In last month’s article, Cement vs Concrete, we had a look at the differences between the two and their intended uses. For this month we decided that going through the cement strength classes and concrete testing would provide a great follow up.
Why Knowing Cement Strength Classes is Important
Cement is manufactured in a rather complex process with a variety of ingredients, which is why testing is essential in ensuring that the specifications and regulations can be met. As cement plays such a vital role in the mixture of concrete and mortar, it needs to be of reliable quality. Concrete is one of the main materials in many building projects. It is a material we need to know a lot about. When erecting a building you want to ensure that it will perform under the expected load and stand for a long time.
Testing concrete is not complicated but does involve a number of different tests to establish the working parameters of the cement.
Compressive Strength Test
Now, this is where concrete shines. Concrete and mortar can withstand high compressive loads in part due to the cement used in the mixture. The performance of concrete under compressive loads is influenced by the cement used.
In order to test the compressive strength of concrete, a compression machine is used. Hardened cubes of cement-sand mortar are placed in the machine. The compressive strength of the cubes is determined by the highest stress applied to the cube that causes it to fracture.
Tensile Strength Test
If you have read one of our previous articles, Overview of Rebar, you will be aware that concrete has a low tensile strength. This is why reinforcing steel is used in concrete structures, but it is nonetheless important to know how concrete performs under tensile loads. This helps to understand the cohesion between the particles.
The most common test used when testing for tensile strength is conducted by the application of uniaxial tension. Cement-sand mortar briquettes are placed into a machine which applies a tension load. The tensile strength is then calculated by measuring the load needed to split the load in half over the fracture point.
These are just two of the main testing points for concrete or mortar. Fineness, soundness, flow and consistency are all part of the varied testing required to fully understand the material’s performance. Mistakes in these aspects of the building material and process can lead to severe budgetary losses and can endanger the lives of the people building the structure or those using it thereafter.
Cement Strength Classes
The cement strength is determined by the varied tests done on the mortar and concrete cubes at specified intervals of 2 days, 7 days, and 28 days of hardening. There are three main strength classes for cement: 32,5, 42,5 and 52,5 followed by a R or N. The R refers to rapid or early strength development and the N to normal or standard strength development. While 32,5 is the low strength, 42,5 is the middle strength, and 52,5 is the highest strength.
When choosing the cement right for your project you will want to keep in mind that the compressive strength reached by the 42,5N and the 42,5R will be the same once completely cured. However, the 42,5R will reach a higher initial compressive strength. As it is for all the classes.
Class 32.5 is typically used for applications where a high initial strength is not needed, at average ambient temperatures (10 – 15°C) and with constructions of standard thicknesses (
Class 42.5 is most often used in builds that require the compressive strength of concrete at 28 days exceed 30 N/mm² , while also being suitable for use in lower temperatures.
Class 52.5 is used for structures where a higher initial compressive strength is required. e.g. for the rapid stripping of prefabricated elements.