How can you select the right concrete grade for your application?

There are number of points to take into account before deciding the grade of concrete 1) What is purpose e.g.residential, industrial , national importance etc 2)Environment-- In aggressive environment like coastal area,higher grade is chosen 3) Pre-stressed or traditional cast-in -situ Minimum grade for prestressed concrete is M 40 while for cast -in -situ is M 20 PT slab give us more carpet area & headroom 4) Economics - Design the mix considering locally available materials 5)Higher the grade of concrete ,reduces column size Opting M60 in stead of M25 reduces concrete qty by half.

Also the higher concrete grade results less needed area of steel so in case of high straining action it is preferable to use high concrete grade instead of high area of steel wich lead to steel congension

I think as a baseline, you can select the concrete strength per structural element. Columns and beams, especially when used as moment frames, may require min C40 depending also on how complex/large/high your structure is. Walls may also require high concrete strength than others like slabs, screed, blinding concrete etc. On top of this, durability might also be something to consider, as well as presence of chemicals in the soil e.g. min C50 (type SR) is required when acid sulphate is present in soil. Not sure if I remember these correctly though but these are the things to be taken into consideration

Grade of concrete is the compressive strength the concrete must achieve after 28 days of construction with proper quality control. There are different standards, such as: 1-British Standards BS 8500-2 where the grade of concrete is C10, C15, C20, C25, etc., ‘C’ means ‘Concrete Strength class’, and the number refers to Compressive strength of Concrete in N/mm2. 2-American Concrete Institute ACI where the grade of concrete is M25, M35 and M45, etc., ‘M’ means ‘Mix’, and the number refers to the desired compressive strength in MPa.

The concrete grading system is normally based on compressive strength of the concrete. It is based on tests of standard size cube or cylinder specimens. Eurocode 2 (EN 1992-1-1) Table 3.1 provides a good summary how concrete parameters are related to concrete grade. Interesting fact that engineers not always are aware: mean value of concrete strength acc. to EN 1992-1-1 is differing from characteristic value by 8 MPa, while concrete grade by producers is defined acc. to EN 206 with different requirements. The knowledge of this fact allows to avoid incorrect applications of mean concrete strength.

As per the ACI 318 standard, the selection of the appropriate concrete grade is heavily influenced by exposure conditions such as sulfates and chlorides. Higher levels of sulfate and chloride exposure require the use of concrete with increased durability and resistance to these aggressive agents. The ACI 318 provides guidelines for different exposure categories, and upgrading the concrete grade, incorporating sulfate-resistant cement or blended cement, and employing measures to enhance chloride resistance are recommended to ensure long-term performance and durability of concrete structures in sulfate and chloride-rich environments.

As a structural engineer I have had considered the following factors when I choose for particular grade of concrete; 1. Class of member 2. Environment In addition to the above, some people may consider the following; 1. Type of architecture finishes 2. Optimization of provided reinforcement 3. Heat of hydration However, we have to ensure the sustainability by choosing the optimum grade of concrete.

One of the most important factors when building in general, is the exposure conditions. And thus testing the soil prior to planning and designing is essential. In coastal areas, the soil has a high percentage of salt which affect the reinforced steel imbedded into the concrete. In this case, its recommended to use a resistant type of cement and use further additive to the concrete mix to enhance its physical properties.

depending on where the concrete will be used, the exposure condition can easily influence the concrete grade required to be used. for example CIRIA CS 137 and 163 documents require strengths of C40 and above for concrete used for special structures with a design life of over 50 years. similarly for structures with shorter design life, but high exposures to chlorides and sulphates which may affect the integrity of the concrete.

According to the exposure conditions, which may vary from minor to severe and extreme, appropriate concrete grades and cover thickness should be chosen to ensure the durability of the concrete structure. As the concrete grade increases, the permeability of the concrete decreases. In simpler terms, higher-grade concrete typically has fewer voids, resulting in improved durability.

Concrete characteristic strength and design strength are two important parameters in structural design. The characteristic strength represents the strength at which a certain percentage, typically 95%, of the tested samples will exceed. On the other hand, the design strength is the strength value used in structural design calculations. The design strength is derived from the characteristic strength by applying a safety factor, which accounts for uncertainties in material properties, construction practices, and load variations. The design strength is typically lower than the characteristic strength to ensure a higher level of safety in practical applications.

