Concrete proportions: how to properly select the mix for a reliable foundation

Concrete is one of the most common and important building materials. Its strength, durability, and resistance to external influences make concrete indispensable when constructing foundations, pouring slabs, walls, paths, and other structures. But what are the proportions of concrete that need to be maintained to achieve the desired grade and characteristics?

What are the proportions of concrete and why are they important

Proportions of concrete are the ratios between the main components of the concrete mix: cement, sand, gravel (or crushed stone), and water. A properly selected concrete formula guarantees:

• Adequate strength (ability to withstand loads).
• Durability (resistance to wear and destruction over a long time).
• Resistance to atmospheric influences (freezing, thawing, moisture).
• Ease of work (the mix is easy to distribute and compact).

If the ratio of cement and sand for concrete, gravel, and water is determined incorrectly, the finished material may turn out to be brittle, insufficiently strong, or conversely – too thick and difficult to work with.

Main components of the concrete mix

1. Cement

   • Acts as the “binding” element that connects all components into a strong monolithic structure.
   • Quality cement should be fresh and free of lumps.
   • The most common brands: PC I-400 (M400), PC I-500 (M500), etc.

2. Sand

   • Fine fraction filler that fills the voids between the gravel.
   • It is important that the sand is clean, free of clay, organic impurities, and debris.
   • The coarseness of the sand affects the proportions of concrete and its subsequent strength.

3. Gravel or crushed stone

   • The main coarse aggregate; usually granite or limestone gravel is used.
   • Granite gravel provides higher strength, but it is more expensive.
   • The size of the fractions also affects the calculation and proportions of concrete.

4. Water

   • Acts as a solvent, activating the cement and providing the necessary plasticity.
   • Excessive water can reduce the strength of the concrete, while insufficient water can complicate the laying process.
   • It is recommended to use clean water without impurities.

5. Additives (if necessary)

   • Plasticizers, accelerators or retarders, hydrophobic additives, etc.
   • Help to adjust the concrete formula, improve its characteristics, and save water and cement.

Concrete grades and their proportions

In construction, the following concrete grades (or classes) are often encountered:

• M100 (B7.5)
• M150 (B12.5)
• M200 (B15)
• M250 (B20)
• M300 (B22.5)
• M350 (B25)
• M400 (B30)

The choice of grade depends on the purpose of the structure:

• M100: suitable for preparatory work, road bases, screeds.
• M150: for light foundations of small structures, floor screeds, etc.
• M200: the most common grade for the foundations of private houses, stairs, paths.
• M300, M350: for more heavily loaded structures, reinforced concrete slabs, supports.
• M400: used for particularly strong structures (columns, beams, industrial buildings).

The higher the grade, the more cement in the mix, but the price also increases.

Concrete proportions in buckets: tables and formulas

To simplify the measurement process, buckets are often used instead of scales. A uniform bucket for all components allows for quick and convenient measurement of the required amount of cement, sand, and gravel. Below is a table of concrete proportions in buckets for popular grades (taking into account approximate values of component densities).

Note: proportions may vary depending on the actual characteristics of the materials (moisture, fraction, type of cement, etc.).

Frequently asked questions:

• “What is the proportion of concrete M400 in buckets?”
  • Refer to the ratio: 1 bucket of cement : 0.8 buckets of sand : 1.6 buckets of gravel. Add 0.3–0.4 buckets of water.
• “Concrete formula”
  • Usually, the formula refers to the ratio of all four components: cement : sand : gravel : water.

Concrete proportion for foundation

The foundation is the most critical part of any building, as it determines the durability and safety of the entire structure. Concrete grades M200 and M300 are often used, as they have sufficient strength for private construction:

• M200 (B15): recommended for small private houses and utility structures.
• M300 (B22.5): for more heavily loaded structures, two- or three-story houses.

Example of proportions for foundation (M200):

• Cement (M400): 1 bucket
• Sand: 2 buckets
• Gravel (fraction 5–20 mm): 3 buckets
• Water: ~0.4 buckets

If it is necessary to make stronger concrete (for example, M300), the ratio of cement increases, while that of gravel and sand decreases.

How to correctly calculate the amount of cement, sand, and gravel

1. Determine the total volume of concrete needed for a specific structure. For example, if you are pouring a foundation measuring 5×5 m with a height of 0.2 m, the volume will be 5×5×0.2 = 5 m³ of concrete.
2. Choose the concrete grade according to the recommendations for your type of structure and load.
3. Take the concrete proportions from the table for your grade.
4. Multiply the proportions from the table by the required number of buckets based on the total volume.
5. Account for adjustments for the moisture of the sand, the fraction of the gravel, and other actual parameters.

Tip: If you need to achieve more accurate results, it is better to calculate by weight (kg). However, in private construction, the “bucket” method is often used due to its simplicity.

Tips and recommendations for preparing the concrete mix

1. Maintain cleanliness of materials

   • Sand and gravel must be cleaned of dirt, clay, and organic substances.
   • The bucket and tools must also be clean.

2. Mixing sequence

   • First, mix the cement with the sand dry to form a homogeneous mixture.
   • Add the gravel and mix again.
   • Then gradually add water or a solution with a plasticizer.

3. Control the amount of water

   • Excessive water reduces the strength of the concrete and contributes to delamination.
   • If mobility of the mixture is needed, it is better to use a plasticizer rather than adding excess water.

4. Usage time

   • The prepared mixture should be used within 1–2 hours (depending on temperature and additives).
   • Do not leave the mixture idle for long, as it will start to set.

5. Compaction and care

   • Be sure to compact the concrete with a vibrator or tamping.
   • After pouring, the concrete should be watered in the first days (especially in high temperatures) to prevent premature drying and cracking.

Conclusion

Concrete proportions directly affect the strength and durability of future structures, whether it is the foundation of a private house or massive industrial buildings. The approximate table of concrete proportions in buckets helps quickly determine how much cement, sand, and gravel is needed to achieve the required concrete grade (M100, M150, M200, M300, M400, etc.). However, it is important to consider factors such as the quality and moisture of the materials, the type of cement, and other real construction conditions.

If you are unsure about the calculations or have doubts about the required concrete proportion for the foundation or other structures, it is best to consult specialists or use professional calculation programs. Properly prepared concrete is the key to the durability and reliability of the entire building.