Proportions of concrete: composition of the mixture, ratio, how to knead

Concrete is the bread of construction. Any construction project cannot do without it, various structures and products are produced from construction – foundations, walls, stairs – to decorative ones – benches, garden paths and small architectural forms. And all this is a simple mixture. True, the proportions of concrete and its characteristics can be very different – for the performance of different tasks.

Composition of concrete: components, their dimensions and characteristics

For the most part, concrete consists of three main components:

• The binder is most often cement, sometimes lime.
• Aggregates – sand, crushed stone, pebbles.
• Water.

A different amount of just three components gives a wide range of qualities and characteristics. To impart special properties, various additives and additives are also used, which further expands the scope of this material many times over.

The main characteristic of concrete is its strength or the load that it can withstand for a long time without loss of strength characteristics. It is this parameter that is key when choosing a brand of concrete for the foundation. Water permeability, frost resistance can also be important. But these are the characteristics of the “matured” material, which depend on the recipe. And when kneading, you may be interested in such a characteristic as workability. It reflects the degree of fluidity of concrete and depends on the amount of water in the composition. It is possible to increase fluidity without adding water with the help of additives, as well as to increase frost resistance and water-repellent properties.

The strength of concrete depends on how accurately the recipes were adhered to, on the quality of the components and on how thoroughly everything was mixed. Only with a homogeneous composition and high-quality components can design characteristics be achieved. Read more about what components can be used and the requirements for them at the end of the article.

Concrete marking principle

The main characteristics of concrete are its strength and compressive class. The compression class is indicated by the letter “B”, followed by class numbers from 3 to 40, the strength grade is indicated by the letter “M”, followed by numbers from 50 to 1000. They indicate the maximum load that this type of concrete can endure. For example, the M300 brand means that the maximum load per 1 square centimeter cannot be higher than 300 kg.

In private construction, the M200-M250 brands are most popular, M300-M350 concrete can be used for the foundations of two-story houses, M400 is poured much less often – for heavy buildings on difficult soils. Higher ones are rare. Their scope is industrial construction and objects with special properties (piers, dams, roads, etc.).

The correspondence between concrete grades for strength and compression is given in the table (used in private construction).

Proportions of concrete of different grades

All this range and range of qualities is obtained by using the same materials, just in different quantities. To achieve the required characteristics, it is necessary to observe the recommended proportions strictly.

When building your house, you want to do everything as best as possible, and therefore, when preparing concrete, there is a desire to add more cement: to make it stronger. You shouldn’t do this. It won’t get any better, but it will get worse. To gain strength, concrete needs a certain amount of water and other components. If there is little water, there is a lot of cement, the bonds between the particles are formed in insufficient quantities, which can cause the concrete to crack and crumble. The same applies to the number of placeholders. And too much of their content, and insufficient, negatively affects the qualities of the concrete stone.

The proportions of concrete are usually displayed in fractions. The amount of cement is taken as a unit, and the remaining components are prescribed in relation to it. The data are given in the form of tables for the respective grades, the units of measurement must be indicated. You can see such a table of concrete components below.

How to determine the required proportions of concrete from this table? In the second column, find the required brand of concrete. For example, you need an M250. Depending on which Portland cement you will use M 400 or M 500, choose one of the two lines. The third column shows the proportions for concrete in kilograms: for 400 cement, this is 1 / 2,1 / 3,9. This means this: to obtain concrete grade M 250, for 1 kg of Portland cement M400, 2,1 kg of sand and 3,9 kg of crushed stone must be added. Similarly, you determine the proportions for concrete M200 – the data for it in the table are slightly higher, or concrete M 300 – slightly lower.

The fourth column shows volume fractions: all components are given per 10 liters. They are chosen in the same way.

Such tables do not indicate the amount of water. It depends on how thick you need the solution. The water-cement ratio is given in separate tables. For example, below are data on the number of inputs in relation to a kilogram of cement, assuming the use of medium-sized aggregates.

For example, to obtain concrete grade M 300, the proportions of cement M 500 and water are defined as 0,61. This means that for 1 kg of cement, 0,61 liters of water (610 ml) are added to the solution. In this case, a medium-plastic solution is obtained, which is used most often. But when pouring foundations or other structures with thick reinforcement, a plastic mortar may be needed. Then, when determining the amount of water, in addition to the brand of cement, it is also necessary to take into account the size of the aggregates and how fluid the mortar should be. These data are presented in the table below.

Sometimes it is necessary to determine how much cement you need for a particular task. To do this, you need to know how much cement is contained in a cubic meter of concrete. Data on grades of concrete and cement can be found in the table below.