The design strength is always equal or lower to the characteristic strength of a material. Actually for concrete, if you use an M20 concrete, the design strength applying a Factor of Safety (FoS) for the material of 1.5 will be 20/1.5 = 13.5 MPa

Choosing the appropriate workability for concrete, measured as slump, is a critical issue. Low workability concrete is often used for applications requiring high strength, such as foundations and pavement, while medium workability concrete finds use in general construction such as walls, columns, and beams. High workability concrete is preferred for hard-to-reach areas and heavily reinforced structures. Factors like the method of compaction, mode of placement, shape of the formwork, and aggregate size and type should be considered while choosing the workability. However, it's important to remember that higher workability can lead to segregation if not properly controlled.

The degree of workability can be classified as low, medium and high. Zero slump concrete shall be used for roller-compacted concrete in road pavements since it gives higher abrasive resistance and medium slump concrete is generally used in in-situ concrete such as slabs, beams and columns. High-slump concrete is used for pile concreting since the flow needs to travel through the concrete pump car.

How easily freshly mixed concrete can be placed, consolidated and finished with minimal loss of homogeneity. The workability affects the strength, durability, and appearance of the concrete.

Make sure that the concrete mix design being used is right for the application. Make sure to consider aggregate size vs reinforcing steel spacing to make sure proper consolidation can be achieved. Make sure you have considered the size of the placement and ensure that the mix proposed will stay plastic throughout the pour. Concrete that cannot be properly placed will not work, it's that simple.

In my opinion, determination of concrete grade for any RCC construction is possible by mathematical expression as F/(L*W), the load falling on the structure in N (Newton) divided by the structure configuration sizes length and depth in mm, the result is coming in Mpa or N/mm2 no matter. The higher the sizes the less the concrete grade.

Concrete price and its availability depends on the market specifics, nevertheless the general tendency is that in some strength range concrete price does not necessarily differ much, while performance may differ significantly, therefore it might be reasonable to consider higher strength concrete and capitalize on the benefits depending on your structure (less concrete needed, etc.). Nevertheless, with some concrete grades, especially high strength / high performance concrete (i.e. higher than C50/60 concrete) the price starts to go steeply up and workability may also be affected, therefore understanding of concrete producers' possibilities, concrete specifics and at least rough levels of concrete price is very beneficial for engineers.

Costs can be dramatically impacted by the components in the mix design, such as admixtures and aggregates used. While those costs may be money well spent, make sure you are looking at the mix design and considering the value added by each item in the mix to ensure it is cost effective. For example, there is no need in adding an expensive retarder to a mix that can be delivered and placed quickly.

In the selection of the right concrete, a lot of factors need to be considered. And with the current climate crisis, the environmental performance of the concrete needs to the considered in the design. The selected concrete should meet both structural and durability performance but should also have a lower carbon footprint and that could potentially mean replacing certain components in the mix.

Selecting the right concrete grade requires a balance between theoretical understanding and practical constraints. It's not just the strength or environmental resistance, but also cost-effectiveness, material availability, and compliance with project specifications. Collaborating with local suppliers can provide insights into readily available solutions, avoiding potential delays. Moreover, understanding the specific needs of the client and the project's environmental impact drives the choice towards sustainable options. Constantly updating one's knowledge and adapting to innovative practices ensures that decisions are grounded in current realities, yielding a structure that stands the test of time and elements.

I think the question should have been ‘How can you select the right concrete for your application?’ Choosing the right concrete is not limited to just the ‘right grade’. As many others point out, it can be many other requirements apart from strength grade: Density (light weight or heavy); Aggregate type; Cement type; Durability; Availability of material; Temporary or permanent; Sustainability consideration; Economics; Appearance in finishes, colour and texture; Handling, pumping, skipping; Workability; Self compacting; Fast/ Slow strength gain; Overseas and Undersea applications; Hot, cold, freezing, dry, wet weather conditions; and many more.

apart from strength, it is duty of structural engineers to ensure the right type of concrete mix and materials used in it which have sustainability considerations. use of materials such as SCM's (supplementary cementitious materials like GGBS and Fly ash can improve the strength, durability also the behaviour of fresh concrete such as workability, setting time and heat of hydration while being environmentally sustainable materials that reduce the use of carbon intensive cement in the manufacturing of concrete.

In my experience while it is important to select and utilize the proper concrete mix design, it is equally important to have good quality control in place to ensure the concrete is mixed, delivered, placed, and cured properly. A deficiency in any of these areas will result in poor performing concrete and a reduced service life.