Concrete grade for the foundation: choice depending on the type of soil, house and type of foundation itself

For the stability and lifespan of your home, selecting the appropriate concrete grade for your foundation is essential. This choice is heavily influenced by the kind of soil you have, the layout of your house, and the type of foundation you want to install. Knowing these things will help you make sure your foundation is sturdy and long-lasting.

The concrete grade you should use is greatly influenced by the soil on your property. The performance of the foundation can be impacted by the load-bearing capacities and moisture content of different soils. For example, compared to clay soil, which retains water and can expand or contract, sandy soil drains well and calls for a different strategy.

The concrete grade you choose is also influenced by the layout and size of your home. The foundation will be under more pressure from a larger, multi-story house than from a smaller, single-story home. This implies that the additional weight and stress must not be too much for the concrete to bear. Seeking advice from an engineer or a concrete specialist can yield insightful information specific to your home’s design.

Finally, the concrete grade required will depend on the kind of foundation you select, such as a slab, basement, or crawl space. Every kind of foundation has different needs and difficulties. For instance, a basement foundation may require higher-grade concrete to withstand the added weight of soil pressing up against the walls and the possibility of moisture intrusion.

You can choose the right concrete grade for a sturdy, long-lasting foundation by taking into account the following factors: soil type, house design, and foundation type. Your investment will be better safeguarded and your home’s stability and safety will be guaranteed for many years to come with this meticulous planning.

Composition of concrete mix for the foundation

Crushed stone and sand are added to cement mortar to create concrete, also known as artificial stone. Additionally, different additives are added as needed, such as fibrous reinforcing components and water repellents.

Water is added last among all the ingredients, and only in the specified amount and order. The concrete’s quality and quantity determine its overall quality.

Cement

Portland cement (PC) is primarily used to prepare the concrete mix for the reinforced concrete base of the building. PC consists of finely ground limestone and sintered clinker clay granules with a 1.3–3% gypsum addition.

The grades of cement vary, ranging from 300 to 600. The value represents the compressive strength, expressed in kgf/sq. cm, of fully matured cement mortar (which takes 28 days to prepare), which is made up of one part cement, three parts sand, and a water volume that yields the ideal water-cement ratio. Cement PC 400 or PC 500 should be used when creating foundation concrete. The first is better since it is more widely available and less expensive.

Even under ideal storage conditions, cement deteriorates 20% in three months, 30% in six months, and 40% in a year. Consequently, it is best to buy new material. In the event that outdated material is required, its proportion in the mixture is raised in line with the degree of strength loss. The mixture’s preparation time is increased fourfold.

Fillers

Concrete can contain a variety of fillers, but traditional fillers are used in foundation concrete.

• fine-grained sand;
• coarse crushed stone or gravel.

Strict specifications also apply to these materials.

Sand

Sand from a quarry that meets the following requirements is the best option:

1. Fineness modulus: 2.0-3.25.
2. Voidness: no more than 30% (all fractions are represented).
3. Average density: 1500 kg/cube. m.
4. No clay or fine sand impurities.

The following may be built as a supporting structure for a one- or two-story cottage and other private construction projects:

1. Use of sand with a grain size of less than 1.6 mm — in a volume of up to 20%. In this case, the permissible share of dusty sand (up to 0.6 mm) does not exceed 3%.
2. Clay impurity — up to 3% by weight.

These kinds of breaks are only allowed for concrete grades M300 and lower. Sand is used in dried form because the volume of water is chosen based on the volume of cement.

Crushed stone and gravel

Crushed stone is made up of sharp pieces of stone that are removed or processed. Gravel is mostly naturally occurring, sloping stones that have weathered or been eroded by water.

Four fractions are present:

• from 5 to 10 mm;
• from 10 to 20 mm;
• from 20 to 40 mm;
• from 40 to 70 mm.

You will need to use multiple fractions. The maximum size of the stones determines their number:

• up to 40 mm: 2 different fractions;
• up to 70 mm: 3 different fractions.