Areas of use

With what materials are required for concrete in what proportions have you decided, but what brand is needed? It depends on the purpose of the structure and the conditions of its operation. It will be easier to navigate if you know what grades of concrete can be used for what (we will name only those that are used in the construction of a private house, its repair or landscaping).

M100 (B7,5). This is the so-called lean concrete. It is used to prepare the site for critical structures. For example, during the construction of a strip foundation, a layer of lean concrete is laid on a gravel-sand bedding, and then reinforcement work begins. The same composition is used when laying a curb stone, for example, in the manufacture of paths or blind areas around the house.

M150 (B12.5). This composition is used in preparation for a slab foundation, for screeds, pouring concrete floors or garden paths. This type of concrete can be used to make foundations for small light buildings such as a wooden bathhouse or a small guest house made of timber or logs.

M200 (B15). One of the most popular brands of concrete. Foundations of any type are made from it for light houses on normal soils, screeds, stairs, blind areas, paths. Cement blocks are made from concrete of this brand at home, it is also used in factories for the manufacture of foundation and building blocks.

Read about how to build your own foundation from FBS blocks here.

M250 (B20). The scope is almost the same, but in more difficult conditions. They make any foundations on difficult soils, or on normal ones, but for houses built from heavy materials. They make blind areas that will be used as paths, outdoor stairs, concrete the porch, fences, etc. It is also used to make floor slabs for light loads.

M300 (B22,5). Also suitable for all the above areas, but in even more severe operating conditions. They make foundations for heavy houses on heaving soils, make monolithic walls, paths, a waterproof blind area, etc. Floor slabs and grillages for pile-grillage foundations are mainly made from this brand of concrete.

M350 (B25). The strength of this brand for private construction is mostly excessive. This concrete is used for the construction of monolithic pool bowls or for the manufacture of foundations at high groundwater levels, for other structures requiring high water resistance. This brand is already more often used in industrial construction.

M400 (B30). This is already an expensive brand of concrete, which is used in facilities with special requirements: for large pools, dams, bank storages, etc.

Concrete preparation

For large volumes of work, it is better to order concrete at the factory. Making a large amount of mortar by hand or even using concrete mixers is a difficult task, and laying in batches requires extra effort to ensure that the layers adhere well. However, concrete can also be prepared by hand. In this case, there are two sequences of actions:

1. First dry mix concrete and sand. It is mixed until the color becomes uniform. Then crushed stone is poured, everything is mixed again, and water is added last.
2. First, water is poured into it – cement. When everything is mixed, sand is added and then coarse aggregate.
   

In the first option, it is possible that with manual mixing at the bottom, near the walls of the container, an unmixed composition will remain, which will lead to a decrease in the strength of concrete. The way out is to mix everything well and thoroughly. But you can’t spend too much time on this: the solution will begin to set.

The second option has its drawbacks: it sometimes takes a long time to obtain a homogeneous cement milk (a mixture of water and cement). As a result, it is simply not enough to form bonds with the backfill: the cement “seizes” and the strength of the concrete also decreases.

All this is not so critical when using concrete mixers, but it is also imperfect. There is another difficulty here. Concrete is delivered to the construction site, usually in carts. The entire volume does not fit into one, and the remainder is left to spin in a concrete mixer. This is better than just leaving it standing, but if it is stirred too long, the mortar may begin to delaminate, resulting in lower concrete strength. The way out is two carts and two people who will carry them. The way of backfilling – the first or the second – choose for yourself.

So all the same, how to prepare concrete. The choice is yours. If the volumes are small, you can knead by hand. Just do it carefully. To pour the foundation, it’s better to order a mixer, but you can also use a concrete mixer (or two, depending on the volume). And to solve problems with the heterogeneity of the batch (although it is better that it be good), process the laid concrete with a vibrator. Most of the problems will go away.

Next, let’s talk about the requirements for concrete components, their dimensions and qualities.

Requirements for cements for concrete

For most construction work, cement concrete is used, where Portland cement is used as a binder. There is also lime, but its scope is limited mainly to finishing work, which is done “the old fashioned way”.

Types of portlancements and their storage

There are several types of Portland cement – slag Portland cement, aluminous and pozzolanic. All of them are slightly different in characteristics, but any is suitable for private construction. The difference can only affect the setting time: Portland slag cement does not harden the longest – up to 12 hours, then comes standard Portland cement – up to 10 hours, and aluminous binder hardens the fastest – no more than 8 hours.

