Foundation concrete: brand, aggregates, proportions

The basis of a reliable home is a solid foundation, and the strength of the foundation is largely determined by the sufficient margin of safety of concrete, as well as its other characteristics: frost resistance, and at a high level of groundwater – water permeability. In order for the house to stand for a long time and without problems, you need a correctly calculated brand of concrete for the foundation. What it is and how to define it will be discussed further.

The composition of concrete for the foundation

Concrete is a material consisting of:

• Astringent. Most often it is cement (Portland cement). There is also non-cement concrete, but it is not used for foundations.
• Placeholders:
  • sand;
  • rubble or gravel.
• Water.

The brand of concrete is determined by the proportions of all these components, as well as the conditions of its hardening (setting). Optimal conditions for concrete to gain strength are created at a temperature of +20°C. Under such conditions, the process is very active in the first 7 days. During this time, concrete gains about 50% strength. With these parameters, it is already possible to continue construction further. The design strength, which is taken as 100% during design, is gained under such conditions in 28-30 days. In reality, the process continues further, but at a very slow speed. The strength gained after 30 days is not taken into account anywhere – it goes “to the reserve”.

When the temperature drops, the setting time increases significantly (at +15°C it takes about 14 days to reach 50% strength). At a temperature of +5°C, the process practically stops, and under such conditions, winter concrete is already needed – with appropriate additives and / or measures to increase the temperature (wrap, heat in a mixer, use heating through the formwork or heat directly by attaching heating cables to the formwork from the inside ).

Cement

Various types of Portland cement are used to make concrete. The most common are:

• Portland cement – begins to set no earlier than 3/4 hours and no later than 3 hours after mixing. The end of setting is after 4-10 hours.
• Portland slag cement – after kneading, depending on the temperatures and parameters of the solution, it begins to set after 1-6 hours, finishes after 10-12 hours.
• Pozzolanic Portland cement – hardening begins after 1-4 hours, ends after 6-12 hours.
• Aluminous cement – begins to harden after 1 hour, finishes after 8 (but not later).
  

Any of these types of binder can be used to prepare concrete. Only you will need to take into account the setting time of the mortar – you need to lay and vibrate it before the start of hardening.

Placeholders

Aggregates also affect the quality of concrete. It is necessary to adhere not only to the recommended proportions, but also to quality indicators – moisture and granularity.

Sand

Depending on the size of the grains, the following types of sand are distinguished:

• large size of grains of sand 3,5-2,4 mm,
• medium – 2,5-1,9 mm,
• fine 2,0-1,5 mm;
• very small 1,6-1,1 mm);
• thin (less than 1,2 mm).
  

For backfilling, mainly large and medium are used, less often – small. The sand must be clean – not contain any foreign inclusions – roots, stones, plant debris, pieces of clay. Even the content of dust and silt is standardized – they should not be more than 5%. If you decide to “mine” the sand yourself, check the amount of contaminants.

To test 200 cu. centimeters of sand are poured into a half-liter container (jar, bottle), filled with water. After a minute and a half, the water is drained, poured again and the sand is shaken. The procedure is repeated until the water is clear. If there are 185-190 cubic meters of sand left. cm, it can be used – its dust content does not exceed 5%.

You need to pay attention to the moisture content of the sand. All proportions are based on dry ingredients. Even dry and loose sand has a moisture content of at least 1%, ordinary – 5%, wet – 10%. This must be taken into account when dosing water.

Crushed stone and gravel

Crushed stone is obtained by crushing rocks. Depending on the size of the fragments, the following fractions are distinguished:

• extra small 3-10 mm;
• small 10-12 mm;
• average 20-40 mm;
• large 40-70 mm.
  

For the preparation of concrete, several fractions are used – so the distribution of crushed stone by volume is more uniform, and strength increases. The size of the largest fragments is normalized: it should not be more than 1/3 of the smallest size of the structure. In relation to foundations, the distance between the reinforcement bars is taken into account. SNiP also determines the amount of small gravel: it must be at least 1/3 of the total volume.

Gravel has approximately the same fractions and sizes, but when using it, the water-cement ratio (water / cement or w / c) increases by 0,05 (5% more water needs to be poured).

Water

For the preparation and watering of concrete, drinking water is used. Including the one that can be drunk after boiling. Sea water can be used with Portland and alumina cement. Any other technical water is not suitable.

Brand of concrete and its choice of strength

Depending on the characteristics of concrete, it is divided into compression classes and their corresponding grades. This correspondence is shown in the table. 

The compressive strength of concrete is determined during testing. This parameter reflects the load that this concrete can endure without signs of destruction during prolonged exposure. According to this characteristic, concrete is selected depending on the load that the house will create (it depends primarily on the weight of the walls and ceilings, but a lot of factors are also taken into account, including snow loads).

When designing the foundation and determining its dimensions, the load from the building is calculated. This figure is one of the brand selection parameters. Find in the middle column of the table a value close to the calculated load and determine the brand.

An example of calculating the load from the house look here.

Grade of concrete for the foundation, depending on the materials of the walls and soils

Projects and calculations are not always carried out. During the construction of dachas or baths, developers prefer not to spend money, and develop the structures themselves. And although there are many brands of concrete, in private housing construction, three are mainly used for the foundation:

• M 200. This concrete is used when pouring the foundation for light houses – from a log or bar, panel type, light building blocks (foam blocks, gas silicate, expanded clay concrete).
• M 250. This brand of concrete for pouring the foundation of one-story buildings made of bricks and other materials of the same density, with concrete slab ceilings.
• M 300 brand of concrete for the foundation of a brick house, but in 2 or 3 floors.
  