There is a maximum size limit for stones:

1. No more than 2/3 of the step between longitudinal reinforcement bars.
2. No more than 1/3 of any smallest size of the foundation, but better no more than 1/5. For example, if a strip structure is erected under a fence 200 mm wide, then the maximum permissible stone size will be 40 mm.

A strong stream of water is used to wash dirty crushed stone. Even with a small amount of earth or clay, the concrete mixture will lose up to 30% of its strength if it is not thoroughly cleaned off.

Water

The document GOST 23732-79 lays out the specifications for water. It controls oxidizability, threshold salt content, and several other factors. This document states that it is not acceptable for the liquid to contain any traces of acids, phenols, petroleum products, or sugars. All requirements are satisfied by water from the well and water supply.

Concrete grade and its choice by strength

The grade and strength class of concrete are its most crucial parameters.

1. Strength grade. Denoted by the letter "M" and a number corresponding to the average compressive strength in kgf/sq. cm. Concretes with a strength grade from M50 to M800 are used.
2. Concrete strength class. Denoted by the letter "B" and a number corresponding to the compressive strength in MPa, which 95% of samples in the form of a cube with an edge length of 150 mm have.

1. Grade is the average value of compressive strength based on the results of testing several samples, and the nearest number is taken from the standard series, even if it is greater than the actual average value. If the compressive strength of 10 samples varied from 280 to 370 kgf/sq. cm, the average value will be 330 kgf/sq. cm. The nearest numbers in the standard series of grades are 300 and 350. The second is closer; accordingly, the M350 grade will be adopted for this concrete.
2. The class displays the load, which is guaranteed to withstand almost all samples (95%), that is, it is minimal or close to the minimum compression strength value. Moreover, from the standard row, the nearest is taken. In the example, the minimum load was 280 kgf/sq. cm (28 MPa), respectively, the concrete class will be B25.

It is indisputable that the M200 brand of concrete will be able to support a 200 kgf/sq. cm load on the casting.

A class of 20 indicates that there is a 95% chance that the railway design will withstand a load of 20 MPa, or nearly 200 kgf/sq. cm. As a result, the following is a correlation between the strength of the different brands and classes of concrete:

Concrete strength class	The nearest brand of concrete
B3.5	M 50
AT 5	M 75
B7.5	M 100
AT 10	M 150
B12.5	M 150
B15	M 200
IN 20	M 250
B25	M350
B27.5	M 350
B30	M 400

Professionals use the term "concrete strength class" most frequently. In the context of individual construction, brand comprehension suffices. M100, M300, and occasionally M400 are well-liked in this market.

Choosing the right concrete grade for your foundation depends on several key factors: the type of soil beneath your home, the specific design of your house, and the type of foundation being used. Different soil types, such as clay or sandy soils, require different strengths of concrete to ensure stable support. Similarly, the design of your house, including its size and weight distribution, influences the concrete grade needed to withstand loads effectively. Lastly, the type of foundation—whether it"s a slab-on-grade, a crawl space, or a basement—dictates the durability and strength requirements of the concrete. By understanding these factors and selecting the appropriate concrete grade, you can ensure a solid foundation that supports your home for years to come.

Concrete grade depending on the materials of the walls and soils

When building a multistory structure, the necessary concrete strength is calculated, beginning with the load collection. This is not required in private single-story construction because all homes have roughly equal loads on the foundation as a result. It is sufficient to choose the wall’s material and consider the kind of soil the construction site will be experiencing:

Wall material	Weakly heaving soil with good bearing capacity	Heaving soil
Sandwich panels (wooden frame houses)	M 200 (B 15)	M 250 (B 20)
Timber, log	M 250 (B 20)	M 300 (B 22.5)
Walls made of aerated concrete, gas blocks, expanded clay and sawdust concrete, other lightweight concrete	M 300 (B 22.5)	M 350 (B 25)
Brick, ordinary concrete, natural stone	M 350 (B25)	M 400 (B 30)

The concrete grade is raised by one step, but not higher than M 400, for a cottage with multiple floors.

