The term grade refers to the strength or the compression capacity of the concrete after it has aged 28 days. Normally, middle east’s symbol is C followed by numbers. But several other countries use symbol M instead of C. If one asks which concrete grade is the best? It depends on where or what type of construction you are building. Just be sure you are using concrete with grades not overly designed for economical purposes.
Before we proceed with the discussion of concrete grades, several factors contribute to its strength and quality. Some of which are water-cement ratio, the workmanship like how the cement and its aggregates where mixed, cast, finished and cured.
As to how to assess the quality of concrete, it is best that to ask a technical person, say, a quality engineer, for a compression test. Slump test may also be conducted to check the on-site quality and determine the workability of the concrete. But if in doubt, get advises from a technical person.
For the determination of what is suitable concrete grade to be used, please take note that there are two common types of concrete mixes, the design mix, and the nominal mix.
Nominal mixes are generally used for small construction like residential house.
Design mixes are used for large-scale construction like buildings and roads. Since we are discussing the concrete grade for building construction, the most suitable mix for this one is the design mix.
Building construction basically uses almost all concrete grades. For Dubai’s setting, here are some of the most commonly used concrete grades for building construction.
Concrete grade C25/20 and C30/20:
These concrete grades are used in for blinding, and foundation or road leveling. Generally, these are low graded concrete with low designed capacity.
Concrete Grade C40/20 and C45/20:
These concrete grades are used for structural members and systems that are directly or indirectly experiencing loads in itself such as slabs, foundations, columns, and footings. Basically, these are middle to high graded concrete with a certain design capacity as these grades have direct impact on the credibility and integrity of the structure.
Concrete Grade C60/20:
This concrete grade is used for the most important part of the structural concrete element which is “Column” and “Shear wall.” The highest quality concrete grade is the high strength concrete grade. This is the grade used for beams and girders, columns and other major structural members. It can be used also for retaining wall. If you can notice that the grade is increasing it means that the most pressured concrete element is column, beam, and wall that needs to possess a huge amount of strength, 60 Megapascal (Mpa or N/mm²). Imagine how much load a column can carry, it means there should be 60 newtons per millimeter square. HUGE!
Again, to save several dimes, it is advisable to use the lowest quality of concrete mix that is C25/20 for leveling of footing instead on 40/20, because that is quite self-explanatory. While for the structural construction elements, the grade as stated by the structural engineer is advised. The required concrete grade for these structural elements fall under the bracket of standard grade concrete or the C40/20 – C45/20 concrete grade. In addition, concrete grade has several mix proportions. These are denoted with 1:2:3 or cement:sand:coarse aggregates.
Normal concrete grades are usually used in the leveling of footing, kerb and blinding mix, domestic floors, and foundations that carry light loads like single story residential. Standard concrete grades are generally used in commercial and domestic constructions. These are basically multi-purpose concrete grades. It may be used as footings for residential floorings. Under this grade bracket lies the lowest standard design grade for roadways.
Normally, a realized plan signed and sealed by the designer has its own concrete design mixes. If it is not stated, you can ask your designer to provide it to you. But generally, it is already stated in the structural note.
In addition to concrete mixes, some commercial admixtures or additive generally boosts a concrete grade. But it is not advisable to depend the concrete grade on these admixtures. To be clear, concrete has to attain its designed strength with or without admixture. That is a rule of thumb.
Anyway, the common admixtures available in the market are plasticisers (added to concrete to promote viscosity, thus, free-flowing), Retardants (added to concrete for it to settle way longer than normal concrete), and Accelerants (opposite to retardants, it accelerates the time the concrete settles).