Still need to make adjustments, taking into account the type of soil under the foundation and the level of groundwater. If the soils are sandy or rocky, and the water is below the freezing depth, then all recommendations remain valid. If the groundwater level is high and the soils are heaving, the concrete grade is taken a step higher: the conditions are more difficult and a larger margin of safety is required.

Selection based on the type of foundation

The grade of concrete for the strip foundation with deep water (below 2 meters) and an average degree of reinforcement is selected according to the previous recommendations:

• for light buildings – M200;
• for one-story brick – M250;
• for two and three storey brick – M300.

In the manufacture of a foundation-slab or tape, but with a high level of groundwater, the solution is taken no lower than M350. Moreover, the aggregate should be only gravel (not crushed stone), and not chalked. For pouring bored piles, M200 or M250 is used.

From all of the above, it follows: what brand of concrete is used for the foundation depends on:

• the material from which the house is built;
• from soils under the foundation;
• from the depth of groundwater;
• of the foundation type.
  

All this seems complicated, yes, in fact it is, but after all the calculations, in 90% of cases in the construction of small houses, summer cottages, baths, garages, concrete grade M200 or M250 is used. And its strength is usually more than enough. The main thing is to maintain the proportions correctly when mixing, mix everything well – only a homogeneous solution can give the required strength. It is also important to treat the solution well with vibrators for concrete when laying. This treatment alone increases the brand of concrete by one step (it becomes more durable, frost resistance increases, water is less absorbed).

How to make a concrete mixer with your own hands, read here. 

Other Features

In addition to the main characteristic – strength, concrete has a number of other indicators that can affect the choice of grade under special construction or operating conditions.

• Frost resistance – denoted by the letter F. Shows how many freeze-thaw cycles a given mortar withstands without loss of strength. It is clear that the higher this indicator, the better (see table). The brand of concrete for frost resistance for the foundation is taken into account during the construction of unheated premises (baths, summer cottages for periodic visits). When building houses of permanent residence, this indicator can be ignored, especially if the foundation is insulated and an insulated blind area is made around the perimeter of the house.
• Waterproof. Denoted by the letter W. This indicator determines how much moisture the concrete will absorb. After the letter W, there are numbers from 2 to 10 (in fact, there are also W20 brands, but they are not used in private construction). W2 and W4 – high and normal absorbency. Concrete grades with such water resistance can only be used on well-drained soils (sands and sandy loams) with deep groundwater. At W6, moisture absorption is already reduced, and a solution with such characteristics can be used for foundations on soils that do not drain water well. With water resistance, W8 absorbs only 4,2% of water by weight, and W10 even less.
  

• Comfortability. It is denoted by the letter P, after which there are numbers from 1 to 5.
  • P1 – draft of the cone 1-4 cm, semi-rigid or low-plastic solution (almost does not slip from an inclined shovel);
  • P2 – draft of the cone 5-9 cm, medium-plastic – slides from an inclined shovel;
  • P3 – draft 10-15 cm, semi-cast – when it flows down from a flat shovel;
  • P4 – draft 16-20 cm – cast – almost fluid.
  • P5 – the cone settles more than 21 cm, the solution flows.

As for workability, it affects how comfortable you will be with the mortar. For foundations, concrete with characteristic P2 is most often used (with frequent reinforcement, P3 is used). It is permissible to use P1 with the obligatory treatment with a vibrator for concrete – it settles the solution well, making it more homogeneous.

Given all these characteristics, we can recommend the following grades of concrete for the foundation for one-story houses.

Remember that when building a two-story house, the concrete grade increases by one step from those shown in the table.

Proportions of components for concrete

In the formulations of the preparation of concrete are given by weight or volume. Moreover, cement is always taken as 1, and all other components are taken in relation to it. Proportions for commonly used brands are shown in the table.

As you can see, the same concrete can be obtained using different brands of cement. What it is, fall asleep like that, only adhering to the recommended mass (or volume) fractions. Basically, when pouring foundations, Portland cement M400 and M500 are used. When composing the mixture, remember that if gravel is used instead of crushed stone, 0,05 more water is added.

How to decipher table instructions? For example, let’s take the Grade of concrete M250, it will be made of Portland cement M400. Let’s focus on mass. Then select the corresponding row in the third column: 1/2,1/3,9. This means that for one kilogram of M400 cement, we take 2,1 kg of crushed stone and 3,9 kg of sand. The amount of water can be taken from the table below – it is 0,65 kg for crushed stone backfill.

If we make the same concrete, focusing on volume, for example, a 10-liter bucket, then we take the proportions from the fourth column: 10/19/34. This means that for 1 bucket of cement, we put 1,9 buckets of sand, 3,4 buckets of crushed stone. The water/cement ratio remains the same: 0,65 pails.

Sometimes you need to calculate how many components are required per cube of concrete. These data are shown in the following table.

Another table summarizes the data on the consumption of M400 and M500 cement per cubic meter of the most popular concrete grades in the construction of the foundation.

The right brand of concrete for the foundation is the key to its strength. But high-quality compositions are very expensive. If, according to all the above recommendations, you need to use the M300 and above, it is better to order a foundation calculation that will confirm or refute your assumptions. The cost of this service is about $100-150, and it can save thousands if you need a lower brand.