Choice based on the type of foundation

When selecting a concrete mix, consideration is also given to the kind of subterranean structure:

1. Concrete preparation: a solution of grade M 100 is used. Permanent concrete formwork is also made from it – hollow blocks, into the cavity of which concrete is poured after their installation.
2. Tapes. Grades from M200 to M400 are used.
3. Slabs. Made of concrete of grade not lower than M300.
4. Pillars and bored piles. The part immersed in the ground is isolated pillars, therefore they are built from stronger concrete: not lower than M250, and better M300 or M350. Concrete M200 is suitable for the grillage.

Other characteristics

• frost resistance;
• water resistance;
• mobility index.

Frost resistance

The designation is the letter "F" followed by a number that represents the maximum number of freeze/thaw cycles that a sample saturated with water can endure before losing its strength. In any condition, the number of cycles for concrete is not limited. A material that has a F 25 frost resistance will, in fact, not crumble after 25 years. Even with a minimum frost resistance of F 25, dry concrete will not even slightly deteriorate after thousands of freeze/thaw cycles.

If there is no way to keep the products and structure dry, this indicator is taken into consideration. For instance, the waterproofing will wear off on paving slabs or other concrete blind areas. Thus, in this case, concrete that has a minimum of F 300 frost resistance is needed. Because cement tiles won’t last long in the water, products are made from a mixture that has a frost resistance of F 600 to F 800.

Since waterproofing materials shield the foundation, F 150 is sufficient and this feature is not essential. Even less for walls (F 50 – F 100).

Frost resistance for concrete in the absence of additional additives is as follows:

• M 100 and M 150: F 50;
• М200 and М250: F 100;
• М300 and М350: F 200;
• М400: F 300.

This concrete parameter is increased to F 1000 by mixing in special ingredients. Using a concrete mixture with a high level of frost resistance does not eliminate the need for waterproofing the foundation.

Water resistance

The designation consists of the letter "W" and a number that indicates the highest pressure in MPa that is acceptable without compromising water resistance. This characteristic’s range is W2-W12. This indicator is meaningless in the case of a foundation because waterproofing is built in at the time of construction.

The water resistance of concrete is as follows in the absence of any special additives:

• М100 and М150: W 2;
• М200 and М250: W 4;
• М300: W 6;
• M 350: W 8;
• M 400: W 10.

Mobility index

Concrete’s workability is characterized by its mobility index (MI), which takes into account its fluidity, plasticity, capacity to fill confined spaces without creating voids, and ability to create a level horizontal surface.

When constructing alone, without the use of a vibratory paver or other specialized tools for concrete laying, this parameter is crucial.

The letter "P" and a number that represents the sample’s subsidence from the raw mixture in relation to its initial height over the designated time period are used to denote the mobility index.

The following mobility indices of concrete can be used to construct the building’s subterranean portion:

1. P4. Flowable concrete, can be supplied by a concrete pump. Suitable for foundations in the reinforcement frame of which the rods are located close to each other. Laying without a vibratory paver is allowed.
2. P3. The mixture is mobile, can be used with tightly spaced reinforcement, but the use of a vibratory paver is mandatory. The mixture flows off a vertically placed bayonet shovel.
3. P2. Concrete of medium mobility, smoothly slides off the shovel. Such a mixture can only be selected with a large distance between the reinforcing bars and using a deep vibrator.

Proportions of components for concrete

Different amounts of PC 400 cement, crushed stone, and sand (cubic capacity/weight) go into making different grades of concrete:

1. M100. 1: 7: 4.6/1: 6.1: 4.1.
2. M150. 1: 5.7: 3.5/1: 5: 3.2.
3. M 200. 1: 4.8: 2.8/1: 4.2: 2.5.
4. M 250. 1: 3.9: 2.1/1: 3.4: 1.9.
5. M 300. 1: 3.7: 1.9/1: 3.2: 1.7.
6. M 400. 1: 2.7: 1.2/1: 2.4: 1.1.

