder) is calculated by dividing the maximum force applied to the specimen by the cross-sectional area. This area shall be calculated from the average of the two measured diameters of cylinders or side dimensions of cubes.

The Compressive strength of concrete is usually tested at intervals of 7 days and 28 days, usually in pairs. Results are then compared to the project specification for conformance.

Table 1- example of Compressive Strength test results for general purpose concrete

Table 1

Compressive Strength of Rapid Set Concrete 

Rapid Set Concrete is specially formulated to achieve structural strength (20MPa) in 2 hours, enabling the contractor to complete a job in a quicker time than when using conventional concrete.

Its special chemistry means the Rapid Set Concrete does not bleed water as conventional concrete does but achieves rapid set and rapid strength gain in minimal hours.

Rapid Set Cement is produced in a similar fashion to traditional cement containing bauxite among other typical products. It is produced at a lower temperature with no accelerating additives and reduces Carbon emissions. 

Rapid Set Concrete is tested for Compressive in the same fashion using the same Australian Standards as traditional concrete. It follows all the same guidelines and testing principles and achieves the initial set in 15 minutes and the final set in 45 minutes. Structural strength is achieved at 1 to 2 hours as per the examples in Table 2.

Table 2

It is clear to see that compressive strength after 2 hours, the Rapid Set Concrete stands out from general purpose cement. As a result, it offers better longevity for fatigue and has higher carbonation resistance than all other cement/concrete in the market. 

Where Antoun Steps In 

Antoun is a pioneer in the E6 Methodology and a leading distributor of RapidSet concrete, the concrete with the highest compressive and flexural strength over the fastest setting and curing time.

Antoun’s rapid set concrete has been proven and tested to guarantee results for projects of all sizes or scales, without the pain of overruns both in time and money.  

If you're on the lookout for a trusted partner for your next infrastructure project that has a long history of innovative solutions around construction and concrete then get in touch with our team today.