Cement is demanding on storage conditions, and especially on humidity. For the manufacture of important structures – foundations, ceilings, etc. it is advisable to use fresh, recently released from the factory. Within a month, it loses up to 10% of its properties, and after 6 months they deteriorate by 30-35%. Therefore, for example, for pouring the foundation, it is better to take it a maximum of two weeks ago, and purchase it shortly before use.

Store in a dry ventilated area. If there is no room, they are folded under a roof or wrapped from moisture with several layers of film. Pay attention – wrapped, not covered. And preferably not on the ground, but on a wooden flooring. The thing is that when moisture gets in, even in a vapor state, the cement becomes lumpy, which greatly worsens the characteristics of concrete. With an abundance of moisture, it simply becomes a stone and there is no way to use it. Therefore, take care of the place for storing cement in advance.

Cement marking

What brand of cement to take is usually indicated in the concrete recipe. It is denoted by the letter M and numbers that indicate the maximum concrete strength that can be achieved with this binder. For example, with cement grade M400, it is possible to obtain concrete grade M400 as much as possible, as well as lower ones.

Next comes the letter “D” and the numbers indicating the amount of impurities. M400 D15, means that the impurities in the binder are 15%. For construction work, this figure should not be more than 20%.

Aggregates – crushed stone and sand

The composition of concrete is determined by the functions and characteristics of concrete that are necessary during its operation. The most common are sand and gravel. They are subject to no less stringent requirements than the quality of cement. Pebbles are sometimes used, but only if they have sharp edges, not round ones. In the presence of broken lines, the adhesion of the aggregate to the mortar is better, as a result, the strength of the concrete is much higher.

Sand

Construction sand can be river or quarry. River is more expensive, but it is usually cleaner and has a more uniform structure. It is best used in the preparation of concrete for pouring foundations, screeds. For masonry or plaster, it is appropriate to use cheaper quarry sand.

In addition to origin, sand is distinguished by fractions. For construction work use large or medium. Small and dusty are not suitable. The normal size of sand grains is from 1,5 mm to 5 mm. But optimally in solution, it should be more homogeneous, with a difference in grain size of 1-2 mm.

The purity of the sand is also important. It definitely should not contain any extraneous organic inclusions – roots, stones, pieces of clay, etc. Even the dust content is standardized. For example, when mixing concrete for a foundation, the amount of contamination should not exceed 5%. This is determined empirically. 300 ml of sand is poured into a half-liter container, everything is filled with water. After a minute, when the grains of sand settle, the water is drained and poured again. This is repeated until it is transparent. After that, determine how much sand is left. If the difference is not more than 5%, the sand is clean and can be used when mixing concrete for the foundation.

For those works where the presence of clay or lime is only a plus – when laying or plastering – there is no need to take special care of the cleanliness of the sand. Organics and stones should not be, and the presence of clay or lime dust will only make the solution more plastic.

Crushed stone

For critical structures – floors and foundations – crushed crushed stone is used. It has sharp edges that better adhere to the solution, giving the structure greater strength.

Fractions of crushed stone are standard:

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

In concrete, several different fractions are used simultaneously. The largest fragment should not exceed 1/3 of the size of the smallest element of the structure to be poured. Let’s explain. If a reinforced foundation is poured, then the structural element that is taken into account is reinforcement. Find the two elements that are closest. The largest stone should not be more than 1/3 of this distance. In the case of pouring the blind area, the smallest size is the thickness of the concrete layer. Crushed stone is chosen so that it is no more than a third of its thickness.

Small gravel should be about 30%. The rest of the volume is divided between medium and large in an arbitrary proportion. Pay attention to the dustiness of rubble. Lime dust is especially undesirable. If there is a lot of it, the crushed stone is washed, then dried, and only after that they are poured into concrete.

Placeholder storage

It is clear that the construction site is not the cleanest and most equipped place. And sand and gravel are often unloaded directly onto the ground. In this case, when loading, it is necessary to ensure that no earth gets into the batch. Even a small amount of it will negatively affect the quality. Therefore, it is desirable to pour aggregates on solid areas.

It is also necessary to protect them from precipitation. In concrete formulations, the amount of components is given in terms of dry components. Considering the moisture content of components is learned with experience. If you do not have one, you have to take care of the condition and cover the sand and gravel from rain and dew.

Water

Drinking water must be used to obtain concrete of normal quality. So it is written in SNiP: “drinkable, including after boiling.” You can’t take water from a river or lake, technical water even more so. No contaminants, acids, salts, alkalis, oils, etc. All these substances negatively affect the strength of concrete, and worst of all, the result is impossible to predict.