The following ratios are utilized while working with PC 500 cement:

1. M 100. 1: 8.1: 5.8; 1: 7.1: 5.3.
2. M 150. 1: 6.6: 4.5; 1: 5.8: 4.
3. M 200. 1: 5.6: 3.5; 1: 4.9: 3.2.
4. M250. 1: 4.5: 2.6; 1: 3.9: 2.4.
5. M300. 1: 4.3: 2.4; 1: 3.7: 2.2.
6. M400. 1: 3.2: 1.6; 1 : 2.8 : 1.4.

For concrete grade, PC 400 water-to-cement ratio:

• M100: 1.03;
• M150: 0.85;
• M200: 0.69;
• M250: 0.57;
• M 300: 0.53.

Likewise, with cement PC 500:

• M100: 1.2;
• M150: 0.99;
• M200: 0.79;
• M250: 0.65;
• M 300: 0.61.

Extra water either evaporates and forms pores in the concrete, or it stays liquid and freezes in the winter, destroying the concrete. Furthermore, if there is insufficient water, some cement will not undergo a chemical reaction.

Combine all the ingredients, making sure that the cement paste covers the entire surface of the crushed stone and sand grains. Follow these steps in order:

1. Wet the concrete mixer with water, or better yet, with liquid cement mortar, mixing it for several minutes.
2. Sequentially load 15% water, sand and cement in full into the mixer.
3. After mixing, pour out all the crushed stone and remaining water.

For a brief period of time—between 0.5 and 1.5 minutes—mix the mass. The majority of the liquid will evaporate and the concrete will become less strong if the process is prolonged.

The preparation of the mixture varies when using a small concrete mixer (up to 0.5 cubic meters):

• dry mix the sand in full and half the crushed stone;
• load the entire portion of cement and mix thoroughly again;
• add water;
• fill in the remaining rubble to break up the lumps.

The filling will be finished in a single shift thanks to the efficient work scheduling.

Soil Type	Recommended Concrete Grade
Stable Soil (e.g., Gravel, Sand)	M15-M20 (Lightweight Houses)
Moderate Soil (e.g., Clay, Silt)	M20-M25 (Medium Houses)
Weak Soil (e.g., Peat, Expansive Clay)	M25-M30 (Heavy Houses)

For the stability and lifespan of your home, selecting the appropriate concrete grade for your foundation is essential. To guarantee a solid base, different concrete grades are needed for different soil types, home construction styles, and foundation types. Knowing these needs enables you to make an informed choice that will give you comfort and a solid foundation for the years to come.

Your choice of concrete is greatly influenced by the kind of soil on your construction site. To stop settling and shifting, higher-grade concrete may be needed on soft or loose soils like sand or clay. On the other hand, lower-grade concrete can be supported on stable soils without sacrificing stability, such as rock or gravel. An essential first step in the foundation process is determining the type of soil you have.

The right concrete grade is also dependent on the structure of your home. Concrete that is stronger is required because larger, heavier houses put more strain on the foundation. On the other hand, lower-grade concrete foundations can support smaller, lighter homes with safety. Balanced support and prevention of potential damage are ensured by matching the weight of your house with the concrete grade.

Selecting the right kind of foundation is equally crucial. The requirements for various types of foundations—such as slab, crawl space, or basement foundations—vary. Every foundation type interacts with the ground differently and distributes weight in a different way. By choosing the appropriate concrete grade for your particular type of foundation, you can preserve structural integrity and prevent future expensive repairs.

You can choose the concrete grade for your foundation with knowledge if you carefully consider these three factors: type of soil, type of house, and type of foundation. This choice will make a big difference in your home’s longevity and safety, ensuring that it will last for many years. Recall that choosing the appropriate concrete grade now can prevent headaches and costs down the road.

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